749
FXUS65 KBOU 040601
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1201 AM MDT Mon May 4 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 (50-60% chance) Tuesday
  night into Wednesday.

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

&&

.DISCUSSION /Through Sunday/...
Issued at 1200 AM MDT Mon May 4 2026

Forecast remains generally on track aside from slightly increasing
QPF amounts for the region, with a late winter storm impacting
our mountains, foothills, and possibly the plains. On Monday, a
weak shortwave in the morning will likely bring scattered showers
and isolated thunderstorms to the mountains before spilling onto
the plains in the afternoon. Gusty outflow winds will be the main
threat. In the evening, a cold front will travel south, which will
kick off an active period of weather through Wednesday.

A 500-mb upper level low in California will begin trekking east into
the Four Corners region throughout the first half of the week.
Farther north, the upper level jet stream will dip south, with an
embedded shortwave progged over Wyoming/Colorado. These two
features, moisture from the low and cold air from the shortwave,
will likely merge together and result in measurable precipitation
for much of the forecast area, with mountain snow beginning late
Monday, and rain for the plains through Tuesday afternoon-ish,
before transitioning to all snow sometime late Tuesday with the
arrival of colder air. As mentioned in the previous AFD, earlier
model guidance trended towards a more western placement of the upper
level shortwave, with overall increasing confidence in higher
amounts of QPF for the forecast area. Here is what has changed (and
has not changed) with 18Z and portions of 00Z guidance:

Global models and their AI counterparts continue to trend towards
a more western placement of the upper level shortwave. This is
consistent with out hi-res models that are now coming in.
Ultimately, this leads to higher confidence of this materializing.
With this westward trend, colder air is more likely to be
advected into the region, with general consensus between global
models of 700- mb temperatures reaching -8 to -9 dg C behind the
cold front for the plains on Tuesday night. However, some hi-res
guidance coming in does have temperatures as low at -12 dg C,
which would result in slightly higher snow-to-liquid ratios, and
thus higher snowfall amounts.

Still some uncertainty with moisture. New guidance is still in
agreement of the northern portions (Boulder and Larimer county
mountains) receiving the most QPF (upwards of 2.00" of QPF!), with
the southern portions of the Front Range mountains closer to 1-1.5"
QPF. The uncertainty comes with the Denver metro and adjacent
plains. For Denver, almost all the ECMWF ensemble members show
~1.00" of QPF by Wednesday afternoon. The GEFS still is not as
enthusiastic, however it is important to note that that the 10th
percentile has 0.50". The spread in QPF amounts remain similar to
the 12Z guidance, leading to increased confidence of over 0.50-0.75"
for that area. This is further supported by hi-res guidance also
indicating over 1.00" for the metro area. One area that is becoming
increasingly concerning, is the stretch of area from Boulder county
northward to the Wyoming border, west of I-25. Ensemble guidance has
indicated potential of QPF amounts greater than 1.5", with hi-res
guidance showing the potential of 2.00". This is likely attributed
to models resolving about 20-30 kts of upsloping from the surface
all the way up to about 700-mb, with the possibility of a mid-level
low forming.

What does all this mean for us? For the mountains and foothills,
snow is likely from Monday night through Wednesday. Highest
amounts will be confined to the Boulder/Larimer county mountains,
where localized snowfall amounts of ~24" is possible. South of the
I-70 mtn corridor should generally see 4-10" aside from high
mountain passes (Loveland pass) and areas above 10,500 feet.
Travel impacts are likely, particularly for the Tuesday evening -
Wednesday morning commute.

For the Urban Corridor, Palmer Divide, and adjacent plains, this is
where the highest uncertainty continues to be. Many factors will be
at play, such as warm road temperatures leading to only minimal
snowfall amounts accumulating on the pavement, sun-angle, and
meso- scale factors like upslope forcing (weaker upslope flow will
result in less snowfall amounts). Would like to see 00Z and 06Z
guidance before making any assumptions, but there are increasing
chances of higher-end solutions verifying for that stretch of of
area from Boulder county to the Wyoming border west of I-25
(50-60% of snowfall amounts >8"). While there are decreasing
chances as you move farther east, there is still a 20-30% chance
of snowfall amounts >8" for Denver metro. As mentioned in the
previous AFD, if the higher-end solutions verify, this would lead
to more significant tree/powerline impacts, as this is expected to
be a wet and heavy snow event for the plains. In addition, the
heaviest snow is expected to fall overnight Tuesday into Wednesday
morning, where road temperatures will be the coldest and sun-
angle would have the least impact. For now, have decided to hold
off on any highlights, but would not be surprised if a watch or
advisory is issued as new data comes in with the same trend.

For areas further east, forecast remains on track that the majority
of this precipitation should fall as rain. Some slushy accumulation
is possible, but not enough to any significant issues.

Little change to the long term forecast as guidance remains in
fairly good agreement of warmer air returning to the region
towards Thursday/Friday, with temperatures ranging from high 60s
to low 70s across the plains. A broad ridge will begin to build,
with embedded shortwaves bringing chances of precipitation to
mountains and portions of the plains.

&&

.AVIATION /06Z TAFS through 06Z Tuesday/...
Issued at 1127 PM MDT Sun May 3 2026

SW drainage flow will continue through mid-morning for all
terminals, before a W and then NW flow pattern develops around
15/17Z respectively as daytime mixing takes hold. Scattered -SHRA
and -TSRA are expected to develop over the mountains early
afternoon, drifting east into the Denver metro near 21Z, +/- about
an hour. This will be roughly coincident with the arrival of the
incoming cold front, with the FROPA slated for ~21-22Z, ushering
in 12-20 kt N/NNE winds. The interaction with afternoon convection
will likely make wind direction rather inconsistent come late
afternoon, with progressively increase chances for variable
outflows and gusts 30-40 kts in the vicinity of the strongest
cells.

Behind the FROPA, expect a steady lowering of CIGS. There`s some
uncertainty as far as the timing of the transition away from VFR
conditions, with potential for CIGS ~060 with any mid-afternoon
convection. Nonetheless, there`s increasing likelihood for CIGS
falling to or below 015 by ~03-04Z, and certainly by 06Z, as -RA
becomes more persistent and fills in from north to south.

&&

.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...MAI
AVIATION...BRQ

NWS BOU Office Area Forecast Discussion