183
FXUS61 KBOX 190242
AFDBOX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOSTON/NORTON MA
942 PM EST WED DEC 18 2024
SYNOPSIS
INCREASING CLOUDS EARLY TONIGHT AS A FAST-MOVING AREA OF LOW
PRESSURE PROGRESSES THROUGH SOUTHERN NEW ENGLAND THIS EVENING
INTO THE PRE-DAWN HOURS ON THURSDAY. RAIN IS EXPECTED IN MANY
AREAS, ALTHOUGH A MINOR ACCUMULATION OF LIGHT SNOW IS
ANTICIPATED IN THE HIGHEST ELEVATIONS. BRIEF DRY WEATHER THEN
RESUMES FOR THURSDAY. MONITORING AN OFFSHORE AREA OF LOW
PRESSURE THAT COULD BRING ADDITIONAL LIGHT PRECIPITATION ON
FRIDAY INTO SATURDAY. THIS WILL BE FOLLOWED BY VERY COLD AND
BLUSTERY WEATHER THIS WEEKEND INTO EARLY NEXT WEEK.
NEAR TERM /UNTIL 6 AM THURSDAY MORNING/
940 PM UPDATE:
FORECAST ON TRACK. WIDESPREAD RAIN HAS OVERSPREAD MUCH OF SNE.
RAIN IS MORE SCATTERED OVER THE ISLANDS, BUT WILL FILL IN AFTER
MIDNIGHT. TEMPS AROUND 32-24 OVER THE NORTHERN BERKSHIRES WHERE
PTYPE IS LIKELY ALL SNOW. ACCUM UP TO 2 INCHES POSSIBLE HERE
BEFORE THE PRECIP EXITS SHORTLY AFTER MIDNIGHT. TEMPS A FEW
DEGREES WARMER OVER THE NORTHERN WORCESTER HILLS BUT IT IS
COOLING A BRIEF CHANGE TO SNOW IS EXPECTED BEFORE THE PRECIP
ENDS BUT ANY ACCUM HERE WILL LIKELY BE LIMITED TO A COATING TO
AN INCH. OTHERWISE, THIS IS A PROGRESSIVE SYSTEM AND PRECIP WILL
BE ENDING BETWEEN MIDNIGHT AND 3 AM IN THE WEST AND 3-6 AM IN
EASTERN NEW ENG.
BRIEF GUSTY SW WINDS EXPECTED THROUGH ABOUT 06Z OVER THE
ISLANDS AND OUTER CAPE WHICH WILL BE ALONG AND JUST SOUTH OF
THE LOW TRACK, THEN WIND SHIFT TO NW AND DECREASING GUSTS.
PREVIOUS DISCUSSION...
KEY MESSAGES:
* RAINY CONDITIONS DEVELOP IN MOST OF SOUTHERN NEW ENGLAND EARLY
TONIGHT, FALLING AT MODERATE RAIN RATES NEAR SOUTH COASTAL NEW
ENGLAND.
* A MINOR ACCUMULATION OF WET SNOW (UP TO 2") FOR THE BERKSHIRES AND
HILLS IN NORTHERN WORCESTER COUNTY.
* PRECIPITATION ENDS WEST TO EAST BY THE PRE-DAWN THURS HOURS.
AREA OF SHOWERS IS STARTING TO REACH THE GROUND IN WESTERN MA
AND CT AHEAD OF A WEAK AREA OF LOW PRESSURE CURRENTLY NEAR
CENTRAL PA. THIS LOW PRESSURE IS EXPECTED TO TREK ENE THROUGH
SOUTHERN NY AND COASTAL CT/RI/SOUTHEAST MA THROUGH THE EVENING
AND OVERNIGHT. THIS FEATURE WILL SPREAD PRECIPITATION INTO
SOUTHERN NEW ENGLAND; WITH TEMPS IN MANY AREAS EXPECTED TO BE
ABOVE FREEZING, THE MAJORITY OF SOUTHERN NEW ENGLAND WILL SEE
PRECIP FALL IN THE FORM OF STEADY LIGHT TO AT TIMES MODERATE
RAINS EVEN WITH THE INITIAL EVAPORATIVE COOLING EFFECTS AT
OUTSET. THE HIGHEST ELEVATIONS IN THE HILLS IN NORTHERN
WORCESTER COUNTY AND THE BERKSHIRES SHOULD BE COLD ENOUGH TO
SUPPORT A MINOR ACCUMULATION OF WET SNOW.
THERE WERE OVERALL NO SUBSTANTIAL CHANGES IN FORECAST GUIDANCE THAT
WOULD REQUIRE A SHIFT IN MESSAGING THIS EVENT. PRECIP EXPECTED TO
BEGIN JUST AFTER THE PM RUSH HOUR IN WESTERN MA AND CT, AND INTO
CENTRAL AND EASTERN PORTIONS OF SOUTHERN NEW ENGLAND BY MID-EVENING.
BULK OF THE PRECIP WILL FALL DURING THE LATE EVENING TO EARLY
OVERNIGHT HOURS, THEN START TO PULL AWAY OFFSHORE FROM WEST TO EAST
DURING THE LATE OVERNIGHT TO THE PREDAWN HRS.
OVERALL QPF RANGES FROM A THIRD OF AN INCH IN NORTHERN MA, AND TO
AROUND HALF TO TWO-THIRDS OF AN INCH, WITH THE HEAVIEST TOTALS OVER
SOUTH-COASTAL RI AND MA CLOSER TO THE TRACK OF THE SFC CYCLONE. AS
IS USUAL WITH EVENTS WITH MARGINAL/BORDERLINE TEMPS, THERE IS QUITE
A BIT OF DIFFERENCE BETWEEN THE USUAL 10:1 SNOW TO LIQUID RATIO
SNOWFALL FORECAST OUTPUTS (A FEW INCHES HIGHER AS IT ASSUMES ALL
PRECIP FALLS AS SNOW AND SOME OF THE INITIAL QPF WILL BE LOST TO
EVAPORATION) VS THE POSITIVE-SNOW-DEPTH-CHANGE FORECAST SNOWFALL
GUIDANCE. THE LATTER OFTEN PERFORMS WELL IN THESE BORDERLINE
SITUATIONS. GIVEN SNOW GROWTH PARAMETERS AREN'T NECESSARILY
SUBSTANTIAL (E.G. WEAK VERTICAL ASCENT NOT LINING UP WITH THE
DENDRITE SNOW CRYSTAL GROWTH LAYER PER BUFKIT X-SECTIONS), I'VE
LEANED TOWARD THE UP TO 2 INCHES OF SNOW FORECAST SIMILAR TO THE
POSITIVE SNOW DEPTH CHANGE GRAPHICS. THIS ALSO CARRIES CONTINUITY
WITH PREVIOUS FORECASTS.
TEMPS MAINLY IN THE 30S, WITH 30-32F READINGS IN THE HIGHER TERRAIN
AND AROUND 33-37F IN THE LOWER ELEVATIONS/VALLEYS.
SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/
345 PM UPDATE:
THURSDAY:
HIGH PRESSURE RIDGE THEN FOLLOWS THE OVERNIGHT STORM SYSTEM, AND
SUPPLIES DRIER WEATHER WITH COOL ADVECTION ON NW WINDS. MAY SEE AN
INCREASE IN HIGH CLOUDS AGAIN LATER IN THE AFTERNOON BUT MOST OF THE
DAY SHOULD FEATURE ABUNDANT SUN. HIGHS THURSDAY SHOULD BE PRETTY
SEASONABLE, IN THE MID/UPPER 30S HIGH TERRAIN AND IN THE LOW TO MID
40S ELSEWHERE.
THURSDAY NIGHT:
1030 MB HIGH PRESSURE CELL OVER QUEBEC WILL SLOWLY TRANSLATE
EASTWARD INTO THE ST LAWRENCE RIVER AREA THROUGH THE OVERNIGHT.
RIDGING WILL STILL MAINTAIN DRY WEATHER FOR THE BALANCE OF THE
EVENING/OVERNIGHT. HOWEVER INCREASING MID TO HIGH LEVEL MOISTURE
SPREADS EAST FROM A DISTANT CLIPPER LOW PRESSURE SYSTEM EXPECTED TO
MEANDER SOUTHEAST THROUGH THE GT LAKES REGION. SO WE WILL SEE CLOUD
COVER ON THE INCREASE AGAIN FOR THURS NIGHT. AS WINDS TURN ONSHORE
DURING THE OVERNIGHT, THERE IS SOME INDICATION FOR SHALLOW MARINE
LAYER OF LOWER LEVEL RH TO ADVECT BACK LANDWARD TOWARDS EAST COASTAL
SOUTHERN NEW ENGLAND. THESE SHOULD JUST LEAD TO A LOW CLOUD LAYER
INTO COASTAL MA BUT DOESN'T APPEAR CAPABLE OF GENERATING ANY PRECIP.
LOWS IN LOW TO MID 20S IN THE INTERIOR, MID TO UPPER 20S FOR THE
RI/MA COASTAL PLAIN, AROUND FREEZING NEAR THE IMMEDIATE COAST, AND
MID 30S OVER THE MID- TO OUTER CAPE.
LONG TERM /FRIDAY THROUGH WEDNESDAY/
KEY MESSAGES:
*LIGHT SNOW ACCUMULATIONS ARE POSSIBLE LATE FRIDAY NIGHT INTO
SATURDAY, BUT UNCERTAINTY REMAINS HIGH
*ARCTIC AIRMASS MOVES IN FOR THE WEEKEND WITH HIGHS BELOW FREEZING
AND LOWS IN THE SINGLE DIGITS AWAY FROM THE COASTS
*NEAR TO WARMER NORMAL TEMPS FAVORED HEADING INTO CHRISTMAS WEEK
WITH LOW CHANCES PRECIP MID-WEEK.
A SHORTWAVE TROUGH WILL INTENSIFY AS IT EXITS THE OHIO RIVER VALLEY
FRIDAY INTO SATURDAY. THE MAJORITY OF THE SHORTWAVE ENERGY WILL PASS
WELL TO THE SOUTH OF THE REGION. THIS ALLOWS A SURFACE LOW TO
INTENSIFY AS IT PASSES OFFSHORE. HOW FAR OFFSHORE THE LOW PASSES
REMAINS UNCERTAIN AT THIS TIME. THIS WILL BE VERY IMPORTANT WITH
HOW MUCH PRECIPITATION/SNOW FALLS IN EASTERN MA. THE LATEST
DETERMINISTIC GLOBAL GUIDANCE HAS TRENDED THE LOW FURTHER WEST OVER
THE LAST 24 HOURS, BUT CONFIDENCE REMAINS LOW. ANOTHER ASPECT OF
THIS SYSTEM IS THAT IT DRAWS DOWN COLD ARCTIC AIR OVER WARM WATERS
IN THE UPPER 40S TO LOW 50S. THIS COULD LEAD TO OCEAN-EFFECT SNOW
SHOWERS ACROSS EASTERN MA. AS FOR SNOW TOTALS, DETERMINISTIC AND
ENSEMBLE MEAN GUIDANCE ONLY SHOWS A COATING TO 2 INCHES. THE BUST
FACTOR FOR THIS FORECAST WOULD BE IF THE SURFACE LOW TRENDS FURTHER
WEST. LOOKING AT THE 95 PERCENTILE NBM AND LREF, A FURTHER WEST
TRACK COULD BRING AS MUCH AS 3-6 INCHES OF SNOW TO EASTERN MA.
AS FOR TEMPERATURES ON FRIDAY, THEY WILL LIKELY BE HOVERING AROUND
FREEZING, EXCEPT NEAR THE COASTS, WHERE HIGHS SHOULD BE NEAR 40F.
WITH A STRONG NNE WIND, TEMPERATURES WILL LIKELY DROP BELOW FREEZING
BY MIDNIGHT ON FRIDAY EVENING. ANY PRECIP THAT DOES FALL FRIDAY
AFTERNOON WILL START AS A RAIN-SNOW MIX BEFORE TURNING OVER TO ALL
SNOW SOMETIME OVERNIGHT.
THE CENTER OF THE SURFACE LOW SHOULD BE TO OUR NORTHEAST BY SATURDAY
MORNING, LEAVING ONLY LINGERING OCEAN-EFFECT SNOW SHOWERS. WINDS
TURN MORE NNW BY SATURDAY AFTERNOON, PUSHING OCEAN-EFFECT SNOW
OFFSHORE AND USHERING IN AN ARCTIC AIRMASS TO THE REGION. HIGH
TEMPERATURES SATURDAY THROUGH MONDAY WILL STRUGGLE TO EVEN THE TOP
30. LOWS DURING THIS TIME PERIOD WILL BE DROPPING INTO THE SINGLE
DIGITS AWAY FROM THE COASTS.
LOOKING AHEAD INTO THE MIDDLE OF NEXT WEEK, NEAR TO ABOVE-NORMAL
TEMPERATURES ARE FAVORED, WITH HIGHS IN THE MIDDLE TO UPPER 30S.
PRECIP CHANCES REMAIN UNCERTAIN, BUT SOME GUIDANCE SOURCES HINT AT A
SLOW-MOVING SYSTEM MOVING THROUGH IN THE TUESDAY-THURSDAY TIMEFRAME.
AVIATION /03Z THURSDAY THROUGH MONDAY/
FORECAST CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MODERATE - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
00Z TAF UPDATE:
TONIGHT: MODERATE CONFIDENCE
RAIN IS STARTING TO REACH THE GROUND IN WESTERN MA AND CT AND
WILL CONTINUE TO SPREAD EAST THROUGH THE EVENING. MVFR CIGS ARE
LAGGING SLIGHTLY BEHIND AS THE LOW LEVEL DRY AIR WILL TAKE TIME
TO SATURATE. STILL THINKING IFR IS LIKELY LATER TONIGHT CLOSER
TO MIDNIGHT, BUT LIFR SHOULD STAY CONFINED TO THE CAPE AND
ISLANDS OVERNIGHT. RAIN COULD SWITCH TO SNOW ACROSS THE HIGH
TERRAIN OVERNIGHT.
WINDS TURN FROM SOUTH TO NORTH TONIGHT BUT STAY UNDER 10 KNOTS
TONIGHT EXCEPT FOR THE CAPE AND ISLANDS. THERE COULD BE SOME
WIND SHEAR OVER THE CAPE AND ISLAND THIS EVENING AS A 50 KNOT
LLJ QUICKLY MOVES THROUGH. WIND SHEAR CONCERNS SHOULD END AROUND
MIDNIGHT AS THE LLJ MOVES EAST.
THURSDAY: HIGH CONFIDENCE.
CONDITIONS IMPROVE TO VFR EARLY, IF THEY HAVEN'T DONE SO BEFORE
12Z THURS. NW WINDS AROUND 10 KT, UP TO 15 KT FOR THE CAPE.
THURSDAY NIGHT: MODERATE CONFIDENCE.
VFR EARLY AND MOST AREAS SHOULD STAY IN THE VFR RANGE. HAVE TO
WATCH EASTERN AND SOUTHEAST MA FOR POSSIBLE SUB-VFR CEILINGS
AFTER 06Z FRI. N WINDS AROUND 5-10 KT, THOUGH WILL DECREASE AND
SHIFT TO NE OVERNIGHT.
KBOS TAF...HIGH CONFIDENCE
LOWERING CIGS TO MVFR-IFR AFTER 02Z AS RAIN DEVELOPS. IMPROVING
TO VFR BEFORE 12Z.
KBDL TAF...MODERATE CONFIDENCE
LOWERING CIGS TO MVFR-IFR AFTER WITH RAIN THIS EVENING.
IMPROVING TO VFR AROUND 09Z.
OUTLOOK /FRIDAY THROUGH MONDAY/...
FRIDAY: MVFR/IFR CONDITIONS POSSIBLE. BREEZY. CHANCE SN, CHANCE
RA, SLIGHT CHANCE FZRA.
FRIDAY NIGHT: MVFR/IFR CONDITIONS POSSIBLE. BREEZY. CHANCE SN.
SATURDAY: MAINLY VFR, WITH LOCAL MVFR POSSIBLE. BREEZY. CHANCE
SN.
SATURDAY NIGHT: VFR. BREEZY. SLIGHT CHANCE SN.
SUNDAY THROUGH SUNDAY NIGHT: VFR. BREEZY.
MONDAY: VFR.
MARINE
FORECASTER CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MEDIUM - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
940 PM UPDATE:
WE EXPANDED SCA FOR NEARSHORE SOUTH COASTAL WATERS FOR A BRIEF
PERIOD OF 25+ KT SW GUSTS ASSOCD WITH THE LOW LEVEL JET. WINDS
WILL BE RAMPING UP SOON, THEN SHIFT TO NW WITH DIMINISHING
GUSTS AFTER MIDNIGHT.
PREVIOUS DISCUSSION...
KEY MESSAGES:
* SMALL CRAFT ADVISORY CONTINUES IN EFFECT LATE TONIGHT THROUGH
THURSDAY FOR THE OUTER OCEAN WATERS OFF SOUTHERN NEW ENGLAND.
TONIGHT INTO THURSDAY: HIGH CONFIDENCE.
SW WINDS 15-20 KT WITH GUSTS TO 25 KT DEVELOPING OVER THE
SOUTHERN OUTER WATERS TONIGHT, WITH BUILDING SEAS TO 4 TO 6 FT
OVER THE SOUTHERN WATERS. FOR THE EASTERN WATERS, WINDS SHIFT TO
E/NE AROUND 10-15 KT TONIGHT, THEN BECOME NW AND INCREASE TO
15-20 KT THROUGH THURS. WIDESPREAD RAINS DEVELOP OVER THE WATERS
TONIGHT INTO EARLY THURS AM WHICH COULD REDUCE VISBYS TO 3
MILES, TRENDING TO DRY FOR THURS.
THURSDAY NIGHT: HIGH CONFIDENCE.
NW WINDS LIGHTEN EARLY THURS NIGHT AND THEN SHIFT TO NE AROUND
10-15 KT. SEAS 2-4 FT.
OUTLOOK /FRIDAY THROUGH MONDAY/...
FRIDAY: WINDS LESS THAN 25 KT. SEAS UP TO 5 FT. RAIN.
FRIDAY NIGHT: MODERATE RISK FOR SMALL CRAFT ADVISORY WINDS WITH
GUSTS UP TO 25 KT. ROUGH SEAS UP TO 10 FT. RAIN, SNOW LIKELY.
LOCAL VISIBILITY 1 TO 3 NM.
SATURDAY: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH GUSTS UP
TO 25 KT. AREAS OF ROUGH SEAS. SNOW LIKELY, CHANCE OF RAIN.
VISIBILITY 1 TO 3 NM.
SATURDAY NIGHT: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH
GUSTS UP TO 20 KT. AREAS OF ROUGH SEAS. CHANCE OF SNOW. AREAS
OF VISIBILITY 1 TO 3 NM.
SUNDAY: WINDS LESS THAN 25 KT. LOCAL ROUGH SEAS. CHANCE OF
SNOW. LOCAL VISIBILITY 1 TO 3 NM.
SUNDAY NIGHT: WINDS LESS THAN 25 KT. AREAS OF SEAS APPROACHING
5 FT. CHANCE OF SNOW.
MONDAY: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING 5 FT.
BOX WATCHES/WARNINGS/ADVISORIES
CT...NONE.
MA...NONE.
RI...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 3 AM EST THURSDAY FOR ANZ232.
SMALL CRAFT ADVISORY UNTIL 1 AM EST THURSDAY FOR ANZ233>235-
237.
SMALL CRAFT ADVISORY UNTIL 7 PM EST THURSDAY FOR ANZ250-254.
SMALL CRAFT ADVISORY UNTIL 7 PM EST THURSDAY FOR ANZ255-256.
SYNOPSIS...LOCONTO/KP
NEAR TERM...KJC/LOCONTO/KP
SHORT TERM...LOCONTO
LONG TERM...KP
AVIATION...LOCONTO/KP
MARINE...KJC/LOCONTO/KP
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