441
FXUS65 KBOU 070546
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1146 PM MDT Tue May 6 2025

.KEY MESSAGES...

- Spring storm continues tonight into tomorrow. Significant
  mountain snow accumulations, with some snow falling as low as
  6500 feet.

- Rain and snow showers taper off gradually Wednesday. A couple of
  thunderstorms also possible over the plains.

- Warmer weather moves into the forecast area on Thursday and
  continues into the weekend. Still a few late day showers and
  storms most days.

&&

.UPDATE...
Issued at 920 PM MDT Tue May 6 2025

We had a temporary reprieve as a patch of dry air rotated around
the low across the Front Range over the last several hours. This
is passing now, and the main precipitation band is lifting north.
There`s still a very shallow layer of westerly winds, but the deep
flow above that is rotating a bit more ENE now. Overnight the
deeper flow will become more northeast, with the low level flow
likely becoming more northerly towards morning. Precipitation
trends should be mainly dependent on the larger scale QG forcing
as that band lifts north over the next few hours before starting
to shift east and diminish. We should see a resurgence of rain in
Denver that`s starting to happen now, and more moderate to heavy
snow in the foothills up to Boulder County. Not sure how much this
will all push north into Weld and Larimer counties, but there will
be some northward push. Whatever is happening there will likely be
shrunk by the usual developing northerly low level winds around
sunrise.

We`re dropping the Winter Weather Advisory for South Park as there
just hasn`t been that much snow in the valley. The easterly flow
hit the mountains up to Kenosha Pass, but the moisture/lift at
higher levels that made it past those mountains wasn`t that
strong. Another inch or two will fall there, but impacts look
minimal so far. Elsewhere road conditions still appear to be tied
to the snowfall rate, with wet roads outside of the heavy snow
areas.

&&

.SHORT TERM /Through Wednesday/...
Issued at 223 PM MDT Tue May 6 2025

Today`s water vapor satellite shows one of the more fascinating
upper level patterns across the CONUS. A well defined upper level
low continues to spin over central New Mexico this afternoon, with
a secondary upper low diving into south/central Nevada.
Meanwhile, a third upper low continues to spin over the eastern
Great Lakes. I`m not sure I can remember a spring pattern quite
like this.

At the surface, the weather across Colorado is also equally
interesting. An expansive precipitation shield remains anchored
across the eastern half of the state... which is unsurprising
given the KFTG/KPUX VAD profiles (along with various ACARS/model
soundings) continue to show an extremely deep easterly upslope
regime. While the heaviest precipitation today has generally fell
south of the Palmer Divide, there has been respectable snowfall
totals already out of the higher terrain (more on that in a
second), along with plenty of rainfall across the I-25 corridor.
Rainfall reports from reliable mesonet stations across the Denver
metro have generally reported 0.4-0.8", with over an inch in
spots across the lower foothills. Some convective elements have
also emerged on radar over the past hour or two with closer to 250
J/kg of MUCAPE east of I-25.

Across the higher elevations, snow has been a little more
impressive, except for South Park where precipitation has
struggled to make it (blocked by the terrain in Teller county).
While reports have been sparse, we have received reliable reports
of 5-6" across Boulder/Gilpin/northwest Jefferson counties.
Webcams also confirm that snow levels dropped far quicker than
guidance (particularly NBM) this afternoon, likely due to the
diabatic cooling effects during the heavier precipitation earlier.
It`s difficult to say how well snow will stick below 7,000-7,500
feet, but we did get snow mentions in across most of the
foothills and higher portions of the Palmer Divide overnight
tonight. Impacts are far from certain, but the current
Warnings/Advisories seem to tell the story well enough.

While the above paragraphs are a fairly good summary of today`s
weather, I unfortunately cannot offer as good of a forecast for
tonight into tomorrow. Composite reflectivity over the region
shows that more consistent precipitation is drifting west-
northwestward into our CWA, as the primary rainfall band with this
upper low gradually lifts north. This should get widespread
precipitation back into the forecast area over the next couple of
hours, with perhaps another burst or two of heavier snow across
the Front Range/Foothills. Guidance continues to be of little
help, with poor model-to-model/run-to-run consistency today. At
this point most of the forecast overnight is based on my
extrapolation of the current radar and general pattern
recognition, rather than any particular deterministic/ensemble
solution.

The upper low should begin to shear out and weaken by Wednesday,
with gradually decreasing upslope flow. As temperatures begin to
warm back up into the 50s or perhaps low 60s, modest instability
will allow for widely scattered showers and a few thunderstorms,
though confidence for any given location/time is fairly low.

&&

.LONG TERM /Wednesday night through Tuesday/...
Issued at 223 PM MDT Tue May 6 2025

Any remaining precipitation should quickly diminish Wednesday
evening as the upper low continues to weaken and drift eastward.

Thursday will start a trend towards warmer and generally dry
weather across the forecast area, as a large ridge builds across
the intermountain west. Temperatures by Thursday afternoon should
be close to normal values for this time of year - upper 60s to low
70s - with isolated to scattered convection across the higher
elevations.

There may be a little more coverage to the convection over the
higher elevations on Friday as warmer temperatures lead to a
little more instability, along with the near/above normal moisture
trapped underneath the building ridge.

Warmer weather is likely this weekend into early next week as the
mid-level ridge continues to strengthen and the thermal ridge axis
shifts closer to the CWA. Highs should reach the mid/upper 70s by
Saturday and the 80s by Sunday. Additional warming is likely into
next week, with mid 80s possible by Monday or Tuesday. With the
low-level moisture remaining trapped, at least isolated/scattered
showers are forecast each afternoon. The pattern turns to a broad
southwesterly flow aloft eventually but we`ll sort those details
out later.

&&

.AVIATION /06Z TAFS through 06Z Thursday/...
Issued at 1145 PM MDT Tue May 6 2025

Occasional rain showers will continue through much of Wednesday,
but forecasting ceilings and to some extent visibility will
remain the largest concerns. We`re starting with ceilings mostly
just above MVFR, but with additional showers and especially a
slight turn in winds to the NNW through 09Z, we expect lower
ceilings to reform with IFR/MVFR conditions developing and
becoming widespread through 15Z. There is some threat of fog but
with the present stratus deck we think most visibility
restriction will be due to light rain, occasionally reducing it to
4-5SM. Ceilings should start to lift with diurnal heating
15Z-18Z, but still remain MVFR. There will also be a greater than
50% chance of thunderstorms due to the moist but slightly unstable
airmass on Wednesday, and MLCAPE near 200 J/kg and virtually no
cap as soon as we start to warm. Thus, we`ll have TEMPO -TSRA in
the 19Z-23Z period, but could start as early as 18Z or so.

Overall, light north to northwest winds should persist through
about 03Z, then light drainage winds, less showers, and gradually
rising ceilings expected by 03Z-05Z Thursday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 AM MDT Wednesday for COZ033-034.

Winter Weather Advisory until 6 AM MDT Wednesday for COZ035-036.

&&

$$

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