Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 120532

Area Forecast Discussion
National Weather Service Cheyenne WY
1132 PM MDT Fri Aug 11 2017

.SHORT TERM...(This evening through Sunday night)
Issued at 120 PM MDT Fri Aug 11 2017

Forecast challenges with this package deals with severe thunderstorm
potential both today and Saturday.

Currently...Upper ridge remains stationary over the northern
Rockies from northern Idaho to northern Utah into southwestern
Colorado this afternoon. Water vapor imagery showing a small
disturbance moving through the ridge into south central Wyoming
this afternoon thats interacting with low level moisture and the
tail end of a stationary front on surface analysis. A stronger low
pressure system can be seen on water vapor imagery over north
central Montana that will have some impacts on us tomorrow. Latest
mesoanalysis showing surface based CAPE up to 1500 J/KG over
Converse and Niobrara Counties this afternoon at 1PM. Radar is
starting to light up on convection mainly in this area, extending
south into northern Platte County.

Latest HRRR simulated radar showing the area of convection over
Converse County dropping south as the afternoon progresses and
this is what the SPC mesoanalysis page also shows with increasing
CAPE. We could see a few severe thunderstorms down here in Laramie
County late this afternoon just east of Cheyenne if the HRRR
forecast comes true. Forecast CAPE up to 1500 J/KG by 23Z across
Laramie County.

Better chances for severe thunderstorms Saturday as that Montana
low tracks southeast into western South Dakota. By late Saturday
afternoon, upper low near Rapid City with a cold front moving
south into Converse and Niobrara Counties. Afternoon MUCAPE from
NAM up to 2500 J/KG. Pretty high shear values as well with
northeast surface winds at 10kts and upper level winds at 65kts at
350mbs. Agree with SPCs Slight Risk area they have identified for
the Panhandle and would only maybe shift it a little further west
into Wyoming. Good shear means we will likely see a few severe
thunderstorms with very large hail. Did introduce severe
thunderstorms into the weather grids for the Panhandle and extreme
southeastern Wyoming for tomorrow afternoon/early evening. Also
updated HWO and weather story to reflect current thinking.

Sunday may be a little drier with upper ridge starting to move
into western WYoming. Most of the precip should be down over
Colorado Sunday, but we could see a few thunderstorms coming off
the mountains Sunday afternoon, so kept low chance PoPs going.

.LONG TERM...(Monday through Friday)
Issued at 120 PM MDT Fri Aug 11 2017

Monday...Expect to see a decrease in afternoon and evening shower
and thunderstorm coverage as a ridge aloft moves overhead. Still
though, enough low and mid level moisture for isolated to widely
scattered afternoon and evening showers and thunderstorms. Definite
warming trend with more sunshine and a moderating airmass.

Tuesday...With a shortwave trough aloft moving overhead in the
afternoon, and adequate low and mid level moisture, expect to see
scattered showers and thunderstorms develop, especially east of
Interstate 25.

Wednesday...In the wake of the shortwave trough aloft, will see some
low and mid level drying, thus minimal chances for showers and
thunderstorms in the afternoon and evening. Warming trend continues
with more sunshine and downslope winds.

Thursday...Continued mild per thicknesses and mid level
temperatures. Flow aloft becomes northwest and with limited low and
mid level moisture, minimal chances for showers and thunderstorms.

Friday...Continued dry and seasonably mild with the flow aloft
backing to zonal, west to east, and limited low and mid level
moisture to fuel any showers or thunderstorms.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1127 PM MDT Fri Aug 11 2017

Hi-res guidance continues to show a consistent signal for fog
formation primarily along the Cheyenne Ridge from KCYS to KSNY
between 08Z and 13Z. Brought in IFR visibility with the potential
for LIFR near KSNY where a more moist low level airmass is in
place. IFR also likely at least briefly near KBFF and KAIA as
winds will be very light and temperatures cool early this morning.
Any fog will burn off quickly Saturday with scattered
thunderstorms developing off the high terrain by 18-19Z and
spreading into the Panhandle by late afternoon. A few of these
storms may contain hail and wind gusts to 50kt, with the highest
threat for the Nebraska terminals.


Issued at 120 PM MDT Fri Aug 11 2017

Minimal fire weather concerns as a cool and unsettled weather
remains in place across southeast Wyoming and Nebraska Panhandle.
Upper ridge over the northern Rockies will keep a northwest flow
over the area with disturbances moving across our area from the
northwest on a daily basis. Rainshowers and thunderstorms look to
occur on a daily basis with these disturbances, so most areas
should see measurable rainfall at least Tuesday before drying out
towards the end of next week.




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