Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 171814
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1214 PM MDT Mon Apr 17 2017

.SHORT TERM...(Today and Tuesday Night)
Issued at 335 AM MDT Mon Apr 17 2017

Low clouds currently extend from northeast Colorado into the
southern Nebraska Panhandle under moist east to northeast llvl
winds. Kept patchy fog in the fcst for the southern Panhandle and
eastern Laramie county through mid morning. Shortwave trough will
lift northeastward into western Montana through the aftn.
Southwest winds will increase for areas along and west of the
Laramie Range after 18Z, with gusts to 35-45 kts from the
Arlington area to Rawlins and the Ferris/Seminoe mtns.

A marginal risk of severe storms is still expected by late this
aftn through the early evening for much of the western Nebraska
Panhandle. Southerly winds will advect llvl moisture northward
(dewpoints of 45-50) to the east of the dryline that will be
located near the WY- NE border by 18-21Z. Instability and deep
layer shear look more than sufficient for rotating storms, the
question is how much convective initiation and storm coverage will
we see. The NAM does show a cap persisting into at least through
21Z at most Panhandle sites, and then perhaps eroding some between
21-00Z. But 700 mb temps are also increasing through the evening.
Thus, current thinking is that the storm coverage will be
isolated with any initiation occurring later in the aftn or even
the early evening. Threat is definitely there though for strong to
severe discrete storms given that winds aloft and shear is
increasing through the evening. Tstm threat will end by later in
the evening as the sfc trough pushes east of the CWA. Another
shortwave will move into WY by late Tuesday with increasing
chances of precipitation over northern and western portions of the
CWA. Temps will continue to be above normal through Tuesday.

.LONG TERM...(Wednesday through Sunday)
Issued at 335 AM MDT Mon Apr 17 2017

Fast moving upper shortwave will pass across WY on Wednesday.
Period of upper diffluence within the left front quadrant of a fast
Pacific jet over CO will will provide support for widespread precip
over much of the CWA. Best low level moisture will remain over the
western and northern CWA with westerly flow and downsloping effects
keeping areas in the lee of the Laramie Range in a relative minimum
in the precip.  At the surface, a cold front will push southward and
across SE WY and western NE during the day Wednesday bringing breezy
north winds.  Gradients in the wind prone areas such as ARL increase
Wednesday morning to 40 to 50 m so could see some gusty conditions
there as well with the passage of the shortwave.

After some brief ridging early on Thursday, attention turns to a
potentially more significant system diving into the central Rockies
on Thursday night and Friday.  GFS is aggressive in closing off the
mid level center in a favored location over northern CO on Friday
morning.  GEFS ensembles indicate PW values in the the percentile
for this time of year on Friday with up to 35 kt of east to
southeast flow at 700 mb which would favor a significant
precipitation event.  European is weaker and much further north with
the mid/upper level center with a track over central WY before
progressing quickly eastward.  Unfortunately model track records
with this type of system this spring have not been great and due to
the spread in the solutions, elected to make minimal changes to the
previous forecast at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1214 PM MDT Mon Apr 17 2017

MVFR cigs are hanging on at a few sites across the Nebraska
Panhandle, but expect those to continue lifting over the next
couple of hours. There remains a chance for showers and storms to
develop near the WY/NE border after 20Z today, spreading east
across the Panhandle through the evening. Some could be strong,
with gusty and erratic winds and hail, but uncertainty as to where
these storms develop is high at the moment. Gusty southwest winds
will continue west of the Laramie Range this afternoon as well
with gusts around 40kts likely.

&&

.FIRE WEATHER...
Issued at 335 AM MDT Mon Apr 17 2017

South to southwest winds will increase across the region from late this
morning through the afternoon.  Minimum humidity values will drop to 15 to
20 percent across lower elevations of southeast Wyoming during the afternoon.
Thus, an elevated fire weather threat will occur across fire weather zones 306
and 309 where dry fuels would support fire growth.  There will be increasing
chances for widespread wetting precipitation across the region from Tuesday night
through Friday.

&&

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

$$

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



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