Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 280257
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
857 PM MDT Mon Mar 27 2017

.UPDATE...
Issued at 840 PM MDT Mon Mar 27 2017

Evening water vapor imagery showed a strong upper trough axis
from ID south to the Desert Southwest. Deep layer QG ascent
extended from UT into western CO and southern WY, producing
widespread showers. There were a few lightning strikes detected
over extreme northeast UT. A surface low was located northeast of
Rawlins with a front over southeast UT with a cold front trailing
southwest to a deeper low over southeast UT. Just over a tenth
of an inch of rain has fallen at Cheyenne this evening. Showers
will continue to spread northeast into the Nebraska Panhandle this
evening. West of the Laramie Range, rain was mixing with or
changing to snow above 7500 feet. Snow levels will lower to 6500
feet tonight, with 1 to 4 inches expected, heaviest above 8500
feet. Went ahead an issued a Winter Weather Advisory for Tuesday
and Tuesday night for Rawlins, Sinclair and the northern Carbon
County mountains for snow and blowing snow. Still some uncertainty
as to snowfall amounts for Laramie Valley and southern Laramie
Range.

&&

.SHORT TERM...(Tonight - Wednesday)
Issued at 325 PM MDT Mon Mar 27 2017

Main forecast concern continues to be the strong upper level trough
currently over the Great Basin. It still is expected to move
southeast across the four corners region and into New Mexico tonight
and Tuesday, with midlevel disturbances rounding the trough and
moving north/northwest on the eastern edge of the trough. 12Z models
are still a bit inconsistent on temperatures through the day as
these disturbances move overhead, lending to continued uncertainty
on locations within the southern Laramie Range (Including the I-80
summit) and the Laramie Valley as to how much snow they`ll receive
Tuesday afternoon through the early evening. Soundings support a mix
of rain and snow at Laramie through the day along with some
instability, which would likely translate to snow up at the Summit.
Sfc winds/mslp model analysis favor the inverted trough slightly
more over the Laramie Valley for best sfc convergence and upslope
flow though, but the differences between models are just enough to
create a few degrees temperature difference across the area which
will affect actual snow rates and accumulation, and thus  resultant
impacts. Hopefully with one more model cycle, confidence will
increase whether the Summit and Laramie Valley will need winter
weather alerts of some variety, especially if winds increase any
further. Temperatures should cool off enough through the evening for
all snow in these areas.

Further north along the northern Laramie Range, think this pattern
is good for that area to get a foot or more of snow with strong
winds to warrant inclusion into the Winter Storm Watch. Further
west, think the Winter Storm Watch for Snowy Range and the Arlington
area looks good. Less snow is forecast elsewhere in the high country
(around 2-4") so think that can be handled with an Advisory later
on. However, if the next model suite comes in colder, certainly can
see more snow accumulation. East of the Laramie Range, temperatures
will remain warm enough for light to moderate rain during the day
Tuesday and Tuesday evening as plentiful upper level diffluence and
isentropic ascent develop over this area. Could see snow down to
roughly 5000ft Tuesday night as the sfc trough steadily shifts east
and we see colder northerly flow move in. Wednesday will see precip
end steadily from west to east through the early part of the day,
with temperatures rebounding pretty quickly.

.LONG TERM...(Wednesday Night - Monday)
Issued at 325 PM MDT Mon Mar 27 2017

A brief period of quiet weather early in the forecast period as an
upper-level low departs across the central/southern plains, giving
way to short-wave ridging over the central Rockies. The GFS and EC
are in general agreement showing an upper low digging toward the 4
Corners on Friday, with precipitation spreading from W-E across SE
Wyoming and the western Nebraska Panhandle through Saturday. Model
guidance differs significantly with this system, so our confidence
is quite low in precipitation amounts. The GFS is bringing the low
south along the Mexican border by Sunday, while the ECM ejects the
system northeast into the central plains at the same time. Temps &
resulting precipitation types will be heavily dependent on how the
storm system evolves, so made few changes this forecast period.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 453 PM MDT Mon Mar 27 2017

Rain is expected to increase in coverage and intensity across much
of the area after 03z, and continue through Tuesday. A change-over
to snow will occur along/w of the Laramie Range overnight, w/ some
potential to spread into CYS on Tuesday morning. VFR prevails into
the early evening hours, but conditions will rapidly worsen in the
06z-12z time frame w/ IFR to LIFR ceilings and visibilities likely
for all terminals from 12z through much of the day on Tuesday.

&&

.FIRE WEATHER...
Issued at 325 PM MDT Mon Mar 27 2017

No fire weather concerns through the middle of this week with
widespread precipitation expected across the district along with
cooler temperatures.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from 6 AM MDT Tuesday through late Tuesday
     night for WYZ103-110-114.

     Winter Weather Advisory from 6 AM Tuesday to 6 AM MDT Wednesday
     for WYZ104-109.

NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...RJM
LONG TERM...CLH
AVIATION...CLH
FIRE WEATHER...RJM


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