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fxus65 kbou 191102 

Area forecast discussion
National Weather Service Denver/Boulder Colorado
402 am MST Tue Nov 19 2019

Short term...(today and tonight)
issued at 402 am MST Tue Nov 19 2019

Dry and mild weather will continue for one more day today as high
pressure aloft remains over the state today and tonight. An
unseasonably warm airmass remains in place over the entire state
today which will result in temperatures climbing to about the same
levels as yesterday. Sky cover will vary from partly to mostly
sunny as mountain wave clouds come and go through day. A surface
Lee trough of low pressure will be in place over the plains
which will keep winds out of the southeast through the afternoon.
Flow at higher elevations will be weak to moderate, so winds
should light to moderate in the mountains as well. Moisture aloft
is expected to begin increasing over mountains later tonight as an
upper disturbance approaches the state from the Desert Southwest.
Precipitation over the central mountains is not expected to begin
until tomorrow as the upper level disturbance advances northward.

Long term...(wednesday through monday)
issued at 402 am MST Tue Nov 19 2019

On Wednesday two short wave troughs off the West Coast of California
and Baja California begin to phase. By Wednesday afternoon models
agree that phasing will be complete, with a fairly robust closed
circulation across Southern California. Overall forecast of the
evolution of the trough and precipitation timing/amounts has not
changed much since the last forecast update. Southwest flow will
advect a decent amount of moisture into Colorado, favoring
southwest facing slopes for best orographics in the high country.
Snow should begin by late Wednesday morning across the mountains
via orographics and moisture. Lapse rates over the mountains are
pretty weak, but mid-level qg dynamics get better overnight into
Thursday as the trough moves into the Great Basin. The trough
will move slowly across our area late Thursday, generally west to
east, and should impact our weather through Friday. With the
trough just upstream Wed-Friday morning, then overhead throughout
a good chunk of Friday, clouds and some precipitation will be in
the forecast. The devils are in the details in terms of how cold
it could get and how much precipitation may fall over the two day

First, for the mountains, despite decent synoptic scale lift via the
approaching trough and eventual trough movement right overhead, weak
lapse rates combined with weak orographics (ssw flow 5-10 kts)
should result in light snow rates, mostly tied to the presence of
synoptic qg lift from midday Wednesday through overnight into very
early Friday. Temperatures should remain fairly warm throughout
the event until late Thursday when cold advection gets going in
the low to mid levels of the atmosphere. It may remain right
around freezing Wednesday afternoon and through much of Thursday
(especially during daylight hours) roughly near and below 10,000
ft elevation. This may reduce potential impacts from snowfall on
the roadways. We are not expecting significant 2-day totals to
begin with given so many things arguing against decent snowrates.
Given this, we do not anticipate needing any highlights for the
mountains. Lift, moisture, and orographics are best across the
southwest 1/3 of Colorado, outside of our area of responsibility.
It will most likely be light snow off and on for 2 days Wednesday
midday through morning hours Friday, amounting to a total of 2-8
inches over that lengthy timeframe.

Next, for the plains, there looks to be a strong cold front that
traverses the area during the late morning Wednesday. High
temperatures across the I-25 corridor and much of the plains will
likely occur a little before noon. Temperatures are expected to
drop slowly throughout the afternoon into the mid to upper 30s by
sunset. Not surprisingly, the low-levels also don't saturate
until around sunset/early evening, so precipitation amounts will
be limited until then. P-type should start off as rain initially,
but change fairly quickly to snow. Model soundings indicate
saturation aloft well colder than -10 degc, so we will not have to
deal with any freezing drizzle this time around. The northeast
upslope flow is fairly deep with the front, up to about 10kft mean sea level
from Wednesday evening into Thursday morning. By mid-morning
Thursday, only shallow northeast upslope remains. The best chance
for precipitation will likely be associated with the front. There
is a massive inversion at and above the 9,500 ft mean sea level level, so
convective potential is nil. It will be cloudy all day Thursday
with the low- level moisture trapped beneath the inversion, but
with some mid- level drying working in above the inversion via the
southwest flow aloft, it also may not be precipitating much
Thursday afternoon. Weak qg lift exists across the plains through
midnight Friday morning but it will be fighting some drying above
10 kft msl, so probably just light snow not accumulating much. The
European model (ecmwf) has the trough axis moving across east. Colorado by late
morning Friday, while the GFS is much more progressive, ending
precipitation by sunrise Friday or so. Suffice to say our forecast
will be dry by midday Friday, but pops will be kept low Friday
morning as well given strong subsidence and drying on the backside
of the trough. With clouds, weak upslope, and light
precipitation, temperatures will not warm much if at all on
Thursday across the plains. We expect highs in the upper 20s to
around freezing. Sun may finally make an appearance Friday
afternoon, but temperatures will remain chilly with highs in the
low to mid 30s. Throughout the late Wednesday into early Friday
period, no significant impacts to travel are expected, though some
roads may see some slush occasionally, and some light
accumulation is possible on grassy areas and overpasses.

Saturday and Sunday there will be dry northerly flow over our area
as the trough tracks northeast across the conus, shifting to
northwest flow aloft by Sunday. It should remain dry across our
area with a warming trend resulting in above normal temperatures
with daily highs in the mid 50s across the plains, with upper 30s
to mid 40s in the high country. Winds should be fairly light for
just about everywhere other than early Sunday at and above
timberline and the higher elevations of The East Slope with gusts
over 40 mph. A mountain wave may develop when flow becomes more
west and a weak inversion develops just above Ridgetop.

Models continue to be all over the place regarding the evolution of
a Pacific storm coming onshore across the Pacific northwest early
next week. Very low confidence in any particular solution or even
ensemble solutions because those too are spread out quite a bit. The
ensembles are highlighting decent moisture advection and lower
than normal thickness values early next week. There is enough of
a signal in the available model guidance to say that a cold front
will drop temperatures below normal Monday afternoon and Tuesday,
and snow chances will increase across our area. For now the nbm
solution is as good as any so we did not modify much on days 6 and
7. It will be interesting to see how the forecast evolves for
early next week given the massive increase in travel over
Thanksgiving week.&&

Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 402 am MST Tue Nov 19 2019

Few, if any, aviation impacts over the next 24 hours as moderate
westerly flow aloft continues today and then becomes more
southwesterly tonight. The airmass over Denver will remain dry and
warm until a cold front arrives late tomorrow morning. Surface winds
will be out of the southwest this morning and then more
southeasterly through the afternoon. Drainage southerlies will
prevail tonight. Any clouds and ceilings should be above 15,000 ft
mean sea level.


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