Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 091137
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
437 AM MST Fri Dec 9 2022

.SHORT TERM...(Today and tonight)
Issued at 330 AM MST Fri Dec 9 2022

Currently, GOES-16 mid level water vapor showing moisture
entering western Colorado. Our short-lived ridge pattern comes to
an end today as a shortwave arrives in the mountains this morning.
Weak QG fields show ascent briefly this afternoon and moisture
remains meager as noted by the previous shift. With the
combination of weak lift, moisture, and dry air still linger in
the 500-700mb levels, expecting low snowfall accumulation under 1
inch for Rabbit Ears Pass and mainly flurries for Rocky Mountain
National Park and Park Range.

Winds from NAMNEST soundings display 30-40kt winds mixing to the
surface this afternoon across the plains. This pattern will lead
to gusty winds between 25-35 mph over a few areas of the plains.
Additionally, this may lead to elevated fire weather conditions in
the Palmer Divide and adjacent plains but winds will not remain
above 25 mph for a 3-hr period. See fire weather discussion for
more. Although there isn`t strong flow aloft yet, 45-50kts at
700mb late this morning through this afternoon will be enough to
produce wind gusts up to 45-55 mph for parts of the eastern
slopes.

NAM cross sections indicate 65-75kt westerly flow at 600-700mb
starting early this evening which will likely lead to brief
downslope winds given the favorable wind direction and increasing
subsidence entering through overnight. This means the foothills
and parts of the mountains could continue to see 45-55 mph gusts
overnight into early Saturday morning. Expect lows tonight to be
similar to yesterday; plains near mid to upper teens and the
mountains in the single digits to a few degrees below zero.

.LONG TERM...(Saturday through Thursday)
Issued at 330 AM MST Fri Dec 9 2022

...MILD WEEKEND FOLLOWED BY POTENT WINTER STORM IN THE AREA FOR
EARLY NEXT WEEK...

The main concern for this forecast period will center on the storm
system for late Monday-Tuesday Night and possibly into Wednesday
for parts of the plains. But first, we`ll discuss this weekend`s
mild and dry weather.

A flat upper level ridge will be building across the forecast area
Saturday, with mostly sunny and dry conditions prevailing. Winds
will also weaken with some decrease in the southwest flow aloft.
We`ll see temperatures respond to the sunshine and warm
advection, although low lying areas like eastern Larimer and Weld
Counties will likely struggle to reach maximum potential with
inversions holding in place and a little high cloud possible by
afternoon. That cooler air may attempt to surge into the northern
sections of Denver again with a weak shear zone setting up between
the warmer southwesterlies coming off the Palmer Divide and the
cold pool to the north. Therefore, the southern half of Denver
could very well reach 50F Saturday, while depending on amount of
high clouds we could struggle to reach 40F in Greeley and Fort
Collins.

Sunday will see high temperatures warm further, and we`re still on
track to reach 60F from about Denver across much of the Palmer
Divide toward Limon. Somewhat cooler temperatures again to the
north but even there should see modification with the warm
advection pattern and generally weaker inversions. Winds will be
increasing as the southwesterly mid level gradients strengthen
ahead of the deep upper low sliding down the Pacific Coast.

On Monday, all eyes will be pointing to that upper low as it moves
into the Great Basin. Despite the high amplitude pattern
developing, models and ensembles have remained quite consistent
and relatively tightly grouped for this far out per WPC ensemble
cluster guidance. That said, there has been a slight southward
drift in the overall ensemble averages over the last 24 hours,
which maximized in yesterday`s 12Z runs. This has been noted in
the average plume QPF/snow output, with the ECENS increasing
Denver QPF to around 0.35" on yesterday`s 12Z run, but only to
decrease it again to 0.20" on the most recent 00Z. Similar
ups/downs have occurred on the plains. It will be interesting to
see if these gyrations in the ECENS continue, or if the 12Z shift
was just a temporary trend to a more southern track. Meanwhile,
the GEFS has remained quite consistent with the upper low track
very near or just south of Denver, and near 0.40" liquid output at
Denver and amounts closer to 0.60" over the northeast plains.
While the averages are fairly steady, there are still and will
likely continue to be large run to run differences as the exact
track of the upper low will make all the difference in QPF output.
That can be seen in the individual member output with the vast
majority showing a lower end snow accumulation, but a few runs
really getting excited. Right now, it still appears that odds
favor the northeast plains for the higher likelihood of more
significant snowfall totals and impacts, while a few things would
need to come together (like a slower solution, farther south
track, and arrival of TROWAL all the way back to the Front Range)
to see a bigger storm along the I-25 Corridor. That`s a low end
probability at this point (as also seen in the ensemble output),
but still one worth watching as this storm is a few days away.
Even the northeast plains (e.g. Akron, Julesburg, Holyoke areas)
have a considerable amount of uncertainty as the overall highest
probabilities remain from portions of the Nebraska panhandle into
the Dakotas. While the bulk of the snow is expected to fall Monday
night and Tuesday, the northeast plains could see some snow
linger as late as Wednesday. Winds will be the other factor to
consider, as the deep, slow moving/ vertically stacked low will
bring a prolonged period of strong, gusty north/northwest winds to
the plains. If we do receive several inches of snow, then blowing
snow will also be a big problem with winds likely gusting to
around 50 mph.

One final note, there should be enough southerly flow to delay
the frontal passage a bit on Monday. Therefore, Monday could very
well be a sharp transition day from warm, windy, and heightened
fire weather concerns ahead of the front to much colder
temperatures and gusty northerly winds late in the day.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 432 AM MST Fri Dec 9 2022

Southerly winds will continue through Friday morning for all
terminals. Wind speeds will likely continue under 11kts through
the entire TAF period but KAPA and KBJC could receive 12-14kt
winds between 15-20Z as the shortwave passes. Otherwise, wind
direction will shift to the north mid afternoon shifting to south
by this evening. Expect drainage winds to occur overnight into
Saturday morning.

&&

.FIRE WEATHER...
Issued at 330 AM MST Fri Dec 9 2022

Elevated fire weather conditions are possible for the Palmer Divide
and adjacent plains as an incoming shortwave increases winds off the
Front Range. The combination of minimum relative humidity of
13-17 percent and wind gusts up to 19-23 mph could occur this
afternoon but winds are not likely to remain above 25 mph through
a 3-hr period thus no fire weather headlines in this forecast
package.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...AD
LONG TERM...Barjenbruch
AVIATION...AD
FIRE WEATHER...AD


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