827 
FXUS65 KBOU 192053
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
253 PM MDT Tue Oct 19 2021

.SHORT TERM...(This evening through Wednesday)
Issued at 245 PM MDT Tue Oct 19 2021

The center of a cut off trough is progressing eastward over
north-central Colorado this afternoon. Strong QG ascent along with
low to mid level frontogenesis are creating widespread 
precipitation over southeast Wyoming and into Weld and Larimer 
counties. Across the far northern mountains and foothills of 
Larimer County the precipitation is falling as snow. At lower 
elevations of Larimer and Weld counties, the precipitation is 
falling as rain. Webcams on I-25 just south of the WY/CO border 
confirm this although temperatures are in the mid 30s and could 
fall another couple of degrees. It's not out of the question that 
some snow starts to mix in with the rain right along the WY/CO 
border but no snow accumulation is expected. Further south, there 
have been some showers over the urban corridor and even a few 
lightning strikes in southern Weld County. The SPC mesoanalysis 
indicates a small area of 500 j/kg of surface based CAPE in the 
Denver area which is likely overdone but indicative of the weak 
instability that exists. Isolated showers will continue into the 
early evening and PoPs were increased to show that 20 t0 40 
percent of the area may get wet. 

There hasn't been much going on in terms of precipitation in the
mountains today as that area was in the dry slot. As the trough
continues eastward tonight, there will be some light snow showers
along the western slopes of the northern mountains. Any additional
snow accumulation should be less than an inch. 

The final impact to talk about from this system will be the wind
gusts. A strong pressure gradient will develop across the
northeastern plains this evening and gusts could reach up to 40 
mph as a result. A spot or two of blowing dust can't be ruled out 
in Lincoln and Washington counties. 

A ridge will quickly develop over Colorado tomorrow on the heels
of the departing trough. This will increase temperatures aloft and
result in much more stable conditions. Dry weather is expected
with a good amount of sunshine although there may be some cirrus
clouds by the afternoon. Highs will be 5 degrees warmer in the
mountains tomorrow and will be roughly the same as today across
the plains. 

.LONG TERM...(Wednesday night through Tuesday)
Issued at 245 PM MDT Tue Oct 19 2021

On Thursday through the weekend our weather will continue to be 
dominated by a ridge of high pressure at 500 mb. Starting with 
Thursday the ridge axis will be over western Colorado with WNW 
flow aloft. Temperature advection at 700 mb is neutral to just 
barely warming into the +4 to +5 degC range, so expect 
temperatures to warm a little vs Wednesday at the surface with 
mid 60s to near 70 across the plains and I-25 corridor, and 40s to
mid 50s for the mountain valleys and foothills. No chance of 
precipitation. In general the pressure gradient looks benign so 
winds should be fairly light other than across the northeast 
corner of Colorado.

On Friday the ridge amplifies thanks to a short wave trough 
moving onshore in northern California. This time there is 
appreciable warm advection at 700 mb with southwest flow, 
resulting in surface temperatures warming well into the 70s below 
6,000 ft elevation, with 50s for the mountain valleys and 
foothills. Drier air will advect from the southwest later on 
Friday into Saturday so there will be zero chance of 
precipitation. Winds should be a bit weaker than Thursday with a 
broad lee trough developing in the lee of the Rockies.

Models and ensemble solutions begin to diverge on Saturday and 
Sunday as the Pacific trough continues to move across the Rockies.
The GFS has a much deeper and more progressive trough moving 
across Wyoming by late Sunday. The EC has a long wave trough off 
the Pacific coast with embedded short wave troughs moving to the 
northwest of Colorado. In terms of the ensemble data, the EC only 
has a handful of members that generates mountain snow, and only 
light amounts at that, by late Sunday. The GEFS has roughly 1/4 of
it's members with some light mountain snow in that timeframe. 
Needless to say chances are low for mountain snow by Sunday 
evening, but will keep blended PoPs in place. Every ensemble 
member and available piece of guidance keeps the east slope and 
plains dry due to downslope flow through this weekend. The EC and
ensembles keep it quite warm all weekend with highs in the 70s 
across the plains, while the GFS and ensemble members hint at a 
cool down with the chance of a glancing blow from the cold pool of
a short wave trough. Will keep the blended solution which mirrors
the EC better at this junction with continued highs in 70s for 
Sunday across the plains, and 50s in the mountain valleys. Flow 
aloft increases significantly by late Sunday, leading to strong 
winds above 9,000 ft, and with incoming stability at mountain top,
ingredients are in place for a mountain wave event early Monday. 
Details and wind speeds way too early at this point however.

On Monday the models agree should be a warm advection day with 
strong southwest winds aloft and 700 mb temps warming to +11 to 
+13, which would result in highs in the low 80s (and dry east of 
the mountains). It's a long ways off yet but that's approaching 
record warmth for late Oct. There is a chance some moisture works 
ahead of the deep trough along the Pacific coast and the NBM 
indeed starts increasing PoPs throughout Monday, mainly for the 
west slope and higher eastern foothills. The main worry early next
week will be fire wx related with the strong and dry southwest 
flow coupled with unusually warm temps.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 245 PM MDT Tue Oct 19 2021

A few virga showers may create periods where ILS is needed this
afternoon. Ceilings should stay at or above 6 kft but the rain 
below the cloud deck may reduce slantwise visibility. Some of the
showers could create brief sporadic winds but overall, winds will
be out of the north. This evening, the low pressure system will
move out of the area and VFR conditions will return for the rest
of the TAF period. Wind speeds will decrease this evening and will
be light overnight and tomorrow.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 6 PM MDT this evening for COZ247.

&&

$$

SHORT TERM...Danielson
LONG TERM...Schlatter
AVIATION...Danielson