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Area Forecast Discussion (AFD)

Issued by Cheyenne, WY (CYS)

FXUS65 KCYS 231707

Area Forecast Discussion
National Weather Service Cheyenne WY
1107 AM MDT Mon Sep 23 2019

.SHORT TERM...(Today and tonight)
Issued at 338 AM MDT Mon Sep 23 2019

Main highlight today will the near-critical to briefly critical
fire weather hazard and the above normal temperatures of 5-9F

Shortwave H5 ridging is occurring from south-central Canada
southward along the Rockies and Front Range this morning per water
vapor and RAP analysis. A notable shortwave trough is positioned
across OR to NV farther west but will be shifting southeast to CA
and AZ with minimal impacts to WY/NE. The shortwave ridging and 
resultant subsidence will aid dry west winds and above normal 
temperatures today with lots of sun. Afternoon relative humidity 
will fall into the 10-15 percent range across much of Southeast WY
and with wind gusts reaching near 25 mph at times, near-critical 
to briefly critical fire weather conditions will occur. See fire 
weather discussion below for more details. Outside of the elevated
fire weather hazard, it will be a warm and pleasant day across 
the region with above normal temperatures. A quick moving 
shortwave trough will pass over WY late tonight into Tuesday with 
no sensible weather impacts given the dry air in place other then 
a few more mid- and high-level clouds.

.LONG TERM...(Tuesday through Sunday)
Issued at 338 AM MDT Mon Sep 23 2019

Continued warm Tuesday with a quick cool off Wednesday followed by
a warmup Thursday and then a much more active pattern for the late
week and weekend into next week. Precipitation chances will be low
until late Thursday into Friday and stay elevated through the
upcoming weekend and early next week. Some wintry precipitation
could be possible late weekend into early next week for portions
of the High Terrain in SE WY. 

A quick moving front will brush far NE WY and the north Nebraska 
Panhandle Tuesday for a slight cool off but most of the region 
will see similar temperatures tomorrow as they did today with 
readings perhaps 1-3F degrees cooler. A broad, low-amplitude 
longwave trough will take shape over the northern CONUS late 
Tuesday into Wednesday and will aid in another cold front passing
through Wednesday that will drop temperatures 10F degrees or so 
to more normal values for this time of year. However, WAA will 
likely occur Thursday with above normal readings again. All the 
while, precipitation chances will remain low due to the lack of 
a deeper vertical lift mechanism.

The vertical lift dynamics will change by late Thursday, late 
week, and weekend as a stronger front and greater divergence 
aloft is advertised per GFS/ECMWF. Precipitation should primarily 
be rain through this period until late weekend when a greater 
amplified stronger western CONUS trough develops over the Pacific 
NW and shifts towards the inter-mountain west. Will need to keep 
close watch on surface boundary timing vs. deeper lift timing for 
High Plains rain/storms and high terrain snow showers a week from 
now. It is too far out to speculate on impacts or snow amounts but
multi-model guidance does suggest wintry precipitation being
possible in the mountains.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday morning)
Issued at 1107 AM MDT Mon Sep 23 2019

VFR expected at all terminals through the period with just some high 
clouds passing over into this evening.  Winds generally light though 
a few gusts around 20 kts this afternoon from KRWL to near KCYS.


Issued at 338 AM MDT Mon Sep 23 2019

Near-critical to briefly critical fire weather conditions anticipated 
this afternoon to early evening and again Tuesday. Relative humidity 
values will fall to 10 to 15 percent with wind gusts 18 to 23 mph and
occasional gusts to 25 mph possible across the high terrain and High
Plains of Wyoming today. Most likely zones of greatest chance for a Red
Flag Warning would be Zones 302-303. Spatial coverage of marginal winds
will preclude the issuance of a Red Flag Warning at this time but this
will be monitored closely through the day. Tuesday will have higher 
winds but slightly higher humidity. Wednesday will be cooler with
greater moisture.