Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 151724

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1024 AM MST Thu Feb 15 2018

Issued at 1024 AM MST Thu Feb 15 2018

Current conditions show a tremendous amount of variability in the
weather across the forecast area. Even in the mountains, some
areas have seen only light snowfall and even a few periods of
sunshine, while others are getting heavy convective showers with
snowfall rates upwards of 2" per hour. The main band of convection
is clearly indicated on radar and GOES 16 satellite imagery from near
Grand Junction up the I-70 Corridor to around Eagle and then into
eastern Grand county. The band is slipping slowly southward (with
the jet and mid level cold front) and should reach the I-70
mountain corridor through Summit County from Vail Pass to the
Eisenhower Tunnel through early afternoon. Worst travel
conditions can be expected there this afternoon into early
evening. Elsewhere, there is still large scale lift occurring
ahead of a weak trough moving through western Wyoming and northern
Utah, so expect snow to continue through early evening. Will keep
current warnings for the mountains going with expected travel
impacts, locally heavy snow, and blowing snow into early evening.

On the plains, a strong cold front has pushed through central
Wyoming and the northern Nebraska panhandle, and is on schedule to
move across northeast Colorado this afternoon. There may be slight
slowing during the peak heating hours in the afternoon, but still
expect a good push as very strong 3 hour pressure rises of 6 mb
are noted behind the front. Temperatures will quickly fall into
the lower 30s behind the front, with snow showers also developing
in unstable upslope regime. We have adjusted PoPs upward on the
plains since the airmass is rather unstable and convective snow
showers are likely. Could see anywhere from a trace to 3 inches
from Denver south/southeast across the Palmer Divide, with the
heavier accumulations favoring Elbert and northern Lincoln


.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 18Z TAFS through 18Z Friday afternoon)
Issued at 1024 AM MST Thu Feb 15 2018

Anticyclonic flow had developed as suspected this morning. We`re
still not certain how winds will turn this afternoon, but we do
favor a more northeasterly flow through the period. Winds should
gust to 22-30 knots when stronger northeast winds arrive behind
the cold front around 21Z-22Z. Then look for snow showers to
develop and become more widespread with up to 1 inch of
accumulation possible given the convective nature of showers, and
up to 2 inches from KAPA southward. Visibilities could briefly dip
to 1/2SM with the heavier showers. Stratus deck with IFR/MVFR
ceilings will also develop behind the front, and should then erode
later in the night as winds turn back more southerly 06Z-12Z.


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



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