380
FXUS65 KBOU 041830
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1230 PM MDT Mon May 4 2026

.KEY MESSAGES...

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

- A significant snow storm will impact the mountains and foothills
  late tonight through Wednesday, most pronounced north of I-70
  where the snowfall could be damaging and travel conditions
  exceptionally difficult at times.

- Several inches of heavy/wet snow are expected late Tuesday into
  Wednesday for much of the I-25 corridor, especially on colder
  surfaces. Damage to tree limbs will be possible, as well as
  locally slick conditions for the Wednesday morning commute.

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

&&

.UPDATE...
Issued at 1259 AM MDT Mon May 4 2026

Following the arrival of the full 00Z suite of guidance, the
overall picture remains unchanged - if anything, ensembles
continue to coalesce around a more consistent solution and hack
away at the outliers on either end, with the NBM interquartile
range sitting at roughly 1.20-2.10" of QPF over a broad portion of
the forecast area.

After assessing the above, have opted to expand our winter weather
highlights into some of our lower elevations. Of particular
relevance, a Winter Storm Watch was hoisted for the urban corridor
for late Tuesday evening through Wednesday afternoon. Strictly
speaking, current forecast snowfall amounts aren`t really
commensurate with those associated with Winter Storm criteria as
defined on paper for the majority of our I-25 corridor zones (i.e.
8"+). That likelihood remains on the lower side (20-30% as
mentioned below, slightly higher closer to the foothills),
especially when it comes to accumulation amounts for roadways.
Having said that, this will be a heavy and wet snow, considerably
increasing the risk of damage to tree limbs following an early and
widespread leafout, as well as localized power outages. Moreover,
with the heaviest snowfall rates projected to be coincident with
the colder temperatures Tuesday night/early Wednesday morning,
there certainly is potential for a difficult commute. This late in
the season, even lighter accumulations on roads tend to be much
more problematic. In any case, there`s still room for things to
trend in either direction, and the Watch gives room to maneuver
accordingly.

Along a similar vein, Advisories have been issued for adjacent
plains locations as well as our high mountain valleys. The extent
of impacts is considerably more uncertain in these areas.

Lastly, have upgraded our northern mountains and foothills
(Boulder County northward) to a Winter Storm Warning. There is
high confidence at this point in substantial travel impacts and a
damaging Spring snow event. Some of the higher elevations
surrounding RMNP and the Medicine Bow Range have noteworthy
chances of locally exceeding 2 feet (50-70% probability).

&&

.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 /18Z TAFS through 18Z Tuesday/...
Issued at 1228 PM MDT Mon May 4 2026

A pretty strong cold front over the northern border area of
Colorado is pushing southward at this time. I went with a 21Z
arrival at DIA. I left the PROB30 going for -TSRA late this
afternoon. So with the convection and decent north winds behind
the front, wind gusts to 35 knots are possible. Generally, will go
with ceilings in the BKN-OVC020-030 range this evening, then get
them to or below OVC010 from 06Z onward. Visibilities aren`t
expected to be very low this evening and overnight with a range of
3-5SM with -RA BR tonight. There could be some -RASN late tonight
into Tuesday morning.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 8 PM Tuesday to 3 PM MDT Wednesday
for COZ030-032-037-043-045-046.

Winter Storm Warning from midnight tonight to 3 PM MDT Wednesday
for COZ033-035.

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

Winter Storm Watch from Tuesday evening through Wednesday
afternoon for COZ038>041.

&&

$$

UPDATE...BRQ
DISCUSSION...MAI
AVIATION...66

NWS BOU Office Area Forecast Discussion