Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 211819
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1119 AM MST Tue Nov 21 2017

.UPDATE...
Issued at 1118 AM MST Tue Nov 21 2017

Our morning shower band is falling apart as the warm advection
aloft is kicking in. This is evident by the motion of the band
which has started to come back east onto the plains. Still a few
showers near the mountains for the next hour or two, but just a
few hundredths of an inch of rain if even that much. Character of
mountain precipitation has changed too. Now we have shallow shower
bands that should also gradually diminish as the subsidence
inversion lowers. There should be a quick end to these around
sunset. Temperatures cooled near the mountains with the showers
and frontal surge this morning, but there should be enough
clearing to get near our forecast highs this afternoon.

&&

.SHORT TERM...(Today and tonight)
Issued at 315 AM MST Tue Nov 21 2017

150 kt jet in northwest flow aloft is pushing into the state
early this morning, with the left exit region positioned over the
forecast area. Strong winds continue to create difficult travel
conditions in blowing snow in the high terrain. These winds are
also pushing down the lee slopes over the foothills and urban
corridor. Gusts of 30 to 50 mph will be slowly diminishing through
the morning. Snowfall rates will be at their max until about 6 AM,
then start trending downward as the best dynamics head south.
Looking at Snotel data, currently around 1 to 2 inches of snow has
likely fallen around the area, except over near Rabbit Ears Pass
where perhaps around 4 inches have fallen. Steamboat cameras
showing around 4 or 5 inches, favored by northwest flow. Expect
another inch or two to fall between now and 6 AM, then snowfall
rates will back off with lighter snow to fall through the rest of
the morning and afternoon.

The surface cold front associated with this jet has now just
pushed through Scottsbluff and Torrington in Wyoming. This front
will push into the plains around of after sunrise with cooler air
on northeasterly winds. This front will likely kick off isolated
to scattered rain showers mid-morning, with clearing by the
afternoon. Temperatures today will be 10 to 20 degrees cooler than
yesterday, back near seasonal normals. The plains should see upper
40s to low 50s while the high country will be in the 30s to low
40s as the snow tapers off. Some rain may mix in in the late
afternoon in the high valleys.

Mostly clear skies this evening will cloud up again through the
night as another disturbance in the northwest flow aloft pushes
down. Another lee side surface trough will deepen to bring more
gusty winds in the high terrain and foothills overnight. Gusts of
25 to 40 mph will moderate minimum temperatures to warmer than
average for late November. A few light orographic snow showers
will continue in the high country, mainly along western slopes
with little to no accumulation.

.LONG TERM...(Wednesday through Monday)
Issued at 315 AM MST Tue Nov 21 2017

Pattern becomes somewhat benign in the long term as the deep
ridge currently over the pacific will make its way NW pushing the
jet to the north of the state.

For Wednesday remnants of the jet will still impact portions of NE
CO with moisture moving over the top and bringing continued light
orographic rain and snow to the higher, west facing slopes of the
Park range and portions of the northern Front Range. Models show 700
mb temperatures staying mild so a mix of rain and snow with little
to no additional accumulation is expected. Temperatures will
gradually warm from Tuesday with highs in the mid to upper 60s. For
Thursday, high pressure will build with continued subsidence over
the region. A lee side low will develop Thursday allowing for a
continued downslope pattern to persist allowing for much warmer
then average temperatures. Model 700mb output puts temperatures 3
to 4 degrees above Wednesday highs with values in the low 70s.
Current record high for thursday stands at 74...this will be
flirted with.

For Friday into the weekend the jet will drop south bringing a piece
of energy over the northern mountains giving another round of rain
and snow to that region. On the plains, with the NW orientation of
the system areas will remain dry with gusty downslope winds.
Conditions will remain partly cloudy with highs in the low 60s for
Saturday and upper 60s on Sunday.

Monday will see a slight breakdown in the ridge bringing an upper
level tough into northern CO along with a cold front to the plains
by early Tuesday. Some precipitation will be possible in the
mountains will cooler temperatures on the plains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1118 AM MST Tue Nov 21 2017

Scattered showers near the mountains are keeping areas of MVFR
ceilings into the early afternoon. Expect drier air and the
dissipating showers to result in improving conditions by 21z.

&&

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

$$

UPDATE...Gimmestad
SHORT TERM...Kriederman
LONG TERM...Bowen
AVIATION...Gimmestad


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