Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 270340
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
1040 PM CDT Thu Apr 26 2018

.DISCUSSION...
Aviation discussion for the 06 UTC TAFs is below.

&&

.AVIATION...
VFR conditions are expected to continue.

Plan to go with VCSH at KSPS before 07 UTC Friday. -SHRA should
be well south of KLAW and KCSM by 06 UTC Friday.

Brief MVFR or lower conditions with BR may occur near KCSM now
through 09 UTC Friday, but do not plan on mentioning due to low
confidence of occurrence.

Light winds will generally switch to the south at most sites
between 21 UTC Friday and 02 UTC Saturday.

MBS

&&

.PREV DISCUSSION... /issued 956 PM CDT Thu Apr 26 2018/

..Update...

DISCUSSION...
We significantly increased winds across far southwest Oklahoma
and a small section of western north Texas due to interaction of
cold pool/meso-high and weak surface trough. Localized pressure
gradient has been strong enough for wind gusts up to 50 mph at
Hollis mesonet and 45 mph at Goodlett mesonet. These winds will
subside later tonight. Otherwise, the forecast remains on track.

BRB

PREV DISCUSSION... /issued 749 PM CDT Thu Apr 26 2018/

.Update...

DISCUSSION...
We have completed a couple of minor updates so far this evening
following trends. Weak instability, generally <400 J/kg MLCAPE,
will support a small thunderstorm cluster near Elk City as it
continues moving southeast. Occasional lightning will be the main
threat, although this convection may be just strong enough to
augment post-frontal winds slightly to result in gusts of around
40 mph. These stronger winds are mainly driven by post-frontal
pressure rises and resultant gradient confined to far western
Oklahoma and further west into the Texas Panhandle. Locally around
two tenths of an inch of rain may occur, before heavier
convection weakens later this evening, but most locations will
receive less than a tenth of an inch. Besides adding more temporal
detail to PoPs/Wx grids, changes to other fields were minor.

BRB

PREV DISCUSSION.../issued 354 PM CDT Thu Apr 26 2018/

DISCUSSION...
Primary forecast issue continues to be the risk for severe weather
next week.

For this evening, a quick-moving shortwave trough and attendant
cold front will result in scattered showers and isolated storms
across western Oklahoma. Forecast soundings indicate some MUCAPE,
so some isolated thunder can`t be ruled out. Also, some gusty
winds will be possible in vicinity of any showers/storms with
very dry boundary layer conditions and steep low-level rates. This
wave and associated precipitation should exit tonight.

In the system`s wake, another cold night is expected across the
area. Low temperatures will be well below average again with upper
30Fs to mid 40Fs. A seasonable day is expected tomorrow with
abundant sunshine and temperatures in the 70Fs.

The upcoming weekend will be a transition period as mid-level
ridging occurs across the Plains. In response, a warming trend is
expected for Saturday and Sunday. Southerly winds will become
breezy by Sunday, which may elevate fire weather conditions across
far northwest Oklahoma.

By early next week, the eastern U.S. trough will begin to exit
toward the western Atlantic Basin. As a result, the GFS and ECMWF
(and their ensemble means) continue to indicate a longwave trough
will develop across the western half of the contiguous U.S. This
will place the Southern Plains within southwest flow aloft.
Synoptically, this is a favorable pattern for strong to severe
thunderstorms, with increasing vertical wind shear and instability
across the Southern Plains. More specific details follow below.

For Monday: Low-level moisture may limit the magnitude of the
severe weather threat as low-level trajectories may not be most
favorable with some influence from the departing eastern U.S.
trough and attendant surface ridge across the eastern Gulf of
Mexico. Nevertheless, some high-based thunderstorms may develop
across the Texas panhandle along a dry line. These thunderstorms
could affect western Oklahoma late Monday afternoon into Monday
evening.

For Tuesday and Wednesday: Low-level trajectories will be more
favorable for increasing low-level moisture with the complete
departure of the eastern U.S. trough/Gulf of Mexico surface ridge.
Thunderstorms are expected to develop along a dry line both days
across western and/or central Oklahoma. The overall risk on
Wednesday might be slightly more to the east than on Tuesday.

It is important to note that while confidence is increasing for
the potential for severe weather on Tuesday and Wednesday, there
is still uncertainty on the mesoscale details that will determine
the magnitude of the severe weather threat. One significant factor
will be the timing of embedded shortwave troughs and any
attendant surface cyclones. This will modulate the low-level winds
(i.e., backed vs. veered low-level flow). Therefore, it is still
too early to get specific on which day will be more significant
and what will be the primary hazards.

For Thursday: Strong to severe thunderstorms will remain possible,
especially across southern Oklahoma and north Texas, as a cold
front passes the area. Forecast confidence does decrease for
Thursday.

Quieter weather is expected for the following weekend as there
are indications on both the GFS and ECMWF of a transition back
toward northwest flow aloft.

Mahale

&&

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

$$

12/17



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