Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 240244
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
844 PM MDT Fri Oct 23 2020

.UPDATE...
Issued at 844 PM MDT Fri Oct 23 2020

Strong winds are thus far confined to above 10,500 ft across the
Continental Divide. Berthoud Pass gusted to 64 mph recently,
indicating the strength of the flow aloft. Latest RAP/HRRR model
cross sections continue to show a very weak mountain wave, which
is good news for a good portion of the fires. What this means is
the strongest west winds are not likely to punch below about
9,000 ft tonight. However, the wind forecast was tweaked a little
upwards given recent observational and high res model trends.
Gusts to 60 mph are possible down to 9,000 ft through the early
morning hours. The lower elevations will need some mixing during
the day on Saturday to get strong wind gusts. Still looking like
gusts 40-50 mph across the west/east slope foothills and mountains
above 6,000 ft during the early afternoon hours. Wind grids were
also just barely tweaked to account for this. RFW was updated with
the most current expected winds. It it still going to be active
tomorrow for most of the fires given the winds and lower RH. The
rest of the forecast looks on track for tonight/tomorrow.

Just wanted to give a brief update on the potential evolution of
the storm system late Saturday into Monday. Still a good deal of
uncertainty in QPF in the latest model runs and ensembles. GFS and
ensemble members are much quicker than EC, brining the deep cold
air in (and deeper NE upslope) midday on Sunday, while the EC and
ensemble members are slower with the deep cold air/deeper NE
upslope (late afternoon Sunday), and they keep the snow going
well into Monday hanging a trough to our west much longer. Will
still need banding across the plains and I-25 corridor to get the
higher amounts, and the 18Z runs have not helped improve any
confidence on the favored location(s). Confidence is as high as it
can be that it will be very cold Sunday through Tuesday morning.
It could get to -20C at 700mb Sunday evening, normal for the
winter months. Brrrr.


&&

.SHORT TERM...(This evening through Saturday)
Issued at 330 PM MDT Fri Oct 23 2020

The cool and moist airmass over the plains and foothills is
holding in place this afternoon as expected. That will begin to
erode tonight across the higher elevations as stronger westerly
flow aloft develops. Cross mountain flow impinging on the
mountains increases to 40 to 50 knots, but we don`t see much
mountain wave amplification so the strongest gusts should stay up
high. However, they should reach 60-70 mph over higher mountains
considering the brute force of the flow. Meanwhile, we should see
increased mixing leading to gusty winds to around 40 mph and
higher fire danger in the high mountain valleys. Some of the
higher foothills could see gusts of 40 to 50 mph, but the lack of
lee troughing should keep the stronger winds confined to the
higher elevations.

There should be some breaks in clouds after this evening`s upper
level wave move through. Temperatures will probably be a little
colder than guidance at least on the plains considering some
breaks in the clouds and current readings. Tomorrow`s high will
show decent warming ahead of the storm, with highs pushing into
the 60s along the Front Range, but cooler temperatures the farther
north and east you go across the plains where the cold airmass
will be slow to scour out due to the large surface anticyclone
only slowly retreating across the Central Plains.

.LONG TERM...(Saturday night through Friday)
Issued at 400 PM MDT Fri Oct 23 2020

A system will bring snow and much needed moisture and higher
humidity to our forecast area beginning Saturday night. This will
drastically decrease fire activity for all fires in our CWA.

A strong, positively tilted trough will progress southeastward from
the Pacific Northwest towards Colorado Saturday night into Sunday.
As this trough approaches, it will slow its progression with a
portion of the trough breaking off and becoming a closed low over
Arizona. This is not the typical mid level setup for a major winter
storm in the Denver area but other factors are more favorable with
regards to snow. At the surface, a strong cold front will move
across northern Colorado Saturday evening with sharp temperature
drops behind it. Frontogenetical forcing will create a band of
snow showers that moves southeastward across the area Saturday
night. During the day on Sunday and into Sunday night, persistent
QG ascent along with Pacific moisture will create widespread snow.
In addition, a developing jet streak over the Northern Plains
will position our forecast area under the lift from a right
entrance region which may lead to heavy bands of snow especially
throughout the afternoon and evening Sunday. It will be tough to
predict the exact location of the heaviest snow bands and we may
not know their location until only a few hours before they
develop. This is where a lot of the uncertainty in the snow amount
forecast will come from. Not surprisingly, models disagree about
the location of the snow bands with the ECMWF keeping the heavier
bands north of Denver while the GFS keeps the heavier bands south
of Denver. The QPF forecast is basically a smoothed out version of
the different model solutions. As for snow ratios, model
soundings indicate a very large dendritic growth zone potentially
reaching from the ground to 500mb during the afternoon on Sunday.
This should result in snow ratios of up to 15-16 to 1 during the
latter half of Sunday. With all that being said, we are expecting
8 to 14 inches of snow in the mountains, 5 to 12 inches across the
mountain valleys and foothills, 4 to 8 inches across the Urban
Corridor, and 4 to 6 inches across the plains. Therefore, we
issued a Winter Storm Watch for all of our CWA except for southern
Lincoln and Elbert Counties. It begins in the northern mountains
and mountain valleys at midnight Saturday night and begins at 6AM
elsewhere. It ends at 6AM Monday morning for all locations.

Another important aspect of the forecast will be the cold
temperatures. The high on Sunday will end up being in the low 20s
or upper teens in Denver which could set a record cold max
temperature. Lows will reach below zero in the mountains Sunday
night with wind chills as low as -25 degrees. Across the plains,
lows will be in the single digits and could reach below zero in a
couple spots. Any fire personal or evacuated people sleeping
outside will need to be aware of these very cold temperatures to
avoid adverse affects.

There is uncertainty in the ending time of the snow on Monday
morning but any additional snow showers that do occur will be
light. Highs Monday will almost certainly set the record cold max
temperature in Denver since the record is much warmer than the
previous day.

A mid to upper level ridge will develop over the Intermountain
West starting and Tuesday and lasting through the following
weekend. This will lead to a warming trend with dry weather
throughout. Fire danger should remain on the lower side during the
extended but could start to increase if the snow cover melts by
the end of the week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 844 PM MDT Fri Oct 23 2020

VFR conditions should persist throughout the night and until late
Saturday evening. Southeast winds currently at DEN 10-12 kt will
gradually transition to SSW at about 10 kts. APA is trending
towards drainage (SSW at APA), while BJC is almost calm. Expect
BJC to have very light drainage 5 kts or so out of the W. Tomorrow
winds will briefly pick up out of the west or WNW at APA and BJC
as mixing increases in mid-afternoon. Expect 10-15 kts. DEN is
most likely to remain E or ENE from late morning through the
afternoon, increasing to about 10-12 kts during the afternoon
hours. By early evening all terminals should be NE or N. A strong
cold front will move across the terminals between 7-9 PM,
increasing NNE winds 15-20 kts with gusts to 25 or 30 kts. Stratus
and ILS conditions will follow rapidly behind the front, but snow
may hold off until 3 - 6 AM Sunday. Very cold temperatures,
accumulating snow, and ILS to maybe IFR conditions will be the
norm Sunday into early Monday.


&&

.FIRE WEATHER...
Issued at 330 PM MDT Fri Oct 23 2020

High clouds have kept temperatures a little cooler, therefore
reducing mixing and gusty winds in most areas west of the Front
Range today. However, those winds will become stronger over the
mountains tonight, and then spread across the valleys on Saturday
with less significant inversions than we saw today. Humidity will
actually be a little higher (20-30%), but considering the
magnitude of winds and fires burning in heavy fuels we have a Red
Flag Warning in effect until 7 PM Saturday. Peak gusts could reach
50-60 mph in higher forested areas, with gusts up to 40 mph
expected in lower elevations and high mountain valleys.

Meanwhile, a cooler and moist airmass remains across the foothills
and plains. That will begin to retreat across the foothills late
tonight and Sunday as flow aloft strengthens.  Warmer and drier
air along with gusty west winds should progress slowly eastward
down the east slopes of the higher foothills, mainly above
8000-8500 feet late tonight, but then possibly into lower
elevations to around 7500 feet on Friday. The strongest winds
develop very late tonight and Saturday, and a Red Flag Warning
will remain in effect for the foothills, starting at midnight
tonight.

Snow and much colder temperatures are then expected to spread
from north to south across the area Saturday night and Sunday.
Snow is then expected to decrease during the day Monday. Most
likely, we`ll see several inches of snow in the mountains and
foothills with local accumulations of a foot or more possible. On
the plains, a few inches will be possible.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday morning through late Sunday night
for COZ034-036>046-048>051.

Red Flag Warning from midnight tonight to 7 PM MDT Saturday for
COZ215.

Winter Storm Watch from late Saturday night through late Sunday
night for COZ030>033-035.

Red Flag Warning until 7 PM MDT Saturday for COZ211>214-217-218.

&&

$$

UPDATE...Schlatter
SHORT TERM...Barjenbruch
LONG TERM...Danielson
AVIATION...Schlatter
FIRE WEATHER...Barjenbruch



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