Special Weather Statement, NWS Cheyenne, WY

Area Forecast Discussion
National Weather Service Cheyenne WY
511 PM MDT Mon May 15 2017

.UPDATE...
Issued at 448 PM MDT Mon May 15 2017

Severe Tstrm Watch #215 effective until midnight for Niobrara,
Goshen, Sioux, Dawes and Box Butte counties. Tstorms were
developing from Whitaker north-northeast into western Goshen
County, as well as north and west of Douglas. Threats from
stronger tstorms in and close to the watch area will be large
hail and damaging winds.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 311 PM MDT Mon May 15 2017

Current obs and GOES 16 visible satellite loop shows a llvl
boundary just east of the Laramie Range near Interstate 25 this
afternoon. East to southeast winds have been observed across the
far eastern plains of Wyoming and western Nebraska, with south to
southwest winds further to the west. This boundary will likely be
the focus for deep convection later on this afternoon as daytime
cumulus has developed along and west of this boundary. Dewpoints
are in the 50's east of this boundary even though there still
remains a significant cap inversion just off the surface. Expect
isolated thunderstorms to develop along and east of Interstate 25
over the next 2 to 3 hours and gradually move northeast across the
plains. These thunderstorms will enter a more favorable
environment for strong to severe weather as they move towards the
WY/NE stateline. A few of these thunderstorms may contain damaging
winds, large hail, and brief heavy rainfall with frequent
lightning through late this evening. The most favorable area for
severe tstorms will be north of a line from Douglas to Alliance
Nebraska. This activity will likely subside after 9 PM with
models showing occasional rain after midnight as a shortwave
disturbance aloft moves overhead. Added patchy fog to the forecast
in these areas due to light winds and saturated llvls.

Tuesday will be a little cooler as the broad upper level trough
begins to move eastward. High temperatures will still be pleasant
and in the 60's to low 70's across the area, coolest north and
west. Models show an area of midlevel subsidence associated with a
weak ridge aloft. This will be short-lived however as the upper
level trough currently over the southwestern Great Basin region
will accelerate and lift northeast towards the forecast area
Tuesday afternoon and evening. All models are showing an increase
in convection during the afternoon. Some models are showing
organized multi-cell convection due to the additional dynamic lift
across southeast Wyoming with a bulls eye near Cheyenne and
Torrington. Increased POP up to 45 percent in this area for
scattered showers and thunderstorms, with values around 50-60
percent further east across the Nebraska panhandle due to more
favorable soundings and greater amounts of moisture.

For Wednesday, temperatures will continue to trend 5 to 10 degrees
lower compared to Tuesday as the main Pacific upper level low
begins moving towards the area. This system will have to be
watched closely for impacts due to snowfall. Models all show
precipitation (rain below 8000 feet) developing over the mountains
and expanding into the valleys and high plains of southeast
Wyoming for Wednesday. Continued increasing POP between 30 to 65
percent across most locations with rain possibly mixing with snow
down to 6500 feet west of Interstate 25 as early as late Wednesday
afternoon.

.LONG TERM...(Wednesday night through Monday)
Issued at 311 PM MDT Mon May 15 2017

The pattern starts to get interesting Wednesday night as a strong
upper low digs southeastward into northern Utah and a cold front
pushes south of our area and into eastern Colorado and central
Nebraska. Rain showers changing over to snow for elevations above
5500 ft msl is expected late Wednesday night and as the upper low
stalls over northern Colorado Thursday into early Friday. There
could be some travel issues by Thursday morning across
southeastern Wyoming along and west of I-25 where periods of
heavy snow is possible Thursday into Friday.  One thing that
sometimes the models do not handle well with the projected pattern
is convection that can fire up near the surface front to our
southeast, cutting off the deep moisture inflow on the east and
northeast side of the upper low, something that we have seen this
spring with most of our storm systems. For now we kept expected
snow amounts in the extended forecast closer to what the ensemble
members would suggest. Models in fair agreement on at least likely
PoPs most of the area Thursday with chance PoPs tapering off
Friday into Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 511 PM MDT Mon May 15 2017

Widely scattered thunderstorms expected into this evening mainly
over and around the northern Neb Panhandle.  Cool front will pass
across the area this evening switching winds out of a northerly to
northwesterly direction.

&&

.FIRE WEATHER...
Issued at 311 PM MDT Mon May 15 2017

No Fire Weather concerns this week due to green fuels and moderate
to high humidities. It will be mostly dry and mild this afternoon
and into this evening with widely scattered thunderstorms east of
Interstate 25. Mild conditions will likely extend into Tuesday
before a significant spring storm system is forecast to push into
the area Wednesday through the end of the week resulting in much
cooler temperatures, widespread precipitation, and snowfall above .

&&

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

UPDATE...MAJ
SHORT TERM...TJT
LONG TERM...JG
AVIATION...RE
FIRE WEATHER...TJT

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