041
FXUS65 KRIW 191059
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
359 AM MST Sun Jan 19 2025

.KEY MESSAGES...

- Well below zero temperatures this morning across much of the
  Cowboy State.

- Little warming is expected during the day with highs ranging
  from the single digits to low teens.

- Scattered light snow showers are possible starting late Sunday morning
  across western and northern WY. Snow showers are expected to
  gradually shift east/south during the afternoon and evening.

- Skies are expected to gradually clear overnight Sunday into
  Monday morning with dangerously cold temperatures forecasted.

&&

.DISCUSSION...
Issued at 257 AM MST Sun Jan 19 2025

The arctic air made its presence known Saturday with many
locations seeing high temperatures in the single digits to low
teens. Skies cleared much quicker than expected in some parts of
the CWA Saturday night and as a result, temperatures are really
starting to plummet this morning.

Forecasting the extent of this cold is going to be tricky.
First, there are multiple small scale factors that can greatly
influence temperatures. These factors are cloud cover, wind,
snow cover/depth, and elevation. The other aspect of
forecasting the cold is the lack of consensus among models. They
are really struggling to get a grasp on these cold
temperatures, with many of the forecasted lows for this morning
being already 5 to even 10 degrees too warm as of 0100 MST. So
due to this the forecasted lows have been adjusted, with many
locations likely seeing temperatures ranging in the negative
teens. As mentioned earlier, there are a lot of small scale
factors at play here and there is a good chance that lows will
be even colder than what is currently forecasted.

A shortwave will make its way into the region today, gradually
shifting flow to a more northerly direction and ushering clouds over
the area. Increasing clouds today will hinder daytime heating,
keeping many locations with well below normal values. Chances
for light snow showers will also be associated with this
shortwave. Snow showers look to first develop across northern
and western WY this morning. Snow chances will gradually shift
east/south during the day and linger through the evening.
Overall, impacts will be minimal as not much moisture will be
associated with this disturbance. One thing to monitor will be a
quick burst of snow across central basins as a brief period of
northerly upsloping may develop. Snowfall amounts look to range
from a trace to around 0.5 inches. Depending on the location of
these showers, some isolated areas may see higher amounts near
an inch. The disturbance moves out of the area during the
evening Sunday with skies expected to clear out overnight Sunday
into Monday morning.

Another morning with dangerously cold temperatures is forecast for
Monday. Current forecasted lows range from the negative twenties to
negative teens. Probabilistic guidance is pinpointing portions
of the central Wind River Basin, Natrona/Johnson Counties, Upper
Green River Basin, and parts of Jackson Hole as having the best
chance of seeing lows near or below negative 20. One thing to
monitor will be the clearing of clouds as temperatures may end
up being colder if clouds can clear out quicker. Due to these
colder temperatures, daytime highs on Monday will struggle to
warm above sub-zero values. Another cold morning is forecasted
for Tuesday with lows once again ranging from the negative teens
to negative twenties. Tuesday morning is looking quite brutal
across portions of the state, particularly the Wind Corridor
from Casper to Rock Springs along with Johnson County. The main
concern will be sub-zero temperatures being combined with breezy
southwesterly winds of 20 to 30 mph

Cold Advisories that were issued Saturday evening remain in
place. These Advisories have been expanded to cover portions of
the Bighorn Basin, Wind River Basin, eastern Sweetwater County,
the Upper Green River Basin, and Jackson Hole/Tetons. The main
concern will be some breezy conditions which, when combined with
already cold temperatures, may lead to apparent temperature
values ranging from - 25F to even -35F in some areas on Monday
and Tuesday morning.

Tuesday onward will see much of the arctic air begin to moderate, as
a brief period of ridging tries to build back in over the region.
Winds will increase ahead of this ridge with periodic gusts of 25 to
35 mph. Gusty winds look to be mainly from Casper to Rock Springs as
well as portions of the upper Wind River Basin. These winds will aid
in mixing warmer air towards the surface during the day.
Temperatures are looking to warm back up across much of the CWA on
Tuesday, but there may be some exceptions to this. Lower elevation
basins with fresh snowpack will need to be monitored as stubborn
inversions may limit how much mixing makes it down to the surface.
One area of note is the central Wind River Basin, which based off
previous events, has had difficulty mixing out after inversions
develop. So while many will see warmer temperatures for the second
half of next week, some locations may be socked in with cold
temperatures prevailing until those inversions can be broken. The
remainder of the week is looking to see the active weather pattern
continue. However, confidence regarding impacts is still very low,
as models are still uncertain on tracks and timing of these
potential disturbances.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 347 AM MST Sun Jan 19 2025

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

Skies will remain mainly VFR through much of the day, but mid
to high clouds will be common throughout. A weak system will
bring a brief round of light snow to KJAC and KRKS during the
day, but confidence is still low on persistent impacts lowering
conditions into MVFR range. Have kept PROB30 for KJAC,
adjusting timing, and have added to KRKS during the afternoon. A
second wave of snow will arrive at KRKS around 04Z, as another
arctic front moves through and turn winds to the northeast.
Conditions will drop to MVFR with the snow, with MVFR ceilings
continuing through the rest of the period. Otherwise, cigs will
gradually rise overnight, with some thinning after 00Z.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

High clouds, SCT-BKN FL150-200, have increased from the
northwest over the last couple of hours. These clouds will
continue to increase, becoming OVC and lowering below FL100
through the rest of the morning. Another round of lower clouds
and light snow returns during the day, but chances of
conditions falling to MVFR remain low to moderate during the
afternoon at KCOD. Snow chances at KCPR and KWRL remain lower
during the afternoon, so have not added PROB30 at this time.
However, do expect IFR/MVFR conditions at KLND/KRIW around
23Z/00Z. MVFR ceilings are expected at these two sites through
the rest of the period, once the snow ends. There is the
possibility of sustained winds of 15-20kt developing from KPOY
to KWRL after 00Z, crossing over the Wind River Canyon to KRIW
around 06Z at around 13kt. These winds will add to the threat of
bitter cold temperatures tonight.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Cold Weather Advisory from 10 PM this evening to 10 AM MST
Tuesday for WYZ003>006-010-011-017>020-025-026-030.

Cold Weather Advisory from 10 PM this evening to 5 AM MST
Tuesday for WYZ008-009.

Cold Weather Advisory from midnight tonight to 10 AM MST
Tuesday for WYZ012-013.

&&

$$

DISCUSSION...Dziewaltowski
AVIATION...LaVoie/Straub