606
FXUS65 KBOU 042349
AFDBOU

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

.KEY MESSAGES...

- Scattered showers and thunderstorms this afternoon, gradually
  transitioning to steadier rain and snow tonight into Tuesday
  morning.

- 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.

&&

.DISCUSSION /Through Monday/...
Issued at 223 PM MDT Mon May 4 2026

Today`s weather has progressed about as expected. A weak cold
front has been the focal point for most of the showers and
isolated thunderstorms so far today. This front continues to
slowly sink southward, though the mid-level front is still rather
poorly defined at this point in time. A gradual increase in fgen
is expected later this evening/tonight, which will serve as the
focal point for an east-west oriented band of rain/snow showers
and perhaps an embedded thunderstorm or two during the overnight
hours. As snow levels fall from >10kft this afternoon to ~7500ft
by Tuesday morning, snow should begin to accumulate across the
northern Front Range mountains and foothills.

A complex winter (spring?) storm remains on track to produce
significant snowfall across the forecast area Tuesday into
Wednesday. As was the case yesterday, the overnight shift provided
an excellent summary of the synoptic pattern through Wednesday
evening. A majority of the discussion below will focus on a brief
"state of the models" along with some forecast/impact
uncertainties as we draw closer to the start of the event.

All things considered, model guidance has remained relatively
consistent over the past few forecast cycles. While yesterday`s
12z suite had some rather aggressive QPF from a few deterministic
models, ensemble mean QPF yesterday was around 1-2.25" across our
forecast area... and that remains the case today. The main trend
of note today has been to slow down the northern stream shortwave
a touch, though this still eventually leads to a phase with part
of the southern stream cutoff low. The main effect from this trend
has been to hold on to snow a bit longer Wednesday morning or
early Wednesday afternoon, which of course leads to more questions
about impacts (more on that soon). The additional short term
guidance available today also adds a bit more noise that could
subsequently be interpreted as a general trend up/down in
precip/snow. Still, even the short term models appear to be slowly
converging towards the QPF range of the global
deterministic/ensemble solutions.

One thing not discussed in much detail so far is that a
significant portion of the event is driven by mid-level
convergence/frontogenesis before adding in some QG ascent (via
positive vorticity advection) and a slow strengthening of upslope
flow below 700mb. Banded precipitation events are notoriously
difficult to forecast, where the exact location of a persist band
of heavier precipitation typically sets up... but not exactly the
way that higher resolution guidance may suggest. There will almost
certainly be some sharp gradients in QPF/snow early on in the
event that are not captured particularly well our deterministic,
gridded forecast. I have far lower confidence in the forecast
south of the Denver metro (southern Foothills, South Park, Palmer
Divide) compared to places like the Highway 34 corridor for that
reason.

Finally, while forecast confidence in QPF is rather high, that
does not lead to a high confidence forecast in impacts. First, the
translation from QPF to snow is not very straightforward. Higher
resolution guidance varies wildly with surface temperatures below
6500ft during the rain to snow transition Tuesday
afternoon/evening. Is that a reflection of diabatic effects from
falling precipitation (i.e., melting and/or refreezing), or a
symptom of imperfect radiation/boundary layer schemes within the
model itself? I would suspect it`s probably a bit of both. The
rain to snow transition remains pretty similar to our previous
forecast and is perhaps a little conservative. We don`t expect
much accumulating snow before midnight for any of our lower
elevation forecast zones. However, even after the snowfall
begins... we still have several questions left to answer. How long
will it take for snow to accumulate on surfaces? Trees? Roadways?
This will likely be well correlated to snowfall rates... which are
of course uncertain too.

We have fairly high confidence in a couple of inches of snow
accumulating overnight across the Denver metro, but questions
don`t end there. As noted earlier, the slower storm system leads
to higher QPF amounts from roughly 12-21z Wednesday. Will
additional snow manage to accumulate on the existing snowpack from
the overnight hours? Even if snow simply compacts on itself, do
trees/power lines feel the added weight of the snow, even if
accumulations on grass and your patio furniture remain constant?
Is it cold enough for snow to affect the morning commute? The
answer to these questions may again be dependent on snowfall rates
which will likely take another few cycles of higher resolution
model cycles to get a better grasp on.

That leads us to what most people are here to read about, the
storm total snowfall forecast. Our forecast is once again nudged
up a little, though that generally leads to about 3-7 inches of
snow in the forecast. Generally 1-2 feet of snow looks on track
for the Front Range foothills above 7500ft and north of I-70.
Guidance still hints at a few pockets of closer to 2-3 feet,
though this may be overdone with significant compaction likely
through the duration of the event. While our forecast is actually
a little lower than NBM/WPC guidance, it is a closer match to
model positive snow depth change and the HRRR`s variable density
snowfall output.Given the considerable uncertainty in impacts
across the I-25 corridor, we`ve left the Winter Storm Watch as is
for now. The biggest change was to upgrade the southern Front
Range mountains/foothills into a Winter Storm Warning with a
slightly later start time (12 PM Tuesday).

Snow is expected to wrap up quickly on Wednesday afternoon or
evening as the upper trough axis slides off to the east and any
upslope flow quickly diminishes. However, we`ll have one night of
"wintry" temperatures. Guidance over the last 24 hours has become
far more bullish, tanking temperatures early Wednesday night into
the upper teens to mid 20s across a large chunk of the I-25
corridor and plains. While we have had several nights near or just
below freezing, the well below freezing temperatures was enough to
justify a Freeze Watch for the lower elevations.

By Thursday afternoon, guidance is in good agreement that the
colder air will quickly retreat off to the north and east, as a
ridge begins to build west of our forecast area. Temperatures
should rebound into the upper 50s to 60s across the plains, though
this may be somewhat dependent on how much snow is left to melt
across the I-25 corridor and adjacent plains.

The gradual warming trend is expected to continue into the
weekend, with a few shortwaves rippling through the northwesterly
flow aloft. With enough moisture lingering in the region, that
should be enough for daily chances of showers and a few storms.
We`ll have to work out the timing for the better chances of
precipitation here... but given the complexity of the short term
forecast, NBM appeared to be good enough for now.

&&

.AVIATION /00Z TAFS through 00Z Wednesday/...
Issued at 550 PM MDT Mon May 4 2026

Will keep a northerly direction to the winds going through the
evening with a TEMPO group in for light rain showers and variable
wind gusts to 25 knots. Models show more due easterly winds
overnight with just a PROB30 group in for light rain showers.
Will go with ceilings in the BKN-OVC020-030 range from late this
evening continuing overnight. On Tuesday, ceilings below OVC015
are expected along with visibilities in the 3-5SM range along
with -RA BR. It gets cold enough by mid afternoon Tuesday to
warrant snow by 21Z. The heaviest snow is expected from about
03Z-09Z Tuesday evening/night.

&&

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

Winter Weather Advisory from noon Tuesday to noon MDT Wednesday
for COZ031.

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

Winter Storm Warning from noon Tuesday to 3 PM MDT Wednesday for
COZ034-036.

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

Freeze Watch from Wednesday evening through Thursday morning for
COZ038>051.

&&

$$

DISCUSSION...Hiris
AVIATION...66

NWS BOU Office Area Forecast Discussion