SPC Severe Thunderstorm Watch 339

WW 339 SEVERE TSTM IA IL 170855Z - 171600Z
      

URGENT - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Watch Number 339
NWS Storm Prediction Center Norman OK
355 AM CDT Wed Jun 17 2026

The NWS Storm Prediction Center has issued a

* Severe Thunderstorm Watch for portions of 
  Iowa
  Northwest Illinois

* Effective this Wednesday morning from 355 AM until 1100 AM CDT.

* Primary threats include...
  Scattered large hail and isolated very large hail events to 2.5
    inches in diameter possible
  Scattered damaging wind gusts to 60 mph possible
  A tornado or two possible

SUMMARY...Severe thunderstorms are expected to develop through the
predawn hours across a large part of Iowa with large hail possible,
potentially including some significant hailstones (larger than
golfballs) on a localized basis. Storms may organize as they spread
east-southeastward toward the I-80 corridor of eastern Iowa and
eventually northwest Illinois toward/after sunrise, with damaging
wind potential also developing and possibly even some tornado risk
later this morning.

The severe thunderstorm watch area is approximately along and 75
statute miles north and south of a line from 30 miles west of Storm
Lake IA to 10 miles north northeast of Moline IL. For a complete
depiction of the watch see the associated watch outline update
(WOUS64 KWNS WOU9).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch area.
Persons in these areas should be on the lookout for threatening
weather conditions and listen for later statements and possible
warnings. Severe thunderstorms can and occasionally do produce
tornadoes.

&&

AVIATION...A few severe thunderstorms with hail surface and aloft to
2.5 inches. Extreme turbulence and surface wind gusts to 50 knots. A
few cumulonimbi with maximum tops to 550. Mean storm motion vector
27045.

...Guyer


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SPC Severe Thunderstorm Watch 339 Status Reports

WW 0339 Status Updates
      

STATUS REPORT ON WW 339

SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 40 NNE SUX
TO 25 NE FOD TO 15 SSE MCW TO 25 NE MCW.

..BROYLES..06/17/26

ATTN...WFO...DMX...DVN...FSD...ARX...


STATUS REPORT FOR WS 339 

SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS 

ILC015-071-073-085-131-161-187-195-171240-

IL 
.    ILLINOIS COUNTIES INCLUDED ARE

CARROLL              HENDERSON           HENRY               
JO DAVIESS           MERCER              ROCK ISLAND         
WARREN               WHITESIDE           


IAC007-009-011-013-015-017-019-021-023-025-027-031-035-037-039-
043-045-047-049-051-055-057-061-065-067-069-073-075-077-079-083-
087-091-093-095-097-099-101-103-105-107-111-113-115-117-121-123-
125-127-135-139-151-153-157-161-163-169-171-177-179-181-183-185-
187-193-197-171240-

IA 
.    IOWA COUNTIES INCLUDED ARE

APPANOOSE            AUDUBON             BENTON              
BLACK HAWK           BOONE               BREMER              
BUCHANAN             BUENA VISTA         BUTLER              
CALHOUN              CARROLL             CEDAR               
CHEROKEE             CHICKASAW           CLARKE              
CLAYTON              CLINTON             CRAWFORD            

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SPC MD 1162

MD 1162 CONCERNING SEVERE POTENTIAL...WATCH LIKELY FOR EASTERN IOWA...FAR NORTHEAST MISSOURI...CENTRAL AND NORTHERN ILLINOIS...FAR WESTERN INDIANA

Mesoscale Discussion 1162
NWS Storm Prediction Center Norman OK
0721 AM CDT Wed Jun 17 2026

Areas affected...Eastern Iowa...Far Northeast Missouri...Central and
Northern Illinois...Far Western Indiana

Concerning...Severe potential...Watch likely 

Valid 171221Z - 171415Z

Probability of Watch Issuance...95 percent

SUMMARY...Severe wind gusts potentially above 80 mph, tornadoes and
isolated large hail will be likely this morning from eastern Iowa
southeastward across northwest and and central Illinois. Weather
watch issuance is expected along this corridor.

DISCUSSION...Radar imagery shows a well-developed severe bowing line
segment ongoing over eastern Iowa. This line will move quickly
southeastward into northwestern and central Illinois this morning,
where damaging wind gusts will be likely. The line is located near
the nose of a strong low-level jet, which will shift eastward into
western Illinois over the next few hours. In response, the bowing
line segment is expected to become very organized and will lay down
a swath of wind-damage across northwest and central Illinois, where
wind gusts in the 80 to 100 mph range will be possible. In addition,
RAP forecast soundings increase 0-3 km storm-relative helicity into
the 400 to 450 m2/s2 range across western and central Illinois by
mid morning, suggesting that a tornado threat will develop with the
bowing line segment. Tornadoes will be possible near the apex of the
bow, and with any supercell embedded in the line. Isolated large to
very hail could also occur within the more intense cores of
supercells.

..Broyles/Guyer.. 06/17/2026

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...IND...LOT...ILX...LSX...DVN...DMX...

LAT...LON   39308747 39048810 39088873 39428972 39949092 40519207
            40839246 41249260 41589250 41899226 42249172 42339108
            41868958 41568894 40998785 40578738 40198717 39688722
            39308747 

MOST PROBABLE PEAK TORNADO INTENSITY...140-170 MPH
MOST PROBABLE PEAK WIND GUST...65-80 MPH
MOST PROBABLE PEAK HAIL SIZE...2.00-3.50 IN

SPC MD 1161

MD 1161 CONCERNING SEVERE THUNDERSTORM WATCH 339... FOR CENTRAL AND EASTERN IOWA...NORTHWEST ILLINOIS

Mesoscale Discussion 1161
NWS Storm Prediction Center Norman OK
0542 AM CDT Wed Jun 17 2026

Areas affected...Central and Eastern Iowa...Northwest Illinois

Concerning...Severe Thunderstorm Watch 339...

Valid 171042Z - 171245Z

The severe weather threat for Severe Thunderstorm Watch 339
continues.

SUMMARY...A threat severe wind gusts and isolated large hail will
continue across central Iowa over the next hour, and is expected to
increase over eastern Iowa and northwest Illinois as daybreak
approaches.

DISCUSSION...A cluster of severe storms is currently located over
central Iowa, along the eastern edge of an unstable airmass. The RAP
shows moderate instability over much of western and central Iowa,
where MUCAPE is currently estimated in the 1000 to 2000 J/kg range.
As an 850 mb jet strengthens across the mid Missouri Valley early
this morning, MUCAPE will steadily increase and should reach the
2000 to 3000 J/kg range around daybreak from central Iowa into
northwest Illinois. This will provide additional fuel to the ongoing
convection, helping the storms to become more organized with time.
The instability combined with a strong wind field (effective shear
55 to 65 knots) will result in a potential for supercells with
isolated large hail. Supercells will be embedded in the ongoing line
segment. The line segment may begin to take on a bowing shape is it
move across southeast Iowa. This could mean that severe wind gusts
will become the greatest threat as the line moves across the eastern
part of weather watch 339. A potential will exist for significant
wind gusts in excess of 65 knots.

..Broyles/Guyer.. 06/17/2026

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...LOT...ILX...MKX...DVN...ARX...DMX...

LAT...LON   40539069 41048984 41698960 42178979 42519041 42689131
            42839236 42919343 42699400 42329428 42039433 41789418
            41389349 40719177 40539069 

MOST PROBABLE PEAK WIND GUST...65-80 MPH
MOST PROBABLE PEAK HAIL SIZE...1.00-1.75 IN