Preliminary July 12 2013 Storm Assessment

...THIEF RIVER FALLS MINNESOTA PRELIMINARY STORM ASSESSMENT...


DURING THE EARLY EVENING HOURS OF FRIDAY JULY 12TH DAMAGING WINDS
FROM A SEVERE THUNDERSTORM STRUCK PORTIONS OF THIEF RIVER FALLS
MINNESOTA AND THE SURROUNDING AREA. AN NWS SURVEY TEAM
ACCOMPANIED THE PENNINGTON COUNTY EMERGENCY MANAGER AND LAW
ENFORCEMENT TO SURVEY THE DAMAGE.

THREE RELATIVELY DISCRETE AREAS OF DAMAGE WERE SURVEYED. THE
FIRST IN THE CITY OF THIEF RIVER FALLS...THE SECOND JUST EAST OF
THE CITY PROPER ALONG THE RED LAKE RIVER...WITH THE THIRD
APPROXIMATELY 4 TO 8 MILES EAST OF THIEF RIVER FALLS.

ALL OF THE WIND DAMAGE CATEGORIZED BELOW WAS FROM A MACROBURST. A
MACROBURST IS DEFINED AS DAMAGING STRAIGHT LINE WINDS ASSOCIATED
WITH A SEVERE THUNDERSTORM WHICH AFFECT AN AREA LARGER THAN 2.5
MILES IN AREAL EXTENT. THROUGHOUT THE SURVEY PROCESS THERE WERE
NO INDICATIONS OF TORNADIC ACTIVITY.

AREA ONE...STARTED APPROXIMATELY 600 PM TO 605 PM.

THE MAJORITY OF THE DAMAGE TO THE CITY OF THIEF RIVER FALLS
ENCOMPASSED THE SOUTHEAST HALF OF THE CITY...GENERALLY SOUTH AND
EAST OF THE CONFLUENCE OF THE THIEF RIVER AND RED LAKE RIVER. THE
VAST MAJORITY OF THE DAMAGE WAS TO TREES OR DIRECTLY RELATED TO
TREES CAUSING DAMAGE. BASED ON THE TYPE OF TREES...THE DAMAGE TO
THE TREES AND ASSOCIATED STRUCTURES THE PRELIMINARY DAMAGE RATING
IS EF0 WITH WIND SPEEDS OF 60 TO 80 MPH.

FARTHER TO THE SOUTHEAST OF THE EDGE OF THE CITY LIMITS...
SLIGHTLY GREATER DAMAGE WAS NOTED. DAMAGE TO SEVERAL TRAILERS...
GARAGES AND A SINGLE WIDE MOBILE HOME SUGGEST EF1 SCALE WINDS
WHICH WERE ESTIMATED AT 95 TO 105 MPH.

AREA TWO...STARTED APPROXIMATELY 605 PM TO 610 PM.

THE SECOND AREA SURVEYED LAY ALONG AND EAST OF THE RED LAKE RIVER
JUST EAST OF THIEF RIVER FALLS. MOST OF THE DAMAGE WAS FROM
MINNESOTA STATE HIGHWAY 1 SOUTHEAST FOR APPROXIMATELY 3 MILES. THE
DAMAGE SURVEYED INCLUDED NUMEROUS LARGE HARDWOOD TREES WHICH WERE
UPROOTED OR HAD TRUNKS SNAPPED. BASED ON THE DAMAGE OBSERVED AND
TYPES OF TREES THE PRELIMINARY DAMAGE RATING IS EF1 WITH ESTIMATED
WIND SPEEDS OF 100 TO 110 MPH.

AREA THREE...STARTED APPROXIMATELY 615 PM TO 620 PM.

THE THIRD AREA SURVEYED EXTENDED ALONG AND SOUTH OF
MINNESOTA STATE HIGHWAY 1 FROM 4 TO 8 MILES EAST-SOUTHEAST OF
THIEF RIVER FALLS. DAMAGE IN THIS AREA INCLUDED A DOZEN SINGLE
WOOD POLE ELECTRICAL TRANSMISSION LINES...ONE SNAPPED OFF NEAR
THE BASE AS WELL AS SEVERAL LARGE CAPACITY GRAIN BINS. BASED ON
THE DAMAGE OBSERVED...THE TYPE...SIZE AND MOORING OF BINS AS WELL
AS DAMAGE TO THE POWER POLES THE PRELIMINARY DAMAGE RATING IS EF2
WITH ESTIMATED WIND SPEEDS OF 110 TO 120 MPH.

REMEMBER THIS IS A PRELIMINARY ASSESSMENT. AS ADDITIONAL
INFORMATION IS RECEIVED FROM THE PUBLIC...LAW ENFORCEMENT...THE
EMERGENCY MANAGER AND NATIONAL WEATHER SERVICE MAY MODIFY SOME OF
THE ESTIMATED WIND SPEEDS.

FOR ADDITIONAL INFORMATION CONTACT MARK EWENS AT THE GRAND FORKS
NWS.


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