578
FXUS61 KBOX 182315
AFDBOX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOSTON/NORTON MA
615 PM EST SAT JAN 18 2025
SYNOPSIS
AN APPROACHING COLD FRONT WILL RESULT IN SOUTHERLY FLOW AND A PERIOD
OF RAIN SHOWERS LATE THIS AFTERNOON INTO THE EVENING. A WAVE OF LOW
PRESSURE MOVING IN LATER SUNDAY AND SUNDAY NIGHT WILL BRING
ACCUMULATING, PLOWABLE SNOWFALL TO SOUTHERN NEW ENGLAND. AN
ARCTIC AIRMASS WILL BRING FRIGID TEMPERATURES AND WIND CHILLS ON
THE ORDER OF 5 TO 15 BELOW ZERO MONDAY EVENING THROUGH THURSDAY
MORNING. TEMPERATURES BEGIN TO WARM AGAIN THURSDAY AND FRIDAY.
NEAR TERM /UNTIL 6 AM SUNDAY MORNING/
KEY MESSAGES...
* RAIN SHOWERS CONTINUE, BECOMING MORE WIDESPREAD BY EARLY EVENING.
TODAY THE PATTERN TRANSITIONS FROM MORE ZONAL FLOW BENEATH A MID
LEVEL RIDGE TO MERIDIONAL FLOW AHEAD OF AND AN APPROACHING SHORTWAVE
TROUGH AND SURFACE COLD FRONT. WHAT THIS MEANS FOR SOUTHERN NEW
ENGLAND IS TWO FOLD. ONE, WARM ADVECTION KICKS IN ON SW FLOW AHEAD
OF THE COLD FRONT SO 925 MB TEMPS RISE FROM -2C AT 7AM TO +5-7C BY
EVENING. AT THE SURFACE THIS SHOULD TRANSLATE TO HIGH TEMPERATURES
REACHING AS WARM AS THE MID TO EVEN UPPER 40S! THE WARMEST LOCATION
WILL BE SOUTHEAST NEW ENGLAND. THE SECOND RESULT OF THIS APPROACHING
SHORTWAVE/COLD FRONT WILL BE A ROUND OF WARM ADVECTION/ISENTROPIC
LIFT GENERATED PRECIPITATION, AND GIVEN THE PREVIOUSLY MENTIONED
ABOVE AVERAGE TEMPERATURES IT WILL ALL FALL AS RAIN. THE
PRECIPITATION SHOULD BEGIN TO OVERSPREAD THE REGION FROM SOUTH TO
NORTH BY 2-4PM, EXITING BY 9PM. BY THE TIME ALL IS SAID AND
DONE ANYWHERE FROM A FEW TENTHS TO A QUARTER OF AN INCH OF RAIN
ARE LIKELY. THE HIGHER AMOUNTS TOWARD A QUARTER INCH OR MORE ARE
MOST LIKELY OVER THE CAPE AND ISLANDS WHERE A ROBUST 45-50 KT
925MB JET WILL PROVIDE MORE FORCING FOR ASCENT COLOCATED WITH
THE PLUME OF HIGHEST PWATS. THE BULK OF THIS JET SHOULD BE KEPT
ALOFT OVERNIGHT THANKS TO A STOUT LOW LEVEL INVERSION.
SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/
KEY MESSAGES...
* PLOWABLE SNOW LATE SUNDAY AND THROUGH SUNDAY NIGHT.
* A WINTER STORM WARNING IS IN EFFECT FOR MUCH OF THE INTERIOR,
WHILE THE WINTER STORM WATCH HAS BEEN EXPANDED INTO SOUTHEAST
MA AND NORTHERN RI WHERE POTENTIAL FOR HIGHER SNOWFALL TOTALS
HAS INCREASED.
DETAILS...
FOCUS THEN RETURNS TO A WINTER STORM WHICH WILL PRODUCE A SWATH OF
HEAVY SNOW FROM THE NORTHERN MID-ATLANTIC TO THE NORTHEAST INCLUDING
SOUTHERN NEW ENGLAND. BEFORE WE GET TO THAT, THOUGH, THE FIRST HALF
OF THE DAY ON SUNDAY WILL BE LESS EVENTFUL, WITH QUIET WEATHER AND
LINGERING MILD AIR, EVEN AS WARM AS THE UPPER 30S AND LOW 40S IN
SOUTHEAST MA. BREAKS OF SUN ARE LIKELY IN THE MORNING BEFORE MID
CLOUDS OVERSPREAD MID MORNING THROUGH NOON.
SUNDAY AFTERNOON AN AREA OF LOW PRESSURE EMERGES OFF THE CAROLINA
COAST OUT JUST AHEAD OF A BROAD MID LEVEL TROUGH DIGGING INTO THE
EASTERN SEABOARD. THIS LOW THEN QUICKLY DEEPENS AS IT RACES UP THE
EAST COAST FROM THE CAROLINA COAST SUNDAY AFTERNOON TO THE GULF OF
MAINE BY EARLY MONDAY MORNING. THE EXACT TRACK THAT THE SYSTEM TAKES
BETWEEN THESE TWO POINTS IS SLOWLY COMING INTO FOCUS, AND WILL BE
THE GREATEST DETERMINER OF WHO SEES SNOW AND HOW MUCH.
GUIDANCE SHOWS INDICATORS OF POTENTIAL FOR A SIGNIFICANT SNOW MAKER
WITH THIS SYSTEM. FOR ONE, SNE WILL BE PLACED BENEATH THE LEFT EXIT
REGION OF A 160 KT UPPER JET AS THE LOW PASSES BY; AND WHILE THERE
REMAINS DISCREPANCIES AMONGST THE GUIDANCE AS TO HOW STRONG THE 850
MB LOW IS AS WELL AS THE TRACK, MUCH OF THE GUIDANCE INDICATES A
CLOSED OFF 850 MB LOW WITH A SWATH OF 850-700M FRONTOGENESIS TO THE
NORTHWEST OF THE LOW CENTER THAT WILL HELP TO INCREASE SNOWFALL
RATES (LIKELY JUST TO THE NORTHWEST OF WHEREVER THOSE BANDS OF FGEN
SET UP). THE HREF INDICATES A RATHER HIGH PROBABILITY OF GREATER
THAN 1"/HR SNOWFALL RATES IN THE INTERIOR (AS HIGH AS 50-60%). THIS,
TOGETHER WITH THE SURGING IN OF COLDER AIR THROUGHOUT THE EVENT,
STEADILY RAISING SNOW-TO-LIQUID RATIOS EVENTUALLY ABOVE 10:1
(ESPECIALLY IN THE INTERIOR) AND A GOOD LOW END WINTER WEATHER
WARNING EVENT (6-7 INCHES OR MORE) REMAINS A DECENT POSSIBILITY. THE
FLY IN THE OINTMENT AND WHY THE WINTER STORM WATCHES REMAIN IN
EFFECT IS THE LINGERING UNCERTAINTY AS TO THE EXACT TRACK AND
STRENGTH OF THE SYSTEM.
12Z GUIDANCE PROVIDED SOME CLARITY AS MODELS BEGAN TO SHIFT A
CLOSER, WITH THE RGEM MOVING A BIT SOUTHEAST, CLOSER TO THE GFS
SOLUTION WHILE THE ECMWF AND NAM SHIFTED FURTHER NORTHWEST, CLOSER
TO THE CONSENSUS AND INSIDE THE BENCHMARK. THIS INCREASES THE
LIKIHOOD OF A LOW END END WINTER STORM WARNING EVENT VS THE
PREVIOUSLY POSSIBLE WIDESPREAD 3-4 INCH STORM.
OVERALL THE DIFFERING CAMPS HAVE COME A BIT CLOSER TO CONVERGING.
STILL, MORE UNCERTAINTY THAN YOU'D LIKE (OR EXPECT) TO HAVE GIVEN
BEING 24 HOURS OUT FROM THE ONSET. TAKING A LOOK AT THE ENSEMBLE
GUIDANCE, THE GEFS MEMBER LOW LOCATIONS INDICATE MUCH MORE
CLUSTERING AROUND ITS STRONGER SOLUTION VS THE MORE SPREAD OUT EPS
POTENTIAL LOW LOCATIONS WHICH WOULD SEEM TO INDICATE THAT THE ECMWF
HAS YET TO CONVERGE ON ONE SOLUTION. FOR NOW, WE CONTINUE WITH A
COMPROMISE APPROACH (BETWEEN THESE TWO SCENARIOS) IN WHICH WE DO SEE
MIXING/PTYPE ISSUES (I.E. RAIN MIXING IN) OVER SOUTHEAST MA
INITIALLY, FLIPPING TO SNOW AT SOME POINT IN THE OVERNIGHT HOURS
(SAVE FOR PERHAPS CAPE COD AND THE ISLANDS). THE STRONGER/CLOSER
TRACK COULD ACTUALLY HURT SNOWFALL TOTALS IN SOUTHEAST MA BY KEEPING
STRONG ONSHORE/WARMER FLOW GOING PREVENTING A QUICKER FLIP TO SNOW.
THAT THEN PLACES THE SWATH OF HEAVIEST SNOW, GENERALLY 4-8 INCHES
NORTH AND WEST OF I-95, WITH THE CAVEAT THAT EXPECTED BANDING COULD
LEAD TO LOCALIZED AMOUNTS CLOSER TO 10 INCHES. OVERALL CONFIDENCE IN
SPECIFIC SNOWFALL TOTALS AND LOCATION OF BANDING IS ON THE LOWER
END, BUT THE POSSIBILITY OF WARNING LEVEL SNOWS HAS SHIFTED
SOUTHEAST, SO THE WINTER STORM WATCH HAS BEEN EXPANDED INTO
SUFFOLK, NORFOLK, AND PROVIDENCE COUNTIES. WHAT IS MORE SOLIDLY
KNOWN IS THAT SNOW WILL BE ENDED BY SUNRISE ON MONDAY FOR MOST,
THOUGH FRIGID TEMPERATURES TO FOLLOW SHOULD SOLIDIFY THINGS AND
CAUSE SLICK CONDITIONS ON UNTREATED ROADS.
LONG TERM /MONDAY THROUGH SATURDAY/
KEY POINTS
*ARCTIC AIRMASS MOVES IN FOR MONDAY PM THROUGH THURSDAY AM
*LOWS NEAR ZERO WITH WIND CHILLS BETWEEN NEGATIVE 5 TO 15
*HIGHS IN THE LOW TEENS WITH WIND CHILLS NEAR ZERO
*WATCHING SNOW POTENTIAL WEDNESDAY, THEN AGAIN FRIDAY, BUT
CONFIDENCE IS LOW AT THIS TIME
AS SNOW COMES TO AN END EARLY MONDAY MORNING, THE COLD ARCTIC
AIRMASS FOLLOWS QUICKLY BEHIND. 850MB TEMPS FALL FROM -12C EARLY IN
THE DAY TO -22C BY MIDNIGHT ON TUESDAY. HIGH TEMPERATURES ON MONDAY
ARE LIKELY TO OCCUR EARLY IN THE UPPER TEENS TO LOW 20S BUT WILL
DROP INTO THE SINGLE DIGITS SHORTLY AFTER SUNSET MONDAY EVENING. THE
ARCTIC AIRMASS STAYS OVER SOUTHERN NEW ENGLAND THROUGH THURSDAY
MORNING. OVERNIGHT LOWS TUESDAY THROUGH THURSDAY MORNING WILL BE
NEAR OR BELOW ZERO FOR MUCH OF THE REGION, WITH WIND CHILL VALUES IN
THE 5-15 BELOW RANGE. HIGH TEMPERATURES ON TUESDAY AND WEDNESDAY
WILL STRUGGLE TO REACH THE LOW TEENS, WITH LOW PROBS FOR HIGHS IN
THE SINGLE DIGITS. BLUSTERY WINDS AROUND 10-20MPH DURING THE DAY
WILL KEEP THE WIND CHILL INDEX NEAR ZERO DURING THE DAY.
AS FOR SNOW POTENTIAL NEXT WEEK, SOME GUIDANCE HINTS AT A COASTAL
LOW PASSING OFFSHORE SOMETIME ON WEDNESDAY, THEN ANOTHER ONE
POSSIBLY ON FRIDAY. STARTING WITH WEDNESDAYS SYSTEM, THE ENSEMBLES
25TH TO 75 PERCENTILE SNOW SHOWS A COATING TO 2 INCHES OF SNOW
ACROSS THE CAPE AND ISLANDS, WITH A COATING POSSIBLE INLAND TO THE I-
95 CORRIDOR BETWEEN BOSTON AND PROVIDENCE. WITH THIS SYSTEM STILL
SEVERAL DAYS OUT, ANY SHIFT IN THE TRACK OF THE LOW WILL RESULT IN
CHANGES IN EXPECTED SNOW ACCUMULATIONS; HENCE, CONFIDENCE IS LOW.
CONFIDENCE IN FRIDAYS COASTAL LOW AND PRECIPITATION IS EVEN LOWER,
WITH A VERY LARGE SPREAD BETWEEN THE DIFFERENT DETERMINISTIC AND
ENSEMBLE GUIDANCE SOURCES, BUT SOMETHING TO MONITOR AS THE WEEK
PROGRESSES. TEMPERATURES LATE IN THE WEEK MODERATE BACK INTO THE
UPPER 20S TO LOW 30S.
AVIATION /00Z SUNDAY THROUGH THURSDAY/
FORECAST CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MODERATE - 30 TO 60 PERCENT.
HIGH - GREATER THAN 60 PERCENT.
00Z TAF UPDATE:
TONIGHT...HIGH CONFIDENCE IN TRENDS. MODERATE CONFIDENCE IN
TIMING OF CEILING IMPROVEMENTS.
RAIN ENDS BETWEEN 00-02Z, BUT CIGS COULD BE SLOW TO IMPROVE,
REMAINING IFR/MVFR UNTIL 08-12Z.
SUNDAY...HIGH CONFIDENCE IN TRENDS.
VFR CEILINGS THROUGH THE AFTERNOON. WINDS W BECOMING NWW LESS
THAN 10 KTS. LIGHT SNOW OR RAIN/SNOW INCREASES FROM WEST TO
EAST AFTER 18Z TO 21Z.
SUNDAY NIGHT...LOW TO MODERATE CONFIDENCE.
IFR/MVFR IN SN FOR MOST OF THE OVERNIGHT HOURS, ENDING FROM WEST
TO EAST 08-14Z, IMPROVING TO VFR BEHIND. N WINDS BECOMING NW 10
TO 15 KTS, GUSTING 20 TO 25 KTS FOR MOST TERMINALS.
KBOS TAF...HIGH CONFIDENCE IN TAF.
MVFR WITH RAIN THROUGH 03Z, THEN CIGS SLOWLY IMPROVE TO VFR
OVERNIGHT BY 12Z. VFR AND DRY MOST OF SUNDAY. PRECIPITATION
SHOULD START AS A RAIN/SNOW MIX BETWEEN 21-23Z, BEFORE FLIPPING
TO ALL SNOW BETWEEN 23-01Z.
KBDL TAF...HIGH CONFIDENCE IN TAF.
IFR/MVFR THIS EVENING, BECOMING VFR OVERNIGHT. VFR MOST OF
SUNDAY. LIGHT SNOW AND MVFR CIGS MOVE IN BETWEEN 21-23Z.
OUTLOOK /MONDAY THROUGH THURSDAY/...
MARTIN LUTHER KING JR DAY THROUGH TUESDAY: VFR. BREEZY.
TUESDAY NIGHT: VFR. SLIGHT CHANCE SN.
WEDNESDAY: VFR. BREEZY. SLIGHT CHANCE SN.
WEDNESDAY NIGHT THROUGH THURSDAY: VFR. BREEZY.
MARINE
FORECASTER CONFIDENCE LEVELS:
LOW - LESS THAN 30 PERCENT.
MEDIUM - 30 TO 60 PERCENT.
HIGH- GREATER THAN 60 PERCENT.
HIGH CONFIDENCE...
S TO SW WINDS GUST 25-30 KTS TODAY. SEAS 4-6 FT FOR THE OUTER
COASTAL WATERS. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE OUTER
COASTAL WATERS INTO EARLY SUNDAY MORNING. WINDS/SEAS GRADUALLY
DECREASE TOWARD MIDNIGHT. SUNDAY WINDS ARE LIGHT MOST OF THE DAY,
INCREASING TO 15-20 KTS SUNDAY NIGHT WITH GUSTS TO 30 KTS AS A LOW
PRESSURE PASSES FROM SOUTHWEST TO NORTHEAST.
OUTLOOK /MONDAY THROUGH THURSDAY/...
MARTIN LUTHER KING JR DAY: MODERATE RISK FOR SMALL CRAFT
ADVISORY WINDS WITH GUSTS UP TO 30 KT. ROUGH SEAS UP TO 10 FT.
CHANCE OF FREEZING SPRAY.
MONDAY NIGHT: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH
GUSTS UP TO 25 KT. LOCAL ROUGH SEAS. CHANCE OF FREEZING SPRAY.
TUESDAY: WINDS LESS THAN 25 KT. SEAS UP TO 5 FT.
TUESDAY NIGHT: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING
5 FT. CHANCE OF SNOW.
WEDNESDAY: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING
5 FT. CHANCE OF SNOW, CHANCE OF FREEZING SPRAY.
WEDNESDAY NIGHT: WINDS LESS THAN 25 KT. AREAS OF SEAS
APPROACHING 5 FT.
THURSDAY: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING 5 FT.
BOX WATCHES/WARNINGS/ADVISORIES
CT...WINTER STORM WATCH SUNDAY AFTERNOON FOR CTZ002-003.
WINTER STORM WARNING FROM 1 PM SUNDAY TO 7 AM EST MONDAY FOR
CTZ002-003.
WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY
MORNING FOR CTZ004.
MA...WINTER STORM WATCH SUNDAY AFTERNOON FOR MAZ002>012-014-026.
WINTER STORM WARNING FROM 1 PM SUNDAY TO 7 AM EST MONDAY FOR
MAZ002>012-014-026.
WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY
MORNING FOR MAZ013-015-016.
RI...WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY
MORNING FOR RIZ001-002.
MARINE...SMALL CRAFT ADVISORY UNTIL 7 PM EST THIS EVENING FOR
ANZ230>237-251.
SMALL CRAFT ADVISORY UNTIL 2 AM EST SUNDAY FOR ANZ250-254>256.
SYNOPSIS...BW/KP
NEAR TERM...BW/KP
SHORT TERM...BW
LONG TERM...KP
AVIATION...BW/KP
MARINE...BW/KP
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