Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 251910 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 310 PM EDT TUE AUG 25 2015 .SYNOPSIS... A COLD FRONT WILL BRING SCATTERED SHOWERS AND THUNDERSTORMS THIS AFTERNOON...THEN CROSSES SOUTHERN NEW ENGLAND TONIGHT. HIGH PRESSURE SLOWLY BUILDS IN FOR MID AND LATE WEEK WITH MAINLY DRY AND SEASONABLE WEATHER AND COMFORTABLE HUMIDITY LEVELS EXPECTED WEDNESDAY INTO SATURDAY. WARMER TEMPERATURES AND HIGHER HUMIDITIES RETURN FOR THE BEGINNING OF NEXT WEEK. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... THROUGH 7 PM... COLD FRONT IN THE HUDSON VALLEY WITH WARM AND VERY HUMID AIR OVER SOUTHERN NEW ENGLAND. OBSERVATIONS SHOW A LIGHT WIND FIELD...LIGHT ENOUGH FOR WINDS TO TURN EAST ALONG THE MASS EAST COAST NORTH OF CAPE COD BAY. THIS IN TURN HAS BROUGHT FOG ASHORE ALONG THIS STRETCH OF COASTLINE. WITH BREAKING CLOUDS ACROSS MUCH OF THE REST OF SOUTHERN NEW ENGLAND...SOLAR HEATING HAS STARTED TO DESTABILIZE THE AIR MASS WITH SBCAPE 1500-2000 J/KG AND MIXED LAYER CAPE 1000-1500 J/KG. LAPSE RATES ABOVE THE SURFACE HAVE MEDIOCRE THROUGH EARLY AFTERNOON BUT LOW LEVEL RATES HAVE JUST REACHED 7C/KM AND FURTHER HEATING MAY HELP BUILD THESE RATES FURTHER. WINDS ABOVE THE SURFACE HAVE REACHED 20-25 KNOTS AT 850 MB AND 35-40 KNOTS AT 500 MB. THE 500 MB WINDS MAY INCREASE FURTHER THROUGH THIS EVENING. SCATTERED SHOWERS/STORMS HAVE FIRED IN THE CT VALLEY AND MOVED NORTHEAST. THIS WILL CONTINUE THROUGH THE AFTERNOON WITH SOME AREAL EXPANSION POSSIBLE. THE THUNDERSTORMS MAY BE ABLE TO TAP SOME OF THE WIND ALOFT AND BRING STRONG WIND GUSTS TO THE SURFACE. THIS MAY CAUSE ISOLATED DAMAGE. PRECIPITABLE WATER VALUES ARE AROUND 1.5 INCHES...HIGHEST IN EASTERN MASS. THIS IS NOT TREMENDOUSLY ABOVE NORMAL BUT MAY HELP SUPPORT SOME LOCALLY HEAVY DOWNPOURS. TONIGHT... COLD FRONT SLOWLY MOVES ACROSS SOUTHERN NEW ENGLAND...REACHING THE EAST MASS COAST TOWARD MORNING. CHANCE OF SHOWERS WILL LINGER UNTIL IT MOVES THROUGH...BUT INCOMING MID LEVEL DRY AIR WILL LIMIT THE ABILITY OF ANY SHOWER. DEW POINTS WILL REMAIN HIGH MUCH OF THE NIGHT...BUT BEGIN TO LOWER OVERNIGHT AFTER THE COLD FRONT MOVES THROUGH. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... UPPER LOW LINGERS OVER QUEBEC WITH CYCLONIC FLOW AND COLD ADVECTION ALOFT OVER NEW ENGLAND. SURFACE HIGH PRESSURE SUPPLIES SUBSIDENCE NEAR GROUND. PLENTY OF DRY AIR NEAR THE SURFACE AND A MOIST LAYER ABOVE 850 MB. EXPECT THERE WILL BE SOME CLOUDS WEDNESDAY...BUT OTHERWISE A FAIR DAY WITH MIXING REACHING UP TO 825 MB AND POSSIBLY A LITTLE HIGHER. TEMPS AT THAT LEVEL WILL SUPPORT MAX SFC TEMPS IN THE LOW TO MID 80S. DEW POINTS WILL TREND LOWER /MORE COMFORTABLE/ WITH AFTERNOON VALUES IN THE MID 50S TO AROUND 60. FAIR WEATHER CONTINUES WEDNESDAY NIGHT WITH A LIGHT NORTHWEST WIND. MIN TEMPS WILL REACH NEAR DEW POINT MOST PLACES WITH VALUES IN THE MID 50S TO LOW 60S...A LITTLE HIGHER IN BOSTON AND ALONG THE CAPE AND ISLANDS. && .LONG TERM /THURSDAY THROUGH TUESDAY/... OVERVIEW... MEDIUM RANGE MODELS AND ENSEMBLES REMAIN IN VERY GOOD AGREEMENT DURING THE THU-SAT TIMEFRAME...WEAKENING THE EASTERN LONG WAVE TROUGHING AS CUTOFF LOW LIFTS NE OVER EASTERN CANADA. UPPER FLOW BECOMES MORE ZONAL ACROSS THE EASTERN SEABOARD BY LATE THIS WEEK. LARGE SURFACE HIGH PRESSURE WILL BE THE DOMINANT FEATURE ACROSS THE EASTERN HALF OF THE U.S. INTO THIS WEEKEND. DO NOTE DIVERGENCE IN SOME OF THE MODEL SOLUTIONS DURING THIS WEEKEND...NAMELY THE 06Z/12Z GFS...IN HANDLING LINGERING H5 SHORT WAVE WEAKNESS. MOST MODELS AND ENSEMBLES KEEP THIS HANGING AROUND THE GREAT LAKES INTO OHIO VALLEY...BUT THE GFS SHIFTS IT E MUCH FASTER AND OVERDOES THE PRECIP FIELD. ON SATURDAY...IT TRIES TO WORK MOISTURE UP THE EASTERN SEABOARD AROUND THE SUBTROPICAL RIDGE OFFSHORE...THEN SHIFTS SHORT WAVE ENERGY E AROUND THE SUNDAY/SUNDAY NIGHT TIMEFRAME. THE 00Z ECMWF/GGEM KEEP THIS WEAKNESS WELL W OF THE REGION AND NOT NEARLY AS MUCH CONVECTION WITH IT AS THE HIGH PRES OFFSHORE REMAINS IN CONTROL. GEFS/ECENS MEANS ALSO KEEP A MAINLY DRY FORECAST GOING...SO LEANED TOWARD THE DRIER SOLUTION DURING THIS TIMEFRAME. INTO EARLY NEXT WEEK...LOOKS LIKE HIGH PRES RETURNS AS W-NW FLOW ALOFT TAKES OVER WITH HIGH AMPLITUDE UPPER RIDGE BUILDING INTO HUDSON AND JAMES BAYS. SW SURFACE WINDS TAKE OVER...WHICH BRING WARMER TEMPS AND HIGHER HUMIDITY. LEANED TOWARD AVAILABLE GUIDANCE FOR THE EARLY PORTION OF THIS FORECAST...THEN TRANSITIONED TOWARD THE ENSEMBLE MEANS FROM SUNDAY ONWARD. DETAILS... THURSDAY THROUGH SATURDAY...HIGH CONFIDENCE. LARGE HIGH PRES RIDGE BUILDS E OUT OF THE MIDWEST/GREAT LAKES. EXPECT DRY CONDITIONS. A GENERAL W-NW FLOW IN PLACE FOR THURSDAY-FRIDAY...THEN RIDGE AXIS SHIFTS E ON SATURDAY BRINGING RETURN TO SW WINDS AND GRADUALLY INCREASING HUMIDITY. SUNDAY...LOW CONFIDENCE. AS MENTIONED ABOVE...MODEL SOLUTION DIVERGENCE DUE TO TIMING OF SYNOPTIC FEATURES ACROSS THE REGION. LEANED TOWARD THE HPC GUIDANCE FOR THIS PERIOD. KEPT A DRY FORECAST GOING. WILL ALSO SEE TEMPS RISE...WITH HIGHS POSSIBLY AS MUCH AS 5 TO 10 DEGREES ABOVE NORMAL. MONDAY AND TUESDAY...LOW TO MODERATE CONFIDENCE. UPPER LEVEL RIDGE BUILDS ACROSS THE CENTRAL U.S. INTO CENTRAL CANADA...KEEPING MAINLY DRY FLOW IN PLACE. SOME UNCERTAINTY DUE TO DIVERGENCE WITH EARLIER SOLUTIONS...THOUGH LOOKING A BIT MORE CONFIDENT. TEMPERATURES WILL CONTINUE TO TREND ABOVE NORMAL DURING THIS TIMEFRAME...WHICH IS BACK TO SCHOOL FOR SOME. BOTH MODELS AND ENSEMBLES ARE HINTING AT UPPER 80S TO AROUND 90 AS UPPER LEVEL RIDGE CONTINUES TO BUILD ACROSS THE CENTRAL U.S. && .AVIATION /19Z TUESDAY THROUGH SUNDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH WEDNESDAY NIGHT/... THIS AFTERNOON...MODERATE CONFIDENCE. AREAS OF IFR FOG ALONG THE MASS EAST COAST UNTIL WINDS BECOME SOUTH. OTHERWISE VFR WITH PATCHES OF MVFR CIGS IN SCATTERED SHOWERS/TSTMS...MOSTLY IN CENTRAL MASS AND NORTHEAST CT. TONIGHT...MODERATE CONFIDENCE. LINGERING SHOWERS/TSTMS EARLY WITH BRIEF MVFR/IFR POSSIBLE. PATCHY FOR DEVELOPING OVERNIGHT WITH MVFR AND SOME IFR. WIND SHIFT WITH COLD FRONT PASSAGE...WINDS BECOMING WEST BY MORNING MOST AREAS. WEDNESDAY AND WEDNESDAY NIGHT...HIGH CONFIDENCE. VFR. PATCHY IFR IN FOG LATE WEDNESDAY NIGHT. KBOS TERMINAL...MODERATE CONFIDENCE IN TAF. DIFFICULT FORECAST FOR THE FIRST 3-4 HOURS. WINDS SHIFTED EAST JUST BEFORE 18Z AND DREW FOG ONSHORE WITH VSBYS 1/2 AT TIMES. THIS EAST WIND WILL CONTINUE FOR A COUPLE OF HOURS AND SHOULD KEEP THREATENING FOG DURING THIS TIME. THIS SHOULD END FOR GOOD AS WINDS BECOME SOUTH AHEAD OF SCATTERED TSTMS APPROACHING FROM THE WEST. WIND SHIFT TO WEST LATER TONIGHT AS A COLD FRONT MOVES THROUGH. KBDL TERMINAL...MODERATE CONFIDENCE IN TAF. OUTLOOK /THURSDAY THROUGH SUNDAY/...MODERATE TO HIGH CONFIDENCE. THURSDAY THROUGH SUNDAY...MAINLY VFR CONDITIONS. COULD SEE WEAK SEA BREEZES ALONG THE COASTLINES. MAY SEE BRIEF PATCHY FOG OVERNIGHT THU NIGHT ACROSS NORMALLY PRONE INTERIOR VALLEYS. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH WEDNESDAY NIGHT/...HIGH CONFIDENCE. WINDS AND SEAS WILL REMAIN BELOW 25 KNOTS AND 5 FEET THROUGH THE PERIOD. A COLD FRONT MOVES ACROSS SOUTHERN NEW ENGLAND TONIGHT AND MAY BRING A FEW SHOWERS TO THE WATERS...POSSIBLY A TSTM. AREAS OF FOG ON THE EASTERN MASS WATERS WILL DIMINISH WITH THE WIND SHIFT. ADDITIONAL FOG WILL AFFECT THE WATERS IN THE LIGHT FLOW TONIGHT. OUTLOOK /THURSDAY THROUGH SUNDAY/...HIGH CONFIDENCE. EXPECT WINDS AND SEAS TO REMAIN BELOW SMALL CRAFT CRITERIA AS HIGH PRESSURE TAKES CONTROL. EXPECT W-NW WINDS THU-FRI...SHIFTING TO S-SW FRI NIGHT-SUNDAY. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/EVT MARINE...WTB/EVT

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