21 August – severe weather

 

Synoptic Setup: A large upper level trough approached the region as a strong mid-level jet rotated through its base. A rather potent surface cold front approached from the Great Lakes region, crossing the CWA Sunday night.

 

There were two distinct rounds of severe weather. The afternoon round was well expected ahead of the cold front. However, the late morning into early afternoon was not. The first round ended up being the more significant of the two. The first round was associated with mid-level short wave, low and mid-–level thermal advection along with mixed layer instability. The models including the HISRESWFR and HRRR-3km did not pick up on the evolution of organized lines with the late morning convection.  

 

From 10:28 am NWS Albany, AFD:

OUR HIRES WRF BASE REF PRODUCT SEEMS TO HAVE A GOOD HANDLE ON

THINGS AT THIS POINT...AND IT SHOWS A RATHER INTENSE LINE OF

CONVECTION FORMING TOWARDS 20Z...AND CROSSING THE FORECAST AREA BY

0100Z.

 

THE 12Z KALB SOUNDING SHOWS ONLY 0-6 KM SHEAR VALUES OF 24 KTS

NOW...BUT WE EXPECT THE DEEP SHEAR TO INCREASE TO 40-50 KTS BY THE

LATE PM WITH A MULTICELL LINE...AND PERHAPS SOME SUPERCELL

STRUCTURES WITH MLCAPE/SBCAPES GENERALLY 1000-2000 J/G OVER THE

FCST AREA. THE 12Z SOUNDING ALL SHOWS H850-500 LAPSE RATES OF 6.8

C/KM WITH A WET BULB ZERO HEIGHT OF 10.3 KFT. THERE IS LOW-LEVEL

VEERING IN THE BOUNDARY LAYER...BUT WITH THE STRONG CYCLONICALLY

CURVED MID/UPPER LEVEL JET STREAK APPROACHING LATER TODAY WE

BELIEVE THE PRIMARY SEVERE THREAT WILL BE WIND...WITH SOME HAIL.

 

THE LATEST MAGLENTA SEVERE WEATHER INDEX CAME OUT WITH A MAJOR

SEVERE WEATHER EVENT POSSIBLE. THIS IS A CONDITIONAL POINT INDEX

FROM PAST COMET RESEARCH.

 

HERE ARE THE PARAMETERS INPUTTED:

CAPE = 1500 J/KG

MAX SOUNDING WINDS = 65

EHI = 1.0

SRH (0-3 KM) = 150 (M/S)2

STORM SPEED = 30 KTS

 

Convection initiated before 9 am over the southern Adirondacks.  Cells really began to fire up along and just to the west of the Hudson River late in the morning as the moist air mass continued to destabilize.  These cells quickly evolved in multi-cellular lines with few individual “stand-out” cells becoming brief supercells .

 

The first SVR warning was issued at about 12:30 pm. That cell developed an inflow and became a supercell with not one, but two gate to gate couplet signatures over 30kts.   The SVR was upgraded to a TOR for Dutchess County at 1:21 pm. Thus far, no tornado was confirmed, but we did receive a report of a funnel cloud which was documented in our LSR.

 

Other severe thunderstorm warnings were issued for the Capital District and southern Vermont.  The first set of thunderstorms quickly exited to our east after 300 PM.  There was only a brief break.  Just as the last storm in the first round exited Litchfield County around 3:30 pm cells began to develop over the Lake George Saratoga region and western Adirondacks. The radar really began to blossom by 5 pm, mainly to the south and east of the Capital District which was expected.  Some of these become quite potent. One cell on the Saratoga/Warren/Washington line produced wind damage in and around Queensbury. This cell briefly weakened before pulsing up and produced a significant wind field on the velocity scans. This storm also began rotating quite a bit just as quickly exiting Washington County.

 

The other significant cell indicated gate to gate rotation in Berkshire County. Due to reported sighting of a funnel cloud near Pittsfield, another tornado warning was issued shortly after 7 PM.      

 

Coordination with SPC for Severe Thunderstorm Watch #795 came while we were in the thick of issuing warnings.  Some storms produced torrential downpours including here at the office where we picked 0.96 inches of rain in about 15-20 minutes along with dime size hail. Three flash flood warnings were issued. 

 

 

Above:  Soundings from Albany, NY at 12Z UTC 21 August (left) and 18Z 21 August (right).  Note the instability and moisture suggesting possible severe weather.

 

 

Above:  Soundings from Upton, NY at 12Z UTC 21 August (left) and 18Z 21 August (right).  Note the instability and moisture suggesting possible severe weather.

 

 

Above:  Loop of surface maps showing the passage of the cold front.

 

 

Above:  Storm reports from 21 August.

 

  

 

Above:  Pictures of damage in Selkirk, NY