Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 251100 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 700 AM EDT FRI JUL 25 2014 .SYNOPSIS... HIGH PRESSURE CENTERED OVER THE OHIO VALLEY TODAY WILL PASS TO THE SOUTH OF NEW ENGLAND SATURDAY. A WARM FRONT WILL MOVE ACROSS THE REGION SUNDAY BRINGING WARM AND HUMID AIR. A COLD FRONT THEN SWINGS ACROSS THE REGION MONDAY...THEN LINGERS ALONG THE MASSACHUSETTS COAST TUESDAY AND WEDNESDAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM UPDATE... BAND OF CIRRUS MOVING ACROSS CAPE COD AND ISLANDS WILL MOVE OFF TO THE EAST THIS MORNING. CROSS SECTIONS FROM 06Z MODELS SHOW A LAYER OF 70+% RELATIVE HUMIDITY BETWEEN 750 AND 850 MB...WHICH WOULD BE TAPPED BY EXPECTED MIXING TODAY. THE UPDATE ADDS SOME DIURNAL CUMULUS...BUT MAINTAINS SKY COVER LESS THAN 20 PERCENT. TEMPS AT 800 MB 7-9C...WHICH SUPPORTS MAX SFC TEMPS 80-85F. SEA BREEZE WILL TAKE EFFECT AT THE COAST...KEEPING TEMPS COOLER THERE THIS AFTERNOON. PREVIOUS DISCUSSION... THICK CIRRUS FROM CAPE ANN TO THE EASTERN TIP OF LONG ISLAND WAS PROGRESSING STEADILY EASTWARD AND WILL EXIT THE COAST BY DAYBREAK. THEN... WOW WHAT A SPECTACULAR DAY TODAY. HIGH PRESSURE CENTERED OVER THE OHIO VALLEY WILL BE IN CONTROL OF OUR WEATHER. WE ARE FORECASTING BRIGHT SUNSHINE...WARM TEMPERATURES FROM 80-85...AND LOW HUMIDITY WITH DEWPOINTS MAINLY 50-55 EXCEPT UPPER 50S ON THE CAPE/ISLANDS. A WEAK PRESSURE GRADIENT WILL ALLOW FOR SEA BREEZES TO DEVELOP ON THE IMMEDIATE COAST...RESULTING IN HIGHS A FEW DEGREES COOLER IN THOSE LOCATIONS.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... TONIGHT...AS THE HIGH PASSES TO OUR SOUTH...SKIES WILL BE CLEAR WITH VERY LIGHT SOUTHWEST WINDS. OVERNIGHT LOWS MAINLY IN THE UPPER 50S TO LOWER 60S. SATURDAY...SOUTHERN NEW ENGLAND WILL BE IN A WARM SOUTHWESTERLY AIR FLOW BETWEEN HIGH PRESSURE OVER THE OFFSHORE WATERS AND LOW PRESSURE ADVANCING THROUGH THE GREAT LAKES. SKIES WILL BE SUNNY TO START BUT WITH ZONAL FLOW ALOFT...SOME HIGH LEVEL CLOUDINESS WILL MOVE INTO THE REGION IN THE AFTERNOON. DEWPOINTS WILL STILL BE IN THE COMFORTABLE RANGE...CLOSE TO 60. HIGHS WILL BE IN THE LOWER TO MID 80S. LATE IN THE AFTERNOON...A WEAK LEE-SIDE TROUGH WILL BE APPROACHING THE REGION FROM THE SOUTHWEST AND THIS COULD BE ENOUGH TO TRIGGER A WIDELY SCATTERED SHOWER OR THUNDERSTORM IN WESTERN MA AND WESTERN CT. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... BIG PICTURE... MUCH OF THE UPPER FLOW REMAINS UNCHANGED THROUGH THE FORECAST PERIOD. DEEP RIDGES WILL EXTEND ACROSS WESTERN NORTH AMERICA AND THE WESTERN ATLANTIC. CLOSED LOW OVER NORTHERN QUEBEC ANCHORS A TROUGH OVER EASTERN NORTH AMERICA. SHORTWAVE EJECTING FROM THE PACIFIC NORTHWEST WILL PLOW OVER THE TOP OF THE WESTERN RIDGE...THEN MERGE WITH THE EASTERN NORTH AMERICA TROUGH OVER THE WEEKEND. THIS WILL DEEPEN THE TROUGH AND PULL ITS FEATURES FARTHER SOUTH. THE TROUGH BEGINS TO EJECT NORTH THROUGH EASTERN CANADA BY THURSDAY. THE DEPTH OF THE TROUGH AND THE EXTENT OF THE RIDGES ON EITHER SIDE WILL MAKE STRAIGHT WEST-TO-EAST MOVEMENT DIFFICULT NEXT WEEK. THE MASS FIELDS AND THERMAL FIELDS OF THE GFS AND ECMWF LOOK SIMILAR IN OVERVIEW. SOME DIFFERENCES IN DETAILS EMERGE DURING THE FIRST HALF OF NEXT WEEK. THE SIMILARITY BUILDS CONFIDENCE IN THE OVERALL FLOW OF THE FORECAST. THE LONGER RANGE DIFFERENCES IN DETAIL SUPPORT USING A BLEND OF THE FORECAST DATA. THE DAILIES... SATURDAY NIGHT-SUNDAY... AS THE SHORTWAVE PHASES WITH THE EASTERN UPPER TROUGH...THE SUPPORTING UPPER JET EXTENDS EAST TOWARD NEW ENGLAND. MEANWHILE WEST TO SOUTHWEST LOW LEVEL WINDS 30 KNOTS WILL BRING INCREASING HUMIDITY...WHICH THEN RIDES OVER THE SURFACE AIRMASS. ISENTROPIC FIELD SUGGEST THE BEST LIFT WILL BE OVER NORTHERN NEW ENGLAND. BUT LOW LEVEL WIND FIELDS VARY FROM ONE MODEL TO ANOTHER. ONE FEEDS THE LOW LEVEL JET TOWARD NORTHERN NEW ENGLAND...A SECOND TOWARD MASSACHUSETTS...AND THE THIRD TOWARD LONG ISLAND. IN ANY OF THE SCENARIOS...THIS WILL MEAN INCREASING CLOUDS AND AN INCREASING CHANCE OF SHOWERS/THUNDER OVERNIGHT INTO SUNDAY AS THE RESULTING WARM FRONT APPROACHES. WE WILL FAVOR HIGHER POPS TO OUR NORTH BUT MODERATE CONFIDENCE AT BEST. EXPECT DEWPOINTS TO INCREASING DURING THIS TIME AND REACH 65-70. AS WE MOVE INTO THE WARM SECTOR ON SUNDAY...THE QUESTION WILL BE HOW MUCH THE AIRMASS DESTABILIZES. MODEL STABILITY VALUES SUGGEST A FAIR AMOUNT OF ENERGY TO FEED UPON. SB CAPES CLIMB ABOVE 1000 J/KG AND BETTER THAN 60 PCT CHANCE OF LI VALUES LESS THAN -4. PRECIPITABLE WATER VALUES ARE SQUIRRELLY ON THE GFS BUT EITHER SIDE OF 1.5 INCHES. THIS IS WELL BELOW 2 STD DEV...SO CONFIDENCE IN HEAVY DOWNPOURS IS LOW AT THIS TIME. WINDS AT 500 MB ARE FORECAST AROUND 40 KNOTS. OVERALL...A CONCERN FOR STRONG WIND GUSTS IN ANY THUNDERSTORMS WHICH WILL NEED TO BE MONITORED. MONDAY-TUESDAY... UPPER SHORTWAVE MOVES INTO POSITION ALONG THE TROUGH AXIS OVER THE EASTERN GREAT LAKES. STRONG UPPER JET LEADING THE SHORTWAVE SETS UP OVER NEW ENGLAND WITH OUR AREA UNDER THE RIGHT ENTRANCE REGION /FAVORABLE FOR GENERATING LIFT/. MODELS HINT AT A WAVE DEVELOPING ALONG THE APPROACHING COLD FRONT AND PROVIDING AN EXTRA PUSH TO GET THE COLD FRONT INTO NEW ENGLAND. INSTABILITY REMAINS WITH TOTALS UPPER 40S AND AROUND 50. EXPECT MORE SHOWERS/TSTMS MONDAY...SEVERE POTENTIAL WILL NEED TO BE MONITORED. THE COLD FRONT AND SUPPORTING UPPER JET DYNAMICS WILL STILL BE IN THE VICINITY ON TUESDAY. THE ECMWF HOLDS THE FRONT OVER INTERIOR SOUTHERN NEW ENGLAND WHILE THE GFS AIMS TOWARD THE EAST MASS COAST. WE WILL SHOW CHANCE POPS FOR MOST AREAS ESPECIALLY IN THE AFTERNOON...WITH PCPN PULLING OUT TO THE NORTHEAST IN THE EVENING. WEDNESDAY-THURSDAY... UPPER TROUGH REMAINS IN PLACE WITH AXIS TO OUR WEST. THE AIRMASS DRIES WITH JUST A MOIST LAYER BETWEEN 750 AND 850 MB...WHICH MAY CONTIBUTE TO PERIODIC CLOUDS ESPECIALLY DURING THE DAY. SURFACE HIGH PRESSURE BUILDS INTO THE REGION...WHICH SHOULD ALSO WORK AGAINST ANY SHOWERS. WILL GO WITH DRY WEATHER IN THE FORECAST. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/...
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FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH SATURDAY/...HIGH CONFIDENCE. TODAY THROUGH SATURDAY... VFR CONDITIONS RIGHT THROUGH SATURDAY OTHER THAN VERY LOCALIZED LATE NIGHT/EARLY MORNING FOG. SEA BREEZE ALONG THE COASTLINE THIS AFTERNOON. KBOS TAF...HIGH CONFIDENCE. SEA BREEZES ALONG THE IMMEDIATE COAST BY THIS AFTERNOON. SOUTHWEST WINDS SHOULD BE STRONG ENOUGH ON SATURDAY TO PRECLUDE SEA BREEZE DEVELOPMENT. KBDL TAF...HIGH CONFIDENCE. OUTLOOK...SATURDAY NIGHT THROUGH TUESDAY...MODERATE CONFIDENCE. VFR BUT WITH AREAS OF MVFR IN SHOWERS/TSTMS. AREAS OF IFR/LIFR IN LATE NIGHT/EARLY MORNING FOG ESPECIALLY ALONG THE SOUTH COAST.
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&& .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/...HIGH CONFIDENCE. TODAY AND TONIGHT...EXCELLENT BOATING WEATHER THROUGH TONIGHT WITH WEAK PRESSURE GRADIENT KEEPING WINDS/SEAS WELL BELOW SCA THRESHOLDS. VISIBILITIES SHOULD BE EXCELLENT MOST OF THE TIME. SATURDAY...SOUTHWEST WINDS WILL INCREASE TO 15-20 KNOTS AND MAY GUST TO NEAR 25 KNOTS OVER THE WATERS IN THE AFTERNOON. A SMALL CRAFT ADVISORY MAY BE NEEDED. OUTLOOK...SATURDAY NIGHT THROUGH TUESDAY...MODERATE CONFIDENCE. SATURDAY NIGHT-SUNDAY...A WARM FRONT APPROACHES THE WATERS WITH WARMER MORE HUMID AIR AND WITH CLOUDS/SHOWERS/TSTMS. INCREASING SOUTHWEST WINDS WITH GUSTS NEAR 25 KNOTS SUNDAY AND SEAS BUILD TO 5-6 FEET ON THE EXPOSED WATERS. A SMALL CRAFT ADVISORY MAY BE NEEDED. MONDAY...WARM HUMID AIR OVER THE WATERS WITH CLOUDS/SHOWERS/THUNDERSTORMS. SOUTHWEST WINDS GUSTING 20 KNOTS. THIS SHOULD KEEP SEAS AT NEAR 5 FEET ON THE EXPOSED WATERS. THE POTENTIAL FOR SMALL CRAFT ADVISORY WILL CONTINUE. TUESDAY...A COLD FRONT MOVES ACROSS THE WATERS...BUT FORECAST MODELS DIFFER ON THE TIMING OF FRONTAL PASSAGE. CONTINUED CHANCE OF SHOWERS/THUNDERSTORMS. SOUTHWEST WINDS DIMINISH AND TURN WEST- NORTHWEST AFTER THE FRONT MOVES THROUGH. SEAS REMAIN AROUND 5 FEET MUCH OF THE DAY. SMALL CRAFT ADVISORY POTENTIAL CONTINUES. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. NH...NONE. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...WTB/GAF NEAR TERM...WTB/GAF SHORT TERM...FRANK/GAF LONG TERM...WTB AVIATION...WTB/GAF MARINE...WTB/GAF

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