924
FXUS65 KBOU 110248
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
848 PM MDT Wed Jul 10 2024

.KEY MESSAGES...

- Mostly dry with continued warming throughout the week. 20-40%
  chance of showers and thunderstorms

- A heat wave is expected Friday through Monday with temperatures
  reaching 98 to 105 degrees across the urban corridor and plains.
  Overnight lows will be near 70 across the urban heat islands,
  resulting is limited relief from the heat.

- A heat advisory is in effect Friday, Saturday, and Sunday from
  10a-8p for the I-25 corridor. It will be hot everywhere,
  including outside of the advisories, so take steps to limit
  outdoor activity and find ways to cool off and stay hydrated as
  much as possible.

&&

.UPDATE...
Issued at 846 PM MDT Wed Jul 10 2024

Radar shows a couple of weak showers and a remnant outflow
boundary or two across the forecast area, but otherwise it`s a
quiet evening. With surface-based instability rapidly decreasing,
any remaining showers should diminish in the next hour or two. The
rest of the overnight should be pretty quiet. No significant grid
updates were needed.

&&

.SHORT TERM /Through Thursday/...
Issued at 250 PM MDT Wed Jul 10 2024

Satellite imagery shows development of scattered cumulus across
portions of the plains this afternoon. Slightly higher instability
over the higher terrain this afternoon. Showers and thunderstorms
have developed over the mountains so far this afternoon, moving
from NNW to SSE. Expect scattered showers and weaker storms to
continue over the mountains and Foothills through early this
evening. Some may bring brief outflow gusts up to 30 mph. The
adjacent plains are more likely to stay dry, but can`t rule out a
few outflows extending off the Foothills early this evening.

The strong ridge aloft will continue to dominate the pattern
tomorrow. The center axis makes a slight eastward shift bringing a
ridge of warmer temperatures aloft. This will warm highs a couple
degrees from Wednesday. Highs will likely rise into the low to mid
90s for the urban corridor and mid to upper 90s for the plains.
70s and 80s are likely in the mountain valleys/foothills and 60s
in the mountains. It will be dry again tomorrow given the ridge
aloft with model soundings on the plains showing large temp-
dewpoint spreads and inverted V profiles. For the mountains, can
see a similar outcome as Wednesday with marginal instability
contributing to terrain forced showers and weak thunderstorms. A
weak wave moves across the far northeast corner of Colorado as
well giving a low chance for a shower. Otherwise, main takeaway
for tomorrow is that it is getting hotter and it will keep going
in that direction for the days following. See long term discussion
below.

&&

.LONG TERM /Thursday night through Wednesday/...
Issued at 250 PM MDT Wed Jul 10 2024

It will be flat out hot in the extended, at least through Monday.
The ridge at 500 mb could reach 600dm, and even it it does not
reach that strength it will be very close. On Friday it will be
centered over the Great Basin and through Monday it will move
slowly east. The center of the ridge should be east of us by
early Tuesday, which mercifully means significantly less heat and
better afternoon thunderstorm chances by midweek. Here are the
details:

On Friday, ensemble guidance puts >70% chance of exceeding 100 deg
along the South Platte Valley from Brighton to the Nebraska
border, and any of the plains below 5,500 ft elevation, while
along the I-25 corridor it`s more like a 40-50% chance. 700 mb
temps look to exceed the magic number of +20 degC, which invites
the potential for highs around 100 in the well mixed atmosphere.
A potential wrinkle in reaching 100 always is convective
potential/clouds, as clouds can sometimes thwart a run at 100.
Across the plains, it will be very dry at the surface, and well
mixed up to 500 mb or higher. Very little to effectively zero
instability is expected anywhere across the plains, and with the
ridge just upstream there will be synoptic scale subsidence to
boot. Thus, the only convective clouds are expected to be over the
mountains, and with weak NNW flow aloft, those clouds won`t make
it too far east. Thus, we have hoisted a heat advisory for the
I-25 corridor from 10 AM - 8 PM Friday to account for the heat of
the day where most of the impacts on human health occur in our
area. We are doing this just for the I-25 corridor to provide a
heads up for vulnerable populations where overnight lows may only
drop to the upper 60s. Elsewhere across the plains it will be
equally hot during the day, between 98-104 deg, but overnight lows
will be more tolerable and overall impacts should be a little
less vs the urban areas. The high country has a 10-25% chance of a
shower/thunderstorm which will provide relief during the
afternoon hours.

On Saturday and Sunday the heat continues, in fact might be a
degree or two warmer on Sunday per ensemble guidance. We have
issued heat advisories both days from 10 AM - 8 PM for the urban
corridor, but as with Friday it will be hot everywhere. A little
subtropical moisture works into Colorado from the southwest each
afternoon, resulting in slightly better shower/thunderstorm
chances across the high country (30-40%). Light rain and gusty
winds would be the main impact, along with cooler temperatures
near the showers/storms. But once again steering flow aloft on
Saturday is 10-15 kts out of the North, and on Sunday it is 10 kts
out of the east. Thus, it`s unlikely any storms move off the
mountains at all on Sunday, and any that do on Saturday or Sunday
will not get far with the very dry low-levels. PoPs are <10% for
the plains below 6,000 ft Friday-Sunday. Saturday could be a
degree or two cooler along the Front Range due to clouds, but only
within 30 miles of the Front Range.

Something we`ll be watching closely are the low temperatures in
the urban areas Saturday through Monday mornings. They look to be
close to 70, which does not provide much relief especially if
it`s 80+ deg past midnight. If the lows creep up into the 70s, we
may consider extending the heat advisories overnight for zone 40
(metro Denver below 6,000 ft).

Monday the heat continues though with a little bit more uncertainty.
Ensemble guidance has a lot more spread in high temperatures, mostly
due to cloud cover, outflow, and increasing chances of precipitation
and hints of a front as the ridge moves east of us.  There is about
a 20% chance of exceeding 100 deg in metro Denver, and 40% across
the lower elevations of the plains. Granted, 100% of the guidance
has a high of at least 90 for Denver, so definitely still hot. For
these reasons, we have not decided on an advisory for Monday.  PoPs
increase a bit in the mountains as well as the plains but
instability looks very limited.  PW still could be below normal even
though it increases to 0.75-1" across the plains and 0.5-0.75" in
the high country.

On Tuesday-Wednesday there is an influx of moisture both aloft
and near the surface, such that PW could become above normal
(120-150%). With northeasterly low-level flow and potentially Tds
in the 50s, good convective coverage and cooler temperatures
should be the norm into midweek. However, we aren`t getting our
hopes up that the heat wave is short lived. From an ensemble
perspective, there are still high chances (40-50%) of exceeding
90 degrees in Denver and the northeast plains despite the increase
in moisture.

&&

.AVIATION /00Z TAFS through 00Z Friday/...
Issued at 545 PM MDT Wed Jul 10 2024

VFR through the TAF period.

Main area of interest is the wind forecast. DEN has remained in
fairly consistent NNW flow this afternoon while APA/BJC have
bounced between light east and north. Outflow from storms across
the foothills has been hard to come by, but there is a boundary
near Boulder as of the time of this AFD. May be a brief period of
westerly winds before drainage takes over this evening.

Similar wind patterns are expected on Thursday with generally some
sort of N/NE wind with light speeds.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from 10 AM to 8 PM MDT Friday for COZ038>040-043.

Heat Advisory from 10 AM to 8 PM MDT Saturday for COZ038>040-043.

Heat Advisory from 10 AM to 8 PM MDT Sunday for COZ038>040-043.

&&

$$

UPDATE...Hiris
SHORT TERM...Schlatter
LONG TERM...Mensch
AVIATION...Hiris