107 
FXUS65 KBOU 080252
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
852 PM MDT Thu Oct 7 2021

.UPDATE...
Issued at 824 PM MDT Thu Oct 7 2021

Broad southwest flow aloft associated with the longwave trough
currently off the West Coast continues to push mid and high level
moisture into Colorado, with ample clouds tonight keeping
temperatures a few degrees milder than last night. Still on track
for an increase in moisture for Friday, which should bring some
isolated showers in the afternoon primarily to portions of the
high country. No significant forecast changes this evening.

&&

.SHORT TERM...(This evening through Friday)
Issued at 259 PM MDT Thu Oct 7 2021

An upper level trough currently off the Pacific West coast has 
dominating southwest flow over the state. There is slight ridging 
over the county warming area with weak QG subsidence in place 
bringing party cloudy skies and dry conditions. Conditions will be 
similar overnight and into early Friday with lows dipping into the 
upper 40s to lower 50s on the plains and mainly 30s in the high 
country. 

By Friday afternoon the upper trough will have moved onshore to CA 
keeping the region in southwest flow aloft. Through the morning and 
into the afternoon the jet circumventing the base of the trough will 
push into western CO with increasing moisture. This will start light 
showers over the higher terrain of mainly the northern mountains by 
the afternoon with areas above 11,000 ft seeing some light snow. As 
the trough and jet feature continues to push northeast the 
precipitation will become more widespread and continue into the 
overnight hours across the mountain region. At lower elevations for 
Friday conditions will remain dry with some increasing mid and upper 
level clouds by the afternoon. Temperatures will be similar to
today. 

.LONG TERM...(Friday night through Thursday)
Issued at 259 PM MDT Thu Oct 7 2021

...STRONG AUTUMN STORM POTENTIAL TUESDAY - WEDNESDAY NEXT WEEK...

We'll be watching next weeks strong storm system carefully over
the next few days, more on that below. In the meantime, we'll 
have some unsettled weather to contend with, mainly over the 
mountains with rain and snow showers. Gusty winds Saturday, as 
well. 

For Friday night, a strong 140 knot upper level speed max and 
associated short wave will kick out of the Desert Southwest. There
is enough moisture to provide more numerous showers in the 
mountains, especially along and north of a Rocky Mountain National
Park to Vail line. Orographics are best suited for the Northern 
Gore and Park Ranges, while the rest of the forecast area is 
pretty strongly blocked by southwest flow aloft. Thus, we don't 
expect much more than a dusting to 2" in most of our high alpine 
areas as snow levels dip to around 10,500 feet. 

On Saturday, the upper level speed max sinks to our south, putting
us in a slightly more unstable airmass with cold advection aloft
and steepening lapse rates. Snow levels should continue to fall, 
dropping to around 9500 feet or so during the day. We should see 
at least a couple more inches of snow in the northern mountains, 
and perhaps slightly higher than that over the northern Gore and 
Park Ranges which again will be setting up better from an 
orographic perspective (more westerly flow). Winds will also be 
increasing with the cold advection, mixing, and downslope pressure
gradient in place. We'll likely see wind gusts around 30-35 mph 
across most of the plains, with 40-50 mph peak gusts in the 
mountains and wind prone areas of the foothills. Most of the 
plains should stay dry given this downslope component, although 
not impossible for an isolated shower to survive. 

For Saturday night and Sunday, the upper level trough splits and
weakens as it drops southeast across Colorado. As a result, we
should see a gradual decrease to mountain snow showers and winds,
while the plains remain on the dry side. Perhaps a few showers for
the Palmer Divide area but overall appears the trend is for a
mainly dry forecast for much of Sunday. Temperatures will be
cooler, with highs by Sunday only being in the mid 60s across the
plains, with 40s to lower 50s most of the high country. 

Now, onto the main forecast problem...the large storm dropping 
into the Southwest U.S. early next week, and then lifting 
northeast across the Central and Northern Rockies late Tuesday 
into Wednesday. The ensembles have been quite consistent with 
regard to the placement and timing of the upper level low and 
driving that into the Utah/Arizona border by 12Z Tuesday. From 
there, things become more uncertain. That said, about 60% of the 
ensemble members are leaning toward a deeper and slightly slower 
southern solution, while about 40% lean toward the north. The
eventual track may actually end up dependent on the exact  
timing/amount of latent heat release that occurs as this storm 
taps into Gulf of Mexico moisture, and then heads northeast. We 
don't see much downstream blocking, but a high amplitude ridge 
should keep it on the slower side. Overall, the ECMWF and its
ensemble members have remained fairly consistent, with that track
showing a near miss for much lower elevation snow, but a bigger
shot of accumulating snow and gusty winds/blowing snow for the 
mountains and even the northern foothills late Tuesday into 
Wednesday. It's certainly hard to get caught up in the details 
given just a small change in track will mean a big change in 
expected weather. It was interesting to note the large variations
in the ensemble output, from the dry and windy conditions to a 
significant snow event (upwards of a foot) for parts of northeast 
Colorado. We have a feeling we won't know a whole lot more 
regarding the resultant weather for at least a couple more days 
given that a very small change in track would have a significant 
difference in expected weather. One thing is for sure, this will 
be a dynamic system with significant winter impacts somewhere 
across the Central/Northern Rockies. Stay tuned for the latest 
forecast updates! 

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 824 PM MDT Thu Oct 7 2021

Southeast winds over the next few hours will transition to
prevailing southwest drainage flow overnight. Friday morning,
winds should be relatively light and variable, with a gradual 
shift to east/southeast winds in the afternoon and evening. SCT 
mid level clouds at/above FL080 likely for much of the period, 
along with abundant high clouds. VFR conditions likely to prevail 
through Friday evening at all terminals.

&&

.FIRE WEATHER...
Issued at 259 PM MDT Thu Oct 7 2021

Fire danger will increase Saturday as strong, gusty winds develop
across most of the forecast area. Peak wind gusts will likely 
range between 25 and 35 mph across the plains, and between 30 and
45 mph over most of the mountains and foothills. However, humidity
readings will be increasing. At this time, the lowest humidity
readings and thus elevated fire danger is expected to occur over
the Palmer Divide area eastward through Lincoln County. 

&&

.HYDROLOGY...
Issued at 204 PM MDT Thu Oct 7 2021

Light showers will likely move over the burn areas late Friday
afternoon through Friday evening, and then again Saturday, but 
rainfall amounts and rates should be rather light. Thus, no hydro
impacts are expected for this period. Looking ahead, a large storm
will move into the area next Tuesday and Wednesday. However,
temperatures will turn cold enough to produce mostly snow over the
burn areas. There would be just a small chance of heavier rain
showers on the front end of this system late Monday or Tuesday. 

&&

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

&&

$$

UPDATE...Rodriguez
SHORT TERM...Bowen
LONG TERM...Barjenbruch
AVIATION...Rodriguez
FIRE WEATHER...Barjenbruch
HYDROLOGY...Barjenbruch