Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Cheyenne, WY (CYS)

FXUS65 KCYS 211129

Area Forecast Discussion
National Weather Service Cheyenne WY
529 AM MDT Wed Aug 21 2019

.SHORT TERM...(Today through Thursday) 
Issued at 440 AM MDT Wed Aug 21 2019

With last night's impressive line of severe thunderstorms lingering
past midnight across the Nebraska panhandle, will keep the 
discussion short for today and Thursday. Fog has started to form
west of the cloud shield associated with the MCS pushing east
across Nebraska early this morning. Expect clearing skies across
portions of Nebraska shortly, so kept fog in the forecast. Will
likely lower the coverage though since sunrise will occur 
shortly. Pretty dense fog has been recently observed along I-80
west of Cheyenne with visibilities below one quarter of a mile. 

Cooler temperatures are expected today with daytime highs around
10 to 20 degrees cooler than yesterday across a large portion of
the area. Thankfully, this should limit the coverage of strong to
severe thunderstorms. The area that needs to be watched is along
and west of the cool front/differential heating boundary, which 
is currently along the border to Carbon and Albany counties. 
Expect this boundary to nudge eastward into this afternoon and 
provide enough convergence for scattered thunderstorms. Some of 
these thunderstorms will be strong or severe across Albany county,
and eventually into Platte and Laramie counties as well. The 
limiting factor will be environmental shear which will be 
marginal. Current high res models show this activity quickly 
weakening as it moves off the Laramie Range, so expect any severe 
weather threats to be short-lived into this evening due to cooler
temperatures and potential for thick cloud cover as you head 
further east across far eastern Wyoming and western Nebraska.
Another round of thunderstorms are possible later this evening,
but do not expect these to be severe.  Another chance for 
thunderstorms are expected on Thursday, but severe weather risk 
should be very low with limited shear and a stronger capping 
inversion across the eastern plains.

.LONG TERM...(Thursday night through Tuesday)
Issued at 410 AM MDT Wed Aug 21 2019

Models continue to trend slower with the Pacific upper level
trough forecast to push across Montana and northern Wyoming late
this week. This trend is mainly due to models initializing the 
base of the trough further to the south as it is about to move 
onshore across the Pacific northwest today. As a result, increased
POP and chances for thunder on Friday. In contrast to 48 hours
ago, Friday no longer looks like a dry day across most of
southeast Wyoming and western Nebraska. Daytime highs will also
remain a few degrees cooler, but afternoon readings should be warm
and in the 80s across most locations. Once this Pacific system 
moves east, all models show drier weather across the area during 
the weekend along with some pretty warm temperatures in the 80s to
low 90s. However, the upper level trough takes its time ejecting
to the east, so kept a slight chance of thunder north of I-80
across east central Wyoming and the northern Nebraska panhandle 
on Saturday. 

There is reasonable confidence that the first late-summer cold 
front will impact the region as early as Monday of next week. 
Models are still struggling on the timing, showing FROPA sometime
between early Monday morning and Tuesday afternoon. This front is
pretty strong for this time of the year with even extended MOS 
guidance showing high temperatures lowering into the 60s 6-7 days 
out. All models show 700mb temperatures lowering between -2c to 
+5c across the area, which is around 2 standard deviations below 
climatology. In addition, there will likely be a decent coverage 
of precipitation across the area as this front plunges southward,
but confidence is low on what time periods that may take place. 
Kept POP on the lower end of chance (20-40 percent) for now. 


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 525 AM MDT Wed Aug 21 2019

MVFR conditions will develop and continue this morning in the 
wake of a thunderstorm complex over central Nebraska. Easterly 
upslope flow will drive a moist air mass westward through today 
brining periodic low clouds. While fog has been limited, early 
morning fog is possible across all of the plains terminals. 
Scattered convection is expected to develop this afternoon along 
the laramie range and spread eastward into the plains. Max tops to
FL450 along with strong gusty winds and hail is to be expected. 
Pilots use cation in and around terrain this morning and be aware 
of convection later this afternoon and evening.


Issued at 354 AM MDT Wed Aug 21 2019

Dry conditions will continue across most of Carbon
County today, but winds should remain pretty light. Daytime 
humidities further to the east will be moderate to high, with
relative humidities generally above 40 percent. West to southwest
winds will increase during the day on Thursday, resulting in 
hazardous fire weather conditions across portions of Carbon 
County. Issued a fire weather watch for zones 303 and 304 (western
and southwestern Carbon County) for Thursday for elevated to 
low-end critical conditions with wind gusts around 25 mph and
relative humidity as low as 10 percent. Additional fire weather 
concerns are expected this weekend as temperatures increase and 
daytime humidities trend lower.


WY...Fire Weather Watch from Thursday morning through Thursday 
     evening for WYZ303-304.