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FXUS65 KCYS 181057
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
357 AM MST Fri Jan 18 2019

.SHORT TERM...(Today - Sunday)
Issued at 400 AM MST Fri Jan 18 2019

The main near-term concern continues to be the potential for heavy
mountain snow, along w/ minor accumulations in lower elevations of
Carbon County including Rawlins, Saratoga and Elk Mountain. SNOTEL
observations suggest 15 inches of snow has already fallen over the
Sierra Madre range, assuming a 15:1 ratio and 1 inch of QPE. Snowy
Range amounts have been in the 8 to 12 inch range so far over some
of the higher peaks. Additional snowfall amounts are not likely to
be particularly impressive. Rates have actually slowed quite a bit
over the last few hours as well. Nonetheless, dynamic support will
remain strong through the AM as the trough axis translates from W-
E across the CWA over the next several hours. UVV in the left exit
region of a 140+ knot H25 jet will support additional accumulation
today. Snow is likely to decrease rapidly in coverage/intensity by
5 PM MST today as forcing mechanisms exit to the east & drier mid-
level air works into the area. No changes were made to our ongoing
Winter headlines. If anything, we may be able to cancel Warnings &
Advisories prior to the 5 PM expiration.

Our next focus becomes the potential for strong winds on Saturday/
Sunday over the southeast Wyoming wind corridors. H7-H8 CAG to CPR
gradients increase into the 50 to 60 meter range early on Saturday
and remain elevated through mid-day Sunday. Flow aloft should also
be favorable around 50 knots, but only briefly on Saturday morning
before decreasing to around 40 knots for the remainder of the time
period of interest. The surface pattern is not all that convincing
for warning-criteria wind gusts either with no real pressure falls
other than some very weak troughing in the vicinity of the Laramie
Range. This troughing is what is driving the gradients upward with
a less-than-optimal orientation over Arlington. Subsidence appears
to be fairly modest, and low-level lapse rates are not expected to
be all that steep by any stretch. This does have some similarities
to Wed night & early Thu, which largely failed to produce warning-
level gusts in the wind prone areas. Confidence is not high enough
to warrant High Wind Watch/Warning issuance at this time, but wind
gusts at least up to 55 MPH do appear possible. The next shift can
re-evaluate and issue highlights as deemed appropriate if over all
confidence increases.

.LONG TERM...(Sunday Night - Thursday)
Issued at 400 AM MST Fri Jan 18 2019

A brief period of ridging and relaxing winds as we move into
Sunday night. However, clouds streaming in at the mid and upper
levels over the course of the night indicate that these conditions
won't be here to stay for very long as the next deep upper-level
trough drops quickly out of the Pacific northwest and digs south and
east, through the region. This disturbance will bring widespread
rain and snow to the region as temperatures drop to 5-10 above
normal on Monday. Little confidence exists in snow amounts or QPF at
this time as this event is still rather far out, however, with
limited upper dynamics and a weak moisture tap, the possibilities in
a significant storm are rather low. This will be a forecast element
that will have be watched closely as it approaches.

Breezy conditions will develop in the trough's wake Tuesday, with
temperatures approaching normal for this time of year. A cold
northwesterly flow aloft will set in across the region as a
deepening ridge builds off the Pacific coast. This pattern looks
mainly dry, with winds becoming the main forecast concern. High Wind
headlines will need to be considered beginning late Tuesday and
continuing for the duration for the long-term forecast.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1003 PM MST Thu Jan 17 2019

An upper level shortwave is making its way across the western
portion of the CONUS leaving a mess of weather conditions across
southeastern Wyoming and the Nebraska panhandle. Ceilings and
visibilities have been fluctuating up and down between MVFR and
IFR over the last few hours. Parts of the northern Nebraska
panhandle have been experiencing freezing fog, and it could
continue overnight with moisture sticking around. Currently,
moisture across southeastern Wyoming is lacking at the lower
levels, but with moisture in the mid-levels, it is expected that
the lower levels will saturate bringing ceilings down and snow and
rain showers possible in the early morning hours and into the day
tomorrow. Anticipate categories to fall to IFR for most sites and
down to LIFR for a select few due to lowering ceilings. In
addition, winds will be increasing across the area tomorrow
to 20 to 30 knots.


&&

.FIRE WEATHER...
Issued at 400 AM MST Fri Jan 18 2019

No fire weather concerns. Snow continues today, mainly west of the
Laramie Range with some accumulations possible especially over the
mountains of southeast Wyoming. Snowfall amounts along and east of
the Laramie Range should generally be one inch or less. Occasional
windy conditions can be expected over the next several days, but a
relatively cool pattern will keep RH outside of critical threshold
values.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Warning until 5 PM MST this afternoon for WYZ112-
114.

Winter Weather Advisory until 5 PM MST this afternoon for
WYZ109>111-113.

NE...None.
&&

$$

SHORT TERM...CLH
LONG TERM...AB
AVIATION...CW
FIRE WEATHER...CLH
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