081
FXUS63 KDVN 300535
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1235 AM CDT Wed Jul 30 2025

.KEY MESSAGES...

- Much higher chances (70-90%) of a round of thunderstorms late
  tonight through Wednesday. Some storms may be severe with
  damaging winds the main hazard.

- Rainfall amounts tonight through Wednesday night could be in
  the 1-2 inch range with localized amounts near 3 inches. This
  could cause localized flooding on Wednesday.

- Confidence is high that much cooler and drier conditions will
  arrive for late week into the weekend.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 230 PM CDT Tue Jul 29 2025

Current Conditions and Synoptic Pattern: Water vapor and upper
air analysis continues to show a broad upper level high pressure
cell/ridge over the central US. Iowa remains along the northern
periphery of this and therefore continues to be susceptible to
ridge runners. 12z KDVN sounding showed strong inhibition
(convective temp 106F) in the wake of the early morning MCS and
therefore has created another tight instability gradient across
the area. The mesoscale features were quite messy with multiple
things going on this afternoon. The early morning MCS has pushed
an outflow boundary well south of the area into Missouri and
has essentially become diffuse. A surface low across southern MN
has draped another boundary east/southeast into northern
portions of the CWA where temps are slightly cooler with
northwest winds. Lastly, a weakening cluster of showers north of
Des Moines has an associated outflow boundary moving into the
southwestern CWA.

The airmass was slowly beginning to recover/destabilize early
this afternoon, especially along and south of I-80 where ML CAPE
was now up to around 2500 J/kg. Much higher instability exists
just west/southwest of the CWA. Temps were increasing into the
middle to upper 80s with dews in the middle to upper 70s along
and south of I-80.

This afternoon through this evening: low confidence forecast
for this time period as CAMS have struggled with morning/early
afternoon isolated storms across northern/central Iowa. One
thing that has occurred is that storms seem to weaken as they
approach our area which makes sense given the inhibition. We
will maintain low pops (10-30%) through the afternoon and
evening hours mainly due to presence of several mesoscale
features. However, it`s likely many places remain dry. Last few
runs of the Warn On Forecast model shows a signal for a few pop
up storms south of I-80, closer to the Missouri border in the
3-6pm time frame. If a storm were to get organized, then
damaging winds would be the most likely hazard. Temps and heat
indices are generally on track with peak heat indices in the 95
to 105 range this afternoon/early evening. Highest values south
of I-80 where temps and dews are higher. Heat Advisory remains
in effect until 8pm.

Overnight through Wednesday: There is a good signal in the high
res guidance that a low level jet will develop overnight across
western Kansas into Nebraska. This combined with an incoming
shortwave and favorable upper level jet structure will allow for
another MCS to form. 12z HREF reflectivity paintball plots
suggest that this will likely form in/near Nebraska and then
roll/develop east overnight. Additional warm air advection type
activity may develop out ahead of it across the frontal
boundary that will be slowly sagging south into central Iowa.
Instability and shear would be favorable for this to be severe
as it approaches western portions of the CWA as early as 3am.
However, several CAMS show it moving in later. There is more
consensus that the MCS moves in closer to sunrise, lasting
through the morning hours. Instability will decrease with
eastern extent; therefore there is some question as to the
severe magnitude. Damaging winds gusts of 60-70mph will be a
concern as this MCS moves in, especially if it moves in as a
line segment as some of the 12z CAMS suggest.

Behind the complex, a lingering (but likely weakening) low
level jet, combined with a surface front and upper level lift
should allow for scattered showers and storms to last at times
through the morning and perhaps afternoon. If the morning MCS has
a strong enough cold pool, this could shunt the storms farther
south. However if it does not, then we will need to watch the
position of the front for additional strong to severe storms by
the afternoon if enough instability can return. Confidence is
low in the severe threat for tomorrow afternoon.

The other component is the heavy rainfall/localized flood
threat. HREF mean PW values increase to around 2.0in late
tonight through Wednesday. Therefore, plenty of moisture is
available for heavy rainfall/efficient rates. Furthermore, the
ECMWF EFI/Shift of tails tool suggests the potential for an
anomalously high rainfall event somewhere across
central/southern Iowa (generally the I-80 corridor). 12z HREF
qpf data suggests a footprint of 1-2 inches with localized
pockets of 3 inches across our area with higher amounts farther
west of the area. Currently, this looks to fall across a long
enough time period to not cause widespread flooding. However,
if that time window becomes shorter, then the flooding threat
could increase. We will continue to monitor the qpf forecasts
over the next 12-18 hours. Currently NBM high temps suggest
upper 70s to lower 80s tomorrow given the clouds/precip and
increasing northerly winds.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 230 PM CDT Tue Jul 29 2025

Ensembles are in strong agreement that a pattern change will
occur with the upper level high getting shunted farther
southwest, with northwest flow aloft developing over our area.
This will allow for a much cooler and drier air mass to move
into the area Thursday into the weekend. NBM high temp spreads
are quite small (2-4 deg) which leads to high confidence in
highs in the 70s and lows in the 50s. This is 10 degrees below
average for late July. Pops remain below 20 percent through
until Monday when the next disturbance moves in.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1231 AM CDT Wed Jul 30 2025

Storm complex over central Iowa is progged to reach the
Mississippi River by 11z/30. After 11z/30 another storm complex
is progged to move through eastern Iowa and northern Illinois
that may have wind gusts of 40 knots or higher. After 00z/31 a
widespread rain will overspread eastern Iowa and northern
Illinois with periods of embedded TSRA. CIGS will be collapsing
to MVFR if not IFR.

&&

.HYDROLOGY...
Issued at 839 PM CDT Tue Jul 29 2025

New river flood warnings have been issued for the Skunk
River at Sigourney and the English River at Kalona. There are
also news watches for all sites on the Iowa River south of
Columbus Junction.

Higher levels are expected on many area rivers based on
the latest 24 hours of forecast rainfall. The current forecast
has widespread 1 to 2 inches of rain across all of eastern Iowa,
northwest Illinois and far northeast Missouri. The current
placement of the heaviest rainfall is on the Wapsipinicon,
Iowa, Cedar, Skunk, and English River Basins. Have decided not
to issue river flood watches for sites that go above flood
stage beyond 48 hours in case the heaviest rain falls in a
different location.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...None.
IL...None.
MO...None.

&&

$$

SHORT TERM...NWS
LONG TERM...NWS
AVIATION...08
HYDROLOGY...Cousins