182
FXUS61 KBOX 200547
AFDBOX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOSTON/NORTON MA
1247 AM EST FRI DEC 20 2024
SYNOPSIS
AN OFFSHORE COASTAL STORM IS LIKELY TO BRING LIGHT SNOW
ACCUMULATIONS LATE FRIDAY 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.
NEAR TERM /UNTIL 6 AM FRIDAY MORNING/
9 PM UPDATE...
* DRY AND QUIET TONIGHT WITH SEASONABLY COLD TEMPERATURES
HIGH PRES TO THE NORTH AND LOW PRES OFF THE CAROLINAS WILL
RESULT IN EASTERLY FLOW DEVELOPING OVER THE OCEAN. WHILE THERE
WILL BE CONSIDERABLE HIGH CLOUDINESS, INCREASING LOW LEVEL
MOISTURE WILL LEAD TO SOME LOWER CLOUDS DEVELOPING LATE TONIGHT
ACROSS EASTERN MA. BUT IT WILL REMAIN DRY THROUGH TONIGHT. LOWS
WILL BE GENERALLY IN THE MID-UPPER 20S, EXCEPT FOR THE CAPE AND
ISLANDS WHERE LOW 30S ARE MORE LIKELY THANKS TO THE MODERATING
INFLUENCE OF THE OCEAN.
SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/
KEY MESSAGES...
* WIDELY SCATTERED OCEAN EFFECT SHOWERS ARE POSSIBLE DURING THE DAY
ON FRIDAY WITH MORE LIGHT SNOW OVER THE INTERIOR IN THE
EVENING/EARLY OVERNIGHT
AS EARLY AS SUNRISE ON FRIDAY WE SHOULD BEGIN TO SEE WIDELY
SCATTERED OCEAN EFFECT SNOW (INTERIOR) AND RAIN (IMMEDIATE
COAST/SOUTHEAST MA). THIS, THANKS TO A SURFACE LOW LIFTING NORTH
FROM THE MID ATLANTIC AND DEEPENING AS IT PASSES SOUTHEAST OF THE
BENCHMARK FRIDAY NIGHT. THE TRAJECTORY WILL KEEP THE BULK OF THE
MOISTURE AND FORCING (INCLUDING THE DEEP MID LEVEL TROUGH) WELL
SOUTH OF OUR REGION. THUS, NOT EXPECTING MUCH IN THE WAY OF IMPACTS
FROM THIS WINTRY SYSTEM, MORE OF A FESTIVE PRE-CHRISTMAS SNOWFALL
THAT WILL DROP ANYWHERE FROM A COATING TO AN INCH OR TWO FROM FRIDAY
INTO SATURDAY AFTERNOON. NEW GUIDANCE TODAY CONTINUES TO KEEP MOST
OF THE QPF OFFSHORE IN CLOSER PROXIMITY TO THE LOW/FORCING. ENSEMBLE
CLUSTERING RETAINS SOME SPREAD IN THE SPEED OF THE LOW, BUT SHOWS
HIGHER CONFIDENCE OF A WELL OFFSHORE TRACK. HIGH PRESSURE TO OUR
NORTH WILL HELP TO KEEP TEMPERATURES DOWN, HIGHS LIMITED TO THE MID
30S TO LOW 40S (HENCE, THE STRUGGLE TO ACCUMULATE SNOW FOR MANY
LOCATIONS). AS THE TROUGH BEGINS TO MOVE OVERHEAD THIS LARGER SCALE
FORCING WILL BRING MORE WIDESPREAD LIGHT SNOW TO THE INTERIOR
OVERNIGHT, BUT AGAIN, LACK OF MOISTURE WILL BE A BIG LIMITING FACTOR
TO US GETTING ANY MORE THAN A COATING TO AN INCH.
LONG TERM /SATURDAY THROUGH THURSDAY/
KEY MESSAGES
* DRY THIS WEEKEND BUT WITH VERY COLD TEMPERATURES
* TEMPERATURES TRENDING BACK TOWARD NORMAL EARLY NEXT WEEK
* MODEST RAIN/SNOW EVENT POSSIBLE FOR CHRISTMAS EVE/CHRISTMAS DAY
SATURDAY THROUGH MONDAY NIGHT
A FAIRLY DEEP LOW-PRESSURE SYSTEM OFF THE EAST COAST OF SOUTHERN NEW
ENGLAND WILL CONTINUE TO DEPART EAST ON SATURDAY. A FEW LINGERING
SNOW SHOWERS MAY BE POSSIBLE DURING THE DAY IN A COLD NORTHWEST
FLOW, BUT GENERALLY EXPECTING DRY/CHILLY CONDITIONS. DEEP NORTHWEST
FLOW BEHIND THE DEPARTING LOW WILL ADVECT A POLAR AIR MASS OVER THE
REGION ON SATURDAY WITH 925 HPA TEMPS DIPPING TO -7 TO -10 CELSIUS
BY SATURDAY EVENING. THIS WILL SUPPORT CHILLY TEMPERATURES SATURDAY
THROUGH MONDAY WITH HIGH TEMPERATURES IN THE AFTERNOON BELOW THE
FREEZING MARK FOR MOST LOCATIONS IN SOUTHERN NEW ENGLAND. LOW TEMPS
WILL ALSO BE SUBSTANTIALLY BELOW NORMAL SATURDAY NIGHT THROUGH
MONDAY NIGHT WITH INTERIOR LOCATIONS FALLING TO SINGLE DIGIT
TEMPERATURES. ELSEWHERE, LOW TEMPS IN THE 10 TO 20 DEGREE RANGE WITH
THE MILDER TEMPERATURES FOCUSED NEAR THE IMMEDIATE COAST. PEAK OF
THE COLD WILL BE MONDAY MORNING WHEN SOME LOCATIONS IN WESTERN MA
MAY FLIRT WITH THE 0F MARK. THIS TIME PERIOD WILL BE DOMINATED BY
SURFACE HIGH PRESSURE, SO EXPECT DRY CONDITIONS THROUGH MONDAY WITH
THE EXCEPTION OF PERHAPS A FEW SNOW SHOWERS ON SATURDAY.
TUESDAY NIGHT THROUGH THURSDAY
AS HIGH PRESSURE SHIFTS EAST EARLY TUESDAY, SHORT-WAVE ENERGY DIGS
DOWN FROM THE GREAT LAKES, SUPPORTING A MODEST PRECIPITATION EVENT
ON CHRISTMAS EVE. THE COLUMN SHOULD BE COLD ENOUGH TO SUPPORT
SNOWFALL ACROSS THE INTERIOR, BUT RETURN FLOW FROM THE SOUTH WILL
RESULT IN SOME TEMPERATURES ABOVE FREEZING ACROSS SOUTHEASTERN MA
AND RI WHERE RAIN WILL BE MORE LIKELY. STILL SOME DETAILS TO IRON
OUT, BUT IF WE CONSULT ENSEMBLE GUIDANCE WE FIND THAT THE MODEL
EXCEEDANCE PROBABILITIES FOR GREATER THAN 1 INCH OF SNOW ON
CHRISTMAS EVE RANGES FROM 20 TO 40 PERCENT FOR LOCATIONS NORTH OF
CT/RI. PROBABILITES DROP TO 10 TO 20 PERCENT FOR RI AND SOUTHEASTERN
MA. THERE IS AGREEMENT AMONG MODEL GUIDANCE FOR THIS SYSTEM TO BE
PROGRESSIVE AND TO BE FOLLOWED BY SURFACE HIGH PRESSURE WHICH SHOULD
SUPPORT A QUIET CHRISTMAS DAY AND END OF NEXT WEEK.
AVIATION /06Z FRIDAY THROUGH TUESDAY/
FORECAST CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MODERATE - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
THROUGH 12Z...
LOWERING VFR CIGS, WITH AREAS OF MVFR DEVELOPING ACROSS PORTIONS
OF EASTERN AND NE MA TOWARD 12Z, WITH A LOW RISK FOR LIGHT SN.
TODAY...MODERATE CONFIDENCE.
WIDESPREAD MVFR CIGS EXPANDING ACROSS SNE DURING THE MORNING.
AREAS OF LIGHT SNOW DEVELOPING ACROSS CENTRAL AND EASTERN MA,
EXPANDING WESTWARD DURING THE AFTERNOON WITH AREAS OF IFR VSBYS.
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.
KBOS TAF...MODERATE CONFIDENCE. CIGS LOWERING TO MVFR WITH
PERIODS OF LIGHT SNOW DEVELOPING WHICH MAY REDUCE VSBYS BELOW 3
MILES AT TIMES, ESPECIALLY DURING THE AFTERNOON AND EVENING.
SNOW COULD BE MIXED WITH RAIN INITIALLY. NNE GUSTS TO 20-25 KT
DEVELOPING LATE AFTERNOON AND EVENING.
KBDL TAF...MODERATE CONFIDENCE. CIGS LOWERING TO MVFR WITH
SPOTTY LIGHT SNOW IN THE AFTERNOON AND EVENING.
OUTLOOK /SATURDAY NIGHT THROUGH TUESDAY/...
SATURDAY NIGHT THROUGH SUNDAY NIGHT: VFR. BREEZY.
MONDAY: VFR.
MONDAY NIGHT: VFR. SLIGHT CHANCE SN.
TUESDAY: MAINLY VFR, WITH AREAS MVFR POSSIBLE. CHANCE SN.
MARINE
FORECASTER CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MEDIUM - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
KEY MESSAGES:
* SMALL CRAFT ADVISORY ISSUED FOR FRIDAY AFTERNOON THROUGH
FRIDAY NIGHT.
TONIGHT: HIGH CONFIDENCE.
NW WINDS LIGHTEN AND THEN SHIFT TO NE AROUND 10-15 KT. SEAS
BETWEEN 2 AND 4 FEET.
FRIDAY: 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. WILL
LIKELY NEED TO REISSUE A SMALL CRAFT ADVISORY FOR THESE
CONDITIONS.
FRIDAY NIGHT: MODERATE CONFIDENCE.
NE WINDS 25-30 KTS WITH SEAS 6-9 FEET.
OUTLOOK /SATURDAY THROUGH TUESDAY/...
SATURDAY: MODERATE RISK FOR SMALL CRAFT ADVISORY WINDS WITH
GUSTS UP TO 25 KT. ROUGH SEAS UP TO 10 FT. CHANCE OF SNOW.
SATURDAY NIGHT: MODERATE RISK FOR SMALL CRAFT ADVISORY WINDS
WITH GUSTS UP TO 25 KT. AREAS OF ROUGH SEAS. CHANCE OF SNOW.
SUNDAY: WINDS LESS THAN 25 KT. LOCAL ROUGH SEAS. CHANCE OF
SNOW.
SUNDAY NIGHT: WINDS LESS THAN 25 KT. AREAS OF SEAS APPROACHING
5 FT. SLIGHT CHANCE OF SNOW.
MONDAY: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING 5 FT.
MONDAY NIGHT: WINDS LESS THAN 25 KT.
TUESDAY: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING 5 FT.
CHANCE OF RAIN, CHANCE OF SNOW.
BOX WATCHES/WARNINGS/ADVISORIES
CT...NONE.
MA...NONE.
RI...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 4 PM FRIDAY TO 7 AM EST SATURDAY FOR
ANZ231>235-237-251.
SMALL CRAFT ADVISORY FROM 1 PM FRIDAY TO 7 AM EST SATURDAY FOR
ANZ250-254>256.
SYNOPSIS...BW/RM
NEAR TERM...KJC/BW
SHORT TERM...BW
LONG TERM...RM
AVIATION...KJC/RM
MARINE...BW/RM
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