331
FXUS63 KDMX 131749
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1249 PM CDT Sat Jul 13 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Hot and humid temperatures with heat index values of 100 to
  105F today through Monday.

- Possibility for thunderstorms tonight into tomorrow morning,
  with strong winds and heavy rainfall the main concern.

- Additional shower and thunderstorm chances on Monday, with a
  few strong storms and heavy rain again possible in the
  afternoon.

&&

.DISCUSSION...
Issued at 410 AM CDT Sat Jul 13 2024

Upper level ridging continues to build in from the southwest US, as
multiple areas of convection are seen throughout the plains region
as weak waves propagate along the peripheral of the high pressure.
Fortunately, much of the activity remains north and west of the
state where the stronger theta-e advection wing resides and stronger
moisture convergence is present. That being said, should the
convective activity to our north and west (particularly the MCS
currently over western Minnesota as of the time of writing this)
hold on through the morning hours, could potentially see some
convection leak into northern Iowa today. Models arent necessarily
capturing this well, so will be monitoring convective trends closely
through the early morning hours.

Our first day of hot and humid temperatures begins today, as the
influence of the heat dome begins to take hold of the state. This is
indicated by the 850 temperatures of 20+ celsius now in place over
western Iowa. Warm 850 temperatures will expand east through the
morning, pushing highs into the 90s over roughly the southwestern
half of Iowa. These temperatures will combine with some sticky 70+
dewpoints to put heat indices near 100F for most and up to 105F for
some. Therefore, the heat advisory remains in place for much of
western, central and southern Iowa today, with a slight expansion
further north during this forecast cycle.

Looking ahead to tonight, the upper level ridge and associated low-
level moisture plume continues further east, which will open up the
chances for some storms tonight into Sunday. As mentioned
previously, weak waves will be kicking off convection through the
overall northwest flow, but a definitive forcing mechanism isnt
really apparent as this occurs. This lack of forcing will negate
storm development during the day, thanks to the warm layer aloft
putting a stop to any parcels that are displaced. However,
uncertainty increases greatly when you consider the convection
occurring across the north today, especially as the atmosphere
decouples this evening and the LLJ ramps up overhead. CAMs have been
all over the place for thunderstorm progression, ranging anywhere
from an MCS dropping into northern Iowa to nobody in the forecast
area seeing a single drop of rainfall. Therefore, without any strong
forcing mechanism to keep an eye and lack of reliable model
guidance, confidence is low on how convection will play out tonight.
Storm prediction will rely heavily on identifying mesoscale features
and monitoring trends through the day today and into tonight. If an
MCS were to drop into the state tonight/tomorrow morning, strong
winds and brief heavy rainfall would be the main severe weather
concerns, which SPC has highlighted with their day 1 slight risk for
severe weather across far northeast Iowa.

The hot and humid conditions continue through Sunday, with slightly
warmer highs in the low to mid 90s and widespread heat index values
of 100 to 105F. A heat advisory will likely be needed once again on
Sunday, but have continued the day by day strategy for issuing
headlines given the uncertainty of convection tonight and how that
may affect highs Sunday. The heat dome then begins to break apart on
Monday, as a cool front pushes the warmer temperatures further
south. That being said, expecting temperatures on the warm side of
the front to again be nearing advisory criteria with more high
temperatures in the 90s on Monday. The extent of these temperatures
will be contingent on the location of the front on Monday, as well
as any convection and resultant cloud cover that develops along it.
Guidance continues to show thunderstorms developing along the
boundary as it pushes south on Monday, but has backed off on the
extent of storms further west in the state. This is likely due to
the warm layer still present along the boundary and large amounts of
CINH preceding it. That being said, bulk shear values around 30 to
35 kts and plenty of instability will make Monday worth keeping an
eye on, especially with PWATs of 2+ pooling along the front. SPC
currently has a marginal risk area wide, with a slight risk clipping
our far northeast.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1248 PM CDT Sat Jul 13 2024

More expansive MVFR cu field developing along boundary in
northern Iowa; affecting mainly FOD and ALO for now. Some
potential for brief MVFR cu at DSM this afternoon. Projected CAP
at H700 expected to hold, but still uncertain. If cap doesn`t
hold, will have an area of storms breaking out somewhere near
US20 corridor by 21 to 02z. CAMS have not handled well past 24
hours, but forcing still struggling over our area. Winds have
increased with deeper mixing south of boundary at DSM/OTM and
should continue with gusts nearing 23kt through 23z. Will
monitor future any convective trends that might occur and update
as needed. /rev

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for IAZ004-015-023-
024-033>035-044>048-057>062-070>075-081>086-092>097.

&&

$$

DISCUSSION...Dodson
AVIATION...REV