218
FXUS65 KBOU 181801
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1201 PM MDT Mon May 18 2026

.KEY MESSAGES...

- Rain and mountain/foothill snow increases in coverage this
  morning with much colder temperatures arriving.

- Accumulating snow for the mountains and 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.

&&

.UPDATE...
Issued at 933 AM MDT Mon May 18 2026

Snow was increasing over the mountains, with a heavy accumulating
drizzle along the Front Range I-25 Corridor. Precipitation rates
will increase as expected this morning through mid afternoon with
the heart of QG forcing and band of heavier precipitation on the
western slope moving in. It was thinning a bit as it moves this
way, but still expect a 2-3 hour period of enhanced precipitation
rates and even a possibility of a couple thunderstorms as this
moves in. Given snow levels are lower already (down to about
6500-7000 feet in Larimer County), we added a Winter Weather
Advisory for the northern Foothills as snow intensity should pick
up to >1" per hour. Also there was even a narrow corridor of
freezing drizzle in the higher foothills of Boulder County
(7,500-8,500 feet) where deeper seeder/feeder action was not in
play yet. That will change soon as deeper moisture arrives. We
also added North Park to the Winter Weather Advisory considering
heavier snow had begun falling there this morning and will
continue through mid afternoon with rates 1" per hour or more at
times.

Finally, still can`t rule out a few snowflakes mixing in with our
rain in the I-25 Urban Corridor despite high surface wet bulb
temps. Main opportunity would be with convective showers early to
mid afternoon, and then into evening if precipitation lingers long
enough under the influence of further cold advection aloft.

&&

.DISCUSSION /Through Sunday/...
Issued at 1222 AM MDT Mon May 18 2026

An upper level trough over the Great Basin will move across the area
on Mon with favorable mid level ascent thru the aftn hours.  At the
sfc, upslope flow will be in place which will combine with the
upper level trough to produce widespread precip across the area.
Snow will occur in the mtns and higher foothills with several
inches of accumulation. Have upgraded areas north of I-70 in the
mtns to a warning as east facing slopes could see from 12 to 18
inches in some areas. Elsewhere will keep amounts in the advisory
range. In the foothills, some of the higher areas may see 4 to 8
inches above 8000 ft with lessor amounts below 8000 ft.

By Mon night precip should gradually decrease over the area during
the evening hours.  Overnight lows late Mon night into early Tue
morning may drop down to freezing or slightly below across
portions of the plains.

For Tue,  a disturbance will move across the area late in the aftn
thru Tue night.  As this feature moves across, there will be another
round of precip over the higher terrain and across portions of the
plains.  Highs on Tue will remain cool as readings stay in the 50`s
across the plains.

Looking ahead to Wed, an unsettled pattern will continue as decent
lapse rates will combine with lingering moisture to produce a good
chc of showers and thunderstorms over the higher terrain and
portions of the plains.  Most of the activity will occur in the late
aftn and early evening hours.  Highs will remain below normal as
readings only reach the upper 50`s to mid 60`s across the plains.

By Thu into Fri, latest data shows another upper level trough moving
southeast towards the area.  Not sure about the timing of this
system, however, as it moves across should see another good chance
of showers and thunderstorms.  Temperatures both days will be in the
mid 60`s to lower 70`s across the plains.

For next weekend, will see a return to a drier and warmer pattern
based on latest data.

&&

.AVIATION /18Z TAFS through 18Z Tuesday/...
Issued at 1159 AM MDT Mon May 18 2026

Model cross sections indicate that the ceiling should be at or
below BKN-OVC015 at DIA the rest of the today and all of tonight.
Will leave a PROB30 in for TSRA this afternoon for 3 or 4 hours
based on the instability available. Visiblities should range from
1SM-5SM, lower with the better rain showers through around 00Z
this afternoon. Will get visibilities to P6SM after that. There
could be patchy fog developing after 06Z tonight, but there is
enough uncertainty at this time so I will it out for now.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 9 PM MDT this evening for COZ030-
031-034-035.

Winter Storm Warning until 9 PM MDT this evening for COZ033.

Frost Advisory from midnight tonight to 8 AM MDT Tuesday for
COZ038-042>051.

Freeze Warning from midnight tonight to 8 AM MDT Tuesday for
COZ041.

&&

$$

UPDATE...20
DISCUSSION...RPK
AVIATION...66

NWS BOU Office Area Forecast Discussion