Area Forecast Discussion
Issued by NWS Cheyenne, WY

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700
FXUS65 KCYS 141739
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1039 AM MST Sun Jan 14 2018

.UPDATE...
Issued at 949 AM MST Sun Jan 14 2018

We sent out a quick update to adjust the timing of the
precipitation chances in the northeast section of our forecast
area this afternoon. Latest satellite loop was showing the high
clouds starting to thicken a bit, but the mid layers are still
trying to saturate. Therefore, we decided to tweak pops a bit with
the bulk of the precip. saturating after 21z mainly northeast of
an Alliance to Douglas line. &&

.SHORT TERM...(Today through Monday night)
Issued at 315 AM MST Sun Jan 14 2018

Current KCYS radar loop shows a band of snow showers moving
southeast across the southern Nebraska panhandle and far eastern
Wyoming near the Nebraska border. This band is mainly in response
to jet dynamics aloft along with some marginal warm air advection
just off the surface. Not much snow accumulation, if any at all,
has been observed via webcam with maybe a dusting to half an inch
on grassy surface. This band should push south into Colorado over
the next few hours. The arctic front has exited the area for now,
as seen with temperature trends over the last several hours.
Chadron Nebraska is currently at 29 degrees early this morning
after being down in the teens and single digits yesterday
morning. The arctic front is currently over far eastern Montana
and extends south-southeast into the western Dakotas and central
Nebraska. This front is expected to push southwest later today.
Before that though, temperatures will be pleasant today and
generally in the 40`s with a few locations along I-80 around 50
degrees. Daytime highs further north will occur late this morning
or early this afternoon before rapidly falling.

By tonight, models show the arctic front rapidly moving southwest
around the I-25 corridor between sunset and 9 pm. Temperatures
will quickly lower into the teens to low 20`s behind the front
with snow developing and moving south across the eastern plains
of Wyoming. Precipitation will mainly be due to jet dynamics
across the Right Entrance quadrant of the main jet streak. In
addition, some warm air advection aloft, marginal upslope flow,
and moderate but brief frontogenesis will be present. Values are
slightly less impressive compared to 24 hours ago, so will not go
with any Winter Weather Advisories quite yet, especially with
areas further north and west which may not get much snowfall at
all. There is still some concern with heavy snow bands, but model
soundings are showing a layer of drier air around 700mb which is
in the snow growth region. Tweaked our forecast snow amounts and
mainly going with 1 to 2 inch amounts across the High Plains at
this time with slightly higher amounts along the Laramie Range.
With no blowing snow expected, and drifting snow expected to be
minimal, will let the day/evening shifts make a final decision on
potential advisories and/or travel impacts.

Snow will gradually end from northeast to southwest early on
Monday. High temperatures in the teens to mid 20`s will be common,
with locations further east near Chadron, Lusk, and Alliance
remaining in the single digits. With clearing skies Monday night,
low temperatures below zero are expected across most of the area.
May need Wind Chill Advisories for most of the eastern plains and
maybe the Laramie Valley for Wind Chills around -20 to -25.

.LONG TERM...(Tuesday through Saturday)
Issued at 224 AM MST Sun Jan 14 2018

Main focus for this period will be the upper level trough which
moves through at the end of next week.

Wednesday a closed upper level low will move over the forecast area.
Confidence is getting lower as to where the trough will track. The
latest two runs of the ECMWF have shifted the trough over Montana
instead of Colorado, while the GFS has changed little. Despite the
low confidence of its track, condensation pressure deficits are
quite high, even under the closed low.  As a result have low
confidence of the trough producing any precipitation outside of the
mountains.

The best chance for widespread precipitation will begin Friday as a
stronger upper level trough moves in from the northwest.  A cold
front will accompany the trough passage.  Precipitation will develop
behind the cold front as condensation pressure deficits lower
considerably.

The colder air behind the front will move in Saturday.  If this
timing holds, temperatures will be warmest in the morning, then fall
through the rest of the day.

Models have been fairly consistent with the track/speed of this
trough at the end of the week.  This increases confidence that the
forecast area will receive precipitation from this trough.  However
this is still a week away, and much can change in that amount of
time.

Friday through Saturday, very strong wind gusts may occur due to
700mb winds of 50kts developing over the central and southern part
of the forecast area.  These winds could lead to blowing snow and
reduced visibilities on these two days.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday morning)
Issued at 1039 AM MST Sun Jan 14 2018

VFR for all terminals this afternoon with some gusty winds affecting
KRWL.  Cold front will move south across much of the area this
evening with cigs lowering to MVFR/IFR east of the mtns as the
evening progresses.  Area of light snow will move sw across the area
overnight into Monday morning with conditions returning to VFR from
north to south late tonight through Monday morning.


&&

.FIRE WEATHER...
Issued at 224 AM MST Sun Jan 14 2018

No Fire Weather concerns expected this week with an arctic front
impacting the area early in the week. Very cold temperatures, high
relative humidities, and some light snow are expected until
Tuesday. A slight warm up is then expected through the middle of
the week, with daytime humidities lowering between 30 to 50
percent.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...REC
SHORT TERM...TJT
LONG TERM...JTL
AVIATION...RE
FIRE WEATHER...TJT



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