Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 140258 AAA
AFDOUN

Area Forecast Discussion...UPDATED
National Weather Service Norman OK
858 PM CST Tue Feb 13 2018

.UPDATE...
Lowered overnight lows in many locations tonight. Also, made
minor adjustments to hourly elements tonight, mainly temperature
and sky cover.

&&

.DISCUSSION...
Fog potential tonight into Wednesday morning is the main concern.

Latest satellite imagery and surface observations at 845 pm CST
depicted abundant low cloud cover east of an Archer City, Texas
to Watonga to Medford line. Some high level cloud cover was over
Oklahoma and north Texas as well.

Rest of tonight, low level moisture/surface dewpoints will
continue to increase across much of Oklahoma and western north
Texas. The increase in moisture will allow for the expansion of
the low level cloud shield westward and northwestward. This cloud
cover should lower closer to the ground as well. Thus, think some
patchy fog will occur across much of Oklahoma and western north
Texas later tonight into Wednesday morning. Do not think the fog
will get too dense (visibilities staying above 1/4 mile) due
increasing low level winds which will veer from the southeast to
the southwest through the night into Wednesday morning.

The highest potential for dense fog may be across parts of south
central and southeastern Oklahoma (southeast of a Ardmore to Ada
line) where low level moisture will be deepest, deep enough to
support some drizzle. Noticed the latest surface observation from
Durant depicted 3 miles of surface visibility, which should
continue to fall in the next several hours. Will not issue a
Dense Fog Advisory at this time due to low confidence of
widespread dense fog.

Temperatures tonight will not fall too much more in most
locations and may actually rise by sunrise in some locations.

Wednesday will be much warmer across the area compared to today.
Fire weather conditions will be near critical levels Wednesday
afternoon across northwestern Oklahoma, but due to abundant
mid/high level cloud cover which may limit vertical mixing and
keep 20 ft winds below 20 mph, will not issue a Fire Weather
Watch or Red Flag Warning at this time.

Products will be updated shortly.

MBS

&&

.PREV DISCUSSION... /issued 553 PM CST Tue Feb 13 2018/

DISCUSSION...
00Z TAFs.

AVIATION...
MVFR stratus is widespread across central Oklahoma, but has eroded
(at least temporarily) across western Oklahoma and western north
Texas. The stratus should fill in again, lower, and spread
northwest this evening encompassing all the TAF sites except KGAG
and KWWR, although there is a chance that the stratus could
briefly brush these sites as well according to the HRRR model. It
appears that ceilings will be the main issue, although the HRRR
and RAP do show for potential across western Oklahoma (KHBR, KCSM
and perhaps KLAW) as moisture pushes back in to areas where skies
will have cleared temporarily this evening.

PREV DISCUSSION... /issued 320 PM CST Tue Feb 13 2018/

DISCUSSION...
Stratus has been persistent across much of the area today in
warm/moist southerly low level flow regime. This will continue
through the night and upstream moisture will advect northward
resulting in low clouds and fog across much of the area. Model
soundings show deep enough low level saturation for drizzle across
roughly the southeast third of the area. This will mostly be a non-
measurable precipitation event, but a few locations in far south-
central/southeast Oklahoma could receive a couple hundredths of an
inch.

Low clouds should gradually clear tomorrow as low level flow veers
and strengthens. Temperatures will warm substantially as a result.
This pattern will favor increasing concern for wildfire spread (see
fire weather section below). The closed low off the California coast
will open/deamplify and shift east at least partially phasing with
northern stream wave. This consolidated trough will approach
Thursday with one more warm day before it forces a cold front
through. Windy conditions behind the front Thursday night into early
Friday can be expected. Large scale ascent may be enough for showers
to develop across mainly the eastern portion of the area Thursday
night, but coverage/amounts should be limited.

Positively tilted trough/shear axis Friday is expected to be
oriented east-northeast to west-southwest across the area during the
day Friday and could provide just enough mid level forcing for some
precipitation across the south. A better chance may occur Friday
night as another shortwave results in veering/strengthening
southwesterly low level flow and isentropic ascent. There is some
model disagreement, however. ECMWF is more amplified with a stronger
low level response along with more robust moisture advection and
QPF. If this trend continues, we may need to increase probabilities,
but recent systems have proved to be less productive at rainfall in
the short term than medium range.

We`ve trended warmer for the weekend as mid-level height rises have
trended more dramatic and southwesterly downsloped/adiabatically
warmed plume should advect over the area again. We may need to
increase temperatures even more if the trend continues. Although
details in geometry and amplitude of western trough are yet to be
resolved (still differences in deterministic guidance and a decent
spread in the ensembles), the general pattern should support
southerly low level moist advection beneath capping EML (a familiar
pattern) again which could bring stratus and drizzle to the
southeast Sunday night. As the trough approaches, assuming it
doesn`t deamplify and weaken like has been the trend the past few
weeks, warmth/moisture could contribute to enough instability for
thunderstorms Monday-Tuesday. We`ve kept probabilities lower than
we otherwise would in the pattern depicted in medium range
guidance in case we do trend toward a less favorable scenario
again.

BRB

FIRE WEATHER...
Dormant fuels with ERC-G values near the 90th percentile and
increasing southwesterly winds, warmer temperatures, and low RH
values (as low as 10-15 percent) will all combine to result in
near-critical to possibly critical fire weather conditions
tomorrow afternoon across northwest Oklahoma. There is some
uncertainty with how thick cirrus will be and how that may impact
temperatures, and more importantly fine fuel heating.
Nevertheless, the wind/RH combination will support rapid fire
spread. In case any fires are ongoing, RH recovery Wednesday night
is expected to be 50-60 percent.

Winds may not be quite as strong and RH values may not be as low
Thursday afternoon, although they will be sufficient for elevated
fire weather conditions at least.

The other concern for area fire managers will be the wind shift
Thursday night. Strong northerly winds are expected behind a cold
front that will progress through the area Thursday evening and
overnight.

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  42  71  56  78 /   0   0   0   0
Hobart OK         40  73  52  80 /   0   0   0   0
Wichita Falls TX  42  74  56  82 /   0   0   0   0
Gage OK           38  79  53  74 /   0   0   0   0
Ponca City OK     42  71  57  76 /   0   0   0  10
Durant OK         40  66  58  74 /  20  20  10  10

&&

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

$$

17/26



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