Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211218
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
524 AM MDT SAT MAY 21 2016

.SHORT TERM...(Today through Monday)
Issued at 340 AM MDT Sat May 21 2016

Stationary upper low/trough just west of the Great Basin continued
to advect abundant Pacific moisture northward across the
Intermountain West. Downstream upper ridge axis was situated over
the high plains. A stationary front extended from central MT south
through central WY to a low over northern UT and trailing south
into western AZ. Breezy south to southeast surface winds and
considerable mid/high clouds kept temperatures mild overnight with
40s and 50s. Moistening boundary layer resulted in areas of low
clouds...patchy fog and light drizzle along the I-80 corridor from
just west of Cheyenne east through Kimball and Sidney. Similar to
last night...isolated showers developed along weak convergence
boundary over northwest NE and moved northeast.

Good model agreement during the short term with a slow eastward
progression of the upper trough across the Intermountain West and
upper ridge across the Great Plains this weekend. The upper low
will slowly lift northeast into southwest Canada on Sunday. The
trough axis will become more elongated northeast-southwest and
remain west of the CWA Monday.

HRRR model progs have performed well with the timing and location
of the low clouds and fog the past few days. These low conditions
will likely linger over the southeast WY plains and NE Panhandle
through midday.

00Z instability/shear progs suggest a marginal risk for isolated
strong to severe thunderstorms this afternoon and early evening
across east central WY and northern NE Panhandle...similar to
yesterday. Mid level thermal axis will be situated across the
high plains this afternoon with low level southeast flow advecting
50-degree dew points as west to a Lusk-Pine Bluffs line. NAM
lapse rates progged near 8.5 deg C/km with surface based CAPEs
as high as 3500 j/kg over the high plains between 21Z and 00Z.
The GFS progs 1500-2200 j/kg for the same time frame. 0-6km bulk
shear values generally on the order of 30-40 kt over the northern
CWA. Primary threats from any severe thunderstorms that develop
would be large hail and strong winds. It will be unseasonably warm
this afternoon with highs in the 70s to near 80 for the lower
elevations...with 50s and 60s for the higher terrain. Convective
coverage decreases tonight and shifts north and east into the
plains.

The front to the west will finally push through the CWA Sunday
with winds shifting to a westerly direction. Cooler air will
likely not be felt across the high plains with highs in the
70s. Western areas can expect high temperatures 10-15 degrees
cooler compared to today. Instability/thermal axis will shift
east into the central Great Plains Sunday afternoon. Still cannot
rule out an isolated strong to severe thunderstorm or two over
the NE Panhandle.

Near seasonal temperatures are forecast Monday along with a
drier air mass. There will be a little moisture and instability
present for isolated afternoon showers and thunderstorms for
the southeast WY mountains and foothills.

.LONG TERM...(Monday night through Friday)
Issued at 300 AM MDT Sat May 21 2016

Monday night...Aside from isolated evening thunderstorms southeast
of a Cheyenne to Alliance line, the rest of the night will be quiet.

Tuesday...With southwest flow aloft and a low level convergence
boundary across our eastern counties, expect isolated afternoon and
evening thunderstorms primarily southeast of a Laramie to Lusk line,
with storms ending and dissipating by late evening.

Wednesday...With the addition of a bit more low and mid level
moisture, and the presence of a shortwave, expect an uptick in
coverage of afternoon and evening thunderstorms, mainly along and
east of Interstate 25.

Thursday...Even more coverage of afternoon and evening thunderstorms
expected as troughiness aloft develops along with an attendant
increase in low and mid level moisture. Coverage most numerous along
and east of Interstate 25.

Friday...Coverage of afternoon and evening thunderstorms will be
similar to Friday with about the same amount of low and mid level
moisture and a shortwave nearby.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 521 AM MDT Sat May 21 2016

Southeast Wyoming TAFs...IFR at Cheyenne until 16Z...then VFR with
isolated thunderstorms from 19Z to 02Z. Rawlins and Laramie with
isolated thunderstorms from 19Z to 01Z. South to southwest winds
gusting to 33 kt this afternoon and early evening.

Nebraska Panhandle TAFs...IFR continues until 15Z to 18Z...then VFR.
IFR redevelops this evening and continues tonight. South winds
gusting to 35 kt this afternoon and early evening.

&&

.FIRE WEATHER...
Issued at 239 AM MDT Sat May 21 2016

No fire weather concerns through the weekend with green fuels
and expected moisture from isolated to scattered showers and
thunderstorms. Above normal temperatures will continue today...
followed by cooler temperatures on Sunday with the passage of
a cold front. It will be breezy with southerly winds gusting
between 30 and 40 mph across the high plains and western Carbon
County today. Winds will shift to a westerly direction Sunday with
gusts to 30 mph west of the Laramie Range. Minimum humidities this
afternoon will range from 25 to 30 percent along and west of
Interstate 25...with higher humidities to the east. Minimum
humidities Sunday will range from 25 to 30 percent districtwide.
Near average temperatures and isolated to scattered afternoon and
evening showers and thunderstorms are forecast for next week.

&&

.HYDROLOGY...
Issued at 239 AM MDT Sat May 21 2016

Isolated to scattered afternoon and evening showers and thunderstorms
will develop along and east of the Laramie Range today and Sunday...
with minor impacts on area rivers...creeks and streams. Brief heavy
rainfall will be possible through the lower north platte river
valley...especially Sunday afternoon and evening. Mountain snowmelt
will increase today with the warmer temperatures and gusty winds.
Cooler temperatures Sunday through much of next week will decrease
snowmelt rates.

&&

.CYS Watches/Warnings/Advisories...
WY...None.
NE...None.
&&

$$

SHORT TERM...Jamski
LONG TERM...Rubin
AVIATION...Rubin
FIRE WEATHER...Jamski
HYDROLOGY...Jamski


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