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Area forecast discussion
National Weather Service Denver/Boulder Colorado
1000 PM MDT sun Sep 23 2018

issued at 955 PM MDT sun Sep 23 2018

Spotty high based mainly virga showers are now popping up over
the far northwest corner of the County Warning Area (extreme Jackson County around
the Ryan wildfire). Radar returns are weak at most. Main concern
with this leading batch of showers will be gusty winds which the
Ryan fire imet has been made aware of. Farther west over northwest
Colorado, showers are a bit more numerous and cloud bases are a
bit lower with smaller temp/dewpoint spreads. But, the underlying
cloud layer with these showers would support more wind than
wetting rainfall. Expect this shower activity to reach western
portions of the County Warning Area between 12-3 am tonight. However, the main
batch of showers associated with the mid-level trough and jet are
still back over northeast Utah at this hour. They don't look all
that impressive on radar. At their present rate of forward motion,
should see main batch of showers passing over the northern mtns
between 1-5 am and over the N-central mtns around Summit County
area and the northern Front Range foothills between 3-7 am MDT.
Furthermore, the sfc cold front now moving ewrd thru south-
central Wyoming is still timed to jump the Continental Divide and
then slide south and east acrs nern Colorado during the pre-dawn
hours. A shift to gust northerly winds and a modest jump in
dewpoint will mark it passage thru the Denver Metro area around 5
am. Really do not see much of a chance for precip east of the mtns
as temp/dewpt spreads should remain relatively large. What may
be more noticeable will be the wildfire smoke that will spread
down from Wyoming behind the front. Do not expect to see
visibilities in the smoke much below 5-6 miles, although can't
rule out pockets of heavier smoke and vsbys down to 2-3 miles
early to mid morning.

Have made adjustment to the forecast grids to account for this
time line.


Short term...(this evening through monday)
issued at 322 PM MDT sun Sep 23 2018

Lots of shallow convection upstream over western Colorado and
Utah, but it's still dry enough to limit cape and there is still
very dry air above this convective layer. As a result these clouds
shouldn't produce any precipitation. Deeper bands further north
will shift eastward, clipping northern Colorado later tonight into
Monday morning as the associated trough moves past. The
environment is still pretty dry overnight. Around sunrise the cold
front will drop across the plains, bringing a bit more moisture
but still nowhere near saturation. This moisture and convergence
will help keep cloud cover through the day, but it will still
probably not produce much shower activity. Current forecast has
chance pops over the northern mountains and isolated
showers/storms mainly near the northern border on Monday. This
still looks good. It should be cold enough for the precip that
does fall to be snow in some areas above timberline, but not
expecting it to go any lower. Forecast temperatures look good. We
did increase winds a little and lower humidities a little in the
mountains again--more in the fire weather section below.

Also of note is that the cold front will likely bring in smoke at
low level that the Ryan wildfire produces overnight. The hrrr
smoke model shows this pattern well, but is likely overdoing the
smoke production, especially the plume from the more southern
Silver Creek fire. Even given this, the pattern of low level smoke
being advected from the Laramie/Cheyenne area southward behind the
front seems likely. Laramie has still had visibilities of 5-10
miles and we're further away, so probably not that low here, but
it may well be the smokiest we've been this Summer in Denver. The
smoke should gradually mix out and be blown away as winds turn
more toward the east and southeast in the afternoon, but this
could be a slow process. For now, we'll add the mention of smoke
in the morning for Denver northward across Weld County.

Long term...(monday night through sunday)
issued at 322 PM MDT sun Sep 23 2018

Monday night into Tuesday, a moderate to strong west/northwesterly
flow aloft will be over the area. The first short wave trough will
pass to the north and east of Colorado by 00z Tuesday. The best chance
of showers at that time will be over the far northeast plains. Mid
level cold air advection will drop the 700 mb temperatures from
10c Monday afternoon to 4c Tuesday aftn. Although it will be
cooler it will remain dry in the mountains west of the Continental
Divide. A cold front Post-frontal upslope along the Front
Range/Palmer Divide may allow for some light showers in and near
the foothills southwest of Denver by late Tuesday. It will remain
cool on Wednesday over the northeast plains, with only a slight
chance of showers over the Palmer Divide early. Dry elsewhere
across the cwa. Weak to moderate mid level qg descent sweeping
across the state Wednesday afternoon and evening. Mid level
warming will start to develop late Wednesday and continue with a
gradual warming trend the rest of the week. A dry northwesterly
flow aloft with a ridge to the west through Friday then more of a
zonal flow aloft over the weekend.


Aviation...(for the 06z tafs through 06z Monday night)
issued at 955 PM MDT sun Sep 23 2018

Light and variable winds in the Denver Metro area as of 2145z
should transition to a more typical south-southwest drainage wind
pattern in the next couple of hours with pressure falls across
southern Wyoming ahead of a cold front. This cold front is
expected to slip across southeast Wyoming after midnight, then
drop south across northeast Colorado late tonight. Latest model
guidance points to a 11z arrival time in the Denver Metro area.
The air near the ground behind this front appears quite dry. So,
the chance for precipitation in the Denver area with frontal
passage appears quite low at this time. What most likely will
accompany the front will be northerly winds bringing smoke down
from wildfires upstream from here, such as the Ryan wildfire. It
is possible that visibilities in the Denver area could fall to the 4-6
mile range due to the smoke during the early to mid-morning hours.
By late morning, a shift to southeast wind should dispatch most
of this smoke before noon.


Fire weather...
issued at 955 PM MDT sun Sep 23 2018

The red flag warning for fw zones 211..213 and 217 in Jackson,
grand and Summit counties expired at 8 PM MDT this evening. Its
possible this warning could be reissued for this same area
tomorrow afternoon should the latest model data support this
action. Otherwise, high based mostly dry showers passing over the
Ryan and Silver Creek wildfires overnight may produce gusty and
erratic wind gusts to 30 mph. Fortunately, rising relative humidity values
overnight in advance of the weather system presently passing over
Utah should reduce the risk of large fire growth.


Bou watches/warnings/advisories...


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