Archive March 18-20th, 2018 Severe Weather

[Taylor Campbell]

It needs its own thread.

 

[Taylor Campbell]

ZCZC SPCSWOD48 ALL
ACUS48 KWNS 140756
SPC AC 140756

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0256 AM CDT Wed Mar 14 2018

Valid 171200Z - 221200Z

...DISCUSSION...
Medium-range models are in general agreement that a strong
short-wave trough will flatten a mid-continent ridge during the day4
period. However, considerable differences emerge in regards to the
speed of the short wave and the degree of height falls, or lack
thereof, over the Mid-South. As a result, trailing surface front
should extend across the northern Gulf states into the Red River
region where weak height rises are expected. While a seasonally
moist air mass should encompass much of the Southern States through
the medium-range period, there is considerable uncertainty regarding
the timing of short waves. Organized severe may ultimately develop
along the western-northern fringe of this moisture/instability plume
but predictability remains low regarding the timing and areal
extent.

..Darrow.. 03/14/2018

 

[Taylor Campbell]

The threat level for this period has trended up.

 

[Kory]

The threat level for this period has trended up.

I agree. Global models were having trouble resolving the low level jet for Sunday/Monday. The pattern has come in a bit more amplified ahead allowing a more SWerly and stronger LLJ. Things look to be discrete given the shear vectors. One caveat, which is showing on the GFS, is a small disturbance that gets entrained ahead of the main trough. I don't see that on the Euro/UKMET.

 

[rolltide_130]

I agree. Global models were having trouble resolving the low level jet for Sunday/Monday. The pattern has come in a bit more amplified ahead allowing a more SWerly and stronger LLJ. Things look to be discrete given the shear vectors. One caveat, which is showing on the GFS, is a small disturbance that gets entrained ahead of the main trough. I don't see that on the Euro/UKMET.

The Euro took quite a while to actually get an LLJ going but it's doing better now.

However, I am concerned about that disturbance just ahead of the trough. It could really throw a wrench in the system if that verifies. The STJ has still been wanting to stick around some the last time I checked the models as well.

 

[MattW]

If the timing doesn't change much, I should be able to chase after work on Monday.

 

[Taylor Campbell]

Models show everything setting up for prime time across Alabama on Monday.

 

[Taylor Campbell]

Mid level lapse rates have improved tremendously.

 

[Taylor Campbell]

This is an all mode severe weather threat. Large hail, damaging winds, and tornadoes.

 

[CheeselandSkies]

Looks at GFS soundings valid for Monday the 19th.

*raises eyebrow

Methinks reports of this threat's demise were greatly exaggerated.

 

[KoD]

There are some pretty impressive parameters, but will the thermodynamics all line up with the forcing & precipitation? Right now it's looking a bit messy and unpredictable but I think we'll get a good believable and hopefully consistent picture of what we can expect soon

 

[MichelleH]

My son has all-day orientation at his new job on Monday. Just hope it waits until he gets home.

 

[Taylor Campbell]

The mean supercell composite trend over the last 10 runs of the GFS ensembles.

What's impressive about this mean is the number of ensembles supporting the high numbers in the same general area. Also, there looks to be about 7 individual members that show the supercell composite in the 10-15 range in a pretty sizable area.

 

[champal3003]

First legitimate severe threat of the year across Ms and Al for Monday into Tuesday.

Forecast Discussion
ZCZC SPCSWOD48 ALL
ACUS48 KWNS 160900
SPC AC 160900

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0400 AM CDT Fri Mar 16 2018

Valid 191200Z - 241200Z

...DISCUSSION...
Numerical models are in relatively good agreement with the movement
of a shortwave trough from the southern Plains across the lower MS
Valley on Monday, with a surface low moving from AR into TN during
the day or evening. The ECMWF is faster/farther east with the low
than the GFS, which lends uncertainty. However, the risk area can be
adjusted in later outlooks.

On Monday Day 4, a cold front is forecast to stretch roughly from
middle TN southward across MS by 00Z Tuesday, continuing eastward
across AL and into GA by Tuesday morning. A warm front will also
lift north across the region, stretching from northern AL into
central GA at 00Z. Dewpoints in the 60s F and cool midlevel
temperatures will result in around 1500 J/kg MUCAPE, with strong
deep-layer shear profiles supporting organized convection. Low-level
shear will be maximized near the warm front, and forecast wind
profiles do support supercells. Conditional on storm mode, a tornado
threat may exist. The northern threat into TN will depend on
instability, but otherwise the synoptic setup appears most favorable
there. To the south, instability will be much greater and one or
more clusters of storms are expected to spread across AL and GA with
damaging winds likely given strong mean wind profiles.

 

[Kory]

Morning MCS, afternoon atmospheric recovery behind it...many moving parts to Monday's threat. LLJ looks weak at first, but strengthens during the afternoon. Two of the biggest questions for me...how does the morning MCS progress and degree to which the LLJ strengthens behind the MCS? Some pretty ridiculous lapse rates move in behind the MCS (associated with an EML) which would allow for recovery...

 

[Richardjacks]

One thing about this that stands out to me is the size of the upper low-it is very compact. I seriously doubt the models are handling the track of such a small feature correctly...but there is already overall agreement. As for the morning convection, it seems there is time for recovery and the overall flow doesn't appear to be one to support the idea of a slow moving decaying complex to spoil the afternoon activity...unless the time/distance between the two begin to collapse.

 

[akt1985]

I thought I saw the GFS had snow on the back side of this system in Alabama. Is this true?

 

[MichelleH]

Well, now I'm getting kinda concerned. "Conservative forecasting severe weather" James Spann posted this a few minutes ago on Facebook:

Concern is growing over the potential for severe storms in Alabama Monday, especially during the afternoon and evening hours. For now it looks like all modes of severe weather will be possible, including tornadoes. But, it is too early to be really specific about the threat this far in advance. Just be aware of the potential and we will keep you updated...

 

[CheeselandSkies]

Was eager to get his take on the Weather Extreme video this morning and was disappointed to see that it wasn't him.

 

[WesL]

Well, now I'm getting kinda concerned. "Conservative forecasting severe weather" James Spann posted this a few minutes ago on Facebook:

Concern is growing over the potential for severe storms in Alabama Monday, especially during the afternoon and evening hours. For now it looks like all modes of severe weather will be possible, including tornadoes. But, it is too early to be really specific about the threat this far in advance. Just be aware of the potential and we will keep you updated...

JP Dice has chimed in as well-

FIRST ALERT: There is a growing threat we will see severe storms on Monday. The storms will develop as early as mid-morning and continue through the afternoon. The primary risk is hail, damaging winds, and tornadoes. This is a situation you need to monitor closely. Stay with WBRC Fox6 for updates.