Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 201041

Area Forecast Discussion
National Weather Service Norman OK
541 AM CDT Wed Sep 20 2017

The September 20-21, 2017 12 UTC TAF discussion follows:


Unrestricted visibility and ceilings are generally expected to
continue through the TAF period.

A cold front with a northerly wind shift has entered northwest
Oklahoma, and will continue to shift southeastward this morning.
The front will progress into west-central Oklahoma (perhaps to
KCSM) before stalling early this afternoon. Ahead of the front,
southerly winds are expected through the TAF period.

The probability of showers and thunderstorms along the front this
afternoon/evening is too low to include in TAFs; however, there
is a low chance some storms may develop across southwest
Oklahoma/western north Texas and/or far north central Oklahoma.
KPNC, KSPS, and/or KHBR could be impacted if storms do develop.
Gusty winds will be possible with any storms.



.PREV DISCUSSION... /issued 353 AM CDT Wed Sep 20 2017/

For this morning, a cold front (currently located in the
panhandle) will enter northwest Oklahoma. The front is expected to
stall by early afternoon along a line from near Alva to Clinton
to Hollis. Opted to raise high temperatures a few degrees above
blended guidance. Forecast low-level thermal temperatures (e.g.,
925-850 mb) are progged to be as warm or even warmer than
yesterday because of a low-level thermal ridge ahead of the cold
front. Therefore, would expect high temperatures ahead of the
front to be at least as hot as yesterday (Wichita Falls tied a
record at 100F yesterday afternoon).

There is uncertainty on whether convective initation will occur
along the cold front. No synoptic scale ascent is forecast for
this afternoon; subtle mid-level height rises even suggest
synoptic scale subsidence. BUFR soundings indicate two potential
areas where the cap will be weakest. One potential area is across
north central Oklahoma--near the Kansas/Oklahoma border--where
mid-level temperatures are cooler. Another potential area is
across southwest Oklahoma and adjacent parts of north Texas. Very
hot temperatures may allow for convective initation in this area.
Between these two areas, (i.e., Central Oklahoma), the cap is
forecast to be the strongest.

Out of the two potential areas, convective-allowing models are
most aggressive across western north Texas. The NSSL WRF and 3 KM
NAM both initiate convection in this area. A few members of the
NCAR Ensemble initiate thunderstorms as well. Nevertheless, a
slight chance of showers/storms was included in both areas.

If thunderstorms develop, environmental conditions will be
sufficient for strong to severe thunderstorms with MLCAPE >=3500
J/kg and effective bulk shear up to ~20 knots. Damaging wind gusts
will be the primary threat with large DCAPE (>=1700 J/kg);
however, the moderate instability and large CAPE in the -10C to
-30C layer indicates hail will also be a hazard.

The cold front is expected to lift back north as a warm front
tonight. For Thursday, a subtle shortwave trough may initiate
showers/storms across western Oklahoma and adjacent parts of north
Texas. Mid-level heights are forecast to fall (slightly) with
limited convective inhibition. Convective allowing models (WRF-
ARW/HRW NMMB) and the parameterized 12 KM NAM indicate the
possibility of convection. Therefore, opted to add a slight
chance Thursday afternoon/evening.

Otherwise, another hot day is expected Thursday afternoon.
Temperatures might be slightly cooler than today as the center of
the low-level thermal ridge shifts northwestward with the retreat
of the front. The pressure gradient will be stronger, which will
increase the southerly winds--especially across northwest
Oklahoma. This may result in elevated fire weather conditions
Thursday afternoon.

From Friday and Saturday, very warm and dry conditions are
expected. Oklahoma and north Texas will be on the western
periphery of a mid-level ridge centered across the Midwest. This
should keep the affects of a high amplitude longwave
trough/closed-low across the western contiguous United States to
the west of our area these two days. High temperatures will
continue to be above average in the upper 80Fs to low 90Fs.

By late Sunday into Monday, the trough/closed-low across the west
is forecast slowly to shift eastward. There are indications in
both the ECMWF and GFS that the trough may separate into two
waves (one to the north and one to the south). The southern trough
/closed-low will provide a favorable pattern for widespread
showers/storms for Monday and Tuesday. Locally heavy rainfall may
become a hazard during this period. The northern trough/closed-
low`s attendant cold front may bring much cooler (below average)
temperatures by Tuesday.



Oklahoma City OK  93  75  91  72 /  10  10  10  10
Hobart OK         96  74  94  73 /  10  10  10  10
Wichita Falls TX 100  76  95  73 /  20  20  10  20
Gage OK           88  66  94  70 /  10  10  20  20
Ponca City OK     94  75  93  72 /  10  20  10  10
Durant OK         94  75  92  71 /  10  10  10  10




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