21 August 2009 Severe Weather Outbreak and Flooding

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This was a combined severe weather and flooding event, associated with a system and cold front prior to the approach of Hurricane Bill. The meteorological set-up was described in Area Forecast Discussions from NWS Albany, NY.

Large CAPE and High Shear, along with a very moist atmosphere lead to an active severe weather/flash flood day. Here are excerpts from well written AFD's:

Early morning AFD: AS OF 3 AM EDT...IT IS A BALMY MORNING ACROSS THE ALY FCST AREA. THE REGION ENTERED A TROPICAL WARM SECTOR WITH THE PASSAGE OF THE WARM FRONT TO THE NORTH YESTERDAY. SFC DEWPOINTS HAVE RISEN INTO THE 60S TO L70S WITH EVEN SOME MID 70S POSSIBLE IN THE LOWER AND MID HUDSON VALLEY BASED SFC OBS. THE NEXT 12 TO 48 HOURS LOOK TO BE ACTIVE IN TERMS OF THE SEVERE WEATHER AND RAIN THREATS. THE FIRST PROBLEM WILL BE THE POTENTIAL SEVERE WEATHER.

A SFC WAVE...AND AN ASSOCIATED SHORT WAVE IMPULSE RIDING ALONG A PREFRONTAL TROUGH TRIGGERED SOME STRONG TO SEVERE THUNDERSTORMS OVER THE NIAGARA FRONTIER LAST NIGHT. THIS MORNING...SOME SCT SHOWERS AND EMBEDDED THUNDERSTORMS HAVE MOVED INTO THE TUG HILL REGION...AND THE WRN ADIRONDACKS AHEAD OF THE PREFRONTAL SFC TROUGH. THE PRE-CONVECTIVE ENVIRONMENT IS LOOKING CONDUCIVE FOR A POSSIBLE SQUALL LINE OR TWO WITH EMBEDDED QUASI LINEAR CONVECTIVE SEGMENTS /QLCS/ LATE THIS MORNING THRU THE AFTERNOON. SBCAPE VALUES RISE TO 1500-2000+ J/KG WITH SFC DEWPOINTS IN THE L70S ...AND APPRECIABLE SOLAR HEATING. THE BEST 0-6 KM DEEP SHEAR IS 35-50 KTS FROM BOTH THE NAM/GFS WEST OF THE HUDSON RIVER VALLEY. ABOVE THE BOUNDARY LAYER INTO THE HIGH TROPOSPHERE THE FLOW IS UNIDIRECTIONAL. DCAPE VALUES RISE INTO THE 500-1000 J/KG RANGE. THE 0-1 KM BOUNDARY LAYER BULKS SHEAR VALUES DO GET TO 20-25 KTS. THERE IS COPIOUS MOISTURE LOADED INTO THE BOUNDARY LAYER. OUR 0600 UTC LOCAL HIRES WRF COMPOSITE REFLECTIVITY PRODUCT SHOWS A FAIRLY CONTINUOUS BROKEN LINE OR TWO OF INTENSE CONVECTION EVOLVING BTWN 16Z AND 03Z ACROSS THE FCST AREA.

WE HAVEN/T MENTIONED THE THREAT IN SOME TIME...BUT WITH THE APPRECIABLE SOUTHWESTERLY BOUNDARY LAYER SHEAR...AND THETA-E RICH AIR IN THE SFC-H850 LAYER...AND LOWERING LCL HEIGHTS TO 1-3 KFT AGL...A SPIN UP ISOLD TORNADO WITH A QLCS IS POSSIBLE IN THE FCST AREA. WET MICROBURSTS SHOULD DOMINATE...BUT THE CONDITIONAL TORNADO THREAT IS THERE. HAIL SHOULD BE LESS DOMINATE WITH WET BULB ZERO HEIGHTS EXTREMELY HIGH AT AROUND 14 KFT AGL AND VERY WEAK MID LEVEL LAPSE RATES. OVERALL...WE AGREE WITH THE SPC DAY 1 GRAPHIC...THOUGH THE BEST THREAT FOR SEVERE WITH THE INTERSECTION OF THE BEST SHEAR AND CAPE LOOKS TO BE ACROSS ERN NY IN THE VICINITY OF THE HUDSON RIVER VALLEY.

PRECIPITABLE WATER VALUES ARE ANOMALOUSLY HIGH IN THE TROPICAL AIR MASS. THE 00Z KALB SOUNDING HAD 2.2 INCHES ON IT. THE GLOBAL ENSEMBLES SHOW THESE VALUES TO BE 2-3 STANDARD DEVIATIONS ABOVE NORMAL. SOME HEAVY DOWNPOURS ARE LIKELY...AS THESE VALUES ARE ROUGHLY 200% ABOVE NORMAL. WE BELIEVE THE FLASH FLOOD THREAT IS ISOLD TODAY WITH THE HIGH FFG VALUES...AND THE VERY DRY ANTECEDENT CONDITIONS SINCE AUG 1ST ACROSS THE VAST MAJORITY OF THE FCST AREA. THE WIDESPREAD RAINFALL THREAT INCREASES IN THE OVERNIGHT PERIOD THROUGH SATURDAY WITH POSSIBLE TRAINING ECHOES.

930AM AFD: AS OF 930 AM...12Z SOUNDINGS ARE IN WITH IMPRESSIVE PWATS IN PLACE AS VALUES ARE >2SD ABOVE NORMAL. MODIFIED SFC PARCELS YIELDS CAPES IN EXCESS OF 3K J/KG WITH THE 40 KNT 0-6KM SHEAR SUPPORTS EHI/S BETWEEN 1-3 M2/S2. UPSTREAM SFC DEWPOINTS ARE BETWEEN 75-80F AND THIS MOISTURE TRANSPORT CONTINUE TO ADVANCE NORTHWARD. THE COMPLICATED FACTOR IS THE EXTENSIVE CLOUD SHIELD ACROSS MOST OF NY AND UPSTREAM INTO WESTERN HALF OF PA. PER SPC COLLABORATION...WE EXPECT SOME BINOVC/S TO OCCUR WHICH WILL ONLY HELP DESTABILIZE THE COLUMN FURTHER /THIS MORE LIKELY SCENARIO WOULD BE SOUTH OF THE CAPITAL REGION/ FOR CONVECTION TO MATURE. THE POTENTIAL FOR A TORNADO DOES EXIST BUT THE MORE IMMINENT THREAT WILL BE FROM WET MICROBURSTS AND IMPRESSIVE RAINFALL RATES /K-INDEX NEAR 40 THIS MORNING/ WITH THOSE HIGH PWATS IN PLACE.

TO SUPPORT SEVERE WEATHER OPS AND IMPACTS FROM HURRICANE BILL...SPECIAL UPPER SOUNDINGS WILL OCCUR THROUGH AT LEAST SATURDAY. GOES-E RSO REQUEST HAS BEEN APPROVED AND WILL BE IN PLACE FROM 1326Z-0326Z.

1210 PM AFD Update: ...SEVERE THUNDERSTORM WATCH IN EFFECT UNTIL 7 PM... LINE OF CONVECTION CONTINUES TO EVOLVE OVER THE WESTERN CWA WITH SIGNS OF A QLCS UNDERWAY. LOCAL LAPS DATA REVEALS CAPES NEAR 3K J/KG FROM THE HUDSON RIVER VALLEY...CATSKILLS AND ADJACENT WESTERN NEW ENGLAND WITH CORRESPONDING EHI/S APPROACHING 3 M2/S2. SO WIND POTENTIAL IS HIGH INCLUDING THE FLASH FLOOD POTENTIAL. CAN NOT RULE OUT A TORNADO OR TWO GIVEN THE INCREASED SHEAR AND BASED ON THE RUC13 HODOGRAPHS...LOW LEVEL SHEAR INCREASE THIS AFTERNOON.

Satellite loops

 

Visible loop

 

LAPS CAPE, instability and EHI

 

 

Radar Loops

 

KENX loop from 1600-1800 UTC of Reflectivity and lightning

 

 

KENX loop from 1800-2000 UTC of Reflectivity and Severe Thunderstorm Warnings

 

 

KENX loop of Reflectivity, Severe Thunderstorm and Tornado Warnings for the southeastern portion of the NWS Albany, NY forecast area

 

 

KENX loop of Storm Relative Velocity, Severe Thunderstorm and Tornado Warnings for the southeastern portion of the NWS Albany, NY forecast area

 

 

Individual KENX radar images

 

 

KENX FSI image 1915Z 3 body scattering - hail spike in reflectivity, KENX 1901Z

 

Precipitation estimates

 

 

KENX River Forecast Ceneter

 

SPC soundings

 

21 August Soundings from ALY

 

 

12Z 18Z 00Z

 

AWIPS D2D Modified Sounding ALY 12Z 21 August

 

 

SPC Convective Outlooks 12Z and 13Z 21 August

 

 

Probabilities

 

 

Tornadoes Hail Wind

 

SPC Convective Outlook 1630Z 21 August

 

 

Probabilities

 

 

Tornadoes Hail Wind

 

SPC Mesoscale Discussions

 

 

 

 

Severe weather reports courtesy of The Iowa Environmental Mesonet division of Iowa State University Department of Agronomy

 

NWS Albany, NY

 

 

Any questions or suggestions, please e-mail Brian.Montgomery@noaa.gov or Neil.Stuart@noaa.gov