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Office: CYS

FXUS65 KCYS 211713

Area Forecast Discussion
National Weather Service Cheyenne WY
1113 AM MDT Tue Aug 21 2018

.SHORT TERM...(Today and tonight)
Issued at 402 AM MDT Tue Aug 21 2018

...Localized flash flooding and debris flows for the Badger Creek
burn scar possible this afternoon and evening...

Main hazard focus will center on the increasing risk of localized
flash and creek flooding due to heavy rain and thunderstorms this
afternoon and evening across the southern Snowy and Laramie Ranges
near the WY/CO border. Localized pockets of 1-2 inches could occur
and lead to local flooding impacts. A flash flood watch has been
issued for southwest Laramie County that incorporates the Badger
Creek Burn Scar area. The heavy rainfall and flash flood risk will
diminish through the mid to late evening hours as activity shifts
farther east into the High Plains.

Several H7-H3 impulse perturbations are noted in latest RAP
analysis over SW WY with modest divergence evident per water vapor
satellite imagery. PWATs across Southern and southwestern WY are
near 0.9" and near 50% above normal per SSMI/ASMU data. The
combination of above average moisture in presence of increasing
lift aloft from a traversing shortwave trough and weak elevated
instability near a stalled mid-level boundary in the right
entrance region of the H25 jet will set the stage for a few rounds
of heavy showers and thunderstorms this afternoon and into the
evening. Concern has increased this morning with several hi-res
CAMs indicating 1-2 inches possible over the higher terrain of the
southern Snowy Range in a short time frame. Pockets of quick 0.5
to 1 inch totals will also be possible from an area along and
south of a Cheyenne to Seminoe Dam line from noon through midnight
today. Expansion of the flash flood watch could occur later in
the day as trends and new guidance becomes available. Shower and
storms will slowly shift east through the evening to the High
Plains with the flash flood risk diminishing. Will need to watch
for the development of fog Wednesday morning given the likely
saturated boundary layer from all the rain today and this evening.

.LONG TERM...(Wednesday through Monday)
Issued at 402 AM MDT Tue Aug 21 2018

Additional rounds of showers and thunderstorms are expected again
Wednesday and into portions of Thursday before an elevated front
moves through Thursday afternoon. Temperatures will continue to
warm through the week and weekend as the regions stays just south
of cooler airmasses in Montana.

The SW flow aloft will persist Wednesday and Thursday as the
positively tilted H5 low over the Pacific NW opens up and shifts
across northern WY on Thursday. A few marginally strong storms
could form off the Laramie range and adjacent High Plains
Wednesday afternoon. Scattered showers and few thunderstorms
could also linger Wednesday night into Thursday morning for
western areas as yet another impulse rides over the area.

A larger pattern change will come Thursday afternoon with the
passage of the shortwave trough over and then east of the region.
An H5-H7 drier airmass will shift over the region and shunt the
deeper moisture east prior to the strong height falls, generally
leading to a dry frontal passage. Limited to no cold air advection
is being advertised per the models so temperatures should not
drop off behind the boundary into the weekend. A large scale H5
ridge looks to form over the Mississippi River valley over the
weekend and into early next week placing the region under SW flow
and the return of some embedded shortwave troughs could enhance
shower activity by Saturday afternoon with a possibly calmer
Sunday as we could be between waves. Will need to watch early next
week for a possible stronger trough developing across the
northern Rockies and a side-swipe of a colder airmass.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday morning)
Issued at 1113 AM MDT Tue Aug 21 2018

Mainly VFR through the period but some MVFR/IFR cigs possible mainly
from KCYS to KSNY later tonight into Weds morning.  Should see sctd
showers and a few tstms develop over se Wy this afternoon and
persist through tonight mainly near the Colorado border.


Issued at 402 AM MDT Tue Aug 21 2018

Widespread wetting rain is expected across the southern half of
the forecast area from Seminoe Dam towards Kimball, Nebraska
through the day and especially this afternoon and evening. Some of
the higher terrain of the Snowys and southern Laramie Range could
see upwards of 1 to 1.5 inches through the next 24 hours in
isolated locations. The Badger Creek burn scar is included in this
higher precipitation potential zone and a flash flood watch as
been issued for the burn scar area as rapid flash flooding and
debris flows could occur between noon to late this evening.
Caution is urged in this area.

Isolated to scattered showers are expected again tomorrow and
Thursday but overall coverage will be less then today and this
evening. fire weather concerns will be minimal with high relative
humidity values.

Fire weather concerns return Friday with a drier airmass moving
into place but recent rainfall could mitigate overall fire weather
danger. None-the-less, elevated to near-critical parameters could
be in place with lower humidity and higher wind speeds.


WY...Flash Flood Watch from noon MDT today through this evening for




Office: RIW FXUS65 KRIW 211737 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1137 AM MDT Tue Aug 21 2018 .SHORT TERM...Today through Thursday night Issued at 112 AM MDT Tue Aug 21 2018 Pacific low pressure trough over the Pacific Northwest and Northern Rockies will move slowly east today and Wednesday bringing colder temperatures, higher relative humidity, clouds and showers with isolated afternoon and evening thunderstorms. Temperatures will be significantly colder in the north with highs in the upper 50s to lower 60s around Cody and in the mid 60s around Buffalo. Weak ridging will follow the Pacific low pressure trough with warming and a decrease in showers and thunderstorms Thursday. Overall winds will be fairly light through noon Thursday. In the afternoon Thursday expect gusty conditions in the wind corridor from Sweetwater to Natrona Counties. .LONG TERM...Friday through Tuesday Issued at 112 AM MDT Tue Aug 21 2018 The main weather concern during this extended period will be elevated fire weather conditions across portions of south and central Wyoming; where gusty west wind and dry conditions will prevail. The upper air pattern will feature strengthening ridges across the northeast Pacific (roughly along 150W) and across the mid/lower Mississippi valley. A trough is expected to amplify across the Intermountain West early next week between the two ridges. Medium- range models continue to show run-to-run inconsistencies on the placement and progression of this trough with forecast giving more weight to further west and slower progression. Thus, southwest flow aloft will prevail with several shortwaves and associated cold fronts clipping the north and west. The first front will move across the north on Friday night, the second on Sunday and the third on Tuesday. Only the Tuesday cold front looks like it could bring more significant precipitation to the northwest, while the preceding fronts look mainly dry. Fire weather concerns will be at a maximum in warm sector ahead of the respective fronts Friday afternoon/evening, and occasionally from Sunday through Tuesday across mainly the south half of Wyoming. Friday and Sunday will be the warmest days of the period with some 90s in the lower basins; with cold fronts bringing slight cooldowns on Saturday and Monday. We could see a significant cooldown on Tuesday, but confidence in this scenario remains low eight days out. && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 1135 AM MDT Tue Aug 21 2018 Low clouds with areas of MVFR conds will prevail east of the divide through 21z before clouds lift. A few thunderstorms will move across the southwest and far south through tonight with local mvfr vsbys in rain. The best chance for this will be at KRKS with a lesser chance at KBPI and KPNA. Patchy late night fog and MVFR conds are possible around KBPI, KPNA and KCOD between 10z and 15z Wed. Wednesday afternoon and evening will see a better chance of thunderstorms across much of the area. Some of these could be quite strong. && .FIRE WEATHER... Issued AT 112 AM MDT Tue Aug 21 2018 An upper level system will bring colder temperatures, higher relative humidity, clouds and showers with isolated afternoon and evening thunderstorms today and Wednesday. Temperatures today will be about 5 to 10 degrees cooler across most of the area with only a slight change in the northwest. Humidities will also be higher in most areas with some drying in the northwest. As the system leaves the area Wednesday expect a return to dry conditions. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...Hulme LONG TERM...Meunier AVIATION...Skrbac FIRE WEATHER...Hulme