Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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000
FXUS64 KOUN 071238
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
638 AM CST Wed Dec 7 2016

.UPDATE...
Made a minor adjustment to precipitation type and timing for this
evening (see below).

&&

.DISCUSSION...
First of all, weak very light echos on Vance radar were limited to
a couple thousand feet and above as they passed over the radar.
This suggest it is unlikely that snow flurries reached the ground,
though not totally out of the question that flurries were briefly
observed at a few locations. So, the near term forecast remains on
track.

After discussing the setup later this evening with another
forecaster on shift, we decided to scale back slightly on
precipitation this evening. Locations ahead of the front across
the far southeast will have deep enough moisture for the
possibility of light drizzle, but temperatures should remain above
freezing. Sub-freezing temperatures behind the front will probably
not align with when the moisture is deep enough. Even though model
forecast soundings show a brief period of freezing drizzle, they
are probably too deep with the post-frontal moisture. We have
removed freezing drizzle mention and ended the drizzle a little
earlier than what we originally had.

BRB

&&

.PREV DISCUSSION... /issued 544 AM CST Wed Dec 7 2016/

DISCUSSION...

AVIATION... /For the 12z TAFs/
The western edge of MVFR stratus has reached OKC and OUN. There
may be an opportunity for VFR conditions by around midday
persisting until the front comes through later this afternoon.
Once the front moves through, MVFR stratus will likely return and
persist for a few hours into at least early evening. Otherwise
VFR conditions are expected at all sites with most locations
experiencing MVFR stratus behind the front for at least a few
hours. Snow flurries are possible at Ponca City later this morning
through afternoon. There is an outside chance of drizzle or
flurries at OKC and OUN, but the deeper moisture and better chance
is just east of those two sites.

PREV DISCUSSION... /issued 430 AM CST Wed Dec 7 2016/

DISCUSSION...
Precipitation type today will be the biggest forecast challenge.
Impactful winter weather is not expected and measurable amounts
are in doubt. Nevertheless, at least some flurries/sprinkles or
drizzle will be possible, mainly across the eastern and northern
portions of the area. Other forecast challenges include cold air
mass timing and how cold we will be over the next few days. Most
of the rest of the forecast period is fairly benign.

Early this morning, 11-3.9 micron satellite imagery showed
stratus expanding northward across eastern Oklahoma. Short term
model guidance seems have have a decent handle spatially on this
with near zero condensation pressure deficits/saturation on 285k
potential temperature surface. Stratus is evident in satellite
imagery further west across northwest Oklahoma as well, and model
forecast soundings show a shallower but higher saturated layer
associated with this.

On the synoptic scale, a broad band of weakly cyclonic westerlies
is noted in upper air analysis across much of the country. A deep
cyclone is moving east across Ontario with a broad area of fairly
strong northwesterly flow behind it. This has resulted in an
equatorward shift of mature arctic air mass. Very cold
temperatures are now spreading southeastward across the northern
High Plains. Embedded within the westerlies a low-amplitude broad
trough will progress eastward with its axis crossing the Rockies
by this afternoon. This process will help significantly
strengthen/sharpen a deep baroclinic zone across the central
Plains and aid in southward progression of arctic air mass. By
evening, an increasingly sharp surface cold front will have
passed through the area with growing 6 hour post-frontal pressure
rises of 10-12 mb supporting strong gusty winds in its wake. Wind
speeds of around 20-25 mph sustained with gusts to around 35 mph
will be common from late afternoon through about mid-evening,
especially across the western half of the area.

Specific details with regard to moisture magnitude and vertical
distribution will have significant implications for precipitation
type. Our forecast represents the best attempt at applying the top
down method to forecast soundings from various short term models,
but any spatial or temporal inaccuracies or errors in the
depiction of moisture characteristics will result in a different
outcome. The good news is that there is high confidence in light
amounts. In fact, saturation and lift may not be adequate for
measurable precipitation at all, rather just trace amounts.

Already early this morning regional radar mosaic shows light echos
across southern Kansas extending south into north central
Oklahoma associated with virga. A very dry layer in the low
levels was observed in both KOUN 00z sounding and the 06z Lamont
sounding. This will be difficult to overcome. We have included
snow flurries and sprinkles across the far northeast portion of
the area mid-late morning where low level moisture is greater as
these echos move over, but we have refrained from inclusion of
measurable precipitation during the morning. From midday into
early afternoon, sufficiently deep saturated layer and slowly
diminishing low level dry layer may support a better chance of
measurable precipitation, but we still kept the probabilities of
measurable precipitation in the slight chance range and confined
to the northeast and far east. Low level saturation is shown in
RAP and NAM to be more significant further south across the far
eastern part of the area into eastern Oklahoma, but the depth of
saturation is much less. The thermal/moisture profiles in those
areas are more supportive of drizzle. We have only gone with
exclusively snow/flurries in the north where saturation is deep
enough for activation of ice nuclei in the dendritic growth zone.
Further south, this is more in question. In fact, we have even
included some light freezing drizzle in the far east by evening as
the cold front progresses south and temperatures fall below
freezing. Low level saturation should diminish shortly after the
front passes so duration of any freezing drizzle is not expected
to be long enough for any impacts.

Temperatures will struggle to rise above freezing in the north on
Thursday after many locations fall into the teens tonight. Friday
morning may be the coldest as surface high pressure moves over
resulting in light winds and stronger thin near surface
inversion across north central Oklahoma. Single digit low
temperatures are possible there, especially in low lying typically
cold spots.

Southerly surface winds will begin Friday starting a warming trend,
especially in the west. The entire area will experience a
noticable warmup Saturday. By this time medium range model
guidance differs in the amplitude of the next shortwave embedded
within persistent west-northwesterly flow aloft. GFS has been
consistently more amplified and quicker compared to the slower
less amplified ECMWF, but this is slightly quicker than its
ensemble mean. In either case, conceptually, this progressive
pattern would not support much if any precipitation. We have
retained low chances in the far southeast Saturday night but
confidence is not particularly high. The passage of this wave will
result in another cold front. The timing differences in the models
lead to an uncertain temperature forecast for Sunday, and our
current forecast represents a blend of the two models. Adjustments
to temperatures upward or downward will probably be needed once
models come into better agreement.

Early next week the synoptic scale pattern remains pretty much the
same with strong west-northwesterly mid level flow and embedded
fast moving shortwaves. Another warm up should occur quickly by
early next week followed by another cold front sometime around mid
week.

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  42  19  35  15 /  10  10   0   0
Hobart OK         45  18  35  16 /   0  10   0   0
Wichita Falls TX  50  24  37  19 /   0  10   0   0
Gage OK           32  13  32  14 /  10  10   0   0
Ponca City OK     37  15  33   9 /  20  10   0   0
Durant OK         50  28  40  21 /  10  20   0   0

&&

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

$$

17/12



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