Initial Ohio Valley Chase Forecast for Friday 20 July, 2018

  It's pretty late tonight and I've been looking over everything I can trying to put together the pieces for this weekend's setup and I thought why not go ahead and try to dive into it with everyone for a bit. I will be breaking everything down in a lot more detail Thursday night because I expect to have it all narrowed down even more over the next 24 hours but I'm confident enough to at least get into it so let's go. Side note: Although I will do the usual "target forecast" blog post within 18 hours of the event the reality is that I'm likely not going to have to go far at all since I have a feeling based on how it looks to me that my driveway will be the staging point while waiting for initiation. I think supercells will be likely and tornadoes, large hail, and damaging winds are very possible. Currently the plan is to be patient, see how things set up, and once CI (Convective Initiation) seems imminent position just East then ride along with the storms as the greatest dynamics come into play, stair-stepping southward if needed.

  First of all, it's my first chase since the end of June in the when we chased everywhere from South Dakota/Minnesota to Kentucky to the Oklahoma/Texas panhandles so it's nice to be back in the saddle. I've been busy with college but there hasn't been much more than diurnally-driven Summer storms anyway, save for a few supercells tonight in Nebraska and occasionally some stuff up the Atlantic Coast. Currently the SPC has delineated much of western Kentucky and southern Indiana/Illinois in a "slight" risk for severe weather but I fully anticipate this will be upgraded by Thursday or Friday (as of typing time of 10pmCDT). Do not discount the threat regardless though because this is just my opinion based on my own analysis. I think the greatest challenge is outlining the areal extent and duration of severe weather. I also want to mention that there is always increased anxiety when it looks like my home and family/friends are in the same area I'm targeting for dangerous storms so I certainly want them to be paying attention as well. It's not that I'm not anxious when I'm chasing anywhere else in the Country for the folks involved, just that anxiety is even higher when I'm out chasing and trying to watch everyone I know at the same time.

Current Analysis

Regular followers know that I always like to start a forecast by looking over what's going on in the here and now. I know I probably spew this to you guys until we're all blue but it's just how it is. That said, since I'll be updating things again in the final target area post Thursday evening and in the purpose of brevity I'm not going to post every single thing I've been looking at but I'll post a few keys that sum up the current pattern.

Current radar imagery shows pretty intense convection over Nebraska and the Dakotas associated with our cyclone upstream. Some of these storms have been maintaining supercellular characteristics for awhile now with several reports of very large hail and even a few tornadic storms. Elsewhere, we can see scattered convection lingering along a stalled frontal zone across the southeast CONUS. Much of this activity is dissipating at this time.

Above is the 0z 250mb obs. We can easily pick out our upstream speed max associated with a relatively far southward dive in the Polar Jet as it flows across the Pacific coast and into Nebraska. It's a fairly amplified streak with a core of 80kt flow. It's not the 100kts or more you expect in the "cool season" but sufficient nonetheless. We can also see a broad ridge over the SW CONUS and a pronounced trough over the NE. These features will serve to provide the general track for our mid-level wave.

Dropping down to 500mb, pictured above. Our shortwave trough over South Dakota is evident with a 50kt speed max entering its base at 0z. This speed max will play a big role Friday I belive as we often see very strong instability but weak shear during the "warm season" but this will help procide plenty of shear for more organized storms. I expect this shortwave to amplify further, given the fact that the jet streak is still located in its upstream region. It should track generally East as it is steered by the ridge over the SW and trough over the NE, before eventually phasing with the deep eastern trough this weekend.

This is the geopotential height tendency over the past 12 hours. This is just a good resource to asses the amplitude of  mid-level waves as well as track their movement, and areas of height falls can also be linked with ascent given the temperature advection process that occurs. 30m falls isn't particularly strong at all but this shortwave just recently developed and has yet to mature and undergo intensification so I think greater height falls will overspread the Ohio Valley on Friday. I have been tossing around the idea of doing a detailed tutorial on dynamics such as this but just haven't had the time. One day...

We'll drop on down to 850mb now. In the Summer moisture is rarely an issue but I don't think it's a good idea to leave anything to climatology. I'm pretty sure a major goal of forecasting is to out perform climatology actually so we should assess moisture quality. We can see the middle bound of our surface-low associated with our developing upper-low over SD with a strong area of high-pressure over the Ohio Valley. Deep moisture, in many locations over 16C, extends well into the central plains after rounding the periphery of the high-pressure system and surging through the warm sector. A strong temperature gradient associated with the frontal zone aloft is also evident.

This is just another product I like to use to assess moisture. My favorite product is mixing ratios which is the ratio of moisture in grams per kilogram of air but this is precipitable water (PWAT) and is a parameter that attempts to calculate the amount of precipitable water in a vertical column. PWATs of 1.5" extend up into the Dakotas with very high PWATs of 2"-2.2" still waiting to be tapped into in the source region. 

Now we get to the surface. I will try to post my manual hand analysis for the next blog but since it's already late it was just way easier to copy the WPC analysis. It's worth pointing out that there have been some pretty major storms over Nebraska this evening so there are some influences of that in the obs. We can still see our 1008mb surface-low over South Dakota with a narrow warm sector. That is heavily modified by convective reinforcement of the frontal boundaries. A stalled front extends East-West across the SE into TX/OK near the Red River. Like I said, moisture isn't expected to be a problem in terns of how much moisture exists but rather determining where it will end up over time. We've got 70+ dew points across much of the southern and central CONUS, only suppressed where the area of high-pressure is sitting near the Great Lakes. 1008mb isn't terribly deep but as the shortwave deepens and temperature/moisture gradients tighten  this low will deepen a few mb as well. This low is well connected to the trough aloft (meaning it's not a shallow system) so it will take a generally eastward track as well and this will lift a warm front northward and drag a cold front eastward. With the low progged to be near the Great Lakes by Friday we can infer that we'll have at least a narrow warm sector over some portion of the Ohio Valley. 

I'm not going to share a bunch of sounding analysis yet as I will do that in the next post but I like to analyze many observed soundings from the upstream air mass at this point.
We've already developed a solid conceptual model of what the general setup will be. Now part of the challenge is determining when/where/if CI (Convective Initiation) will occur as well as what the preferred storm mode will be. We'll look at some model data in a second but before we do let's review our setup. Understand that there's all of Thursday in between now and our event so I have had to forecast for it too as I wouldn't recommend forecasting an event without knowing what leads up to it because the overnight storms on Thursday could play a key role in how Friday plays out. (I could go on about how an overnight MCS in Kansas recently forced me to shift my target area about 90mi southward before finally seeing storms initiate along the dryline but I'll spare you).
We know we've got a shortwave over South Dakota sandwiched in between a broad SW ridge and NE trough with a slightly amplified speed max in its upstream flank. A surface-low and attendant frontal boundaries are located beneath this shortwave with deep moisture riding the periphery of a SE high-pressure area. Steep lapse rates are located over the central plains. Our shortwave will amplify and track generally eastward with height falls spreading over the Ohio Valley by Friday afternoon coincident with the exit region of a northwesterly 60kt 500mb speed max nosing into Missouri/Kentucky/Illinois. The surface-low will also track eastward toward the Great Lakes while establishing a warm sector across much of the Ohio Valley while the westerly component of flow aloft will draw the plume of steep lapse rates atop the warm sector. That's a typical severe storm environment: strong moisture extending well throughout the boundary-layer, steep lapse rates overlapping the warm sector, strong boundary-layer heating steepens low-level lapse rates and aids destabilization, speed max digging in with upper-trough providing sufficient shear profiles, focused ascent along frontal zone and large-scale ascent with jet streak exit region should provide enough forcing for CI.

Forecast: How Does This Go?

Enough of that though, you guys want to know WHY I think this setup bears chasing. As I said, we have already inferred that we'll have an environment certainly supportive of of severe storms. Even the models agree and have been pretty consistent both with each other and from run-run. The biggest trick is trying to anticipate how Thursday's storms will affect the setup. Strong-severe storms are expected to fire across Missouri and the Mississippi River Valley Thursday afternoon and these storms will track ESE. They will be crossing Kentucky through Friday morning and this may act to position the risk area a bit. Here's how I see it playing out though. These storms should likely be over central Kentucky by mid-morning on Friday and should be rooted above the boundary-layer by then when they'll clear I65. If this convection is stronger or weaker than anticipated or has greater/less areal coverage than anticipated then that could shift the threat area in either direction because this can affect the degree of destabilization. The corridor of steepest lapse rates aloft looks like it will be located primarily over western Kentucky into extreme southern Indiana and as it couples with deep moisture and strong heating (forecast temps near 90F) MLCAPE values should be in the 2,000-3,000j/kg range. The region should be relatively uncapped except for areas East of I65 where the morning convection may factor in. It seems likely that storms will fire by mid-afternoon as soon as low-level lapse rates steepen sufficiently, likely in Illinois and toward the Ohio River near the "tongue" of the theta-e axis after which they will track ESE into Kentucky where stronger thermodynamics and shear profile supportive of both persistent rotating updrafts and low-level circulations should result in primarily supercell storms until late evening when upscale growth seems more likely. The aforementioned mid-level speed max will be nosing into western Kentucky by noon on Friday and this will help increase deep-layer/effective shear to over 40-50kts. Although low-level SRH appears weaker during the early afternoon very strong buoyancy in the lowest level (0-3km MLCAPE over 200j/kg) as well as critical angles near 90 degrees and strong pressure pertubations associated with strong mesocyclones could compensate and support a tornado threat given vortex stretching and forecast hodos still show sufficient curvature with veering wind profiles, but the "golden hours" will be when the storms are likely in western Kentucky and the strengthening low-level jet will interact with supercellular storms in a already favorable environment and late-afternoon could pose the greatest risk for tornadoes. This is part of the reason this area is my current target. Be aware though that regardless of the tornado outcome any of these storms will be capable of large hail and damaging winds along with intense rain and dangerous lightning.

NAM3k forecast for 0-1km storm-relative helicity just as the LLJ is expected to be increasing. Values are forecast by this model to be in the 200m^2/s^2 range which is more than capable of supporting tornadoes. Values may even increase more as the LLJ intensifies.

You can pretty much see the mid-level speed max in the model forecast 0-6km shear graphic as deep-layer shear will be supportive of organized storms where this core noses in.

Pictured above is the NAM model trend for 500mb heights over the last few runs. There is general agreement but notice that it has been trending with a more amplified wave with each successive run (thicker contours=more recent run)

This is a forecast graphic from the GFS model depicting the height tendency equation (Like the height falls graphic from earlier). Notice that it has an area of height falls spreading over the Ohio Valley preceding the trough as it moves into the region on Thursday night. 

NAM3k output for MLCAPE just prior to when CI is expected. You can see the heavy influence of the morning convection East of I65 with strong instability developing West of it. These morning storms are something I'm going to be following along with in real-time very intently because they could modify where this instability axis is located.

MLCIN from the NAM3k model shows the stabilizing influence of the morning convection as well.

A great deal will be riding on mesoscale details and I'm going to be very curious to see how the mesoscale features evolve so I'll be tightening up my chase forecast tomorrow as well as posting videos breaking down the threat so stay tuned because there's at least the potential for a substantial severe weather event.

Widespread Severe Weather Returns to the Plains

An early Summer severe weather outbreak looks to be taking aim at the central plains this weekend into early next week. Severe storms capable of damaging wind and hail, and perhaps a low-end tornado threat although high-based, elevated storms should temper the overall tornado risk, are expected across portions of Oklahoma and Texas eastward. The "peak" of activity appears to be tomorrow (Sunday 24 June) however. A more significant scenario has the potential to unfold across most of Kansas into the Oklahoma/Texas panhandke region. All hazards are possible tomorrow as initial supercells will be capable of tornadoes, large hail, and damaging wind. Through the evening, an intense MCS is expected and extreme wind gusts as well as embedded mesovortices will be possible. A flood threat may develop too if storms end up backbuilding.
From our perspective as chasers we're considering two possible target areas. Target #1 is the sfc-low/triple-point in Southwest KS to the OK panhandle. Target #2 is near Hays, KS along I70.
I'm leaning toward the triple-point play because, for one it's the triple-point, a classic target for us. Also, I feel fairly confident that storms will be more discrete here and the environment should support strong supercellular storms. I'm thinking the cap will break across the panhandle by 21-23z (4-6pm) and this would put us in the vicinity of the "burst point". I'm also somewhat concerned with early-day convection and a possible upstream cluster moving in from Nebraska with the northern target.

A substantial overlap of ingredients favorable for severe storms is suggested in the vicinity of the triple point in southern KS. The area circled in black is the region I'm targeting for supercells and possibly a few tornadoes. (Note: Significant severe weather is possible outside the circled area, that is just the region we are targeting)

Either way, significant storms are expected across the region Sunday with both the initial supercells and the eventual powerful QLCS and anyone in the area should monitor multiple sources of weather information and listen to all watches and warnings.

The Setup

So what is coming together for this fairly noteable late-June severe weather? Unlike the diurnally-driven "pulse" storms that are oh so common this time of year, we actually have a relatively strong upper-low/"kicker" swinging around the Rockies. Normally during the Summer we see extreme instability and very rich atmospheric moisture but weak flow aloft owed to the northern retreat of the polar jet results in high CAPE-low shear setups, in other words, a lot of disorganized convection. In this case however, we have a 50kt speed max associated with our mid-level "kicker" that's going to lead to stronger deep-layer shear atop our Summer instability. Steep lapse rates overlapping a very moist & unstable boundary layer with plenty of effective shear sets the stage for more organized severe storms. While not impossible to see this type of setup at all, it's also not the most common thing during late-June.

GOES East water vapor imagery shows an upper-low over Wyoming with an embedded shortwave trough pivoting around it. This shortwave will eject over the central plains tomorrow with a 50kts speed max overspreading the KS/OK/NE region. Surface cyclogenesis will occur and surface-low will deepen and lift northward, bringing a frontal boundary with it. 

A "theta-e" axis associated with out moist sector will be established across the target area providing plenty of "juice" to work with. 

A plume of very steep lapse rates atop a very moist boundary-layer will aid in strong instability developing tomorrow afternoon. Effective wind shear of 40kts and backed surface winds, especially along the boundary, may support supercells and decent curvature of low-level hodos suggests tornadoes are possible. Low-level thermodynamics also seen seem to support the stretching of low-level circulations to the ground, should they develop. 

Storm-relative helicity values support rotating updrafts, particularly after 0z (7pm) when the low-level jet gets cranking. 

There are uncertainties regarding the exact evolution of tonight's convection as well as location of outflow boundaries tomorrow. The NAM has also been a well-noted consistent outlier. We'll tighten up the details tonight and in the morning at the hotel so stay tuned for that. It looks like a busy day tomorrow though. Severe chances, though not as substantial as Sunday, will continue through Tuesday and I'll have chase forecasts for those days as well following tomorrow's chase. See ya soon!

Chasing Severe Storms in Missouri & Arkansas

Good morning! I've got to be brief today because I'm already in Missouri and I'm westbound to position for severe weather in the next couple of hours. The target is along the AR/MO border in western MO where I think a low-end tornado threat is coming together. I personally suspect an increase of severe probs from the SPC but we'll see.


So the setup today is very similar to what we saw Thursday (which did produce a brief, but photogenic tornado near Cape Girardeau, MO). It's been yet another tricky forecast with another overnight MCS blasting through the target area during the morning. That said, the atmosphere seems to be recovering well, no doubt aided by deep, tropical moisture, dew points ~70ºF+. Temperatures are in the mid 80's already as well and this has helped increase the degree of destabilization despite convective cloud cover. A semi split-flow jet stream regime is in place and ridging locked in across the area has resulted in strong heating. A speed-max upstream appears to be timed out favorably to cross over southern NO by this evening.
Today's chase really lies in the details though, particularly on the mesoscale. Fairly steep lapse rates (8ºC/km+) associated with the EML are located across OK/KS and as our speed max enters the nose of the southern stream ridge it will spread this plume of steeper lapse rates atop the moist sector, which we already mentioned is very moist and quickly destabilizing.

The real kicker today is the position of a major outflow boundary laid down from the early-period MCS, and other more subtle boundaries as well. Morning surface analysis showed this boundary was located just West of Springfield, MO extending NE/SW. I expect this will serve as the trigger for convective initiation within the next 1-3hrs. Any storms that develop will likely be severe and capable of damaging winds and large hail, given steep lapse rates (low-level and aloft), a well-mixed boundary-layer, and large MUCAPE (4000-5000j/kg) including sufficient CAPE above the -10ºC layer.

Like I said, I think the greatest risk is along and South of I44 to Little Rock, AR. Morning observed soundings showed a strong cap in place but that's been eroding throughout the day due to both cooling aloft and heating below it. A "loaded gun" thermodynamic profile with a somewhat curved hodograph suggests strong rotating updrafts may be supported. Although the tornado threat is low, it's still realistic.
Good luck to others out here today, I think there's one clear target so it should be fun in the jungle.

Alberto's Landfall and Severe Weather Chase Potential through the Weekend

It's been somewhat active the last couple days hasn't it? Tornadoes in the high plains, flooding in the East, and Alberto finally made landfall in Florida. Tonight we're going to dive in to Alberto's impact as he tracks Northward through the Tennessee Valley first, including where I think a severe storm and flood threat will exist, then we'll break down tomorrow's severe threat in the plains and identify some target areas for that.

Subtropical Storm Alberto's Landfall and Impacts

As of the time I'm typing this subtropical storm Alberto just recently made landfall in the Florida panhandle and was continuing his track northward, turning slightly northwest.  

I expect this will be the track from here on out as he moves northwest into western Tennessee and Kentucky on Tuesday and Illinois/Indiana toward the Great Lakes on Wednesday. While he will be generally weakening over the next 48-72hrs his impacts will not go unnoticed. 

Flood Threat Across The Tennessee Valley

The most prominent threat associated with Alberto is the heavy rainfall and flooding. When forecasting a flood threat I'm essentially looking for a few key factors; a source of deep, rich moisture, a degree of instability to trigger convection and enhance rates, and a source of mesoscale/synoptic lift (preferably slow moving). That said, flood/flash flood potential certainly exists over Tennessee/Kentucky as a very moist air mass, characterized by dew points in the upper 60's-low 70's and PWAT values in the 1.8"-2" range, is already established across the region upstream from Alberto. 

Southerly flow throughout the eastern half of the surface-low will help advect even more moisture into the area and convergent flow associated with Alberto's circulation along with interaction of an upper-level trough will definitely be enough to bring swaths of heavy rainfall to the area. Rain has already been ongoing well in advance of Alberto but the heaviest rain will begin with the arrival of the low-pressure center itself timed favorably with afternoon heating Tuesday afternoon. We'll get into the severe threat shortly but this rain will continue through the evening, and into Wednesday morning in Kentucky, before finally exiting to the North. Stray showers are possible in Alberto's wake as well but there is often strong subsidence following the passage of troughs like the one upstream and regardless, the heaviest rain will exit with Alberto. I think a swath of significant rainfall is possible in the western Carolinas and another 2"-3" of rainfall likely, particularly along I-65, and obviously the stronger storms will be capable of producing higher totals. This will likely result in flash flooding issues with heavy rain occurring the previous leading to saturated soil and flood guidance already being approached.

GFS Accumulated Precip through 12z Wednesday Morning. The GFS is a little more West with the corridor of highest totals than the ECMWF is but it's been handling the current track of Alberto pretty well. More importantly, it appears to account for the fact that the heaviest rainfall has been occurring nearest the center of Alberto's circulation and along a pseudo warm front in the Northeast quadrant since landfall. The ECMWF and UKMET models seem a bit more broad with the precip

High-Res CAM guidance has been supporting the recent GFS runs as well. Based on recent trends with Alberto, I chose to use a blend of GFS and NAM3k guidance to forecast heavy rain. This is not to say heavy rain won't occur elsewhere, it will, it just points toward where the longest duration of heavy rainfall will occur.

The SREF (Under-utilized in my opinion) shows similar output as above with swaths of heavier rainfall in central TN/western KY associated with the center of Alberto and another swath of heavy rain from northern GA into the Carolinas. Graphic above shows 700-500mb Vertical Velocity and 3hr QPF. This is meant to identify areas where ascent and precip are suggested concurrently by the model, hopefully helping to increase certainty. 

Severe/Tornado Threat As Alberto Moves Northward

I'll start this part off by saying that it's somewhat different forecasting severe weather, and specifically tornadoes, with a tropical cyclone. For one, you don't typically see the high instability you see when forecasting more "conventional" tornado setups, often mitigated by extensive cloud cover/rainfall. Another is the lack of deep-layer shear. Deep-layer shear is very unfavorable for tropical systems, and even though we aren't looking at a tropical system per se anymore, the environment that spawned it still needed to support tropical development to even get this far. This usually results in embedded convection in feeder bands of tropical cyclones that quickly devlop low-level circulations without a deep, persistent mesocyclone (although occasional mesocyclones are noted). They are most likely going to be identified by tighter velocity couplets, sometimes briefly preceded by a convergent velocity signature. 

The northeast quadrant (on a Cartesian plane) of tropical cyclones is the climatologically favored zone for these tornadoes to develop, and this is due to a few reasons.

This diagram, actually taken from one of my first college textbooks (Understanding Weather and Climate 7th Ed. by Aguado & Burt), plots the position of 373 embedded tornadoes between 1948-1972. 

Perhaps later in the hurricane season when I'm preparing to intercept a hurricane/tropical storm I'll really get into the nuts and bolts of all that but it basically has to do with the fact that shear profiles are traditionally much more favorable for both non-mesocyclonic tornadoes and mini-supercells. There are also subtle baroclinic zones that tend to develop here (think a storm ingesting vorticity as it rides along a frontal boundary).

Now that we've got a little bit of foundation put down let's see how it applies in the short-term. As Alberto's remnants drift forward areas in the northeast quadrant will have some increased potential for tornadoes, based on everything we just mentioned. We still need to assess the environment though and better delineate the risk area. It's no help to just blanket half of the Country with a threat, and it's definitely no use to do that when targeting for a chase, a target area of 1000^2mi isn't going to help anyone. For tomorrow's situation I timed out where Alberto will be when diurnal heating really gets going. Today's analysis shows that he's slowed down quite a bit so this looks most likely to be in central TN as he will move through there through mid-afternoon. Like I said, you don't typically see the intense instability you see in other setups but steep low-level buoyancy will enhance convective cell development and low-level vortex stretching. Forecast models are progging marginal, but sufficient instability across KY/TN for Tuesday and it's in line with what we saw in a similar setup today so it seems realistic. Plus, any breaks in cloud cover will help localized buoyancy build even more. 

HRRR shows sufficient destabilization across the region tomorrow. 

The main reason the tornado threat will exist tomorrow is due to the enhanced low-level shear profiles we mentioned. This looks to be maximized from North Georgia into East/Central Tennessee and Kentucky. Forecast soundings show around 100-150m^2/s^2 0-1km SRH in this region and this will be supportive of low-level rotation, particularly in the stronger cells that are able to redistribute this rotation back to the surface. 

Above: HRRR and SREF model guidance showing 0-1km SRH and ensemble probability and mean 0-1km SRH, respectively.

There may very well be instances tomorrow with relatively unimpressive rain showers that show signs of low-level rotation, I've seen this multiple times when chasing inland tropical systems. I think the greatest tornado threat is going to be in central TN but I can't rule them out anywhere in western KY either. Understand that I'm not forecasting major tornadoes and that's not what I'm chasing. A phrase you hear a lot with squall lines is "brief, weak tornadoes", and I hate that because I don't want to undermine any severe weather threat and build complacency, but we're not expecting huge tornadoes, at least not in the Tennessee Valley. 

Central Plains Severe Weather Potential

The most classic chase target tomorrow is the central plains. While there are a few different locations someone could target I really see two primary areas to be. They both center around the dryline across western KS into the OK panhandle. A large, elongated area of low-pressure is established from the southern plains all the way into Minnesota and there will be severe storms associated with this entire area given steep lapse rates advecting over a moist boundary-layer however the focus for chasing tornadoes seems to be OK/KS. Don't misread this, ALL severe weather is serious and I don't want other severe storms to be ignored or "written off" because they aren't tornadoes. I'm choosing to focus on forecasting supercells and tornadoes here for the purpose of chasing supercells/tornadoes. 

Now that that's out of the way let's get back to it. The boundary-layer is already sufficiently moist across the plains and continued southerly flow associated with the surface-low will only act to further moisten the boundary-layer. Dew points are already near 70F into OK and are in the low 60's northward.

0z observed sounding indicate deep moisture,14-16C dew points at 850mb, across eastern OK and TX, extending into southeast KS. 

 The big question mark tomorrow is the current convection and how it will modify the environment. Given their current eastward progress however, I don't think the current storms will limit things. The bigger uncertainty arises with the potential for morning convection. Should storms initiate in central KS Tuesday morning then they could overturn the atmosphere a bit and this may come into play. I would most certainly get up early in the morning to perform some nowcasting and assess the environment because these storms will likely limit the areal extent of later, more robust convection and if someone was playing the northern target then they may need to make some quick adjustments. Either way, the strong theta-e advection and diurnal heating will result in moderately strong destabilization ahead of the dryline (2000-3000j/kg MLCAPE) by afternoon.

HRRR guidance indicates moderate-strong destabilization taking place by mid-afternoon across western KS/OK. Convective influence is noted in NE and MO. Also plotted is 0-6km shear. The deep-layer shear is more parallel to the surface boundary in KS which suggests to me that storms will become linear muvh more quickly as opposed to western OK where boundary-relative shear will allow storms to initiate along the boundary and move away, remaining semi-discrete. It's important to note that storms in OK may face problems with short residence time along the boundary and if they move out of the area of forcing too quickly they may have trouble staying organized.

 Deep-layer shear associated with an ejecting shortwave will be on the order of 30-40kts, sufficient for organized convection. Forecast hodographs tomorrow are supportive of supercells, at least initially, and low-level curvature suggests low-level rotation is supported. 0-3km MLCAPE is forecast to be between 75-100j/kg across south central KS (I usually want to see about 100j/kg 0-3km CAPE or more for tornadoes).

SREF "composite" plot of  showing the ensemble mean MUCAPE, effective shear vectors, and 0-3km SRH. Areas where favorable values of these ingredients overlap suggest supercells may be supported, assuming you actually have storms in that area.

This graphic can get a little noisy but it's one of my personal favorite "secret weapons". It shows low-level MLCAPE and I don't usually expect tornadoes with less than 100j/kg. That's just a rough guideline though and I will never bail on a chase because of any one parameter. Regardless, it shows 0-3km CAPE in the range I like to see anyway.  

 This suggests the potential is there for tornadoes, maybe a strong tornado or two. My hang up tomorrow however is whether or not storms will even be able to initiate and take advantage of the ingredients. I always want to see lid strength indices of -1C and below and tomorrow they are in the +2 and above range. This could be an issue. I would not bank an entire forecast on one index at all but this is certainly something to monitor in real-time if following along tomorrow, especially considering there is some reason to buy this given the morning convection. 

This can also appear like a noisy product due to the high resolution but I like to use it from time to time. It depicts areas of moisture convergence. It is subject to influences from terrain but if you have a knowledge of the area you're looking you can apply it easily. Looking across western OK and KS you can see what almost looks like the areas of enhanced cumulus (cloud streets) we'd expect to see when convective initiation is imminent. This corresponds with moisture surging northward here and warmer colors indicate convergence, and you can infer ascent from this. This is a decent proxy for identifying the "burst point". This data supports the thinking that storms will fire in central KS (where convergence is strongest) and then fire in western OK.

All in all I expect the earlier storms to be confined to the area of Hays, KS into NE. The target options, as I see them, will be either the area near Hays where earlier initiation would allow for the opportunity to chase storms that will likely be supercellular before upscale growth occurs, or the "burst point" further to the South between Woodward, OK and Pratt, KS where storms would have the benefit of "cleaner" inflow and one could "stairstep" to continue playing the tail-end charlie. Any of these storms will be capable of large hail and damaging winds of course and will be dangerous regardless of whether tornadoes occur or not. It's also worth mentioning that I always get up early on chase day because things happen overnight and the best target the night before may not be the best target the day of. I've seen tornadoes on days when I changed my target by 100mi at 7am on chase day based on the position of boundaries or other mesoscale features. This is why it is imperative to perform solid analysis using current observations, radar, and satellite data. A good forecast won't mean anything if you bank on last night's HRRR and sleep until 1pm and wake up thinking storms are about to fire only to realize the dryline surged a little more eastward than expected and now you're sitting 80mi West of it, chilling in 30 degree dew points and clear skies. With all of this in mind, I personally think the southern play is the better option. That would put me somewhere near Woodward, OK or just along the OK/KS border. I expect linear forcing along the boundary and boundary-relative flow to result in upscale growth and I think that although initiation is more difficult to anticipate in this area it's the area where discrete supercells will be most likely for the longest time.

I hope everyone enjoyed tonight's post, I tried to be as detailed and informative as possible. As always, thanks for following along and we'll start picking apart this weekend's setup next time.

Chase Forecast for Holiday Weekend Tropical Storm

  Good evening everyone! We actually do have a little bit to cover tonight with this subtropical system in the Gulf of Mexico to isolated severe storms across the Plains, so let's do it. I'll start off by saying that I am packed up and ready to hit the road but I am still debating on whether I'm going South to cover the subtropical-low or heading West to chase occasional severe storms out there so today's chase forecast will simply break down all of it and take a look at all the options on the table. I'm probably targeting a marginal, but realistic tornado threat in the Gulf States as our subtropical system makes landfall. An areal tornado threat can't really be outlined at this point because there is too much uncertainty regarding where the storm goes but I expect at least some increase in tornado probs as we get closer to "go time" and he's making landfall. Enough of my rambling though and let's dive in to the interesting stuff...the analysis.

Subtropical Storm "Alberto"

Coincidentally, during the typing of this blog, the NHC issued advisory #1 at 15z and dubbed Invest 90L as "Alberto". This isn't unexpected as it has been organizing slowly for a few days and myself and others were eyeballing this potential for over a week now but nonetheless it means that things are developing as anticipated, at least up to this point. Alberto is currently located just East of the Yucatan peninsula of Mexico (19.5N 88.0W as of 13z). GOES East shows a broad area of convection in the Caribbean associated with this storm and the low-pressure circulation is more defined than it was even last night. It's not the tightest circulation by any means and the entire western half of the storm is fairly dry, only broken cumulus fields indicated the center if the low, but that is likely due to the land-interaction of the peninsula.

I expect conditions to become a little more favorable for organization over the next 36-48hrs, coinciding with Alberto's emergence into open water. When analyzing/forecasting tropical weather I prefer to assess water temperatures after I make an initial analysis & diagnosis of the storm itself using satellite imagery, radar, etc. (This is always preceded by the usual analysis and diagnosis of the atmosphere at-large) This is just the way I prefer to do it because, for one, it's fairly easy to analyze by simply looking at buoy obs and maybe some remote sensing data to fill in the gaps, but also the release of latent heat that occurs in tropical/subtropical storms over warm water is essentially the "fuel" that they use to survive & thrive.

Recent buoy observations, along with NOAA-19 and GOES satellite-derived data to fill in gaps where observations don't exist (errors occur due to cloud cover, etc) show SST's across the GofM in the 26C-28C range with a corridor of warmer water through the central Gulf and relatively cooler water eastward toward Florida. Some slight warming is possible with air temps potentially reaching 30ºC+, this could support a 1-2ºC increase. You generally want to see SST's of 27C+ to support tropical development, although like anything else in meteorology these values are just rough guidelines. The track of the storm will be key because it will determine if Alberto can hang out over the warmer open water or if it will be influenced by slightly cooler water near Florida. Our subtropical storm is currently parked in between two ridges with one to its West over Mexico and another off the East coast of Florida and these will be big players in steering Alberto as he drifts generally northward. The general motion should be to shoot the gap between these ridges, so to speak.

 Windsat satellite-derived wind fields. Errors can occur because this technology isn't perfect but areas of 30kts+ are evident.

Special "Nighttime Visible" satellite imagery from the NPP VIIRS satellite via the Naval Research Lab. Note the broad area of enhanced convection near the Yucatan peninsula.

Satellite-derived 6hr precipitation totals. Retrieved from the Naval Research Lab NexSat Program.

The above image shows the 700-850mb streamlines and is a good proxy for the steering layer of 1000mb-1010mb tropical cyclones. It seems reasonable that the Bermuda high will shift slightly eastward while Alberto drifts northward. I am not going to take a stab at landfall or anything yet and please forgive me for not explaining EVERYTHING that affects a tropical cyclone's track, like trochoidal motions, beta effects due to vorticity, and other factors like the Coriolis force, but the likely track seems to be through the central Gulf, although an embedded synoptic trough across the southern CONUS may draw the storm toward it, as often happens, so that is a variable worth monitoring. Either way, at this point I'm tentatively watching somewhere from the Florida panhandle to Louisiana. Heavy rainfall, strong winds, and dangerous rip tides will be likely along the West coast of Florida and the southern Alabama, Mississippi, and perhaps Louisiana coasts.

Now that I have a rough area for this thing to go let's look at how it may evolve. Wind shear has been relatively strong across the region, which has been part of the reason it was slow to organize, however it is expected to weaken somewhat. Honestly, the wind shear is probably going to be the main inhibiting factor going forward. We should see further intensification but I think the regional shear will be enough to mitigate a more intense storm. There will still be impacts and I don't think it's wise to write off any storm but it's just to say that I think this is one environmental influence that will prevent a more robust threat. 

I also assessed upper-level divergence and lower-level convergence. These factors tie in together and are related to mass-removal within the column of the cyclone. Continued removal of mass within the storm will result in pressure falls and I believe we will see a slow deepening/intensification. I'm not convinced we'll even see a sub 1000mb low yet but there appears to be enough ventilation to help sustain the cyclone as it moves northward to the CONUS.

Graph depicting output from 21 different numerical models suggesting possible trends in overall wind shear across the Gulf of Mexico in the coming days.

GFS output for wind shear over the next 30hrs. It does show a weakening of shear aloft but it's still painting a corridor of flow resulting in 30-40kts of deep-layer shear in some areas. I like to see roughly < 30kts to feel confident in increasing potential of a stronger tropical storm. This is not the "be all, end all" and as I said earlier, these guideline values are simply bases to refer to. Assigning any rigid, arbitrary "limits" to the fluid processes of which we're dealing with can and will get you in trouble at some point as nature doesn't recognize our thresholds. 

Another factor to consider is the possibility of tropical cyclone induced tornadoes. The SPC hasn't been able to delineate an areal risk for tornadoes associated with Alberto yet because of the variability in the track but I anticipate they will eventually. Tropical cyclones are often able to produce embedded tornadoes, particularly in the northeast quadrant. In the simplest terms, this results from favorable wind fields, especially in the low-levels where southeasterly flow and surface friction often enhances the probability for circulations to develop. You can usually identify enhanced hodograph curvature and favorable wind profiles in this region. This is something I've assessed when I've chased hurricanes/tropical storms in the past and it is currently the area I'm leaning most toward trying to identify and targeting for my chase this weekend. My initial target looks to be in the region of the extreme southeast Louisiana coast to the western tip of the Florida panhandle but that may change.

The GFS (and other guidance as well) picks up on this increased low-level shear and is painting an area of pretty high storm-relative helicity values (500+m^2/s^2 0-3km) in southern Mississippi. This isn't to show exactly where I think the greatest tornado potential will be but is just to demonstrate the enhanced low-level shear that often occurs in tropical cyclones.

So before we compare the model guidance with what I've put together here, let's summarize. I'm thinking our subtropical storm Alberto will finish crossing the Yucatan Peninsula of Mexico before emerging over open Gulf water at which point he'll drift northward, undergoing a slow deepening as he does. Some wobble is possible given mesoscale eccentricities and a nearby synoptic trough but overall it looks like a generally northward track. Heavy rainfall, flooding, gusty winds, and dangerous rip tides are likely this weekend along Florida's West coast and by late weekend/Memorial Day along the southern Gulf coast. Once making landfall it is uncertain how quickly Alberto, eventually an extratropical low, will phase with the mean westerly flow but heavy rainfall/thunderstorms will be possible across the southeast CONUS into the Tennessee Valley. 

"Spaghetti Plot" of numerous forecast models showing simulated tracks of our cyclone. It seems that some of the models are picking up on the tendency for tropical cyclones to be drawn toward synoptic troughs, which is producing decent clustering of tracks supporting my forecast, however, there is still considerable spread with just as many models diverging from that track too. There are several outliers which need to be noted as well. Remember, the trend is your friend! If the models are trending toward a certain outcome then there is always a reason why and it should be investigated further. That said, a solid conceptual model needs to exist and a forecast should ALWAYS come from detailed analysis, diagnosis, and prognosis with model data being used to help guide us along *after* we've already made our own assessments ofhow the atmosphere will evolve over space and time.   

Deterministic model guidance from the GFS and even the NAVGEM. These are only isolated model graphics but they are a good proxy for the forecast I've come up with because they have "Alberto" taking a slight northwest jog before landfall and that's the way I'm leaning. I don't think this is going to be such a strong storm that it shows a lot of "deviant motion" so it seems it will be even more susceptible to mid-tropospheric flow and features. Like I mentioned earlier, given the tendency for these cyclones to be "drawn in" toward synoptic troughs I am anticipating that the trough to its northwest will influence its track. There's also a tendency for tropical cyclones to propagate toward areas where they are producing maximum latent heat release and that could play a role in this outcome too.

Above is the GFS output for total QPF through Tuesday evening (7pm CT). This is just to show the potential for flooding in many locations because widespread heavy rain is likely regardless of how "Alberto" evolves, although areas that see a more direct hit will of course see higher precip totals. After landfall the remnant extratropical-low will likely still be responsible for rainfall across the TN Valley but I didn't go past 102hrs because, 1: a passing frontal system begins to influence QPF totals in the output and I haven't assessed this system much yet so I don't want to lend any credence to it until I have, and 2: after 96hrs I start questioning the legitimacy of the GFS even more than usual because it starts to show the effects of mounting uncertainty stemming from each previous forecast hour (related to "chaos theory")

That will do it for tonight, I'll post updates throughout the weekend because at this point we're in the monitoring stage. It's going to be all about tracking in real-time as "Alberto" emerges over the open Gulf, plus we'll have more Air Force recon data to work with so that'll help. I said several days ago that this system could flirt with category 1 hurricane strength, and even though confidence is higher that it maintains tropical storm intensity, I still think there's an outside chance that it can if it manages to stay over the central Gulf where the longer residence time over warmer water may compensate for the influence of shear. Regardless of what anybody calls it, do not ignore this storm if you're in any if the threatened zones. There will be flooding risks, strong wind gusts, and dangerous ocean currents. Simply monitor official sources for information, assess your situation, and be smart. I'll get into the upcoming severe risk in the northern plains and upper Mississippi River Valley with tomorrow's post.
Stay cool folks, the next adventure is right around the corner.
DKK