Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 040945
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
345 AM MDT Tue Oct 4 2022

.SHORT TERM...(Today through Wednesday)
Issued at 345 AM MDT Tue Oct 4 2022

A weak midlevel circulation that had separated from the main Pacific
low currently centered over the Northern Plains remains just to our
south early this morning. This shortwave will lift across southwest
Colorado this afternoon, which, acting on continued above normal
precipitable water values, will result in another round of scattered
shower and thunderstorm activity this afternoon. The bulk of
activity will be centered over the San Juan Mountains where PWATs
remain near 150% of normal. Even so, other areas along and south of
I-70, particularly the higher terrain, will still have the potential
to receive storms as PWATs remain elevated for early October. As the
lower levels of the atmosphere gradually dry out, gusty outflow
winds will begin to take over as the primary threat from convection
though brief heavy rain will certainly still be possible. Additional
light snow accumulations are expected mostly above 11,000 feet.
The shortwave will skirt east of the area by this evening as a
ridge of high pressure building over the western CONUS continues
to progress further inland. With the lack of support aloft, shower
activity will quickly come to an end by sunset, though similar to
tonight an isolated shower cannot be ruled out. Drier air will
filter into the region from the north with PWATs sitting at or
potentially slightly below normal by sunrise Wednesday for much of
the region. With the drier air, mostly clear skies are expected
for most areas beginning Tuesday night and continuing into
Wednesday morning. Given the clear skies, strong radiational
cooling will occur overnight which will see low temperatures
plummet in response. I say plummet...values will still sit near
and even slightly warmer than the typical norm for this time of
year. However, this will be a drop by at least 10 degrees for some
areas compared to just a couple nights ago. All the same, low
temperatures will reach the freezing mark for some of the lower
valleys in northwest Colorado. Guidance has come into better
agreement regarding this, though there are still some discrepancies
regarding coverage and duration. Based on the latest projections,
opted to issue a Freeze Watch with this morning`s package for the
Upper Yampa River Basin. This may need to be expanded to include
other zones but we`ll give the models one more go before making
the final call for highlights later this afternoon. Either way, as
we progress further into the fall season it`s not a bad time to
start making preparations all the same for your vegetation, swamp
coolers, and the like.

An exception to the notable drying trend will be portions of
southwest Colorado where PWATs will remain around 125% of normal.
This is due to continued weak moisture advection from the previously
mentioned shortwave which will linger across eastern New Mexico
through Wednesday evening. As a result, look for another round of
afternoon convection centered over the higher terrain as orographics
will be the primary lifting mechanism for activity. Therefore,
once the sun sets, showers will dissipate.

.LONG TERM...(Wednesday night through Monday)
Issued at 345 AM MDT Tue Oct 4 2022

Deterministic model guidance continues to grapple with a west coast
ridge and a pinched off low over the southwest Wednesday night.
Complications arise as well when a series of waves drops in over the
Great Lakes and pushes height falls back southwestward across the
plains. For now, it looks like the ridge holds its own in this
grudge match. The cutoff upper low feature will keep a pool of
tropical moisture on our doorstep along the southern edge of the CWA
through the extended period, while mostly northerly flow aloft dries
out the northern three fourths of the CWA. The southern mountains
and higher terrain along the continental divide should hold a chance
of afternoon shower/thunderstorm development in their forecasts
through the remainder of the week and into Monday. Ensemble
guidance, mentioned here earlier, did trend drier in the latest
runs, which gives pause to the hopes of shower/storm development.
For now, the ample surface moisture from daily convection this past
week seems like a bird in hand for afternoon storms across the
mountains. Afternoon high temperatures will hold at or above
climatology through the extended period. Overnight lows will trend a
little cooler in the north thanks to the intrusion of drier air up
that way. Higher dewpoints down south will help keep freezing nights
at bay, even in cold spots like Gunnison.

Model agreement really comes apart Saturday and Sunday with another
Canadian system trying to whittle away at the ridge. A frontal
passage Sunday and Monday would likely bring some more cooler air,
along with a widespread round of precipitation if this solution can
hold together. Shoulder season transitions and a nonexistent jet
stream for any support leaves one with low confidence in a significant
weather producer. Nonetheless, our station on the front side of
the ridge leaves us a thread of hope for a few shortwave weather
producers and hopefully a more decisive shift in the overall
pattern late this weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1048 PM MDT Mon Oct 3 2022

Dry weather will prevail across the region tonight with some
passing clouds. These will remain above ILS breakpoints into
Tuesday. An exception is the potential development of stratus
which may form by sunrise as moisture lingers in the valleys. High
pressure will begin to build overhead on Tuesday resulting in
continued quiet conditions. The exception will be the southern
mountains where residual moisture will fuel isolated to scattered
afternoon convection around midday and continuing into the early
evening hours. The main threat from any storms would be gusty
outflow winds to 30 kts.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Freeze Watch from late tonight through Wednesday morning for
     COZ005.

UT...None.
&&

$$

SHORT TERM...MMS
LONG TERM...LTB
AVIATION...TGJT


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