Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 291034
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
434 AM MDT Fri Mar 29 2024

.KEY MESSAGES...

- Accumulating snow and minor travel impacts across the northern
  mountains through Friday night. More significant accumulations
  in the Rabbit Ears Pass and Park Range, and lighter
  accumulations in the I-70 Mountain Corridor.

- A few showers spill onto the plains today and tonight.

- Next storm system late Sunday into Monday night with mountain
  snow, and a mix of rain and snow likely for the plains. Mostly
  light precipitation amounts are expected but there could be some
  travel impacts for the mountains and higher elevation foothills.

&&

.SHORT TERM /Through tonight/...
Issued at 434 AM MDT Fri Mar 29 2024

Well, we`ve been fiddling with trying to pin down the details of
shortwaves riding along the weak mid level front west and north of
Denver, and sure enough the result today isn`t exactly what was
expected. The showers are 6-8 hours late and it looks like they`re
going to be about 50 miles further south too. This means the best
mountain snow from this round is happening now through about mid
morning, and there will be a chance of showers on the plains that
will probably start late this morning north of Denver, and
a bit more of a chance around Denver this afternoon. The best lift
might be in the middle of the day so not perfect timing to tap
diurnal instability. Low levels on the plains remain dry, though
cooler highs from the extra cloud cover will reduce evaporation a
bit. So we`ll stick with scattered PoPs on the plains due to
evaporation. So the main adjustments for today were to raise/shift
southward the PoPs, add clouds, and cool highs a couple of
degrees.

For tonight there`s another ripple in the jet producing a wave
riding along that mid level front. Current expectations are that
this will run a bit further north, but we`ll see how that goes. In
any event, it will keep showers going in the mountains with some
chance of rain on the plains mainly north of Denver. Temperatures
may bottom out close to freezing on the plains, but the snow level
should stay above 6000 feet and probably closer to 7000.

&&

.LONG TERM /Saturday through Thursday/...
Issued at 434 AM MDT Fri Mar 29 2024

On Saturday, moist southwest flow aloft continues with a potent
trough just off the West Coast of the U.S. The moisture plume and
upslope flow, combined with steep lapse rates, should result in
light snow during the day, mostly across the Park Range and
northern Front Range. The I-70 mountain corridor doesn`t look
great for accumulating snow given the less favorable wind
direction and the best moisture remaining to the north. One thing
to mention is the Euro ensembles crank out a tenth or two across
the plains of Colorado during the late morning hours into early
afternoon. This is a new development and we are scratching our
heads how this is possible with significant downslope flow at 700
mb. There is some convective instability during the afternoon but
low levels look too dry for much accumulation, so for now we are
discounting the Euro model/ensembles for Saturday. However, along
the Wyoming border PoPs will remain in the 20-40 percent range
given the influence of the 120 kt jet overhead. There isn`t any
cold air around to generate snow below 6 kft in the morning, and
with the near-April sun angle and warming temps, snow levels rise
to around 8 kft by mid-afternoon. Travel impacts are expected to
be limited on Saturday during the day due to the sun angle and
light precipitation rates, even across Rabbit Ears Pass where a
couple inches could fall, but the roads will most likely be wet
into the early evening hours. Temperatures across the plains and
urban corridor should reach the mid 60s, while the mountain
valleys should be in the 40s.

On Sunday the West Coast trough splits into two, with one trough
moving across the Great Basin, and the other diving south and
remaining offshore of SoCal.  Moisture in southwest flow, combined
with lapse rates, should continue the light snow across the
mountains and northward along the Wyoming border.  There is better
QG lift and overall moisture that there should be a little more snow
falling across the higher elevations, this time extending further
south to the I-70 mountain corridor but still not all the way
into South Park. Temperatures Sunday will be similar to Saturday,
limiting the impacts to travel during the day, even across the
higher passes, but by Sunday afternoon convective instability is
introduced and brief moderate snow may occur, and if it occurs
late enough in the day slush/slick spots are possible across the
mountain roads Sunday afternoon/evening. Several inches of snow
are expected on Sunday along and north of I-70, especially above
9,000 ft. The previously mentioned jet and QG lift traverses the
Wyoming border Sunday evening, increasing rain chances for the
norther tier of Colorado Counties. Snow levels should remain above
6 kft once again while it is precipitating, but can`t rule out a
late change over with light snow by sunrise right along the
border. Elsewhere across the I-25 corridor, light showers are
possible (15-40 percent) but for now it looks too warm for snow,
and precip accumulations should be light through Monday morning.
Temperatures Sunday should again be in the 60s across the Plains,
and maybe even 70s across Lincoln County where much less cloud
cover should exist. There will be strong southwest winds gusting
30-35 mph during the afternoon, deep mixing across the far eastern
plains of Colorado, and combined with RH dropping below 15%, we
may need fire wx highlights for Lincoln County and maybe
Washington County too. It`s too early and marginal to issue
anything just yet.

Forecast uncertainty increases with Monday`s forecast, and I would
be a fool if I was confident in what will happen at this stage of
the game. Deterministic models and their ensemble output
continue to be all over the place with the evolution of the cut
off low ejecting east across the area, and thus with QPF and snow.
There are a few aspects of the forecast that at least seem to be
a little more certain. One is a cold front. All models/ensembles
have a pretty good cold front moving south across the plains, and
a strong connection to a 700 mb front moving southeast across the
mountains too. EC and members are quicker and weaker with the
front, while GFS and members bring it through late morning Monday
and it packs a punch. 700 mb temps drop to -10 degC and surface
wet bulb temps drop below freezing by mid afternoon if not sooner.
There is also significant frontogenesis associated with the front
as it sweeps across the plains, if you believe the GFS and some
of it`s ensemble members. Upslope potential across the east
slopes is limited in the GFS with a deepening surface cyclone and
700 mb low across the NE panhandle. That results in strong
northwest winds. However, the GEFS has several members crank out a
bunch of QPF due to instability and frontogenesis. Every single
member has some precip for the plains and urban corridor, and
about half are cold enough for snow. The EC ensemble system on the
other hand, despite it not deepening a surface cyclone and 700 mb
cut off trough like the GEFS, has a better upslope component to
the low level winds, and also has plenty of instability but less
cold air and a weaker frontal passage. Every member of the EC also
generates precipitation across the I-25 corridor and eastern
plains. Snow accumulation across the lower elevations (below
6,500 ft) is not out of the question, though it`s less likely with
the EC ensembles. Instability is going to be the wild card here,
because the synoptic/mesoscale pattern just doesn`t look favorable
for more than light QPF totals. With marginal temperatures and
April sun angle, travel impacts should be limited to Monday
evening and only with the higher elevation roadways, especially
above 8 kft across the east slopes.

Aside from very low confidence in Monday`s weather, confidence
increases once again from Tuesday morning onward. It looks dry
across the area with a rapid warming trend, going from the 50s
Tuesday to near 70 on Wednesday, and over 70 on Thursday across
the plains and I-25 corridor. Even the mountain valleys will see a
significant warming trend, with temperatures going from the 40s
on Tuesday to the low 50s the rest of the week. The NBM forecast
looked reasonable after Monday next week.

&&

.AVIATION /06Z TAFS through 06Z Friday/...
Issued at 1240 AM MDT Fri Mar 29 2024

VFR through Friday. There will be some variability in the light
winds overnight, but S to SE winds should prevail at KDEN/KAPA.
More uncertainty at KBJC which may be on the edge of west winds
coming off the foothills at around 10 knots and lighter southeast
winds. A shift to northeast winds is expected between 10z-12z.
Isolated rain showers are expected after 18z, but ceilings are
generally expected to remain above 6000 ft.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM MDT Saturday for COZ031.

&&

$$

SHORT TERM...Gimmestad
LONG TERM...Schlatter
AVIATION...Gimmestad


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