433
FXUS65 KGJT 170942
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
242 AM MST Fri Jan 17 2025

.KEY MESSAGES...

- A pattern change arrives by the weekend as an Arctic front
  traverses the area this evening into Saturday bringing snow
  and very cold temperatures.

- A strong arctic outbreak is expected to close out the weekend
  into early next week across the central CONUS. The coldest air
  should remain east of the Divide but temperatures on this side
  are forecast to be well-below normal. Overnight lows have a
  high probability of dropping well-below zero in many mountain
  and high valley/basin areas.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 242 AM MST Fri Jan 17 2025

The flow will shift to west-northwest as a cut off low over
SoCal opens up and tracks across Arizona early this morning and
into New Mexico. This is evident with low clouds passing through
the southwest Colorado valleys this morning but not much else.
Elsewhere, clear skies are leading to efficient radiational
cooling, especially over those deeply inverted cold pool basins
with Gunnison already well below zero with temps in the negative
teens, while the Yampa River valley sites are in the single
digits to around 5 degrees below zero at Craig. Westerly flow
ahead of a strong arctic front dropping in from the north this
evening will usher in one more mild-ish day before temperatures
come crashing down this weekend into next week as an arctic air
mass settles in over much of the region with the coldest
temperatures along the Divide and areas east. To account for
very cold persistent temperatures in the cold pool basins and
particularly Gunnison and Yampa River Basin, trended highs and
lows through the full 7 day period to the colder guidance to
better reflect the colder air trapped here, anywhere from the
5th percentile in the near term to the 25th to 50th percentile
for the Day 3 through 7 period.

Snow showers will develop on the leading edge of this arctic
front late this afternoon and evening with potential for snow
squalls across northwest Colorado as this front drops southward.
The best period of lift looks to be overnight tonight into
Saturday morning but unstable northerly flow behind this arctic
front in addition to some weak instability and steep lapse rates
should lead to snow continuing through Saturday afternoon. The
combination of convective element as well as favorable
orographics and cold temperatures down to -20C at H7 should lead
to efficient dendritic snow continuing to fall over the Park
Range, Gore and Elk mountains with high SLRs and efficient snow
accumulation despite the lower QPF numbers...even though these
numbers are trending a bit upwards as higher res guidance comes
in. Therefore, decided to include the Flattops (COZ013) and
Gore and Elk Mountains (COZ010) to the Winter Weather Advisory
for 4 to 8 inches of snow expected. Amounts also came up for the
Park Range (COZ004) where 5 to 12 inches of snow is expected
with locally higher amounts above timberline where the
orographics and efficient temperatures could lead to a bit more,
so the Winter Weather Advisory here looks good. However, decided
to extend the time out to 5 pm Saturday to account for the
continued impacts to travel given the very cold air mass and
potential for continued efficient snowfall beyond what the
models can resolve. Another area of concern to watch out for is
the Uncompahgre Gorge area of the Northwest San Juans where
light north-northwest flow and favorable temperatures in the
dendritic range could lead to a bit more snowfall here Saturday
morning into the early afternoon behind this arctic front.
Confidence is lower for advisory amounts here but can see this
scenario as a possibility.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 242 AM MST Fri Jan 17 2025

While the bulk of precipitation should be past us by sunset on
Saturday, flurries to mild accumulation could continue into the
evening and overnight hours. While snowfall during this period
will have minor to negligible impacts, we`ll be turning our eyes
to the frigid temperatures settling in over the region.
Mountain regions and upper valleys are likely to drop to sub-
zero temperatures Saturday night with some locations expected to
reach -20 to -30F. Upper-level flow remains north to
northwesterly into early next week and cold air advection will
keep our temperatures well below normal. Deterministic models
suggest a weak shortwave will drop southwards across western
Colorado around late morning Monday. Some flurries may be
expected with this, but more notably, this little bit of upper-
level support could keep dense cloud cover over the region late
Sunday into Monday morning, potentially bumping low temperatures
up a few degrees, but still remaining below normals. This brief
warm-up doesn`t seem to be much of a trend though. While
confidence in exact details is shaky at the moment, Tuesday
morning is shaping up to hold most extreme bout of abnormally
cold temperatures. Upper valleys could be sitting under strong
inversions thanks to the recent snowfall, and skies are expected
to clear as ridging begins to build over the area. This sets up
for a potentially nasty night of wind chills around -40F over
multiple high-elevation locations across the CWA. Models aren`t
agreeing too well on what follows this, though a relatively
strong warm-up shortly after this chilly period seems likely.
Some models are also hinting at some late-week precipitation,
but little can be commented on this at the moment given the lack
of model agreement.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 933 PM MST Thu Jan 16 2025

Mostly clear skies,and light, terrain driven winds will prevail
through 18z tomorrow. After that point, look for increasing
cloud cover, and lowering ceilings for higher elevation TAF
sites and those north of I-70. Winds south of I-70, particularly
at KDRO, with be southwesterly and gusting 20-25 knots during
the afternoon. VFR conditions will prevail through the period.
Snow showers will begin creeping in late tomorrow evening, with
drops below ILS breakpoints and into MVFR conditions possible
after 00z, mainly for KVEL, KHDN, KEGE, and KASE.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Winter Weather Advisory from 4 PM this afternoon to 5 PM MST
     Saturday for COZ004-010-013.
UT...None.

&&

$$

SHORT TERM...MDA
LONG TERM...BW
AVIATION...TGJT