National Weather Service Forecast Discussion

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928
FXUS65 KBOU 182024
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
224 PM MDT Sat May 18 2024

.KEY MESSAGES...

- Isolated to scattered high based showers and storms continue
  into this evening with gusty outflow winds

- Slight chance of a severe storm or two northeast plains Sunday
  afternoon

- Cooler, wetter pattern late Monday into Tuesday as a couple weak
  systems track across the region

- Warming/drying trend for the second half of next week

&&

.SHORT TERM /Through Sunday/...
Issued at 225 PM MDT Sat May 18 2024

Scattered high based showers and a couple storms have developed, a
bit earlier than originally anticipated on the plains with one
batch shifting to the east across the Palmer Divide. And that`s
despite the relatively cool/stable boundary layer in place across
the lower elevations. That stability was evidenced in the latest
ACARS soundings taking around KDEN, with near 100 J/kg of CIN
still noted.

Behind that first wave of showers, there will still be additional
chances of shower/storm development given sufficient moisture and
cloud breaks for further solar insolation and destabilization.
We`ve already seen isolated gusts of 40-45 mph with the first
showers, and similar rounds of gusty winds/light rain can be
expected with any of the additional showers or storms that
develop. Eventually later this evening, most of these will work
to the east with the best QG forcing headed that way. By midnight,
most of the precipitation is expected to have ended with weak
subsidence building in and gradual stabilization.

On Sunday, the main concern will shift to the northeast plains.
That`s where low level moisture is expected to increase with
dewpoints pushing into the lower 50s in deeper moist southerly
flow. At the same time, a dryline will organize in the stronger
westerly flow pushing off the Front Range, and shift eastward in
the late morning to mid afternoon hours. That feature along with
weakening CIN during max heating should be enough to get a few
stronger storms going, with the highest probability generally
along and east of a line from Sterling to Akron. MLCAPE near 1500
J/kg are expected, along with sufficient deep layer shear profiles
supporting mainly a large hail and wind threat. SPC`s Slight Risk
comes right up to the border, with a Marginal Risk farther west
toward Sterling. This looks like a good starting point for now.

Meanwhile, the immediate Denver area looks drier Sunday behind the
dry line, with mostly virga if anything. Farther to the north,
there`s still a decent mid level moisture plume - enough to keep
isolated to scattered showers and high based storms in the
forecast. This would include the northern tier of mountains, Fort
Collins, and the northern border area on the adjacent plains.
Temperatures will warm back into the lower 80s for most off the
plains considering the downslope flow and slight warm advection
aloft.

&&

.LONG TERM /Sunday night through Saturday/...
Issued at 225 PM MDT Sat May 18 2024

Main changes in the long-term forecast focus on Monday-Tuesday
system precipitation chances across the mountains to the plains.
Starting Sunday night, lingering showers are possible mainly in
the northern tier of counties.  A positive tilted upper level
trough enters Utah Monday. It appears there is a slight chance of
scattered showers and storms west of the Divide and along the WY-
CO border late Monday afternoon with model guidance favoring a
swath of moisture extending to these areas before the trough axis
enters Colorado. Although these areas are favored across guidance,
this will critically depend on if the trough axis can deepen. If
this outcome doesn`t occur, showers and storms would likely be
delayed until overnight Monday into Tuesday morning. As additional
forcing enters the region, ensembles indicate hefty QPF amounts
especially along the eastern plains Tuesday morning through late
evening. NBM QPF fields between 0.30-1 inch seem reasonable across
the foothills and plains. 700mb temperatures drop between 1C to
-2C throughout the high country Tuesday afternoon. Snow levels
will likely sit near 8500 ft. Given QPF fields displaying
0.30-0.60 inches Tuesday morning through Tuesday evening, light
snowfall accumulations between 2-5 inches are possible for areas
above 9 thousand feet.

By Wednesday, drier air enters the region but there is enough
lingering moisture for an isolated shower or two in the foothills
and plains late Wednesday afternoon. Afternoon highs remain
generally warm Thursday through next weekend with
isolated/scattered PoPs focused for the higher elevations.

&&

.AVIATION /18Z TAFS through 18Z Sunday/...
Issued at 1041 AM MDT Sat May 18 2024

A cold front moved through earlier this morning, with post-frontal
easterly flow dominating across the TAF sites. Those winds should
trend a bit more southeasterly through 21Z, and increase with
gusts 20-25 knots possible. After 21Z, it appears surface winds
will likely be interrupted by high based shower/storm outflows,
making it difficult to pin down any particular direction. That
said, the higher coverage of showers/storms just to the south
would favor more southerly outflows prevailing than any other
direction. The dry airmass will lead to the potential for gusts
up to 35-40 knots. At this point, will lean toward TEMPO VRB gusty
winds between 21Z-01Z. Still can`t rule out VRB winds thereafter
with a few lingering showers, but overall winds should settle
back toward enhanced southerlies after 01Z.

Winds tomorrow afternoon could get variable again, or at least
will be hard to pinpoint any changes. Odds would favor a push from
the north/northwest late in the day given more convection to our
north. VFR conditions will persist with the generally dry low
levels holding firm.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...AD
AVIATION...Barjenbruch

NWS BOU Office Area Forecast Discussion