Winter Storm Warning 6, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
545 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 /00Z TAFS through 00Z Tuesday/...
Issued at 542 PM MDT Sun May 17 2026

Models are showing east and northeasterly winds at DIA overnight
well into Monday. Upslope winds are expected to continue after 18Z
Monday, but with a more due northerly direction. Will leave a
PROB30 in later this evening for -SHRA based on area radars and
high resolution models. Widespread rain is expected from Monday
late morning into Monday evening. Will go with PROB30 for a few
hours for thunder Monday afternoon. For ceilings, will get them
down in the OVC006-012 range from just after midnight tonight and
continue that through 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...66

Special Weather Statement
National Weather Service Denver CO
625 PM MDT Sun May 17 2026

COZ032>034-180100-
Grand and Summit Counties Below 9000 Feet CO-South and East
Jackson/Larimer/North and Northeast Grand/Northwest Boulder
Counties Above 9000 Feet CO-South and Southeast Grand/West
Central and Southwest Boulder/Gilpin/Clear Creek/Summit/North and
West Park Counties Above 9000 Feet CO-
625 PM MDT Sun May 17 2026

...A STRONG THUNDERSTORM WILL IMPACT NORTHWESTERN SUMMIT AND CENTRAL
GRAND COUNTIES UNTIL 7 PM MDT...

At 625 PM MDT, Doppler radar was tracking a strong thunderstorm 6
miles north of Green Mountain Reservoir, moving northeast at 40 mph.

HAZARD...Wind gusts up to 40 mph and half inch size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
  Granby, Kremmling, Hot Sulphur Springs, Williams Fork Reservoir,
  Parshall, Hot Sulphur Spring, Green Mountain Reservoir, Elkdale,
  Tabernash and Heeney.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Monitor the weather situation closely and be alert for threatening
weather conditions.

&&

LAT...LON 3998 10647 4002 10651 4004 10649 4032 10617
      3998 10582 3981 10633
TIME...MOT...LOC 0024Z 237DEG 36KT 3996 10633

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH

Hiris

Special Weather Statement
National Weather Service Denver CO
656 PM MDT Sun May 17 2026

COZ045-180130-
Central and East Adams and Arapahoe Counties CO-
656 PM MDT Sun May 17 2026

...A STRONG THUNDERSTORM WILL IMPACT ARAPAHOE AND SOUTHEASTERN ADAMS
COUNTIES UNTIL 730 PM MDT...

At 656 PM MDT, Doppler radar was tracking a strong thunderstorm near
Byers, or 38 miles east of Denver, moving northeast at 35 mph.

HAZARD...Wind gusts up to 30 mph and half inch size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
  Byers.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Monitor the weather situation closely and be alert for threatening
weather conditions.

&&

LAT...LON 3958 10421 3965 10433 3993 10412 3975 10378
TIME...MOT...LOC 0056Z 231DEG 31KT 3966 10424

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...30 MPH

Hiris

Special Weather Statement
National Weather Service Denver CO
722 PM MDT Sun May 17 2026

COZ034-035-039-180145-
Larimer and Boulder Counties Between 6000 and 9000 Feet CO-
Boulder And Jefferson Counties Below 6000 Feet/West Broomfield
County CO-South and Southeast Grand/West Central and Southwest
Boulder/Gilpin/Clear Creek/Summit/North and West Park Counties
Above 9000 Feet CO-
722 PM MDT Sun May 17 2026

...A STRONG THUNDERSTORM WILL IMPACT EAST CENTRAL GRAND...BOULDER AND
SOUTHERN LARIMER COUNTIES UNTIL 745 PM MDT...

At 722 PM MDT, Doppler radar was tracking a strong thunderstorm over
Ward, or 39 miles northwest of Denver, moving northeast at 35 mph.

HAZARD...Wind gusts up to 40 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
  Boulder, Jamestown, Ward, Peaceful Valley, Raymond, Gold Hill,
  Allenspark, Brainard Lake, Summerville, Wallstreet, Sunshine,
  Salina, Meeker Park, Mt Audubon, Crisman, Arapahoe Peak, Riverside
  and Caribou.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Monitor the weather situation closely and be alert for threatening
weather conditions.

&&

LAT...LON 3997 10563 4013 10571 4017 10566 4016 10560
      4017 10557 4018 10557 4019 10554 4021 10555
      4020 10559 4022 10555 4025 10555 4026 10552
      4025 10549 4027 10548 4029 10549 4030 10547
      4030 10546 4031 10546 4003 10523
TIME...MOT...LOC 0121Z 245DEG 30KT 4005 10554

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

Hiris

Special Weather Statement
National Weather Service Denver CO
750 PM MDT Sun May 17 2026

COZ035-038-039-043-180215-
Central and South Weld County CO-Larimer County Below
6000 Feet/Northwest Weld County CO-Larimer and Boulder Counties
Between 6000 and 9000 Feet CO-Boulder And Jefferson Counties
Below 6000 Feet/West Broomfield County CO-
750 PM MDT Sun May 17 2026

...A STRONG THUNDERSTORM WILL IMPACT WESTERN WELD...EASTERN BOULDER
AND SOUTHEASTERN LARIMER COUNTIES UNTIL 815 PM MDT...

At 750 PM MDT, Doppler radar was tracking a strong thunderstorm 5
miles south of Lyons, or 30 miles south of Fort Collins, moving east
at 35 mph.

HAZARD...Wind gusts up to 40 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
  Northern Boulder, Longmont, Firestone, Frederick, Berthoud, Mead,
  Lyons, Niwot, Hygiene and Campion.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Monitor the weather situation closely and be alert for threatening
weather conditions.

&&

LAT...LON 4004 10535 4015 10531 4024 10538 4036 10502
      4011 10492
TIME...MOT...LOC 0150Z 251DEG 32KT 4015 10525

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

Hiris