Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 202124
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
324 PM MDT Tue Oct 20 2020

.SHORT TERM...(This evening through Wednesday)
Issued at 315 PM MDT Tue Oct 20 2020

Mid to upper level moisture in west-northwest flow aloft continues
to produce wave clouds, most prominent over and downwind of the
Front Range Mountains. A dry cold front has made its way all the
way west across the plains and into the very lower foothills,
concentrating a lot of the smoke from the wildfires against the
foothills. This front should remain where it is this evening,
before a wiggle in the flow aloft will begin to induce a lee
surface trough bringing westerly winds back out of the higher
terrain. Not as strong as last night, so lows should be able to
cool off more tonight.

Winds aloft will be turning more west-southwesterly tomorrow as up
upper shortwave trough drops south into the Pacific Northwest.
This will bring in warmer and drier weather, with winds remaining
gusty. This will increase fire danger for the mountains, see more
in the discussion below. Less wave cloud action will help for
warmer temperatures over the plains, however with active fire
behavior expected again, smoke plumes may limit some readings from
getting too high mainly over Larimer and Boulder counties.


.LONG TERM...(Wednesday night through Tuesday)
Issued at 315 PM MDT Tue Oct 20 2020

Falls arrives in the extended but first have to get through a little
more elevated fire weather concerns. 500 mb flow starts with broad
nw flow over the western US and a weak shortwave trough diving
southwest out of the pacific nw. Trough axis reaches Idaho/Wyoming
border by 12z Thursday and brushes northern Colorado during the day
Thursday and moves on Thursday  night. Models pessimistic about
precip on Thursday afternoon now with this feature, but have left
chc pops in the northern mountains and northern plains. Have never
been bullish on precip amounts with this feature these last few
days, and although I still think we`ll see a dusting of snow in the
higher terrain and rain/snow mix on the plains, will be light qpf
amounts. Regardless, any precip and higher RH will be a help to
firefighters.

Of more note with the Thursday system is the first cold front that
will begin our cooldown to more late fall-like temperatures. Lows
Wednesday night in the 30s and 40s will give way to teens and 20s
Thursday night. One more day to wrap up that cold weather prep.

Low level flow on the plains turns east during the day Friday, then
back to south, while higher elevation winds tap more into the upper
westerly flow and stay that direction through the end of the week.

Heading into the weekend our next system starts the process of
carving out a deeper trough in the west. Initial shortwave trough on
Saturday with surface front passes by mainly to the north, with
another shot of northern mountain/northern plains snow. this
shortwave is kicked on east as a second shortwave deepens a
positively tilted trough over the western US. ECMWF deeper/slower as
it has been all week, and even wants to close off a low near
southern CA/AZ Monday. Jet dynamics will come into play
Sunday/Monday but right now best 100 Kt jet is forecast to be over
southern CO out of our forecast area with better pops that area.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 315 PM MDT Tue Oct 20 2020

Winds are easterly to northeasterly behind a weak front. Expect wind
speeds to slowly decrease through the afternoon and turn to normal
drainage patterns later tonight, leading into southerly winds
tomorrow morning. West winds may mix down again during the day and
afternoon.  The East Troublesome fire is still burning hot and
putting up a smoke plume. There have been brief, sporadic periods of
reduced surface visibility and pilots have reported reduced
slantwise visibility at DEN. The front did help to clear out some of
the smoke at the surface though it seems to have now concentrated
much of that smoke in the foothills. Easterly winds in the low
levels behind the front should keep smoke at the surface away from
most terminals for a few more hours at least, though smoke aloft
will persist. VFR conditions expected through the rest of the TAF
cycle except at terminals very close to the foothills, such as BJC
where reduced visibility is still being reported. We will continue
to keep smoke in the TAFs without visibility reduction this
afternoon, except in the current line of BJC.

&&

.FIRE WEATHER...
Issued at 315 PM MDT Tue Oct 20 2020

Near critical fire conditions over the high terrain with gusty
wind and low humidity should be improving this evening as winds
decouple in the high mountain valleys, and an cool front pushes
into the foothills from the east to improve humidity. However, a
surface lee trough will be forming later this evening to begin the
westerly winds again decreasing humidity recovery into the early
morning hours. Warmer and much drier conditions are expected
tomorrow, including less wave cloud activity. Winds will be
slighty weaker, however will still be at criteria. Therefore, have
issued a Red Flag Warning for Middle and South Parks for humidity
near 10% and winds gusting 25 to 35 mph. Over the northern
foothills, conditions may be marginal, but with a trend involving drier
humidities expected, and the ongoing fire activity, have included
Fire Weather Zone 215 in the Red Flag Warning. In addition, even
drier dewpoints are expected to move in tomorrow evening and
overnight, and a mountain wave signature will show up overnight.
This will result in extremely poor humidity recoveries overnight
with winds reaching 40 to 50 mph over the mountains and higher
foothills. Have therefore extended the Red Flag Warning into
Thursday morning before a cold front moves down from the north.

Elsewhere Wednesday night, modest RH recovery into the 30s in the
mountains and 40s-50s on the plains. On Thursday min RH in South
Park back down to the mid teens, with 20s and higher elsewhere. A
weak upper trough to move through late Thursday will bring light
snow over the higher terrain and northern plains. Liquid amounts a
few hundredths at best but will help RH recovery into the 60-90
percent range. Westerly flow over higher terrain will continue
with wind 20-30 mph, although improving humidity. Seasonably cold
temperatures to continue from Thursday onward, and a second trough
will bring more moisture to the region for the weekend. Minimum
RH during the day improves from Friday onward, with good RH
recovery during the nightime hours.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon to 7 PM MDT Wednesday for COZ213-214.

Red Flag Warning from noon Wednesday to 8 AM MDT Thursday for
COZ215.

&&

$$

SHORT TERM...Kriederman
LONG TERM...Hanson
AVIATION...Kriederman
FIRE WEATHER...Kriederman/Hanson


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