Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211720
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1120 AM MDT SAT MAY 21 2016

.UPDATE...
Issued at 1100 AM MDT Sat May 21 2016

The main forecast challenge for this afternoon and evening will be
the potential for strong to severe thunderstorms. A fair amount of
moisture across the high plains today, evidenced by widespread fog
and low clouds this morning. Inherited forecast dew points were 5-
10 degrees too low at 16z as much of the CWA is seeing readings at
or above 50F. We will probably mix out some this afternoon, but SE
flow in the low-levels should keep moisture intact. Latest visible
satellite loop has shown clouds rapidly eroding along and to the E
of the Laramie range, particularly on the WY side. This is quite a
bit faster than models were showing, so expect capping will not be
as much of an issue with limited storm coverage. The SLGT risk has
been extended into our CWA as the HRRR continues to develop storms
along the Laramie Range later today.

Strong surface heating and good low-level moisture (by high plains
standards) with dew points in the 50s will support steep mid-level
lapse rates and CAPES between 1500-2500 J/KG. Wind profiles should
favor supercells with pronounced veering/increasing winds in the 0
to 6 km layer (0-3 km SRH over 200 M2/S2 and 30-40 knot bulk shear
per the GFS). Large hail/damaging winds are possible. Heading into
the evening, some concern for a couple of tornadoes with excellent
veering wind profiles in the sfc to 1 km layer. EHIs from both the
GFS and NAM range from 2-4, especially over our usual problem area
of central Platte county near Chugwater.

The only change was to expand mention of SVR thunderstorms. All of
the remaining aspects of the inherited forecast look great. Update
has been sent.

&&

.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.
&&

$$

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



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