Severe WX Severe Weather Threat 11/30-12/2/18

Taylor Campbell

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Forecast Discussion
ZCZC SPCSWOD48 ALL
ACUS48 KWNS 260845
SPC AC 260845

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0245 AM CST Mon Nov 26 2018

Valid 291200Z - 041200Z

...DISCUSSION...
Models depict a shortwave trough digging across the southwestern
states on Thur/D4, though solutions vary as far as how amplified it
will be, with the GFS less so than the ECMWF. Low-level moisture
will begin returning north across eastern TX into the lower MS
Valley by this time, but in either scenario instability will be too
weak for any severe weather threat.

On Fri/D5, this wave will move into the southern Plains, with
aforementioned model tendencies holding. It does appear that the GFS
is becoming a bit more amplified with time, which is closer to the
ECMWF guidance. In either case, low pressure is likely to form over
the southern High Plains during the day, with dewpoints into the mid
60s F across most of eastern TX and LA, and perhaps as far north as
central AR Friday night if the more amplified ECMWF solution
verifies. Rapid height falls and a low-level jet increasing to 50 kt
will favor severe thunderstorms mainly Friday evening and overnight
from eastern TX into LA and AR. Strong shear profiles will favor
supercells and/or QLCS structures, depending on upper trough
geometry. The less amplified GFS solution would result in a more
focused area of potential, and perhaps more favorable for
supercells, while the more meridional ECMWF solution would support
more of a linear storm mode. Regardless, the threat level is great
enough to depict a 15% area for Fri/D5 for damaging winds and
tornadoes.

Some manner of severe threat will likely persist into Sat/D6 across
MS, AL, GA and TN, but predictability is too low to depict the
greatest threat area given the likelihood of widespread
precipitation and model differences in how the upper trough will
eject.

..Jewell.. 11/26/2018
 
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#2
Concerns upon first glance for the setup.

1) Degree of moisture return/northward warm frontal retreat inland, which hinges greatly upon...

2) Degree of convection in the warm sector and Gulf as there is currently a signal for this in guidance (including some influence from the sub-tropical jet).

3) How much the trough amplifies or deamplifies in subsequent iterations, this was a big issue with the last system that looked like it had potential to be significant in Dixie. Right now, heights over the warm sector look pretty decent and may suggest decent lapse rates assuming less convective contamination, but that could easily change.

This isn't your classic scenario that you would assume to have major issues with Gulf coast convection given its latitude, but the active sub-tropical jet will likely throw some wrinkles into the potential.

There also doesn't look like there will be a particularly strong WAA regime in advance of this trough, which can be key in the cold season (no "sacrificial lamb" to help with establishing moisture/warm air advection trajectories).

I am actually also interested to see what happens to the second trough that looks to come onshore shortly after if this wave flattens a bit (i.e. becomes less significant).
 
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#4
00z CIPS analogs for the SE on Saturday had notable fall Dixie events (yielding several sig severe/tornado reports) in 4 out of the top 6 matches, including 11/24/2001 (#2) and 11/10/2002 (#3).

Obviously an event of that magnitude is not expected, but if Gulf convection remains at a non-crippling level, there should be something with this type of trough orientation aloft and the strength of the wind fields. One issue could be that the bulk of the forcing for ascent may swing northeast if the trough matures too quickly.
 

Taylor Campbell

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#5
00z CIPS analogs for the SE on Saturday had notable fall Dixie events (yielding several sig severe/tornado reports) in 4 out of the top 6 matches, including 11/24/2001 (#2) and 11/10/2002 (#3).

Obviously an event of that magnitude is not expected, but if Gulf convection remains at a non-crippling level, there should be something with this type of trough orientation aloft and the strength of the wind fields. One issue could be that the bulk of the forcing for ascent may swing northeast if the trough matures too quickly.
I looked at them last night, and was wowed at how many matched dates of big severe weather outbreaks. It’s something you can say that hasn’t been so strongly signaled on CIPS analogs with our other severe events this fall.
 
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#6
00z CIPS analogs for the SE on Saturday had notable fall Dixie events (yielding several sig severe/tornado reports) in 4 out of the top 6 matches, including 11/24/2001 (#2) and 11/10/2002 (#3).

Obviously an event of that magnitude is not expected, but if Gulf convection remains at a non-crippling level, there should be something with this type of trough orientation aloft and the strength of the wind fields. One issue could be that the bulk of the forcing for ascent may swing northeast if the trough matures too quickly.
12Z NAM verbatim would be a respectable, if not particularly high-end severe threat over the lower MS valley on Friday. Possibly something on the order of the early November event.

Hey Andy, why don't you share your thoughts on Americanwx as well? I always know it's going to be a quality analysis when I see a post with your Tuscaloosa couplet avatar next to it over there.
 
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#8
00z HRRR lights up the dryline with supercells tomorrow in OK and TX near/east of the I-35 corridor.

Addendum: Strong cyclonic flow aloft remains over the SE all the way through Sunday and the front never gets shunted to the Gulf so plenty of instability remains. Going to need to watch that, any ripple in the flow could amplify the low level wind fields and set off new convection.

This is not to discount Saturday's potential if adequate destabilization inland can take place.
 
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Taylor Campbell

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#11
mcd1667.gif

Mesoscale Discussion 1667
NWS Storm Prediction Center Norman OK
0334 PM CST Fri Nov 30 2018

Areas affected...northeast Texas...southeast Oklahoma...western
through central Arkansas and northwest Louisiana

Concerning...Severe potential...Watch possible

Valid 302134Z - 302330Z

Probability of Watch Issuance...60 percent

SUMMARY...Thunderstorms should undergo a gradual increase in
coverage and intensity over the Arklatex region through about 00Z,
with a more substantial severe threat expected to evolve after 01Z.
While a tornado watch is possible in the short term (next 1-2 hours)
it is more likely that WW issuance will be closer to 01Z. All severe
modes including large hail, tornadoes and damaging wind will be
possible. Another mesoscale discussion will likely be needed closer
to WW issuance time.

DISCUSSION...As of late afternoon a quasi-stationary/warm front
extends across northern AR through central OK and westward into the
TX panhandle where it intersects a dryline. The warm sector
continues to moisten with mid-upper 60s F dewpoints as far north as
northeast TX and southern AR. Within the moistening warm sector,
scattered convection has been increasing in coverage from LA through
southern AR, but lack of lightning suggests this activity remains
shallow, and indeed special 18Z RAOBs indicate presence of modest
capping inversions between 800 and 500 mb. Objective analysis shows
the atmosphere has become moderately unstable with MLCAPE from
1000-1500 J/kg, and further destabilization is likely as zone of
ascent and cooler temperatures aloft spread eastward above the
western half of the moistening boundary layer. WV imagery and RAP
analysis shows a potent negative-tilt shortwave trough moving
eastward into the southern High Plains. The zone of deeper ascent
associated with this feature (manifested by plume of expanding
cirrus) is currently moving through northwest TX into western and
central OK. By early-mid evening this zone of ascent will begin to
interact with corridor of greater instability from northeast TX into
eastern OK and eventually AR and LA, resulting in removal of any
remaining inversions and deepening updrafts. Moreover, wind profiles
will strengthen with the low-level jet increasing to 50+ kt and 0-1
km hodographs increasing significantly by mid evening. These
processes suggest storms will undergo a substantial increase in
organization and intensification, especially after 01Z with
supercells capable of large hail, damaging wind and tornadoes
becoming likely.


..Dial/Hart.. 11/30/2018

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...JAN...LZK...SHV...TSA...FWD...

LAT...LON 35969489 35949295 35729160 34759126 32579217 31779377
32499527 34509559 35969489
 

Taylor Campbell

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#12
tornadowatch.gif

SEL9

URGENT - IMMEDIATE BROADCAST REQUESTED
Tornado Watch Number 429
NWS Storm Prediction Center Norman OK
520 PM CST Fri Nov 30 2018

The NWS Storm Prediction Center has issued a

* Tornado Watch for portions of
central and eastern Oklahoma
parts of North Texas

* Effective this Friday afternoon from 520 PM until Midnight CST.

* Primary threats include...
A few tornadoes possible
Scattered large hail likely with isolated very large hail events
to 2 inches in diameter possible
Scattered damaging wind gusts to 70 mph possible

SUMMARY...Strong/severe thunderstorms are forecast to continue to
develop over the next few hours, and move quickly northeastward
across Oklahoma and North Texas. Large hail and damaging winds will
be possible with stronger storms, as well as risk for a few
tornadoes.

The tornado watch area is approximately along and 85 statute miles
east and west of a line from 20 miles west southwest of Fort Worth
TX to 35 miles north northeast of Chandler OK. For a complete
depiction of the watch see the associated watch outline update
(WOUS64 KWNS WOU9).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A Tornado Watch means conditions are favorable for
tornadoes and severe thunderstorms in and close to the watch
area. Persons in these areas should be on the lookout for
threatening weather conditions and listen for later statements
and possible warnings.

&&

AVIATION...Tornadoes and a few severe thunderstorms with hail
surface and aloft to 2 inches. Extreme turbulence and surface wind
gusts to 60 knots. A few cumulonimbi with maximum tops to 500. Mean
storm motion vector 22040.

...Goss
 

Taylor Campbell

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#13
Mid level lapse rate of 8.2 with special weather balloon launch at NWS Fort Worth. 6.7 in Little Rock, AR. Mesoscale analysis shows the very significant lapse rates all the way into southern MO. Dewpoints in the mid 60s, plenty of shear for storm organization, and rotation. It's going to be a long night. I'm glad it's Friday.

FWD.gif
 

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