668
FXUS65 KGJT 262242
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
342 PM MST Tue Nov 26 2024

.KEY MESSAGES...

- An Atmospheric River continues to impact the area today
  through Wednesday bringing a prolonged period of moderate to
  heavy snowfall to the mountains and higher valleys of eastern
  Utah and western Colorado.

- If you plan to travel over the mountains tonight or Wednesday,
  be aware that very difficult, if not impossible, driving
  conditions are possible.

- If possible, plan alternative routes ahead of time, or delay
  travel if possible until Thanksgiving Day. Stay up-to-date
  with the latest forecast as this significant storm continues
  to impact the area.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 342 PM MST Tue Nov 26 2024

Models have struggled mightily with temperatures with this
Atmospheric River event and as a result, precipitation type
forecasts were not good for the morning period. Forecast highs
for the afternoon were way overdone and adjusted those this
morning. Though surface temperatures haven`t changed a lot where
precipitation was falling, it appears enough warming has
occurred aloft to cause snow to change over to rain for areas
near or below 6,000 feet. Meanwhile, most mountain areas
received considerable snowfall with amounts up to 18 inches at
select automated sites.

For tonight, the short wave mid-level trough now moving across
the northern Great Basin will sweep east-southeastward moving
over the area late tonight through Wednesday morning. The jet
moving just ahead of the trough will sharpen the remnant
baroclinic zone in the vicinity of the I-70 corridor and push it
south as a cold front later tonight. Models indicated that
frontogenetic forcing is likely to bring increased snowfall
rates across the central and southern portion of the forecast
area as this occurs. Meanwhile, orographic forcing and mid-
level lift ahead of the trough will keep widespread mountain
snow going through midday Wednesday. Consequently, Winter Storm
Warnings will continue over the mountains with an additional 10
to 20 inches anticipated with up to 2 additional feet or more
across favored peaks and ridges. Aside from the south, areas
with rain are likely to see a transition to snow or a rain/snow
mix, though timing is difficult to nail down and as a result,
snowfall amounts as well. That said, felt that the possibility
for advisory level snow in the central I-70 corridor and the
Uncompahgre River basin was high enough to warrant adding those
areas to the existing slate of highlights.

Models differed with regard to drying and subsidence on
Wednesday afternoon behind the trough. The ECMWF has been
consistently shallower and faster with the passage of this
feature, and therefore generated significantly lower, and less
widespread PoPs. The NAM and GFS in contrast, showed a deeper
and colder trough and therefore, held onto PoPs a bit longer.
Expect the end result will be somewhere in the middle and model
blends seemed to handle this well enough for now. Highs on
Wednesday are a bit of mixed bag. Those areas, the northern and
central zones in particular, which were suppressed by steady
precipitation are likely to be a bit warmer than today as
drying takes over during the afternoon and allows some sun to
peak through. Meanwhile, the south which spent much of the day
noticeably warmer should see lower temperatures Wednesday
afternoon.

On Wednesday night, subsidence and drying in northwest flow will
bring an end to showers for much of the forecast area. Models
were split as to whether enough moisture remains across the
northern Colorado mountains to generate some light showers
during the night, but regardless, any additional accumulations
should be light.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 342 PM MST Tue Nov 26 2024

Despite the ongoing storm impacting pre-Thanksgiving travel,
conditions look much better for the holiday itself, as well as
the post-holiday travel. We remain under cold, dry northwesterly
flow through much of the period, keeping temperatures 5-10
degrees below normal into the weekend. A gradual warming trend
will kick in Sunday into early next week as ridging pushes into
the Great Basin, allowing temperatures to return to near-normal
values. The one thing that could put a wrench into this
forecast is the development of valley inversions in the wake of
the current system. If these inversions develop, then many lower
valleys will see below normal temperatures persist into next
week. So stay tuned. With much drier air in place through the
weekend, skies will remain mostly clear. The only place where
light snow showers could be possible during this period will be
the northern Colorado mountains. This area does well under
northwesterly flow, even when there`s hardly any moisture to
work with. And with a few ripples in the flow aloft currently
forecast to clip the northern mountains, some light and widely
scattered snow showers might form on Saturday and Sunday. If
this does come to pass, accumulations will be light, if any.
Looking out into the first week of December, quiet weather looks
to persist as ridging dominates the west and troughing controls
the east.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1029 AM MST Tue Nov 26 2024

Widespread rain and snow showers have reduced ceilings and
visibilities at nearly all terminals this morning. This trend
will continue, with LIFR/IFR prevailing at most sites. A few
locations, including KTEX, KDRO, and KCNY, will see MVFR to VFR
conditions prevailing over the next 6 or so hours. However
conditions deteriorate tonight as the cold front moves in,
affecting all terminals. Low level wind shear will impact higher
elevation terminals, including KASE, KEGE, and KGUC. In
addition, some gusty westerly winds will impact the higher
elevation sites this afternoon. Significant improvements are not
expected until after 18z tomorrow.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Winter Weather Advisory until noon MST Wednesday for COZ002-
     008.
     Winter Storm Warning until midnight MST Wednesday night for
     COZ003-004-009-010-012-013-017>019.
     Winter Storm Warning until noon MST Wednesday for COZ005-014.
     Winter Weather Advisory until noon MST Wednesday for COZ007-
     011.
UT...Winter Storm Warning until 6 PM MST Wednesday for UTZ028.

&&

$$

SHORT TERM...NL
LONG TERM...BGB
AVIATION...TGJT