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Area Forecast Discussion (AFD)

Issued by Cheyenne, WY (CYS)

FXUS65 KCYS 191719

Area Forecast Discussion
National Weather Service Cheyenne WY
1119 AM MDT Mon Aug 19 2019

Issued at 1115 AM MDT Mon Aug 19 2019

Fog/low clouds over Platte River Valley earlier this morning have
dissipated. Only made minor maximum temperature adjustments in a
few areas based on trends and MOS guidance. Updates sent.


.SHORT TERM...(Today through Tuesday night) 
Issued at 350 AM MDT Mon Aug 19 2019

Quiet and benign weather expected today as a shortwave ridge moves
eastward across the region. After some patchy fog or low clouds
early this morning east of I-25, it will be very warm or hot with
afternoon temperatures in the mid to upper 90s across the lower 
elevations, and around 90 degrees even across interior southeast 
Wyoming west of I-25. A few locations across western Nebraska or 
far eastern Wyoming may see the century mark as temperatures push 
towards 100 degrees. Although most of the area will remain dry, 
higher dew point values are expected across portions of the 
western Nebraska panhandle. Can't rule out an isolated 
thunderstorm for this area today, but confidence is pretty low so 
left out any mention of thunder for now. 

Models continue to trend later with a cold frontal passage
Tuesday. Earlier model runs had this Canadian cool front moving
into the eastern Wyoming plains as early as tonight, but all
models have now backed off and delayed FROPA until Tuesday. This
front will not push into the I-25 corridor until late Tuesday
afternoon/Tuesday evening at this earliest. Unless models trend in
the other direction, kept out the mention for fog behind the front
and increased high temperatures on Tuesday to account for this
trend. Expect most locations to remain in the upper 80s to middle
90s ahead of the front, although it will become a few degrees
cooler further north towards Chadron and Lusk. Also, delayed the
onset of thunderstorms until Tuesday evening and mostly Tuesday
night. Expect a good chance of evening and nocturnal thunderstorms
Tuesday night, so kept POP between 40 to 60 percent along/east of
Interstate 25. There is a slight risk for severe thunderstorms due
to a reasonable amount of shear and impressive MLCAPE between 2000
to 3500 j/kg above the stable layer at 700mb. Expect enough
forcing along the front, and ahead of an upper level shortwave 
trough, to break the 700mb cap Tuesday night. Large hail looks to
be the primary threat east of I-25. 

.LONG TERM...(Wednesday through Sunday night) 
Issued at 335 AM MDT Mon Aug 19 2019

Showers and thunderstorms will continue Wednesday and possibly
Thursday ahead of a slow-moving Pacific system which will push
east along the Canadian/US border through the middle of the week.
Some stronger storms are possible Wednesday afternoon, but with
the lack of decent 0-6km shear near the upper level ridge axis 
any concerns are limited at this time. Once this Pacific system
moves east, drier air will filter into southeast Wyoming and
western Nebraska for next weekend. Extended forecast then becomes
uncertain into the last week of August as some models are hinting
towards the first polar cold front of late summer moving 
southward out of Canada and into the northern Great Plains. The 
GFS and Canadian both show this cold front moving across the Front
Range and bringing much cooler temperatures. but this feature is still
8 to 10 days out.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 526 AM MDT Mon Aug 19 2019

Temporary IFR conditions are expected this morning across the
panhandle of Nebraska. Cool and moist air associated with a front
has surged back west brining low ceilings and fog this morning. 
This will be short lived with ceilings and vis expected to break 
by 1500z. There after, VFR conditions are expected at all 
terminals thanks to high pressure and dry west winds. Aviation
hazards will be low threat after IFR conditions clear later this


Issued at 240 AM MDT Mon Aug 19 2019

Elevated Fire Weather concerns will continue today although wind 
gusts should not be nearly as bad as the last few days across
southeast Wyoming. Model forecast soundings suggest west to
southwest winds around 10 to 20 mph with some gusts around 25 to
30 mph. Expect minimum daytime humidities to easily reach Red Flag
criteria, but wind speeds are less likely due to the lack of 700mb
to 500mb flow aloft. Will keep the Red Flag Warning out for today,
with zone 303 (western Carbon county) the most likely area to
reach wind criteria. 

Low level moisture will begin pushing back into the region on
Tuesday, with fire weather concerns lowering by midweek. 


WY...Red Flag Warning until 8 PM MDT this evening for WYZ303-304-310.