Severe Thunderstorm Warning 42, NWS Quad Cities, IA

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1232 PM CDT Tue May 20 2025

.KEY MESSAGES...

- Windy conditions will be seen this morning with sustained east
  winds of 20 to 30 mph, gusting up to 45 mph at times.

- A conditional severe risk (level 1 out of 5) is in place this
  afternoon along/south of Interstate 80 and east of U.S.
  highway 218. The primary risks look to be damaging winds and
  tornadoes.

- Much cooler than normal temperatures will be seen across the
  area Wednesday and Thursday.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 304 AM CDT Tue May 20 2025

The large rain shield across the area with embedded thunderstorms
will slowly move north through the morning. Breaks in the clouds
should then develop south of I-80 that will allow differential
heating to occur.

Some diurnally driven isolated convective cells will probably
develop by early afternoon. However, there is a respectable vort max
that will catch up to the attendant surface low that will be
responsible for the more substantial convection this afternoon.

That raises the question regarding the severe potential.

The differential heating will help initiate the diurnal storms and
the associated surface low and upper level vort max will help
strengthen them.

Based on output from the CAMs, the overall scenario that is being
suggested is low-topped supercells (i.e. storm tops not much above
30 kft). With progged freezing level heights of 10-11 kft, there
will probably be a fair amount of small hail (<0.75 inch) associated
with the storms. The strongest storms and associated updrafts would
be capable of producing 1 inch hail.

What is interesting is the progged corridor running from Keokuk, IA
up to the northeast of DVN/MLI that has non-supercell tornado
parameter numbers greater than 3. Numbers this high suggest that any
horizontal vorticity roll would get quickly stretched if pulled into
an updraft producing a tornado.

The 1-5 PM time frame looks to be the most favored for any severe
but initiation/continuation before/after those times is possible.
Thus a padded 12-6 PM time frame is more realistic.

The area of concern looks to be along/south of the I-80 corridor and
generally east of U.S. highway 218.

The primary severe risks look to be damaging winds and tornadoes.
These will be most likely along any mesoscale boundary and near the
triple point of the passing surface low.

Tonight the diurnal convection will dissipate with sunset but the
proximity of the upper level system suggests very isolated
convection may persist well past sunset.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 304 AM CDT Tue May 20 2025

Wednesday through Thursday
Assessment...very high (>90%) confidence of cooler than normal
temperatures.

The proximity of the upper low combined with steep mid-level lapse
rates will keep plenty of clouds across the area on Wednesday with
clouds slowly breaking up Thursday. As a result, temperatures will
be well below normal.

Wrap around rain will be seen across the area Wednesday into
Wednesday evening as the storm system slowly exits the area. The
best chances for rain will be north of I-80 with little if any rain
south of an Ottumwa, IA to Galesburg, IL line.

Rain may linger into late Wednesday night across the area.

On Thursday, mid-level lapse rates remain quite steep with cold air
aloft. Thus diurnal clouds are expected to develop during the day
and produce some isolated showers, mainly north of I-80.

Thursday night through Friday night
Assessment...High (>80%) confidence on cooler than normal
temperatures

The model consensus has cool Canadian high pressure moving through
the Midwest bringing cooler than normal and dry conditions to the
area.

Saturday through Monday
Assessment...Medium (40-60%) confidence on precip for Saturday and
Saturday night. Low (<25%) confidence beyond Saturday night

Although there are differences in the details, the global models
general show boundaries, either at the surface or aloft, that a
storm system will follow through the Midwest Saturday/Saturday
night. Subtle timing differences between the solutions have resulted
in somewhat lower pops but the model consensus of 20-50 percent rain
chances look reasonable this far out.

The bigger questions is what happens Sunday into Monday.

The CMC slows the Saturday system to a crawl and has it lingering
over the Midwest for the remainder of the holiday weekend. The ECMWF
and ICON models are essentially dry Sunday into Monday.

The GFS/UKMET have a very weak system right on the heels of the
Saturday system that has rain for Sunday/Sunday night. The UKMET has
a tertiary system, albeit very weak, in the cyclonic flow aloft
moving through the area on Monday producing some rain while the GFS
is dry.

Some but certainly not a majority of the ensemble members from the
global models linger precipitation Sunday through Monday across the
area.

Thus the model consensus is biased by the CMC/UKMET solutions and
the ensembles of the global models in keeping 30-40 percent rain
chances Sunday through Monday across the area.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1214 PM CDT Tue May 20 2025

A warm/quasistationary front near I-80 will serve as a
demarcation between VFR and MVFR/IFR ceilings, with the higher
ceilings to the south. As the surface low pressure system lifts
through the IA/IL/MO tri-state region through the afternoon,
prolific scattered thunderstorms will continue. Depending on
confidence at each the TAF site, PROB30 or TEMPO groups were
added to highlight the period of greatest concern for lightning;
the strongest storms will be capable of brief erratic winds and
small hail. Through the afternoon, winds will primarily blow
from the east along and north of the warm front, and from the
south further south. This evening into tonight, those winds will
veer to west-northwest, picking back up and featuring some
gusts after around 14/15z (9-10am) Wednesday.

&&

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

&&

SHORT TERM...08
LONG TERM...08
AVIATION...Bumgardner

Hazardous Weather Outlook...CORRECTED
National Weather Service Quad Cities IA IL
329 AM CDT Tue May 20 2025

IAZ040>042-051>054-063>068-076>078-087>089-098-099-ILZ001-002-007-
009-015>018-024>026-034-035-MOZ009-010-210830-
Buchanan-Delaware-Dubuque-Benton-Linn-Jones-Jackson-Iowa-Johnson-
Cedar-Clinton-Muscatine-Scott-Keokuk-Washington-Louisa-Jefferson-
Henry IA-Des Moines-Van Buren-Lee-Jo Daviess-Stephenson-Carroll-
Whiteside-Rock Island-Henry IL-Bureau-Putnam-Mercer-Henderson-
Warren-Hancock-McDonough-Scotland-Clark-
329 AM CDT Tue May 20 2025

This Hazardous Weather Outlook is for portions of north central
Illinois...northwest Illinois...west central Illinois...east
central Iowa...northeast Iowa...southeast Iowa and northeast
Missouri.

.DAY ONE...TODAY AND TONIGHT

There is a Marginal to Slight Risk (level 1 and 2 out of 5 risk)
for severe storms this afternoon. The risk area is generally south
of Highway 30 and east of Highway 218. Damaging winds and
tornadoes are the primary risks. Localized flooding is also
possible.

Non-thunderstorm winds may gust up to 45 mph at times this
morning.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is possible along and south of Interstate 80
and east of Highway 218 this afternoon.

GENERAL STORM MOTION OF THE DAY:

To the east northeast at 20 mph.

Preliminary Local Storm Report
National Weather Service Quad Cities IA IL
923 AM CDT Tue May 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0649 AM     Rain             2 NNE Credit Island     41.52N 90.60W
05/20/2025  M2.04 Inch       Scott              IA   Trained Spotter

            Corrects previous rain report from 2 NNE
            Credit Island. 24 hour rainfall.

&&

TJG

Preliminary Local Storm Report
National Weather Service Quad Cities IA IL
945 AM CDT Tue May 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0945 AM     Rain             2 NNW North Liberty     41.76N 91.62W
05/20/2025  M2.10 Inch       Johnson            IA   Trained Spotter

            Current storm total rainfall.

&&

TJG

Preliminary Local Storm Report
National Weather Service Quad Cities IA IL
1031 AM CDT Tue May 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1030 AM     Rain             2 SW Saint Donatus      42.35N 90.57W
05/20/2025  M1.53 Inch       Jackson            IA   Trained Spotter

            Storm total rainfall so far.

&&

TJG

Preliminary Local Storm Report
National Weather Service Quad Cities IA IL
1115 AM CDT Tue May 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1114 AM     Rain             Walcott                 41.59N 90.77W
05/20/2025  M1.75 Inch       Scott              IA   Public

            Storm total rainfall.

&&

TJG

Preliminary Local Storm Report
National Weather Service Quad Cities IA IL
1133 AM CDT Tue May 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1130 AM     Rain             1 SSW Geneseo           41.44N 90.16W
05/20/2025  M1.16 Inch       Henry              IL   Trained Spotter

            Storm total.

&&

JRB

Preliminary Local Storm Report
National Weather Service Quad Cities IA IL
1145 AM CDT Tue May 20 2025

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1145 AM     Rain             1 ENE Green River       41.47N 90.32W
05/20/2025  M1.30 Inch       Henry              IL   Trained Spotter

            Storm total.

&&

JRB