03/27/05 FCST: AL/GA/SC/TN

Following what will likely have been an active overnight period, a volatile environment remains in place across the southeast with model forecast shear and instability capable of a significant severe weather event. Certainly still too far out to discuss the mesoscale aspects that could tip the balance between marginal and extreme severe weather potential, but the latter certainly looks possible at this stage. Continue the discussion below.

Glen
 
Wow. I have been so busy all morning looking at tomorrow that I hadn't even considered looking at Sunday yet. But after a quick glance over, Sunday looks very potent as well. It appears that severe weather will be very likely over AL, TN, GA and SC. And I noticed a big CAPE bullseye with values up into the 3000 j/kg range over central GA by evening. Helicity values are also very high over SRN TN and NRN GA, with adequate values surrounding.

As you say, we'll have to wait (likely until Sunday morning) to get a real idea for what may happen, but it appears conditions may be primed for yet another medium to high end event Sunday.

I made a prediction back around the start of the year (actually more of a guess to be honest) that the south would be much more active than in recent years. If both Saturday and Sunday's events meet their potential, that prediction may well be on it's way to being verified. But that is unfortunate as it seems there's a high number of deaths and injuries over these areas. Hopefully everyone in the south this weekend will be on guard and paying attention, as two very dangerous back toback days are shaping up.
 
For some reason I can't edit my post in this forum, but I just wanted to note one other thing: Isn't Sunday the anniversary of the big Palm Sunday tornado outbreak across the south in 1994? Seems climatolgy also heavily favors potent severe weather events in this area during this timeframe, as there have been many over the years. Also, I believe the Carolinas Outbreak of 1984 was on March 28th. Not saying this event will be the same, just noting that this general area has experienced quite a number of large severe weather episodes around this time of year.
 
Tomorrow still looks to be a significant severe weather day - in fact I'd be a little surprised if it is not the first high risk day of the year. While the low-level shear has been slow to set up tonight - just looping the sfc obs from the last few hours shows the sfc flow starting to respond from the approaching upper level system still back over Texas. Given the lack of interest in this forecast - I won't go into the details. Of course, this is far from the traditional alley and I suspect few folks will be out there chasing this event - but for any that do be safe and we look forward to seeing your reports.

Glen
 
It looks like the limiting factor yesterday (meager instability) won't be AS much of a factor today... The warm sector should advance more inland, allowing for rich Gulf moisture to move into the southeast. Winds at all levels will be quite impressive, resulting in strong deep- and low-layer shear, certainly supportive of strong tornadoes. Cloud-cover appears to be an issue right now, with current vis sat indicating widespread clouds at various heights across the area, though there are some "holes" here and there. 0z NAM indicates 1500-2500 j/kg sbCAPE in southern AL by afternoon, though it wasn't so hot in verifying yesterday, so I guess we'll have to see. Otherwise, it appears that instability will be a touch greater than yesterday, though still will be on the low side. The widespread nature of the precip doesn't appear to be helping either, cuasing widespread cell interactions and increased cloudcover / reduced insolation.
 
Hey Jeff - I don't think the instability will be that great an issue today either. Warming is evident despite the cloud cover. There is also a new low developing just west of Montgomery AL which should lift neward and lift a warm front across central AL into GA. Ahead of this flow is strongly backed - and convection has been slowly increasing this morning (likely slightly elevated at this stage). Behind the boundary - sfc dewpoints in the low 70's should aid in a narrow corridor of favorable conditions within the next few hours. RUC maintains area of strong convergence across cental GA - will be interesting to see if this can happen with the stubborn regeneration further south.

Glen
 
Well, it appears that limited instability has again been an issue here... Current SPC Mesoanalysis at 23z indicates that sbCAPE remains in the 500-1000 j/kg range across central AL, southwestern GA, and the FL panhandle, and relatively nonextistant elsewhere. Widespread cloudcover has limited insolation. Yes, low-level warm air advection continues, but this has thus far been insufficient to build up much potential instability. The best moisture (and instability) remains over sw GA and the FL panhandle, though winds are veered to the south of that warm front.

Deep-layer shear continues to be very strong across the northern part of the mdt risk area, ranging from 60-85kts in the warm sector (or between the two warm fronts if you think of the situation as a double-warm front scenario). In addition, low-level shear appears to be quitte strong in GA, just where there is very little instability.

I do expect the instability axis to build into western GA with time as the cold front march across AL, though we'll have to see if the storms can be surface-based...

Nonetheless, I really do feel that the 15% torn prob in the northern part of the mdt risk area is a bit much given the very marginal surface-based potential instability in the risk area. So far, we've had several PDS tornado boxes (and other outlets, media and NWS alike, calling for significant tornadoes), and only 1 tornado report (yesterday) plus 3 tornado reports today (though all three were within a few miles of Montgomery). I'm not saying anything about the issuance of the watches or dogging on the SPC, but rather just that the models haven't been handling the near-surface environment very well at all. This isn't entirely uncommon, since the problems with convective parameterization are relatively well-known. It just makes forecasting that much more difficult when the near-surface model forecasts aren't very good, since it negatively affects other forecast elements, such as model forecast instability, etc.

EDIT: Not worth an additional post, but, as an update, SPC mesoanalysiss not indicating a shrinking area of 500 j/kg sbCAPE, so it appears that the tornado threat is shrinking, despite very impressive shear profiles (both deeplayer and low-level). In fact, the 0z FFC RAOB (central GA) shows ~1300 m2/s2 0-3km SRHelicity given a storm motion from 240 (west-southwest) at 44kts (which isn't entirely realistic, but noteworthy nontheless). The hodograph looks wicked to say the least.

At any rate, I did find it interesting the the TLH radar did a Tilt 1 scan at 0049z, 0051z, 0052z, 0052z, and 0058z. That's 5 scans in the course of 9 minutes... There were even two scans in the 0052z minute. I'm wondering if the TLH NWSFO is trying to see what it will feel like when (if, technically) the phased array radars are deployed next decade... ;)
 
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