FXUS65 KBOU 111746

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1046 AM MST Mon Dec 11 2023


- Areas of light snow and freezing drizzle developing over the
  northern plains (mainly north of Denver) this evening and

- Limited travel impacts expected, except northern border area

-  Still some uncertainty around cutoff low through the Southern
   Rockies Wednesday - Wednesday night, but most impacts should
   stay south of the area.


Issued at 1004 AM MST Mon Dec 11 2023

Forecast is moving along pretty much as expected, although there
is more sunshine initially this morning. That may also allow us to
warm a couple more degrees so have nudged the sky and temperature
forecast accordingly for today.

We're still keeping a close eye on the threat of precipitation and
type tonight. There is an organized short wave noted in southeast
Idaho, with fairly widespread snow, which is slowly tracking
east/southeast. This appears to be along an area of weak
frontogenesis and QG lift which is forecast to move across the
Wyoming border area this evening. That should bring an dusting to
two inches of snow to the far northern mountains, foothills, and
Wyoming border area of the lower elevations. Farther south,
moisture is far too limited. However, the precipitation in the
north this evening combined with a backdoor cold front and
shallow upslope could be just enough to moisten the low levels
sufficiently for light freezing drizzle. With generally east or
southeast flow, the highest odds/coverage of that should stay
right next to the foothills roughly from Boulder northward to Fort
Collins and the Wyoming border, as already indicated in the
forecast. Air temperatures only slip just a few degrees below
freezing overnight, and with quite warm pavement temperatures we
think any impacts would be minimized with the exception being the
northern Front Range Foothills and I-25 Corridor north of Fort
Collins. Bridges and overpasses farther south to about Boulder
could still be impacted however.

That light freezing drizzle could last into Tuesday morning but
any upslope forcing is really light. Points from Denver to the
east and south would likely stay dry with only low clouds.


.SHORT TERM...(Today and tonight)
Issued at 215 AM MST Mon Dec 11 2023

Water vapor imagery showing a persistent and moderate northwest
flow pattern across Colorado with a steady stream of mid/high
level moisture. There is some orographic impact for the mountains
with enough low level moisture for occasional light snow over the
mountain passes. Observations and web cams showing a general 1-3
inches of new snow has fallen since this afternoon. Given the
moisture and upstream disturbance, far northern mountains could
see up to 3 inches of additional snowfall.

Water vapor imagery also showing next weak wave now moving across
far northern California and northwest Nevada at this time. This
feature is progged to move across Northern Colorado later this
afternoon and this evening. Ahead of the wave, enough mid/high level
moisture to keep at least partly cloudy skies ongoing today across
lower elevations. Temperatures today will be similar to Sunday's
readings, except a bit cooler over far northern sections.

There is some weak QG ascent tonight, especially over the far
northern border areas with Wyoming and Nebraska. There is a weak
associated cold front which will backdoor into the Front Range
overnight tonight with some shallow upslope flow developing.
Airmass saturates in the low levels, then becomes more shallow
overnight and into Tuesday morning. This means a mixed bag of
precipitation types with possible rain/snow in the evening,
leading to some freezing drizzle later in the night as moisture
depth becomes more shallow. Forcing and moisture will favor precip
chances over Larimer and Northern Weld counties, mainly staying
north of Denver. In addition, the southeast winds will favor
additional lift over Larimer and Weld counties with the better
chances for precipitation. Mountain moisture decreases tonight as
the wave moves east of the mountains with drying expected west of
the Continental Divide.

.LONG TERM...(Tuesday through Sunday)
Issued at 215 AM MST Mon Dec 11 2023

An upper level trough will move into the Great Basin on Tue with SW
flow aloft across the area.  At the sfc, cold high pres will be over
the Central Plains.  Soundings show some potential for light
freezing drizzle thru mid morning on Tue from Denver north to the WY
border and in the nrn foothills.  Otherwise cross-sections show mid
and higher moisture embedded in the flow. Could see a few snow
showers over the higher mtn peaks but the rest of the area should
be dry in the aftn. Highs will be in the mid 30s to lower 40s
across the plains.

For Tue night into Wed, the upper level trough will move SSE from
the Great Basin into the swrn US.  This will allow for a more
southerly component to the mid level flow.  Cross-sections show
abundant moisture from 700 mb to 400 mb, however, orographics are
unfavorable over the higher terrain so any snow shower activity that
does occur will be very light.  Across the plains, the low level
flow will be southeast.  Can't rule out a slight chc of rain or snow
showers over the far sern portions of the CWA by aftn, however the
rest of the plains will be dry.  Highs over nern CO will be in the
upper 30s to lower 40s.

Looking ahead to Wed night and Thu, the GFS tracks an upper level
low from sern AZ across srn NM with most of the precip staying south
of the area.  The ECMWF has a slightly further northward track
across central NM which allows some influx of precip across sern
areas of the CWA mainly Wed night.  Still not sure about the exact
track of the low so will leave in 30%-50% pops over sern areas.
Highs on Thu will be near seasonal normals.

For Fri into Sat an upper level ridge will begin to build into the
area.  This will lead to a dry pattern with warmer temperatures.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1046 AM MST Mon Dec 11 2023

VFR conditions will persist through 04Z-06Z with only increasing
middle and high level cloudiness. We do think ceilings will lower
to around 5000 feet AGL toward 03Z-04Z, and then drop to MVFR
ceilings closer to 06Z-08Z, a little sooner at KBJC. With regard
to precipitation, the airmass overall is quite dry in the Denver
area, but enough weak upslope to support just a slight chance of
flurries or light freezing drizzle once the lower stratus deck
develops. KBJC would see a little higher chance and enough to
warrant a VCSH there.

With regard to winds, we're starting this period with southeast
flow, eventually pushing more easterly through this afternoon
(about a 30% chance we go more northeast). Then easterly should
hold most of this evening until the backdoor cold front moves
through and Denver cyclone matures 03Z-08Z. Winds will turn more
north or northwesterly at the TAF sites through that time frame,
and should hold there through much of Tuesday as long as the
cyclone is displaced to the southeast as expected.




SHORT TERM...Entrekin