Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 170925
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
325 AM MDT Thu May 17 2018

.SHORT TERM...(Today through Saturday)
Issued at 325 AM MDT Thu May 17 2018

Overnight water vapor loop revealed a upper ridge axis centered
over the high plains and upper low over the central Great Basin.
A weak surface ridge from AZ to western WY, with a trough from
eastern MT to south central WY. The nearest convection was located
across southeast MT. Skies were clear with temperatures in the
40s and 50s. Prevailing winds were variable 5-15 kt.

Forecast challenges during the next 24 to 36 hours will focus
on convective timing, coverage and intensity.

Upper ridge will continue its slow eastward progression into
the Great Plains today. Diffluent southwest flow aloft will
increase this afternoon, with south-southeast low level flow
along surface trough. Model sounding parameters are supportive
of strong to severe thunderstorms this afternoon and evening
from the eastern WY plains east into the NE Panhandle where SPC
has highlighted with a SLIGHT risk. Convective initiation will
occur just after midday over the Laramie Range, with storms
intensifying as they move east of the I-25 corridor. NAM progs
2500-3500 j/kg surface-based CAPE (SCAPE) from mid-late afternoon.
0-6km bulk shear values exceed 50kt across the NE Panhandle.
Primary threats will be large hail up to 2.5" diameter, damaging
wind gusts to 65 mph, and heavy rainfall (precipitable water
values near an inch). The tornado threat is low due to high LCLs,
however an isolated spin-up or two is possible over the NE
Panhandle. Showers and tstorms will taper off late this evening
into the overnight across western NE.

Friday will be unsettled once again, with widely scattered
showers/tstorms in the morning, increasing in coverage and
intensity in the afternoon and evening. Cold front will be
moving through the CWA Friday afternoon. Northeast low level
winds topped by southwest flow aloft will create sufficient
shear. SCAPEs rise to 1500-2000 j/kg with 0-6km shear of
35-40 kt Friday afternoon. SPC Day2 outlook has a SLIGHT risk
extending south and east of a line from Cheyenne to Torrington to
Alliance, surrounded by a MARGINAL risk from the Laramie Range
north and east to Lusk and across the northern NE Panhandle.
Primary threats will be large hail up to 1.5" diameter, damaging
wind gusts to 60 mph, and heavy rainfall. Severe threat ends
by evening, with widespread rain showers and isolated tstorms
Friday night and Saturday. Models are at odds with snow levels,
with the NAM up to 4000 feet lower than the GFS. Snow levels
will drop to 9000 feet, with several inches of wet snow possible
over the Snowy Range. The Sierra Madre and north Laramie ranges
could measure 2-4". After highs in the 60s and low 70s Friday, it
will be a raw day Saturday with highs in the 40s and low 50s.
It will be blustery across the plains with gusty northerly winds
up to 30 mph.

.LONG TERM...(Saturday night through Wednesday)
Issued at 325 AM MDT Thu May 17 2018

Upper trough situated over the central high plains Saturday night
will progress slowly east-northeast toward the Upper Midwest
through Sunday. Showers will gradually diminish Saturday night and
Sunday morning along and east of I-25. Scattered showers/isolated
tstorms will redevelop along and west of the Laramie Range Sunday
afternoon with the passage of a weak disturbance in the SW flow
aloft. High temperatures Sunday will be 10 degrees warmer compared
to Saturday.

For the Monday through Wednesday period, weak diffluent south-
southwest flow aloft prevails as a upper low meanders over the
Great Basin. Weak disturbances will periodically track across the
CWA, producing isolated to scattered convection each day. Above
normal temperatures are expected with highs in the 70s lower
elevations.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1142 PM MDT Wed May 16 2018

VFR conditions prevail as of 06Z and will persist through the
next 24 hours for LAR/RWL sites. Farther east however, greater
coverage of showers and thunderstorms are expected to begin near
19-20Z Thursday near the Laramie Range with quick expansion
eastward. Greatest TSRA coverage is expected to be across the NE
TAF sites from 22-03Z as TSRA grows into an semi-organized line.
Some TSRA is expected to be strong and severe with localized high
winds, hail, and lightning. Localized IFR visibilities will be
likely in vicinity of strongest activity. TSRA tops to FL450
probable. TSRA will shift east of the NE panhandle by 03Z (9pm
MDT) Thursday night with a return to VFR. However, some guidance
indicates MVFR ceilings to make a return by 12Z Friday morning.

&&

.FIRE WEATHER...
Issued at 230 AM MDT Thu May 17 2018

Scattered thunderstorms will develop shortly after midday over the
Laramie Range, increasing in coverage and intensity through the
afternoon and evening across the plains. Strong to severe storms
producing large hail and damaging winds are likely east of
Interstate 25 into the Nebraska Panhandle late this afternoon and
evening. Lowest humidities will develop across lower elevations of
Carbon County this afternoon, over however fuels and winds will be
non-critical. Isolated to scattered strong to severe storms will
develop ahead of a cold front Friday afternoon and evening across
much of western Nebraska into far southeast Wyoming. The cold front
and a upper level low pressure system will usher in much cooler
temperatures Friday night and Saturday, as well as widespread rain
showers, high elevation snow showers, and isolated thunderstorms.
Somewhat drier and warmer conditions return Sunday through early
next week, with widely scattered afternoon and evening showers and
thunderstorms.

&&

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

$$

SHORT TERM...MAJ
LONG TERM...MAJ
AVIATION...JSA
FIRE WEATHER...MAJ



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