177
FXUS65 KBOU 070248
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
848 PM MDT Mon May 6 2024

.KEY MESSAGES...

- Very active weather pattern through the next couple days, with
  Winter Weather, Fire Weather, and High Wind highlights in effect

- Periods of snow and blowing snow through Tuesday with some
  travel impacts, especially across the Park Range/Rabbit Ears
  Pass.

- Widespread gusty winds continue through at least Tuesday, with
  the strongest winds expected across the higher Foothills.

- Cool and unsettled weather along with much lighter winds
  Thursday through Saturday.

&&

.UPDATE...
Issued at 848 PM MDT Mon May 6 2024

We mainly just updated the highlights that were getting close to
expiration this evening. First, the RFW for southern Lincoln
County was ended given RH is over 25% in the area, and winds have
come down significantly. We also opted to cancel the high wind
warning for the adjacent plains below 6 kft MSL in Larimer,
Boulder, and Jefferson Counties. The only remaining places that
are gusty per latest observations are near the foothills in a few
pockets and at best are gusting a little over 40 mph. Over 98% of
the remaining area in that part of the warning have much weaker
winds, more like 20-25 mph gusts. There are still ingredients in
place for a mountain wave to develop now through 3 AM, but none of
the CAMS bring the winds out of the foothills overnight. Still
think there is enough signal for wind gusts 40-50 mph in the wind
prone areas along CO-93 and north of Fort Collins, and the
existing forecast reflects this, but even if mountain wave
enhanced winds occur they should remain well below warning
criteria. Strong winds should continue and possibly pick up in
the foothills with the approach of the next weak trough, so will
keep the HWW in place there until 6 AM. Overall the details of the
forecast were not updated much, mainly just to clean up (i.e.
remove) some of the PoPs before noon where measurable precip
isn`t expected in the face of strong subsidence and strong
downslope flow (i.e. below 6500 ft elevation) until potentially
after noon Tuesday.

&&

.SHORT TERM /Through Tuesday/...
Issued at 135 PM MDT Mon May 6 2024

High winds will continue to be the main concern for impacts across
northeast Colorado this afternoon through Tuesday. Based on
surface observations, the stronger winds are across the Front
Range mountains/foothills and the adjacent plains with gusts 40-60
mph and locally higher for the Front Range windy spots. Radar
shows snow continuing across the mountains with CDOT cameras shows
minor impacts to the higher passes. A band of showers remains over
northern Weld County; however, large dewpoint depressions will
make it difficult for any liquid to hit the ground.

Starting with the highest impact..wind. Strong winds continue
through Tuesday. Toward the early evening, winds decrease some
across the urban corridor with high winds still across the Front
Range mountains/foothills. More dry subsident flow moves in
overnight. Model cross sections show a period of decreased
moisture in the high country before the next wave approaches.
There is also indication of a wave mountain wave set up during
this time with cross sections showing a stable layer with
increased flow aloft, and decreased moisture. This supports the
potential for wind enhancement across the Front Range mountains
and foothills in the 03-09z timeframe. If this were to set up,
those areas could see gusts 60-75 mph and locally up to 80 mph in
the windy spots, further justifying the ongoing High Wind
Warning. Moisture increases again in the high country by sunrise
and the pressure gradient relaxes across the Front Range which
will decrease the higher magnitude winds Tuesday morning. Tuesday
will still be windy, although not as windy as Monday with HREF
showing mean gusts 40-60 mph for the Front Range and 30-45 mph
elsewhere.

Regarding snow. Snow continues in the mountains through early
this evening with minor impacts to travel possible. Later in the
evening, moisture decreases along with lapse rates promoting a
lull in snow for a period overnight. The next wave increases and
deepens moisture across the high country with steepening lapse
rates early to mid AM Tuesday. This will increase snow showers
across the mountains. Tues AM through late afternoon will be the
best timeframe. Can`t rule out minor impacts, although May sun
angle will make it difficult to accumulate on most trafficked
roads. Mountain passes will have the higher chances of any impact.
With windy conditions, patchy blowing snow is possible. From
tonight through tomorrow, we`ll see closer to advisory level
amounts. Held off for now on issuing any additional headlines
with hesitation on actual travel impact. Could see up to another
foot in the Park Range in spots and 4-8 inches for the Front Range
mountains and mountains of Summit County with some locally higher
amounts.

&&

.LONG TERM /Tuesday night through Monday/...
Issued at 135 PM MDT Mon May 6 2024

Winds will gradually subside Tuesday evening as gradients relax
slightly and we lose daytime heating and mixing. There is still a
moderate gradient aloft, however, so gusty winds could linger off/on
overnight into early Wednesday morning in/near the foothills but to
much less magnitude than tonight. With less wind, clearing skies,
and a very dry airmass, there is a greater than 50% chance of
sub-freezing temperatures on the plains. With the growing season
beginning, we`ve opted for a Freeze Watch in all but the northeast
corner where winds will stay up a bit more. There`s a greater
chance we could remain above freezing in some of the western
suburbs depending on wind, but opted to keep it simple for now
with a contiguous area across the plains and I-25 Corridor. It`s
also been quite some time since it`s frozen in the western
suburbs.

For Wednesday, generally lighter breezes are expected in most
areas, with temperatures still averaging a few degrees below
normal. We`ll be mostly caught in between moisture sources
(departing to our east and another staying to our north) during
the day, so only a few mountains snow showers expected.

That will start to change Wednesday night and Thursday as an
elongated trough slips southward into our forecast area. And yes,
that will be the remnants of the deepening (current) storm system
moving through the western Dakotas tonight and Tuesday. Given the
increase in moisture and instability associated with the upper
trough, at least scattered showers are expected Thursday and
Thursday night. Temperatures will be several degrees below normal,
and in fact snow levels could hover around 7,000 feet during the
day, and a little lower than that at night. We don`t see any big
precipitation amounts at this time, but perhaps a couple more
inches for the northern mountain snowpack.

For Friday into Saturday, there is still considerable uncertainty
regarding how the upper trough further elongates and even shears
south/southwest. The EPS members are showing strong agreement with
the shearing/deeper trough into the Desert Southwest, while the
GEFS is almost entirely opposite showing the elongated trough
essentially getting squashed to our south and building a ridge
quicker into our area. Even the EPS, however, is a little too far
south and west so overall a drier weather pattern is expected to
develop sometime around Friday, and then hold through most of the
weekend. It`s hard to buy the EPS solution of lifting the low back
to the northeast and across us toward Sunday, but definitely
worth keeping an eye on as that could slow our warming trend and
drier weather forecast. At this point, it`s hard to argue with the
ensemble forecast which shows gradual moderation and above normal
temperatures by Sunday and Monday. Low odds (~20%) that we would
stay cooler with more numerous showers.

&&

.AVIATION /00Z TAFS through 00Z Tuesday/...
Issued at 535 PM MDT Mon May 6 2024

Not an overly complicated forecast through Tuesday but
operationally there will continue to be impacts via strong west
winds much of the day Tuesday. First, VFR conditions will prevail
through Tuesday. Winds out of 280/290 should finally weaken
through 01 to 02Z at DEN and APA, with winds coming down to 10-15
kts out of the SW after 02Z. From there, they should remain
around 10-15 kts until 15Z. Winds out of 280 increase across the
terminals 20G30KT after 15Z, and should remain that way all the
way until weakening around 03Z Wednesday. There is a low chance
(25%) that winds are 5-7 kt less than we have in the TAFS during
the afternoon, but for the most part the CAMs all have sustained
west winds 20 kts or higher from 15Z Tue - 03Z Wed.

There is an additional thing to mention regarding BJC. There are
several ingredients in place for a short-lived mountain wave to
develop after 03Z. This would keep the winds up at BJC through
about 08Z. For now, have just kept the 28020G30KT going until 08Z
because of the strong chance of a mountain wave keeping the winds
elevated. CAMs actually keep the stronger west winds in the lower
foothills during this time and away from BJC, but we can`t ignore
the favorable ingredients. If the ingredients come together we`d
likely see gusts 40-50 kts late tonight. But after 08 or 09Z those
ingredients weaken significantly, so we are pretty confident
there will be a lull at BJC from 08-15Z tomorrow with winds 10 kts
or less.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 AM MDT Tuesday for COZ035-036.

Freeze Watch from late Tuesday night through Wednesday morning
for COZ038>049.

&&

$$

UPDATE...Schlatter
SHORT TERM...Mensch
LONG TERM...Barjenbruch
AVIATION...Schlatter