366
FXUS65 KBOU 262051
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
251 PM MDT Wed Jun 26 2024

.KEY MESSAGES...

- Scattered to numerous thunderstorms later this afternoon and
  evening. Some storms will be strong to severe with heavy rain,
  large hail, and strong winds, mainly east of a Fort Morgan to
  Limon line.

 - Scattered-numerous thunderstorms return Thursday, some strong
   to possible severe, especially in the plains. The best chance
   of severe storms (30%) is across the far northeast corner.

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

- Potential for significant heat will rise again next week.

&&

.SHORT TERM /Through Thursday/...
Issued at 150 PM MDT Wed Jun 26 2024

A broken line of strong to severe storms could be moving across the
eastern plains of Colorado this evening. The best instability
remains east of the I-25 corridor, and and mostly east of a Fort
Morgan to Limon line. The latest CAMS generate showers and
thunderstorms from the higher terrain and move them east. While
across the I-25 corridor instability is more limited, brief
moderate rain and small hail can`t be ruled out. Overall coverage
shouldn`t be that great which is the reason for just 50/50 chances
of measurable rain. However, once the outflows from these storms
make it east into the better moisture and instability, expect a
broken line of strong to severe storms to develop. Main impact
would be large hail, maybe up to golfball size based on the
instability and vertical wind shear. The I-25 corridor should be
storm/shower free by 9 PM, and by midnight the eastern plains
should be clear of the showers and storms.

Tonight, a really subtle 700 mb trough moves across the northern
half of Colorado. CAMS want to break out light rain showers
between 3-7 AM, probably related to this feature. No significant
impacts are expected from these showers but a few hundredths of
rain could fall. It will be partly to mostly cloudy much of the
night across our area, resulting in a warm night (but thankfully
not anywhere near "hot", the central and eastern US can keep their
hot nights). Lows will be in the low to mid 60s across the
plains, upper 60s for the urban heat islands and along the base of
the foothills, and believe or not, close to 50 degrees in the
mountain valleys like in Summit County, South Park, and
North/Middle Parks.

Tomorrow, the ridge shifts eastward and we have southwest flow
aloft.  There is a fair amount of subtropical moisture that gets
advected from the southwest, resulting in PW values up to 1.2"
across the plains below 7,000 ft, and 0.5-0.75" in the northern
and central mountains/valleys and foothills. There is plenty of
moisture around combined with the power of the late June sun to
get storms going across the high country and Palmer Divide by
early afternoon. South to southeast winds off the deck will bring
in 60+ deg dewpoints across the far eastern plains, generally east
of a Sterling to Burlington line. Initial storms should be
capable of small hail and heavy rain. Coverage again looks to be
spotty and not well organized given the weaker winds aloft. As the
storms move east, a weak dry line should be in place across the
far northeast corner. East of this boundary, any storm has a good
chance of being severe with large hail and severe winds the most
likely impacts. SPC has an enhanced risk out for this potential
across the far northeast corner of Colorado. The main threats look
to be after 6 PM for the far eastern plains. Before the storms
get going we should have another day of 90+ deg heat across all of
the plains below 6,000 ft elevation, but the heat impacts will be
short lived with cloud cover and rain/cool outflows all
afternoon, but especially after 3 PM for the I-25 corridor. The
mountain valleys will also warm into the 70s before thunderstorms
arrive after 12 PM.

&&

.LONG TERM /Thursday night through Wednesday/...
Issued at 150 PM MDT Wed Jun 26 2024

Thursday night into Friday a flat upper level ridge will stretch
from the Desert Southwest into the Southeastern U.S. with an upper
level shortwave moving across the Northern Rockies and Great Plains
States. The combination of deep subtropical moisture, lift from the
passing upper level shortwave/50KT upper level jet and adequate
instability should result in continued scattered to numerous shower
and storms through the evening hours. With anomalous PWAT`s of 0.35"
above normal, some of the storms could produce heavy rainfall and
localized flooding. Further east across the plains, one or two of
the storms could be strong to severe during the early evening.

Storm coverage and intensity are expected to decrease on Friday due
to less available moisture and more stability. However, there still
will be the potential for locally heavy rainfall across the forecast
area along with one or two strong to severe storms across the far
northeastern plains.

On Saturday, a cooler and more stable airmass will be in place east
of the mountains behind the passage of a cold front. Therefore, the
Northeastern Plains will see some relief in the heat along with
limited precipitation chances. Higher precipitation chances will be
possible across the Front Range Mountains, foothills and Palmer
Divide where a few storms could produce brief heavy rain and small
hail.

On Sunday, the upper ridge will be centered over the Texas/Oklahoma
border with an upper trough over the Western States. This pattern
may allow another surge of subtropical moisture to move into
Colorado from the southwest. As a result, could see an increase in
precipitation chances, mainly south of I-70, where locally heavy
rainfall may be possible. Hot temperatures are also expected to
return to the CWA on Sunday with highs climbing back into the 90s on
the plains.

Drier weather is expected along with continued hot temperatures
early next week as the the upper flow becomes more zonal and the
subtropical plume of moisture gets pushed south of our CWA. The main
threat during this period will be gusty winds due to high based
convection and some possible fire weather concerns across portions
of plains and southern foothills.

&&

.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 150 PM MDT Wed Jun 26 2024

Enhanced levels of PW will remain in place through Thursday
evening. Precipitation efficiency could even be slightly higher
in the mountains given a more favorable low-level moisture profile
and overall there are skinny CAPE profiles. Storm motion should
ample to keep storms moving with 15-20 kts of steering layer flow
out of the west-northwest. So while a few storms Thursday
afternoon may be capable of heavy rain, they should be moving fast
enough that the overall threat of flash flooding is very low.
Over the burn scars, it`s only a tad higher (maybe a 10-20% chance
of issuing a flash flood warning). PW`s are expected to decrease
somewhat by Friday. However, there could still be localized areas
of heavy rain with a limited flash flood threat across the burn
scars.

&&

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

&&

$$

SHORT TERM...Schlatter
LONG TERM...JK
AVIATION...Schlatter
HYDROLOGY...Schlatter