149
FXUS65 KBOU 261728
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1128 AM MDT Wed Jun 26 2024

.KEY MESSAGES...

- Scattered to numerous thunderstorms today.

- Some storms will be strong to severe with heavy rain, large
  hail, and strong winds.

- Scattered-numerous thunderstorms return Thursday, some strong to
  possible severe, especially in the plains.

- Turning drier into the weekend, with some moderation in
  temperatures Saturday.

- Potential for significant heat will rise again next week.

&&

.UPDATE...
Issued at 1128 AM MDT Wed Jun 26 2024

Still a good deal of uncertainty about convective initiation and
evolution. The best low-level moisture and thus instability
remains over the northern quarter of Colorado, westward all the
way to the Larimer County foothills. Given this, the best be for
initiation would be across the Larimer County foothills where
there is a nice cu field that has already developed, and then the
storms would move move east across the plains during the late
afternoon. Lightning, small hail, gusty winds are possible from
1-5 PM. GOES-16 also showing better instability across the
Mosquito and Front Range mountains/ridges near and south of
Berthoud Pass. Storms should get going there and also move east,
but when the get to metro Denver instability is less vs Larimer
and Weld Counties. Gust winds and brief showers may be the norm
this afternoon for much of metro Denver from 2-6 PM. However, when
the storms from the north and make it east of Denver, CAMs want
to develop a broken line of strong to severe storms that moves
across much of the plains of northeast Colorado. Those storms
could contain severe hail up to golfball size. If that happens,
it`s most likely to occur after 6 PM. Storms should move east of
our area by 10 PM. The existing forecast has these trends well
handled.

&&

.SHORT TERM /Through tonight/...
Issued at 243 AM MDT Wed Jun 26 2024

Upper level high that`s been centered over the Central and Southern
Rockies will slowly sink south today. At the surface, a weak cold
front drops south across eastern Colorado this morning. Temperatures
today drop to near normal with highs in the upper 80s to lower 90s
across northeast Colorado. We see a good increase in moisture today.
Water vapor satellite imagery showing a plume of moisture across the
Four Corners region. This moisture will rotate northeast around the
upper level high today. Also, low level moisture will increase
behind the cold front this morning with dew points in the 50s to
lower 60s across the Front Range and plains. This will result in
precipitable water values of 125-150% of normal east of the
mountains and over the higher terrain precipitable water values near
200% of normal. Heavy rain will be possible with the stronger storms.
Mid level winds of 20-30 knots will help to keep storms moving along.

There are small differences on the amount and placement of
instability and shear today. Models generally agree we will see
MLCAPE of 1000-1500 J/kg over a good part of northeast Colorado. The
main difference is how far west the better instability will be. As
far as shear goes, where southeast surface winds reach 15-20 knots,
bulk shear (0-6km) reaches 30-40 knots. This is enough instability
and shear for severe storms. Hi-Resolution models show scattered
storms forming over the higher terrain this afternoon. The storms
then progress eastward across the Front Range. Once this convection
reaches the plains (east of the metro area), it is expected to
increase in intensity and coverage with a broken line tracking across
the plains. Main threats will be hail up to golf ball size and
strong damaging winds.

&&

.LONG TERM /Thursday through Tuesday/...
Issued at 243 AM MDT Wed Jun 26 2024

Of the extended forecast period, Thursday certainly carries the
higher potential for severe weather, together with more expansive
coverage of late day convection. Ample moisture will remain in
place, only negligibly lower than today`s values, with precipitable
water peaking between 1.10-1.40" over the urban corridor and plains,
the bulk of this above 600mb. The weakening 500mb ridge will
continue to flatten, but a 700mb shortwave becomes more noticeable
in advance of broader troughing to our northwest. Sustained warm
temperatures into the lower 90`s for much of our lowlands will make
for drier near-surface conditions, but with dewpoints in the 50`s
along the urban corridor and 60`s in the eastern plains, and
steepening lapse rates as cold air advection increases aloft, we`ll
have sufficient instability to support scattered to numerous
afternoon showers and thunderstorms.
Rainfall will likely be more efficient in the high country relative
to today given richer low-level moisture, becoming less productive
due east of the Front Range, especially along the I-25 corridor
where precipitation should be more limited. As far as timing is
concerned, a strengthening upper-level jet will help provide for
some enhanced dynamic support in the evening, with a few
showers/storms likely lingering in the eastern plains and high
country.

The trough axis traverses overhead on Friday with robust westerly
flow prevailing aloft, resulting in a marginal moderation of
temperatures and breezier conditions across our higher elevations.
Subsident flow will keep drier conditions in place below 600mb, and
help reduce thunderstorm coverage and intensity east of the
mountains. Nonetheless, a chance of afternoon storms (35-60%) will
persist.

A cold front Friday evening/night will usher in milder
temperatures for Saturday, with highs descending into the low to mid
80`s for the lower elevations - most likely the coolest day of the
forecast period. More stable conditions will also develop as a
ridging pattern begins gaining prominence, with notably lower
precipitation chances region-wide. ML CAPE will be much more
marginal, outside of perhaps the southernmost row of our plains
counties and Palmer Divide where chances for afternoon/evening
convection will be highest.

Warming and drying then accelerates into Sunday under the
expanding ridge, quickly returning us to the 90`s for highs and
keeping precipitation chances quite low outside of the mountains.
Come Monday, there are some signs of troughing over the northern
plains with enhanced west-northwest flow along its leading edge,
which carries potential for some fire weather concerns depending
on just how dry we stay. Regardless, expect the heat to linger and
precipitation chances to remain on the low side. Depending on the
timing and amplitude of the trough, we could see a slight reprieve
in temperatures for Tuesday, but moisture still appears rather
lacking and any cooling is likely to be short-lived, with hotter
temperatures favored to take hold for the mid to late portions of
the week.

&&

.AVIATION /18Z TAFS through 18Z Thursday/...
Issued at 1128 AM MDT Wed Jun 26 2024

VFR conditions expected through Thursday. Thunderstorms are
expected to develop across the high country after 12 PM, and then
move east. There is uncertainty about how well the storms can stay
together across metro Denver and out to DIA. There are enough
ingredients in place for severe storms east of DIA, but storms
will be less intense if they move over any of the terminals this
afternoon. Given the expected spottiness of the storms, TEMPO
groups seem to be the best way to handle the potential of -TSRA.
Showers and storms should clear the terminals by 7 PM. Gusty
winds to 35 or maybe 40 kts are possible with the strongest storms
this afternoon/evening.

In the wake of the showers/storms, southeast winds may briefly gust
over 25 kts, but by midnight drainage winds at the terminals
should take over (SSW DEN and APA 8-12 kts, WNW at BJC 5-8 kts).
Tomorrow winds should shift to the north by midday at 10 kts or
less, and there will be another round of thunderstorms that
either impact the terminals (40-50% chance) directly or generate
gust fronts that result in winds shifts and gusty winds.

&&

.HYDROLOGY...
Issued at 243 AM MDT Wed Jun 26 2024

With an increase in moisture, thunderstorms will be capable of
producing heavy rainfall today. Mid level is expected to be strong
enough to push storms along. However, where storms train or remain
anchored, flash flooding will be possible. Also, flood prone areas,
such as burn scars could see flooding if heavy rain is slow to leave
the area. Conditions will be similar Thursday with ample moisture
remaining in place. Precipitation efficiency could even be
slightly higher in the mountains given a more favorable low-level
moisture profile, although storm motion should still be decent
enough to keep most storms moving fast enough to limit the scope
of flash flooding concerns. Expect the pattern to turn notably
drier heading into the weekend.

&&

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

&&

$$

UPDATE...Schlatter
SHORT TERM...Meier
LONG TERM...Rodriguez
AVIATION...Schlatter
HYDROLOGY...Meier