607 
FXUS65 KCYS 182322
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
420 PM MST Fri Feb 18 2022

.SHORT TERM...(This evening through Sunday) 
Issued at 230 PM MST Fri Feb 18 2022

Visible Satellite loop across the Front Range shows mid to high
clouds moving southeast across the area under northwest flow
aloft. Some lower clouds will likely sneak into Niobrara and Dawes
county over the next several hours as a weak boundary shifts south
and stalls. Can't rule out a few rain/snow showers near this 
boundary with enough convergence in the low levels, mainly near
Lusk, Harrison, and Chadron. Showers should be short-lived and
brief with clearing skies after midnight. 

Otherwise, strong winds this morning across the Wind Prone areas
have subsided with gusts dropping below 45 mph. Expect these winds
to continue to decline this evening; but this lull in the winds
will be brief as all models are showing low to midlevel pressure 
gradients increasing between midnight tonight and early Saturday
morning. Gusts over 50 mph can not be ruled out prior to sunrise
Saturday along I-80 between Rawlins and Laramie.

Primary forecast concern this weekend will be another round of
strong gusty winds, not only for the Wind Prone areas but possibly
spreading into the high valleys and immediate high plains zones
near the Laramie Range. Models show the Arctic front near the
Canada/US border gradually drifting south into Idaho, Montana, and
the Dakotas late Saturday and Saturday night. Further south, a
115 knot upper level jet axis will gradually shift south into
Wyoming. Pressure gradients from the surface up to 500mb will also
increase Saturday through Saturday night. Most models show a core
of 700mb winds between 50 to 70 knots across most of southeast
Wyoming extending east into western Nebraska. Models also show the
typical mountain wave signature at 700-500 mb, which is further 
confirmed on the 12z 700mb OMEGA fields. Greater confidence in 
high wind criteria west of Interstate 25 compared to the eastern 
zones. Therefore, due to increasing confidence with timing, 
upgraded the Arlington and Elk Mountain zone to a High Wind 
Warning for gusts around 75 MPH. Bordeaux, the Laramie Valley, and
the I-80 Summit also look good, but since the timing of the winds
appear to be later in the day, or possibly Saturday night, 
decided to hold off on those upgraded for now since there are some
subtle timing differences on the onset of the winds. Thought 
about extending the High Wind Watch further north to including 
northern Carbon, Shirley Basin, and Converse County, but after 
coordinating with nearby offices will hold off since it appears 
pretty marginal. Not as confident with high winds east of 
Interstate 25 since subsidence is not all that strong and peak 
700mb winds are not colocated with the best subsidence. Low level
lapse rates could also be better with more substantial mixing. 
Regardless, still expect it to be windy across far eastern Wyoming
and into most of western Nebraska Saturday and Saturday night. 

Mild temperatures are still forecast for this weekend with highs
well into the 50s to maybe low 60s on Sunday for the lower
elevations. Increased high temperatures on Sunday ahead of the
front since guidance typically has a cold bias during the daytime 
pre-FROPA.

.LONG TERM...(Sunday night through Friday)
Issued at 114 PM MST Fri Feb 18 2022

The main focus of the long term forecast is a substantial and 
prolonged outbreak of arctic air expected beginning Sunday night 
and continuing through the work week. Accumulating snowfall is 
likely Monday through Tuesday morning, with light snow possible 
through Thursday. 

A broad trough is expected to dive down along the coast of British 
Columbia into the Northwest CONUS on Sunday. A strong surface high 
will also push down the eastern side of the Rockies, ushering in the 
coldest air of the season towards our area. Frontal passage is 
currently expected roughly between midnight and sunrise on Monday 
morning, so a midnight high is likely for Monday. During the 
daylight hours, temperatures are likely to remain fairly steady or 
even drop as the arctic airmass deepens through the day. Northeast 
winds will help saturate the lower atmosphere through the morning on 
Monday, leading to snow filling in from northwest to southeast as 
the day progresses. A period of pretty good snowfall rates is 
possible with model soundings showing a saturated isothermal layer 
in the dendritic growth zone Monday afternoon near Cheyenne. Much 
colder and drier air pushing in Monday night is likely to slow down 
snow rates, but light snow will remain possible. For totals in the 
first round, looking like we might approach advisory criteria in 
several areas, with the mountains most likely. A secondary shortwave 
is then expected to dive down slightly further south and west than 
the initial wave on Tuesday into Wednesday. The 12z model suite has 
shifted slightly north with the best ascent for this second round, 
so nudged PoPs up slightly all the way through Thursday morning. 
With moist SW flow aloft, the Sierra Madre mountains would have the 
highest odds of snowfall continuing through the middle of the week. 
The Tuesday-Thursday timeframe will continue to be watched for the 
possibility that light snow continues across a broader portion of 
the area, but confidence is quite low at this time. 

The cold temperatures may be the most significant story with this 
event. 700-mb temperatures could bottom out around -25C Tuesday 
morning, which will lead to some of the coldest temperatures of the 
season. Strong consistency among the ensembles boosts confidence in 
extreme cold, with the worst Monday night into Wednesday. Tuesday's 
highs may struggle to get out of the lower single digits, and record 
low maximums look likely. The record low maximum for Cheyenne is 
11F. Even some low minimums may be challenged. Cheyenne's record low 
on Tuesday and Wednesday is -13F. This is currently close to the 
lower end of ensemble guidance, but certainly a possibility to keep 
watching closely. The bitter cold and dreary weather is likely to 
continue Wednesday, with single digits highs possible once again. 
The upper level trough finally starts to clear out by Thursday, 
which will start to weaken the arctic air. Currently expect some 
moderation Thursday, but we should be still significantly below 
average. Much more uncertainty introduced late in the week regarding 
how quickly the trough will clear out and how much we will be able 
to warm. 

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday afternoon)
Issued at 420 PM MST Fri Feb 18 2022

W-NW surface winds will remain gusty at times early this evening 
before diminishing, only to come back up again Saturday afternoon. 
Weak front slips south into eastern WY and far northwest NE this 
evening with CIGS declining to MVFR at KCDR and KAIA through late 
tonight. This front will to the north and east by early Saturday 
morning with CIGS improving to VFR. W-SW surface winds will become 
gusty Saturday morning and continue into the afternoon. Gusts of 30 
to 40 kt will be prevail. Stronger winds and mountain wave 
turbulence will develop in the vicinity of the higher terrain in 
southeast WY through the day Saturday.

&&

.FIRE WEATHER...
Issued at 144 PM MST Fri Feb 18 2022

No fire weather concerns this weekend through next week. It will
trend a little warmer with increasing winds, but fuels should
remain wet or snow covered. For next week, a strong arctic cold 
front will push south across the Front Range and adjacent plains 
Sunday night and Monday, resulting in periods of additional 
snowfall and a very cold last week of February.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning from 5 AM Saturday to 11 AM MST Sunday for 
     WYZ110.

     High Wind Watch from late tonight through Sunday morning for 
     WYZ106.

     High Wind Watch from Saturday afternoon through late Saturday 
     night for WYZ107-115-118-119.

     High Wind Watch from Saturday morning through Sunday morning for 
     WYZ116-117.

NE...None.
&&

$$

SHORT TERM...TJT
LONG TERM...MN
AVIATION...MAJ
FIRE WEATHER...TJT