AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 651 AM EDT THU MAY 12 2011 .SYNOPSIS... AN AREA OF HIGH PRESSURE ELONGATED DOWN THE EAST COAST WILL REMAIN IN PLACE TODAY AND TONIGHT BEFORE GRADUALLY SHIFTING TO THE EAST FRIDAY. AN AREA OF LOW PRESSURE ACROSS THE PLAINS WILL GRADUALLY SLIDE EASTWARD TO THE OHIO VALLEY SATURDAY THEN TO THE SOUTHEAST STATES SUNDAY. A FRONTAL BOUNDARY AHEAD OF THIS SYSTEM MAY SETTLE SOUTHWARD ACROSS OUR AREA FRIDAY NIGHT AND SATURDAY, THEN BEGIN MOVING NORTHWARD AS A WARM FRONT SUNDAY. A SLOW MOVING LOW PRESSURE SYSTEM THEN SETTLES INTO THE EAST MONDAY THROUGH WEDNESDAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A 06Z SURFACE ANALYSIS PLACED HIGH PRESSURE OVER THE CANADIAN MARITIMES WITH A SECOND BUT WEAKER HIGH DOWN ALONG THE MID ATLANTIC COAST. A STATIONARY FRONT WAS DRAPED FROM THE SOUTHEASTERN U.S. BACK UP TO THE UPPER OHIO VALLEY THEN INTO IOWA WHERE WEAK LOW PRESSURE WAS LOCATED. ANOTHER LOW WAS OVER WESTERN KANSAS WITH A COLD FRONT TRAILING TO THE SOUTHWEST. AN UPPER AIR ANALYSIS PLACED A SHARP RIDGE FROM THE GREAT LAKES DOWN TO THE GULF COAST. A CLOSED LOW WAS IN THE WEST-CENTRAL PLAINS. A 300 MB JET WAS FROM WEST TEXAS UP TO THE NORTHERN PLAINS THEN ACROSS SOUTHERN CANADA AND BACK DOWN ACROSS THE NORTHEAST. AN AREA OF ENHANCED LOW-LEVEL MOISTURE PIVOTING SOUTHWESTWARD ON THE WESTERN SIDE OF THE PERSISTENT OCEAN STORM HAS BEEN DEEP ENOUGH UNDER AN INVERSION TO ALLOW THE STRATOCUMULUS TO EXPAND ACROSS THE EASTERN ZONES EARLY THIS MORNING. BASED ON FORECAST SOUNDINGS, THIS MOISTURE DOES NOT APPEAR ALL THAT DEEP, THEREFORE WE HAVE LESS CONFIDENCE ON HOW LONG IT PERSISTS OR EVEN EXPANDS. FOR NOW, WE LEANED MORE TOWARD IT MIXING OUT SOME BASED ON THE LACK OF VERTICAL DEPTH TO THE MOISTURE FIELDS. HOWEVER, WE SHOULD GET TO THE CONVECTIVE TEMPERATURES TODAY WHICH SHOULD PROMOTE AT LEAST SOME CUMULUS/STRATOCUMULUS DEVELOPMENT. AN UPPER-LEVEL RIDGE TO OUR WEST WILL SLOWLY BE PUSHED EASTWARD. SINCE THIS FEATURE STILL REMAINS TO OUR WEST, SURFACE HIGH PRESSURE IS MAINTAINED TO EXTEND DOWN INTO OUR AREA IN A WEAKENED STATE. TO THE WEST OF THE RIDGE, CONVECTIVE CLUSTERS CONTINUE AND THE CLOUD DEBRIS FROM THESE WILL RIDE OVER THE RIDGE AXIS TODAY, PRODUCING AREAS OF HIGH LEVEL CLOUDS. AS THE UPPER-LEVEL RIDGE SLOWLY MIGRATES EASTWARD, THE AIRMASS IS FORECAST TO MODIFY SOME THEREFORE AWAY FROM THE COAST WE SHOULD HAVE DAYTIME HIGHS A FEW DEGREES WARMER THAN YESTERDAY. AS A RESULT OF THE LOW-LEVEL FLOW, THE WARMEST TEMPERATURES SHOULD BE ALONG AND ESPECIALLY NORTHWEST OF I-95. THE OVERALL FLOW REMAINS ON THE WEAKER SIDE IN THE LOWER LEVELS, THEREFORE WITH INCREASING THERMAL DIFFERENCES FROM THE COAST TO INLAND, RIVER, BAY AND SEA BREEZE BOUNDARIES ARE EXPECTED TO DEVELOP THIS AFTERNOON. AS FOR TEMPERATURES, WE TOOK A GFS/NAM BLEND FOR MOST OF THE CWA BUT DID MAKE SOME TWEAKS CLOSER TO THE COAST BASED ON THE SEA BREEZE EXPECTED TO DEVELOP. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/... THE OMEGA BLOCK THAT HAS BEEN IN PLACE WILL REMAIN THROUGH ABOUT FRIDAY, HOWEVER UPSTREAM SHORT WAVE ENERGY WILL HELP DISLODGE THE NEARLY STAGNANT PATTERN. THIS WILL FORCE THE RATHER STRONG TROUGH IN THE WESTERN ATLANTIC EASTWARD AND WEAKEN. HOWEVER, THE FLOW REMAINS ON THE AMPLIFIED SIDE THEREFORE THE RIDGE WILL BE SLIDING TO OUR EAST WITH TIME AS A LARGE TROUGH MIGRATES WESTWARD SOME ACROSS NORTHEASTERN CANADA. THIS WILL FORCE SHORT WAVE ENERGY INTO THE OHIO VALLEY ALLOWING ANOTHER FAIRLY SUBSTANTIAL TROUGH TO BECOME ESTABLISHED, THEN MOVE EASTWARD THROUGH THE END OF THE PERIOD. GIVEN THE AMPLIFIED FLOW THOUGH STILL OCCURRING, THE SYSTEM SHOULD BE IN NO HURRY TO MOVE FROM WEST TO EAST. FOR TONIGHT, A TROUGH CONTINUES ACROSS THE CENTRAL PLAINS WITH DOWNSTREAM RIDGING GETTING CLOSER TO OUR AREA. THIS RIDGE AXIS IS MOSTLY STILL JUST TO OUR WEST, THEREFORE AT THE SURFACE A WEAK HIGH PRESSURE AREA REMAINS ELONGATED ALONG THE EAST COAST. ANY CUMULUS THAT DEVELOPS DURING THE DAY SHOULD FLATTEN AND DISSIPATE WITH THE LOST OF DAYTIME HEATING. HOWEVER, AS THE BOUNDARY LAYER COOLS AND MOISTURE INCREASES A BIT UNDER AN INVERSION, WE MAY HAVE THE DEVELOPMENT OF SOME LOWER CLOUDS AND PERHAPS PATCHY FOG. FOR NOW, WILL RESTRICT ANY PATCHY FOG MENTION TO THE FAR NORTHWESTERN ZONES. ANY RIVER, BAY AND SEA BREEZE BOUNDARIES WILL TEND TO MIX OUT AND DISSIPATE THROUGH THE EVENING AS THE BOUNDARY LAYER COOLS. SINCE WE ARE STILL JUST EAST OF THE UPPER-LEVEL RIDGE, SOME HIGH LEVEL CLOUDS FROM UPSTREAM CONVECTION WILL CONTINUE TO MOVE THROUGH IN THE NORTHWEST FLOW. FOR FRIDAY, PERHAPS PATCHY FOG AND/OR SOME LOW CLOUDS TO START OTHERWISE A STORM SYSTEM MIGRATES TOWARD THE OHIO VALLEY WHICH SLOWLY BRINGS THE UPPER-LEVEL RIDGE AXIS OVER OUR CWA. AS THE RIDGE BECOMES ORIENTATED A BIT DIFFERENTLY AND LEANS MORE TOWARD THE NORTHEAST, A WEAKNESS LOOKS TO DEVELOP ON ITS NORTHWEST SIDE AS AN UPPER-LEVEL TROUGH EXTENDS BACK ACROSS EASTERN CANADA. AT THE SURFACE, THIS IS IN THE FORM OF A FRONTAL BOUNDARY THAT WILL TRY AND DROP SOUTHWARD. DURING THE DAY THOUGH, THIS FEATURE SHOULD STAY TO OUR NORTH DUE TO THE SHARPNESS OF THE UPPER-LEVEL RIDGE. AGAIN, SOME MODIFICATION OF THE AIRMASS ALOFT THEREFORE PERHAPS A DEGREE OR TWO WARMER FOR HIGHS AWAY FROM THE COAST. A POSSIBLE FLY IN THE OINTMENT IS SOME INCREASE IN CLOUDS/MOISTURE UNDER A SOUTHERLY FLOW. THE COMBINATION OF INCREASING MOISTURE SHOULD ALLOW FOR MORE CLOUDS TO DEVELOP AND ALSO THERE COULD BE SOME CONVECTIVE CLOUD DEBRIS STILL OVERTOPPING THE RIDGE. AS THE ATMOSPHERIC COLUMN MOISTENS UP FRIDAY NIGHT SOME, A SOUTHERLY WIND MAY BE JUST ENOUGH IN CONJUNCTION WITH BOUNDARY LAYER COOLING AND SOME WAA ALOFT ARRIVING FROM THE WEST TO HELP DEVELOP SOME LOW CLOUDS AND FOG. FOR NOW, WE WILL JUST CARRY PATCHY FOG ACROSS THE CWA AS THERE IS SOME UNCERTAINTY WHETHER FOG FORMS OR MOSTLY SOME STRATUS DEVELOPS. THE INCOMING WAA AND ISENTROPIC LIFT MAY BE ENOUGH TO PRODUCE A FEW SHOWERS THAT ARRIVE FROM THE WEST. WE WILL MAINTAIN SOME LOW POPS FROM WEST TO EAST MAINLY AT NIGHT. FOR SATURDAY, THE UPPER-LEVEL RIDGE IS FORECAST TO SLIDE TO OUR EAST AS THE UPPER-LEVEL FLOW MOVES THE SYSTEMS ALONG A BIT MORE. THE STORM IN THE OHIO VALLEY WILL CONTINUE TO SLOWLY MOVE EASTWARD. AS THE MOISTURE DEEPENS ACROSS THE REGION, SOME SHOWERS WILL BE POSSIBLE ESPECIALLY IN THE AFTERNOON AND EVENING. THE INSTABILITY DOES NOT LOOK ALL THAT IMPRESSIVE, BUT WILL CONTINUE WITH A SLIGHT CHC OF THUNDER IN THE GRIDDED DATABASE FOR AREAS AWAY FROM THE COAST. THERE IS SOME UNCERTAINTY ON THE TRUE TIMING OF THE SYSTEM TO OUR WEST, HOWEVER BASED ON THE CLOSE PROXIMITY OF THE DEPARTING UPPER-LEVEL RIDGE, THERE MAY NOT BE MUCH CONVECTION ACROSS OUR AREA. AS FOR TEMPERATURES, WE GENERALLY SIDED WITH AN EVEN BLEND OF THE GFS/NAM MOS BUT DID MAKE SOME TWEAKS ESPECIALLY CLOSER TO THE COAST WITH SOME MARINE INFLUENCE ANTICIPATED. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... A CHANGE IN THE OVERALL PATTERN IS EXPECTED DURING THIS TIME FRAME AS THE PRIOR OMEGA BLOCK BREAKS DOWN, HOWEVER THE FLOW REMAINS RATHER AMPLIFIED. THIS ALLOWS A POTENTIALLY STRONG UPPER-LEVEL TROUGH TO GRADUALLY SLIDE INTO THE EAST WITH TIME. HOWEVER BASED ON THE AMPLIFIED FLOW, THE SYSTEMS INVOLVED MAY NOT MOVE ALL THAT QUICKLY. THIS GENERALLY SPELLS FOR UNSETTLED CONDITIONS DURING THIS TIME FRAME, ALTHOUGH THERE IS SOME UNCERTAINTY REGARDING THE TIMING AND AMPLIFICATION OF THE INCOMING SYSTEM/S/. THERE MAY BE SEVERAL LOW PRESSURE SYSTEMS SLIDING EAST OR NORTHEAST WITH THE INCOMING UPPER-LEVEL THROUGH, AND THESE FEATURES ARE TYPICALLY MORE DIFFICULT TO TIME THIS FAR OUT. A BACKDOOR TYPE FRONT SHOULD BE SITTING NOT TO FAR TO OUR SOUTH DURING THE START OF THIS TIME FRAME. THIS MAY TEND TO WORK NORTHWARD SOME AS A WARM FRONT SUNDAY AND MONDAY AS INCREASING WAA AND ISENTROPIC LIFT OCCURS. THE POSITIONING OF THIS FRONT WILL LIKELY DEPEND ON WHERE THE SURFACE LOW OR SERIES OF SURFACE LOWS TRACK. IN ADDITION, THE SHARPNESS OF THE UPPER-LEVEL TROUGH AND IF IT CLOSES OFF FAIRLY QUICKLY WILL IMPACT THE TRACK OF THE SURFACE FEATURES. IT APPEARS THERE IS SOME BUILDING MODEL CONSENSUS FOR A BIT MORE WIDESPREAD SHOWERS AND STORMS ON SUNDAY AS AN INITIAL IMPULSE MOVES THROUGH. ALL IN ALL, A STRONG SOUTHERLY FLOW SHOULD BE ABLE TO TRANSPORT INCREASINGLY MOIST AIR ACROSS THE ENTIRE REGION DURING THIS TIME FRAME /PW VALUES NEAR 1.5 INCHES/. THIS IN COMBINATION WITH A SERIES OF SURFACE LOWS AND SHORT WAVES WILL PROVIDE LIFT TO GENERATE NUMEROUS SHOWERS AT TIMES. AS COOLING OCCURS ESPECIALLY ALOFT, WE SHOULD BE ABLE TO GENERATE SOME INSTABILITY SO WE WILL MAINTAIN A CHC OF THUNDER. THE MORE WIDESPREAD CONVECTION MAY BE TIED TO THE DAYTIME HEATING CYCLE. BASED ON THE ADVERTISED SYNOPTIC FLOW /MORE DEEP SOUTHERLY/, CONVECTIVE ELEMENTS MAY BE SLOW MOVING OR TEND TO REDEVELOP IN THE GENERAL SAME AREAS /BACKBUILDING?/. THIS MAY LEAD TO A LOCALLY HEAVY RAIN THREAT, HOWEVER THE SPECIFICS ARE NOT ALL THAT CLEAR ATTM. WE WILL CONTINUE TO MENTION THIS POTENTIAL IN THE HAZARDOUS WEATHER OUTLOOK. REGARDING POPS, WITH LOWER CONFIDENCE IN THE TIMING OF THE SURFACE LOWS AND SHORT WAVES AND THEIR ASSOCIATED LIFT, WE WILL CONTINUE TO BROAD BRUSH CHC POPS ACROSS THE AREA FOR MOST OF THIS TIME FRAME. INITIALLY, THE MORE FOCUSED ACTIVITY MAY BE INLAND AS THE FLOW COULD BE MORE SOUTHEASTERLY HELPING TO ENHANCED LIFT, BEFORE EVERYTHING GRADUALLY SHIFTS EASTWARD. OVERALL, WE DID NOT MAKE SIGNIFICANT CHANGES ALTHOUGH WE DID BLEND IN A SMALL PERCENTAGE OF THE 00Z MOSGUIDE TO INTRODUCE AT LEAST SOME NEW GUIDANCE INTO THE OFFICIAL FORECAST. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. GFS/ECMWF COMBINATION WAS FAVORED FOR THE RH SECTIONS WITH LESS CONSIDERATION GIVEN TO THE 00Z/12 NAM IN THIS FCST. AFTER 11Z...VFR WITH CIGS NEAR 5-6000 FT THIS MORNING DISSIPATING SLOWLY DURING MID AFTERNOON. LIGHT SW WIND BECOMING AFTERNOON SEA BREEZES ALONG ALL WATER LOCATIONS...IE WIND EVENTUALLY BECOMING SSE DURING THE AFTN. KPHL/KPNE/KILG SHOULD HAVE MOSTLY SW WIND THRU 18Z BEFORE THE WIND BACKS TO S OR SSE SOMETIME THIS AFTERNOON. TONIGHT...VFR TO START. MVFR CONDS IN SC/FOG SHOULD DEVELOP VCNTY KABE AND KRDG NEAR OR AFTER 06Z. VSBY 2-5MI IN FOG MAY END UP A BIT MORE EXTENSIVE THAN CURRENTLY FCST IN THE UPCOMING 12Z TAFS. DEPENDS ON WHETHER A DECK OF CLOUDS DEVELOPS BETWEEN 5000 AND 15000 FT TOWARD MORNING. OUTLOOK... FRIDAY...VFR WITH CIGS DEVELOPING BY MIDDAY AOA 4000 FT. LIGHT S OR SSE WIND. WEEKEND...WIDESPREAD MVFR CIGS AND OCNL IFR CONDS IN SHOWERS. EMBEDDED ISOLATED TSTMS CONFINED MAINLY TO THE SUNDAY PORTION OF THE WEEKEND. LOWEST CIGS/VSBY EXPECTED SAT NIGHT AND SUNDAY MORNING AHEAD OF THE WARM FRONT. E-SE WIND UNDER 20 KTS. CONFIDENCE: ABOVE AVG. MONDAY...CONDS MAY IMPROVE TO VFR CIGS DURING THE DAY THEN DETERIORATE AT NIGHT TO MVFR CONDS OR LOWER IN FOG AS BRIEFLY WSW FLOW DURING THE DAY BACKS TO SLY AT NIGHT? CONFIDENCE LESS THAN AVG. TUESDAY...VARIABLE CIGS BUT PROBABLY MOSTLY MVFR OR IFR IN SHOWERS/ISO TSTMS. E-SE FLOW. CONFIDENCE AVG. && .MARINE... HAZARDOUS SEAS SCA CONTINUES ON THE ATLC WATERS AND EXTENDED IN THIS 340 AM FCST INTO MIDDAY FRIDAY FOR MOST OF THE WATERS. THE PROBLEM: 4-5 FT E SWELL AT 44025 AND 44065 WITH A 6-7 FT NE SWELL AT 44009 MAKING IT MORE LIKELY THAT THE DEL AND S NJ COASTAL ZONES WILL REMAIN IN THE HAZARDOUS SEAS ADVY REGION THRU AT LEAST MIDDAY FRIDAY. IT MAY NOT BE CORRECT TO EXTEND ADVY ANY FURTHER IN TIME THAN IT IS NOW IN THE MONMOUTH CTY COASTAL ZONE /10Z FRIDAY/. SOME MODEL GUIDANCE CONTS 44009 LINKED SCA HAZARDOUS SEAS 5 FT OR HIGHER AT LEAST INTO THE 16TH OF MAY WHILE 44025 IS MORE OF A MARGINAL 1 FT EITHER SIDE OF 5 FT FOR THE NEXT COUPLE OF DAYS. NO ADVISORY IS NEEDED FOR THE DELAWARE BAY AT THIS TIME AS WINDS ARE EXPECTED TO REMAIN BELOW CRITERIA LEVELS THROUGH SATURDAY MORNING. THIS WEEKEND...SE WINDS INCREASE LATE SATURDAY AND BECOME MORE SLY SOMETIME SUNDAY OCNL NEAR 20 KNOTS. LIKELY THAT A HAZ SEAS ADVY WILL BE NEEDED. FOG OVER THE WATER MIGHT BECOME A MARINE HAZARD. MONDAY...VARIABLE MAINLY S WINDS. LIKELY THAT A HAZ SEAS ADVY WILL BE NEEDED. TUESDAY...ANOTHER PULSE OF SE FLOW OF 20 TO 25 KTS POSSIBLE. LIKELY THAT AN SCA WILL BE NEEDED. && .TIDES/COASTAL FLOODING... SO FAR THE ONLY MINOR INUNDATION TIDAL FLOODING THAT OCCURRED AT MONDAY IN OUR AREA OF RESPONSIBILITY WAS CAMBRIDGE MD...3/4 OF AN INCH ABV FS AT 1030AM WEDNESDAY. MANY MORE OPPORTUNITIES FOR AT LEAST MINOR CF THROUGH THIS COMING WEDNESDAY. BEST CHC FOR EXTENSIVE ADVY THRESHOLD EXCEEDENCE IS SUNDAY EVENING AND NEXT TUESDAY-WEDNESDAY. THERE IS A LOW PROBABILITY OF EXTENSIVE MODERATE COASTAL FLOODING OCCURRING ON THE TUESDAY EVENING THE 17TH HIGH TIDE CYCLE IF THE 00Z/12 ECMWF FCST FOR 00Z/18TH IS ONLY MARGINALLY IN ERROR. PERSISTENT SLY TO SE INFLOW OVER THE NEXT 5 DAYS BUILDS THE SEAS AND WATER LEVELS. IN THE MEANTIME...POCKETS OF MARGINAL MINOR MIGHT OCCUR LATER TODAY /CHES BAY/ OR THIS COMING THURSDAY NIGHT IN UPPER DEL BAY AND CHES BAY BUT NO STMT AT THIS TIME DUE TO MARGINALITY. FULL MOON THIS COMING TUESDAY THE 17TH SO ASTRO TIDES ARE INCREASING DAILY. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 2 PM EDT FRIDAY FOR ANZ451>455. SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 6 AM EDT FRIDAY FOR ANZ450. && $$ SYNOPSIS...GORSE