601
FXUS65 KGJT 241309
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
609 AM MST Tue Dec 24 2024

.KEY MESSAGES...

- Mild conditions continue today ahead of the next storm which
  will begin to impact the region tonight through Christmas Day.

- The Christmas Day storm is expected to bring accumulating
  mountain snow with amounts ranging from 2 to 6 inches with
  locally higher amounts on favored peaks and ridges.

- A series of disturbances will bring periods of snow during the
  latter part of the week, through the weekend and into early
  next week with the northern mountains favored.

&&

.UPDATE...
Issued at 609 AM MST Tue Dec 24 2024

Issued a Dense Fog Advisory for northwest Colorado through 10 AM
MST this morning. KHDN, KCAG and K4V0 were all reporting
visibilities below 1/2 mile in fog which is likely to persist
until daytime warming causes it to dissipate.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 322 AM MST Tue Dec 24 2024

Subsident northwest flow settled over the region in the wake of
yesterday`s system. Very light snow continued over the northern
and central portions of the Continental Divide according to ski
webcams, but should be coming to an end by sunrise. Meanwhile,
low clouds were slow to clear over northwest Colorado and
northwest facing slopes in the central mountains. Areas of fog
were also present at KCAG, KHDN, KSBS and KEEO as a result of
rain showers last evening which served to saturate the near
surface layer.

A transitory ridge moves over the region ahead of a deep Pacific
trough which moves ashore this morning. As this system moves
farther inland this afternoon the ridge axis is forced east of
the forecast area causing flow aloft to shift to the southwest.
This will allow temperatures remain mild for most locations,
though high clouds pushing out ahead of the western trough are
likely to inhibit warming to some degree. The Gunnison Valley
remains the exception where the shallow surface inversion should
reform and temperatures are expected to fall back into the 20s.

Light snow is expected to begin over the higher terrain of
northeast Utah this evening then into western Colorado later
tonight as the trough moves from the Great Basin to just east of
the forecast area by 12Z Christmas Day. Modest atmospheric
river remnant moisture moving ahead of the system fuels light
showers during the night in the area of ascent ahead of the
trough. Increased cloud cover and showers should bring somewhat
milder overnight lows.

The system then moves across the region on Christmas Day with a
low circulation forming in the base of the trough moving along
the southern UT/CO border. Favored lift over the southern
mountains north of this feature should lead to better snowfall
totals as compared to the rest of the forecast area. That said,
amounts associated with this system aren`t expected to be
significant with generally 3 to 6 inches and locally higher
amounts expected across the southwest San Juans, and roughly 2
to 4 inches elsewhere. Temperatures in the valleys will largely
be warm enough to support rain. In the Gunnison Valley and other
higher elevation valleys snow amounts are expected to be less
than an inch.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 322 AM MST Tue Dec 24 2024

Snowfall will continue over the western Colorado mountains
Wednesday night into Thursday morning, but snowfall rates drop
off considerably as the Christmas Day storm exits onto the
Front Range and Plains. This system is fairly progressive as
another Pacific low upstream pushes into the PacNW, helping
drive this trough eastward with dry northerly flow pushing in on
the backside by Thursday morning. We will see a bit of a break
Thursday morning and afternoon as a shortwave ridge slides over
the region with increasing cloud cover ahead of the next Pacific
trough. A series of waves moves through the PacNW and into the
Intermountain West late this week into the coming weekend, with
the storm track and jet shifting north. This active pattern
looks to result in persistent rounds of snowfall favoring the
northern Colorado mountains under a favored west to northwest
flow. Timing these waves appears problematic where snowfall
rates could be enhanced, but regardless, favored orographic flow
should keep snow piling up in the northern Colorado mountains,
especially the Park Range with some of these waves potentially
reaching the central mountains. Temperatures throughout this
late week period heading into the coming weekend remain mild
despite these shortwaves across the north with highs averaging
around 10 degrees above normal for late December. Will have to
keep an eye on model output and the strength of these shortwaves
in terms of snowfall accumulation for the northern mountains as
this pattern would favor them, but timing, strength and model
discrepancy leads to lower confidence in forecasted snowfall
totals until we get closer to nailing these details down.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 451 AM MST Tue Dec 24 2024

Low clouds and fog will impact portions of northwest Colorado
resulting in IFR/LIFR conditions at KHDN through 15Z. Similarly,
low clouds in the upper Colorado River basin and the Roaring
Fork Valley will keep ceilings below ILS breakpoints through
midmorning or later at KEGE and KASE. Thereafter, VFR conditions
will prevail with high clouds pushing through the ridge over the
area. Clouds increase and lower tonight ahead a system poised
to move through the area Christmas Day, though ceilings should
stay above ILS breakpoints. Winds will remain light.

Special note, ceilings at KEGE indeterminate due to erroneous
sky sensor, so amendments will only be issued for wind,
visibility and weather.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Dense Fog Advisory until 10 AM MST this morning for COZ001-002.
UT...None.

&&

$$

UPDATE...NL
SHORT TERM...NL
LONG TERM...MDA
AVIATION...NL