709
FXUS61 KBOX 201751
AFDBOX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOSTON/NORTON MA
1251 PM EST FRI DEC 20 2024
SYNOPSIS
AN OFFSHORE COASTAL STORM IS LIKELY TO BRING LIGHT SNOW
ACCUMULATIONS LATER TODAY INTO SATURDAY FOLLOWED BY VERY COLD
WEATHER INTO EARLY NEXT WEEK. THE WEATHER PATTERN BECOMES
UNSETTLED AGAIN AS WE HEAD TOWARD CHRISTMAS DAY BUT TEMPERATURES
WILL BE MODERATING THROUGH THE MIDDLE AND END OF NEXT WEEK.
NEAR TERM /THROUGH TONIGHT/
10:30 AM UPDATE...
THE MAIN CHANGE WITH THIS MID MORNING UPDATE HAS BEEN TO
INCREASE SNOWFALL TOTALS A BIT, ESPECIALLY IN NORTH
CENTRAL/NORTHEAST MA WHERE WE'RE LOOKING AT ANYWHERE FROM 2 TO 4
INCHES POSSIBLE. THE IMPETUS HAS BEEN RECENT RADAR AND
OBSERVATIONAL TRENDS AS WELL AS THE NEW 12Z HI-RES GUIDANCE.
WE'RE DEALING WITH A COASTAL FRONT OBVIOUS IN THE SURFACE OBS
DRAPED ALONG THE I-95 CORRIDOR. NW OF THE FRONT PLACES LIKE
WORCESTER ARE IN THE UPPER 20S WHILE SE OF THE BOUNDARY IN
TAUNTON TEMPS ARE IN THE UPPER 30S. HI-RES GUIDANCE IS KEYING IN
ON SOME CONVERGENCE ENHANCEMENT NEAR/NW OR THIS FRONT MAXIMIZING
QPF (BOTH RAIN AND SNOW, WHERE TEMPS ARE COLD ENOUGH TO
ACCUMULATE). THIS ENHANCED QPF IS MAXIMIZED OVER EASTERN AND
NORTHEASTERN MA. THE QUESTION WILL BE HOW MUCH SNOW CAN
ACCUMULATE GIVEN WHERE TEMPERATURES ARE JUST ABOVE FREEZING.
EVEN WHERE TEMPS ARE JUST ABOVE FREEZING LIKE BOS/OWD DEWPOINTS
ARE SEVERAL DEGREES LOWER LEAVING ROOM FOR DYNAMIC COOLING IF
PRECIPITATION INTENSITY IS ENOUGH. 12Z HREF GUIDANCE DOES
INDICATE 30-40% PROBABILITIES OF 1 INCH/HOUR SNOWFALL RATES
AFTER SUNSET WHEN MODEL SOUNDINGS SHOW GOOD FORCING IN THE
DENDRITIC GROWTH ZONE. ALL THAT TOGETHER WOULD LEAD US TO
BELIEVE THAT THE MOST LIKELY OUTCOME WILL BE ANYWHERE FROM AN
INCH TO 4 INCHES, WITH THE HIGHEST AMOUNTS IN EASTERN AND
NORTHEAST MA. A WINTER WEATHER ADVISORY HAS BEEN ISSUED FOR
AREAS MOST LIKELY TO RECEIVE MORE THAN 3 INCHES. WHILE THIS
EVEN ISN'T REMARKABLE FROM A SNOWFALL AMOUNT PERSPECTIVE, IT
WILL BE ONGOING DURING THE FRIDAY EVENING COMMUTE AHEAD OF A
HOLIDAY WEEK, SO IMPACTS WILL BE AMPLIFIED...TAKE IT SLOW!
PREVIOUS DISCUSSION...
KEY MESSAGES:
* COASTAL LOW PRESSURE SYSTEM BRINGS LIGHT RAIN AND SNOW
SHOWERS TO THE REGION WITH MINOR SNOWFALL ACCUMULATIONS.
NO MAJOR SHIFTS IN THE FORECAST FOR TODAY, A MID-LEVEL TROUGH
CROSSES THE NORTHEAST LATE TODAY AND THROUGH TONIGHT. COMING
AROUND THE TROUGH, A ROBUST SHORTWAVE ENERGY, WITH THE BEST
FORCING OVER THE SOUTHEAST WATERS. THE 20/00Z MODEL GUIDANCE HAS
CONSENSUS THE CENTER OF THE LOW REMAINS OFF SHORE AND IS
SOUTH/EAST OF THE BENCHMARK. A WRINKLE THAT IS DIFFICULT TO IRON
OUT, THE QPF, WHICH IF THIS WERE A WARM SEASON EVENT WOULD NOT
BE AS PROBLEMATIC. HIGHEST QPF IS OVER FAR SOUTHEAST
MASSACHUSETTS; CAPE COD, AND THE NEIGHBORING ISLANDS, QPF AROUND
A QUARTER TO A THIRD OF AN INCH, LOCALIZED HIGHER AMOUNTS OF
HALF OF AN INCH NEAR NANTUCKET. FURTHER NORTHWEST VALUES LOWER
CONSIDERABLY, WHILE THE CONNECTICUT RIVER VALLEY ONLY RECEIVES A
FEW HUNDREDTHS OF AN INCH. WHILE THIS DOESN'T SEEM PROBLEMATIC,
IT DOES BECOME AN ISSUE WHEN IT COMES TO SNOW AS IT CAN
DRASTICALLY CHANGE SNOW TOTALS. ANY SHIFT IN LATER GUIDANCE TO
THE NORTH/WEST MAY LEND TO MORE QPF AND POSSIBLY HIGHER
SNOWFALL, WHILE A SHIFT FURTHER OFFSHORE LENDS TO LESS QPF AND
LESS SNOW.
FOR TODAY INTO TONIGHT PERIODS OF LIGHT RAIN AND SNOW, FOLLOWED
BY A CHANGE TO ALL SNOW PERHAPS LATE THIS AFTERNOON. THE SNOW
ACTIVITY WANES OVERNIGHT INTO EARLY SATURDAY MORNING. IN TERMS
OF HOW MUCH SNOW, NO BIG CHANGES FROM THE PREVIOUS FORECAST,
ACCUMULATIONS ARE MAINLY FOUND IN THE EASTERN HALF OF SOUTHERN
NEW ENGLAND, 1-3" WITH LOCALIZED 4" ACROSS INTERIOR NORTHEAST
MASSACHUSETTS AS THERE IS AN AREA LOW LEVEL FRONTOGENESIS FROM A
COASTAL BOUNDARY/FRONT. THINKING THIS AREA WOULD BE FOCUSED
THIS AFTERNOON FOR INTERIOR ESSEX COUNTY AND PORTIONS OF
MIDDLESEX COUNTY. AS FOR THE WESTERN HALF OF THE CWA WITH
LACKING QPF TOWARDS THE CONNECTICUT RIVER VALLEY, SNOW
ACCUMULATIONS WILL BE LIMITED TO A DUSTING TO PERHAPS A FEW
TENTHS OF AN INCH. ONE FORECAST CHALLENGE, WHETHER OR NOT WE ARE
ABLE TO FLIP THE RAIN TO SNOW EARLIER ON IN THE AFTERNOON FOR
CAPE COD AND THE NEIGHBORING ISLANDS DUE TO THE HIGHER RAINFALL
RATES, WHICH CAN HELP TO COOL THE SURROUNDING AIR AND LOWER THE
TEMPERATURES AND ALLOW FOR SNOW.
HIGHS THIS AFTERNOON AREA LARGELY IN THE LOW AND MIDDLE 30S,
WITH UPPER 30S TO 40F AREAS EAST OF NEWPORT TO PLYMOUTH.
TONIGHT, THE LOWS FALL WELL BELOW FREEZING INTO THE 20S.
NORTH/NORTHEAST WINDS TODAY WILL FLIP TO THE NORTH OVERNIGHT.
SHORT TERM /SATURDAY/
UPDATED: 3:50 AM
KEY MESSAGES:
* SCT'D SNOW SHOWERS, CLOUDY, AND COLD! AFTERNOON HIGHS
STRUGGLE TO GET TO ABOVE FREEZING AND WITH GUSTY NORTHWEST
WINDS, THE AFTERNOON APPARENT TEMPERATURE WILL BE IN THE LOW
TO MIDDLE TEENS.
SCATTERED SNOW SHOWERS, ALBEIT 'FESTIVE' CONTINUES WITH LITTLE
TO NO ADDITIONAL ACCUMULATION AS THE LOW PRESSURE SYSTEM EXITS
THE REGION. OTHERWISE THE DAY IS CLOUDY AND COLD WITH INCREASING
WINDS FROM THE NORTHWEST, GUSTING TO 25 MPH. BECAUSE OF GOOD
MIXING IN THE BOUNDARY LAYER UP TO 875MB, SHOULD TAP INTO THE
COLDER TEMPERATURES, BETWEEN - 12C TO -15C, THUS HIGHS RANGE
FROM THE UPPER 20S AND LOW 30S WITH APPARENT TEMPERATURE WILL BE
IN THE LOW TO MIDDLE TEENS. KICKING OFF THE FIRST OF SEVERAL
DAYS, FOR MOST, WITH HIGH TEMPERATURES BELOW THE FREEZING MARK.
LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/
UPDATED: 3:50 AM
KEY MESSAGES...
* COLDEST AIR OF THE SEASON SAT NIGHT INTO MONDAY WITH DRY WEATHER
* MODERATING TEMPS TUE THROUGH THU RECOVERING TO NEAR SEASONABLE
NORMALS
* SOME LIGHT SNOW POSSIBLE CHRISTMAS EVE WITH MAINLY DRY WEATHER
CHRISTMAS DAY AND THU BUT LOW CONFIDENCE
SATURDAY NIGHT THROUGH MONDAY...
COLDEST AIR OF THE SEASON WILL DESCEND ON SNE DURING THE SAT NIGHT
THROUGH MONDAY PERIOD WITH DRY CONDITIONS AS STRONG CANADIAN HIGH
PRES BUILDS INTO THE REGION. THE ONLY EXCEPTION WILL BE THE
POTENTIAL FOR SOME OCEAN EFFECT FLURRIES OR SNOW SHOWERS ON THE
OUTER CAPE SAT NIGHT INTO SUN NIGHT AS VERY COLD AIRMASS MOVES OVER
WARMER SST. INITIALLY THE COLD WILL BE ACCOMPANIED BY GUSTY NW WINDS
SAT NIGHT AND SUNDAY WHICH WILL HAVE AN IMPACT ON WIND CHILLS, THEN
WINDS DROP OFF SUNDAY NIGHT AND MON AS THE HIGH PRES MOVES OVERHEAD.
THE CORE OF THE COLDEST AIR WILL BE SUNDAY AS 850 MB TEMPS DROP TO
-15 TO -16C.
LOOKING AT HIGHS ONLY REACHING UPPER TEENS TO MID 20S SUNDAY, THEN
MID 20S TO NEAR 30 MON. LOWS SAT NIGHT WILL DROP INTO THE SINGLE
NUMBERS AND TEENS BUT WIND CHILLS WILL BOTTOM OUT TO ZERO TO -10
INTERIOR AND ZERO TO +5 NEAR THE COAST BY EARLY SUNDAY. THE COLDEST
TEMPS WILL OCCUR SUN NIGHT AS THE HIGH MOVES IN FROM THE WEST WITH
RADIATIONAL COOLING BRINGING LOW TEMPS INTO THE SINGLE NUMBERS
(POSSIBLY NEAR ZERO NW MA) AND TEENS ALONG IMMEDIATE COAST.
TUESDAY THROUGH THURSDAY...
THE PATTERN BECOMES SOMEWHAT UNSETTLED BY TUE AS SHORTWAVE ENERGY
APPROACHES FROM THE GT LAKES. IT APPEARS WE MAY BE DEALING WITH SOME
SORT OF CLIPPER SYSTEM BRINGING A LIGHT SNOWFALL WHICH COULD
TRANSITION TO RAIN ALONG THE SOUTH COAST IF THE SYSTEM TRACKS TO THE
NORTH ALLOWING FOR A SOUTHERLY WIND TO DEVELOP. SPECIFIC DETAILS
STILL NEED TO BE WORKED OUT BUT THIS WILL NOT BE A SIGNIFICANT
SYSTEM. GEFS AND EPS ENSEMBLES SHOWING LOW TO MODERATE PROBS OF 1"+
SNOWFALL FOCUSED IN THE INTERIOR. TIMING IS ALSO UNCERTAIN AND SNOW
COULD DEVELOP AS EARLY AS LATER MON NIGHT. DETERMINISTIC GLOBAL
GUIDANCE FAVORS A MOSTLY DRY PERIOD CHRISTMAS DAY AND THU, BUT THERE
IS A LOT OF SPREAD IN THE 500 MB PATTERN AND ENSEMBLE DATA HAS
MODERATE TO HIGH PROBS FOR MEASURABLE PRECIP BOTH WED AND THU SO
FUTURE FORECAST ADJUSTMENTS WILL LIKELY BE NEEDED. TEMPS WILL BE
MODERATING THROUGH THE MIDDLE AND END OF THE WEEK, EVENTUALLY
GETTING BACK TO NEAR NORMAL BY WED AND THU.
AVIATION /18Z FRIDAY THROUGH TUESDAY/
FORECAST CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MODERATE - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
THIS AFTERNOON...MODERATE CONFIDENCE.
WIDESPREAD MVFR CIGS ACROSS SNE WITH POCKETS OF IFR IN HEAVIER
SN. RAIN OR MIXED RAIN/SNOW NEAR THE COAST, CHANGING TO SNOW
TOWARD EVENING. MINOR ACCUMS LIKELY WEST OF I-95 AND 128 IN MA.
NE WIND 8-15 KT WITH 20+ KT GUSTS DEVELOPING NEAR THE COAST
TOWARD EVENING.
TONIGHT...MODERATE CONFIDENCE.
MVFR CONDITIONS WITH AREAS OF IFR SE MA AND CAPE/ISLANDS. SNOW
BECOMING MORE FOCUSED ACROSS EASTERN MA DURING THE EVENING, THEN
GRADUALLY MOVING OFFSHORE TOWARD 12Z WITH SOME IMPROVEMENT TO
VFR AWAY FROM SE NEW ENG COAST. N-NE WIND BECOMING N 10-20 KT
WITH 25-30 KT GUSTS ALONG EASTERN MA COAST AND CAPE/ISLANDS.
SATURDAY...MODERATE CONFIDENCE.
MAINLY VFR CIGS, BUT LOWER MVFR CIGS POSSIBLE EARLY
CAPE/ISLANDS. SCATTERED SNOW SHOWERS DEVELOPING WHICH MAY
REDUCE VSBYS AT TIMES, ESPECIALLY INTERIOR. NW WIND 10-20 KT
WITH SOME HIGHER GUSTS.
SATURDAY NIGHT...MODERATE.
VFR. NW WINDS 5-10 KTS GUSTING TO 20 KTS.
KBOS TAF...LOW TO MODERATE CONFIDENCE. CONTINUED LIGHT TO
MODERATE SNOW OFF OF THE OCEAN WILL REDUCE VSBYS AT TIMES BELOW
3 MILES THROUGH THE EVENING. NNE GUSTS TO 20-25 KT.
KBDL TAF...MODERATE CONFIDENCE. CIGS LOWERING TO MVFR WITH
LIGHT TO MODERATE SNOW IN THE AFTERNOON AND EVENING WHICH MAY
REDUCE VSBYS BELOW 1 MILE AT TIMES.
OUTLOOK /SATURDAY NIGHT THROUGH TUESDAY/...
SATURDAY NIGHT THROUGH SUNDAY: VFR. BREEZY.
SUNDAY NIGHT THROUGH MONDAY: VFR.
MONDAY NIGHT: MAINLY VFR, WITH LOCAL MVFR POSSIBLE. SLIGHT
CHANCE SN.
TUESDAY: MAINLY VFR, WITH AREAS MVFR POSSIBLE. CHANCE SN,
CHANCE RA.
MARINE
FORECASTER CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MEDIUM - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
UPDATED: 3:50 AM
KEY MESSAGES:
* SMALL CRAFT ADVISORY IN EFFECT EARLY AFTERNOON THROUGH SATURDAY.
TODAY AND TONIGHT...MODERATE CONFIDENCE.
SURFACE LOW PRESSURE SYSTEM MOVES UP THE COAST AND REMAINS OUT TO
SEAS. INCREASING EAST/NORTHEAST WINDS 15 TO 20 KNOTS WITH GUSTS UP
TO 30 KNOTS ON THE OUTERMOST WATERS, NEARSHORE WATERS GUSTS ARE
CLOSER TO 25 KNOTS. SEAS BUILDING 4 TO 7 FEET FOR THE SOUTHERN
WATERS, WHILE 5 TO 9 FEET FOR THE EASTERN WATERS. PERIODS OF RAIN
AND SNOW TODAY AND TONIGHT, DIMINISHING EARLY SATURDAY MORNING.
SATURDAY...HIGH CONFIDENCE.
LOW PRESSURE SYSTEM LIFTS NORTHEAST FROM SOUTHERN NEW ENGLAND, BUT
WITH HIGH PRESSURE BUILDING TO THE WEST, IT WILL LEAD TO NORTHWEST
FLOW WITH GUSTS CONTINUING THROUGHOUT THE DAY. HAVE EXTENDED THE
SMALL CRAFT ADVISORY AS A RESULT THROUGH THE DAY. SEAS REMAIN ROUGH
3 TO 6 FEET ON THE SOUTHERN WATERS AND 5 TO 8 FEET ON THE EASTERN
WATERS.
OUTLOOK /SATURDAY NIGHT THROUGH TUESDAY/...
SATURDAY NIGHT: MODERATE RISK FOR SMALL CRAFT ADVISORY WINDS
WITH GUSTS UP TO 25 KT. AREAS OF ROUGH SEAS. SLIGHT CHANCE OF
SNOW SHOWERS. LOCAL VISIBILITY 1 TO 3 NM.
SUNDAY: WINDS LESS THAN 25 KT. LOCAL ROUGH SEAS. SLIGHT CHANCE
OF SNOW SHOWERS. LOCAL VISIBILITY 1 TO 3 NM.
SUNDAY NIGHT: WINDS LESS THAN 25 KT. AREAS OF SEAS APPROACHING
5 FT. SLIGHT CHANCE OF SNOW SHOWERS.
MONDAY: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING 5 FT.
MONDAY NIGHT: WINDS LESS THAN 25 KT. SLIGHT CHANCE OF SNOW.
TUESDAY: WINDS LESS THAN 25 KT. CHANCE OF RAIN, CHANCE OF SNOW.
BOX WATCHES/WARNINGS/ADVISORIES
CT...NONE.
MA...WINTER WEATHER ADVISORY UNTIL 4 AM EST SATURDAY FOR MAZ005>007-
012>018.
RI...WINTER WEATHER ADVISORY UNTIL 4 AM EST SATURDAY FOR RIZ001-002.
MARINE...SMALL CRAFT ADVISORY FROM 4 PM THIS AFTERNOON TO 7 PM EST
SATURDAY FOR ANZ231-232-251.
SMALL CRAFT ADVISORY FROM 4 PM THIS AFTERNOON TO 7 AM EST
SATURDAY FOR ANZ233>235-237.
SMALL CRAFT ADVISORY UNTIL 7 PM EST SATURDAY FOR ANZ250-
254>256.
SYNOPSIS...KJC/DOOLEY
NEAR TERM...BW/DOOLEY
SHORT TERM...DOOLEY
LONG TERM...KJC
AVIATION...KJC/DOOLEY
MARINE...KJC/DOOLEY
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