502
FXUS65 KBOU 032054
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
254 PM MDT Sun May 3 2026

.KEY MESSAGES...

- One last mild day across the region Monday, with scattered
  afternoon showers and a few thunderstorms.

- Significant winter (spring) storm still on track across most of
  the Front Range mountains and foothills from Monday night
  through Wednesday.

- Increasing confidence in seeing snowfall across most of the I-25
  corridor and adjacent plains (>80% chance). Closely watching the
  potential for several inches of snow (40-60% chance) Tuesday
  night into Wednesday.

- Return to a warmer pattern likely again by the latter half of
  the week.

&&

.DISCUSSION /Through Sunday/...
Issued at 144 PM MDT Sun May 3 2026

It`s been a relatively quiet early May day today, with some widely
scattered high-based showers noted on radar this afternoon.
Temperatures have warmed into the low to mid 70s across most of
the plains despite the earlier cloud cover.

Monday will be a bit of a transition day, as our two main players
this week get a little closer to the area - the stronger northern
stream trough located over the northern Great Plains near the
Canadian border, and a cutoff upper low over California. A weak
shortwave will attempt to squeeze between these two features
during the afternoon Monday, leading to isolated to scattered
showers/thunderstorms across the high country, which should
eventually spread into the plains during the latter half of the
afternoon hours. A cold front is expected to push in Monday
evening... setting the stage for one last significant winter
storm.

For the sake of the reader, I`ve attempted to organize several
hours of thoughts into a couple distinct sections below:

Synopsis...
A rather complex winter storm setup is still advertised by today`s
12z model suite. The general evolution of both the northern stream
trough and the southwestern cutoff low has been discussed at
length in previous discussions, so today most of our focus is
related to model trends/sensitivity (i.e., what could go wrong)
and how that translates to the actual gridded forecast. While
guidance has generally been consistent from day to day, there was
still a rather large spread in QPF and snowfall amounts across a
large chunk of our CWA. Why was that?

A crude sensitivity analysis suggested that this was almost
entirely due to uncertainty regarding the strength and position of
a shortwave rotating around the west/southwestern periphery of
the northern stream trough. There was a fairly large shift to the
west with this shortwave today, with nearly every model and
ensemble mean trending towards a much cleaner phase, and (perhaps
more importantly) phasing occurring much further west than
previously advertised. This is shown well in 500mb
height/vorticity d(model)/dt charts between 00-12z Wednesday. We
now have much better agreement across the deterministic/ensemble
suite... leading to a general increase in QPF with today`s runs,
while simultaneously shrinking the overall spread in QPF. All but
one ECMWF ensemble member has over 1" of liquid for KDEN... with
49/51 members producing >1.5" of QPF at BJC/BDU.

That leaves two pressing questions as we get into the short-range
model period. First, are the model snow outputs seen today
realistic (or even possible?). The simple answer is yes, but not
the standard 10:1 snow ratio maps available online. Marginal
temperatures and a May sun angle will limit accumulation
potential in the lower elevations, but a majority of the snow in
the metro would fall Tuesday night when temperatures would
support accumulations, especially during periods of heavier
rates. A rather large portion of CIPS analogs in this period
produced lower elevations snowfall, including a handful of
warning-criteria snowfall events. Second, will we see any
significant shifts in the QPF axis? Recent rapid shifts to storm
tracks inside 48 hours will of course lead to some (deserved)
skepticism of today`s forecast. Was today`s model shift a
correction, or the start of a more significant trend? Only time
will tell. However, it would be rather difficult to shift the
slow-moving Fgen zone away from where its been modeled for the
last several days. In this forecaster`s opinion, the overall
forecast pattern supports both a higher floor and ceiling for
precipitation across nearly all of our CWA compared to the last
several spring storms.

The actual forecast & impacts...
So, that`s a lot of words. What does it mean for you, the reader?
In general, the forecast grids have nudged snowfall totals upward
across all of the Front Range and urban corridor. A Winter Storm
Watch has been issued for all of the Front Range mountains and
foothills. Our storm total snowfall forecast generally calls for
over a foot of snow for most locations along/north of I-70 and
above 8000 feet. With 90th percentile QPF values exceeding 2", it
wouldn`t be surprising to see isolated totals of 18-24" over the
Boulder/Larimer county mountains. Travel impacts are likely across
these areas, especially late Tuesday into early Wednesday.

For the urban corridor, the snowfall forecast remains highly
uncertain. As it stands now, we generally have about 2-4" in the
grids, but this will be highly dependent on snowfall rates Tuesday
night with marginal temperatures through the event. That much snow
could lead to some impacts, especially to the now fully-leafed out
trees around the metro. Higher end solutions (>8" of snow, 20-30%
chance) would lead to more significant tree/power line impacts and
potentially some Wednesday morning travel issues. We did consider
a Winter Storm Watch for the I-25 corridor, but ultimately passed
for reasons noted in the previous section. On the other hand, any
decrease in QPF or increase in temperatures would lead to
little/no impacts for most of the lower elevations.

Further east...a majority of the precipitation should fall as
rain. Some slushy accumulation would be possible, but not enough
for any significant issues.

After the storm... guidance remains in fairly good agreement that
the colder airmass will quickly peel away from the Front Range,
leading to a fairly quick return to near/above normal temperatures
for the latter half of the week. Broad northwesterly flow aloft is
expected as a ridge builds to the west, but a few shortwaves may
bring at least a chance at a few more showers across the region.


&&

.AVIATION /18Z TAFS through 18Z Monday/...
Issued at 1205 PM MDT Sun May 3 2026

DIA`s winds are slow in going northwesterly as per model guidance.
The VAD wind profiler shows wind directions in the 300-310 degree
range about 4000 feet above the surface. Most likely not mixed
down yet because of the cloudiness. It should eventually. Will go
with drainage winds to kick in around 05Z late this evening, with
models showing some westerly downsloping mixed in. For tomorrow
afternoon, we went with a PROB30 after 21Z for -TSRA and
VRB20G35KT. There will be no ceiling issue today and tonight.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Monday night through Wednesday
morning for COZ033-035.

Winter Storm Watch from Tuesday morning through Wednesday
afternoon for COZ034-036.

&&

$$

DISCUSSION...Hiris
AVIATION...66

NWS BOU Office Area Forecast Discussion