603
FXUS65 KBOU 020253
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
853 PM MDT Wed May 1 2024

.KEY MESSAGES...

- Showers, along with isolated storms on the eastern plains, tapering
  off this evening

- Another round of showers and storms late Friday/early Saturday.

- Warmer and breezy over the weekend.

&&

.UPDATE...
Issued at 852 PM MDT Wed May 1 2024

Radar shows convective coverage is on the wane this evening as the
airmass stabilizes. There are still isolated to scattered showers
ongoing, but a rather significant downward trend in both coverage
and intensity has occurred over the last couple hours. The only
thunder left is on the far eastern plains. Meanwhile, in the
mountains, snow showers will be winding down as the airmass
becomes more subsident and much drier air moves in from the west.
This is noted by single digit and teens dewpoints already showing
up in northwest Colorado and southwestern Wyoming.

Overall, the forecast was nudged in the direction of more drying
through the late evening hours and ending of any thunderstorms.

&&

.SHORT TERM /Through Thursday/...
Issued at 253 PM MDT Wed May 1 2024

The forecast is still going according to plan this afternoon. The
surface cold front has advanced into the I-25 corridor over the
past hour or so, and should continue to sink southeastward through
the evening hours. Meanwhile, there`s been a decent burst of rain
and snow showers across the high country, associated with the
mid-level front with some minor snow accumulations across the
higher elevations.

As the better lift arrives and the low-level wind gradually turns
to a more favorable upslope component, we should see widespread
showers and a few thunderstorms develop across the urban corridor
and plains. There`s still a chance of a stronger/severe storm or
two out in the far eastern plains where modest instability
exists... but chances of this remain low.

Thursday will be mostly dry, with cool northwesterly flow aloft.
Warmer air does try to nudge back into the region by the
afternoon, but high temperatures are expected to stall out in the
low 60s across the plains. There`s just enough moisture for a few
weak showers during the afternoon, but impacts from these would be
minimal.

&&

.LONG TERM /Thursday night through Wednesday/...
Issued at 253 PM MDT Wed May 1 2024

A broad trough continues to move east across the northern Plains
early Friday, with a short wave trough broken off of the main
trough well to the west over Idaho. Colorado will be under pretty
strong west flow aloft. There is some moisture in the flow aloft,
and combined with relatively steep lapse rates, a few snow
showers should occur along and north of I-70 as soon as midnight
Friday morning. The better chance of precipitation across much of
our area occurs when the lingering, broken off part of the trough
swings across the CO/WY border Friday afternoon and evening.
There is a fair amount of QG forcing associated with the trough
that precipitation is likely (>50%) for much of the northern third
of Colorado. In addition to the QG lift, a robust cold front is
expected to move across the plains sometime in the afternoon.
There will be some instability (300-700 J/kg) to support showers
and thunderstorms along and behind the front during the afternoon.
700 mb temps don`t drop well below 0 deg until the cold front
sweeps across the area, meaning snow levels throughout the day
should remain above 8,000 ft MSL, dropping down to 7,000 ft MSL
early Saturday morning. However, significant subsidence on the
backside of the fast moving trough will spread over the area after
midnight, greatly dropping PoPs Saturday morning across all but
the west facing slopes of the northern mountains. The air on the
backside of the trough is also dry, so by sunrise all
precipitation across our area should be over. QPF amounts are not
overly impressive mountains or plains, though can`t rule out a
few slick spots across the highest, less traveled mountain passes.
Before the cold front arrives temperatures should warm into the
mid 60s across the plains, upper 40s to lower 50s across the
mountain valleys.

Saturday should be dry across most of the area as a ridge builds in
from the west ahead of a strong/deep trough off the West Coast.
Afternoon convective clouds are expected to form over the
mountains and move east, but measurable precip is unlikely other
than across the higher ridges and east slope foothills. The winds
at and above 10 kft MSL are SSW, not great for upslope in our
mountains. An inch or two is possible above 9,000 ft with no
travel impacts anticipated on Saturday.

Sunday through Tuesday our weather will be dominated by a long wave
trough, with embedded shortwave troughs, in place to our north.
Details are quite uncertain this far out, but at a high level it
looks like the main trough will pass over Colorado on Monday, with
the best lift remaining north of us across Wyoming. The main
trough axis looks to move across the state from west to east
during the latter half of Monday. Ahead of the trough axis,
scattered afternoon showers and thunderstorms are possible Sunday
evening to kick of the chances of precipitation associated with
the main trough. In the southwest flow, snow levels will start out
very high, at or above 10 kft MSL Sunday afternoon. At some point
early Monday after the passage of the trough axis, a cold front
is reasonably likely to move across the area as well, lowering
snow levels to about 7500 ft MSL. However, there is a strong
signal for strong subsidence post-trough axis passage the last
half of Monday, which would result in strong winds across most of
the area. So the impacts from this broad trough, and embedded
short wave troughs are uncertain. A few things that stick out:
-> Severe potential Sunday. There will likely be a dry line that
 sets up along or east of an Akron to Limon line. East of the
 dryline strong moisture advection with southeast winds will
 result in strong instability (SBCAPE > 1500 J/kg), and with 50+
 kts of 0-6 km shear, supercells would be the dominant storm type
 with any storms that can break the cap. The best chance for
 initiation would be along the northern tier of counties nearest
 the QG lift and cold pool aloft.
-> Winds/Fire Wx. A strong pressure gradient across the plains
 will result in strong SE winds Sunday afternoon, gusting 35-40
 mph, but the air will be moist enough to preclude the need for
 fire wx concerns. Winds should be less in the foothills and
 mountains until winds aloft turn more west late Sunday. With
 increasing west flow aloft, and pre-trough passage, ingredients
 are in place for a mountain wave enhanced flow late Sunday into
 early Monday, with 70 kts flow are ridge top and a decent
 inversion in place. The unfavorable thing is flow will be
 southwest. In this kind of set up South Park may see the
 strongest winds early Monday. After trough passage around the
 middle of Monday, strong subsidence will drive winds toward the
 surface across the area. Winds across the area should be out of
 the west, and gusting 20-35 mph across much of the area. Tuesday
 will likely see a repeat of elevated west-northwest winds. With
 deep downslope flow east of the Divide, conditions will be dry,
 probably in between 15-20% across most of the plains.

Overall will keep the NBM forecast as is in the Sunday-Tuesday
timeframe given the uncertainty in timing of embedded shortwaves.
The trends look okay, i.e. the warmest day by far will be Sunday
with temps approaching 80 across the plains and urban corridor,
followed by seasonal highs in the 60s Mon-Wed given the proximity
to the cool air aloft with the broad trough to the north. PoPs
will remain near climatology given dry downslope flow east of the
Divide, but >50% each afternoon for the mountains given the
persistent west flow and steep lapse rates. So far though, this
does not yet look like any significant amounts of precipitation
(rain or snow, looking like a few tenths of liquid) and available
ensemble output from the EC and GEFS agrees.

&&

.AVIATION /00Z TAFS through 00Z Thursday/...
Issued at 605 PM MDT Wed May 1 2024

VFR will prevail through the TAF period, although there will be a
period of lower ceilings around 5000-6000 feet AGL in the Denver
area TAF sites with scattered showers this evening. Those ceilings
will then lift and break closer to 06Z.

Winds are currently from the northeast, and should generally
remain that way through about 06Z although scattered showers could
still interrupt this wind regime. Eventually we expect to settle
to more of an east/southeast component that will last through
Thursday, leaning more east/northeast after 18-20Z. Gusts up to 20
knots possible this evening and then again Thursday afternoon.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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