8 September 2012 Severe weather

 

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For more detailed analysis click here to go to the Storm Prediction Center website

 

An unseasonably warm and humid air mass advected north into the CWA preceding a rather seasonably strong cold front and potent upper level dynamics. Numerical models, while differed on timing by a few hours, performed rather well with both the expected dynamics and strength of the cold front. It started with SPC placing our region into a "See Text" on the day 3 outlook with a mention of possible upgrades in later issuances. Our forecast staff indicated the possibility of severe weather beginning on Wednesday September 5th. Our staff was is close coordination with SPC as our region was placed into a high end slight risk with the initial day 2 outlook issuance. Analyzing model guidance several meteorological features were identified that would require additional future upgrades to the SPC outlooks. The day prior to the severe weather, we closely coordinated with SPC with our region being upgraded to a Moderate Risk. There was a review of several meteorological parameters that warranted the upgrade and we began adding additional staffing to accommodate the severe weather and the request for an 18Z sounding Saturday afternoon.

 

There was one main ingredient that was missing for this event, and that was steep mid level lapse rates within an elevated mixed layer. Models and our upper air soundings revealed those mid level lapse rates were less than 6 C/KM. However, low level lapse rates were sufficient along with helicity values in excess of 200 m2/s2. It was discussed that despite the less than favorable mid level lapse rates, the combination of strong dynamics and stronger surface convergence signatures would be more than enough to overcome the lack of mid level lapse rates. Forecast CAPES were primarily in the lower levels of the atmosphere but that changed somewhat during the late morning hours across the mid-lower Hudson valley and Northwest Connecticut.

 

It is important to understand the various sources of RAOB data displayed in SKEW-T plots.  The calculated parameters such as CAPE, LCL and other parameters is different in NSHARP (from the Storm Prediction Center) and AWIPS (old equations from the NWS AFOS era).  NSHARP uses more current calculation methods so these should be consulted more closely than AWIPS soundings.  Some discussions and coordinatio referred to AWIPS sounding parameters rather than NSHARP sounding parameters, which might have resulted in a perception of a greater tornado threat than what really existed.

 

During the morning of the severe weather event, a couple of squall lines developed across Western NY. Meanwhile, individual cells were developing across NYC and tracking north into the Mid Hudson Valley and NW CT. Several Tornado warnings were issued (and verified via pictures and video on the Weather Channel) which prompted SPC to issue a small TOR Watch that initially went as far north as Berkshire County. After coordination, we only placed Dutchess and Litchfield counties where the highest threat would be as expectations for another larger watch to be issued in the next few hours.

 

The radar signatures across NYC using both the Terminal Doppler Weather Radar and 88D did not reveal strong couplets. Initial thoughts were these began as waterspouts before moving inland and convection developing along the sea breeze boundaries that were seen in the visible satellite imagery. NYC did issues a few more Tornado Warnings just south of Dutchess and Litchfield where there was a brief couplet signature.  No tornado events occurred in the NWS Albany, NY forecast area with both the initial watch and the preceding larger watch.

 

A larger Tornado watch was issued that replaced the initial smaller watch as the combination of the approaching squall line and supercell potential ahead of the main line. Surface dew points climbed 3-6 degrees with most of the region well into the 60s and mid 70s. There was a strong southerly wind that was sub advisory, where we did receive reports of trees down across portions of the Capital Region and the Adirondacks. We discussed issuing a short term Wind Advisory or High Wind Warning but we did not want to take away from the main threat. We could have issued a Special Weather Statement but that too was used for the moderate risk and expected hazards. It was decided not to issue and focus on the main threats.

 

The main event included a Quasi Linear Convective System (QLCS) squall line and several cells preceding the line that prompted a few initial warnings. However, those warnings increased in size as the squall line approached. During several times within the squall line, the Tornado Vortex Signature algorithm was activated and Storm Relative Velocity signatures suggested rotation. However, continuity and depth of the mesocyclone were less than ideal. As those dewpoints quickly climbed into the lower 70s, the LCL dropped to 2000 feet or less and LAPS Surface Based CAPES exceeded 2000 J/KG so the potential for a TOR was there but not realized. 

 

There were widespread wind damage reports but only one significant wind report (65 kt or greater) and 2 tornadoes that occurred neat New York City prior to Tornado Watch occurrence.  From one perspective, the density of damage reports verified the moderate risk of severe thunderstorms but the tornadoes and significant wind reports were never realized.

 

 

Above:  4-panel displays of predicted CAPE (upper left), 850 hPa theta-e (upper right), 850 hPa winds and isotachs (lower left) and 700-500 hPa lapse rates (lower right) from the GFS initialized at 1200 UTC 7 September (left) and 0000 UTC 8 September (right).

 

 

 

Above:  4-panel displays of predicted CAPE (upper left), 850 hPa theta-e (upper right), 850 hPa winds and isotachs (lower left) and 700-500 hPa lapse rates (lower right) from the NAM Initalized at 1800 UTC 8 September.

 

 

Above:  NSHARP soundings from 1200 UTC 8 September from Albany NY (ALB, left) and Upton, NY (OKX, right).

 

 

Above:  NSHARP soundings from 1800 UTC 8 September from Albany NY (ALB, left) and Upton, NY (OKX, right).

 

 

Above:  NSHARP soundings from 0000 UTC 9 September from Albany NY (ALB, left) and Upton, NY (OKX, right).

 

 

Above:  AWIPS D2D sounding from 1200 UTC 8 September from Albany NY (ALB).

 

 

Above:  Plots of heights, winds, temperatures and dew points at 850 hPa from 1200 UTC 8 September (left) and 0000 UTC 9 September (right).

 

 

Above:  Plots of heights, winds, temperatures and dew points at 500 hPa from 1200 UTC 8 September (left) and 0000 UTC 9 September (right).

 

 

Above:  Plots of heights, winds, temperatures and dew points at 250 hPa from 1200 UTC 8 September (left) and 0000 UTC 9 September (right).

 

Above:  Surface plot of MSLP, winds, temperatures and dew points at 1200 UTC 8 September (left) and 0000 UTC 9 September (right).

 

 

Above:  Loop of surface based CAPE and Convective Inhibition.

 

 

Above:  Loop of Downward CAPE.

 

 

Above:  Loop of 0-1 Km shear.

 

 

Above:  Loop of 0-6 Km shear.

 

 

Above:  Loop of supercell composite and Bunkers Storm Motion.

 

 

Above:  Loop of the significant tornado parameter and mixed layer Convective Inhibition.

 

 

Above:  Water vapor satellite imagery loop.

 

 

Above:  Visible satellite imagery loop with station plot and isobars overlayed.

 

 

Above:  Radar loops from KENX of Base Reflectivity (left), Base Velocity (center) and Storm Relative Velocity (right).

 

Above:  Two of the Tornado Watches issued.

 

 

Above:  Plot of severe weather overlayed on the convective outlook for verification purposes.