Winter Weather in Colorado, September 7 to 9, 2020

Area Forecast Discussion
National Weather Service Denver/Boulder CO
806 PM MDT Mon Sep 7 2020

.UPDATE...
Issued at 755 PM MDT Mon Sep 7 2020

A couple different pre-front-cold-fronts came down across the CWA
late this afternoon and earlier this evening. The real front is
now across the northern CWA border, heading south. Real data
indicates that the northeasterlies, from the first front made to
the the Cameron Peak Fire. It is already showing signs of
"decrease-ment". Its smoke plume has tamed down consierably
already. The grids are in pretty good shape with the significant
weather change expected.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 348 PM MDT Mon Sep 7 2020

...SNOW AND RECORD COLD EXPECTED TUESDAY INTO WEDNESDAY...

The weather expectations remain unchanged as the north winds ahead
of the cold front are now spreading across Weld County and towards
the Denver/Boulder metro area. Strengthening north winds should
continue through the evening as cold air moves into the state.
Fire activity on the Cameron Peak fire remains vigorous to
extreme. Warnings and Advisories remain in effect for the
mountains, foothills and out onto the plains. Rain on the plains
and snow in the mountains should develop by midnight. Rain on the
plains will continue through about 6 AM, when temperatures cool
enough and the airmass becomes cold enough through the column for
precipitation to change over to snow. Mountain areas should see
accumulating snow overnight and through the day, but accumulations
on the plains will be harder to realize due to warm ground
temperatures and air temperatures in the lower 30s. Snow sill fall
through the day, but will mostly melt until later tomorrow
evening.

.LONG TERM...(Tuesday night through Monday)
Issued at 348 PM MDT Mon Sep 7 2020

The early winter storm will continue to impact the area Tuesday
night and possibly into Wednesday before finally loosening its
grip on the forecast area. We would expect mostly snow (perhaps a
mix in the far northeast corner) continue into Tuesday night with
continued upslope, but gradually weakening QG ascent. Models have
excellent agreement on the synoptic positioning of the upper level
low settling into the Utah/Arizona border/Four Corners region, but
the details on the mesoscale forcing and melting still make this
a difficult forecast, as usual for Colorado. Most of the models do
have one good batch of precipitation rotating around the upper
low and across the Front Range and northeast plains during the
evening. Apparently there is a weak embedded short wave kicking
out of the upper low, but there is little QG signature or other
mesoscale forcing like frontogenesis indicated. There has been
reasonable consistency with regard to this feature, so we do think
a period of heavier snow is possible during the evening. Snow
should continue overnight with overall weak QG lift and upslope,
although the intensity will decrease as both of these support
mechanisms weaken. Tuesday night will provide the highest chances
of meaningful accumulating snowfall across the plains, given loss
of any solar insolation and continued cooling of the near surface
soil temperatures. We'll likely see our largest storm impacts in
this period as we accumulate heavy, wet snow on trees and elevated
surfaces. Thus, we'll continue to message potential for broken
tree limbs and scattered power outages.

Snow will continue to taper off Wednesday as we lose our upper
level forcing and flow turns deep south/southeasterly. That said,
this flow pattern would continue to produce accumulating snow in
Park County through much of the day.

Total snow accumulations are still very tricky considering the
heat ahead of this storm and the amount of melting that will
occur, especially during the day tomorrow. The higher elevations
of the foothills and mountains should be able to hang onto most of
the falling snow as accumulation due to the very cold temps in
the teens to mid 20s, while the I-25 Urban Corridor is still a
wildcard, and will be through this entire event. We did like the
newest HRRR variable snow accumulation product, which shows 8-18
inches with locally higher amounts in the foothills, anywhere
from 2-6 inches in the I-25 Corridor (locally higher amounts
possible right near the foothills and Palmer Divide). Most of the
eastern plains would be lighter in the Trace to 4 inch range, but
if the heavier band materializes Tuesday evening then a few spots
could get more.

The upper level low will eventually fill and most likely kick
northeast across the state Thursday and Thursday night. This will
likely bring one last round of showers to the mountains with light
snow accumulations possible, and a chance of rain showers to the
lower elevations.

Record low temperatures can be expected Tuesday night, with
readings dropping below freezing over most of the plains. If
precipitation continued all night, lows could hover near 30-32F,
otherwise some upper 20s will occur. If it did drop below 30F in
Denver Wednesday morning, it would be the earliest 20s ever
recorded, and only four days after our latest 100F ever. Wednesday
looks like another cold day with highs on the plains only reaching
the 30s.

Temperatures will then gradually moderate through the end of the
week as ridging and drier westerly flow aloft develops.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 755 PM MDT Mon Sep 7 2020

Northeasterly winds currently, should gradually give way to more
due northerly winds at DIA by around 0330Z. Initially, after the
front moves through, wind gusts could reach 35-40 knots for a
couple of hours. Ceilings should come down into the BKN-
OVC020-030 range by 06Z. IFR conditions can be expected at DIA
sometime around 12Z Tuesday morning. Snow is expected as the
precipitation type from that time on into Wednesday, but it will
be unlike for any of the snow to accumulate in runways.

&&

.FIRE WEATHER...
Issued at 755 PM MDT Mon Sep 7 2020

The RFW has expired at 02Z. Hopefully, the expected incoming
moisture will have a serious impact on reducing our short/medium
range fire danger and, perhaps, putting our ongoing fires out.
&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Freeze Warning from 6 PM Tuesday to noon MDT Wednesday for
COZ038>051.

Winter Storm Warning from midnight tonight to noon MDT Wednesday
for COZ033>037-041.

Winter Weather Advisory from midnight tonight to 6 AM MDT
Wednesday for COZ031.

Winter Weather Advisory from midnight tonight to noon MDT
Wednesday for COZ038>040-042-043-045-046.

&&

UPDATE.........RJK
SHORT TERM.....Dankers
LONG TERM......Barjenbruch
AVIATION.......RJK
FIRE WEATHER...RJK

Hazardous Weather Outlook
National Weather Service Denver/Boulder CO
1114 AM MDT Mon Sep 7 2020

COZ030>051-081715-
Jackson County Below 9000 Feet-
West Jackson and West Grand Counties Above 9000 Feet-
Grand and Summit Counties Below 9000 Feet-
South and East Jackson/Larimer/North and Northeast Grand/
Northwest Boulder Counties Above 9000 Feet-
South and Southeast Grand/West Central and Southwest Boulder/
Gilpin/Clear Creek/Summit/North and West Park Counties Above
9000 Feet-Larimer and Boulder Counties Between 6000 and 9000 Feet-
Jefferson and West Douglas Counties Above 6000 Feet/Gilpin/Clear
Creek/Northeast Park Counties Below 9000 Feet-
Central and Southeast Park County-
Larimer County Below 6000 Feet/Northwest Weld County-
Boulder And Jefferson Counties Below 6000 Feet/West Broomfield
County-
North Douglas County Below 6000 Feet/Denver/West Adams and
Arapahoe Counties/East Broomfield County-
Elbert/Central and East Douglas Counties Above 6000 Feet-
Northeast Weld County-Central and South Weld County-Morgan County-
Central and East Adams and Arapahoe Counties-
North and Northeast Elbert County Below 6000 Feet/North Lincoln
County-
Southeast Elbert County Below 6000 Feet/South Lincoln County-
Logan County-Washington County-Sedgwick County-Phillips County-
1114 AM MDT Mon Sep 7 2020

...SIGNIFICANT CHANGE TO WINTER WEATHER STARTS THIS EVENING...

This hazardous weather outlook is for northeast and north central
Colorado.

.DAY ONE...Today and Tonight

Warm temperatures, very dry conditions, and wind gusts to 35 mph
will result in critical fire danger today over the mountains.
Areas of smoke from the Cameron Peak Fire will also affect air
quality across the plains.

A strong cold front will drop south across Colorado this evening.
North winds are expected to gust up to 50 mph and temperatures
will plummet as the cold front pushes through. Rain will move
south into northern Colorado during the evening and continue to
spread southward overnight. As temperatures fall tonight, snow
levels will also fall with a rain/snow mix expected along the I-25
Urban Corridor by sunrise Tuesday morning.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Heavy snow will fall in the Front Range Mountains and Foothills
Tuesday and Tuesday night, with accumulations of 8 to 14 inches in
most areas, and locally heavier amounts around 20 inches possible.
The I-25 Corridor will likely see mostly snow on Tuesday, with
some slush accumulation possible on trees and grass. That snow
will continue into Tuesday night when more significant
accumulations will be possible on trees and elevated surfaces,
resulting in tree damage and possible power outages. If
significant accumulation occurs, be mindful of where you park your
car - not under trees, and be alert for falling tree branches.

On the eastern plains, lighter snow amounts can be expected, with
accumulations of 1 to 3 inches by the time snow decreases late Tuesday
night or Wednesday morning.

Roadways could become slushy, especially over the higher terrain
where the heaviest snowfall accumulations and coldest temperatures are
expected. Record cold temperatures are expected by Tuesday night,
with lows in the upper 20s to lower 30s over the plains, and teens
to lower 20s in the mountains and foothills.

Conditions will gradually improve Wednesday through Thursday with
a gradual warming period into the weekend.

There is still moderate uncertainty with regard to potential
snowfall amounts and impacts. Stay tuned to the National Weather
Service for the latest updates on this early season storm and
drastic change in temperature.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be be needed today and tonight. Any
reports of snowfall or rain changing to snow tonight and Tuesday
will be appreciated.