Alert archive / Iowa / Severe Thunderstorm Watches in 2012

Severe Thunderstorm Watch 618

September 4, 20128 counties in Iowa

The Storm Prediction Center issued this severe thunderstorm watch at Sep 4, 2012, 11:16 PM CDT for 8 counties in Iowa. It was in force for 4 hr.

The area

The outline as issued.

The facts

Issued
Sep 4, 2012, 11:16 PM CDT (04:16 UTC)
Expired
Sep 5, 2012, 2:53 AM CDT
In force
4 hr
Issued by
the Storm Prediction Center
Largest hail expected
2 in
Strongest gust expected
60 kt

Counties Allamakee County, IA, Chickasaw County, IA, Clayton County, IA, Fayette County, IA, Floyd County, IA, Howard County, IA, Mitchell County, IA, Winneshiek County, IA

The radar

The La Crosse radar (KARX), 115 km from the middle of the area, began a volume at 04:17 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

What SPC expected

The probabilities SPC published with the watch, for the watch area over its life.

Two or more tornadoes
10%
One or more EF2 to EF5 tornadoes
5%
Ten or more severe wind reports
30%
One or more gusts of 65 knots or more
20%
Ten or more severe hail reports
30%
One or more hailstones of 2 inches or more
20%
Six or more combined severe hail and wind reports
60%

Warnings issued inside it

8 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.

IssuedProductOfficeCountiesLasted
Sep 4, 11:36 PM CDTSevere Thunderstorm WarningARXAllamakee, Clayton, Fayette, Winneshiek47 min
Sep 4, 11:59 PM CDTSevere Thunderstorm WarningARXClayton61 min
Sep 5, 12:12 AM CDTSevere Thunderstorm WarningARXFloyd, Mitchell63 min
Sep 5, 12:31 AM CDTSevere Thunderstorm WarningARXChickasaw, Howard44 min
Sep 5, 1:06 AM CDTSevere Thunderstorm WarningARXFayette, Winneshiek24 min
Sep 5, 1:28 AM CDTSevere Thunderstorm WarningARXAllamakee32 min
Sep 5, 1:51 AM CDTFlash Flood WarningARXClayton, Fayette3 hr
Sep 5, 1:59 AM CDTFlash Flood WarningARXAllamakee, Clayton3 hr

Discussions around it

SPC mesoscale discussions over the same counties from four hours before the watch to its end.

The nearest sounding before it

Davenport, (KDVN), launched 00:00Z, 4.3 hours before the alert went out and 218 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-pSkew-T log-p diagram, Davenport, at 00:00Z on 2012-09-05Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel. CAPE 1890 joules per kilogram.LCLLFCEL10008507005003002001001 km2 km3 km6 km9 km12 km15 km-30°-20°-10°0°10°20°30°40°
Temperature in red, dew point in green, the mixed-layer parcel dashed.
HodographHodograph, Davenport, at 00:00Z on 2012-09-05The wind through the lowest 10 kilometers. Zero to 1 km storm-relative helicity -23 square meters per square second.19395878Sfc1 km3 km6 km9 kmRMLM
The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.
Mixed-layer CAPE
1890 J/kg
Mixed-layer CIN
-49 J/kg
Mixed-layer LCL
1662 m above ground
0-6 km bulk shear
32 kt
0-1 km storm-relative helicity
-23 m²/s²
0-3 km storm-relative helicity
44 m²/s²
Significant tornado parameter
-0.0
Supercell composite
-0.9

Open this sounding in Storm Lab

The text

As transmitted, in the capitals the NWS used until 2016.

357 
WWUS20 KWNS 050414
SEL8
SPC WW 050414
IAZ000-MNZ000-050900-

URGENT - IMMEDIATE BROADCAST REQUESTED
SEVERE THUNDERSTORM WATCH NUMBER 618
NWS STORM PREDICTION CENTER NORMAN OK
1115 PM CDT TUE SEP 4 2012

THE NWS STORM PREDICTION CENTER HAS ISSUED A
SEVERE THUNDERSTORM WATCH FOR PORTIONS OF

       NORTHEAST IOWA
       EXTREME SOUTHEAST MINNESOTA

EFFECTIVE THIS TUESDAY NIGHT AND WEDNESDAY MORNING FROM 1115 PM
UNTIL 400 AM CDT.

HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 40
STATUTE MILES NORTH AND SOUTH OF A LINE FROM 45 MILES NORTHWEST
OF WATERLOO IOWA TO 75 MILES EAST NORTHEAST OF WATERLOO IOWA.
FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
OUTLINE UPDATE (WOUS64 KWNS WOU8).

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.

OTHER WATCH INFORMATION...CONTINUE...WW 615...WW 616...WW 617...

DISCUSSION...ELEVATED STORMS WILL LIKELY PERSIST IN THE WAA REGIME
ABOVE THE SHALLOW COLD POOL ACROSS NE IA...WHILE ADDITIONAL STORMS
MOVE INTO THIS SAME AREA FROM N CENTRAL IA IN THE NEXT COUPLE OF
HOURS.  MODERATE INSTABILITY REMAINS INVOF THE OUTFLOW
BOUNDARY...AND VERTICAL SHEAR IS SUFFICIENT FOR ORGANIZED SEVERE
STORMS.  THUS...THE RISK FOR AT LEAST ISOLATED LARGE HAIL/DAMAGING
WINDS IS EXPECTED TO CONTINUE FOR A FEW MORE HOURS.

AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT
TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60
KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM
MOTION VECTOR 29025.


...THOMPSON

SPC that day

Everything else SPC issued on the same UTC day.

IssuedProductOfficeCountiesLasted
Sep 4, 7:27 PM CDTMesoscale DiscussionSPCBuena Vista, Cherokee, Clay, Dickinson, Lyon, O'Brien and 3 more63 min
Sep 4, 7:45 PM CDTSevere Thunderstorm WatchSPCBuena Vista, Cherokee, Clay, Dickinson, Ida, Lyon and 5 more6 hr
Sep 4, 8:30 PM CDTMesoscale DiscussionSPCAllamakee, Black Hawk, Bremer, Buchanan, Butler, Chickasaw and 8 more60 min
Sep 4, 8:51 PM CDTSevere Thunderstorm WatchSPCBuchanan, Clinton, Delaware, Dubuque, Jackson, Jones5 hr
Sep 4, 9:51 PM CDTMesoscale DiscussionSPCBuena Vista, Calhoun, Cerro Gordo, Clay, Dickinson, Emmet and 12 more54 min
Sep 4, 10:19 PM CDTSevere Thunderstorm WatchSPCBlack Hawk, Bremer, Butler, Calhoun, Cerro Gordo, Emmet and 14 more5 hr
Sep 5, 1:14 AM CDTMesoscale DiscussionSPCAllamakee, Audubon, Benton, Black Hawk, Boone, Bremer and 54 more91 min
Sep 5, 3:43 AM CDTMesoscale DiscussionSPCBenton, Cedar, Clinton, Des Moines, Dubuque, Henry and 14 more92 min

Nearby

Severe Thunderstorm Watches in Iowa, 2012Every severe thunderstorm watch in 2012

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.