406
FXUS65 KGJT 121157
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
457 AM MST Wed Feb 12 2025

.KEY MESSAGES...

- A fast moving trough will exit the region this afternoon with
  an additional 2 to 5 inches for the northern and central
  Colorado mountains along the divide through this morning.

- Temperatures turn markedly cooler today and tonight in the
  wake of the cold front associated with last night`s storm.

- A stronger system will move in at the end of the week, with
  widespread valley rain and significant mountain snow
  accumulations expected. A Winter Storm Watch has been issued
  for the southern mountains and portions of the central
  mountains for Thursday evening through Saturday where
  confidence is highest.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 350 AM MST Wed Feb 12 2025

Snow stake cams at this early morning hour show about 4 to 6
inches or so at Steamboat, Aspen and Vail Ski areas which
appears on track with our current winter weather advisories.
Roads at the higher elevations and passes remain snow covered.
Temperatures are in the single digits in the northern and
central mountains so temperatures are plenty cold with high
efficient dendritic snow ratios of 18 to 20:1. Snow rates are
higher along the frontal boundary which is over the central
mountains at this time, with the latest hi res guidance
indicating snow to continue through the morning with rates
lessening towards late morning. As the positively tilted trough
moves onto the Plains and Front Range, light snow may linger
over the northern and central divide mountains but drier
conditions and clearing skies will be evident this afternoon
from the west.

Stronger cold air advection works in behind this departing
trough with clearing skies leading to a very cold night tonight
given strong CAA and efficient radiational cooling. H7
temperatures bottom out at -20C or so Thursday morning before
southwest flow returns Thursday afternoon to usher in much
milder afternoon temperatures. This milder air for Thursday
afternoon following our brief cold spell is all thanks to a
shortwave ridge of high pressure and milder Pacific air bringing
in deep Atmospheric River moisture to the California coast and
into the western Great Basin. We will see mostly sunny skies
Thursday morning turn increasingly cloudy as mid and high level
clouds increase ahead of this deep AR moisture laden storm
system set to bring significant snowfall potential to the
mountain areas as well as widespread lower valley rain.
Confidence is increasing for significant mountain snowfall so
issued some Winter Storm Watches to address this potential. More
details on this storm and the Winter Storm Watch is in the Long
Term section.

Temperatures today will be 10 degrees colder than yesterday
following last night`s cold front, with highs today and lows
tonight around 10 to 15 degrees below normal on average. Some
cold pool valleys and mountains will be 20 or so degrees below
normal. This cold spell won`t last long though as highs on
Thursday warm up about 10 to 15 degrees with highs closer to
normal or slightly below...but the difference will be felt.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 350 AM MST Wed Feb 12 2025

A transitory ridge is on the way out Thursday evening, as
southwesterly flow on the nose of a SOCAL jet streak begins to
fill the atmospheric column with Pacific moisture from our next
atmospheric river event. Early ensemble predictions are sticking
to AR moisture values in excess of 200 percent of normal for
much of southeast Utah and southwest Colorado. While the
majority of the CWA is expected to see a precipitous rise in
precipitable water, the San Juans stand first in line for this
tap. Rightly so, given the scarcity of snow there this winter.
Early estimates for this event give the high country of
southwest Colorado 1 to 2 feet of snow by Saturday morning.
Ensemble members are grouped tightly around 1 feet of snow for
spots like Red Mountain Pass and Pagosa Springs. Precipitation
is expected to begin late Thursday night, when temperatures will
likely still be on the mild side from a warmup Thursday
afternoon. But, things should be cold enough to get some
measurable snow on the terrain of the Bookcliffs and southward
on the mountains of the southern half of the CWA. Shortwaves
will keep snow flying into Friday morning and continuing to add
up, with some inch an hour rates turning up in the San Juans.
Most of this activity will ride along the frontal boundary
draped to our south, with our southern counties capitalizing on
the rich moisture and frontal forcing. Northward of the San
Juans, moisture will still be good for plenty of orographics and
the Central and Northern mountains should be able to get in on
the bounty too. The frontal boundary settles southward Friday
afternoon and evening, but the gut of the trough remains
anchored overhead, which keeps additional light snow falling
into Saturday. The backside of the trough and the northerly jet
slide through Saturday afternoon and likely bump up snow showers
along the Continental Divide and deliver a parting gift of an
additional few inches of snow for the high country.

Due to increasing confidence and model consistency the last few
days, decided to issue a Winter Storm Watch for the La Sals and
Abajos in southeast Utah as well as the southwest San Juan
Mountains from Thursday evening through Saturday morning where 1
to 2 feet of snow is possible, with locally higher amounts
upwards of 3 feet in the SW San Juans. Also, included the
central Colorado mountains that are more favored in SW flow
where confidence for 1 to 2 feet is higher (Grand Mesa, West
Elks and Sawatch Mountains) as well as the Uncompahgre Plateau
and NW San Juans from Thursday evening through 5 pm Saturday as
northwest flow on the backside of this trough will keep snow
ongoing through the day Saturday for the NW favored slopes.
Anticipate more highlights will be needed, but confidence for
the northern mountains and some higher valley zones is lower as
details are still in flux...so day shift can reassess potential
for additional winter weather highlights.

A few orographic showers will likely continue Sunday behind the
system for our northern mountains, but dry air will likely
intrude on much of this activity. Models coalesce around a
progressive pattern setting up Monday and Tuesday, with
additional rounds of precipitation working through from the
PACNW. Some early signs are there for this wave digging deeper
south than we`ve seen with these systems this winter, but the
origins for this one keep me cautious on snow south of the
Central Mountains Monday and Tuesday. Plenty of time to worry
about that system. The Valentine`s storm will command more
attention in the coming forecast packages, as this is the first
notable snowfall for the southern mountains in some time and
travel impacts can be expected.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 445 AM MST Wed Feb 12 2025

Light snow continues across the northern and central divide
through the morning hours with MVFR/IFR conditions at KHDN and
KASE with VCSH possible at KEGE and KGUC. ILS breakpoints more
likely to be reached at KHDN and KHDN due to low CIGS and VSBY
in showers. Conditions improve by afternoon to VFR with few to
scattered clouds and breezy conditions with gusts up to 25 kts
possible. Some low clouds could hang around the divide and
mountain TAF sites like KASE before skies clear this evening.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Winter Weather Advisory until 11 AM MST this morning for
     COZ004-010.
     Winter Storm Watch from Thursday evening through Saturday
     afternoon for COZ009-012-017-018.
     Winter Storm Watch from Thursday evening through late Friday
     night for COZ019.
UT...Winter Storm Watch from Thursday evening through late Friday
     night for UTZ028.

&&

$$

SHORT TERM...MDA
LONG TERM...MDA/LTB
AVIATION...TGJT