June 30, 2015 – Local Supercells
Wall cloud and updraft base of storm northwest of Philadelphia. At this time there was little rotation on radar. The cell had produced s couple of tornado warnings earlier. Taken in downtown Philadelphia at 4:11PM.
The forecast for today didn’t look particularly promising for severe weather. The early morning area forecast discussion from the Philadelphia NWS forecast office in Mt. Holly NJ said:
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 1030 AM EDT TUE JUN 30 2015 .SYNOPSIS... A WARM FRONT OVER THE OHIO VALLEY IS PREDICTED TO LIFT SLOWLY NORTHEAST INTO AND THROUGH THE FORECAST AREA TONIGHT. IT WILL BE FOLLOWED BY A SLOW MOVING COLD FRONT LATE WEDNESDAY OR THURSDAY. THAT FRONT IS EXPECTED TO STALL JUST SOUTH OF OUR REGION FOR THE REMAINDER OF THE WORK WEEK. ANOTHER COLD FRONT SHOULD PASS THROUGH OUR REGION ON INDEPENDENCE DAY. A WEAK HIGH PRESSURE SYSTEM SHOULD FOLLOW ON SUNDAY AND MOVE OFFSHORE AS A WARM FRONT APPROACHES FROM THE OHIO VALLEY ON MONDAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... LATEST RADAR TRENDS INDICATE -SHRA WITH AN OCCASIONAL RUMBLE OF THUNDER MOVING THRU THE POCONOS AND LEHIGH VALLEY AS ISENTROPIC LIFT AND ELEVATED INSTABILITY PRECEDE A WARM FRONT INVOF DE BAY ATTM. AS MID-LEVEL FLOW BECOMES MORE MERIDIONAL IN RESPONSE TO S/WV TROUGH APPROACHING FROM THE WEST...THE WM FRONT WILL MAKE SLOW PROGRESS NORTH THRU THE DAY. THE MAIN CONCERN FOR TODAY`S FORECAST IS THE POTENTIAL FOR CONVECTION. THERE IS STILL A LOT OF UNCERTAINTY WITH REGARD TO CONVECTIVE POTENTIAL. APPROACHING S/WV WILL PROVIDE A SOURCE OF LIFT...AS BEST FORCING OCCURS NEAR THE WARM FRONTAL BOUNDARY. AMPLE LOW- LEVEL MOISTURE IS IN PLACE...AS DRIER MID-LEVEL AIR ADVECTS IN ALOFT. THE MAIN UNCERTAINTY IS INSTABILITY...AS LAPSE RATES ARE LESS THAN IDEAL...WITH PERHAPS EVEN SOME CAPPING. SAT TRENDS ALSO INDICATE CONSIDERABLE CLOUDINESS. THEREFORE ML CAPE MAY HAVE A TOUGH TIME REACHING 1 KJ. ALTHOUGH INSTABILITY IS LIMITED...BULK SHEAR IS 40 KNOTS NW OF I-95 ALONG W/ AMPLE LOW-LEVEL HELICITY...AS 0-1 KM SHEAR REACHES 20 KNOTS. SO THERE IS SVR POTENTIAL TODAY MAINLY NW OF I-95...ESP THE POCONOS AND LEHIGH VALLEY...W/ THE MAIN THREAT BEING DAMAGING WINDS AND SMALL HAIL GIVEN INVERTED-V PROFILES AND FAVORABLE WBZ HEIGHTS. WILL ALSO HAVE TO WATCH ACTIVITY NEAR THE WARM FRONT... WHERE A SUPERCELL CANNOT BE RULED OUT...PARTICULARLY IF GREATER INSTABILITY IS REALIZED. FLASH FLOODING IS ALSO A CONCERN MAINLY NW OF I-95...WHERE CELLS WILL HAVE A TENDANCY TO TRAIN. By early afternoon discrete cells were developing in places in southeastern PA and the Storm Prediction Center was considering the possibility of tornadic supercell in it's mesoscale discussion:
MESOSCALE DISCUSSION 1247 NWS STORM PREDICTION CENTER NORMAN OK 0157 PM CDT TUE JUN 30 2015 AREAS AFFECTED...PORTIONS OF FAR ERN PA...FAR WRN NJ CONCERNING...SEVERE THUNDERSTORM WATCH 372... VALID 301857Z - 302000Z THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 372 CONTINUES. SUMMARY...THE RISK FOR A COUPLE OF TORNADOES WILL EXIST ACROSS A SMALL CORRIDOR OF SEVERE THUNDERSTORM WATCH 372 -- ACROSS PORTIONS OF FAR ERN PA AND FAR WRN NJ. DISCUSSION...SFC OBSERVATIONS INDICATE PRESSURE FALLS AROUND 1.5 MB PER 2 HOURS NEAR AND E OF AN AXIS OF DISCRETE CELLS EXTENDING FROM NEAR ALLENTOWN TO W OF WILMINGTON DE WHERE DESTABILIZATION CONTINUES. OCCASIONAL MESOCYCLONES HAVE BEEN EVIDENT BASED ON SINGLE-SITE RADAR DATA. CONTINUED DIURNAL HEATING AMIDST MIDDLE-60S SFC DEWPOINTS WILL MAINTAIN PERSISTENCE OF THIS ACTIVITY. DYNAMIC PERTURBATION PRESSURES AT THE STORM-SCALE MAY ALLOW EWD PROPAGATION OF THIS ACTIVITY IN AN OTHERWISE SWLY-FLOW-ALOFT REGIME...PERHAPS TAKING IT TOWARD FAR WRN NJ AS WELL AS PARTS OF THE PHILADELPHIA AREA IN THE SHORT-TERM. DOX/DIX VWPS SAMPLE A VERTICALLY VEERING WIND PROFILE IN THE LOWEST COUPLE OF KM AGL THAT MAY CONTINUE TO ENHANCE LOW-LEVEL SRH SUFFICIENTLY FOR THE RISK OF ONE OR TWO TORNADOES. POOR MID-LEVEL LAPSE RATES AND ONLY MODEST LOW-LEVEL FLOW STRENGTH MAY MITIGATE A MORE SUBSTANTIAL TORNADO RISK OVERALL.
MESOSCALE DISCUSSION 1250 NWS STORM PREDICTION CENTER NORMAN OK 0315 PM CDT TUE JUN 30 2015 AREAS AFFECTED...PORTIONS OF THE MID-ATLANTIC REGION CONCERNING...SEVERE THUNDERSTORM WATCH 372...377...378... VALID 302015Z - 302215Z THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 372...377...378...CONTINUES. SUMMARY...THE RISK FOR SVR TSTMS CONTINUES ACROSS THE SEVERE THUNDERSTORM WATCH AREAS. DISCUSSION...SCATTERED CONVECTION CONTINUES FROM PARTS OF ERN PA TO CNTRL NC WITHIN A BROAD ZONE OF 500-1500 J/KG OF MLCAPE. AREA VWPS SUGGEST THAT 25-40 KT OF DEEP SHEAR WILL CONTINUE TO FOSTER ORGANIZED CONVECTIVE STRUCTURES...AND POSSIBLY A FEW TRANSIENT SUPERCELLS -- ESPECIALLY IN NRN PARTS OF THE MID-ATLANTIC /PA AND NJ/ WHERE SFC WINDS ARE SLIGHTLY MORE BACKED ENHANCING LOW-LEVEL SRH. ISOLATED TO WIDELY SCATTERED DMGG WIND GUSTS AND PERHAPS HAIL WILL BE THE MAIN CONCERNS...THOUGH A BRIEF TORNADO CANNOT BE RULED OUT IN ERN PA AND NJ. SFC SLY/SSELY FLOW WILL CONTINUE TO FOSTER INLAND INFLUXES OF OCEAN-MODIFIED AIR WITH WEAKER BUOYANCY -- PROBABLY RESULTING IN SOME WEAKENING OF CONVECTION AS IT NEARS THE COAST.
