Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 180522

Area Forecast Discussion
National Weather Service Norman OK
1122 PM CST Sun Dec 17 2017

...Aviation Discussion...


For the 06z TAFs:

Fog has been dense across portions of central Oklahoma late this
evening, and 1/4th mile visibility or less will likely continue
through the night. This may expand into portions of north Texas
and southwest Oklahoma. The most uncertain TAF sites are SPS, CSM,
PNC since these sites may be on the edge of the lower
visibility/ceilings. VFR conditions should return by late morning
or midday.


.PREV DISCUSSION... /issued 859 PM CST Sun Dec 17 2017/


Based on trends we have issued a Dense Fog Advisory for portions
of central and southern Oklahoma.

Despite cirrus, we can faintly see back edge of low stratus
working northeast across south-central Oklahoma in IR shortwave
imagery. As this occurs, radiative cooling will increase enough
for fog formation given how moist the low levels remain. Cirrus
may temper radiative cooling some, but not enough to prevent
several locations from falling to around or possibly below one
quarter mile later tonight.

Further east where low stratus remains, low level saturation has
been deep enough for persistent light drizzle in/around Shawnee
and Chandler to near Ada. This should gradually diminish as depth
of saturation decreases per RAP and HRRR forecast soundings. Once
this happens, and especially when/if low clouds clear, dense fog
may occur across these areas and we will probably need to expand
the advisory eastward.

Across southwest Oklahoma, light winds and at least modest
radiative cooling beneath cirrus will probably eventually result
in fog, but we kept the advisory out of these counties for now
given the fairly large T/Td spread. We may need to expand west to
include a few more counties later tonight.

Otherwise the forecast remains on track. Earlier we lowered
temperatures considerably across the northwest where cirrus is
not present and good radiative cooling is expected.
Minor/insignificant adjustments were made elsewhere to various
weather elements.


PREV DISCUSSION... /issued 355 PM CST Sun Dec 17 2017/

For tonight, fog will be the primary issue. Dew points are
currently in the low to mid-40Fs across the eastern two-thirds of
Oklahoma and adjacent parts of north Texas. As skies clear from
the west, temperatures are expected to cool info the mid-30Fs to
40Fs in this area. With a such a large difference in dew points
and forecast low temperatures, fog (potentially dense) is expected
to develop later tonight across the eastern two-thirds of the area.

By Tuesday, a closed-low is forecast to lift northeastward
into the Southern Plains. Rain is likely across southeast Oklahoma
as this system passes by the area. Some thunder is still possible
as well with some elevated instability. Rain chances decrease
with north and west extent. Dry and above-average temperatures
are expected in the system`s wake for Wednesday.

For the frontal timing on Thursday, both the 17/12Z GFS and
ECMWF have slowed the cold front down due to a slower passage of
the attendant shortwave trough. Therefore, another warm day is
expected for Thursday with the cold front entering northwest
Oklahoma late afternoon. The slower timing may allow greater
moisture advection ahead of the front, which may result in a
better chance of rain across eastern Oklahoma Thursday night.

The air mass behind the initial cold front may not be as cold as
previously forecast with the primary longwave trough remaining to
the west as the initial shortwave lifts northeastward. Therefore,
forecast high temperatures are slightly warmer for Friday and
Saturday afternoons.

By Sunday, there continues to be disagreement between the GFS and
ECMWF. The previous runs of each model don`t even agree with each
other. Even within the GEFS (GFS Ensemble Forecast System), there
is high variance among the 20 members--so overall predictability
continues to be very low in this period.

At this time, the ECMWF and Canadian models are in closest
agreement with a high-amplitude ridge off the east coast and a
high-amplitude trough across the western United States. This
places the Southern Plains within a baroclinic zone/faster flow
aloft. A secondary surge of colder air may advect southward on
Sunday as well. Will maintain a low chance of snow for Sunday,
primarily across northwest Oklahoma, as embedded waves within the
southwest flow could result in snow.

With the continued uncertainty, the bottom line is exactly the
same as yesterday: Colder weather is expected late this week with
at least a low chance of winter precipitation next weekend--but
specifics are yet to be determined.

There are some indications that even colder weather will be
possible beyond this forecast period as another surge of cold air
moves southward.



Oklahoma City OK  47  36  59  45 /  10   0   0  10
Hobart OK         49  34  55  41 /   0   0  10  10
Wichita Falls TX  54  38  56  47 /   0   0  10  10
Gage OK           48  24  60  37 /   0   0   0   0
Ponca City OK     49  39  56  41 /  10   0   0   0
Durant OK         50  43  56  50 /  10  10  10  20


OK...Dense Fog Advisory until 9 AM CST Monday for OKZ018-019-023>031-



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