763
FXUS63 KDMX 150918
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
418 AM CDT Mon Jul 15 2024

.KEY MESSAGES...

- A few strong storms are possible this morning with additional
  development expected again this afternoon and into the early
  evening. Severe weather is likely with any storms that develop
  today, with damaging wind gusts the primary concern.

- Hot and humid temperatures continue south today, with heat
  index values over 105F possible.

- Quieter conditions with the return of more seasonal
  temperatures through the remainder of the week.


&&

.DISCUSSION...
Issued at 414 AM CDT Mon Jul 15 2024

Our upper level ridge to the southwest remains in place early Monday
morning but an upper trough has begun to dig south into the northern
plains region. The resultant forcing has produced convection
currently over eastern North Dakota/western Minnesota at the time of
writing. This complex of storms will bring the first possibility for
storms in the area today as the MCS makes its way south southeast
through Minnesota and potentially into northern Iowa later this
morning. The current expectation is for this to roughly follow the
edge of the more capped air but remain rooted to the LLJ and
associated theta-e advection wing. Therefore, if the MCS can sustain
itself along this path, it would arrive in the far north to
northeastern portions of the area later this morning, with damaging
winds being the greatest concern. However, recent trends in model
guidance have been for this complex to weaken as it nears the state,
likely as it loses support from the LLJ in the morning and succumbs
to the warmer air aloft. If this scenario plays out, the MCS itself
would not be of immediate concern, but its remnants (whether that
be an outflow boundary or an MCV), would be a potential point for re-
development as they pass through less capped air over northeast Iowa
this morning. With plenty of instability present and improving wind
fields allowing for a more favorable shear environment today, any of
this redevelopment could become severe, with strong winds still
being the primary area of concern. If its not already apparent from
the scenarios laid out above, confidence in the morning convective
trends is low, and its certainly also a possibility for the
forecast area to remain dry through the morning hours.

CAMs continue to struggle with how convective trends will play out
into the afternoon as well, and itd be nearly impossible to hone in
on any one solution. Therefore, instead of trying to get surgical
with saying one model run is better than another, well focus on the
features present and the potential scenarios that may result. As we
move through the day, the trough digs further south with a surface
boundary arriving in northern Iowa around 17 to 18z then tracking
through the area and departing to the south in the evening around 04
to 05z. This boundary will be the primary focal point for additional
storm development today, especially as we progress into the
afternoon and the warm layer aloft begins to mix out. The initial
development will favor areas further east where temperatures aloft
are cooler, while areas further west will rely on erosion of the
warm layer aloft to produce storms, likely later in the day and
further south. Guidance indicates that the boundary may also slow
down some as it reaches southern Iowa, which could lead to brief
periods of training storms over southern Iowa. Based on this
expected progression, anticipating that the most favorable areas for
storms will initially be further north and east in the early
afternoon hours, with increasing chances slightly further west and
in the southern portions of Iowa by late afternoon and evening. Of
course, this is the expected scenario. Failure modes/alternate
scenarios would include convection developing west along the nose of
the LLJ in the morning hours and tracking east through the area, a
faster/slower eroding cap allowing for more/less storms further
north and west, and outflow boundaries producing convection ahead of
the synoptic front.

The environment available to storms out ahead of the boundary is
quite favorable for severe weather. MLCAPE values are around 3000 to
4000 J/kg or higher, and stronger midlevel winds ahead of the upper
trough will allow for 0-6 km bulk shear values of 40 to 50 kts.
Moisture will be plentiful as well, with warm, moist air pooling
along the boundary resulting in PWATs up to 2.5 or more in some
locations. Therefore, any storms that fire will become organized and
have the ability to produce efficient rainfall. However, despite the
moisture present within the profile, relatively dry conditions will
be in place at the lower levels thanks to the abnormally warm air in
place over the state. Therefore, DCAPE values of 1000 to 1500 J/kg
are expected with cold pool development likely. This, in conjunction
with the strong shear, will promote upscale growth of initially
discrete storms as they congeal into an MCS, making damaging winds a
strong concern for today. Storm motions will generally be to the
east, which will be along to slightly south of the boundary
orientation. This would suggest the possibility for cell mergers and
ingestion of any boundaries produced by convection further east,
which could locally amplify wind gusts within the MCS. Therefore,
not completely out of the realm of possibility to see winds nearing
70kts with these storms, and could potentially see some localized
wind gusts even higher than that. That being said, with convective
initiation favoring the eastern portions of the area and the 30 to
35 kt storm motions, storms may depart the area prior to reaching
peak intensity, which  echoes both the SPC and machine learning
severe probabilities that favor the eastern portions of the area.
Regardless, this will all depend on where storms initiate and grow
upscale.

In addition to strong winds, hail is possible initially with any
discrete convection, but will  become less likely as the storm
transitions to a linear mode. Similarly, a tornado or two cannot be
ruled out, whether that be with a rotating discrete storm, or with
any embedded QLCS tornadoes that may develop. The 0-3 km bulk shear
will be borderline around 25 to 30 kts, but there will be plenty of
stretching, so will need to monitor for mesovortex development along
any MCSs as well. SPC has maintained the enhanced risk for severe
weather over the eastern portions of our forecast area and into
eastern Iowa, with a risk for significant winds over the same area.
This all being said, want to once again stress the uncertainty in
location and evolution of storms today and into this evening. While
the environment is primed, there are a lot of factors that could
influence storms today. Therefore, our recommendation is to prepare
as if your area could see significant weather, but bear in mind that
some may see little to no storms at all.

Finally, not to be forgotten with the severe weather expected today,
temperatures will once again be hot and humid, mainly over southern
Iowa where the boundary won`t reach until late in the day. With
850mb temperatures in the upper 20s to near 30 celsius, this looks
like it could be the warmest day of the past couple for those in the
southern half of Iowa with highs reaching the mid 90s and apparent
temperatures over 105F. The only caveat will be how convective
debris affects the temperatures over southern Iowa today.
Regardless, a heat advisory remains in effect for these areas this
afternoon.

After the frontal passage today and tonight, temperatures return to
normal through the week and conditions quiet down, with minimal
precipitation in the forecast through the rest of the period.


&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1205 AM CDT Mon Jul 15 2024

Fog is anticipated to develop over KMCW and KALO in the early
morning with IFR restrictions possible until sunrise.
Thunderstorms expected throughout the day, although location and
severity remains at low confidence. Have included VCTS to
reflect window of highest confidence for each site. KMCW to have
storms nearby in the morning, but confidence is low in their
sustainability as they sink into the state so left out of KALO
and KFOD for now. Secondary storms are possible for the rest of
sites in the afternoon, primarily after 20z. Will continue to
refine timing with upcoming issuances. Important to note that
storms may be severe with high winds in excess of 50kts possible
throughout the day.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from 1 PM this afternoon to 8 PM CDT this
evening for IAZ057>062-070>075-081>086-092>097.

&&

$$

DISCUSSION...Dodson
AVIATION...Jimenez