AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 330 PM EDT THU AUG 18 2011 .SYNOPSIS... A WEAK COLD FRONT WILL MOVE INTO THE AREA TONIGHT. THIS FRONT MAY THEN STALL IN OUR REGION FRIDAY INTO SATURDAY AS A WEAK LOW PRESSURE SYSTEM DEVELOPS ALONG IT. A WEAK DISTURBANCE MAY MOVE THROUGH SATURDAY NIGHT INTO SUNDAY, THEN A STRONGER COLD FRONT IS FORECAST TO CROSS THE AREA LATER SUNDAY. THIS FRONT IS ANTICIPATED TO BE EAST OF OUR ON MONDAY AS HIGH PRESSURE BUILDS INTO THE OHIO VALLEY. THIS HIGH THEN DOMINATES OUR WEATHER TUESDAY AND WEDNESDAY BEFORE SETTLING TO THE SOUTH THURSDAY. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... SEE THE HYDROLOGY SECTION BELOW FOR ADDED DETAIL ON THE FLASH FLOOD WATCH FOR MUCH OF OUR AREA FROM LATE TONIGHT INTO OR THROUGH FRIDAY NIGHT. A BIT OF A BREAK WAS OCCURRING THIS AFTERNOON FROM THE MORNING CONVECTION THAT WAS ASSOCIATED WITH WARM ADVECTION PROCESSES. AN INSTABILITY LINE WAS NOTED IN SATELLITE IMAGERY FROM NORTHEASTERN PENNSYLVANIA SOUTH-SOUTHWESTWARD INTO CENTRAL MARYLAND AT MID AFTERNOON. NOT MUCH ACTIVITY WAS OCCURRING IN MOST OF THE PENNSYLVANIA PORTION OF THE LINE. THIS DIURNAL ACTIVITY MAY TRY TO PLAY OUT EARLY THIS EVENING. HOWEVER...AS LOW PRESSURE MAINLY ALOFT ADVANCES EASTWARD OVERNIGHT...ADDITIONAL SHOWERS AND SOME THUNDERSTORMS ARE EXPECTED TO RETURN...AFTER MIDNIGHT OR TOWARD MORNING ESPECIALLY NORTH AND WEST. ANY OF THE THUNDERSTORMS COULD PRODUCE LOCALIZED HEAVY RAIN WITH HIGH RAIN RATES...EXCEEDING TWO INCHES PER HOUR. PERHAPS THIS LATTER WILL BE MORE LIKELY FRIDAY. LOW TEMPERATURES MAY NOT FALL MUCH AS DEW POINTS WILL BE CREEPING UP AND INCREASING CLOUDS WILL INHIBIT TEMPERATURE LOSS. MOS GUIDANCE LOOKED ACCEPTABLE. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... LOW PRESSURE SURFACE AND ALOFT WILL MOVE INTO WESTERN PORTIONS OF OUR AREA DURING FRIDAY MORNING...WITH THE NAM INDICATING A SURFACE LOW OVER THE CHESAPEAKE AND NORTHEASTERN MARYLAND REGION FRIDAY AFTERNOON. ONGOING SCATTERED TO BROKEN SHOWERS AND THUNDERSTORMS FROM LATE TONIGHT WILL BLEED INTO FRIDAY MORNING...WITH ACTIVITY EXPECTED TO GROW SOUTH AND EAST INTO THE AFTERNOON. AGAIN...WITH HIGH PRECIPITABLE WATER VALUES AND RATHER SLOW-MOVING CELLS WHICH MAY TEND TO CLUSTER...EXCESSIVE RAINFALL IS NOT OUT OF THE QUESTION...ALTHOUGH THE FLASH FLOOD WATCH IS JUST ABOVE THE LOW-CONFIDENCE THRESHOLD. NAM MOS WAS MORE AGGRESSIVE WITH CLOUDS AND PRECIPITATION...AND HENCE FORECAST LOWER HIGH TEMPERATURES. A BLEND WITH THE WARMER GFS MOS WAS USED. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... THIS TIME FRAME CONTINUES THE UNSETTLED THEME WITH UPPER-LEVEL TROUGHING ACROSS THE NORTHEAST AND MID ATLANTIC. IN ADDITION, A SURFACE BOUNDARY WILL BE IN THE AREA TO START BEFORE A STRONGER FRONT MOVES THROUGH LATE SUNDAY. WHILE THE LARGE SCALE PICTURE IS GENERALLY AGREED UPON BY THE GUIDANCE, THERE IS LOWER CONFIDENCE REGARDING THE DETAILS. THE STRONGER FRONT SHOULD SWEEP EAST OF THE AREA BY EARLY MONDAY WITH LARGE HIGH PRESSURE BUILDING IN FROM THE WEST TUESDAY AND WEDNESDAY. THIS SHOULD BRING MUCH DRIER AIR INTO THE REGION FOR A FEW DAYS BEFORE SOMEWHAT OF A RETURN FLOW STARTS DEVELOPING TOWARD THURSDAY. FRIDAY NIGHT AND SATURDAY...AN UPPER TROUGH REMAINS ACROSS THE AREA WITH A SURFACE BOUNDARY STALLED IN THE VICINITY. A WEAK SURFACE LOW SHOULD TEND TO TRACK GENERALLY ACROSS THE DELMARVA REGION AS WELL. SEVERAL SHORT WAVES EMBEDDED IN THE WEST/NORTHWEST FLOW WILL TEAM UP WITH SURGES OF WAA AND MOISTURE ADVECTION TO INITIATE CONVECTION. THE GREATEST CONCENTRATION OF CONVECTION SHOULD BE IN THE AFTERNOON AND EVENING HOURS. HOWEVER, DEPENDING ON THE TIMING OF ANY SUBTLE SHORT WAVES TEAMING UP WITH AT LEAST MID LEVEL INSTABILITY COULD KEEP ISOLATED TO SCATTERED CONVECTION GOING DURING THE NIGHT. PW VALUES ARE FORECAST TO BE AROUND 1.5 INCHES, THEREFORE LOCALLY HEAVY CONVECTIVE RAIN WILL BE POSSIBLE. THE CONVECTION MAY TEND TO BE LESS ON SATURDAY AS THE TROUGH AXIS SHIFTS EAST. THE GFS MAY BE OVERDONE AS A RESULT WITH THE CONVECTION. BASED ON THE FLOW, CONVECTIVE ELEMENTS MAY STILL BE SLOW MOVERS FRIDAY NIGHT THUS LOCALLY ENHANCING THE RAINFALL RATES. GIVEN THE WEALTH OF RAIN THAT HAS FALLEN ACROSS MANY AREAS THUS FAR THIS MONTH, A PORTION OF THE FLASH FLOOD WATCH FROM THE NEAR/SHORT TERMS WILL BE IN EFFECT UNTIL 10Z SATURDAY. SUNDAY AND MONDAY...A MORE SIGNIFICANT UPPER-LEVEL TROUGH WILL MOVE THROUGH SUNDAY AND SUNDAY NIGHT. THIS WILL DRIVE A STRONGER SURFACE COLD FRONT ACROSS THE REGION. AS THE HEIGHTS FALL MORE SUBSTANTIALLY AND A BELT OF STRONGER MID LEVEL FLOW ACCOMPANIES THE TROUGH, A MORE ORGANIZED CONVECTIVE THREAT MAY UNFOLD. AT LEAST SCATTERED SHOWERS AND THUNDERSTORMS ARE ANTICIPATED SUNDAY AND SUNDAY NIGHT WITH THE COLD FRONT AND POSSIBLY EVEN INITIALLY WITH A LEE SIDE TROUGH OR LEAD WEAK SURFACE DISTURBANCE. LOCALLY HEAVY RAIN WILL AGAIN BE POSSIBLE WITHE SOME SIGNALS IN THE MODEL GUIDANCE POINTING TO SOME ORGANIZED CLUSTERS OF STRONGER CONVECTION. AT THIS POINT, THE 12Z NAM LOOKS OVERDONE WITH ITS STRONGER AND MORE CONSOLIDATED SURFACE REFLECTION SUNDAY AFTERNOON. THE FRONT SHOULD BE SHUNTED TO OUR EAST EARLY MONDAY MORNING AS LARGE HIGH PRESSURE ARRIVES FROM THE WEST. SOME MODEL GUIDANCE LINGERS THE FRONT BACK SOME MONDAY WHICH WOULD ALLOW FOR ISOLATED CONVECTION, HOWEVER FOR NOW WE WILL PUSH THE FRONT FAR ENOUGH EAST AND JUST KEEP A SLIGHT CHC POP MAINLY INLAND. TUESDAY THROUGH THURSDAY...HIGH PRESSURE IS FORECAST TO SETTLE ACROSS THE AREA TUESDAY AND WEDNESDAY BEFORE SETTLING TO THE SOUTHEAST THURSDAY. THIS ALONG WITH THE ASSOCIATED AIRMASS WITH THE LARGE SURFACE HIGH WOULD BRING MUCH DRIER AIR INTO OUR REGION. THEREFORE, WE FAVORED A DRY FORECAST AND LOWER DEW POINTS. WE MAY START TO GET SOME RETURN FLOW BY THURSDAY AS THE SURFACE HIGH BEGINS TO ELONGATE AND SHIFT TO OUR SOUTH AND EAST. FOR SUNDAY NIGHT THROUGH THURSDAY, WE INITIALIZED WITH HPC GUIDANCE /A BLEND OF THE GEFS AND ECMWF ENSEMBLE MEAN/ THEN MADE SOME MAINLY MINOR TWEAKS BASED ON ADDITIONAL COLLABORATION. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. THE WARM ADVECTION-ASSOCIATED CONVECTION EARLIER THIS MORNING DISSIPATED BY MIDDAY...AND DIURNALLY-INDUCED CUMULUS CLOUDS WERE PREVALENT AROUND THE AREA THIS AFTERNOON. SOME OF THE CUMULUS WAS ABLE TO TOWER INTO ISOLATED CONVECTION...AND SATELLITE IMAGERY SHOWED AN INSTABILITY LINE FROM NORTHEASTERN PENNSYLVANIA SOUTH- SOUTHWESTWARD INTO CENTRAL MARYLAND. SOME OF THIS WILL ADVANCE EASTWARD...BUT FOR NOW JUST A CB MENTION WAS USED TO ALLUDE TO THE SCATTERED AFTERNOON AND EARLY EVENING CONVECTION. VFR CONDITIONS SHOULD PREVAIL EARLY TONIGHT...BUT THEN MODEL GUIDANCE...ESPECIALLY THE GFS...IS HINTING AT ANOTHER ROUND OF SCATTERED TO POSSIBLY BROKEN CONVECTION OVER OUR CENTRAL AND PERHAPS NORTHERN TAF SITES AFTER 06Z FRIDAY AND EXTENDING INTO ABOUT MID OR LATE FRIDAY MORNING...WITH THE FIRST IN A SERIES OF SHORT WAVES MOVING THROUGH. A TEMPO GROUP FOR MARGINAL VFR CONDITIONS IN SHOWERS AND SOME THUNDERSTORMS WAS USED TOWARD AND AFTER SUNRISE. IF THIS COMES TO PASS...IT IS EXPECTED TO MORPH INTO ADDITIONAL SHOWERS LATER FRIDAY AND INTO FRIDAY NIGHT AS SURFACE LOW PRESSURE COALESCES OVER THE CHESAPEAKE AREA AND NORTHEASTERN MARYLAND FRIDAY AFTERNOON. GENERAL VFR WEATHER WITH EXCURSIONS TO MARGINAL VFR OR LOWER IN PRECIPITATION CAN BE EXPECTED FRIDAY AFTERNOON. OUTLOOK... FRIDAY NIGHT...LOCALLY SUB-VFR DUE TO ANY SHOWERS AND THUNDERSTORMS AND PERHAPS SOME FOG. SATURDAY...GENERALLY VFR WITH LOW CONFIDENCE REGARDING ANY LINGERING CONVECTION. SUNDAY...A STRONGER COLD FRONT MOVES INTO THE AREA WITH AT LEAST SCATTERED SHOWERS AND THUNDERSTORMS. SUB-VFR CONDITIONS ARE ANTICIPATED WITH ANY SHOWERS AND THUNDERSTORMS. MONDAY AND TUESDAY...VFR AS HIGH PRESSURE BUILDS IN, HOWEVER CANNOT RULE OUT SOME LOCALIZED EARLY MORNING FOG. && .MARINE... CONDITIONS ARE FORECAST TO REMAIN BELOW SMALL CRAFT ADVISORY CRITERIA TONIGHT AND FRIDAY. LOW PRESSURE APPROACHING FROM THE WEST TONIGHT WILL MOVE INTO WESTERN PORTIONS OF OUR AREA FRIDAY. A SOUTHERLY FLOW IS EXPECTED THROUGH THIS PERIOD. GUSTS BETWEEN 15 AND 20 KNOTS MAY OCCUR AT TIMES...BUT 925 HPA WINDS ARE FORECAST TO BE GENERALLY 15 KNOTS OR LESS. SCATTERED SHOWERS AND SOME THUNDERSTORMS ARE FORECAST TO DEVELOP IN RESPONSE TO THE LOW PRESSURE SURFACE AND ALOFT....POSSIBLY SOME LATE TONIGHT AND EARLY FRIDAY MORNING BUT MORE AS FRIDAY PROGRESSES. ANY OF THE THUNDERSTORMS MAY PRODUCE LOCALLY GUSTY WINDS AND LIGHTNING BUT WOULD BE MORE LIKELY TO YIELD LOCALLY HEAVY RAIN WHICH WILL REDUCE VISIBILITIES SIGNIFICANTLY NEARBY. OUTLOOK... THE WINDS AND SEAS ARE ANTICIPATED TO REMAIN BELOW SMALL CRAFT ADVISORY FRIDAY NIGHT THROUGH TUESDAY. A WEAK FRONT NEAR THE AREA FRIDAY NIGHT SHOULD FOCUS SOME SHOWERS AND THUNDERSTORMS WITH POTENTIALLY LOCALLY ENHANCED WINDS AND SEAS. SOME WEAKNESS MAY STILL BE IN THE VICINITY SATURDAY, THEREFORE CANNOT RULE OUT AN ISOLATED SHOWER OR THUNDERSTORM. A STRONGER COLD FRONT WILL APPROACH SUNDAY THEN SWEEP EAST OF THE AREA BY EARLY MONDAY. THE SOUTHERLY FLOW AHEAD OF THE FRONT SHOULD INCREASE SOME SUNDAY BUT SHOULD REMAIN UNDER 25 KNOTS. THE NAM SEEMS TO ROBUST WITH THE SURFACE REFLECTION ALONG THE FRONT. HIGH PRESSURE BUILDS IN TUESDAY AND WEDNESDAY WITH A GENERAL OFFSHORE FLOW. OCEAN SEAS SHOULD TOP OUT NO HIGHER THAN 4 FEET. && .HYDROLOGY... A FLASH FLOOD WATCH HAS BEEN ISSUED FOR ALL OF OUR AREA EXCEPT THE FAR SOUTHERN ZONES. WITH AN UPPER TROUGH OF MODERATE STRENGTH MOVING ONLY SLOWLY THROUGH THE AREA LATE TONIGHT FRIDAY AND FRIDAY NIGHT... AND WHICH TRIES TO TAKES AN ALMOST NEGATIVE TILT...COMBINED WITH A SLOW-MOVING SURFACE FRONT...SCATTERED CONVECTION MAY GROW INTO SOME CLUSTERS OF HEAVY RAINFALL LATE TONIGHT AND FRIDAY. WITH HIGH ANTECEDENT PRECIPITATION HAVING RENDERED THE GROUND SATURATED... RUNOFF WILL BE HIGH FROM ANY STORMS AND WILL CAUSE URBAN AND SMALL STREAM FLOOD PROBLEMS TO RENEW. GRIDDED FLASH FLOOD GUIDANCE INDICATED THAT AREAS ALONG THE I-95 CORRIDOR AND INTO EAST- CENTRAL NEW JERSEY ARE THOUGHT TO BE MOST VULNERABLE. 1/2 TO 1 INCH OF MORE GENERAL PRECIPITATION THROUGH FRIDAY NIGHT IS EXPECTED TO BE LOCALLY 2 TO 4 INCHES WITH ANY HIGH RAIN RATE THUNDERSTORMS. PRECIPITABLE WATER VALUES LOOK TO BE BETWEEN 1 AND 1.8 INCHES. THE WATCH WILL EXTEND THROUGH 02Z SATURDAY NORTH...BUT THROUGH FRIDAY NIGHT /10Z SATURDAY/ CENTRAL AND SOUTH AS LOW LEVEL FORCING MAY HAVE A LONGER DURATION THERE ACCORDING TO MODEL OMEGA. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH FRIDAY EVENING FOR PAZ054-055-060>062. FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH LATE FRIDAY NIGHT FOR PAZ067>071. NJ...FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH FRIDAY EVENING FOR NJZ001-007>010-012. FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH LATE FRIDAY NIGHT FOR NJZ013>027. DE...FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH LATE FRIDAY NIGHT FOR DEZ001. MD...FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH LATE FRIDAY NIGHT FOR MDZ008-012. MARINE...NONE. && $$ SYNOPSIS...GORSE