Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 212024 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 324 PM EST SUN DEC 21 2014 .SYNOPSIS... SCATTERED SNOW/RAIN SHOWERS CONTINUE THROUGH THIS AFTERNOON. 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 MONDAY MORNING/... 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. && .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/... HIGHLIGHTS... * WEAK SHORTWAVE ON MON NIGHT/TUESDAY COULD BRING WINTRY MIX/FREEZING RAIN TO THE INTERIOR * ANOMALOUS SYSTEM ACROSS THE GREAT LAKES WILL BRING HEAVY RAIN...GUSTY WINDS AND COASTAL FLOODING CONCERNS AROUND CHRISTMAS EVE * BLUSTERY BUT AIRMASS WILL BE DRYING OUT ON CHRISTMAS DAY OVERVIEW... THE MODELS AND ENSEMBLES OFFER A REASONABLY SIMILAR AND PREDICTABLE LARGER SCALE MID TO UPPER LEVEL PATTERN EVOLUTION FOR MUCH OF THE UPCOMING WEEK. THIS ACTIVE FLOW REGIME DEVELOPS SEVERAL MID-UPPER LEVEL TROUGH AND ASSOCIATED/UNSETTLING LEAD SURFACE LOW/FRONTAL SYSTEMS AND PCPN SWATHS PROGRESS STEADILY ACROSS THE LOWER 48. THE FIRST AMPLIFIED/DEEP LEAD SYSTEM WORKS ACROSS THE ERN/NERN US INTO ERN CANADA CHRISTMAS EVE INTO CHRISTMAS ALLOWING FOR A MOIST/WARMED AIRMASS IN ADVANCE OF FRONTAL APPROACH WITH HEAVIEST LEAD QPF POTENTIAL OVER NEW ENGLAND. BIGGEST QUESTION IS THE EVOLUTION OF THE SECONDARY LOW PRESSURE SYSTEM. 00Z GUIDANCE HAS BACKED OFF ON THE STRENGTH OF THE LOW AND IS HAVING ISSUES ON HOW TO HANDLE ITS EVOLUTION. THE EC AND GFS HAVE SLOWED DOWN THE SYSTEM SLIGHTLY WITH THE EC HAVING MORE OF A NEGATIVE TROUGH VS THE GFS. BELIEVE THE UKMET IS A GOOD COMPROMISE AT IS WAS THE FIRST DETERMINISTIC GUIDANCE ON TAMPERING DOWN THE FIRST MAIN LOW...WHILE THE CMC IS OUT TO LUNCH. REGARDLESS THIS IS A VERY ANOMALOUS SYSTEM THAT IS ABOUT 4 STANDARD DEVIATIONS AWAY. A SECONDARY SYSTEM DEEPENS ACROSS THE FOUR CORNERS ON FRIDAY AND WITH THE STUBBORN TROUGH OVER THE CENTRAL U.S...IT WILL PUSH A SYSTEM TOWARDS SOUTHERN NEW ENGLAND BY SUNDAY. THIS SYSTEM MAY SUPPORT A MODEST SWATH OF OVERRUNNING SNOW OVER SOUTHERN NEW ENGLAND. DETAILS... TUESDAY...MODERATE CONFIDENCE. A WEAK WAVE OF LOW PRESSURE DEVELOPS ALONG THE SOUTHEAST COAST MONDAY...AND WILL BE ON A DEVELOPING WARM FRONTAL BOUNDARY. THIS LOW REMAINS WEAK AS THE WARM FRONT MOVES SLOWLY NORTH MONDAY NIGHT INTO TUESDAY. APPEARS THAT THERE IS DECENT WARM ADVECTION AHEAD OF THIS WAVE. SEVERAL HI-RES GUIDANCE MODELS SHOW THAT THERE IS THE POTENTIAL FOR COLD AIR DAMMING ACROSS THE INTERIOR INTO TUESDAY. BIGGEST QUESTION WILL BE IS THERE ENOUGH MOISTURE FOR PRECIP DURING THIS TIME FRAME. DUE TO A STRENGTHENING LLJ PROVIDING A GOOD SOURCE OF LIFT ANTICIPATE PRECIP TO DEVELOP AND OVERSPREAD ACROSS SOUTHERN NEW ENGLAND. IF THIS PLAYS OUT WITH COLD AIR DAMMING AND WARM AIR ADVECTION...THEN WE COULD SEE FREEZING RAIN/DRIZZLE ACROSS THE I-91 CORRIDOR LATE MONDAY INTO TUESDAY MORNING. DURING THE DAY ON TUESDAY...GOOD AMOUNT OF WARM AIR WILL PUSH SURFACE TEMPS ABOVE FREEZING BY THE LATE MORNING ALLOWING FOR ANY PRECIP TO BE MORE OF THE LIQUID VARIETY AHEAD OF THE CHRISTMAS EVE STORM. HOWEVER GUIDANCE IS ALWAYS TOO FAST IN SCOURING TEMPS OUT ESP ACROSS THE CT VALLEY SO WE WILL NEED TO MONITOR THIS CLOSELY...ESP DUE TO LIGHT WINDS AND THE HIGH IN EASTERN CANADA. A WINTER WX ADVISORY MAY BE NEEDED FOR THE LIGHT ICING POTENTIAL. WE COULD ALSO SEE SOME MINOR COASTAL FLOODING CONCERNS ON TUESDAY. SEE BELOW FOR MORE DETAILS. WEDNESDAY INTO THURSDAY...HIGH CONFIDENCE IN SYSTEM LOW CONFIDENCE ON DETAILS. ANOMALOUS SYSTEM WILL STRENGTHEN OVER THE GREAT LAKES TUESDAY INTO WEDNESDAY. A SECONDARY SHORTWAVE WILL ROUND THE DEEP TROUGH AND DEVELOP A SECONDARY LOW PRESSURE ALONG THE COLD FRONT. THIS SECONDARY LOW WILL STRENGTHEN AND PASS WEST OF SOUTHERN NEW ENGLAND. THIS SYSTEM WILL CONTINUE TO DEEPEN AS IT MOVES TOWARDS EASTERN CANADA SWINGING A COLD FRONT ACROSS THE NORTHEAST. STRONG SOUTHERLY JET OUT AHEAD OF THE FRONT WITH A GULF MOISTURE CONNECTION WILL IMPACT SOUTHERN NEW ENGLAND ON CHRISTMAS EVE. THIS STRONG JET WILL YIELD FOR PCPN IN THE LIQUID VARIETY. PW ANOMALIES ARE BETWEEN +4 TO +5 STANDARD DEVIATIONS ABOVE NORMAL WHICH IS QUITE ANOMALOUS FOR THIS TIME OF YEAR. IN FACT CURRENT PWATS ARE PROGGED ABOVE 1.6 INCHES WITH MODEL QPF OVER 2 INCHES...AT THE VERY LEAST COULD SEE URBAN AND POOR DRAINAGE FLOODING. LATEST EC EVEN SHOWS OVER 2.5 INCHES OF QPF...AND WITH THE TROPICAL CONNECTION AND STRONG SOUTHERLY FLOW COULD SEE HIGHER AMOUNTS DUE TO TOPOGRAPHY ENHANCEMENTS. RAINFALL RECORDS MAY FALL IF EVERYTHING FALLS INTO PLACE. AHEAD OF THE FRONT...SOUTHERN NEW ENGLAND WILL WARM SECTOR WED INTO THUR MORNING ALLOWING FOR TEMPERATURES TO BE WELL ABOVE AVERAGE. IN FACT WE MAY BE CLOSE AT BREAKING RECORD HIGH TEMPS IF TEMPS WARM TO 60F OR ABOVE ON CHRISTMAS EVE. SINCE WE WILL BE IN THE WARM SECTOR...AND SHOWALTERS CLOSE TO ZERO...ISOLATED THUNDER IS STILL POSSIBLE. STRONG LOW LEVEL WINDS WILL ALSO BE A CONCERN ESPECIALLY WEDNESDAY NIGHT. THE ECMWF SHOWS EAST-SOUTHEAST WINDS 30-40 KNOTS ON WEDNESDAY INCREASING TO 60-75 KNOTS BY WEDNESDAY NIGHT. 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. CHRISTMAS DAY...MODERATE CONFIDENCE. AS THE COLD FRONT PASSES THROUGH EARLY CHRISTMAS DAY...THE WEST WINDS WILL HELP DRY THE AIRMASS ALLOWING FOR PARTLY SUNNY SKIES. STRONG CAA ON THURSDAY WILL ALLOW DESCENT 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-SUNDAY...LOWER CONFIDENCE WEAK HIGH PRESSURE BUILDS OVER THE AREA FRIDAY INTO SATURDAY. THE NEXT SHORTWAVE APPROACHES LATE SATURDAY INTO SUNDAY 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 /20Z SUNDAY THROUGH FRIDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH MONDAY NIGHT/... THROUGH TONIGHT...MODERATE CONFIDENCE. MAINLY MVFR/IFR CONDITIONS THROUGH TONIGHT. EXCEPTION RIGHT NOW IS THE CAPE AND ISLANDS WHERE VFR CONDITIONS ARE IN PLACE. BELIEVE CONDITIONS WILL DETERIORATE LATER TONIGHT THERE. WIDESPREAD LIGHT SNOW SHOWERS ARE AFFECTING ALL OF SOUTHERN NEW ENGLAND THIS AFTERNOON. EXPECT THIS ACTIVITY TO BECOME MORE SCATTERED IN NATURE. RAIN SHOWERS ACROSS MOST OF PLYMOUTH COUNTY AND ONTO THE CAPE/ISLANDS WITH SNOW SHOWERS NORTH AND WEST. 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...MONDAY THROUGH THURSDAY...MODERATE CONFIDENCE. 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-SOUTHEAST SURFACE WINDS MAY GUST TO 35-45 KNOTS. 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 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. HAVE ISSUED A SMALL CRAFT ADVISORY FOR THE EASTERN WATERS BEGINNING MIDDAY MONDAY AND CONTINUING THROUGH TUESDAY MORNING. WINDS INCREASE TO NEAR 25 KTS ON THE SOUTHERN WATERS BY 12Z TUESDAY. OUTLOOK...TUESDAY THROUGH FRIDAY...MODERATE CONFIDENCE MONDAY...EAST TO NORTHEAST WINDS GUSTING AROUND 15 KNOTS OR LESS. SEAS WILL HOLD AROUND 3-4 FEET...MAY INCREASE TO 5FT IN NORTHEAST FLOW AS GUIDANCE MAY BE UNDERDONE. 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 7-11 FEET HEIGHTS ON THE EXPOSED WATERS. A GALE WARNING WILL BE NEEDED...WITH A LOW PROB OF A FEW STORM FORCE GUSTS ACROSS THE EASTERN OUTER WATERS. CHRISTMAS DAY... WEST SOUTHWEST WINDS WILL CONTINUE TO GUST 35-40 KNOTS...SO A CONTINUED GALE WARNING MAY BE NEEDED. SEAS OF 7-15 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. TUESDAY...HIGH TIDE AT BOSTON WILL BE 11.6 FT. 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 DECENT PRESSURE FALLS. 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 50-60 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...NONE. && $$ SYNOPSIS...DUNTEN/RLG NEAR TERM...RLG SHORT TERM...RLG LONG TERM...DUNTEN AVIATION...DUNTEN/RLG MARINE...DUNTEN/RLG TIDES/COASTAL FLOODING...

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