Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 190843

Area Forecast Discussion
National Weather Service Norman OK
343 AM CDT Wed Sep 19 2018

The primary forecast issue continues to be the threat for heavy
rainfall and flooding from Friday into Saturday.

For today, hot and humid conditions are expected to continue with
a 590 dam mid-level ridge centered just to the east of Oklahoma.
High temperatures will generally be in the low 90Fs (+5-8 deg F
above average) with dew points in the mid-60Fs to low 70Fs. This
will result in another day with heat index values ranging from the
mid-90Fs to near 100 deg F.

For Thursday, the mid-level ridge will be farther east as a
trough ejects into the Plains. High temperatures might be a degree
or two cooler with the ridge farther east than today. As the
trough lift northeastward, the attendant cold front will approach
northwest Oklahoma late Thursday. The trough axis is forecast to
be somewhat elongated with a secondary vorticity maxima centered
across New Mexico.

Showers and thunderstorms are expected to develop along the front
in southwest Kansas and ahead of the trough across the Texas
panhandle. This convection will spread eastward Thursday night.

By Friday into Saturday, the cold front is expected to become
quasi-stationary. Combined with a slow-moving, elongated trough
with embeded vorticity maxima, numerous showers with embedded
thunderstorms are expected. Anomalously high precipitable water
values between ~1.8 to 2.0" and warm cloud depths >11000 ft will
result in efficient rainfall rates. Therefore, heavy rainfall with
localized flooding will be the primary hazard. Severe weather
still appears unlikely given the tropical-like air mass and the
associated weak instability and moist-adiabatic lapse rates.

The greatest forecast uncertainty is how far south the effective
cold front will move as convection may help reinforce the front
southward with time. The movement of the front will modulate the
location of heaviest rainfall.

For Sunday, rain chances will decrease from west to east as a
shortwave trough moves eastward. There is only a low chance of
showers/storms on Monday across southeast Oklahoma.

By Tuesday into Wednesday, a longwave trough is forecast to
amplify across the Plains. The normalized height anomaly
associated with trough are forecast to be 1 to 1.25 standard
deviations below average. This trough is expected to drive a cold
front southward into the Southern Plains.

The uncertainty is the timing of the cold front. Currently, the
19/00Z ECMWF is faster than the GFS with the cold front passage by
almost 24 hours. The ECMWF solution would suggest a frontal
passage on Tuesday. For now, will include a 20-30% chance of
showers/storms across the northern two-thirds of Oklahoma on
Tuesday to account for a potential frontal passage. The cold front
will result in a cooler and drier air mass toward the end or just
beyond this forecast period.



Oklahoma City OK  92  71  89  71 /   0   0   0  30
Hobart OK         92  71  90  70 /   0   0   0  40
Wichita Falls TX  93  71  92  72 /   0   0  10  30
Gage OK           91  71  89  66 /   0   0  10  60
Ponca City OK     93  72  91  70 /   0   0   0  40
Durant OK         94  73  91  72 /  10   0   0  10




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