Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 201703
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1003 AM MST Mon Nov 20 2017

.UPDATE...
Issued at 1003 AM MST Mon Nov 20 2017

Minor changes to winds/sky cover for today. Latest GOES-16 trends
show high clouds streaming into northeast Colorado. The standing
wave clouds across Larimer/Boulder/Jefferson counties have
decreased thickness over the past hour which should help max temps
reach their current forecast highs. Made updates to the sky cover
to account for mostly cloudy skies this morning into early
afternoon. Bulk of the moisture is across Idaho right now and
should arrive in the mountains of Colorado around midnight.
Forecast for start of snow showers west slopes looks on track
based on latest HRRR and satellite/regional radar trends.

Other story today are the winds. A rotor (one that was
detectable using mesonet obs) set up this morning downstream of
the terrain gradient across Boulder/Jefferson Counties. Winds
along CO-93 were weak for a couple hours this morning while 5
miles east at KBJC they were howling. It`s essentially impossible
to predict a rotor`s movement but suffice to say within 5-7 miles
of the terrain gradient strong and gusty winds are possible all
day today, with gusts to 40 mph. The east slope foothills and
higher mountains will blow all day today with occasional gusts
near 60 mph. Red flag conditions should be met this afternoon
across the northern border with Wyoming as mixing transfers strong
winds to the surface. RH values are already low everywhere given
the strong downslope. Additionally, the 12Z Denver Sounding had a
meager 0.06 PWAT. Will keep the RFW going through this evening
for the northern border zones. Rest of the forecast for today
looks on track.

UPDATE Issued at 534 AM MST Mon Nov 20 2017

Updated for Red Flag warning for fire weather zone 238 this
morning. See fire weather discussion below.

&&

.SHORT TERM...(Today and tonight)
Issued at 343 AM MST Mon Nov 20 2017

Main concerns today will be strong winds in the mountains and
foothills along with elevated fire danger over portions of the
northern plains this afternoon.

Current observations showing wind gusts in the mountains and
foothills mainly in the 25-45 mph range with a few higher gusts of
50 to 65 mph over Berthoud Pass and wind prone areas of the
northern foothills. Radar showing some increase in wave
enhancement of the winds in the past hour and would expect that to
continue through daybreak. Latest model data continues to keep
the higher winds below 70 mph so will not issue any wind hilites,
especially given approach of upper jet and increased shear
profile, and lack of any strong subsidence today.

Temperatures will be a little warmer today but will go a few
degrees below MOS guidance given quite a bit of cloud cover
upstream which will continue to spread over the area. Still
readings into the 60s, well above seasonal normals for this time
of year.

For tonight, moisture depth increases in the mountains with
approach of upper wave/jet max with brunt of snowfall expected
after 06z. Decent orographic flow but lapse rates only in the
3-6C/KM range so will keep accumulations in the 1-4 inch range
tonight. May see a few rain showers on the eastern plains as
strong jet max aloft moves over the area. Main change to the
mountain forecast was to delay chances of snow until mainly later
this evening and overnight.

.LONG TERM...(Tuesday through Sunday)
Issued at 343 AM MST Mon Nov 20 2017

Snow in the mountains will be decreasing Tuesday morning as the jet
max over the forecast pushes southeast. The energy may produce a
few showers over the plains as a cold front pushes down as well.
Maximum temperatures will be similar to todays readings in the
mountains, but about 10 degrees colder behind the front over the
plains. The Jet core will push east of the area Tuesday evening
with the upper ridge over the western states to push closer. A
surface lee trough will develop overnight to keep minimum
temperatures moderated in and against the foothills.

Some light moisture rounding the top of the ridge will push into
the mountains in the favored northwest flow Wednesday for isolated
to scattered light snow showers. Otherwise, global models are in
agreement with having a dry and unseasonably warm period Wednesday
through the weekend as the ridge dominates. High temperatures
will generally be 10-20 degrees warmer than climatologically
average for late November. Readings will be in the 60s, and even
70s on Thursday and Friday over the plains. The mountains will see
highs in the 40s and 50s. Models then diverge with the breakdown
of the western US upper ridge for the beginning of next work week.

&&

.FIRE WEATHER...
Issued at 534 AM MST Mon Nov 20 2017

Fire zone zone 238 is already experiencing red flag conditions
so will start the warning now with low humidities and gusty
winds.

For the rest of the area, have upgraded the fire weather watch to
red flag warnings for portions of the northeast plains,
especially near the northern border areas with Wyoming and
Nebraska. Expect low humidity values and enough mixing to result
in gusty winds to 30 mph by late this morning. will not have a
red flag warning over the foothills as modest amounts of qpf
occurred a few days ago and land managers have deemed the southern
foothills not dry enough for rapid fire growth.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1003 AM MST Mon Nov 20 2017

VFR conditions throughout the TAF period. Main impacts will be
winds at the metro area terminals. Expecting a good deal of
turbulence for approaches/departures over the Rockies from now
through Tuesday morning with conditions favorable for mountain
wave development along and east of the Front Range Mountains. BJC
will have strong winds all day today under the influence of a
mountain wave and rotor that has set up. High res models and
observational data suggest the winds will once again stay up
tonight at BJC with ideal conditions supporting a mountain wave,
with gusts occasionally over 40 kts. Primary direction will be WNW
this afternoon and overnight. At DEN mixing throughout the day
should result in winds picking up after 19Z out of the WNW or NW,
but current thinking is to see gusts to around 25 kts. The winds
should come down to 12 kts or less at DEN by 03Z. APA should
remain at or below 15 kts throughout the TAF period, with the
strongest winds starting after 19Z and continuing through 03Z,
again with a WNW direction. On Tuesday wind speeds should remain
down at all three terminals until ~15Z with a surface cold front
passage and a wind shift to NE at 15-20 kts or so.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 5 PM MST this afternoon for COZ242-
248>251.

Red Flag Warning until 5 PM MST this afternoon for COZ238.

&&

$$

UPDATE...Schlatter
SHORT TERM...Entrekin
LONG TERM...Kriederman
FIRE WEATHER...Entrekin
AVIATION...Schlatter


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