042
FXUS65 KBOU 052111
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
311 PM MDT Sun May 5 2024

.KEY MESSAGES...

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

- Snow and blowing snow develops across the high country late
  tonight and continues through Tuesday. Travel impacts likely,
  especially across the Park Range/Rabbit Ears Pass.

- Widespread gusty winds developing tonight and 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.

&&

.SHORT TERM /Through Monday/...
Issued at 309 PM MDT Sun May 5 2024

A busy, mostly windy, short term forecast across our CWA. A potent
trough axis is expected to swing across the region over the next
36 hours, leading to quite a few different hazards across the
state.

GOES water vapor satellite and RAP mesoanalysis show the center of
the upper trough near northern Nevada this afternoon... and this
will quickly become negatively tilted as it ejects north of the
forecast area into the northern plains. Ahead of this trough,
broad but strong south/southwesterly flow has developed, and
widespread gusty winds have developed across most of the lower
elevations, except for a sliver of the western metro/I-25 corridor
where the deeper mixing has yet to kick in. Today`s wind is just a
preview of the next few days, while the warmer temperatures will
be a passing memory by tomorrow.

Later tonight, mountain snow is expected to develop as the flow
aloft turns more westerly and better moisture advects into the
high country. Guidance has gradually become a bit more bullish on
both the snow and wind potential, and we`ve expanded the Winter
Weather Advisories to the I-70 and northern mountain corridors
where a few inches of wind blown snow looks likely tonight into
tomorrow. There`s some uncertainty with regards to impacts given
the current warm temperatures and the early May sun angle, but
there will likely be a period of difficult travel across the
mountain passes during the peak snowfall rates late tonight/early
Monday AM.

Monday morning will also feature an increase in winds as a bora
event develops. Models show good cold air advection/pressure rises
behind the passing trough axis with mid-level/ridgetop flow near
50kt. Winds will quickly increase across the foothills and
adjacent plains early in the day and continue through at least the
evening hours. There is still some question as to the spatial
extent and duration of the strongest winds, with the HRRR notably
stronger across the lower foothills and the typical very windy
spots near Boulder/Highway 93. Machine-learning guidance from CSU
is also split close to 50/50 on if we see higher wind gusts this
far east (though their criteria is slightly different than ours).
Have opted for a High Wind Warning in the foothills and will
message the potential for stronger gusts in the immediately
adjacent plains. Further east, wind gusts will still be fairly
strong (30-50 mph) but not quite at the threshold for any
highlights.

Tomorrow will also be cooler than the past couple of days, with
high temperatures likely remaining in the upper 50s to perhaps the
low 60s across the plains. Still, the airmass is dry enough south
of I-70 for some localized critical fire weather concerns... see
the fire section below for more details.

&&

.LONG TERM /Monday night through Sunday/...
Issued at 309 PM MDT Sun May 5 2024

Monday night through Tuesday evening, a large upper level storm
system will be centered over the Northern Rockies and Northern Great
Plains States with a strong westerly flow aloft over Central
Colorado. Favorable orographic flow combined with decent QG lift and
occasional upper level disturbances should result in periods of snow
across the mountains. The exception may be early Tuesday morning
when moisture and QG lift may be minimized. Overall, the snow
should be generally light due to a lack of cold air advection.
However, with a 115KT+ upper jet over the southern sections of the
CWA, can`t rule out some localized heavier bands. This pattern
will also bring breezy to windy conditions to the high country
with areas of blowing snow possible, especially over mountain
passes. The strongest winds are expected across the Front Range
Mountains/foothills and over the Cheyenne Ridge Monday night with
gusts up to 75 mph possible over wind-prone areas.

Further east across the plains it will be mostly dry, cool and
windy. The winds should be strongest along and near the foothills
Monday night with gusts to 55 mph and most widespread Tuesday
afternoon with gusts to 45 mph. The gusty winds combined with low
relative humidity and dry fuels may lead to fire weather concerns
across Southern Lincoln County Tuesday afternoon and evening. For
more details, see the fire weather discussion below.

On Wednesday, the mid an upper level flow over Colorado decreases as
the strong upper low over the Northern Great Plains weakens and
shifts east into the Upper Midwest. A broad upper trough will still
remain over the Western U.S. with lighter winds across the CWA and
scattered (30-50%) snow showers possible over the Northern
Mountains. However, breezy conditions across much of the forecast
area are still expected to continue.

Wednesday night and Thursday, the large elongated upper trough that
covers much of the Western and Central U.S. splits as a closed upper
low develops over the Intermountain west due to energy rotating
around the back side of the trough.

The weak upper low is progged to track across the Southern and
Central Rockies Thursday through Saturday. In addition, occasional
cold fronts associated with the Central Plains trough are progged
to move across Northeastern Colorado as well. The combination of
upslope flow and increased moisture behind the fronts and some
lift associated with the weak upper low should result in cool and
unsettled weather during this period. Dry and warmer weather is
expected on Sunday as upper level high pressure builds over the
Rocky Mountain Region.

&&

.AVIATION /18Z TAFS through 18Z Monday/...
Issued at 1155 AM MDT Sun May 5 2024

Gusty winds will be the main impacts in the TAF forecast through
Monday afternoon. At DEN/APA, southerly winds have already mixed
down with a few gusts of 20-25kt already noted. Further northwest
(BJC), light/variable winds continue, with a well- defined
boundary between mixed/stable air stretching from downtown Denver
to the NW side of the field at the moment.

Winds should continue to increase through the day today, and also
spread into BJC. As the stronger flow aloft reaches the area later
this afternoon, we should see a few gusts >30-35kt.

Wind forecast gets much more challenging tonight into tomorrow as
a bora event develops. Winds should slowly turn from south to west
tomorrow morning, with westerly flow continuing through the rest
of the day. There may be one or two periods of enhanced flow
Monday AM and again sometime later Monday afternoon, but those
timing details are difficult to determine at this point.

&&

.FIRE WEATHER...
Issued at 309 PM MDT Sun May 5 2024

Critical fire weather conditions are forecast across southern
Lincoln county on Monday as dry and very windy conditions develop.
Relative humidity values should fall to 10-15 percent with wind
gusts of 45-55 mph at times. Overnight humidity recovery will also
struggle to reach 50%... and we may need to continue to extend
that into Tuesday/Wednesday.

Gusty winds and lower humidity will be over the southern sections
of the plains Tuesday and Wednesday. However, there is still some
question as to how dry the fuels are due to decent precip in the
latter half of April. The only area that didn`t receive decent
precip was over portions of southern Lincoln county. Thus that
would be an area to watch for critical fire conditions in the
afternoon and early evening hours.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to 6 PM MDT Monday
for COZ031-033-034.

High Wind Warning from 3 AM Monday to 6 AM MDT Tuesday for
COZ035-036.

Fire Weather Watch from Monday morning through Monday evening
for COZ247.

&&

$$

SHORT TERM...Hiris
LONG TERM...JK
AVIATION...Hiris
FIRE WEATHER...Hiris/JK