Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 240536
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1136 PM MDT Sun Apr 23 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 210 PM MDT Sun Apr 23 2017

The upper level trough is currently over eastern OR/western NV and
steadily moving east toward the Rockies. Ahead of it, a good stream
of Pacific moisture is moving up from the southwest keeping a mid
to upper level cloud deck over most of the forecast area. Sfc low
pressure developing over north central Wyoming today is inducing
breezy south to southwest winds over the forecast area with
widespread gusts 20 to 30 mph and higher speeds out west. Humidity
values in the lower elevations out west have dropped into the
upper teens with winds gusting 30 to 45 mph. Will just keep
elevated fire weather conditions in the forecast as expect cloud
cover to thicken through the afternoon which should help
stabilize humidities. A few showers and t-storms could move up
from the southwest in the late afternoon to impact western Carbon
County, but still feel the best chance for convection will occur
as the front moves west to east this evening. A dry boundary layer
will keep overall instability marginal, but relatively steep
lapse rates should contribute to a few t-storms especially as
moisture increases with the Pacific front. Guidance shows LI`s
decrease to around -1C by early in the evening. Winds behind the
front will veer to the west and remain quite gusty tonight,
especially in the gap wind areas along and west of the Laramie
Range.

Monday will be slightly cooler than today behind the Pacific front.
Another upper trough will approach from the northwest, with
vorticity and increasing moisture moving overhead in zonal flow. In
addition, a deepening leeside sfc pressure trough will amplify sfc
convergence across the plains, so expect a broader coverage of
showers and thunderstorms across the forecast area. GFS shows
precipitable water values increasing to around one half inch, so do
think some showers/storms could produce moderate rain and even small
to marginal hail as bulk shear is progged to be 40+kts. The main
trough will move overhead on Tuesday, and the leeside trough turned
pseudo-cold front will drop south across the forecast area through
the day as well. Expect another round of showers and storms to occur
as the system moves through. Still anticipate moderate snow
accumulations in the Snowy and Sierra Madre Mountains for Monday
night and Tuesday especially as cooler air moves overhead.
Projecting at least 4-7" inches for now.

.LONG TERM...(Tuesday night through Sunday)
Issued at 305 PM MDT Sun Apr 23 2017

Unsettled weather and cooler-than-average temperatures likely for
the mid to late part of this week, and potentially into next weekend
after a brief period of dry weather Sunday and early Monday. Models
are still in disagreement, but are showing excellent run-to-run
consistency with a potentially major storm system late in the week.
This storm will need to be monitored closely Friday and Saturday for
potential high impacts. Regardless, it still looks unsettled with
off/on bands of rain showers and daytime temperatures 10 to 20
degrees below average. For late this week, all models actually show
temperatures 25 to 35 degrees below average with overnight lows well
below freezing.

For earlier in the week, models in good agreement through
Wednesday night showing a pair of Pacific shortwave troughs moving
across Wyoming and into the northern plains. The second trough
will dig southeast across the high plains as a surface cool front
slides southward across the area. Expect any lingering
precipitation late Tuesday evening to be short-lived as a weak
upper level ridge will translate eastward into Wyoming resulting
in a brief drying trend. Models have backed off a bit on the
higher QPF values across southeast Wyoming as the llvl boundary
interacts with the upper level trough. Will have to monitor the
I-80 corridor though since the southern trough across western
Colorado may be further north and slower than models suggest. This
solution would keep rain/snow bands across the I80 corridor east
of Laramie though most of the evening hours. Wednesday is expected
to be mostly dry as the weak ridge axis moves across southeast
Wyoming. However, developing showers and thunderstorms will not be
far behind since the ridge will weaken as it moves into the
plains. Added isolated thunder for the mountains and adjacent
valleys.

Not much has changed since yesterday regarding the potent Pacific
storm system. The only difference is the strength of the initial
shortwave, which the GFS, and to some degree, the Canadian show
as the dominant feature earlier on in the systems evolution. Both
models show this feature develop over far eastern Wyoming with
surface cyclogenesis over the plains Thursday night and Friday.
This is important because this initial system will influence the
main storm for later in the week, potentially forcing it further
south. The ECMWF is the most consistent and actually shows a major
snow storm for most of the forecast area from Albany county
eastward into the eastern plains of Wyoming and western Nebraska,
with a storm track from eastern Colorado northeast into west
central Nebraska. The GFS shows this main storm much further south
and slower compared to the ECMWF with some snow along I80. Most
ensemble forecasts and the ensemble mean favor the ECMWF solution,
but are slightly more progressive. The operational GFS seems to
be the outlier at this time. Increased POP a bit Friday and
Saturday, but did not go too high at this time since it`s still 6
days out. Temperature trends are the one thing all models are in
good agreement on, with high temperatures on Saturday likely
remaining in the 30`s to low 40`s across the area.

&&

.AVIATION...(For the 06Z TAFS through Monday night)
Issued at 1135 PM MDT Sun Apr 23 2017

VFR conditions will prevail tonight and Monday. A cold front will
push through the area tonight. Surface winds will shift from S-SW to
W-NW, with gusts between 30 and 40 kt Monday. Ceilings will lower
throughout the daylight hours Monday, with MVFR developing across
Carbon County including Rawlins Monday evening. Occasional showers
and perhaps a few t-storms will continue tonight and Monday morning
west of the Laramie Range, and Monday afternoon for the WY plains.

&&

.FIRE WEATHER...
Issued at 210 PM MDT Sun Apr 23 2017

Widespread south to southwest winds gusting 20 to 30 mph are
occurring across the district this afternoon. Even stronger gusts of
40 to 45mph have been observed in central and western portions of
Carbon County. Humidities in the lower elevations west of the
Laramie Range have dropped into the mid to upper teens, but
increasing clouds this afternoon should help to increase moisture
and perhaps keep humidities from dropping much further. Regardless,
the fire weather threat will remain elevated for these areas when
combined with the gusty winds. A series of storm systems moving
across the Rockies tonight through midweek will bring a good chance
for wetting rains and a few thunderstorms through this time. The
Sierra Madre and Snowy Mountains could see moderate accumulating
snow as well.

&&

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

$$

SHORT TERM...RJM
LONG TERM...TJT
AVIATION...MAJ
FIRE WEATHER...RJM



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