Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 250208
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
708 PM MST Sat Feb 24 2018

.UPDATE...
Issued at 706 PM MST Sat Feb 24 2018

The lee side pressure gradient continues to weaken, and as a
result wind speeds along the Front Range and out across the
plains have likewise decreased. Should see this trend continue fo
the remainder of the evening. Have also noticed a drop off in
winds in the high country past couple of hours in response to a
steady increase in instability and Q-G forcing for ascent out
ahead of the shortwave trough passing over Utah attm. Low and mid-
level cloud shield spreading out ahead of this trough is finally
reaching western portions of the Denver/Boulder CWA. Precipitation
falling from this cloud cover has yet to reach the forecast area
as per area radars and sfc observations. However, at its present
forward speed, should see the leading edge of this precip in the
form of light snowfall reaching Grand, Summit and Park counties
within the next couple of hours. The heavier snowfall intensities
are still back along the Utah/Colorado as per radar loops.

As the left front quad of a 110 kt jet maxima passes over
southwest Colorado later this evening, mtn areas in north central
Colorado, especially along and west of the Continental Divide
should undergo a steady increase in snowfall coverage and
intensity. a west-southwest upslope flow will combine with
elevated instability to generate snow totals anywhere from 2-6
inches on mtn tops and favored windward slopes, will isolated
amounts of 6-9 inches in the Park Range north of Rabbit Ears Pass
by late Sunday afternoon. The heavily traveled I-70 corridor from
Vail Pass to Georgetown could see anywhere from 1 to 4 inches,
with the higher amounts up around the Eisenhower-Johnson Tunnel
and Vail Pass. In addition, west to southwest winds of 20-30 mph
and higher gusts to as high as 55 mph may also produce areas of
blowing and drifting snow which could further slow travel,
especially over the higher passes.

Elsewhere, conditions will remain dry overnight with gusty west-
northwest winds transitioning to their typical drainage flow
pattern within the next hour or two.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 138 PM MST Sat Feb 24 2018

Very cold air is overhead today as the trough axis from an
underperforming short wave trough departs the state. Another weak
short wave digs south out of the northern Rockies tonight.
Temperature advection at 700 and 500 mb is consistently neutral
through Sunday, and it will remain cold aloft through Sunday
evening when the second trough axis departs Colorado. Based on
GOES-16 water vapor, the cores of both the first short wave
trough currently across western Kansas and the second approaching
trough across Idaho have 500 mb temps around -33 to -34 degC. 700
mb temps remain between -13 and -15 degC across our area. Unlike
the past several events the jet stream or a jet streak will not
play a role in vertical motion across our area, thus increasing
overall confidence in where and how much snow may fall across the
mountains. The remaining challenge is the amount of convective
available potential energy tonight and tomorrow morning. With the
pool of cold air aloft, lapse rates are steep across the
mountains, favorable for convection. Most of the synoptic scale
lift will remain south and west of our area. The west upslope
combined with instability should result in a decent snowfall
across the Park Range, where isolated amounts to 6-8 inches are
possible through Sunday evening. Most areas across the northern
mountains should see a few inches. The I-70 corridor from
Georgetown to Vail Pass could see a trace to 3 inches, with the
highest around across the Eisenhower-Johnson Tunnel and Vail
Pass. The short wave trough responsible for the snow chances
tonight into midday Sunday departs quickly and PoPs drop to near
0 area-wide by Sunday evening. Expect a cold and breezy day at the
ski areas and the rest of the high country with high temps near
20 at and above 9000 feet and wind gusts 30-40 mph. Areas of
blowing snow are likely tonight through much of Sunday across the
high mountain passes and other wind prone areas. The high mountain
valleys will be less windy with high temps in the upper 20s.

Across the Plains, downslope flow will continue eliminating any
chance of precipitation through Sunday evening. The strong west
and northwest winds extend across all of our Eastern Plains,
continuing through 5 PM or so. The currently strong bora winds
will continue for a few more hours before the sun angle decreases
and the boundary layer decouples. Until then areas of blowing snow
mainly west of I-25 (especially northern Larimer County and the
CO-93 corridor from Boulder to Golden) will occur across roadways.
Late this evening and over night winds will be generally light
and temperatures seasonably cold, with the coldest temps across
the lowlands of Weld County where winds are lightest and there is
still a good snowpack. In terms of high temperatures Sunday,
despite second day of downslope the cold air aloft and continued
snow cover (where it exists) will keep high temperatures in check
on Sunday. Mid 30s to low 40s are expected.

.LONG TERM...(Sunday night through Saturday)
Issued at 138 PM MST Sat Feb 24 2018

WLY flow aloft will develop Sun night and then become more SWLY on
Mon.  Cross-sections show limited moisture although there could be a
few light snow showers in zn 31 Sun night.  Otherwise a mtn wave
will develop Sun night and linger into Mon morning before weakening
by midday.  Cross component flow is only in the 30-35 kt range so
may see a few gusts in the 50-60 mph range in the foothills and
higher mtns, however, high winds are not expected. Temps will be
warmer on Mon with highs in the upper 40s to lower 50s over nern
CO.

On Tue the flow aloft will remain SWLY as an upper level storm
system develops over the swrn US.  Cross-sections don`t show much
moisture thru the day so will no mention any pcpn.  Across nern CO a
weak cool fnt may affect the plains, however, highs will stay in
the 45 to 50 degree range in most areas

For late Tue night thru Wed the upper level low over the swrn US
will move eastward across New Mexico as a secondary weaker system
affects nrn CO.  In addition a cold front will move across nern CO
on Wed.  Overall best QG ascent will stay over srn CO into New
Mexico so any pcpn over nrn CO will be on the light side.  As the
weaker system in the nrn branch moves across there will be a period
of light snow in the mtns late Tue night into Wed.  Across nern CO
there could be a chc of light snow mainly south of a Denver to Akron
line on Wed.  Highs on Wed will be in the upper 30s to mid 40s over
the plains.

By Thu a flat upper level ridge will develop across the area with no
pcpn expected.  As a sfc lee trough forms nr the foothills downslope
low level flow should allow for warmer temps as highs rise into the
upper 40s to lower 50s across nern CO.

For Thu night into Fri the flow aloft will become more WSW as the
sfc lee trough strengthens over ern CO.  There is a weak disturbance
along with some embedded moisture in the flow which affects the mtns
Thu night thus will keep in a slight chc of snow.  Over the rest of
the area it will be dry.  Highs on Fri will be in the 50s to around
60 degrees across nern CO.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 706 PM MST Sat Feb 24 2018

Gusty westerly winds within the Denver Metro Area have weakened
considerably during the past hour, and they`ll continue to do so
as they gradually shift to their usual south-southwest drainage
component with speeds generally under 15 kts. It may take a bit
longer for speeds to drop off at KBJC near the foothills. However
they too, should fall off by around 05z this evening under clear
to partly cloudy skies overnight.

&&

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

$$

UPDATE...Baker
SHORT TERM...Schlatter
LONG TERM...RPK
AVIATION...Baker



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