331
FXUS63 KDMX 031923
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
223 PM CDT Wed Jul 3 2024

.KEY MESSAGES...

- Scattered thunderstorms expected during the day on Thursday,
  with some potential for severe weather and locally heavy
  rainfall.

- Rain chances are lower Thursday evening, but a few isolated
  storms will still be possible around sunset and into the early
  nighttime hours.

- Cooler on Friday, with highs only in the 70s and scattered
  showers and storms north and east. Rain and thunderstorm
  chances then return periodically from late Saturday into next
  week.

&&

.DISCUSSION...
Issued at 223 PM CDT Wed Jul 3 2024

A surface high pressure area is currently sliding across the
region, resulting in relatively light winds and only high clouds
streaming across Iowa. Meanwhile, the frontal boundary that
crossed Iowa and brought storms to the area yesterday is now
stalled across central Missouri and is triggering additional
rainfall there. Tonight, a 500 mb trough will swing from eastern
Montana down into the Dakotas and Nebraska, causing steering
flow to turn toward the southwest over Iowa and resulting in
modest warm air/moisture advection aloft, and also push the old
frontal boundary up into northern Missouri by sunrise Thursday.

The aforementioned trough will push fairly quickly eastward
across Iowa during the day on Thursday, with a ribbon of
enhanced moisture and instability streaming up just ahead of the
boundary. A corridor of MLCAPE in excess of 1000 J/KG is
predicted by most high-res models, which combined with the broad
forcing associated with the mid-level trough will be more than
sufficient to trigger convection as the boundary moves through.
Timing is a bit difficult to pin down, but the most likely time
for frontal passage/storms will be from around midday in our far
western counties to mid/late afternoon in our far eastern
counties. In terms of severe weather potential, deep-layer bulk
shear of around 40-50 KT definitely supports some threat of
organized updrafts and large hail, however low-level helicity
and lapse rates are somewhat less conducive to a wind or tornado
threat. Still, if gaps in cloud cover permit sufficient
insolation to strengthen near-surface instability in the
vicinity of the boundary, then all modes of severe weather could
be possible in a brief window of opportunity. Overall, however,
the severe weather threat appears fairly marginal and is well
covered by the current SPC outlook. PWATs of 1.25-1.50 inches
along the boundary will also support a transient heavy rain
threat, but with the storm coverage expected to be scattered and
the system progressive in its eastward movement, the heavy rain
threat is marginal as discussed in the hydrology section below.
Once the primary trough and line of storms clears out to the
east by late in the day Thursday precipitation chances will
diminish, but lingering instability behind the boundary combined
with a subtle secondary impulse aloft may lead to redevelopment
of isolated storms during the evening. Thus for Independence Day
festivities around sunset the overall odds of conducive weather
are favorable, but there is still a low threat of rain that will
need to be monitored.

By Thursday night the 500 mb trough will have closed off as a
low over Minnesota, which will then sink slowly southeastward
into Wisconsin by Friday. A developing precipitation shield
around this low may spread another round of rain/storms into our
northern and northeastern counties during this time, but severe
weather and heavy rain are not expected given the synoptic
scenario and environmental parameters. Any such lingering rain
should clear out of our northeastern counties by late Friday,
but the modest northwesterly surface flow and cloud shield
around the low will keep temperatures unseasonably cool, with
Friday afternoon highs only in the 70s across the service area.
A weak ridge will then move by to the south providing a break
Friday night, but then a mid- level shortwave will approach
Saturday and Saturday night, followed by a deeper layer trough
building down across the north central and northeast U.S. from
Sunday into early next week. This will lead to persistent rain
and thunderstorm chances during the extended forecast period,
beginning late Saturday and continuing at intervals through the
remainder of the forecast. Any details of severe weather
potential during this time remain unclear, but the overall
pattern supports a generally low threat level. Still, this and
any heavy rain potential will be monitored closely in the coming
days.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1211 PM CDT Wed Jul 3 2024

VFR conditions will prevail through the TAF period. Only high
clouds are forecast through tonight, then later on Thursday
morning scattered showers and thunderstorms will move into the
area. Some terminals may be periodically affected right around
or shortly after 18Z Thursday, but will account for that in
subsequent TAF issuances.

&&

.HYDROLOGY...
Issued at 332 AM CDT Wed Jul 3 2024

Most rivers across northern Iowa continue to slowly fall today,
but with a minor secondary crest forecast in parts of the Cedar
River basin in the coming days. Meanwhile the water is flowing
southward and Saylorville Lake continues to rise, but the
forecast crest has eased just a bit to around 878 feet.
Fortunately we have had a break from rain today, but additional
storm chances in the coming days mean continued vigilance
against renewed flooding. The next round of showers and
thunderstorms is expected during the day Thursday and possibly
into the evening, however those appear more progressive and thus
with a lower probability for heavy rainfall accumulations. In
addition, the storms should be more scattered and not
concentrated along any one river basin. All in all, concerns for
additional/renewed flooding remain relatively low through the
end of the week.

&&

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

&&

$$

DISCUSSION...Lee
AVIATION...Lee
HYDROLOGY...Lee