727
FXUS63 KDMX 081947
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
247 PM CDT Tue Jul 8 2025

.KEY MESSAGES...

- An isolated storm possible this afternoon. Patchy fog possible
  tonight over eastern Iowa.

- Thunderstorm chances increasing Wednesday night into early
  Thursday. Low chance of locally heavy rainfall.

- Strong to severe storms, locally heavy rainfall that may lead
  to flash flooding Thursday afternoon into early Friday.
  Another round of storms, heavy rainfall is probable on Friday
  afternoon into Friday evening.

- Rises or renewed river rises and possible river flooding by
  late this week or this weekend on portions of basins in
  central Iowa.

&&

.DISCUSSION...
Issued at 247 PM CDT Tue Jul 8 2025

Clouds have been clearing out of the state after storms rolled
through much of western and southern Iowa early this morning. This
afternoon, with the clearing sky, temperatures have once again
warmed well into the 70s to low 80s. Have seen cumulus development
over southern Minnesota ahead of a weak front and this has dropped
into northern Iowa. The temperatures are pushing towards the
convective temperature, but there is an elevated warm layer. A
glancing shortwave trough to the northeast may provide enough to
overcome this along with the surface boundary, but not expecting
more than an isolated storm or two over northern Iowa where those
cumulus clouds are dropping down this afternoon. High pressure will
pass over the state tonight with very light winds as the surface
front settles just south of the state. With an inversion setting up
and the recent rainfall, this may foster an environment conducive
for fog formation. However, forecast soundings show that full
saturation is not achieved, but surface and low level dewpoint
depressions are just a few degrees. For now, have added some patchy
fog mentions in eastern portions of the forecast area.

As the high moves off to the east Wednesday, will be looking at a
more active period based on pattern recognition along with AI/ML
guidance from late Wednesday night/early Thursday morning through
late Friday. However, there is lack of strong synoptic forcing and
thus the details on hazard type and timing will rely on mesoscale
features like leftover boundaries or depend on the degree of
convective cloud debris that are largely unknown at this time. For
Wednesday night/early Thursday, low level warm air advection may
bring some sort of convective cluster towards the state. While
severity likely will be waning into Iowa, efficient, heavy rainfall
parameters will be favorable with a 30 knot low level jet (LLJ)
funneling moisture into the state. There is the potential for a
corridor of higher rainfall totals, which are reflected in the 12z
GFS, FV3, and CMC along with the latest HREF max field over a small
portion of western into central Iowa. This may not be the most
probable outcome given the rest of the guidance, which shows amounts
around or less than an inch. However, WPC will have a marginal over
portions of western into a bit of central Iowa for this potential.

As we move into the day Thursday, depending on the amount of
clearing and any outflow boundaries lingering, could have several
thousand J/kg of instability by late afternoon with the front south
of the state now lifting back into Iowa. A lead shortwave trough
topping the ridge will drop into the state and provide for some
higher deep layer shear values in the 30 to 40 knot range. This
parameter space would suggest strong to severe storms by later in
the afternoon into the evening hours being possible. The LLJ will be
more pointed into Iowa Thursday night so expecting the severe storm
potential will lessen, but the heavy rainfall and flash flooding
concerns will be increasing into the nighttime hours. The
aforementioned front will be perpendicular to the LLJ axis and thus
provide for repeated rounds, or training, of storms over the same
area with deep, favorable warm cloud depths and precipitable water
values of 1.5 to around 2 inches. This will add additional rainfall,
perhaps several more inches, and lead to flash flooding, especially
if the rain falls over more vulnerable basins from  the previous
night`s rainfall.

The Thursday night/early Friday storms should diminish to some
degree in the morning hours of Friday, but a shortwave trough
translating eastward from the West Coast will bring some additional
forcing into the region. While cannot rule out stronger storms in
the vicinity of the stalled boundary, deep layer shear will not be
as high at just around 30 knots on Friday afternoon/evening with
plenty of instability. The bigger concern will be flash flooding as
the rainfall adds up with little change in the overall favorable
setup for heavy rainfall on Friday into Friday evening. While there
are timing differences in how quickly the shortwave trough exits,
conditions will trend drier on Saturday.

While details such as the location of the heavy rainfall cannot be
certain this far out, the soil capacity is the least in northern
Iowa, but has also now been reduced in southern Iowa with last
night`s rainfall. River channels across the state also have reduced
capacity with USGS streamflows at any of the 7, 14, or 28 day
averaging period showing at least normal if not much above normal,
especially over northeastern into north central Iowa. While the
ensemble QPF hydrographs are likely suffering from not have the full
window of rainfall as it only goes through early Friday, the
experimental five-day Hydrologic Ensemble Forecasting Service (HEFS)
does show more response at the 10% chance exceedance level. Five day
forecast of the high magnitude flow from the National Water Model
(NWM) forced by the GFS, which has the higher QPF, shows low annual
exceedance probabilities (AEP)/higher streamflows on some streams in
central Iowa. This is all pointing toward rises or renewed rises on
some streams in central Iowa late this week if not this weekend.
Whether it is river or flash flooding or severe weather later this
week, Iowans should continue to monitor the forecast and have a
means to receive weather alerts whether NOAA Weather Radio and/or a
notification service like Alert Iowa.


&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1213 PM CDT Tue Jul 8 2025

VFR conditions are forecast through the period, but will be
monitoring the potential for fog development around 12z
Wednesday. FEW to SCT cumulus may develop this afternoon and
perhaps an isolated storm. However, the chance of any storm
impacting a site is too low to include any mention. Winds from
the northwest will be light at or less than 7 knots through the
afternoon. Winds do become a bit variable overnight through the
end of the period, though any prevailing direction will likely
have a northerly component to it. Regarding fog, confidence with
its formation led to no inclusion at this point with
uncertainty in cloud cover and the extent of low level
saturation.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Ansorge