Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 212347 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 647 PM EST SUN DEC 21 2014 .SYNOPSIS... SCATTERED SNOW/RAIN SHOWERS CONTINUE THROUGH MONDAY. WEAK LOW PRESSURE WILL PASS SOUTH OF NEW ENGLAND BRINGING ANOTHER PERIOD OF PRECIPITATION MONDAY NIGHT AND TUESDAY. A MUCH STRONGER STORM PASSING WEST OF NEW ENGLAND WILL BRING HEAVY RAINFALL... GUSTY WINDS AND COASTAL FLOODING CONCERNS TO THE REGION WEDNESDAY AND WEDNESDAY NIGHT. DRY BUT VERY WINDY CONDITIONS FOLLOW FOR CHRISTMAS DAY. HIGH PRESSURE BRINGS DRY WEATHER FRIDAY. ANOTHER DEVELOPING SYSTEM MAY BRING RAIN AND SNOW NEXT SUNDAY. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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645 PM UPDATE... TWO AREAS OF LIGHT RAIN/SNOW SHOWERS ON RADAR...ONE MOVING SOUTHWEST FROM THE GULF OF MAINE TOWARD SOUTHEAST MASS AND THE OTHER MOVING NORTH ONTO THE SOUTH COASTS OF RI AND MASS. WE ADJUSTED POPS TO REFLECT THE CHANCES BEING IN AREAS MAINLY SOUTH OF THE MASS PIKE EARLY TONIGHT. AS NOTED EARLIER...THE LIGHT PCPN SHOULD AMOUNT TO SNOW ACCUMULATIONS OF A DUSTING/COATING UP TO POSSIBLY AN INCH IN A FEW PLACES. NO OTHER CHANGES TO THE EXISTING FORECAST. PREVIOUS DISCUSSION... SHORTWAVE RIDGING BUILDS OVER SOUTHERN NEW ENGLAND OVERNIGHT. IN ADDITION...HIGH PRESSURE IN QUEBEC BUILDS OVER THE REGION AS WELL. THEREFORE...THERE WILL BE NO FOCUSING MECHANISM FOR PRECIP. HOWEVER...ONSHORE FLOW AND DECENT MID LEVEL LAPSE RATES WILL ALLOW FOR A FEW SCATTERED SNOW/RAIN SHOWERS ACROSS THE AREA OVERNIGHT. EXPECT SNOW SHOWERS TO REMAIN ROUGHLY NORTH AND WEST OF A NEWPORT TO TAUNTON TO SCITUATE LINE. ANY ADDITIONAL SNOWFALL WILL AMOUNT TO A DUSTING TO ABOUT AN INCH. TEMPERATURES WILL FALL INTO THE MID 20S ACROSS THE NORTHWEST TO THE MID 30S ON THE CAPE AND ISLANDS.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... MONDAY...CLOUDY...DAMP...AND RAW CONDITIONS WILL CONTINUE THROUGH THE DAY WITH CONTINUED ONSHORE FLOW AND SCATTERED SNOW/RAIN SHOWERS. MONDAY NIGHT...WEAK LOW PRESSURE MOVING UP THE COAST TO JUST OFF THE DELMARVA PENINSULA WILL PRODUCE WARM ADVECTION OVER SOUTHERN NEW ENGLAND. THIS IN TURN WILL LIKELY RESULT IN SOME PRECIPITATION DEVELOPING OVERNIGHT. HIGH PRESSURE MOVING INTO THE MARITIMES WILL ALLOW FOR SOME COLD AIR DAMMING...PARTICULARLY INTO INTERIOR SOUTHERN NEW ENGLAND. THIS SCENARIO IS A GOOD SET UP FOR SOME FREEZING RAIN ACROSS THE INTERIOR. THE MAIN QUESTION IS HOW FAR NORTH THE WARMER AIR GETS AS THIS WILL DETERMINE PTYPE. FOR NOW...WILL MAKE MENTION OF THE POTENTIAL FOR AN AREA OF FREEZING RAIN IN THE HAZARDOUS WEATHER OUTLOOK. IF FREEZING RAIN LOOKS TO BE MORE CERTAIN...WE WILL NEED TO ISSUE A WINTER WEATHER ADVISORY. && .LONG TERM /TUESDAY THROUGH SUNDAY/... BIG PICTURE... TUESDAY STARTS WITH UPPER RIDGES ON EACH COAST AND A DEEP UPPER TROUGH/CLOSED LOW OVER THE PLAINS. BY WEDNESDAY THE CLOSED LOW EJECTS NORTHEAST THROUGH LAKE SUPERIOR INTO CANADA. THIS SWEEPS THE TROUGH AXIS EAST AND PASSES ACROSS NEW ENGLAND CHRISTMAS DAY. ANOTHER SHORTWAVE MOVES ONSHORE IN THE PACIFIC NORTHWEST WEDNESDAY AND PHASES WITH A SECOND DROPPING SOUTH FROM NORTHERN CANADA. THIS REESTABLISHES THE UPPER TROUGH OVER THE NORTHERN PLAINS/GREAT LAKES SATURDAY. CONTOUR HEIGHTS WILL BE ABOVE NORMAL MIDWEEK...TRENDING BELOW NORMAL OVER THE WEEKEND AS THE SECOND UPPER TROUGH SHIFTS EAST. THIS POINTS TO A COOLING TREND AFTER CHRISTMAS. THE GFS AND ECMWF ARE SIMILAR IN HANDLING THE HOLIDAY STORM. THERE ARE DIFFERENCES BETWEEN THE MODELS IN HANDLING THE SUNDAY SYSTEM. WE USED A BLEND MOST OF THE LONG TERM PERIOD...BUT MANUALLY ADJUSTED POPS FOR SUNDAY WITH LOW CONFIDENCE IN MODEL VALUES. THE DAILIES... TUESDAY... HIGH PRESSURE OVER THE MARITIMES WITH RIDGING DOWN THE ATLANTIC COAST. COASTAL LOW PRESSURE WAVE DEVELOPS ALONG STALLED WARM FRONT AND MOVES NORTH. AS THE COASTAL LOW MOVES NORTH IT WILL RUN UP AGAINST THE MARITIME HIGH. THIS WILL INCREASE THE LOW LEVEL PRESSURE GRADIENT AND INCREASE THE LOW LEVEL SOUTHEAST WINDS. RESULT...A SOUTHEAST LOW LEVEL JET RUNS OVER THE RIDGING ALONG THE COASTAL PLAIN INDUCING LOW LEVEL LIFT. THE MOIST LAYER EXPANDS ABOVE 800 MB...NOT DEEP BUT DEEP ENOUGH FOR THE LIFT TO GENERATE LIGHT PCPN. PTYPE WILL BE RAIN MOST PLACES...BUT THERE COULD BE SPOTS OF LIGHT FREEZING RAIN IN NORTHWEST MASSACHUSETTS. WE WILL MAINTAIN CHANCE POPS BUT INCREASE TO AROUND 50 PCT...THE HIGH END OF THE CHANCE RANGE. LOW LEVEL SOUTH JET MOVES UP THE COAST TUESDAY NIGHT REACHING THE COOLER AIR OVER NEW ENGLAND AFTER MIDNIGHT AND BEFORE SUNRISE. THIS WILL GENERATE ADDITIONAL LIFT AS WELL AS INCREASING AND DEEPENING MOISTURE. THIS SUGGESTS INCREASING POPS OVERNIGHT. TEMPERATURES SHOULD REMAIN ABOVE FREEZING SO PTYPE SHOULD BE RAIN. WEDNESDAY-THURSDAY... SOUTH 40-50 KNOT LOW LEVEL JET MOVES OVER NEW ENGLAND WEDNESDAY GENERATING STRONG LIFT AND TRANSPORTING HIGH VALUES OF PRECIPITABLE WATER INTO THAT LIFT. THE MOISTURE VALUES ARE MORE THAN 3 STD DEVIATIONS ABOVE NORMAL FOR DECEMBER...AND ACTUALLY NEAR THE MAXIMUM PREVIOUSLY OBSERVED VALUE. THIS SUGGESTS RAIN WITH SOME HEAVY DOWNPOURS. COLD FRONT APPROACHES LATE WEDNESDAY NIGHT AND CROSSES SOUTHERN NEW ENGLAND CHRISTMAS MORNING. THE FRONT WILL HAVE DYNAMIC SUPPORT FROM A 140-150 KNOT UPPER JET. THE FOREWARD PROGRESS OF THE FRONT WILL SLOW BECAUSE MOST OF THE MOTION IN THE UPPER JET WILL BE PARALLEL TO THE FRONT. THIS WILL MEAN CONTINUED RAIN/HEAVY RAIN THROUGH THE NIGHT INTO THURSDAY MORNING. THE OBVIOUS CONCERN FOR THIS PERIOD WILL BE FLOODING FROM HEAVY RAINFALL. RAIN AMOUNTS OF 2 TO 3 INCHES ARE EXPECTED. THIS SHOULD BE SUFFICIENT FOR AT LEAST URBAN AND POOR DRAINAGE FLOODING. A FLOOD WATCH MAY BE NEEDED. A SECOND CONCERN WILL BE WIND. THE LOW LEVEL JET WEDNESDAY WILL BE SOUTH 40-50 KNOTS...AND INCREASE TO 55-65 KNOTS WEDNESDAY NIGHT. CONVENTIONAL MIXING WILL NOT BE DEEP...BUT HEAVY DOWNPOURS COULD CREATE A MOIST ADIABATIC CHANNEL THAT BRINGS DOWN SOME STRONGER WIND GUSTS. MODEL GRIDS ARE RATHER LIGHT ON WIND GUSTS...15 TO 30 KNOTS. WE WILL MANUALLY ADJUST THESE VALUES UP TO 25-35 KNOTS OVER LAND AND A LITTLE STRONGER OVER WATER. A THIRD CONCERN WILL BE COASTAL FLOODING. MORE DETAILS ON THIS IN THE TIDE/COASTAL FLOOD SECTION. WINDS SHIFT TO WEST BEHIND THE FRONT ON CHRISTMAS DAY. THIS WILL BRING DRIER AIR AND SO END THE RAINFALL. COLD ADVECTION WILL PROMOTE MIXING TO NEAR 850 MB IN THE AFTERNOON AND TAP 25-35 KNOT WIND GUSTS...POSSIBLY 40 KNOT GUSTS. A WIND ADVISORY MAY BE NEEDED FOR THESE WEST WINDS THURSDAY. FRIDAY THROUGH SUNDAY.... HIGH PRESSURE MOVES OVER THE REGION FRIDAY BRINGING DRY WEATHER. SHORTWAVE MOVING THROUGH THE GREAT LAKES SWINGS A COLD FRONT THROUGH NEW ENGLAND SATURDAY. THE SURFACE REFLECTION OF THE SHORTWAVE MOVES UP THE ST LAWRENCE VALLEY AND JET SUPPPORT WILL MOSTLY BE NORTH OF THE BORDER...SO PCPN WILL BE LIMITED. QUESTION FOR SUNDAY WILL BE WHETHER A WAVE FORMS ON THE DEPARTING COLD FRONT...IF SO IT MIGHT BRING PCPN TO NEW ENGLAND. THE ECMWF THINKS SO BUT THE GFS DOES NOT. WE BROUGHT IN A CHANCE OF RAIN AND SNOW SHOWERS. && .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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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 MONDAY NIGHT/... TONIGHT...MODERATE CONFIDENCE. MAINLY MVFR CONDITIONS THROUGH TONIGHT. ONE EXCEPTION IS OVER BOSTON AND ESSEX COUNTY WHERE THE 1500 FOOT CEILING BROKE TO 6500 FEET AND MAY STAY LIKE THAT THROUGH THE EARLY NIGHT. BUT WITH CONTINUED ONSHORE FLOW AND A FEW RAIN/SNOW SHOWERS UPSTREAM IT IS POSSIBLE CIGS WILL DROP BACK TO 1500 FEET IN ANY SHOWERS. THE OVERALL MOIST FLOW SHOULD ALSO BRING LEVELS BACK TO MVFR OVERNIGHT. SECOND EXCEPTION IS OVER CAPE COD AND THE ISLANDS WHERE VFR CONDITIONS CONTINUE IN PLACE. BELIEVE THESE CONDITIONS WILL ALSO DETERIORATE OVERNIGHT IN THE ONSHORE NORTHEAST FLOW. MONDAY AND MONDAY NIGHT...MODERATE CONFIDENCE. MAINLY MVFR CONDITIONS CONTINUE IN SCATTERED LIGHT SNOW SHOWERS. LIGHT FZRA POSSIBLE LATE MONDAY NIGHT ACROSS NW MASSACHUSETTS. KBOS TERMINAL...MODERATE CONFIDENCE. KBDL TERMINAL...MODERATE CONFIDENCE. OUTLOOK...TUESDAY THROUGH FRIDAY...MODERATE CONFIDENCE TUESDAY...MVFR CIGS AND VSBYS IN LIGHT RAIN. EAST SOUTHEAST WINDS...WITH FREQUENT GUSTS TO 25 KNOTS ON THE CAPE AND ISLANDS. LATE TUESDAY NIGHT-WEDNESDAY-THURSDAY... CONDITIONS LOWERING TO IFR/LIFR IN RAIN/HEAVY RAIN. SOUTHEAST WINDS GUSTING 25 TO 35 KNOTS WEDNESDAY AND SOUTH WINDS SIMILAR SPEEDS WEDNESDAY NIGHT...EXCEPT 30 TO 40 KNOTS CAPE COD AND ISLANDS. WINDS AT 2000 FEET MAY REACH 55 TO 65 KNOTS BY WEDNESDAY NIGHT CREATING LOW LEVEL WIND SHEAR. ISOLATED THUNDERSTORMS WEDNESDAY AFTERNOON AND NIGHT. COLD FRONT SWINGS THROUGH ON THURSDAY MORNING BRINGING A WIND SHIFT TO WEST WITH GUSTS 30-40 KNOTS. CONDITIONS IMPROVE TO VFR AFTER THE WIND SHIFT. FRIDAY...VFR.
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&& .MARINE...
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FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. HIGH PRESSURE ACROSS QUEBEC AND THE MARITIMES WILL RESULT IN PERSISTENT NORTHEASTERLY TO EASTERLY WINDS ACROSS THE WATERS THROUGH MONDAY. THIS WILL RESULT IN BUILDING SEAS ON THE EASTERN WATERS DESPITE WIND GUSTS UP TO 20 KNOTS. SEAS BUILD TO AT LEAST 5 FEET ON THE EASTERN WATERS BY MIDDAY MONDAY. A SMALL CRAFT ADVISORY CONTINUES FOR THE EASTERN WATERS FOR THE PERIOD FROM MIDDAY MONDAY THROUGH TUESDAY MORNING. WINDS INCREASE TO NEAR 25 KTS ON THE SOUTHERN WATERS BY 12Z TUESDAY. OUTLOOK...TUESDAY THROUGH FRIDAY...MODERATE CONFIDENCE TUESDAY...EAST WIND GUSTING TO 25 KNOTS...BECOMING SOUTHEAST AT NIGHT. SEAS 5 TO 7 FEET ON THE EXPOSED WATERS. A SMALL CRAFT ADVISORY WILL BE NEEDED. WEDNESDAY-THURSDAY... SOUTH WINDS WITH FREQUENT GUSTS 35 TO 40 KNOTS BY WEDNESDAY AFTERNOON AND CONTINUING THROUGH THE NIGHT. WINDS SHIFT WEST THURSDAY MORNING WITH SIMILAR SPEEDS THROUGH THE AFTERNOON. SEAS WILL BUILD TO 7 TO 12 FEET ON THE EXPOSED WATERS BY WEDNESDAY NIGHT AND THURSDAY. A GALE WARNING MAY BE NEEDED. FRIDAY...WINDS DIMINISHING TO 25 KNOTS FRIDAY MORNING AND BELOW 25 KNOTS BY AFTERNOON. SEAS LINGERING AT 5 TO 7 FEET ON THE EXPOSED WATERS. A SMALL CRAFT ADVISORY MAY BE NEEDED.
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&& .TIDES/COASTAL FLOODING... HIGH ASTRONOMICAL TIDES WILL OCCUR TUESDAY THROUGH THURSDAY. TUESDAY...HIGH TIDE AT BOSTON WILL BE 11.6 FT. WINDS WILL BE FROM THE EAST...SO THERE WILL BE ONSHORE FLOW DURING THIS HIGH TIDE ALLOWING FOR MINOR SPLASHOVER. IF WINDS INCREASE THEN COULD SEE SOME POCKETS OF MINOR COASTAL FLOODING. WEDNESDAY...WINDS SHIFT TO A MORE SOUTHEASTERLY FLOW DURING THE DAY AND INCREASE QUICKLY AS AN APPROACHING SYSTEM BRINGS PRESSURE FALLS OF 1 MB/HR. THE WEDNESDAY AFTERNOON HIGH TIDE IN BOSTON IS 11.7 FT AND STRONG SOUTHEASTERLY WINDS WILL BE JUST OFFSHORE. RIGHT NOW BELIEVE MINOR COASTAL FLOODING IS POSSIBLE ALONG THE EAST COAST. WEDNESDAY EVENING...SOUTHERLY WINDS ARE EXPECTED AND ASTRONOMICAL TIDES INCREASE ALONG THE SOUTH COAST. HIGH TIDE IN PROVIDENCE WILL BE 5.2 FT. 925 MB LLJ STRENGTHENS TO 55-65 KTS WHICH COULD PUSH A DECENT SURGE UP ALONG THE SOUTH COAST DURING THE TIME OF HIGH TIDE. THINKING THAT MINOR COASTAL FLOODING COULD OCCUR BUT CANNOT RULE OUT MODERATE ALONG THE SOUTH COAST...ESPECIALLY IF LLJ CONTINUES TO STRENGTHEN. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. RI...NONE. MARINE...SMALL CRAFT ADVISORY FROM 4 PM MONDAY TO 7 AM EST TUESDAY FOR ANZ250. SMALL CRAFT ADVISORY FROM 10 PM MONDAY TO 7 AM EST TUESDAY FOR ANZ251-255. SMALL CRAFT ADVISORY FROM 1 PM MONDAY TO 7 AM EST TUESDAY FOR ANZ254. && $$ SYNOPSIS...WTB/RLG NEAR TERM...WTB/RLG SHORT TERM...RLG LONG TERM...WTB AVIATION...WTB/RLG MARINE...WTB/RLG TIDES/COASTAL FLOODING...RLG

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