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Area forecast discussion
National Weather Service Norman OK
924 PM CST sun Dec 15 2019

Update...
no significant changes were made to the forecast for tonight.

&&

Discussion...
ascent is beginning to increase downstream of an approaching
trough/pv anomaly (currently located across northwest new mexico).
In response, echoes are beginning to develop across northwest
Oklahoma.

The predominant precipitation type should be snow across
northwest Oklahoma. However, there are indications (at times) that
the dendritic snow growth zone (-12 to -18 deg c) may not be
saturated. In this case, the probability of ice in the cloud
decreases. Instead, supercooled water droplets and "warm-rain"
processes may result in freezing rain and even sleet mixing with
snow. This would occur only if the saturated layer is deep enough
for raindrops to develop through collision-coalescence
(otherwise, it would be freezing drizzle with a shallower layer).
If more of a "wintry mix" occurs here, snowfall totals may be
on the lower end of the forecast.

Farther to the south, including the okc Metro, forecast soundings
indicate a low-level saturated layer that is associated with
freezing drizzle later tonight/toward sunrise Monday morning. This
is associated with isentropic ascent at the ~285 k surface.

With cloud cover overhead and temperatures near saturation, the
primary term in the temperature tendency equation that will result
in colder temperatures tonight is advection. Given that
temperatures across northern Oklahoma are around 27 to 28 deg f,
temperatures should cool another few degrees. One mitigating
factor for drizzle is drier air is also expected to advect
southward, which may increase the surface temperature-dewpoint
spreads. In this case, it would be more difficult from drizzle to
develop. With the aforementioned dry air advection, some forecast
soundings indicate a slightly unsaturated near-surface layer.

Bottom line: there is definitely the potential for slick spots,
as far south as the okc Metro. However, confidence is not high
enough for a Winter Weather Advisory at this time. Trends will
have to be monitored carefully overnight because if temperatures
reach ~27 to 28 deg f with freezing drizzle, travel impacts could
be more significant.

Mahale

&&

Previous discussion... /issued 530 PM CST sun Dec 15 2019/

Discussion...
the December 16-17 00 UTC taf discussion follows:

Aviation...
poor flying conditions due to ceiling and visibility restrictions
are expected through much of the taf period.

Ceilings and visibilities are not expected to fall below Airport
minimums. However, many terminals will likely be below non-
precision approach minimums for several hours. Kwwr, ksps, kcsm,
and klaw will see deteriorating flight conditions late this
evening/overnight while IFR/LIFR conditions continue at the other
terminals.

Freezing drizzle and light freezing rain are possible later
tonight into tomorrow morning across central and north central
Oklahoma (kokc/koun/kpnc). Transition to snow is possible across
north central Oklahoma. Snow is expected to be the predominant
precipitation type across northwest Oklahoma (kwwr).

MVFR conditions should return tomorrow afternoon with even VFR
conditions possible across western terminals (kwwr/kcsm/ksps)
toward the end of the taf. Winds will remain from the north.

Mahale

Previous discussion... /issued 334 PM CST sun Dec 15 2019/

Discussion...

Currently, some light radar echos are being observed across central
Oklahoma. Light drizzle has been reported at some locations within
these echos, however impacts should remain minimal until the
overnight hours. Freezing drizzle has been observed in areas around
Alva, leading to some slick spots on roads being reported within
Woods County.

Cooler temperatures will continue to move into the region, which
will expand the area of freezing drizzle across western, northern
and central Oklahoma Sunday evening. Temperature profiles should
cool in northwestern Oklahoma to where snow should be the dominant
precipitation type by midnight tonight. As the upper level shortwave
trough moves through overnight, there is a decent amount of
agreement between models of snow bands setting up along the
Oklahoma/Kansas border. Forecast soundings indicate a large area of
freezing drizzle and possibly some freezing rain in north central
Oklahoma, in an area primarily between Ponca City and Enid.

Due to the travel concerns with the winter precipitation, a Winter
Weather Advisory has been issued for the northern two rows of
Oklahoma. The limiting factor for the southern boundary of the
advisory is surface temperatures are expected to linger right around
freezing temperature throughout central Oklahoma. If temperatures
fall an additional few degrees, travel conditions will likely
deteriorate due to freezing drizzle in central Oklahoma. Trends will
need to be monitored to determine if the advisory will need to be
expanded southward.

The upper-level shortwave trough will exit the area Monday night,
and ridging behind the trough will result in calm conditions and
seasonal temperatures for the midweek. The next chance of
precipitation will be Friday evening, with an upper level closed low
moving through the region. Models currently differ in the path the
closed low takes to get to the area, with the European model (ecmwf) southerly
trajectory promoting more widespread ascent and precipitation in the
area. The GFS dives the closed low southeastward from the upper
Great Plains and is less favorable for precip, so this pattern
difference will need to be monitored in future forecasts to get a
better idea on rain chances towards the end of the week.

Zwink

&&

Preliminary point temps/pops...
Oklahoma City OK 29 37 26 47 / 20 30 0 0
Hobart OK 27 41 23 47 / 10 20 0 0
Wichita Falls Texas 32 44 27 49 / 10 20 0 0
gage OK 24 38 21 44 / 70 20 0 0
Ponca City OK 26 34 21 44 / 60 60 0 0
Durant OK 37 43 29 48 / 20 30 0 0

&&

Oun watches/warnings/advisories...
OK...Winter Weather Advisory from midnight tonight to noon CST Monday
for okz004>013.

Texas...none.

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