830 
FXUS65 KBOU 081649
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1049 AM MDT Fri Oct 8 2021

.UPDATE...
Issued at 1038 AM MDT Fri Oct 8 2021

The upper level trough has transitioned into central CA and
continues to push northeast keeping Colorado under moderate
southwest flow aloft. Embedded in this flow is the upper level jet
that is circumventing the base of the trough. The nose of the jet
is moving into the northwest portion of the state increasing
moisture. This will being showers and a few embedded storms to the
northern and central mountains with some low end CAPE available
and lapse rates around 7.5 C/km. Some snow could mix in with the
rain above 11,000 ft that will drop into the overnight hours. Have
adjusted timing of pops to match current conditions but overall
the forecast is on track and reflects this well.

For the lower elevations, highs are still expected to reach into
the upper 70s under warm southwest surface flow with a chance of
some virga developing with showers off the higher terrain mainly
for areas north of I-70 by late this afternoon. Some gusting up to
30 mph may accompany these showers. 

&&

.SHORT TERM...(Today and tonight)
Issued at 434 AM MDT Fri Oct 8 2021

Upper level weather pattern will be active over the western U.S. 
through tonight. Satellite showing two strong short wave troughs, 
one off the California coast and a second one off the Pacific 
northwest coast. Ahead of these system strong southwest flow aloft
will prevail over Colorado with a ridge off to the east. The 
trough off the California coast races northeast today and closes 
off near the Montana/Wyoming border tonight. Best lift and 
moisture associated with this system stays to the north and west 
north central and northeast Colorado. However, the north central 
mountains should see some showers this afternoon due to some lift 
from the wave and weak instability. East of the mountains, expect 
a mostly cloudy day with some mid and upper level moisture making 
it over the mountains. Even with the clouds, temperatures will be 
mild today with highs in the mid to upper 70s. 

For tonight, Hi-Res models show banded precipitation over the 
mountains when the left exit region of the jet moves overhead. 
Can't rule out a few showers making it over the divide, but what 
survives will be weak. The strong southwest flow aloft will keep 
mild air over the state and snow levels high, above 10000 feet. 
Could see a few inches of snow above 11000 feet by Saturday 
morning. 

.LONG TERM...(Saturday through Thursday)
Issued at 434 AM MDT Fri Oct 8 2021

Saturday, there will be an increasing southwesterly flow aloft as
the upper trough moves to the Desert Southwest and the jet moves 
overhead. We'll see increasing winds across the area but 
especially in the high country. Expect peak gusts of 40 to 50 mph 
in the mountains and wind-prone areas of the foothills, and up to 
35 mph across much of the plains. As the jet moves to our south 
and east we'll be in an increasingly unstable airmass with cold 
advection aloft and steepening lapse rates, which will allow 
scattered to widespread showers showers to persist in the high 
country. Snow levels will continue to fall to around 9500 feet or 
so by Saturday evening. We should see a couple more inches of snow
in the northern Front Range mountains, and greater amounts are 
possible for the Gore and Park Ranges which will be favored by a 
more westerly flow. Downslope winds should keep the lower 
elevations mostly dry except for some isolated, weak showers. 
During the afternoon, dry conditions combined with strong and 
gusty winds will mean elevated fire danger for much of the plains.
We have issued a Fire Weather Watch for the areas just east of 
the Palmer Divide where critical fire danger is expected through 
the afternoon and into the early evening. This danger will 
decrease through the evening as winds and temperatures decrease.

Saturday night, a cold front will shift winds northerly on the 
plains and bring a hard freeze to the high country overnight, as 
lows fall into the 20s. Expect lows in the upper 30s and lower 40s
across the plains. Models have the trough splitting into two 
shorter waves late Saturday and the southern wave will move over 
the Four Corners early Sunday with strong forcing for New Mexico 
and southern Colorado. We should see a gradual decrease in showers
and wind through Sunday, with any lingering precipitation 
favoring the southern Front Range mountains and Palmer 
Divide. Temperatures will be cooler Sunday, with highs in the 60s
across the plains and 40s and 50s in the the high country. The 
next trough should be coming on shore over the Pacific Northwest 
by Sunday evening. Between troughs, there will be some light 
ridging and subsidence for Colorado through Monday afternoon with 
mostly clear conditions and a few degrees warming.

There is potential for our first strong fall weather system 
Tuesday and Wednesday. Model consensus is pretty good given the 
time range here, but forecast uncertainty remains high as small 
differences in the specific placement and progression of important
features such as the surface low will mean large differences in 
factors like low level flow and upslope forcing, temperatures, and
moisture, which will in turn lead to significant differences in 
our weather and impacts. Generally, it still looks like the trough
will dig quickly south into the Desert Southwest by late Monday 
and close off a low over the Four Corners early Tuesday, which 
will then move northeast over the Central Rockies late Tuesday 
into Wednesday. We could get some more cold air behind another 
cold front late Tuesday, and snow levels down to around 7000 ft by
Tuesday evening. If northeast Colorado does get significant 
precipitation from this system, it looks like it should begin in 
the late Tuesday to early Wednesday timeframe. However, ensembles
are still indicating a wide range of possible outcomes from dry 
and windy to significant snowfall, with even some light 
accumulations for parts of the plains. While the specifics are 
likely to change over the next few days, especially the timing, be
generally prepared for colder temperatures and some snowfall 
Tuesday and Wednesday. This will surely be a dynamic system with 
significant winter impacts somewhere across the Central and 
Northern Rockies, and right now it looks like we have a decent 
chance for northeast Colorado to be included. 

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1038 AM MDT Fri Oct 8 2021

VFR conditions are expected through the TAF period with an
increasing mid-level cloud deck around 080 for DEN and APA after
22z. Southwesterly winds will dominate and turn more south to
southeast after 18z. Continue to track a wind shift that is
expected to occur after 00z to the NNW with speeds from 8 to 10
kts. There will be some virga possible for the northwest gates due
to showers over the higher terrain to the west. Drainage is
anticipated after 04z. 

&&

.FIRE WEATHER...
Issued at 456 AM MDT Fri Oct 8 2021

Fire danger will be elevated for much of the lower elevations 
Saturday afternoon as strong and gusty winds develop across most 
of northeast Colorado. Peak wind gusts of 25 and 35 mph are 
expected across the plains, and between 40 to 50 mph in the 
mountains and foothills. We have issued a Fire Weather Watch for 
the plains just east of the Palmer Divide where these strong winds
are expected to be coincident with low humidity near of below 15 
percent, which could mean critical fire danger through much of the
afternoon and into the early evening. This danger will decrease 
through the evening as winds and temperatures decrease.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Saturday afternoon through Saturday 
evening for COZ246-247.

&&

$$

UPDATE...Bowen
SHORT TERM...Meier
LONG TERM...EJD
AVIATION...Bowen
FIRE WEATHER...EJD