SPC Mesoscale Discussion

mcd1284 SPC Mesoscale Discussion
MESOSCALE DISCUSSION 1284
NWS STORM PREDICTION CENTER NORMAN OK
1147 AM CDT FRI JUN 28 2013

AREAS AFFECTED…PORTIONS CT/MA/RI/NH…SERN NY.

CONCERNING…SEVERE POTENTIAL…WATCH UNLIKELY

VALID 281647Z – 281915Z

PROBABILITY OF WATCH ISSUANCE…20 PERCENT

SUMMARY…WIDELY SCATTERED TSTMS…NEAR AND WITHIN BAND OF INITIALLY NON-SVR CONVECTION OVER DISCUSSION AREA — MAY STRENGTHEN TO NEAR SVR LEVELS THROUGH MID-AFTN. ISOLATED DAMAGING GUSTS OR BRIEF TORNADO CANNOT BE RULED OUT. HOWEVER…POTENTIAL CURRENTLY APPEARS TOO LOW-END…CONDITIONAL AND LIMITED IN AREA FOR WW.

DISCUSSION…16Z SFC MESOANALYSIS SHOWS PRE-FRONTAL CONFLUENCE LINE FROM WARM-FRONTAL INTERSECTION NEAR VT/NH BORDER SWD NEAR CT/VT/NH JUNCTION SSWWD ACROSS CT TO NYC AREA. THIS FEATURE REPRESENTS WIND SHIFT BETWEEN RELATIVELY VEERED/SSWLY-SWLY FLOW OVER LOWER HUDSON REGION TO SSWLY/SLY FLOW OVER SERN NEW ENGLAND. BAND OF CONVECTION HAS PERSISTED INVOF THIS BOUNDARY OVER CT…EXTENDING OFF BOUNDARY INTO CENTRAL MA…WHILE ADDITIONAL/ISOLATED ACTIVITY HAS FORMED ALONG BOUNDARY NEAR SW CORNER OF NH. AIR MASS IS DESTABILIZING FAVORABLY ON BOTH SIDES OF BOUNDARY AMIDST CLOUD BREAKS AND SFC INSOLATION. HOWEVER…GREATEST BOUNDARY-LAYER MOISTURE AND LOW-LEVEL SHEAR ARE EVIDENT E OF BOUNDARY ACROSS ERN CT…CENTRAL-ERN MA AND RI…AND W OF RELATIVELY COOL/STABLE ATLC MARINE LAYER. ACCORDINGLY MODIFIED RAOBS AND FCST SOUNDINGS SHOW WEAK MIDLEVEL LAPSE RATES OFFSET ENOUGH BY SFC HEATING TO YIELD 500-1000 J/KG MLCAPE…AND POCKET OF 100-200 J/KG 0-3 KM CAPE IS EVIDENT JUST AHEAD OF CONVECTIVE BAND OVER ERN CT AND E-CENTRAL MA. ERN FRINGES OF SUBTLE DCVA/DESTABILIZATION ALOFT…RELATED SMALL SHORTWAVE THAT IS EVIDENT IN MOISTURE CHANNEL IMAGERY PIVOTING NEWD ACROSS CENTRAL-ERN PA…ALSO MAY SUPPORT NEW ENGLAND TSTM ENVIRONMENT. PERTURBATION ALOFT ALSO MAY MAINTAIN MID-UPPER GRADIENT FLOW ENOUGH TO MAINTAIN MRGL EFFECTIVE SHEAR MAGNITUDES 30-35 KT. QUASI-LINEAR STRUCTURE ALREADY EVIDENT IN PRIMARY CONVECTIVE BAND APPEARS TO REDUCE POTENTIAL FOR SUSTAINED/DISCRETE SUPERCELL STRUCTURES…AS DOES LACK OF MORE ROBUST DEEP SHEAR…BUT SHORT-LIVED/STORM-SCALE ROTATION MAY OCCUR.

ADDITIONAL TSTMS MAY DEVELOP OVER PORTIONS SERN NY AND WRN CT…MOVING EWD INTO THIS AREA BEHIND INITIAL BAND. THOUGH SUCH ACTIVITY WOULD BE IN WEAKER VERTICAL SHEAR THAN FARTHER E…ISOLATED HAIL AND GUSTS APCHG SVR LIMITS ARE POSSIBLE.

..EDWARDS/MEAD.. 06/28/2013

ATTN…WFO…GYX…BOX…OKX…ALY…

LAT…LON 41097400 41517431 42057398 42837217 43037108 426270934 2237088 41797069 41497105 41247296 41097400

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