719 
FXUS65 KBOU 190438
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
938 PM MST Thu Feb 18 2021

.UPDATE...
Issued at 935 PM MST Thu Feb 18 2021

Widespread fog has developed along the low lying areas near Limon,
affecting I-70 and down US 40/287 down to Hugo. With clear skies
and winds that will keep the moisture pushed into that low lying
area, will issue a Dense Fog Advisory overnight. Motorists should
anticipate poor visibility in that area throughout the night, with
improvement not expected until after sunrise.


UPDATE Issued at 812 PM MST Thu Feb 18 2021

With the fresh snow, clear skies, a few observations of fog, and 
some hints of continued reduced visibility in the high resolution
models, felt confident enough to put in mention of fog along the 
South Platte River Valley (SPRV) and southern Lincoln County 
tonight after midnight. Even though winds will be increasing 
tonight, enough decoupling in the SPRV should lead to light winds,
and southerly winds over southern Lincoln County will lead to 
slight upslope winds to help moisten up the airmass. 

Also increased windspeeds across most of the area for tomorrow as
the jet moves in, and with warmer temperatures expected, those
winds will be able to mix down. Still not a great mountain wave
profile, and with only 60 kt perpendicular winds at mountain top,
not expecting winds high enough and widespread enough to reach the
warning criteria of 75 mph in the high country. Will definitely
still see some 60+mph there, with gusts 30 to 40 across the
foothills, urban corridor and adjacent plains. 

&&

.SHORT TERM...(This evening through Friday)
Issued at 215 PM MST Thu Feb 18 2021

Today and tonight, there may be a few light snow showers on the 
high western slopes of the Front Range but otherwise the area 
should remain dry. Winds will increase in the mountains and high 
foothills late tonight and early tomorrow morning as a stable 
layer builds over the Front Range due to subsidence behind the 
exiting trough. There is a decent mountain wave signature in cross
sections across the Front Range. Forecast lapse rates above the 
mountains are very stable but ridgetop flow is only around 50 kts 
and not yet zonal enough to produce extreme wind. However, gusts 
in the mountains should reach up to 50 mph overnight and some of 
the higher foothills will probably see gusts up to 35 mph as well.
Overnight lows will be in the single digits in the mountains and 
most of the plains, while downsloping winds will warm the 
foothills and adjacent plains. 

Tomorrow, winds and gusts will continue to increase as flow aloft
increases and turns more zonal with the ridgetop stability still 
in place, though maybe slightly weaker. Gusts may reach up to 
around 60 mph in the mountains and 45-50 mph in the higher 
foothills by late morning or early afternoon. Snowfall will likely
increase in the mountains west of the Continental Divide again 
tomorrow morning and afternoon due to increasing northwest 
orographic flow and a broad jet to stream some moisture from the 
Pacific. The mountain peaks may accumulate a few more inches by 
tomorrow evening, but elsewhere very little accumulation should be
expected. Highs tomorrow will be warmer than today, in the mid to
high 20s F in the mountains and high 30s to mid 40s F on the 
plains.

.LONG TERM...(Friday night through Thursday)
Issued at 215 PM MST Thu Feb 18 2021

Will start the period Friday night under a strong (140+kt at 
250mb) upper jet streak. Snow showers over the mountains should 
initially decrease in coverage Friday evening and into the early 
overnight hours Saturday as the first subtle shortwave departs and
a brief period of QG subsidence spreads over the northern 
mountains. Still enough moisture near the surface along with 
favorable upslope winds to promote at least a chance of continued 
light snow showers over the higher mountains. The lull in activity
over the mountains should be short-lived, as a more notable 
shortwave trough enters the region by Saturday morning. Should see
much better lift across the region, combined with fairly steep 
lapse rates and a deepening moisture field, and POPs increase over
the mountains during the day Saturday. Meanwhile, strong winds at
ridgetop will promote the potential for wind gusts of 55 to 65 
mph over the higher mountains and mountain passes. While snow 
accumulations won't be particularly impressive, still could see 
some blowing snow from Friday night through Saturday night.

Forecast gets a little more challenging by late Saturday. 
Mountain snow will gradually wind down through the evening hours 
as the best forcing shifts east towards the plains. Meanwhile, a 
fairly strong surface cold front will push south across the plains
by late Saturday evening. There appears to be decent agreement in
the GFS/ECM/UKMET/GEM that there will be a brief period of snow 
along the urban corridor behind the frontal push. Models show weak
upslope flow (though not nearly as deep as yesterday's snow 
event) with good moisture and continued synoptic scale ascent 
overnight. Have bumped up chances for precipitation up a touch, 
but it's worth noting that both the higher resolution NAM and RGEM
show much in the way of precipitation. Confidence in the current 
forecast is low, and it is worth monitoring the setup for a round 
of light snow across the I-25 corridor.

By Sunday, we transition to northwesterly flow aloft. Cool 700mb 
temperatures will keep highs near or below 40 in most places on 
Sunday, but temperatures will rebound to the 50s, and perhaps even
60, on Monday and Tuesday. The next storm system arrives sometime
by mid to late week. Model solutions continue to vary from run to
run, but an cooler and unsettled weather pattern should be 
setting up for the second half of the work week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 812 PM MST Thu Feb 18 2021

Expect VFR conditions at all terminals through the TAF period. 
A jet will be pushing into the area overnight and into Friday.
This will increase mid to high level cloudiness  tonight and 
tomorrow, with wave clouds expected to the lee of the Rockies.
Between 12z-18z, this wave cloud may deepen and extend all the
way down to near 10kft MSL to the lee of the Front Range
Mountains. Drainage winds tonight, slightly enhanced by a
deepening lee surface trough, will then become gusty and out of
the northwest starting late tomorrow morning through the late 
afternoon. Peak gusts are currently expected up to around 35 kts,
strongest closer to the foothills such as at BJC.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 7 AM MST Friday for COZ046.

&&

$$

UPDATE...Kriederman
SHORT TERM...Direnzo
LONG TERM...Hiris
AVIATION...Kriederman