Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 151112

Area Forecast Discussion
National Weather Service Denver/Boulder CO
412 AM MST Thu Feb 15 2018

.SHORT TERM...(Today and tonight)
Issued at 400 AM MST Thu Feb 15 2018

Radar and web cameras snow picking up in the mountains in the
past few hours. A 160 knot jet is producing bands of heavy snow
across the mountains. Outside of these heavy snow bands, snowfall
is generally light at this time. Cold air advection will help
steepen lapse rates and increase orographic lift across the
mountains today. Additional snowfall amounts of 3 to 7 inches are
expected across the mountains through this evening. Web cameras
indicate many mountain roads are snow covered. This should
continue through the day. The higher passes will see wind gusts to
50 mph which will produce blowing snow and reduce visibility.
Snowfall decreases this evening and will end tonight.

For the Front Range and eastern plains, northwest flow will lead
to cold air advection today and lead to cooler highs. A cold front
will drop south across the area late this afternoon and early
evening. Gusty northeast winds will prevail behind the front.
Areas south of I-70 will see the best chance for snow this evening
due to the northeast upslope flow. The ECWMF shows a couple
inches of snow over the higher terrain southwest of Denver, while
other models show less than an inch. Increased pops for this
evening in this area. Snowfall of up to an inch expected with a
couple inches possible just southwest of Denver. Drier air moves
in late tonight and should bring snowfall to an end shortly after

.LONG TERM...(Friday through Wednesday)
Issued at 400 AM MST Thu Feb 15 2018

Highlights during this period: High country snowfall Friday night,
possible very strong Front Range winds Friday night and Saturday
night, strong warming on the plains this weekend, and then a big
cool down beginning Monday with a good chance of snow most areas
Monday and Tuesday.

First off, drying and subsidence aloft in the wake a departing
shortwave trough embedded in moderate northeast mid-level flow
should end most high country snowfall and clear skies most areas on
Friday. But, still do not see much warming yet since air temps aloft
remain little changed from the previous day. The break in the
snowfall will be short-lived as another disturbance on the nose of
an 85+kt jet speed max swings down from swrn Wyoming late in the day
Friday. Moisture carried along in this flow will combine with
orographic forcing and weak QG ascent over west facing mtn slopes to
produce mainly light snowfall Friday night. Accumulations will be
spotty and light, with most of it occurring before midnight. After
midnight, post trough drying and warming kicks in at mid-levels
which should diminish snowfall on the West slope. This warming aloft
will strengthen the mtn top inversion layer leading to an amplified
leeside mtn wave. Model cross sections indicate a cross barrier
speed maxima of 70-80kts over the upper east slope the Front Range
late Friday night. Fortunately do not see much of the high velocity
momentum working down to the base of the foothills. That said,
ridgetops and high foothill areas could see a 6-7 hour period of
50-75 mph wind gusts.

Over the weekend, the local airmass will warm and dry out even more,
particularly east of the mtns with a downslope gradient flow in
place. Could see a repeat of the strong and gusty winds over higher
elevations of the Front Range Saturday night as the right front quad
of a 100+kt speed maxima passes over the area. Strong mtn top static
stability, the product of warming aloft, should once again produce a
high amplitude mtn wave capable of 50-70 mph gusts on mid and upper
east slopes of the Front Range overnight. The adjacent plains will
benefit from this downslope slope with warmer temperatures Saturday
and Sunday. Looking for highs both days in the 50s, with warmest
readings on Sunday.

Models show this warm and dry pattern breaking down Sunday night
into Monday as the large upper trough carving out over the Pacific
Northwest/northern Great Basin continues to slide south bringing
with it a goodly amount of maritime moisture and cold air. As this
trough deepens and moves closer to Colorado, a surface cold front
slips down over northeast Colorado Monday morning sending temps
downward. Should this scenario come together as most of the guidance
indicates, the high country should see snowfall spreading over the
region through the day on gusty west-southwest winds. Lower
elevations may also get into the act by afternoon, first up along
the Wyoming border, then in areas farther south as a cold nely
upslope flow deepens. This system does not look like a big snow
producer, but the light precip and cold temps it ushers in may could
possibly linger through Tuesday. Right now, Wednesday and Thursday
appear warmer, but not necessarily drier as the GFS and ECMWF show
another fetch of Pacific moisture heading our way from the
California coast. With low confidence in the models that far out,
will only mention a slight chance of snow in the high country and
keep things dry and mild elsewhere.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 400 AM MST Thu Feb 15 2018

Winds will be challenging today. Northwest flow will prevail
today. Some models indicate winds will anti-cyclone and turn
northeasterly ahead of the approaching cold front. Eventually
winds will become northeast behind the cold front, around 01Z.
There will be a chance for snow mainly Denver south and from
00-06Z. The higher terrain to the south of Denver will have the
best chance. KDEN will be on the edge of snowfall. If any occurs
it will be light. Plan on going with a tempo for snow at KDEN and
KBJC between 00Z and 06Z. Will continue with the prevailing snow
at KAPA. Snowfall amounts will be light with a skiff at most
expected at KDEN. Around an inch will be possible at KAPA. Drier
air will move in and clear skies after 06Z.


Winter Storm Warning until 6 PM MST this evening for COZ031-033-



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