Severe Thunderstorm Warning 212, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
219 PM CDT Thu Jul 4 2024

.KEY MESSAGES...

- Scattered thunderstorms north and northeast this afternoon and
  evening. Severe weather possible, primarily in the form of
  hail, along with locally heavy rainfall.

- Relatively cool and cloudy on Friday with scattered light
  showers and isolated thunderstorms.

- Steady temperatures from this weekend through the first half
  of next week, with periodic thunderstorm chances beginning
  around Saturday night.

&&

.DISCUSSION...
Issued at 218 PM CDT Thu Jul 4 2024

Well-anticipated thunderstorms are tracking across northern Iowa
early this afternoon, within a region of enhanced instability
ahead of an approaching cold front. Storm intensity has been
held in check by a relative lack of low-level shear, but should
slowly increase over the next few hours leading to a threat of
severe weather primarily in the form of hail as the front and
storms move eastward across the area. Once they clear out later
this evening we will then see several hours of dry and quiet
weather. However, the 500 mb low over Minnesota will drift
slowly southeastward into Wisconsin by Friday morning, spreading
a shield of clouds and light showers/isolated thunderstorms
across parts of Iowa from later tonight in the north and
expanding through the day Friday. Rain will be most likely
across the northeastern half of the state, closer to the low,
but there is enough signal further southwest that low POPs have
been included for virtually the entire forecast area on Friday
afternoon. The clouds will keep temperatures unseasonably low
during the day, and have continued to nudge high temps down with
low to mid 70s forecast across much of the area. This will also
limit instability and severe weather is not expected with the
isolated storms on Friday.

From Friday night into Saturday weak ridging will briefly move
across the area providing a respite, but by Saturday evening, as
a broad mid-level trough begins to carve out over the north
central U.S., the leading shortwave impulse within that trough
will round the base near the South Dakota/Nebraska border and
generate late day thunderstorms over Nebraska that will move
eastward toward Iowa on Saturday night. Current indications are
that decent flow aloft and broad forcing concurrent with the
approaching mid-level shortwave could support some threat of
strong to severe storms in our area, however, this is mitigated
by only modest instability with such a short airmass recovery
time behind the Friday coolness. Even so, with decent warming
and moistening on Saturday afternoon, Saturday night will be a
period to watch for potential severe weather.

During the first half of next week, from Sunday through around
Wednesday, the slowly deepening northern U.S. trough will
dominate our region and prevent any significant warming, with
daily highs generally in the upper 70s to mid 80s during that
time. Subtle impulses moving through the trough will also
provided opportunities for periodic showers and storms, however,
the timing and placement of any such precipitation is highly
uncertain at this range. There are now increasing signals that a
large heat dome building over the southwestern U.S. by the
middle of next week may finally expand eastward and push away
the trough by the following weekend, supporting generally hotter
and drier weather by that time. However, that is beyond the
scope of the 7-day forecast so we will have to wait to see if it
pans out over the next few days.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1214 PM CDT Thu Jul 4 2024

Scattered thunderstorms will affect the area through this
evening, with MCW, FOD, and ALO the sites most likely to be
impacted. Have included targeted TEMPO groups at those terminals
between 19Z and 01Z, with only VC groups further south at
DSM/OTM. Amendments are anticipated based on short-term storm
trends. Once those clear the area after 00Z however, VFR
conditions are expected for much of the night before low
ceilings and light showers move in from the north on Friday
morning. Have included MVFR ceilings and VCSH during that period
at MCW/ALO where impacts are most likely.

&&

.HYDROLOGY...
Issued at 218 PM CDT Thu Jul 4 2024

Most area rivers continue to gradually fall, excepting a few
areas mainly in the Cedar River basin where modest secondary
rises are occurring. Saylorville Reservoir also continues to
slowly rise, with a peak level still forecast near 878 feet on
July 10-11. With scattered thunderstorms moving across the area
today and seasonally high PWATs around 1.5 inches there is some
risk for excessive rainfall in our northern and northeastern
counties through this evening. However, the storms are expected
to be fairly progressive in their movement which mitigates the
threat somewhat. There is a possibility of two or more rounds of
storms impacting a few areas, but even so, recovery time since
last weeks heavy rains and the state of vegetation across the
area also support a relatively low threat of renewed/flash
flooding, at least outside of urban areas.

&&

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

&&

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

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