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Severe WX Severe Weather Threats 1/21-1/22 2017 (Saturday - Sunday)

Richardjacks said:
One thing about winter severe weather events I have noticed is that the strongest storms aren't able to thrive right in the "belly" of the low. In other words, there needs to be some space from the surface low. For example, if I remember correctly, during the January 24, 1997 tornado in Tuscaloosa, the surface low was up in Missouri...not really sure there is anything to that rule, just something I have noticed over the years.
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That's honestly the case in most of our bigger severe weather events, even during spring and during the warmer part of fall.
 
Fred Gossage said:
That's honestly the case in most of our bigger severe weather events, even during spring and during the warmer part of fall.
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Yes, for the bigger events, for sure, but I am referring to any events in the wintertime...which there aren't as many to go by. I wouldn't classify Jan. 24, 1997 as a big event, just as example of a winter event. When I see the surface low in north Al/Tenn, just makes me wonder how far north the warm front will move-but also have to consider the lapse rates advecting into the area.
 
XUS64 KJAN 171800 AAA
AFDJAN
Area Forecast Discussion...UPDATED
National Weather Service Jackson MS
1200 PM CST Tue Jan 17 2017

Edited For severe emphasis :

Global models continue to indicate that a more significant threat
for severe weather should evolve this weekend. Expect rapid onset
of moisture transport early Saturday into southern portions of the
ArkLaMiss, and with steepening mid level lapse rates > 8 deg C/km
and H850 theta-e values ~330 K, mixed layer CAPE is forecast to
increase to near 2000 j/kg, an extreme value for January. Along with
strong deep layer shear, there is little doubt that the environment
will be supportive of significant severe weather. But due to the
very fast upper level flow and how surface features will respond,
there is a lot of uncertainty regarding lift and specific areas of
deep convection, especially with the tendency for coastal convection
in this sort of pattern with big height falls well to our south.

After coordination with SPC, will indicate a limited severe threat
for mainly the southern half of the area and hold off on elevating
the threat until model details can become more consistent with
convective evolution. Will also broad brush the time window to
include both Saturday/Sunday. Look for quieter and cooler weather
beyond Sunday as the first significant intrusion of CP air tries to
move into the region. /EC/
 
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Afternoon thoughts on the weekend threat from the BMX AFD:

Threat 2 over the weekend still has some disagreement between the
models this time out, but certainly has the potential for some
significant impacts. The GFS has better continuity than the ECMWF
this run, as the ECMWF has trended back toward solutions a few
days ago. The GFS is picking up on several impulses riding through
the flow and the potential of severe storms Saturday, Saturday
night, and Sunday. The ECMWF is a bit more limited to Saturday
night and Sunday. The flow suggest that there will be additional
impulses and am leaning toward the GFS here. Additionally, the GFS
parameters are better for a longer period. ECMWF would suggest a
better risk south. Therefore, we will keep the severe threat going
for the entire area both Saturday and Sunday and not focus
specifically on these parameters. The synoptic pattern is
favorable and the mesoscale will have impacts. Analogues have
indications that the pattern is very conducive to severe storms.
Stay tuned.
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Wow, this threat for Georgia certainly has my attention (the Sunday threat). I know its still ~5 days out, but the large scale setup looks as favorable as I've seen in quite awhile. Good diffluence aloft, plenty of shear, and instability appears to be more than enough (especially for January). We'll see how the mesoscale features play out..
 
00Z run of GFS. Concern increasing from Saturday afternoon all the way till Sunday night. Possible 2 waves attm. Lapse rates are really high for this time of year. Just still wondering about development to the south of coarse.
Early still.....

n0xEsUr.gif


2Pl1C4u.gif




Fcast sounding between Birmingham and Cullman Sat. afternoon from 18Z Run
Kj7CVwx.png
 
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I hate waves of storms unless the first wave can stabilize the atmosphere for the second wave. I was afraid we were going to pay for this warm weather!
 
EXCELLENT discussion by Owen at BMX. Hits all the points I've been looking at and have been discussed in here.

What happens Friday could be instrumental in the forecast for the
weekend. Currently, models agree that ridging builds in over the
region ahead of the upcoming low pressure system. However, the ECMWF
moves the ridging out a little quicker introducing a few rounds of
shortwaves that initiate convection along the Gulf Coast Friday
afternoon. The GFS doesn`t bring in the first shortwave until
Saturday morning. The ECMWF solution would limit instability
recovery ahead of the main system Saturday night into Sunday. The
NAM is leaning closer to the GFS, allowing more instability to
build in Friday evening into Saturday morning. Also, at this time,
both the GFS and ECMWF pick up on dry air in the mid levels coming
off the Mexican Plateau and moving across Central AL Friday
evening, which would further lean thinking towards possible
clearing and potential instability recovery.

For Saturday and into Sunday, models are in good agreement with the
synoptic pattern of a low developing on the lee side of the rockies
and moving into the Ozarks. Both the GFS and ECMWF show a closed low
in the lower levels with more of an open wave in the upper levels,
pulling in a rather strong jet across the southern US. Models are
also now in better agreement with some embedded shortwaves with
relative vort maxes moving through just ahead of the main low
pressure system and associated stronger vort max. The 00z ECMWF has
the main low moving slightly quicker than the GFS output, limiting
the amount of instability recovery for Sunday.

So, to put all this together: Both the GFS and ECMWF show the
possibility of severe weather for the weekend with sufficient
instability, shear, and upper level support from the 500mb vort
max and upper level jet. The GFS is painting a picture for a
stronger severe event Saturday and Sunday - showing instability
recovering to 1500-2200 J/kg MUCAPE on Saturday with 0-6km bulk
shear 55-60 kts. On Sunday, the GFS is showing instability
recovery back into the 1500-1800 J/kg range and 0-6km Bulk Shear
at 65-75 kts as the upper low is closer, allowing for more backing
of the winds. However...the ECMWF doesn`t have as much
instability, considering the convection along the coast and
quicker timing. It`s showing instability closer to 1000 J/kg
MUCAPE for Saturday evening and really not recovering much at all
for Sunday. With the similar synoptic pattern as the GFS, the
ECMWF shows comparable deep-layer shear values.

Considering the GFS has trended more consistently the last few days
than the ECMWF, and with the NAM coming in closer to the GFS with
the clearing on Friday, I`m edging more towards the GFS solution for
severe threat on Saturday and Sunday. With that said, there are a
lot of moving pieces with this event, and plenty of uncertainty and
questions. Does the convection form along the coast Friday like the
ECMWF shows? Does the mid-level dry air off the the Mexican plateau
materialize and clear things out over Central AL Friday evening
ahead of the shortwaves Saturday? Does the main low pressure system
hang back by a few hours, allowing for another round of severe
weather on Sunday?

For now, will keep mention of limited threat in the northern
counties for Saturday and shift the elevated threat slightly
northward. For Sunday, have mentioned an elevated threat area-wide,
hedging closer to the GFS with all modes of severe weather
possible. With that said, depending on how models trend in the
next couple of days, and how we answer the above questions, could
see a change in threat area on both Saturday and Sunday. Stay
tuned.
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As Owen said, what happens Friday will be very important for Saturday and Sunday's threat. Euro, more so than any other model, has been showing a piece of PVA leaking ahead of the main trough. With that, it would spark coastal convection, limit instability, and displace the lower level winds, leading to funky wind profiles. HOWEVER, we are now within the range of mesoscale models, such as a the NAM. That is showing a much more open 500mb trough (important for keeping favorable wind profiles) and it is showing a more intact trough with little PVA leaking ahead and messing with wind profiles. It would likely suggest the event would start Saturday with instability unusually high for any time of the year, much less January. The NAM is the most aggressive solution, but it is a nasty one at that. One would think with the awesome EML forecasted....coastal convection wouldn't be an issue?

I want to preface this....don't overlook tomorrow's event. Not only could it be a litmus test for the NAM (to see if it's handling this pattern correctly), models are showing a discrete/semi discrete storm cluster mode. Instability is sufficient to maintain those updrafts. Hard to rule out a couple tornadoes tomorrow night given the great directional shear profiles.
 
12z nam is confirming what the GFS has been showing in regard to little if any coastal convection on Saturday...it does show some on Friday-if that is underdone, then there may not be much on Saturday, or there could be a longer recovery mode. Otherwise, anyone care to chime in on the risk of long tracked tornadoes later Saturday?
 
The next system will move in on Saturday as strong low level warm
air and moisture advection continue into the weekend. A tropical jet
will bring in a series of shortwave trough across the Southeastern
U.S. over the weekend. Rain chances will increase through the day
Saturday with strong warm air and moisture advection. Models depict
a warm front lifting northward during the afternoon with a good
directional shear along the boundary with SBCAPE values nearing 1000
J/Kg across the Southern Valley and Southern Plateau late in the
afternoon. An upper level trough will close off across the Lower
Mississippi River Valley early Sunday. The continued warm advection
and moisture advection will push SBCAPE values up into the 500-1200
J/Kg range across the forecast area, however, there is a bit of a
capping inversion on the GFS forecast soundings. Low level shear
still remains decent around 150-250 m2/s2. The system is still
several days away but will go ahead and add the mention for strong
to severe storms to the HWO for Saturday and Sunday with the threat
on Saturday limited to the Southern Valley and Plateau.
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