Severe Weather Summary of October 18, 2007

Severe weather reports from October 18, 2007
Nationwide severe weather reports from October 18.  Click here for specific reports.

 

A powerful fall storm system affected a large part of the Midwest on October 18.  Strong low pressure developed out of a deep upper level trough which moved from the Rockies into the Plains overnight, and was strengthened by mid-level jet stream winds in excess of 100 mph.  Surface low pressure developed over the central Plains and quickly deepened to 982 mb and moved northeast into southern Minnesota by early evening. 

Round #1 of severe weather struck central and southeast Illinois during the early morning hours as a 60 mph low-level jet stream brought deep moisture and warm air northward into the area.  Additionally, a mid-level shortwave moved quickly northeast helping to produce widespread showers and thunderstorms.  Intense storms developed just after midnight in west-central Illinois producing strong winds which damaged trees and power lines from Scott county eastward into Sangamon county.  These storms weakened as they moved northeast into a more stable airmass.  New storms erupted around 4 AM in southeast Illinois and developed quickly to the northeast and intensified over the next couple of hours.  Between 4 and 6 AM winds in excess of 60 mph damaged trees and power lines from Effingham eastward through Lawrenceville, and northward through Martinsville, Robinson, and Paris.  In addition to the strong winds these storms were very efficient rain producers, with between 2 and 4 inches of rain falling within one to two hours over a large part of southeast and east central Illinois.

The system’s warm front pushed north of the area by the early morning hours, and the "dry slot" moved in from the west, shutting off precipitation and bringing mostly sunny skies by morning.  The morning sunshine allowed the atmosphere to destabilize ahead of the approaching cold front to the west.  Early afternoon temperatures rose well into the 70s, with dewpoints in the 60s.  With a moderate risk of severe storms for the eastern half of the state, a special 18Z weather balloon was launched from the Lincoln office.  Data showed very strong low-level lapse rates and sufficient moisture in the lower levels to produce a moderately unstable airmass .  Additionally, wind fields were very strong aloft.  Surface winds of 25-35 mph increased to 75 mph at mid-levels, and 125 mph at upper levels.  While the wind direction did not change much with height, the speed difference created an impressive amount of vertical shear which was sufficient to create supercell thunderstorms.  By late afternoon the atmosphere had continued to destabilize as the cold front/dry line plowed in from the west.  Thunderstorms erupted around 5 PM between the I-55 and I-57 corridor and moved to the northeast at 40 to 60 mph.  Frequent lightning, very heavy downpours, hail, and damaging straight line winds accompanied many of these storms.  The activity continued until around 7:30 PM.  Many locations along and east of the I-57 corridor sustained damage to trees and power lines. 

There were no tornadoes associated with this activity, likely due to a lack of directional wind shear locally.  Violent tornadoes did develop later in the evening as these storms pushed into Indiana.  Over 4 dozen tornadoes were reported over a 2 day period across the central U.S.

Preliminary reports from eastern Illinois:

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE LINCOLN IL
909 PM CDT THU OCT 18 2007

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

0525 PM     TSTM WND DMG     DECATUR                 39.85N 88.93W
10/18/2007                   MACON              IL   TRAINED SPOTTER

            TREES DOWN ON LAKE SHORE DRIVE

0526 PM     HAIL             DECATUR                 39.85N 88.93W
10/18/2007  M1.75 INCH       MACON              IL   EMERGENCY MNGR

            GOLFBALL SIZED HAIL ON THE EAST SIDE OF LAKE DECATUR

0535 PM     TSTM WND DMG     1 W CISCO               40.01N 88.74W
10/18/2007                   PIATT              IL   TRAINED SPOTTER

            TREE DAMAGE NEAR THE INTERSECTION OF IL ROUTE 32 AND
            48...TRAMPOLIN IN A TREE AND A SEMI TRUCK FLIPPED OVER ON
            EASTBOUND I-72

0535 PM     HAIL             ARGENTA                 39.99N 88.82W
10/18/2007  E1.00 INCH       MACON              IL   TRAINED SPOTTER

0551 PM     TSTM WND DMG     BETHANY                 39.64N 88.74W
10/18/2007                   MOULTRIE           IL   LAW ENFORCEMENT

            TREES AND POWERLINES DOWN...IN ADDITION FUNNEL CLOUD
            OBSERVED

0553 PM     HAIL             1 S DE LAND             40.11N 88.64W
10/18/2007  E0.88 INCH       PIATT              IL   TRAINED SPOTTER

0602 PM     TSTM WND GST     COWDEN                  39.25N 88.86W
10/18/2007  M74.00 MPH       SHELBY             IL   MESONET

0619 PM     TSTM WND DMG     CHAMPAIGN               40.11N 88.26W
10/18/2007                   CHAMPAIGN          IL   BROADCAST MEDIA

            TREE BLOWN DOWN ON KIRBY STREET JUST WEST OF THE STADIUM.
            FROM WILL TV.

0642 PM     TSTM WND DMG     MATTOON                 39.48N 88.37W
10/18/2007                   COLES              IL   TRAINED SPOTTER

            NUMEROUS 5 INCH TREE BRANCHES DOWN AND ONE CAR DAMAGED
            FROM TREE BRANCH.

0655 PM     TSTM WND DMG     OGDEN                   40.12N 87.96W
10/18/2007                   CHAMPAIGN          IL   CO-OP OBSERVER

            POWERLINES DOWN IN OGDEN...ALSO 6 INCH TREE LIMBS DOWN.
            MEASURES 58 MPH WIND GUST

0657 PM     TSTM WND GST     OGDEN                   40.12N 87.96W
10/18/2007  M58.00 MPH       CHAMPAIGN          IL   CO-OP OBSERVER

0701 PM     HAIL             OAKLAND                 39.66N 88.03W
10/18/2007  M1.25 INCH       COLES              IL   EMERGENCY MNGR

0725 PM     HAIL             8 S SUMNER              38.60N 87.86W
10/18/2007  M1.75 INCH       LAWRENCE           IL   EMERGENCY MNGR

            FROM COUNTY EMA.

0725 PM     HAIL             6 S SUMNER              38.63N 87.86W
10/18/2007  M0.88 INCH       LAWRENCE           IL   EMERGENCY MNGR

            FROM COUNTY EMA.

0730 PM     TSTM WND DMG     BRIDGEPORT              38.71N 87.76W
10/18/2007                   LAWRENCE           IL   EMERGENCY MNGR

            NUMEROUS TREES AND POWER LINES DOWN FROM BRIDGEPORT TO
            THE SOUTHWEST SIDE OF LAWRENCEVILLE. FROM COUNTY EMA.

0829 PM     TSTM WND DMG     COWDEN                  39.25N 88.86W
10/18/2007                   SHELBY             IL   LAW ENFORCEMENT

            TOWER AT FIRE DEPARTMENT DOWN. SEVERAL TREES ON VEHICLES.




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