861
FXUS65 KBOU 132103
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
303 PM MDT Thu Jun 13 2024

.KEY MESSAGES...

- Isolated severe storms with potentially damaging microbursts and
  large hail this evening

- More numerous thunderstorms and a greater severe storm threat
  Friday afternoon and evening

- A return to hot and dry weather this weekend into early next
  week.

- Snowmelt producing minor impacts from high streams. Gradually
  diminishing flows next week.

&&

.SHORT TERM /Through Friday/...
Issued at 249 PM MDT Thu Jun 13 2024

Main concerns are for isolated severe storms this evening, and
more numerous severe storms Friday afternoon.

The "cold" front pushed back into the I-25 Corridor this morning,
but as discussed earlier it didn`t have much impact on
temperatures with highs in the mid 90s! Farther east, at least
there was some cooling with current temps mostly in the upper 80s.
The heating here has eliminated the CIN, with some high based
moderate cumulus. SPC surface analysis shows a ribbon of 1000+
J/kg MLCAPE to the east and northeast of Denver, where post-
frontal moisture depth is larger, with only around 500 J/kg closer
to I-25. Thus, we should get additional convective initiation
closer to the Front Range, and then intensify as it moves into the
more unstable air just to our east. Enough instability and shear
would be present for one or two severe storms with damaging winds
and large hail the primary threats.

Even outside of that area, microbursts will be possible closer to
the I-25 Corridor where DCAPE values are in the 1500-2000 J/kg
range - very impressive values which means isolated damaging gusts
>60 mph will be possible. Also, if deeper moisture is able to push
back (20-30% chance) can`t rule out a more traditional severe
storm bringing a large hail threat.

The bulk of storms should end as we head into the late evening
and overnight hours as the airmass stabilizes. However, with
further moisture/theta-e advection through 700 mb there will still
be a slight chance of storms over the eastern plains overnight.

On Friday, the airmass will destabilize while a shortwave trough
approaches the region from the southwest. This times up well to
produce a greater severe storm threat in the afternoon. MLCAPE
will increase to 1500-2000 J/kg as surface dewpoints hold in the
50s. As southeast flow increases in the afternoon, a Denver
Convergence Vorticity Zone (DCVZ) is expected to develop from the
Palmer Divide/southeast sections of Denver onto the plains. That
will help drive convective initiation (in addition to the elevated
heating source of the mountains), and bring a greater risk of
severe storms including a tornado threat. Large hail, damaging
winds, and heavy rain will all be possible. HREF limited the
amount of updraft helicity, while deep layer shear is a bit
marginal. Nonetheless, the approaching shortwave typically can
make up for that. Therefore, expect at least a few severe storms
on Friday afternoon, continuing into the evening.

&&

.LONG TERM /Friday night through Thursday/...
Issued at 249 PM MDT Thu Jun 13 2024

A return to well above normal temperatures appears likely by this
weekend as a large ridge builds across the eastern U.S. and
gradually strengthens. Broad west-southwesterly flow will likely
be in place by Saturday behind Friday`s shortwave, with
considerable drying through the day. 700mb temperatures are
projected to reach around 15C during the day with upper 80s to low
90s highs likely across the plains. There may be just enough
lingering moisture for an isolated storm or two across the Front
Range mountains/foothills but anything that does develop would
likely produce more wind than precipitation.

The mid-level thermal ridge is forecast to strengthen
considerably Sunday into Monday. As a trough axis approaches to
our northwest and the ridge out east continues to build, this will
likely result in increasing mid/upper level flow and an added
downslope component. Most deterministic, ensemble, and MOS
guidance favor mid to upper 90s across the plains on Sunday and
Monday as 700mb temperatures warm to near 20C. If we are as
hot/dry as advertised, there would certainly be fire weather
concerns at least one or two days this weekend/early next week.
Heat Advisories may also be necessary if the higher end solutions
pan out.

At some point, guidance does bring a cold front into the region
alleviating the heat for most of the region. The GFS is an
outlier, bringing the front in on Monday, while most other
guidance favors Tuesday. Though the aforementioned trough won`t
bring much precipitation, it does appear that we`ll have a few
days with temperatures near or below normal during the mid/late
week next week. Unfortunately, this appears to be short lived,
with a strong ensemble signal for the heat to return by next
weekend.

&&

.AVIATION /18Z TAFS through 18Z Friday/...
Issued at 1148 AM MDT Thu Jun 13 2024

Main concern will be gusty microburst and outflow winds from
passing high based showers and storms. The greatest risk of these
will be from 23Z-02Z, so we`ll have TEMPO VRB winds 25G35-38kts
for this time frame. There`s a small chance (20%) gusts would
reach 45+ kts. Before that, we expect mainly diurnal easterly
winds to prevail behind this morning`s weakening front. However,
there`s a 20-30% chance we could get early outflow or mixing with
a more west/northwest wind at some point this afternoon...before
convective outflows dominate the wind pattern.

Once convection ends this evening, winds should settle down to
more normal southerly flow most likely after 04Z. While low level
moisture increases, this flow is not conducive to any stratus and
airmass is quite warm - also limiting the stratus threat.

On Friday, there will be a greater chance (>60%) of thunderstorms
in the afternoon hours, so TEMPO TSRA and even visibility
restriction is warranted with the potential for strong to even a
couple severe storms nearby after 21Z.

&&

.HYDROLOGY...
Issued at 249 PM MDT Thu Jun 13 2024

Thunderstorms Friday afternoon and evening will be capable of
producing locally heavy rainfall. Precipitable Water (PW) values
increase to between 1 and 1.5", with the highest amounts over the
northeast plains. Storms will be more efficient rain producers
than the last several days, as dewpoints on the plains climb into
the 50s, with even some lower 60s over the northeast corner. Storm
motion is expected to be around 20 mph, which isn`t terribly fast
so given the intensity and ingredients above we think stronger
storms will be capable of producing 1-2 inches in 30-45 minutes.
Considering the low level wind fields, we believe the most focused
area of heavy rain would be from the south/east sides of Denver
metro and Palmer Divide in the mid/late afternoon, and then to
the east and northeast across the northeast plains of Colorado
through the evening hours.

Moisture levels across the high country will be lower; however,
there could be localized areas of heavy rainfall which could cause
flooding issues across the burn scars. Cameron Peak would be most
susceptible given somewhat richer atmospheric moisture profiles there.

Elevated flows continue in the mountain streams, with Flood
Advisories remaining in effect for the streams above Granby and
Grand Lake. Remain alert of the dangers of water that is running
high and cold. Flows are expected to gradually decrease over the
weekend and through next week as the peak snowmelt is passing by.

&&

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

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Hiris
AVIATION...Barjenbruch
HYDROLOGY...Barjenbruch