Severe Thunderstorm Warning 736, NWS Norman, OK

Area Forecast Discussion
National Weather Service Norman OK
208 PM CST Tue Nov 24 2020

...New SHORT TERM, LONG TERM...

.SHORT TERM...
(This evening through Wednesday)
Issued at 208 PM CST Tue Nov 24 2020

A complex and rapidly evolving situation is expected today as 1) a
dryline mixes eastward into our southwest counties, 2) cold front
moves into northwest Oklahoma, 3) Pacific front moves eastward. Both
fire weather conditions and severe convection are near term
concerns. All this is spawn from progressive closed low now that
is now seen clearly in water vapor channel imagery moving into the
Texas Panhandle.

Latest surface observations show eastward mixing dryline encroaching
on our Texas counties. A rapid drop in RH will result, along with
momentum transfer with deep mixing supporting at least a brief
period of elevated to near-critical fire weather conditions generally
southeast of a Sayre to Fredrick to Archer City line through late
afternoon.

The next concern today is intensifying convection along/near the
southeastward moving cold front now across the Panhandles. As strong
DPVA/forcing pushes Pacific front eastward, merging with the
dryline, convection should tend to consolidate into linear segments
and move east. There may be a brief window for quasi-discrete
mode in a small corridor ahead of the aforementioned forcing.
This is supported by some of the CAM guidance.

Although temperatures are not particularly warm and generally
cooler that typically seen with severe events, midlevel
temperatures are quite cold yielding short but sufficiently broad
CAPE profiles for low-top/mini supercells, with a hail threat.
Thermal profiles hint at a tendency for larger quantities of
smaller hail, particularly if storm cores are numerous and
interacting. This could mitigate concerns of significant hail
sizes, though isolated 2+ inch sizes cannot be ruled out.
Fortunately, the core of the low-level jet will be displaced to
the east with latest RAP model data showing more modest low level
shear limiting the tornado concerns in northwest Oklahoma. We'll
still need to watch trends closely, however.

Keep in mind that with this thermodynamic profile we'll need to
downscale our conceptual model some, with high reflectivity cores
not reaching as high and mesocyclones that are smaller/weaker
potentially of more concern.

Later evolution should be strongly forced within an environment
that's otherwise capped. With overall modest instability and
convective vigor not overly impressive, RIJ/surges probably won't be
that substantial and should limit the QLCS tornado threat with
eastward extent. Strong flow aloft and momentum transfer should
still yield a few severe wind gusts. Southward extent is in
question, but at least isolated convection may occur in southern
Oklahoma.

Later in the night, TROWAL and associated midlevel
moistening/ascent may result in a band of light rain traversing
northern counties. Thermal profiles are close to supporting snow
but currently appear to be all rain. We'll need to monitor trends
closely. Only light/brief precipitation amounts are expected.

We increased post-frontal winds tonight through early tomorrow.

BRB

&&

.LONG TERM...
(Wednesday night through next Monday)
Issued at 208 PM CST Tue Nov 24 2020

A brief period of ridging and dry, mild weather will occur
Wednesday and Thursday. With the next wave, the northern stream
will outpace the southern stream component, leaving a NE/SW
oriented shear axis that will traverse the area Friday night
through Saturday. Precipitation should generally be relegated to
the Texarkana region within deeper moisture and positive
precipitable water anomalies. So, the most noticable effect will
be from downslope/warm conditions Thursday to post-frontal
northwesterly flow and cooler (but near normal) conditions Friday
onward. A trend toward deeper more closed low on the southwest end
of aforementioned trough has resulted in an increase in
precipitation chances Friday night into Saturday as modest deep
moisture return occurs. On the northern edge, thermal profiles
could be briefly supportive of snow, although impacts are not
currently expected. Continued warm advection should transition
precipitation to all rain later Saturday.

A southeast moving shortwave trough will pass Tuesday, but
again a dry column doesn't appear to support precipitation.

Just beyond the forecast period, deterministic and ensemble
guidance suggest a trend toward a broad blocking pattern with low
predictability in a couple closed lows at our latitude. Details on
sensible weather will be refined over the next several days.

BRB

&&

.AVIATION...
(18Z TAFS)
Issued at 1201 PM CST Tue Nov 24 2020

IFR ceilings will be prevalent until a dryline and cold front
move from southwestern, and northwestern Oklahoma, respectively
this afternoon and early evening. Thunderstorms are expected,
particulary over the northern half of Oklahoma from later
afternoon until mid-evening. Some severe storms are expected over
northern Oklahoma. The storm system will exit before midnight,
allowing gusty west then gusty northwest winds to spread over
Oklahoma early Wednesday.

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  65  39  57  38 /  90  50   0   0
Hobart OK         72  38  60  39 /  50  10   0   0
Wichita Falls TX  76  41  62  39 /  30   0   0   0
Gage OK           66  33  58  35 /  80  50   0   0
Ponca City OK     60  37  52  34 / 100  80   0   0
Durant OK         68  42  61  38 /  50  30   0   0

&&

.OUN WATCHES/WARNINGS/ADVISORIES...
OK...None.
TX...None.
&&

SHORT TERM...12
LONG TERM....12
AVIATION...09

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