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.