198
FXUS63 KDMX 261905
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
205 PM CDT Fri Jul 26 2024

.KEY MESSAGES...
- Less than 15% chance of an isolated storm this evening
- Gradually turning hotter this weekend into at least the middle
  of next week with heat headlines possible toward midweek.
  Heat index values Monday and Tuesday well into the 90s to near
  100 degrees with more widespread 100 to ~105 degree heat
  index values Wednesday southern into central Iowa
- Storm chances Sunday night/Monday with a low risk of severe weather
- Another round of storms around midweek with severe weather
  possible, but timing and location remain uncertain

&&

.DISCUSSION...
Issued at 205 PM CDT Fri Jul 26 2024

It was rather impressive this morning with the storm that parked
over Appanoose County. The elevated boundary, copious amounts of
moisture with precipitable water values over 1.3 inches and deep
warm cloud depths, moisture transport into the boundary, and hardly
any low level flow kept storms nearly stationary. A CoCoRaHS
observer had over 2 inches west of Centerville with a trained,
reliable spotter north of Centerville recording almost 4 inches with
those reports as of 7 or 8am. PoPs were held into this afternoon
over our southeastern forecast area as these storms continued to
fester, but have now since diminished. Along the boundary, which
extends from southeastern Iowa into north central Iowa, stratus
clouds were stout this morning. However, these clouds have been
breaking up leaving a mixture of sunshine and clouds this afternoon
in many areas of central Iowa. There does remain high level wildfire
smoke over the state, which will likely linger aloft through
Saturday, resulting in hazy, vibrant red and orange sunrises and
sunsets barring any clouds.

All recent runs of convective allowing models (CAMs) except the RAP
are showing isolated storms popping early this evening somewhere
over central Iowa. This would likely be along the weakening elevated
boundary, but with nebulous forcing it raises questions about
whether anything will develop. With all of the uncertainty will just
run with token PoPs/sub 15%, but will need to monitor. Similar to
Thursday evening, if a storm does develop, brief downpours,
lightning, and perhaps gusty winds given dry subcloud air would be
the main concern.

For the remainder of the discussion covering Saturday through late
next week, will break out by first discussing the upcoming heat wave
and then storm chances. Looking at high temperatures, 850mb
temperatures today should peak around 18 to 20C and result in highs
mainly in the 80s over central Iowa. Saturday and Sunday may be a
touch warmer than today, especially if we start the day sunnier than
today. As we move into early next week, 850mb temperatures increase
about 2C each day Sunday to Monday, Monday to Tuesday, and likely
Tuesday to Wednesday. This should place much of central Iowa into
the upper 80s to middle 90s on Wednesday with the latest National
Blend of Models (NBM) showing probability of reaching or exceeding
90 degrees at 50% or higher across central Iowa. ECMWF is showing
very little in the way of any extreme forecast index (EFI) or shift
of tails during the upcoming heatwave meaning that while it will be
hot and humid, this is not unusual for late July or early August.
Still, seasonal humidity levels are likely from this weekend into
next week (dewpoints a few degrees either side of 70). Combined with
the heat, this will result in heat index values well into the 90s to
near 100 degrees Monday and Tuesday with more widespread heat index
values above 100 degrees on Wednesday. A heat headline is certainly
possible Tuesday if not Wednesday, especially considering the
cumulative effect of heat on the body. The experimental HeatRisk is
showing level 3/major HeatRisk (out of a max of 4) in a few places
over southern Iowa Tuesday afternoon with more widespread level
3/major HeatRisk over southern Iowa into parts of central Iowa on
Wednesday.

Now, onto the storm chances. While there is broad scale ridging over
the region today, a shortwave beneath the ridge that is over
Arkansas at midday today will drift northeastward. This shortwave
and weak low level thermal lift could be the impetus for scattered
storms. Some of the more convective happy models are showing this
early afternoon Saturday somewhere over southern or central Iowa.
This, like tonight`s chance, is quite low and have drawn in some low
PoPs over southern Iowa in the afternoon with the main hazard being
gusty winds, lightning, and brief downpours. The more likely period
for any storms from this shortwave will be as it moves into the
state Saturday night into Sunday, particularly over the eastern half
of the state. Shear is lacking so not expecting more than locally
heavy downpours and lightning. As this pulls into the western Great
Lakes Sunday, another upstream shortwave will quickly be approaching
from the Dakotas. This should bring a more widespread chance of
scattered storms later Sunday evening into Monday, including the
possibility of a few severe storms given the instability and higher
deep layer shear values associated with this shortwave compared to
Saturday night/earlier Sunday. This low severe risk is supported by
the SPC day 3 marginal over much of the forecast area along with the
Colorado State University`s machine learning random forest outlook.

Around Wednesday, a more amplified shortwave trough will be moving
through the northern states. The timing of this wave has varied from
Tuesday night to Thursday, but regardless of when this comes through
the region, there will be storm chances. Right now, NBM has too
broad temporal and spatially the PoPs, but that is the uncertainty
of this time. Severe weather may be in play with the stronger
kinematic fields and instability, but where and as already mentioned
when remain big question marks. CSU`s machine learning random forest
outlook has broad brush low probabilities throughout the middle of
next week - not atypical for late July and early August.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1239 PM CDT Fri Jul 26 2024

A mixture of MVFR stratus/cumulus continues across the far
northern terminals this afternoon, with ceilings ranging from
BKN to OVC. Elsewhere, VFR conditions were observed. Expect the
MVFR stratus/cumulus coverage to gradually decrease this
afternoon, raising ceilings to VFR this evening. Ceilings may lower
east of a line from KOTM to KHPT to KMCW early Saturday
morning, potentially approaching MVFR. However, confidence is
low at this time, so have left ceilings at VFR and will
reassess trends at next issuance. Winds will be from the
southeast at 12 kts or less and turn more southerly by the end
of the period. Gusts across the far north around 20 kts are
possible late in the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Ansorge
AVIATION...Castillo/Ansorge