543
FXUS65 KBOU 171953
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
153 PM MDT Sun May 17 2026

.KEY MESSAGES...

- Severe storm chance waning, and mostly confined to far
  southeastern portions of the forecast area into early evening.

- Rain and mountain/foothill snow increases tonight through
  Monday with much colder temperatures arriving.

- Accumulating snow for the mountains, and >70% chance of
  accumulation in the foothills. Only a few wet snowflakes
  possibly mixed in for the I-25 Corridor.

- One last spring freeze possible on some of the plains Monday
  night.

- Delay in the warming and drying trend for the week ahead, but
  still warmer and drier by Friday - next weekend.

&&

.DISCUSSION /Through Sunday/...
Issued at 153 PM MDT Sun May 17 2026

Satellite shows widespread convective development this afternoon
with showers and storms spreading from the mountains and foothills
northeast across the plains. The first cold front had pushed all
the way to the CO/New Mexico border, and thus most of the
instability was much farther south. There has been a slight return
of southeast flow into El Paso County, so a little recovery of
instability could occur with a few hundred J/kg of ML/MU CAPE
over the Palmer Divide. However, showers and a few storms were
already developing and will limit additional heating and
destabilization. Thus, the threat of any severe weather is
decreasing, although an isolated severe storm would still be
possible over Elbert or Lincoln Counties into early evening.

While threat of severe storms is decreasing, the coverage of rain
and mountain snow will be on the increase through the next 24
hours. This will be supported by increasing QG lift and upslope
as an upper level trough digs sharply into the Great Basin
tonight, and then lifts northeast across the forecast area Monday.
The orientation of this trof axis keeps a dry slot over the
eastern plains so after this afternoon`s/evening`s convection we
should see some drying roughly southeast of the I-76 Corridor
overnight. Meanwhile, periods of showers and a couple storms will
continue farther north and west overnight. The atmospheric column
will start to cool especially late tonight through diabatic
cooling and eventual cooling aloft as the trough axis approaches.
That means rain changing over to snow for our higher elevations,
and likely working down to about 8,000 feet in the northern
foothills by daybreak Monday. Snow levels will continue to drop
through the day as further cooling occurs. While 700 mb temps drop
to -5 to -6C by late afternoon Monday, wet bulb zero levels
suggest (all) snow levels only dropping to around 6500 feet.
We`ll also be battling May sun angle and some solar insolation, so
getting snow down into the major metro areas will be challenging.
Nonetheless, a few snowflakes will be possible in the late
afternoon or the evening hours as temperatures continue to cool
and the airmass dries slightly. The chance of any snow
accumulation for most of the Urban Corridor is less than 20%, and
those amounts would be just a dusting to less than one half inch.

The heaviest snow is still expected for areas above 8,500 feet in
the northern tier of mountains and highest foothills including
Rocky Mountain National Park, with totals of 6-15 inches there.
Totals will gradually decrease southward into the mountains of
Summit County (3-8 inches), and then less than that in the
southern foothills and high mountain valleys including those
around Dillon (trace to 3 inches). We`ve opted to add Zone 31
(northern Gore and Park Ranges) to the Winter Weather Advisory,
and also extend that into mid evening for lingering snow and
travel impacts. Storm total liquid precipitation through Monday
evening should range from 0.75-1.5 inches north of Denver, to less
than 0.5 inches southeast of I-76 on the plains and south of I-70
in the mountains.

Temperatures on Monday will be significantly cooler, with near
steady or even slowly falling temperatures along the I-25 Corridor
through the lower 40s when the heaviest precipitation arrives
late Monday morning into Monday afternoon. We`re still eyeing
potential for freezing temperatures on all of the plains for late
Monday night into early Tuesday morning, but this still depends on
whether we clear out and winds subside fully. It appears the
eastern plains could still have a little wind, while I-25 Corridor
should hold onto more clouds. For now, we`ve issued a Freeze
Watch for all of the lower elevations, as temperatures will drop
very close to freezing in urban areas, and likely a degree or two
below that in rural locations.

Overall, there was a delay in ejecting the entire trough axis to
our east. It now appears the trough lingers to our west all the
way through Wednesday with another shortwave potentially for
Thursday in northwest flow. Essentially, this will keep a higher
chance of showers and a couple storms in the forecast through
Thursday, while also delaying our warmup slightly. Once the
kicking wave Thursday passes by, we still expect a warming and
drying trend into next weekend, with above normal temperatures
finally returning for the weekend.

&&

.AVIATION /18Z TAFS through 18Z Monday/...
Issued at 1215 PM MDT Sun May 17 2026

Area radars are showing some weak convection coming off the
foothills and weakening as it moves east. Little to no lightning
has been detected thus far. Models keep the northeasterly winds
going at DIA through much of the TAF period. There could be
variable gusts to 30 knots with the convection this afternoon.
Will leave the -TSRA in this afternoon with the most confidence
for just -RA portion. Will continue the chance of rain showers in
the TAF this evening, tonight and much of Monday. For ceilings,
will go with a BKN035-040 this afternoon and evening. Will get
ceilings down to OVC015 and lower after 06Z tonight and Monday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to 9 PM MDT Monday
for COZ031-033-034.

Freeze Watch from late Monday night through Tuesday morning for
COZ038>051.

&&

$$

DISCUSSION...20
AVIATION...RJK

NWS BOU Office Area Forecast Discussion