Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 271152
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
552 AM MDT Mon Mar 27 2017

.SHORT TERM...(Today through Tuesday night)
Issued at 444 AM MDT Mon Mar 27 2017

Main focus in the short term is the strong upper trough currently
seen on WV imagery moving on shore the NW US coast. Models are in
good agreement that this trough will dig into the SW US by tonight
and form a closed low in/around the four corners region early
Tuesday. Most guidance continues to track this low eastward and into
the Texas Panhandle by Wednesday morning.  While this track is
rather far south, the GFS and to some extent the EC show decent mid
level deformation over SE WY and the NE Panhandle into Tuesday
night.  There is also an elongated inverted trough extending
northward from the main 700 mb low that sits over SE WY during this
time frame as well. These features will combine to make up for the
lack of upper dynamics and produce more precip than would otherwise
be expected for a system that far south.

The main forecast problem is of course, will there be significant
accumulating snow especially for SE WY and if so, how much?  Snow
levels start lowering by late Tuesday as colder air filters in from
the north and west.  Am most confident in precip being all snow for
the Snowy Range and even into the Arlington area so decided to go
with a Winter Storm Watch based on consensus qpf and favorable
upslope regime.  Other areas of concern are the south Laramie Range
including the I-80 summit as well as Laramie proper.  Think the
boundary layer might just be warm enough here during the day on
Tuesday to preclude much accumulation even with the predominant
precip type being snow.  Elected to wait on issuing any headlines
here and let one more shift evaluate.  East of the Laramie range
areas above 5000 ft in SE WY will also most likely see a period
of snow Tuesday night before the precip ends. SREF plumes for
Cheyenne show quite a spread on snow amounts but the average being
around 2 inches. Should the GFS be correct, these amounts may
need to be adjusted up but this model hangs on to precip longer
than the NAM/EC.

As usual, a degree or two difference in temperatures or a subtle
shift in the track of the system could make a big difference in
precip type and amounts.

.LONG TERM...(Wednesday through Sunday)
Issued at 444 AM MDT Mon Mar 27 2017

Could see precip linger along the Colorado Border through Wednesday
morning, especially with the slower GFS soln.  Temps will be cool on
Wednesday with 700 mb temps around -5C.  A shortwave ridge will move
overhead on Thursday ahead of the next upper trough that is digging
into the Great Basin.  The GFS/ECMWF are in rather good agreement at
showing a closed low developing in the vicinity of southern Utah on
Thursday night, which then slowly moves eastward toward the Four
Corners region by Friday.  Precip chances will be on the increase by
Friday with a mix of rain/snow over the CWA.   This system looks
slightly cooler than the storm that will impact the area early this
week, with the snow level 6000-7000 ft.  Saturday will remain cool
and unsettled, particularly in the areas near the Colorado Border.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 549 AM MDT Mon Mar 27 2017

IFR cig/vis will continue at CDR/AIA for another few hours. The
fog/low clouds will dissipate by mid morning. Cloud cover will
increase through the afternoon and evening across the region with
rain showers developing. Winds through the taf period should
mostly be below 15 kts.

&&

.FIRE WEATHER...
Issued at 444 AM MDT Mon Mar 27 2017

No fire weather concerns through the middle of this week.  Minimum
humidity values will mostly be above 25-30 percent this afternoon.
Cooler temperatures and widespread precipitation is in store for
Tuesday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from Tuesday morning through late Tuesday
     night for WYZ110-114.

NE...None.
&&

$$

SHORT TERM...DEL
LONG TERM...ZF
AVIATION...ZF
FIRE WEATHER...ZF



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