The Big Thaw and...Thunderstorms?

  Good afternoon! Snow is melting away a little at a time, although the big melt will be this weekend. With the snow keeping me stuck inside for at least another day or so I thought it would be a perfect opportunity for pining over days with surging dew points and triple points, warm sectors that are actually warm, and when the rumbling you hear outside is from thunder and not just the snow plow going back down the road. Days when you spend hours and hours on the road jacked up on caffeine eating lunch from a Love's Truck Stop in search of a moment in the perfect position on the perfect storm.
  Maybe those days aren't too far away anymore? There are some signals that we could see thunderstorms around East Texas/Arkansas as we round out the weekend. You guys know I like to be a patient forecaster so we won't jump to conclusions, and honestly things aren't necessarily jumping out screaming "severe weather outbreak" or anything but we will look at what we know and examine some of the possibilities going forward and, as always, we will set a "checkpoint" to come back and reassess things. It's something at least, right.


  It's only been two months since I was preparing a forecast for chasing severe storms but it feels like many more. Let's see if I can remember how this goes...I believe best practices dictate that you first analyze and diagnose the current state of the atmosphere so here we go.

GOES16 water vapor imagery shows a series of shortwaves/upper-lows across North America. I've numbered them 1-4 from East to West. The disturbance off of the East Coast (#1) over the Atlantic Ocean was responsible for bringing quite a bit of snow to the Eastern CONUS but it is caught up in the flow of the northern and southern stream and will continue to depart. #2 is a shortwave over the Great Lakes and will track within the northern stream and stay North of our area of concern. #3 has been giving the models fits actually but even with all of the model divergence I expect it to stay South of our area. I'm not saying these shortwaves are insignificant because every detail should always be analyzed, and I have analyzed these as well, but the feature we are focused on for the possible thunderstorms Sunday is #4. Currently located off of the Northwest Coast, it is a very well-defined system already. 12-hour height-falls are significant in the Pacific Northwest in response to this low (200m+). Strong flow is associated with this low as well with 100kts+ sampled in the 00z soundings throughout the region.  

Stepping down to the surface real quick we can see the high-pressure that's dominating our local weather for the next few days, as discussed in yesterday's post. Upstream in the NW CONUS you can see a deep surface-low and attendant frontal boundaries, a clear surface reflection of the impressive mid-upper wave we looked at on water vapor. This surface chart actually says a lot about what may happen, even looking forward 3-4 days. The main thing I'm getting at here is the impact of the surface-high over the Southeast right now. This high is weakening but it's stubborn and has already taken steps to limit severe potential in the near-future. Yes, the southerly flow along the western periphery of it is beginning the return-flow cycle for moisture and warm air advection but I'm concerned if that will be enough to support severe weather. With the divergent flow within the center of the sfc-high straddling the Gulf Coast and an Arctic air mass that has been established for what seems like forever, extensive modification of the air residing over the GofM has taken place. Cooler air has made its way well over the usually warm waters which serve as a common source region for Southeast severe weather. I do think we will see sufficient moisture make its way across the ArkLaTex region as the broad high-pressure slowly makes its way Eastward but it is definitely something to consider going forward, especially if the shortwave over the Southern US is able to develop a related surface system (not highly likely but not impossible either).

Surface dew point observations across the US. The influence of Arctic air is evident and the intrusion of this drier air has resulted in modification extending well into the Gulf of Mexico. This will have to be overcome by sustained return of deep moisture before severe storms would be likely.

We'll take a look at some of the model data next because there is, believe it or not, pretty good consistency regarding the large-scale features that set the stage. Now I always say that you don't just jump on board with the models because they're consistent because they can be consistently wrong, but in this case I agree with a lot of the output regarding the evolution of our shortwave & upper-flow going into Sunday and I don't see anything they are tripping up on either so we'll briefly touch on it. Where I get concerned is with the usual differences in timing. The GFS has initiation of storms a little earlier in the day when diurnal heating could help destabilize things a little more, although there could be showers early on anyway. The Euro's latest "master" run is much later with showers throughout the afternoon and thunderstorms firing up after 9pmCST. These are the details we'll have to work out just like we always do in these situations.

GFS Ensemble Mean 500mb Heights 

Above are the recent GEFS ensemble runs for 72hr/day 3. The valid time of these images is about 18-24hrs before expected thunderstorms across the Southern US. I wanted to show the relative consensus with the upper-level wave as it approaches the Rocky Mountains. You can see that the GEFS has pretty similar solutions in its most recent output. You can also see the struggle with the shortwave over the SE CONUS that I mentioned earlier. From this point I suspect we will see our system round the Rockies and emerge over the Central Plains while also assuming a negative-tilt as it amplifies. I expect amplification in response to a strong upper-level jet passing over the low which will lead to upper-level divergence/mass removal. We should also see a surface-low develop in response to this as well as it interacts with the thermal boundary in place from the retreating cold air and returning warm air. The low-level low-pressure will help advect higher theta-e into the region. With a strong southwesterly jet up top and southerly low-level winds, coupled with a strong kicker at 850mb, wind profiles look favorable for both damaging winds and even a tornado or two. That isn't really in question. Storms should be well-ventilated and deep-layer shear would be supportive of organized updraft/downdraft relationships. Forecast soundings are showing sickle-shaped hodographs supportive of rotation in both the low and mid-levels. Although there are the timing differences I think there isn't much doubt in sufficient forcing for ascent arriving at some point, when will be important though.

 I'm just not sold on instability. There will be a moisture/theta-e axis across East Texas up into Arkansas most likely. It currently looks like we could see surface dew points approaching 60F and dew points at 850mb at 8-10C (I like to see 10-12C). I do prefer to see the deepest moisture established over the target area for about 18-24hrs before storms but it is what it is. The destabilization just doesn't look overly impressive right now. The GFS is suggesting 500j/kg of surface-based CAPE with marginal lapse rates (6.5C/km) during the afternoon centered over ArkLaTex. The ECMWF has slightly more instability and pushes 1,000j/kg of surface-based buoyancy across the area but it shows it after dark with later initiation and also has rain showers throughout much of the day. The ECMWF seems like it's overdoing it personally so I'm leaning more toward the GFS for now. This is one of those "subject to change" things though as I would prefer to lock in the timing better before I make a call on that.

Mutli-Model "Spaghetti" Plot of 500mb Heights at 72hrs (Sunday 6amCST)

Multi-Model "Spaghetti" Plot of 500mb Heights at 96hrs (Monday 6amCST)

GFS MOS (Model Output Statistics) of Max Temp and Probability of Precipitation at 6pmCST Sunday. The Surge of Warm Air Northward Can Be Seen Across East Texas into Missouri. 

Based on everything I've seen I don't see any signals for a "widespread" severe weather outbreak Sunday-Monday. I do see a chance for some storms however with some strong-severe embedded. I won't make a call on specific threats yet either but I can't rule out a tornado or two right now so that's worth watching too. I'd like to see how things go over the next 24hrs regarding moisture return, shortwave evolution, and model output and then make an update Saturday morning so that's the plan. At that point I'd expect to have a better grasp on how smaller-scale details look so we could take a shot at a more specific forecast too.

Regardless of how we see this unfold one thing seems likely; everyone will crawl out of their winter hibernation, throw the Snuggie away, and start making their calls. It's not quite Spring yet but it might be that time for a small taste of chasing model trends, SPC outlooks (and the criticism from the know-it-alls of course), and copious amounts of coffee while impatiently living life on a 12z to 00z schedule!