Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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000
FXUS64 KOUN 181043
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
443 AM CST Sun Feb 18 2018

.DISCUSSION...
Forecast challenges and uncertainties mainly center around ongoing
fog and stratus and aviation impacts, fire weather this afternoon
and Monday afternoon, and potential for some wintry precipitation
by the middle of next week.

Latest observations show significant visibility drops at a few
south-central and southeast Oklahoma sites (i.e., Durant and
Hugo). Easterly winds will continue to transport upstream
moisture into the area early this morning and the area of fog is
likely to expand westward and northward through the early-mid
morning hours. Moisture advection will be significant/deep enough
for low stratus to persist into the afternoon and probably through
the day across a large portion of the area. This may impact
temperatures and so we have lowered them slightly compared to the
previous forecast.

West of the stratus, across far northwest Oklahoma, deep mixing
and momentum transfer will lead to strong winds (25-30 mph with
gusts up to 45 mph). We have issued a Wind Advisory for this area.
We added an area of patchy blowing dust given persistent dry
conditions and the strength of the wind. In addition to the wind
and blowing dust, significant warming into the low-mid 70s and
falling RH values will lead to elevated to critical fire weather
conditions (see Fire Weather Discussion below).

An active, and wet, pattern is expected to develop as
aforementioned western trough deepens and approaches. One weak PV
anomaly within southwesterly flow aloft may be enough for a few
showers across the southern an eastern portions of the area later
today into tonight. By Monday morning, low-level confluence and theta-e
axis will sharpen and mid-level moisture will increase. Showers
should become more numerous across roughly the eastern half of the
area. This will continue through the day and into Tuesday. Monday
afternoon, meager instability may develop contributing to some
threat of thunderstorms. Severe weather seems unlikely, but deep
layer shear is strong. If pockets of modest instability can
develop, some strong to marginally severe thunderstorms are
possible.

PWAT values will increase substantially and become quite
anomalous so that by late Monday night into Tuesday as forcing for
ascent from approaching trough increases, locally heavy rainfall
may occur.

A cold front will move through late Tuesday into Tuesday evening.
This is where the forecast confidence falls considerably. There
remains some discrepancies in model guidance with regard to
timing of the front and magnitude of shallow cold airmass. Most of
the precipitation should have shifted east by the time the cold
air arrives. However, medium range deterministic (ECMWF in
particular), and several GEFS members show a warm/moist advection
regime atop the cold air mass by early Wednesday. This is response
to a rather potent shortwave trough now showing up in New Mexico
rounding the base of longwave. This may result in an isentropic
ascent scenario with sufficient moistening for precipitation
Wednesday across a good portion of our area. With the temperature
profiles models are currently indicating (assuming cold post-
frontal air mass is as cold as models indicate), freezing rain may
be a primary concern, with some sleet a secondary precipitation
type. We tried not to get too specific with precipitation types in
the forecast given uncertainties and the time range, but we did
emphasize mainly rain or rain/sleet near the Red River and some
chance of snow further north near the Kansas border deeper within
the cold air. We`ll need to monitor the trends to see how
impactful this event will be, or if perhaps (with any luck) the
cold shallow air mass can erode more quickly than currently
indicated.

Some precipitation may linger into Wednesday night. There`s now a
decent signal in medium range guidance of another
trough/precipitation event arriving late Thursday and bringing
decent rain chances Friday. This, for the most part, appears to be
a rain event with little or no chance of wintry precipitation.
There could be enough weak instability for thunder in the far
southeast portion of the forecast area. As western longwave begins
to kick out next weekend, mostly dry conditions and near normal
temperatures are expected.

BRB

&&

.FIRE WEATHER...
We have issued a Red Flag Warning for far northwest Oklahoma for
this afternoon. We`ve had scenarios this cold season so far that
are similar with return flow and increasing moisture that have
yielded an increase in initial attack fire. This scenario is a
little more concerning because winds are expected to be so strong.
The response to approaching/deepening western trough is fairly
substantial with lee side pressure falls and a tightening
gradient, so 25 to 30 mph sustained winds with gusts up to 45 mph
are expected. Our approach is weighting the variables for a more
complete weather/fuel consideration. With fuels dormant and dry
(ERC-G above the 75th percentile possibly approaching the 90th
percentile), and such strong winds, a critical day is expected
despite marginal RH values at best for much of the warned area.

Elevated to near critical conditions are expected over much of
western Oklahoma and western north Texas tomorrow (Monday)
afternoon. Deep mixing causing low RH values and breezing
southwest winds are expected.

A cold front will move through late Tuesday afternoon or evening
bringing a wind shift and gusty north winds behind it. Low RH
values and the strongest north winds will be misaligned precluding
a more significant fire weather threat.

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  60  55  71  60 /  10  20  40  70
Hobart OK         63  57  76  58 /  10  20  20  40
Wichita Falls TX  67  61  77  62 /  20  20  30  60
Gage OK           72  54  77  43 /   0   0   0  10
Ponca City OK     62  55  73  50 /   0  20  40  70
Durant OK         58  58  67  64 /  30  40  50  70

&&

.OUN WATCHES/WARNINGS/ADVISORIES...
OK...Wind Advisory from 10 AM this morning to 6 PM CST this evening
     for OKZ004-005-009-010-014.

     Red Flag Warning from noon today to 6 PM CST this evening for
     OKZ004-009-010.

TX...None.
&&

$$

09/12



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