Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 072201 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 601 PM EDT MON JUL 7 2014 .SYNOPSIS... HIGH PRESSURE STRETCHED FROM THE CENTRAL ATLANTIC TO THE GULF OF MEXICO WILL DOMINATE OUR WEATHER THROUGH TUESDAY. A WEAK TROUGH WILL BE EVIDENT IN CENTRAL PENNSYLVANIA THIS EVENING AND WASH OUT OVERNIGHT. A COLD FRONT ARRIVES IN OUR REGION WEDNESDAY. IT SETTLES SLOWLY TO SOUTHEAST DELAWARE THURSDAY AND TO THE VIRGINIA CAPES FRIDAY WHILE HIGH PRESSURE MOVES TO NEW YORK STATE. THAT HIGH MERGES WITH THE CENTRAL ATLANTIC HIGH NEXT WEEKEND AWAITING THE ARRIVAL OF OUR NEXT COLD FRONT LATE SUNDAY OR MONDAY. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... A LINE OF SHOWERS AND THUNDERSTORMS HAVE BEGUN TO FORM TO OUR NORTH AND WEST FROM THE OHIO RIVER VALLEY THROUGH PENNSYLVANIA AND CENTRAL NEW YORK. THIS IS ASSOCIATED WITH A SHORT WAVE/VORTICITY IMPULSE INTERACTING WITH A FRONTAL BOUNDARY APPROACHING OUR AREA FROM THE WEST. THE FRONT SHOULD BE MOVING ACROSS OUR AREA LATER THIS EVENING INTO THE OVERNIGHT HOURS, BEFORE IT BEGINS TO WASH OUT ACROSS THE AREA. THE VORTICITY IMPULSE WEAKENS SOME AS WELL AS IT APPROACHES, ALTHOUGH THE STRONGEST PORTION OF THE IMPULSE SLIDES TO OUR NORTH. WITH THE BEST VERTICAL MOISTURE SLIDING NORTH OF OUR AREA AS WELL, MOST GUIDANCE DISSIPATES THE MAJORITY OF THE SHOWERS AND THUNDERSTORMS BEFORE THEY REACH OUR AREA. THERE STILL REMAINS A CHANCE OF SCATTERED SHOWERS/THUNDERSTORMS MAKING THEIR WAY ACROSS OUR AREA FROM JUST BEFORE SUNSET, TO A COUPLE OF HOURS AFTER. ANY SHOWERS AND/OR THUNDERSTORMS THAT DO MAKE IT ACROSS THE AREA SHOULD END BEFORE MIDNIGHT. A BLEND OF MAV/MET/MOSGUIDE WAS TAKEN FOR OVERNIGHT LOWS, WITH A SLIGHTLY HIGHER USE OF MAV, WHICH KEEPS EVERYONE SEVERAL DEGREES ABOVE NORMAL. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... TUESDAY WILL BE ANOTHER SIMILAR DAY TO TODAY/MONDAY, AS A COLD FRONT WILL BE APPROACHING THE AREA FROM THE WEST, WHILE A PRE-FRONTAL/LEE-SIDE TROUGH SETS UP ACROSS OUR WESTERN AREAS DURING THE DAY. WE EXPECT TUESDAY TO BE A DEGREE OR TWO WARMER THAN TODAY AS THICKNESSES RISE SLIGHTLY AND 925 MB TEMPS INCREASE SLIGHTLY AS WELL. EVEN THOUGH IT WILL BE A WARM DAY WITH HIGHS IN THE LOW-MID 90S ONCE AGAIN ACROSS MUCH OF THE AREA, DEWPOINTS STILL ARE NOT EXPECTED TO RISE MORE THAN THE MID-UPPER 60S, WHICH KEEPS HEAT INDEX VALUES FROM REACHING CRITICAL LEVELS NEEDED FOR AN ADVISORY. NEVERTHELESS, IT WILL BE QUITE WARM, SO IT WILL STILL BE ADVISABLE TO TAKE PRECAUTIONS WITH OUTDOOR ACTIVITIES. IN REGARDS TO SHOWERS/THUNDERSTORMS, THERE WILL ONCE AGAIN BE A FAIR AMOUNT OF INSTABILITY DURING THE DAY, BUT WITH THE MAIN SHORT WAVE/VORTICITY IMPULSE WELL TO OUR WEST DURING THE DAY AND THE FRONT ACROSS CENTRAL PENNSYLVANIA, IT WILL BE HARD TO GET MUCH TO DEVELOP ACROSS THE AREA FOR MUCH OF THE DAY. WE MAY GET SOME SHOWER/STORMS DEVELOPING LATE IN THE AFTERNOON AS THE PRE-FRONTAL TROUGH BEGINS TO INTERACT WITH SOME VORTICITY ADVECTION, SO WE WILL HAVE INCREASING CHANCES DURING THE AFTERNOON, ESPECIALLY ACROSS OUR WESTERN HALF. SHEAR VALUES DO INCREASE, AND WITH AN INCREASING LOW LEVEL JET, SOME GUSTY WINDS COULD BE POSSIBLE. LOOKING AT THE CAPE ALOFT, THERE IS A FAIR AMOUNT OF CAPE IN THE HAIL GROWTH REGION, SO EVEN SOME SMALL HAIL IS POSSIBLE AS WELL. HOWEVER, THE BEST CHANCES FOR STRONGER STORMS WILL BE DURING THE EVENING, WHICH IS DISCUSSED BELOW. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
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500MB: THE GREAT LAKES TROUGH WILL AMPLIFY WEDNESDAY THEN LIFT NORTHEASTWARD TO NORTHEAST CANADA BY FRIDAY NIGHT. HEIGHTS RISE A LITTLE NEXT WEEKEND IN THE WAKE OF THE DEPARTED TROUGH AND IN ADVANCE OF WHAT MIGHT BECOME A SUBSTANTIAL SOUTH CENTRAL CANADA UPPER MIDWEST TROUGH NEXT SUNDAY AND MONDAY. TEMPERATURES: THE FIRST DAYS OF THE MONTH WERE AVERAGING GENERALLY 1 TO 2 DEGREES ABOVE NORMAL EXCEPT NORMAL AT ATLANTIC CITY. THESE POSITIVE DEPARTURES SHOULD REMAIN ABOUT THE SAME OVER THE NEXT WEEK OR SO. CALENDAR DAY AVERAGES FOR THIS FORECAST PERIOD WILL BE ABOUT 5 DEGREES ABOVE NORMAL WEDNESDAY AND NEAR NORMAL THURSDAY- SATURDAY BEFORE PROBABLY BECOMING WARMER THAN NORMAL NEXT SUNDAY AND MONDAY. FORECAST BASIS: UNLESS OTHERWISE NOTED THE LONG TERM PERIOD OF THIS FORECAST IS GENERALLY BASED ON A 50 50 BLEND OF THE 12Z/7 NAM/GFS MOS FOR THE PERIOD TUESDAY NIGHT THROUGH WEDNESDAY NIGHT...THE 12Z/7 GFS MOS THURSDAY AND THURSDAY NIGHT AND THEN THEREAFTER (FRIDAY ONWARD) THE 1522Z/7 WPC GRIDDED GUIDANCE WHICH AT TIMES IS BLENDED 50 50 WITH THE 12Z/7 MEXMOS. ALL THIS IS EVENTUALLY CHECKED AGAINST THE 12Z/7 ECMWF TO SEE IF WE`RE IN THE BALLPARK WITH ECMWF CYCLICALLY PERSISTENT 18Z 2METER TEMPERATURES. THE PRECIPITATION PROBABILITY GUIDANCE IS CHECKED AGAINST THE 09Z/7 SREF POPS FOR .01 IN 3 HRS THROUGH 0Z FRIDAY AND THEREAFTER THE 12Z/7 GEFS 6HR POPS FOR 0.05 . TUESDAY NIGHT...TSTMS IN PROGRESS IN E PA WITH EMBEDDED SVR PER SWODY2 DEPICTION WITH OUTFLOWS CROSSING THE DELAWARE RIVER PROBABLY IN THE 00Z-02Z TIME FRAME. 38KI SWI -2 TT 49 DECENT MLCAPE OF NEAR 1000J AND 30-38 KT 0-6KM BULK SHEAR SIZABLE 0-3KM SHEAR VCNTY KMPO NORTHWARD. CONVECTION SHOULD WIND DOWN AFTER MIDNIGHT AND IF YOU BELIEVE THE ECMWF CONVECTIVE SCHEME WILL BE NON EXISTENT AFTER 00Z. I THINK THAT STRONG CONVECTION UPSTREAM WILL MAKE TO THE DELAWARE RIVER SIMPLY BORNE BY ITS OWN OUTFLOWS IN A HOT AIRMASS. THE COLD FRONT ALOFT THAT I THINK IS THE PRIMARY SQLN DRIVER IS WEAK AND NOT NOTICEABLE AT THE SFC. PWAT 2 INCHES. IF CONVECTION SURVIVES INTO THE I95 CORRIDOR...THEN WE`LL HAVE POOR DRAINAGE FLOODING DURING THE EVENING. PRIOR TO THE TSTMS IT WILL BE HOT IN THE EVENING WITH PROBABLE CONVECTIVE ANVILS FROM UPSTREAM HOLDING IN THE DAYS HEAT AND NAM 2M TEMP OF AROUND 90F AT 00Z WED IN UNDISTURBED SW GRADIENT FLOW LOOKS ACCURATE TO WITHIN 2F. THE OVERALL NIGHTS LOW TEMP WILL IN PART BE CONVECTIVE COOLING RELATED. OTRW AM EXPECTING WARMER THAN GUIDANCE AND USED THE 12Z/7 NAM 2M LOW TEMP TO MODIFY THAT GUIDANCE WARMER. ALSO...WARMER WETTER GFS MOS DEWPOINTS USED (NO MIXING EXPECTED). WEDNESDAY...COLD FRONT BECOMES MORE EVIDENT IN OUR AREA DURING MIDDAY WITH MIXING. WINDS GENERALLY SW GUST 15 MPH IN THE AFTN. ML CAPE 1000+J SNJ AND THE DELMARVA WITH 39KI -4SWI AND 51 TT ALONG WITH 30-40KT 0-6KM BULK SHEAR. LOOKS TO BE AN EARLY SHOW POTENTIALLY FOR TALBOT-CAROLINE COUNTIES MD THROUGH THE S 1/2 DELAWARE AND MAYBE FAR SE NJ. 3RD DAY OF WHAT I THINK WILL BE OUR SECOND HEAT WAVE (572 THICKNESS). ECMWF ADVERTISES 85 2M TEMP AT 18Z AND HAVE OPTED FOR A WARMER SCENARIO AND 90F. NO GUARANTEE BUT I THINK MOS RUNS TOO CONSERVATIVE IN WESTERLY FLOW...ESPECIALLY WESTERLY FLOW THATS ALREADY BEEN BAKED. SO FOR NOW...A 3 DAY HEAT WAVE IS STILL IN THE CARDS FOR I95 WITH GUIDANCE TEMPS RAISED FROM PHL SW TO E MD. ELSEWHERE THERE IS A SMALL CHANCE OF A SHOWER OR TSTM LATE IN THE DAY BUT FOR NOW STAYED WITH CONSERVATIVE APPROACH. WEDNESDAY NIGHT...ANY REMNANT CONVECTION SHOVES OFFSHORE AND OR DIMINISHES EARLY. PATCHY FOG POSSIBLE LATE? THURSDAY AND FRIDAY...P/C AND SEASONABLY WARM. UPPER PART OF THE GUIDANCE MAX TEMPS OR EVEN RAISED SINCE 850MB TEMPS REMAIN NEAR 13-14C. CANT RULE OUT AN AFTERNOON OR EVENING SHOWER OR TSTM IN LEFTOVER TROUGH ALOFT THAT IS LEFT BEHIND AFTER THE PRIMARY GREAT LAKES TROUGH DEPARTS NEWD FOR LABRADOR. FRIDAY NIGHT...FOG/STRATUS POSSIBLE LATE? SATURDAY...BECOMING P/S...CHANCE OF A SHOWER OR TSTM LATE DAY OR AT NIGHT? WAA. SUNDAY AND MONDAY...P/S AND HUMID. MODELED CHANCE STRONG TSTMS WITH HEAVY POOR DRAINAGE FLOODING RAIN POSSIBLE LATE DAY OR NIGHT...POSSIBLY MORE SO PHL S AND W WHERE KI 38 SWI -3 AND TT 51 AND PWAT NEAR 1.75 INCHES. TIMING OF THE TRIGGERING COLD FRONT AND POSSIBLE PREFRONTAL TROUGH ARE IN QUESTION.
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&& .AVIATION /22Z MONDAY THROUGH SATURDAY/...
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THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. MOSTLY VFR CONDITIONS EXPECTED THROUGH THE TAF PERIOD. THE MAIN EXCEPTION IS THE POSSIBILITY OF SOME LIGHT FOG FOR A FEW TAF SITES OVERNIGHT WHICH WE KEEP A TEMPO GROUP OVERNIGHT FOR THESE SITES; RDG/ABE/TTN/MIV. WE EXPECT THE MAJORITY OF THE SHOWER/THUNDERSTORM ACTIVITY TO REMAIN TO OUR NORTH AND WEST THROUGH THIS EVENING, ALTHOUGH IT IS POSSIBLE IT COULD TRICKLE DOWN ACROSS OUR AREA, ESPECIALLY OUR NORTHWEST TAFS. WE HAVE INCLUDED A -SHRA GROUP FOR RDG/ABE, BUT HAVE KEPT IT OUT FOR EVERYWHERE ELSE FOR NOW. WE GO THROUGH THE SAME THING ON TUESDAY AS WE EXPECT THE EARLY PART OF THE DAY TO BE DRY, ALTHOUGH WE HAVE A BETTER PROBABILITY OF SHOWERS/THUNDERSTORMS BY TUESDAY EVENING. ANY THUNDERSTORMS CHANCES WERE LEFT OUT AS THEY ARE BEYOND THE TAF TIME PERIOD FOR THE MAJORITY OF THE TAF SITES, AND AT THE VERY END OF THE PHL TAF TIME PERIOD. WINDS LATE THIS AFTERNOON WILL CONTINUE TO BE GUSTY, BEFORE GUSTS DROP OFF AFTER SUNSET. WINDS MAY BECOME LIGHT AND VARIABLE A SEVERAL SITES, BEFORE INCREASING ONCE AGAIN AFTER SUNRISE TUESDAY. IN FACT, WINDS WILL GUST ONCE AGAIN AROUND 20 KNOTS FOR MOST SITES AGAIN TUESDAY. OUTLOOK... TUESDAY NIGHT...VFR SCT-BKN AOA 5000 FT BUT AT LEAST SCATTERED TSRA ALONG AND WEST OF THE DELAWARE RIVER VALLEY INCLUDING KABE, KRDG, KPHL, KPNE, KTTN, AND KILG. UNDER ANY TSRA THAT BRIEF IFR CONDITIONS WILL BE POSSIBLE ALONG WITH WESTERLY GUSTS 35-40KT. THERE IS SOME DOUBT AS TO HOW FAR EAST THE THUNDERSTORMS WILL MOVE...POSSIBLY SUPPRESSING AND DRYING OUT AFTER MIDNIGHT. WEDNESDAY...VFR SCT-BKN AOA 5000 FT WITH A SW WIND GUST NEAR 15 KT DURING THE AFTERNOON. MVFR AND IFR CONDITIONS POSSIBLE UNDER ANY STORMS THAT DEVELOP. THE THREAT FOR STORMS WILL BE FURTHER SOUTHEAST THAN THOSE OF TUESDAY EVENING, THIS TIME FAVORING KMIV AND KACY. THURSDAY AND FRIDAY...GENERALLY VFR, A SMALL RISK OF TSRA EACH DAY, WITH MVFR CONDITIONS POSSIBLE WITH ANY STORMS THAT DEVELOP. ALSO A SMALL CHANCE OF MVFR BR TO DEVELOP FRIDAY NIGHT AS THE FLOW SHIFTS TO AN ON SHORE FLOW. SATURDAY...SOME UNCERTAINTY REGARDING WHETHER VFR OR MVFR SCT-BKN. MVFR BR POSSIBLE EARLY.
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&& .MARINE... THE SMALL CRAFT ADVISORY REMAINS IN PLACE ACROSS THE COASTAL WATERS AS WINDS ARE LIKELY GUSTING AROUND 25 KNOTS. SEAS ARE ONLY AROUND 3-4 FEET, BUT MAY INCREASE TO 4-5 FEET LATER THIS EVENING INTO THE EARLY PART OF THE OVERNIGHT. WE WILL KEEP THE 2 AM END TIME OF THE ADVISORY AND SEE HOW SEAS REACT THROUGH THE EVENING. WE EXPECT A PERIOD WHEN SEAS AND WINDS WILL BELOW ADVISORY LEVELS EARLY TUESDAY MORNING INTO THE AFTERNOON HOURS, BEFORE THEY INCREASE AGAIN. SO WE HAVE DECIDED NOT TO EXTEND THE ADVISORY FOR NOW, ALTHOUGH WE ANTICIPATE THE NEED FOR ANOTHER BY LATE TUESDAY AFTERNOON. OUTLOOK... TUESDAY NIGHT...A SMALL CRAFT ADVISORY WILL LIKELY BE NEEDED AS WINDS COULD GUST ABOVE 25KT, AND SEAS NEAR 5 TO 6 FT WILL BE POSSIBLE. ALSO GUSTY WINDS COULD BE POSSIBLE WITH ANY THUNDERSTORMS THAT REACH THE COASTAL WATERS...ESPECIALLY TOWARD MIDNIGHT. WEDNESDAY...WINDS SHOULD GENERALLY BE NEAR OR BELOW SMALL CRAFT ADVISORY CRITERIA. ATLANTIC COASTAL WATERS SEAS MAY BE NEAR 5 FEET DURING THE MORNING. THERE IS A RISK FOR STRONG GUSTY WINDS WITH ANY STORMS THAT DEVELOP WEDNESDAY AFTERNOON OR NIGHT FROM THE S NJ COAST SOUTHWARD THROUGH THE DE WATERS. THURSDAY THROUGH SATURDAY...WINDS AND SEAS BELOW SMALL CRAFT ADVISORY CRITERIA. ONLY A SMALL RISK EACH DAY OF A THUNDERSTORM AND IF OCCURS...JUST A VERY SHORT TIME. ODDS FAVOR DRY THROUGHOUT THIS 3 DAY PERIOD. A LITTLE FOG MAY FORM FRIDAY NIGHT OR SATURDAY MORNING. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 2 AM EDT TUESDAY FOR ANZ450>455. && $$ SYNOPSIS...DRAG NEAR TERM...ROBERTSON SHORT TERM...ROBERTSON LONG TERM...DRAG CORRECTED TUE NIGHT LINES 7/8 6P AVIATION...DRAG/ROBERTSON CORRECTED WEDNESDAY LINE 4 6P MARINE...DRAG/ROBERTSON

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