Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 040239 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 1039 PM EDT FRI JUL 3 2015 .SYNOPSIS... HIGH PRESSURE OVER SOUTHERN NEW ENGLAND WILL SHIFT OFFSHORE OVERNIGHT. LOW PRESSURE RACES SOUTH OF NEW ENGLAND ON JULY 4TH AND MAY BRINGING SOME LIGHT RAIN TO SOUTH COASTAL AREAS. HIGH PRESSURE RETURNS SUNDAY AND LINGERS THROUGH MONDAY. A WARM FRONT ON TUESDAY AND COLD FRONT ON WEDNESDAY WILL BRING A RISK OF SCATTERED SHOWERS AND THUNDERSTORMS. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 1039 PM UPDATE... HIGH PRESSURE SHIFTS EAST AND SATELLITE IMAGERY SHOWS HIGH CLOUDS SPILLING OVER SHORTWAVE RIDGE. WILL REMAIN DRY OVERNIGHT...WITH INCREASING CLOUDS ABOVE 10 THOUSAND FEET IN ADVANCE OF AN APPROACHING TROF. WITH THE OVERALL FORECAST STILL ON TRACK...ONLY MADE MINOR TWEAKS TO BRING FORECAST BACK IN LINE WITH OBSERVED TRENDS. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... INDEPENDENCE DAY... LOW PRESSURE ALONG THE MID ATLANTIC COAST RACES SOUTH OF OUR AREA ON SATURDAY. THE LOW WILL BE SUPPORTED BY AN UPPER SHORTWAVE HAVE HAD A CHANCE TO TAKE A LOOK AT THE 18Z GFS RUN AND IT SUPPRESSES THE RAIN A LITTLE FURTHER SOUTH AND MORE IN LINE WITH OTHER MODELS. THE CURRENT FORECAST OF POPS STILL LOOKS REASONABLE. HOWEVER...AT THIS TIME AND HAVE NOT MODIFIED FROM PRIOR FORECAST. PREVIOUS DISCUSSION... MODEL FORECASTS HAVE BEEN BOUNCING AROUND THE PAST 24 HOURS REGARDING THE HANDLING OF COASTAL LOW THAT PASSES SOUTH OF NEW ENGLAND SATURDAY. GFS IS MOST AGGRESSIVE IN BRINGING PCPN ACROSS ALL OF SRN NEW ENGLAND WHILE THE NAM AND GGEM CLIP CAPE COD AND ISLANDS. THE ECMWF COMPROMISES WITH PCPN ACROSS RI AND SE MASS. WE FAVORED THE ECMWF PLACEMENT IN SETTING UP OUR POPS...LIKELY POPS ON THE CAPE/ISLANDS/SOUTH COAST...CHANCE POPS IN SE MASS/RI/NE CT...SLIGHT CHANCE NE MASS AND NCENTRAL MASS. CONVECTION FROM NEW YORK MAY CROSS INTO WESTERN MASS DURING THE AFTERNOON SO WE HAVE CHANCE POPS IN WESTERN MASS. SATURDAY NIGHT... THE COASTAL LOW CROSSES 40N/70W IN THE EVENING AND MOVES SOUTH OF NOVA SCOTIA OVERNIGHT. THIS SHOULD BRING ANY PCPN TO AN END DURING THE EARLY PART OF THE NIGHT. THE SUPPORTING UPPER TROUGH MOVES EAST OF US BY MORNING. THIS SHOULD BRING A CLEARING TREND AFTER MIDNIGHT WITH WINDS TURNING NORTHWEST. && .LONG TERM /SUNDAY THROUGH FRIDAY/... HIGHLIGHTS... * HIGH PRESSURE SHOULD KEEP AREA DRY WITH SEASONABLE TEMPERATURES THROUGH MONDAY. * UNSETTLED CONDITIONS RETURN TUESDAY AND WEDNESDAY...POSSIBLY LINGERING INTO THE LATTER PART OF NEXT WEEK. OVERVIEW...AS ONE WOULD EXPECT IN JULY...ANY ROBUST SHORT WAVES PASS NORTH OF REGION WITH HEIGHTS RELATIVELY HIGH ACROSS SOUTHERN NEW ENGLAND. THE FLOW LOOKS TO BE RELATIVELY ZONAL BUT WITH A TENDENCY FOR WEAK TROUGHING OVER THE EASTERN CONUS. MAIN ISSUES INCLUDE TIMING OF SHORTWAVES...OFTEN MORE PROBLEMATIC IN MORE ZONAL FLOW...AND WHETHER OR NOT WEAK SURFACE WAVES MAY FORM ALONG PASSING COLD FRONTS. DETAILS... SUNDAY THROUGH MONDAY...MODELS ARE IN REASONABLE AGREEMENT FOR A SHORT WAVE RIDGE ALOFT AND WEAK SURFACE HIGH KEEPING THE AREA DRY THROUGH THIS PERIOD WITH SEASONABLE TEMPERATURES. TUESDAY...WARM FRONT WITH POSSIBLY MODEST INSTABILITY BURST ALOFT PER K INDEX SIGNATURE MOVES THROUGH REGION. AIR MASS MOISTENS WITH PWATS INCREASING TO NEAR 2 INCHES ON THE GFS. THINKING CHANCE POPS NW TWO THIRDS OF AREA WHERE A HINT OF A LITTLE BETTER INSTABILITY AND SLIGHT CHANCE MOST OF SE THIRD. BELIEVE LESS CHANCE OF ANY SHOWERS/TSTMS BY LATE AFTERNOON EXCEPT MAY SEE A FEW AIR MASS TSTMS LATE AFTERNOON/EARLY EVENING IN NW MA. WEDNESDAY...UNCERTAINTY IN TIMING OF SHORT WAVE TROF AND ASSOCIATED WEAK COLD FRONT IN TURN LEADS TO SOME ASSOCIATED UNCERTAINTY IN SHOWER/TSTM CHANCES FOR WED. EVEN THE FASTER GFS...HOWEVER...DEPICTS SOME LINGERING SURFACE CONVERGENCE AND RELATIVELY WEAK INSTABILITY THROUGH THE AFTERNOON. THUS...HAVE CHANCE POPS MOST AREAS ON WED. MAY SEE CLUSTERS OF TSTMS THAT MOVE WEST TO EAST DURING THE AFTERNOON BUT RELATIVELY LOW SHEAR SUGGESTS FOR NOW THAT CONVECTION WILL NOT BE ALL THAT ORGANIZED. THURSDAY...A FAIR AMOUNT OF UNCERTAINTY EXISTS FOR THU GIVEN MODEL DIFFERENCES. ECMWF DEVELOPS WAVE ALONG THE FRONT AND HOLDS RAFL BACK ACROSS MUCH OF THE AREA WHEREAS THE GFS IS MUCH MORE PROGRESSIVE. HAVE OPTED TO GO WITH POPS IN THE UPPER END OF THE SLIGHT CHANCE CATEGORY FOR THU...REFLECTING THE RELATIVELY HIGH UNCERTAINTY. FRIDAY...HAVE GONE WITH CONTINUITY OF DRY WEATHER FOR FRI BUT CONFIDENCE IS LOW. THE LATEST RUN OF THE ECMWF AMPLIFIES A PASSING SHORT WAVE TROF AND DEVELOPS STILL ANOTHER SURFACE LOW PRESSURE CENTER ALONG THE FRONT. THIS WAS NOT REFLECTED IN THE 00Z VERSION. IF THE LATEST ECMWF RUN WERE TAKEN LITERALLY...IT WOULD HAVE A SOAKING RAIN SPREAD ACROSS SOUTHERN RI AND SE MA FOR THU NIGHT INTO FRI. THE OPERATIONAL GFS RUN IN CONTRAST KEEPS ANY FRONTAL WAVE ACTIVITY WELL SOUTH OF THE REGION WITH ONLY A FEW ENSEMBLE RUNS SUGGESTING ANYTHING CLOSE TO THE ECMWF SOLUTION. MAY NEED TO ADJUST THE FRI FORECAST AS WE DRAW CLOSER AND GET A BETTER HANDLE ON MODEL TRENDS. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT-TERM /THROUGH FRIDAY NIGHT/... TONIGHT...HIGH CONFIDENCE. VFR. WINDS GENERALLY LIGHT FROM S OVERNIGHT. INCREASING HIGH CLOUDS ABOVE 10 THOUSAND FEET. SATURDAY...MODERATE CONFIDENCE. VFR TO START. MVFR POSSIBLE IN SCATTERED AREAS OF LIGHT RAIN LATE IN THE DAY...BEST CHANCE IS AT TERMINALS ALONG THE SOUTH COAST. WE ARE MONITORING THE POSSIBILITY OF LOW CONDITIONS AT BOSTON TOWARD EVENING DEPENDING ON THE STRENGTH OF DEVELOPING EAST/NORTHEAST FLOW AT THAT TIME. SATURDAY NIGHT...MODERATE TO HIGH CONFIDENCE. RAIN OVER SOUTHEAST MASS/RI SHOULD TAPER OFF DURING THE FIRST PART OF THE NIGHT. MVFR CONDITIONS POSSIBLE IN ANY AREAS OF RAIN/FOG...OTHERWISE VFR WITH CLOUD BASES LIFTING/CLEARING OVERNIGHT. KBOS TERMINAL...HIGH CONFIDENCE IN TAF. EXPECT VFR THROUGH THE PERIOD. KBDL TERMINAL...HIGH CONFIDENCE IN TAF WITH VFR THROUGH THE PERIOD...EVEN IF A FEW SPRINKLES LATE SAT AFTERNOON. OUTLOOK /SUNDAY THROUGH WEDNESDAY/... SUNDAY THROUGH MONDAY...HIGH CONFIDENCE OF VFR CONDITIONS. TUESDAY THROUGH WEDNESDAY...MODERATE CONFIDENCE. ANTICIPATE MOSTLY VFR CONDITIONS BUT WITH SCATTERED MVFR CIGS/VSBYS IN SCATTERED SHOWERS AND TSTMS. POCKETS OF LATE NIGHT/EARLY MORNING IFR CIGS/FOG ALONG THE SOUTH COAST. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. HIGH CONFIDENCE TONIGHT...MODERATE CONFIDENCE SATURDAY AND SATURDAY NIGHT. TONIGHT... HIGH PRESSURE OVER THE REGION MOVES EAST OF THE WATERS BY SATURDAY MORNING. EXPECT LIGHT WINDS AND DIMINISHING SEAS. SATURDAY... LOW PRESSURE DEVELOPING ALONG THE MID ATLANTIC COAST WILL PASS OVER OR SOUTH OF THE SOUTHERN OUTER WATERS SATURDAY AFTERNOON AND EVENING. THIS WILL BRING LOWER VSBYS IN RAIN TO THE SOUTHERN WATERS WITH EAST WINDS INCREASING TO 15-20 KNOTS. FARTHER NORTH THE WINDS WILL BECOME EAST BUT AT LIGHTER SPEEDS. SATURDAY NIGHT...PERSISTENT EAST TO NORTHEAST WINDS WILL BUILD SEAS TO 5 FEET ON THE SOUTHERN OUTER WATERS. THIS MAY EVENTUALLY REQUIRE A SMALL CRAFT ADVISORY ON THOSE WATERS. AS THE LOW PRESSURE AREA MOVES OFF TO THE EAST THE WINDS WILL DIMINISH AND TURN NORTH OVERNIGHT. OUTLOOK /SUNDAY THROUGH WEDNESDAY/... MODERATE TO HIGH CONFIDENCE. SUBSIDING SWELL SUN MORNING...PROBABLY JUST UNDER 5 FEET...OVER THE OUTER S AND SE COASTAL WATERS. OTHERWISE...ANTICIPATE WINDS AND SEAS BELOW SMALL CRAFT ADVISORY THRESHOLDS THROUGH THE PERIOD. AREAS OF VISIBILITIES BELOW 3 MILES IN FOG POSSIBLE TUESDAY AND WEDNESDAY NIGHTS. SCATTERED TSTMS ARE POSSIBLE OVER THE WATERS WED AFTERNOON AND EVENING. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...WTB/THOMPSON NEAR TERM...WTB/BELK/DOODY/THOMPSON SHORT TERM...WTB LONG TERM...THOMPSON AVIATION...WTB/BELK/THOMPSON MARINE...WTB/BELK/THOMPSON

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