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852
FXUS63 KDVN 200530
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1230 AM CDT Fri Sep 20 2024

.KEY MESSAGES...

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- Scattered storms (30-60% coverage) mainly along and north of
  U.S. Highway 30 in Iowa through late evening that may make it
  to the Quad Cities and northwest Illinois before fading.

- Active weather pattern this weekend to bring several chances
  of much needed rain to the area.

- Seasonable temperatures and mostly dry conditions next week.
-- End Changed Discussion --

&&

.UPDATE...
Issued at 838 PM CDT Thu Sep 19 2024

The forecast area will continue under glancing effects from the
deep upper low over southern Manitoba through late evening. A
pronounced cap due to a well-mixed boundary layer was observed
on our 00Z sounding this evening. While that capping is
presumably less north of the U.S. Highway 30 corridor (where
isolated initiation has occurred), there likely is still some
presence of that CIN. With increasing upper and mid-level flow
as seen on water vapor imagery, as well as the system low-level
jet of 25-35 kt developing over the region, storms are likely to
maintain some organization and not fade in overall coverage
until early overnight. In fact, they may even uptick prior to
midnight within general low-level confluence. Also of note,
there was a cell with explosive growth just norht of the CWA in
Fayette and Clayton Counties in northeast Iowa near 8 PM.

As for severe potential, the message continues that the later
any storms are the more minimal potential for severe and more so
small hail and 40 mph wind type hazards. While strong synoptic
systems in a transition season (early meteorological autumn) can
maintain kinematics sufficient for severe potential at night,
that is more ideal if storms are already of an organized mode by
this time of evening moving into that air mass. If there are
any storms to reach severe limits, they look more likely to be
semi-discrete and hail and/or wind and mainly along/north of
U.S. Highway 30.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 228 PM CDT Thu Sep 19 2024

Radar mosaics have depicted scattered showers and thunderstorms
just to our west across north central IA for much of the day.
These storms developed along an instability gradient and have
remained below severe limits, with just a 29 mph wind gust
reported at the Waterloo airport. Latest water vapor and
satellite imagery shows CI occurring near the IA/MN border,
where there is stronger upper level support. 12z DVN sounding
showed a cap around 770 mb that has kept any clouds that
developed locally fairly shallow today. Forecast focus in the
near term is on storm trends and severe weather risk.

Assessing surface analysis and model trends today, I noticed
models were initially overdoing Tds this afternoon. This caused
some of the 06z model runs particularly the NAMNest, to convect storms
too early. In any case, recent CAM runs have matched reality
and show storms developing across southern MN and northern IA
this afternoon in the warm sector, with the greatest upper
level support. Discrete supercells are expected initially, with
some indication of 0-3 km shear values becoming perpendicular to
the storm motion and allowing for some bowing segments. A
narrow 1-3 hr corridor of a tornado threat remains between
23-01z over northeast IA tonight. Otherwise, a fat CAPE sounding
and DCAPE values over 1200 J/Kg may allow for a few severe
storms tonight with marginally severe wind/hail before the sun
goes down and instability quickly wanes. Regarding timing, the
HREF local probability matched mean (LPMM) composite
reflectivity shows storms to move east southeast through the CWA
between 7 pm and Midnight tonight.

With high PWs increasing between 1.7 and 2", any storms will be
proficient rain producers with rain rates 0.5"/hr expected.
Rainfall totals between a quarter and three quarters of an inch
are forecast. This would be much welcomed rain with most of the
area not seeing much rain since Aug 30th. The 12z HREF LPMM
Accumulated QPF even shows some potential for locally higher
amounts approaching 2" in Benton and Iowa counties overnight!
There is some potential for fog along and north of Hwy 30
overnight, but left out of the forecast for now due to expected
cloud cover.

Friday...high pressure will build overhead with clouds
diminishing in the morning leading to dry conditions. Afternoon
highs will top out in the mid to upper 80s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 228 PM CDT Thu Sep 19 2024

Friday Night-Sunday Night...broad ridging aloft will begin the
period, with the latest deterministic and ensemble models in
agreement ejecting the closed low over the southwest CONUS east
of the Colorado Rockies by Sunday afternoon. A lead shortwave is
progged to move along a frontal boundary Saturday night
bringing increasing clouds and precipitation chances. High
moisture levels indicated by PWs in the 1.6 to 1.8" range (200%
of normal) and rich 850mb theta-e air in place to support heavy
rain amounts. ENS/GEFS/GEPS 24-hr probabilities of at least 0.5"
of rain are now in the 50-60% range across the CWA ending 00z
Monday. This is indeed good news for rain prospects! Regarding
the severe potential, there remains too low of probabilities and
uncertainties in instability/shear overlap at this time. That
said, Day 3 CSU machine learning probs suggest there could be
some risk added as we get closer to the weekend.

Early Next Week...the upper low mentioned above to move over the
Midwest, with continued clouds and precip chances through at least
Tuesday. Cooler, more seasonable temps, dry conditions, and
comfortable humidity levels are forecast behind this system,
with highs in the 70s and lows in the 50s Tuesday-Thursday.
Perfect for the 1st full week of astronomical Fall!

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
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Issued at 1228 AM CDT Fri Sep 20 2024

Weak frontal boundary with SHRA/TSRA associated with it will
move through eastern Iowa and northern Illinois through 18z/20.
Brief MVFR/IFR with SHRA/TSRA through 12z/20 but otherwise VFR
conditions will be seen through 06z/21.
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&&

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

&&

$$

UPDATE...Friedlein
SHORT TERM...Gross
LONG TERM...Gross
AVIATION...08