EF0 tornado, Thompson, Nebraska, September 13, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
923 AM CDT MON SEP 13 2010

.DISCUSSION...
SOMEWHAT OF A COMPLEX FORECAST THIS MORNING WITH QUESTIONS
REMAINING WHEN AND WHERE CONVECTION WILL DEVELOP. SUBJECTIVE 12Z
UPPER AIR ANALYSIS REVEALED THAT THE REGION WAS IN THE ENTRANCE
REGION OF AN UPPER LEVEL JET STREAK...ALTHOUGH UPPER LEVEL FORCING
REMAINS GENERALLY WEST OF THE AREA. 13Z SUBJECTIVE SURFACE ANALYSIS
INDICATED A STALLED FRONTAL BOUNDARY GENERALLY ALIGNED ALONG
I80...AND ITS IN THIS SAME AREA THAT AN AREA OF MID LEVEL ACCAS
CLOUDS WERE NOTED. THE 13Z RUC/06Z GFS/00Z ECMWF/09Z SREF ALL SEEM
TO GENERATE CONVECTION ALONG OR JUST NORTH OF THIS BOUNDARY THIS
AFTERNOON...AND THAT DOESNT SEEM THAT UNREASONABLE AS DAYTIME
HEATING PROGRESSES...THUS MAY HAVE TO CONSIDER ADDING A SLIGHT
CHANCE OF THUNDERSTORMS CENTRAL CWFA LATER THIS AFTERNOON...BEFORE
THE MAIN AREA OF CONVECTION ARRIVES LATER TONIGHT. MEANWHILE...
THE 12Z NAM GENERATES CONVECTION IN THIS SAME AREA BY 15Z...AND
THAT IS EXTREMELY OVERDONE...AND MAY HAVE CONTAMINATED THE MODEL
FOR THE REST OF TODAY.

&&

.PREV DISCUSSION... /ISSUED 320 AM CDT MON SEP 13 2010/

DISCUSSION...

SYNOPSIS...00Z UPPR AIR ANLYS INDCD UPPR LVL RIDGE OVR THE WRN HI
PLAINS FM NEB TO W TX. WK SHRTWV ENERGY WAS NOTED OVR THE GREAT
BASIN. H85 ANLYS IDNCD A GOOD SUPPLY OF UPSTREAM MOISTURE WITH DEW
POINTS OF > 15 C FM OK INTO TX. 07Z SFC ANLYS INDCD LO PRES OVR
N-CNTRL KS WITH A WK CDFNT THRU ERN NEB AND CNTRL IA.

FORECAST...PREVIOUS MENTIONED LO LVL MOISTURE WL MAKE A STG RETURN
TO THE REGION TODAY AS SRLY FLOW INCREASES AS HI PLAINS SFC LO
DEEPENS. WK SFC CDFNT SHLD STALL THIS MRNG OVR SE NEB AND SW IA
BEFORE LIFTING NWD THRU THE DAY. LITTLE IN THE WAY OF LARGE-SCALE
SUPPORT FOR ASCENT IS EXPECTED THRU THE DAY TODAY EXCEPT FOR THE
ISENTROPIC LIFT. WITH WARMING MID LVL TMPS AND LITTLE MESOSCALE
SUPPORT /WITH CONVERGENCE WEAK ALNG THE WRM FNT/...THINK MOST OF THE
DAY WL BE DRY ACRS THE FA. ATMOSPHERE WILL BE DESTABILIZING THOUGH
THROUGH THE LATE AFTN AS LAPSE RATES STEEPEN AND MOISTURE
INCREASES WITH MLCAPE AROUND 1500 J/KG POSSIBLE BY LATE AFTN.
THUNDERSTORMS SHLD DVLP LATE THIS AFTN OVR NRN KS AND SRN NEB NEAR
THE SFC FNT AND WITH SOME ADDED ENHANCEMENT FM GREAT BASIN SHRTWV
TROF MOVG INTO THE PLAINS. SOME OF THIS ACTIVITY MAY SNEAK INTO
OUR WRN FA DURING THE EVNG /ALTHOUGH STORM MOTION WL GENERALLY
TO THE SE/...BUT MORE LIKELY SCENARIO ATTM IS FOR THUNDERSTORMS TO
INCREASE IN COVERAGE THRU THE EVNG AS H85 THETA-E ADVECTION
INCREASES AS THE LOW LEVEL JET STRENGTHENS. GIVEN THE STG SUPPORT
FM THE GFS/GEM/EC AND SREF...WL INCREASE POPS TONIGHT INTO THE
CAT RANGE. SUFFICIENT SHEAR DOES EXIST THRU THE CLD LAYER FOR
ORGANIZATION AND ROTATION...THUS WLD NOT BE SURPRISED TO SEE A
FEW SVR CELLS WITH LARGE HAIL BEING THE MAIN THREAT. STORMS MAY
STILL BE ONGOING OVR SE NEB/SW IA ON TUES MRNG...BUT AS THE LO LVL
JET VEERS THRU THE MRNG...ACTIVITY SHLD WEAKEN WITH MOST OF TUES
SHLD REMAINING DRY. HAVE INCREASED GOING HIGHS TODAY AND ON TUES
GIVEN THE EXPECTED DRY DAY TIME CONDITIONS.

LO LVL JET WL ONCE AGAIN INCREASE ON TUES NIGHT...BUT WITH
ADDITIONAL SHRTWV ENERGY MOVG INTO THE HIGH PLAINS...BETTER
FOCUS...AT LEAST INITIALLY...SHLD BE OVER THE WRN HI PLAINS. SOME
OF THIS ACTIVITY MAY MOV INTO THE WRN FA AFTER MIDNIGHT. A BETTER
CHC FOR PRECIP WL COME ON WED AS THE AFOREMENTIONED SHRTWV TROF
MOVS ACRS THE NRN PLAINS AND ALLOWS FOR A SFC LO TO EJECT E ALNG
WITH A CDFNT. CWA SHLD REMAIN IN THE WRM SECTOR FOR A GOOD PART OF
THE DAY ON WED. SOME POTENTIAL FOR ACTIVE CONVECTION AS SHEAR
COMBINES WITH QUALITY MOISTURE...BUT STABILITY IS IN QUESTION ATTM
AS LO CLDS MAY HANG IN. IF CLDS CAN CLEAR SOME IN THE CNTRL AND
S...THERE IS SOME POTENTIAL FOR SVR WX WED ENVG. OTHERWISE STILL A
GOOD CHC OF PRECIP GIVEN THE GOOD FORCING AND MOISTURE. KEPT THE
LIKELY POPS GOING. SYSTEM SHLD MOV SE OF THE FA WITH COOLER AND
DRIER AIR MOVING IN FOR THURS. QUIET WEATHER WILL BE SHORT LIVED
HOWEVER AS NEXT CDFNT WL MOV IN FOR FRI NIGHT WITH A CHC OF PRECIP
INTO THE WEEKEND.

BOUSTEAD

AVIATION...

TAF SITES KOFK/KOMA/KLNK

06Z TAFS

A FRONT THAT HAS STALLED NEAR PLATTE RIVER SHOULD BEGIN TO LIFT
SLOWLY NORTH AS A WEAK UPPER DISTURBANCE MOVES ACROSS THE ROCKIES.
THUS LIGHT WINDS DURING THE PREDAWN HOURS MONDAY SHOULD BECOME SE
AND INCREASE TOWARD NOON.  IN ADDITION...WARMER/MORE MOIST AND
UNSTABLE AIR WILL BE SPREADING NWD TODAY.  INITIALLY THIS WILL BRING
MID CLOUDS TO TAF SITES BUT SHOULD PROVIDE FOR INCREASING TSTM
COVERAGE TOWARD TAIL END OF 06Z TAF VALID PD.  THIS TSTM RISK WAS
ACCOUNTED FOR BY A PROB30 GROUP IN 14/00-14/06 PD AT ALL TAF SITES.

CHERMOK

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...NONE.
IA...NONE.
&&

DEWALD

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
1128 AM CDT MON SEP 13 2010

IAZ043-055-056-069-079-080-090-091-NEZ011-012-015>018-030>034-
042>045-050>053-065>068-078-088>093-140630-
MONONA-HARRISON-SHELBY-POTTAWATTAMIE-MILLS-MONTGOMERY-FREMONT-
PAGE-KNOX-CEDAR-THURSTON-ANTELOPE-PIERCE-WAYNE-BOONE-MADISON-
STANTON-CUMING-BURT-PLATTE-COLFAX-DODGE-WASHINGTON-BUTLER-
SAUNDERS-DOUGLAS-SARPY-SEWARD-LANCASTER-CASS-OTOE-SALINE-
JEFFERSON-GAGE-JOHNSON-NEMAHA-PAWNEE-RICHARDSON-
1128 AM CDT MON SEP 13 2010

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTHWEST
IOWA...WEST CENTRAL IOWA...EAST CENTRAL NEBRASKA...NORTHEAST
NEBRASKA AND SOUTHEAST NEBRASKA.

.DAY ONE...THIS AFTERNOON AND TONIGHT

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON INTO
THE EARLY OVERNIGHT HOURS...MAINLY OVER EASTERN NEBRASKA. THE MAIN
THREAT FROM ANY THUNDERSTORM WOULD BE LARGE HAIL...WITH SOME
LOCALLY HEAVY RAINFALL ALSO POSSIBLE.

WARM AND MOIST AIR WILL START TO RETURN TO THE REGION TODAY
ALLOWING FOR AN INCREASINGLY UNSTABLE ATMOSPHERE BY TONIGHT ACROSS
THE AREA. ISOLATED THUNDERSTORMS MAY DEVELOP ACROSS THE AREA THIS
AFTERNOON ALONG OR NORTH OF A STALLED FRONTAL BOUNDARY ALONG THE
INTERSTATE 80 VICINITY. IN ADDITION...THUNDERSTORMS ARE EXPECTED TO
DEVELOP LATE THIS AFTERNOON OVER CENTRAL NEBRASKA...THE MOVE AND
DEVELOP EAST INTO EASTERN NEBRASKA THIS EVENING INTO THE OVERNIGHT
WITH ISOLATED SEVERE WEATHER POSSIBLE.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

THE UNSETTLED WEATHER PATTERN IS EXPECTED TO CONTINUE INTO TUESDAY
AND WEDNESDAY...WHICH IS LIKELY TO INCLUDE SEVERAL CHANCES FOR
THUNDERSTORMS DURING THE PERIOD. A FEW STORMS COULD BE SEVERE
ON WEDNESDAY ACROSS SOUTHEAST NEBRASKA.

SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL LIKELY NOT BE NEEDED TODAY OR TONIGHT...BUT
SPOTTERS ARE ENCOURAGED TO REPORT ANY HAIL A HALF INCH OR GREATER...
AND ANY SIGNIFICANT RAINFALL.

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