Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210220 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 918 PM EST SAT DEC 20 2014 .SYNOPSIS... SCATTERED SNOW/RAIN SHOWERS WILL AFFECT THE REGION THROUGH SUN. 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. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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915 PM UPDATE... A RATHER TRICKY FORECAST OVERNIGHT WITH INVERTED TROUGH/COASTAL FRONT RESULTING IN SOME MESOSCALE PROCESSES. MAIN BAND OF SNOW THAT HAS DROPPED A NARROW BAND OF 1 TO 2 INCHES OF SNOW ACROSS PORTIONS OF BOSTON/S SOUTH SHORE WAS GRADUALLY PRESSING SOUTHWEST THIS EVENING. THIS PARTLY IN RESPONSE TO A SUBTLE COASTAL FRONT THAT WAS ALSO SHIFTING FURTHER WEST INTO PLYMOUTH AND EVEN FAR EASTERN NORFOLK COUNTY. MILDER AIR WAS ALSO CAUSING SOME OF THESE AREAS TO MIX WITH OR CHANGE TO RAIN. OVERALL...STILL THINK THAT A COATING TO AT MOST 2 INCHES WILL COVER IT FOR MOST OF SOUTHERN NEW ENGLAND INTO SUNDAY. WE DID OPT TO SHIFT THE BEST RISK FOR THE 1 TO 2 INCHES A BIT FURTHER SOUTH AND WEST BASED ON RADAR RETURNS AND LATEST GUIDANCE. LOW RISK OF ISOLATED 3 INCH AMOUNTS REMAINS POSSIBLE. THE RAIN/SNOW LINE WILL LIKELY HANG OUT ACROSS FAR EASTERN NORFOLK/PLYMOUTH AND PERHAPS FAR SOUTHEAST BRISTOL COUNTIES. THE MODELS MAY BE SLIGHTLY UNDER ESTIMATING THE WESTWARD PUSH OF THIS LINE...SO SOMETHING TO WATCH BUT DO NOT THINK IT WILL GET MUCH FURTHER WEST THAN THAT. FINALLY...SCATTERED SNOW SHOWERS MAY ALSO DEVELOP INTO WESTERN MA/NORTHERN CONNECTICUT OVERNIGHT INTO SUNDAY MORNING WITH A APPROACHING SHORTWAVE. MODEL GUIDANCE DIFFERS ON EXACTLY HOW THIS WILL UNFOLD...BUT INCREASING EAST TO SOUTHEAST FLOW JUST OFF THE GROUND SHOULD ALLOW FOR AT LEAST SOME ACTIVITY IN OUR WESTERN ZONES BY DAYBREAK.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... RAIN/SNOW SHOWERS WILL CONTINUE INTO THE MORNING HOURS...THEN TAPER OFF AS INVERTED TROUGH PULLS AWAY FROM SOUTHERN NEW ENGLAND AND HIGH PRESSURE STARTS TO BUILD IN. MOST LIKELY AREAS FOR CONTINUING SHOWERS WILL BE ALONG THE IMMEDIATE EAST COAST OF MASSACHUSETTS. AS THE DAY GOES ON...EXPECT A SLOW DRYING TREND AS HIGH PRESSURE BUILDS INTO THE REGION. CLOUDS WILL STICK AROUND WITH TEMPS GENERALLY IN THE 30S. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... BIG PICTURE... SHORTWAVE ENTERING THE PACIFIC NORTHWEST CROSSES THE ROCKIES OVER THE WEEKEND DRIVEN BY A 160-KNOT JET. THIS DIGS A SUBSTANTIAL TROUGH OVER THE PLAINS MONDAY-TUESDAY WHILE PACIFIC HIGH PRESSURE BUILDS ALONG THE PACIFIC COAST. THIS IN TURN WILL ALLOW AN UPPER RIDGE TO BUILD ALONG THE ATLANTIC COAST AND SLOWLY SHIFT EAST OF THE COAST. THE JET ENERGY WORKS ITS WAY AROUND THE BASE OF THE PLAINS TROUGH AND MOVES UP THE APPALACHANS MIDWEEK. THIS SUPPORTS A TRANSITION OF THE TROUGH TO NEGATIVE TILT AND SLOWER MOVEMENT...WITH WARM MOIST AIR FOR THE ATLANTIC COAST. THE TROUGH AXIS SWINGS OVER NEW ENGLAND CHRISTMAS DAY FOLLOWED BY A WEAK RIDGE. ANOTHER SHORTWAVE MOVES ASHORE FROM THE PACIFIC MIDWEEK AND MOVES ACROSS THE PLAINS BY THE END OF THE WEEK AND APPROACHES THE NORTHEAST USA SATURDAY. THERE IS SOME SPREAD IN DETAILS AMONG THE LONG RANGE MODELS...BUT ALL SHOW A SIMILAR SCENARIO BOTH FOR THE MIDWEEK SYSTEM AND THE END OF WEEK SYSTEM. WE USED A BLEND OF GFS/ECMWF. THE DAILIES... SUNDAY NIGHT-MONDAY... HIGH PRESSURE REMAINS NORTH OF THE REGION BRINGING A MOIST EAST FLOW ACROSS SOUTHERN NEW ENGLAND. UPPER PATTERN SHOWS WEAK RIDGING AS THE PLAINS TROUGH DIGS...BUT SURFACE FIELDS SUGGEST A LINGERING INVERTED TROUGH ALONG OR JUST OFFSHORE. THIS SHOULD MAINTAIN CONSIDERABLE CLOUDS. COULD BE SOME DRIZZLE OR FLURRIES NEAR THE EAST COAST BUT SUPPORTING LARGE SCALE ASCENT IS LACKING SO PCPN SHOULD BE LIMITED. MONDAY NIGHT-TUESDAY... LOW PRESSURE FORMS ALONG A STALLED WARM FRONT OFF THE MID ATLANTIC COAST. AS THE LOW DEVELOPS...IT BRINGS AN INCREASED PRESSURE GRADIENT AGAINST HIGH PRESSURE MOVING THROUGH THE MARITIMES. THE RESULT WILL BE A STRENGTHENING LOW LEVEL EAST- SOUTHEAST JET. THIS WILL INCREASE THE MOIST INFLOW OVER SOUTHERN NEW ENGLAND. ALSO...AS THE JET CONVERGES ON THE INVERTED SURFACE TROUGH IT WILL GENERATE INCREASED LOW LEVEL LIFT AND INCREASE THE CHANCE OF PCPN. WE WILL HAVE CLOUDS AND CHANCE POPS. TEMPS IN NORTHWEST MASS WILL BE COLD ENOUGH SO THAT THERE MAY BE A MIX OF PCPN TYPES DURING THE MORNING. TUESDAY NIGHT-WEDNESDAY... PLAINS UPPER TROUGH DIGS DEEPER AND MOVES EAST...PUSHING THE UPPER RIDGE TO OUR EAST. ALSO...THE STRONG UPPER SOUTHWEST JET MOVES UP OVER THE EASTERN USA. THIS SETUP WILL TAKE THE PRIMARY SURFACE LOW UP THROUGH THE WESTERN GREAT LAKES. SECONDARY LOWS FORMING ON THE SYSTEM COLD FRONT WILL MOVE OUT OF THE GULF OF MEXICO TOWARD THE GREAT LAKES. A SOUTHWEST LOW LEVEL JET WILL MOVE UP THE ATLANTIC COAST ON WEDNESDAY AND OVER THE TOP OF THE EAST-SOUTHEAST FLOW AT THE SURFACE. THE RESULT WILL BE PLENTY OF LOW LEVEL LIFT THE GENERATE PCPN. PRECIPITABLE WATER VALUES ARE FORECAST TO EXCEED 1.5 INCHES...WHICH IS 3 STD DEVIATIONS ABOVE NORMAL FOR DECEMBER. THIS MEANS A LOT OF WATER AVAILABLE AS PCPN. TEMPERATURE PROFILES WILL BE ABOVE FREEZING SO PTYPE SHOULD BE RAIN. MODEL QPF SHOWS POTENTIAL RAINFALL OF OVER 2 INCHES. THIS WOULD SUGGEST POTENTIAL URBAN/POOR DRAINAGE FLOODING ISSUES WEDNESDAY. STRONG LOW LEVEL WINDS WILL ALSO BE A CONCERN ESPECIALLY WEDNESDAY NIGHT. THE ECMWF SHOWS EAST-SOUTHEAST WINDS 30-35 KNOTS ALONG HE EAST MASS COAST AT 1000 MB. THEN WEDNESDAY NIGHT WINDS AT 925 MB INCREASE TO 60-75 KNOTS AND MOVE ACROSS COASTAL RI AND SOUTHEAST MASS. STRONG DOWNPOURS MAY OVERCOME THE LOW LEVEL INVERSION AND BRING STRONG GUSTS OF AT LEAST 30-40 KNOTS ESPECIALLY ALONG THE SOUTH COAST. A WIND ADVISORY MAY BE NEEDED IN SOME AREAS. MAINTAINED A LIMITED MENTION OF THUNDER...AS CONVECTIVE PARAMETERS ARE MARGINAL...SI VALUES WILL BE JUST ABOVE ZERO AND K INDEX WILL BE IN THE LOW 30S WEDNESDAY NIGHT. UPPER LOW WILL SWING THE SURFACE COLD FRONT THROUGH SOUTHERN NEW ENGLAND WEDNESDAY NIGHT. WINDS WILL THEN SHIFT TO WEST AND PCPN SHOULD COME TO AN END AROUND THE TIME THAT SANTA ARRIVES. CHRISTMAS DAY... WEST WINDS WILL BRING A DRYING OF THE AIRMASS DURING THE DAY. SO THE TREND SHOULD BE TO AT LEAST PARTIAL CLEARING. SUFFICIENT COLD ADVECTION AT ALL LEVELS THURSDAY WILL ALLOW SOME MIXING. WINDS IN THE SFC-850 MB LAYER WILL BE 30-40 KNOTS AND POSSIBLY A LITTLE HIGHER...WHICH WOULD REQUIRE A WIND ADVISORY FOR MUCH OF THE AREA. FRIDAY-SATURDAY... WEAK HIGH PRESSURE BUILDS OVER THE AREA FRIDAY AND BRING A SUNNY DAY. THE NEXT PACIFIC SHORTWAVE APPROACHES SATURDAY AND SHOULD BRING INCREASING CLOUDS FROM WEST TO EAST ALONG WITH A CHANCE OF RAIN OR SNOW SHOWERS MAINLY IN THE AFTERNOON AND AT NIGHT. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH SUNDAY/... TONIGHT AND SUNDAY...MVFR CIGS/VSBYS IN RAIN/SNOW SHOWERS MOVING WEST OFF THE OCEAN INTO SOUTHEASTERN MASS. THESE WILL SPREAD FARTHER WEST INTO RI/CENTRAL MASS/NORTHEAST CT. LOCAL IFR CIGS/VSBYS POSSIBLE WITH THE STRONGEST SHOWERS. WINDS ARE ENE ALONG THE IMMEDIATE SOUTHEAST MASS COAST WITH NORTH TO NORTHWEST WINDS ELSEWHERE. THIS WINDSHIFT MAY NUDGE FARTHER WEST WITH THE SHOWERS EARLY TONIGHT. MVFR CIGS MAY REACH THE CT VALLEY LATE TONIGHT BUT LOW CONFIDENCE ABOUT ANY SIGNIFICANT REDUCTION IN VSBY THERE. MVFR CIGS WILL LINGER MOST AREA ON SUNDAY. ANY CIGS THAT RISE ABOVE MVFR SHOULD ONLY REACH AROUND 4000 FEET. KBOS TERMINAL...MODERATE CONFIDENCE IN TAF. FLURRIES AND SNOW SHOWERS THROUGH 06Z WITH POTENTIAL FOR WINDSHIFT TO 040 DURING THIS TIME. KBDL TERMINAL...MODERATE CONFIDENCE IN TAF. OUTLOOK...SUNDAY NIGHT THROUGH THURSDAY...MODERATE CONFIDENCE SUNDAY NIGHT-MONDAY...MVFR CIGS CONTINUE IN RI/EASTERN MASS IN MARINE EAST FLOW. VFR IN THE WEST. TUESDAY...VFR WITH SCATTERED MVFR IN ANY RAIN/SNOW SHOWERS. WEDNESDAY...CONDITIONS LOWERING TO IFR IN RAIN/HEAVY RAIN. ISOLATED THUNDERSTORMS POSSIBLE ESPECIALLY WEDNESDAY NIGHT. EAST-SOUTHEST SURFACE WINDS MAY GUST TO 35 KNOTS ALONG THE EAST MASSACHUSETTS COAST. WINDS AT 2000 FEET AGL WILL REACH 50-60 KNOTS WEDNESDAY AFTERNOON AND EARLY NIGHT CREATING LOW LEVEL WIND SHEAR CONCERNS. CHRISTMAS DAY...WEST WIND GUSTING 30-40 KNOTS. CONDITIONS IMPROVING TO VFR AS THE WIND BRINGS IN COOLER DRIER AIR. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. HIGH CONFIDENCE. HIGH PRESSURE ACROSS QUEBEC WILL RESULT IN PERSISTENT NORTHEAST WINDS ACROSS THE WATERS THROUGH THE WEEKEND. WIND GUSTS UP TO 20 KNOTS WITH SEAS BETWEEN 2 AND 4 FEET ARE EXPECTED FOR MOST WATERS...SO NO HEADLINES PLANNED. THERE IS A LOW PROBABILITY THAT A BRIEF SURGE OF NORTHEAST WIND GUSTS NEAR 25 KNOTS WITH MARGINAL 5 FOOT SEAS COULD AFFECT OUR NORTHERN WATERS LATE TONIGHT INTO SUN MORNING...BUT NOT CONFIDENT ENOUGH TO ISSUE ANY HEADLINES AT THIS POINT. OUTLOOK...SUNDAY NIGHT THROUGH THURSDAY...MODERATE CONFIDENCE SUNDAY NIGHT-MONDAY...EAST TO NORTHEAST WINDS GUSTING AROUND 15 KNOTS OR LESS. SEAS WILL HOLD AROUND 3-4 FEET. TUESDAY... INCREASING EAST WINDS WITH FREQUENT GUSTS 25 TO 30 KNOTS. SEAS WILL BUILD TO 5-6 FEET ON THE EXPOSED WATERS. A SMALL CRAFT ADVISORY MAY BE NEEDED. WEDNESDAY...RAIN WILL BE HEAVY AT TIMES PRODUCING POOR VISIBILITY. INCREASING SOUTHEAST WINDS 20-25 SUSTAINED AND 40 KNOT GUSTS. WINDS SHOULD SHIFT TO SOUTH WEDNESDAY NIGHT AND SOUTHWEST TOWARD THURSDAY MORNING. RAIN WILL TAPER OFF WITH THE WIND SHIFT TO SOUTHWEST. SEAS WILL CONTINUE TO BUILD WITH 5-10 FEET HEIGHTS ON THE EXPOSED WATERS. A GALE WARNING MAY BE NEEDED. CHRISTMAS DAY... WEST SOUTHWEST WINDS WILL CONTINUE TO GUST 35-40 KNOTS...SO A GALE WARNING MAY CONTINUE TO BE NEEDED. SEAS OF 10-15 FEET ARE POSSIBLE ON THE EXPOSED WATERS...AND 5 FEET IS POSSIBLE ON SOME OF THE NEARSHORE WATERS. && .TIDES/COASTAL FLOODING... HIGH ASTRONOMICAL TIDES WILL OCCUR TUESDAY THROUGH THURSDAY. BREAKING IT DOWN...ON TUESDAY HIGH TIDE AT BOSTON WILL BE 11.6 FT. THERE WILL BE EAST WINDS DURING THIS HIGH TIDE ALLOWING FOR MINOR SPLASHOVER ALONG THE MASS COAST. IF WIND GUSTS INCREASE THEN COULD POSSIBLE SEE MINOR COASTAL FLOODING. ON WEDNESDAY...WINDS WILL SWITCH TO A MORE SOUTHEASTERLY FLOW DURING THE DAY AND INCREASE QUICKLY DUE TO THE APPROACHING SYSTEM. WEDNESDAY AFTERNOON HIGH TIDE IN BOSTON IS 11.7 FT...AND AN APPRECIABLE EAST OR SOUTHEAST WIND SHOULD PUSH LEVELS HIGHER THAN ON TUESDAY. THIS WOULD PROBABLY BE THE BEST CHACNE OF MINOR COASTAL FLOODING ALONG THE EAST COAST. THE WEDNESDAY EVENING HIGH TIDE WILL BE A LITTLE LOWER...BUT THE WIND BECOMES SOUTH WITH 50-60 KNOTS AT 2000 FEET ABOVE THE SURFACE AVAILABLE TO BE BROUGHT DOWN IN GUSTS. HIGH TIDE IN PROVIDENCE WILL BE 5.2 FT. MINOR COASTAL FLOODING COULD OCCUR ALONG THE SOUTH COASTS OF RI AND MASS...AND COULD BE CHANNELED IN NORTH-SOUTH AXIS BAYS SUCH AS NARRAGANSETT BAY. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...NONE. RI...NONE. MARINE...NONE. && $$ SYNOPSIS...WTB/RLG NEAR TERM...FRANK SHORT TERM...RLG LONG TERM...WTB AVIATION...WTB/RLG MARINE...WTB/RLG TIDES/COASTAL FLOODING...STAFF

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