040
FXUS65 KCYS 191111
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
411 AM MST Sun Jan 19 2025

.KEY MESSAGES...

- A reinforcing shot of bitterly cold air and light snowfall is
  possible Sunday into Monday.

- An Cold Weather Advisory is in effect from 8pm tonight
  through 11am Tuesday for much of the region.

- An Extreme Cold Warning is in effect for the high valleys of
  southeast Wyoming and the eastern plains north of the North
  Platte River valley starting tonight and continuing through
  early Tuesday morning.

- Strong winds possible this week on Tuesday and again Friday, ahead
  of the next potential arctic air intrusion for the region
  over the weekend.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 315 AM MST Sun Jan 19 2025

Active weather with several forecast concerns and hazards over
the next 48 to 60 hours, including extreme cold...another round
of light snow bringing some travel impacts...and strong gusty
winds for the wind prone areas at the end of the short term
forecast period (Monday night and early Tuesday).

*TEMPERATURES AND WIND CHILLS

Arctic airmass is in place this morning with all locations below
zero for air temperatures. Wind chill temperatures have also
started to trend down with wind chills currently between 20
below and 35 below zero. Air temperatures tonight have also
trended closer to the Canadian NH and the NBM 10th percentile
based on current observations over the last 6 hours. Therefore,
will blend max and min temperatures with both of these over the
next 48 hours. Sunday afternoon will not be quite as cold as
Saturday as we get some slightly WAA out ahead of the next
reinforcing shot of arctic air. Most models and ensembles
show 850mb temperatures climbing over -7c to -15c, coldest over
the eastern High Plains with 700mb temperatures between -18c to -25c.
This will result in surface temperatures in the teens for most
locations. The only places which may remain in the single digits
should be the more sheltered Laramie Valley and areas north of
the North Platte River Valley that should stay locked in the
arctic air. Secondary arctic front then rapidly pushes south
across the high plains and central Wyoming, pushing into the
I-80 corridor by early Monday morning. Models and ensemble
spread show some gusty winds with this secondary push of cold
air. With enough wind to justify wind chills/apparent Temp of
40 below zero, decided to upgrade the Cold Weather Adv to an
Extreme Cold Warning for portions of the forecast area,
including the high plains north of the North Platte River
Valley, and all the high valleys west of the Laramie Range since
some of these locations hit warning criteria already early this
morning and late last night. Started this warning at 11 PM
tonight, which will replace the Advisory. Again, blended
previous forecast with the NBM 10th percentile and the Canadian
NH, which seem to be initializing quite well early this morning.
These cold weather headlines are mainly for wind chill
temperatures, but a few areas may see air temperatures
approaching -20 to -30 as well. These headlines will remain
valid until late Tuesday morning. Expect most locations to climb
above Extreme Cold Apparent T criteria on Monday, but current
forecast values are close enough to justify the warning through
the day (-20 to -30 with air temperatures around zero or
slightly below)

*SNOWFALL AND TRAVEL IMPACTS

As the secondary arctic front moves southward across the Front
Range, models show enough lift and Omega (mainly WAA aloft &
upslope flow) to justify light snow accumulations across the
area, but particularly the I-80 corridor and most of southeast
Wyoming including the Pine Ridge. Model soundings show this WAA
pretty well, between 750mb and 650mb for a good 6 to 10 hours.
Increased POP up to 80 percent tonight through early Monday
morning. This should be a high POP but low QPF event due to the
lack of moisture. There should be just enough moisture for a
gradual inch of snow or 1 to 3 inches in the mountains. Since
this is sub-advisory , decided not to issue any Winter Weather
Headlines since we have enough going on as it is. Even a little
bit of snow will create minor travel impacts. There is some
concern of bands of moderate snow with the GFS showing pretty
good Omega along the I-80 corridor tonight, but with shallow
lift, do not expect this to be a problem for more than an hour
at a time.

*Strong gusty winds as the arctic front retreats eastward

In-house wind models continue to be aggressive with the next
wind event, which may start as early as late Monday evening.
Models show a strong 120kt jet with anticyclonic curvature
developing over Wyoming and Montana and ejecting east with
strong negative pressure tendencies across the plains due to
warm air advection late Monday and Tuesday once the arctic front
ejects eastward. This strong gradient will be most obvious along
the Laramie Range with models show strong surface to 850mb
winds. The 700mb gradient will be delayed somewhat, but all
model show 700mb winds of 50+ knots by early Tuesday morning and
strengthening to 65 knots later Tuesday afternoon. With models
showing decent low level subsidence and in-house wind guidance
showing probabilities of 60+ MPH gusts over 90 percent, decided
to issue an early High Wind Watch since models have continued to
be aggressive. Started the Watch at 11 PM Monday because of low
confidence with how arctic fronts behave in the higher terrain
including the high plains.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 237 AM MST Sun Jan 19 2025

Fairly active pattern expected for the week ahead behind the
departing trough responsible for arctic temperatures this weekend.
The active weather beings early Tuesday morning at an upper-level
ridge pushes through the region with a shortwave trough right on
its heels. As the ridge pushes through in the upper-level, height
gradients strengthen at 700mb as the arctic front pushes off to the
east and temperatures warm significantly across the CWA, with frigid
temperatures still just off to the east. A 55-60kt 700mb jet will
remain in place across the mountain ranges throughout the day
Tuesday into the evening hours. Surface pressure gradients will
strengthen as well, as cold, dense air remains west of the Laramie
Range leading to higher pressure and the arctic air pushes off to
the east with temperatures east of the Laramie Range increasing and
pressure dropping. At the peak, the cross-barrier pressure gradient
will be around 5-6mb with an even strong pressure gradient across
the Snowy Range, around 9-10mb. Based on pressure gradients alone,
high winds are heavily favored for Tuesday. When looking at other
variables, confidence only increases for high winds. Craig to Casper
850mb height gradients are in the 105-110m range during the early
morning hours into the mid-morning hours on Tuesday. 700mb height
gradients are also significant, between 70-80m. This strongly favors
strong winds at Arlington, with in house random forest guidance
pinging 90-95% probability of high winds at Arlington. At Bordeaux,
strong cross-barrier gradients from Arlington to Bordeaux increase
towards 8mb. When combined with the strong 700mb jet, strong
downward omega field in the GFS, and in house random forest guidance
around 80%, high winds are also favored at this location with fairly
high confidence. In house random forest guidance further suggests
that Bordeaux may see wind gusts between 70 and 75mph during this
time with Arlington potentially seeing 65-70mph gusts. Overall,
looks to be a very favorable setup for high winds in the wind prone
regions. Downward omega fields suggest some of these high winds may
spill out into adjacent zones, including the Cheyenne area, though
confidence is not as high for Cheyenne. If Cheyenne were to see high
winds, the best timing looks to be Tuesday evening into the
overnight hours. For now, the High Wind Watch only encompasses the
typical windy areas, but this may need to be expanded in future
updates.

Strong winds will come to an end as another cold front approaches
the region, bringing another chance for cold temperatures. 700mb
temperatures are expected to drop into the -15 to -17C range with
this next cold front on Wednesday morning. This front will be fairly
moisture starved, leading to minimal precipitation chances across
the CWA Wednesday into Thursday. Surface temperatures look to remain
in the teens and 20s west of the Laramie Range and mid-20s to low-
30s east of the Laramie Range. While this cold blast will not be as
potent as the arctic intrusion currently on going, temperatures will
remain fairly cold for the foreseeable future. Additionally, this
front will increase winds across the CWA Wednesday into Thursday,
leading to continued concerns for wind chill temperatures in the
negatives. Northerly flow aloft will develop on the backside of the
upper-level trough associated with the cold front. This northerly
flow will allow cold temperatures to continue to be filtered into
the region, keeping highs on Thursday in the 20s and 30s once more.

A brief warmup is expected Friday as the upper-level trough pushes
out of the area and a weak ridge attempts to develop overhead. 700mb
temperatures will warm into the 0 to -5C range. 700mb heights
gradients will strengthen over the region as the associated
shortwave trough pushes through, leading to elevated to near-high
wind potential for the wind prone regions once more. The additional
downsloping off the Laramie Range will enable locations east of the
Laramie Range to warm into the 40s for Friday. These winds will be
fairly brief, as the next system begins to push into the area by
Saturday morning. This system looks to be more potent than the cold
front that traverses through the region on Wednesday and Thursday.
700mb are progged to be in the -18 to -20C range by late Saturday
night, leading to another chance for very cold temperatures across
the region. Did decrease highs on Saturday and Sunday, but will
likely need to decrease the further over the coming days.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 408 AM MST Sun Jan 19 2025

VFR to near-MVFR conditions ongoing at all terminals as of 11Z.
Light winds at KCYS and all Nebraska terminals will continue
throughout much of the 12Z TAF period. Some lower clouds have
developed over KRWL and KLAR early this morning, but should
remain VFR at these terminals. Primary aviation concern
currently is the potential for low-level wind shear at KCYS.
LLWS should come to an end by 15Z. Scattered, light snow showers
are expected this afternoon and evening across the Wyoming
terminals, with lower chances across the Nebraska terminals.
Snow showers may briefly change conditions to MVFR or IFR in the
heaviest showers.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Cold Weather Advisory until 11 PM MST this evening for WYZ101-
     102-109-110-115-116.
     Extreme Cold Warning from 11 PM this evening to 11 AM MST
     Tuesday for WYZ101-102-104-105-109>111-113-115-116.
     Cold Weather Advisory until 11 AM MST Tuesday for WYZ106>108-
     117>119.
     High Wind Watch from Monday evening through Tuesday afternoon
     for WYZ104.
     High Wind Watch from Monday evening through late Tuesday night
     for WYZ106.
     High Wind Watch from Monday evening through Wednesday morning
     for WYZ110.
     High Wind Watch from Monday evening through Wednesday
     afternoon for WYZ116-117.
NE...Cold Weather Advisory until 11 PM MST this evening for NEZ002-
     003-021-095-096.
     Extreme Cold Warning from 11 PM this evening to 11 AM MST
     Tuesday for NEZ002-003-021-095-096.
     Cold Weather Advisory until 11 AM MST Tuesday for NEZ019-020-
     054-055.

&&

$$

SHORT TERM...TJT
LONG TERM...AM
AVIATION...AM