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afdbou

Area forecast discussion
National Weather Service Denver/Boulder Colorado
432 am MST Mon Nov 18 2019

Short term...(today and tonight)
issued at 432 am MST Mon Nov 18 2019

Gusty winds continue across the higher mountain passes and across
the eastern foothills to the west sides of Metro
Denver/Boulder/Fort Collins in Boulder, Larimer, and Jefferson
counties. The enhanced west- northwest winds are driven by a
strong pressure gradient with a 1022 mb high west of The Divide
and a 1010 mb Lee trough along the I-25 corridor. A little bit of
stability is present above ridge tops to help force the winds down
the east slopes and across the adjacent plains. Gusts exceeding 40
mph are possible for several more hours across the foothills of
Jefferson, Boulder, and Larimer counties before the pressure
gradient relaxes later this morning.

A weak short wave trough continues to shift east of our area today,
with continued northwest flow aloft through tonight. The air on the
backside of the trough within the northwest flow is quite dry and
subsident, and thus any lingering light snow across The Divide and
Park Range should end by 8 am or so. It will remain dry today and
tonight under the influence of dry northwest flow aloft. There is
very weak cold advection at 700 mb across the northeast corner of
Colorado, closest to the long wave trough across the central U.S.
Neutral temperature advection looks to be the norm today and
tonight elsewhere. Thus, with decent sunshine and downslope flow
continuing, expect above normal temperatures today with highs in
the 60s across the plains, and mid 40s to low 50s in the high
country. Northwest flow near the surface will dominate today, with
winds in the 15-25 mph range across the northeast plains. Light
east winds should prevail across the I-25 corridor with the
development of an surface anticyclone across SW Weld County given
northwest flow off the Cheyenne Ridge. With less wind across the eastern
foothills and adjacent plains, expect tonight's low temperatures
to be much cooler than this morning, closer to seasonal norms.

Long term...(tuesday through sunday)
issued at 432 am MST Mon Nov 18 2019

Tuesday will be the last of the warm days for awhile as an upper
level ridge axis moves over the state. Light to moderate westerly
flow aloft in the morning will transitions to southwesterly by
evening as an upper level low over Baja California begins to track
northeastward. Moisture levels over the state will be on the
increase Tuesday night and Wednesday as the upper level weather
system approaches from the south. The upper low from Baja
California is forecast to rapidly shear apart as it moves over
Arizona, but the moisture and forcing for ascent out ahead of the
system will begin producing snowfall over the Colorado mountains
starting early Wednesday. By Wednesday afternoon light
precipitation is expected to have spread out onto the northeast
Colorado plains. Development of precipitation on the plains is
going to be aided by the arrival of a cold front from the north
Wednesday morning. Deep upslope flow is expected to develop behind
the front with surface temperatures falling below freezing in the
evening. High temperatures on Wednesday are expected to be about
20 degrees cooler than Tuesday afternoon, and then Thursday
afternoon will be even colder.

Model cross sections and soundings for Wednesday night show a
saturated airmass up through almost 500 mb with upslope winds up
to 700 mb. All of this will help to produce light snow from the
mountains out across the plains Wednesday night. Thursday and
Friday will see a continuation of cool and unsettled weather as an
upper low and digging trough over California on Wednesday move
inland and toward Colorado through Friday morning. Medium range
models have different timing on when the second upper level
disturbance clear out of Colorado with the European model (ecmwf) solution being
the slowest, and Canadian and GFS being more progressive. General
consensus is that moderation of temperatures and drier weather
will arrive over the weekend, as even the European model (ecmwf) has the upper low
east of Colorado by then.

Snow amounts for the Wednesday through Friday time period are
expected to be light for now since the initial surge of moisture
with the first upper low is expected to have its main impact over
Arizona and New Mexico. Then moisture with the second system will
be limited by the system having its origins over the northeast
Pacific. Accumulations will be light, but the period of snowfall
may be as much as 60 hours, from Wednesday through Friday. No
highlights are expected at this time.

&&

Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 432 am MST Mon Nov 18 2019

VFR through Tuesday. West-northwest winds will remain strong
through mid morning, with gusts to 20 kts at apa and den, and as
high as 25-30 kts at bjc. The pressure gradient weakens later this
morning and the mountain wave enhancement also weakens, resulting in
wind speeds dropping to around 10 kts out of the northwest. By this
afternoon, northwest flow off the Cheyenne Ridge should develop a
Longmont anticyclone, with weak east winds at the terminals.
Drainage winds return tonight.

&&

Bou watches/warnings/advisories...
none.
&&

$$

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