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Area Forecast Discussion (AFD)

Issued by Cheyenne, WY (CYS)

FXUS65 KCYS 171720

Area Forecast Discussion
National Weather Service Cheyenne WY
1120 AM MDT Sat Aug 17 2019

.SHORT TERM...(Today and tonight)
Issued at 310 AM MDT Sat Aug 17 2019

Across the CONUS, a broad belt of cyclonic westerly flow was 
rotating around a large upper low center in northern Alberta. H5
flow of 45 to 55 kts was being observed, with several shortwave
disturbances embedded in the ribbon of flow aloft. This morning, 
a more significant shortwave was located just northwest of the 
international border with Montana moving east southeast. This 
shortwave will be the main weather driver for the day, as the 
stronger mid level flow continues. 

At the surface, a weak area of low pressure was located over 
central Colorado and Wyoming. This low is expected to continue to 
develop as the shortwave trough approaches. As the low 
strengthens, low level wind fields will intensify. Westerly low 
level flow will develop with the increasing gradient, shunting 
low level moisture east into the plains. Continued heating and the
influence of the shortwave and enhanced westerlies aloft will 
promote deep mixing with gusty winds of 25 to 35 mph likely 
reaching the surface this afternoon. Temperatures should also be a
bit warmer in the upper 80s to 90s for most. This in combination 
with the drier surface air from the westerly low level flow will 
create critical fire weather conditions. Fire Weather Watches have
been upgraded Today to Red Flag Warnings for this reason. 

Along with the threat of Fire Weather, the strong lift and wind
fields from the shortwave may be enough for a few strong
thunderstorms, mainly over the panhandle and far eastern Wyoming.
Residual CAPE is small but sufficient for a few sustained
updrafts. Bulk Shear on the order of 60 kts would support storm
organization with a threat for sporadic large hail and strong
downburst winds given low level inverted V soundings. 

By this evening, storms should die off and winds will lessen with
the loss of heating. Mostly clear skies are expected with 
overnight lows in the mid 50s. Precip is not expected with dry air
remaining in place over much of the area. Shortwave ridging will
begin to develop aloft with increasing 700 mb temperatures behind
the departing shortwave. 

.LONG TERM...(Sunday through Friday)
Issued at 310 AM MDT Sat Aug 17 2019

Sunday will be quite similar to Saturday in terms of Fire Weather
potential as the ridge axis slowly strengthens over the area. 700
MB temps approaching 20C will develop, supporting surface high
temperatures in the 90s to near 100. Dry air will remain in place,
creating another round of low humidity. Winds will be weaker but 
still should be sufficient to provide surface gusts of 20 to 25 
mph in the afternoon. Therefore, Fire Weather conditions are again
expected Sunday across much of Carbon County, Wyoming. Red Flag 
and Fire Weather Watches are in effect. See RFW for more info.

The first part of Monday will continue the warm and dry trend of
the weekend. Temperatures are expected to be very warm with mostly
clear skies and quiet weather. An approaching shortwave will skirt the
northern periphery of the ridge and will drag a cold front south 
late Monday and early Tuesday. This will bring cooler and more 
moist air into the region returning chances for precipitation
Wetter weather should continue through Thursday as the jet stream
dips south across the northern US. Shower and thunderstorms will 
remain likely through the end of the week and into the weekend 
where a strong pacific storm system is expected to move onshore. 
Temperatures will be cooler with above average chance of 
precipitation through next week. 


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1121 AM MDT Sat Aug 17 2019

VFR conditions can be expected through Saturday afternoon and
early evening across the region. Gusty winds will be the main
concern this afternoon, especially for Southeast Wyoming terminals
where gusts to 35 kts out of the west are possible through 01Z. 
VCTS is also in play for KCYS and KSNY this afternoon as 
thunderstorm coverage should remain fairly limited to the frontal 
boundary along the CO/WY border. MVFR, possibly IFR, ceilings are 
possible after 05Z tonight for the Nebraska Panhandle terminals 
behind a stronger cold front surging south later this evening.


Issued at 310 AM MDT Sat Aug 17 2019

Upper level winds will increase this afternoon ahead of a 
strengthening pacific storm system. West northwest downslope winds 
will drive dryer air in from the west rapidly lowering humidity 
values. Critical fire weather conditions are expected across much 
of Carbon County Wy. Red Flag Warnings and Fire Weather Watches 
are in effect for zones 303 304 and 310 through this weekend. Fire
weather concerns will be high through monday evening as a cold 
front brings in a cooler and more moist airmass. 


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

     Red Flag Warning from noon to 8 PM MDT Sunday for WYZ303-304.

     Fire Weather Watch from Sunday afternoon through Sunday evening 
     for WYZ310.