119
FXUS63 KDMX 041136
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
636 AM CDT Thu Jul 4 2024

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

* Slight Risks for Severe Storms and Heavy Rainfall today, mainly
  across central into northern Iowa

* Western and southwest areas most likely to have quiet conditions
  for post sunset activities.

* Seasonably cool conditions tend to prevail in coming days,
  along with additional opportunities for showers/storms.

&&

.DISCUSSION...
Issued at 331 AM CDT Thu Jul 4 2024

After a welcome quiet day and evening on Wednesday, the 4th of July
will return to the unsettled weather conditions.

Early this morning an area of showers and weak convection was
approaching the SW CWA and tracking E/NE, tied to a shortwave moving
through the mid to upper level flow. The core of this activity will
remain relegated southward into central and northern Missouri, tied
more closely with the low level boundary and LLJ jet activity. While
this activity will provide periods of light to moderate rainfall
across southern into central Iowa this morning, it does not carry
potential for substantial impacts, wind/hail/heavy rain or otherwise
with its progressive nature and broadly unsupportive environment.
What may be of note to watch is the speed with which this activity
clears out and allows/does not allow sun/insolation to settle in.

Looking to central to northern Iowa, upper level currently sweeping
across the Dakotas will push broad forcing and a surface boundary
into the area and be the impetus for potential severe and heavy rain
activity today. Hi-res/CAM guidance has struggled in recent runs to
depict a consistent solution, adding uncertainty to the potential
today. What is known/should be in place though is supportive deep
layer shear of 40 to 50+ kts, at least marginal SB/MLCAPE values of
1000+ J/kg and PWAT values around 1.5" by late morning into the
afternoon. Leading cloud cover and light rain/showers driven by the
broad lift, in conjunction with the southern shortwave activity
draws into question how much solar insolation and priming can be
done before the deeper lift and boundary drop into the area. Should
a couple/few hours of sunshine breakout, potential for stronger near-
surface instability and general updrafts may increase notably,
elevating hail and wind potential. Tornado potential may remain
limited with weak low level shear and helicity values depicted, but
cannot be ruled out with any longer lived/organized convection. The
current Day 1 SPC Outlook and its components continue to appear
well placed over northern Iowa, closer to the deeper lift and weak
surface low. Heavy rain risk will be largely dependent on areas
experiencing multiple rounds of convection, as storms should be
progressive and training potential limited. The lack of consistency
in the hi-res/CAM runs really mucks up confidence in this area.
Though there have been enough hints showing leading convection ahead
of the surface boundary followed by convection tied more closely
with the surface to warrant at least some risk of heavy rainfall in
excess of 3 inches. Urban areas would be the most at risk in this
situation, as while the area has seen recent rainfalls, rural areas
at this point in the growing season tend to be able to handle fair
amounts of rainfall. Expect most of this activity to occur from
around 18z through 00z within the forecast area, with some lingering
non-severe scattered activity after.

Region will find itself bouncing between NW and semi-zonal flow
aloft in the coming days, largely keeping the area in seasonal to
seasonably cool conditions and multiple opportunities for showers
and storms. NW flow regimes can be sneaky severe weather producers,
and while SPC Day 4 currently depicts predictability too low,
Saturday night could a period to keep a casual eye on given current
synoptic guidance. Otherwise, expect highs predominantly in the
upper 70s to mid 80s across the area Friday onward within the
forecast.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 632 AM CDT Thu Jul 4 2024

Predominantly light rain or rain showers will move through
portions of the area this morning, affecting KOTM and
potentially KFOD/KMCW for prolonged periods. KDSM/KALO may see
more brief periods of this activity before more substantial
storm chances affect all TAF sites later today. Primary window
for convection/storms will be around/after 18z in the west,
translating eastward through the afternoon. Likely to clear out
of all sites by around 00z, with some lingering lighter activity
possible for a couple more hours. With some uncertainty areal
extent of development and timing progression, have opted for
VCTS and tempos at highest confidence sites KFOD/KMCW, and VCTS
elsewhere for the time being. Aside from flight category
reductions during prevailing TSRA, northern sites KFOD/KMCW may
see MVFR ceilings move in toward the latter portions of the
period.

&&

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

There is a Slight Risk for Excessive/Heavy Rainfall over
northern into eastern areas today, driven largely by the
potential for multiple rounds of thunderstorms ahead of and
along an upper level low and surface boundary. Confidence in
this playing out is relatively limited with inconsistent hi-
res/CAM guidance. Should an area experience multiple rounds of
thunderstorms, moderate warm cloud layer depths and PWAT values
of 1.5 inches or so will yield efficient rainfalls and the
potential to receive 3 or more inches of rain. Given the
tendency for rural and agriculture areas to be able to accept
healthy amounts of rainfall this time of the year, impacts
appear most likely to be over urban areas this afternoon and
evening within the risk area.

Rivers within the Des Moines River Basin continue to slowly
fall, though Estherville, Humboldt, and Emmetsburg remain in
Major or Moderate stages. Recent rainfalls across the north have
resulted in the Cedar River Basin experiencing rises, in Minor
Flood stage at Shell Rock, reaching Minor Flood Stage today at
Cedar Falls and potentially at Waterloo over the next day or so.
Southern Iowa rivers, tending to be faster responding, are
quickly receding after recent heavy rains, with none having
reached flood stage. The Saylorville Reservoir lake level
forecast remains relatively unchanged with current forecast
having it peak about 878 feet around July 10th and 11th.
Additional rainfalls within the Des Moines River Basin may alter
that forecast in coming days.

&&

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

&&

$$

DISCUSSION...Curtis
AVIATION...Curtis
HYDROLOGY...Curtis