Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 182249 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 649 PM EDT SAT OCT 18 2014 .SYNOPSIS... A STRONG COLD FRONT USERS IN A CHILLY AIRMASS ACROSS NEW ENGLAND SUNDAY ALONG WITH BLUSTERY CONDITIONS. COLD TEMPERATURES SUNDAY NIGHT WILL GIVE TO SOME MODERATION MON ALONG WITH A DIMINISHING WIND AS HIGH PRES SLIDES SOUTH OF NEW ENGLAND. A LONG DURATION COASTAL STORM IS LIKELY DURING THE MIDDLE TO END OF NEXT WEEK BUT THE AXIS OF HEAVIEST RAIN AND STRONGEST WINDS REMAINS UNCERTAIN AT THIS TIME RANGE. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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7 PM UPDATE... LEAD SHORT WAVE AND ATTENDING SURFACE TROUGH NOW MOVING THROUGH EASTERN MA. THIS FEATURE IS TRIGGERING A BROKEN LINE OF SHOWERS ACROSS THIS AREA. BRIEF HEAVY DOWNPOURS CAN BE EXPECTED OVER SOUTHEAST MA INCLUDING CAPE COD AND THE ISLANDS THROUGH 8-830 PM. BEHIND THE SURFACE TROUGH EXPECT A WIND SHIFT TO THE WNW ALONG WITH COOLER AND DRIER AIR OVERSPREADING THE REGION. HOWEVER THE TRUE COLD FRONT WAITS UNTIL SUNDAY MORNING AS TRAILING SHORT WAVE ENERGY ROUNDS THE BASE OF THE TROUGH. UNTIL MAINLY DRY CONDITIONS WILL PREVAIL WITH JUST A SLIGHT CHANCE OF AN ISOLATED SHOWER OVERNIGHT AHEAD OF THE MAIN FRONT. PREVIOUS FORECAST HANDLES THIS WELL SO NO MAJOR CHANGES WITH THIS UPDATE...JUST UPDATING HOURLY GRIDS TO BETTER REFLECT LATEST TRENDS. EARLIER DISCUSSION BELOW. ================================================================== TONIGHT... INITIAL SHORTWAVE MOVES OFFSHORE...PUSHING A COLD FRONT THROUGH OUR AREA. ANOTHER SHORTWAVE MOVES DOWN FROM ONTARIO TONIGHT AND PUSHES THE TROUGH AXIS OVER NEW ENGLAND BY MORNING. THIS WILL BRING COLDER TEMPS ALOFT WHICH WILL DESTABILIZE THE AIRMASS. THUS EXPECT SHOWER COVERAGE TO DIMINISH EARLY TONIGHT...THEN INCREASE AGAIN OVERNIGHT. EXPECT SCATTERED/CHANCE POPS FOR TONIGHT. MIN TEMPS UPSTREAM IN ONTARIO WERE IN THE UPPER 30S AND 40S...THIS IS A REASONABLE TARGET FOR TONIGHT.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... SUNDAY... SECOND SHORTWAVE AND TROUGH AXIS MOVE OFFSHORE IN THE MORNING. THIS WILL ALLOW LINGERING SHOWERS IN THE MORNING...THEN A CLEARING AND DRYING TREND BY AFTERNOON. NORTHWEST WINDS WILL BRING COLDER AIR AT ALL LEVELS AND SUPPORT DAYTIME MIXING OF THE AIRMASS TO 850 MB. TEMPS AT 850 MB WILL BE -2C TO -5C...WHICH SUPPORTS MAX SFC TEMPS IN THE UPPER 40S TO MID 50S. WINDS IN THE MIXED LAYER WILL BE AT LEAST 25 KNOTS. WE WILL USE THESE VALUES. SUNDAY NIGHT... AS HIGH PRESSURE BUILDS OVER NEW ENGLAND THE WINDS WILL DIMINISH DURNING THE EARLY NIGHT. MOSTLY CLEAR SKIES AND LIGHT WINDS WILL ALLOW FOR RADIATIONAL COOLING. DEWPOINTS IN THE COLDER AIRMASS WILL BE IN THE 20S...EXCEPT 30-35 ALONG THE COASTLINE AND ISLANDS. THIS WILL SET UP THE POTENTIAL FOR WIDESPREAD TEMPERATURES AT AND BELOW FREEZING ACROSS OUR AREA...THE COLDEST NIGHT OF THE SEASON SO FAR. MOST AREAS ALONG THE COAST WILL COLD ENOUGH FOR A FROST. WE WILL ISSUE A FREEZE WATCH FOR ALL AREAS AWAY FROM THE COAST WHERE THE SEASON HAS NOT ALREADY ENDED. && .LONG TERM /MONDAY THROUGH SATURDAY/... HIGHLIGHTS... * TRANQUIL WEATHER MON WITH COOL TEMPS/SUNSHINE AND LIGHT WINDS * A LONG DURATION COASTAL STORM WILL IMPACT NEW ENGLAND FROM TUE THROUGH FRI...HOWEVER AXIS OF HEAVIEST RAIN...STRONGEST WIND AND MAGNITUDE OF COASTAL FLOODING REMAINS UNCERTAIN AT THIS TIME SYNOPTIC OVERVIEW AND MODEL/GUIDANCE EVALUATION... NOT MUCH CHANGE FROM PREVIOUS MODEL RUNS WITH GOOD AGREEMENT ON AN EVOLVING LONG DURATION HIGH AMPLTIUDE COASTAL STORM BEGINNING OFF DELMARVA TUE/WED AND THEN TRACKING NORTHEAST OFF THE NEW ENGLAND COAST THU/FRI. MODELS DIFFER ON FAR SOUTH DOWN THE EASTERN SEABOARD CLOSED LOW INITIALLY DIGS...THEN HOW QUICKLY THE MID LEVEL CIRCULATION CAPTURES THE SURFACE LOW /OCCLUSION/ WHICH WILL DETERMINE TIMING/TRACK AND DURATION OF WARM CONVEYOR BELT RAINS OVER THE REGION BEFORE DRY SLOT ARRIVES. AS EXPECTED THE DETERMINISTIC GUIDANCE DISPLAYS SOME SMALL VARIATIONS FROM RUN TO RUN. THUS AT THIS TIME RANGE THE MOST SKILLFUL SOLUTION IS A MODEL BLEND WHICH IS VERY CLOSE TO THE 12Z GEFS AND 00Z ECENS ENSEMBLE MEANS. DAILIES... MONDAY... OTHER THAN A CHILLY START TO THE DAY WITH MORNING LOWS IN THE 30S REGIONWIDE...IT WILL BE A QUIET WEATHER DAY AS SHORT WAVE RIDGING DEVELOPS OVER NEW ENGLAND AHEAD OF NORTHERN STREAM SHORT WAVE. WHILE TEMPS WILL ONLY BE A FEW DEGS HIGHER THAN SUNDAY IT WILL FEEL CONSIDERABLY WARMER GIVEN MORE SUNSHINE /LESS STRATO-CU/ AND LESS WIND AS HIGH PRES SLIDES SOUTH OF NEW ENGLAND. EASILY THE PICK OF THE WEEK. TUESDAY THROUGH FRIDAY... PROLONG PERIOD OF RAIN AND WIND OF VARYING INTENSITY AS LARGE AMPLITUDE CLOSED MID LEVEL LOW DEVELOPS OVER THE MID ATLC REGION AND EVENTUALLY SLIDES SOUTH OF NEW ENGLAND COAST. AS MENTIONED ABOVE TIMING AND POSITION DIFFERENCES ON EXACT EVOLUTION WHICH CREATES UNCERTAINTY ON STORM DETAILS...SUCH AS AXIS OF HEAVIEST RAIN... STRONGEST WIND AND MAGNITUDE OF COASTAL FLOODING. THE NEW 12Z GEFS INDICATES STRONGEST 925 MB AND 850 MB ONSHORE LOW LEVEL WIND ANOMALIES /+3 STD/ OCCUR FROM 12Z WED TO 12Z THU ACROSS INTERIOR SOUTHERN NEW ENGLAND AND THEN LIFTING INTO CENTRAL AND NORTHERN NEW ENGLAND LATER THU INTO FRI. THIS WOULD SUGGEST THE STRONGEST WINDS AND HEAVIEST RAINFALL LATE TUE NIGHT INTO EARLY THU FOR SOUTHERN NEW ENGLAND BEFORE LIFTING INTO NORTHERN NEW ENGLAND WITH DRY SLOT APPROACHING FROM THE SOUTH. DETERMINISTIC RUNS OF THE 12Z GFS AND ECMWF CONCUR WITH THIS EVOLUTION YIELDING EVENT RAINFALL TOTALS OF 1-2" ACROSS CT/RI AND SOUTHEAST MA...INCREASING TO 3-4" OVER NORTHERN MA INTO SOUTHWEST NH. LOCALLY HEAVIER AMOUNTS POSSIBLE ESPECIALLY IN THE UPSLOPE REGIONS OF THE HIGHER TERRAIN OF MA AND NH. A SUPERBLEND OF ALL MODEL SOURCES YIELDS THE SAME QPF RESULTS...AREAL COVERAGE AND IN MAGNITUDE. THUS AT THIS TIME RANGE ENSEMBLES AND A MODEL BLEND OF DETERMINISTIC GUID SUGGEST GREATEST THREAT OF FLOODING RAINS/STRONG WINDS-DOWN TREE LIMBS-POWER OUTAGES AND COASTAL FLOODING WOULD BE ACROSS NORTHERN MA INTO SOUTHERN NH. GIVEN THE TIME RANGE OBVIOUSLY THIS CAN SHIFT FARTHER SOUTH OR NORTH. THUS NEED TO REMAIN PROBABILISTIC AT THIS TIME RANGE. REGARDING COASTAL FLOODING...AS MENTIONED ABOVE LATEST GUIDANCE AND TRENDS CONTINUE TO SUGGEST LONGEST DURATION OF STRONGEST WINDS WILL OCCUR FROM NORTHERN MA NORTHWARD ACROSS NH AND MAINE COASTLINE. THUS GREATEST RISK OF COASTAL FLOODING AND EROSION IS CAPE ANN/SALISBURY NORTHWARD. HOWEVER AT THIS TIME RANGE IT/S POSSIBLE FOR THIS RISK AREA TO SHIFT SOUTHWARD OR NORTHWARD. WHILE ASTRO TIDES ARE SOMEWHAT LOW THIS WILL LOWER THE RISK OF MORE SIGNIFICANT COASTAL FLOODING. HOWEVER LONG DURATION EVENT OCCURRING OVER MULTIPLE TIDE CYCLES CAN OFFSET THIS. FOR THIS REASON THE RISK OF EROSION APPEARS GREATER THAN INUNDATION GIVEN LARGE SURF WILL BATTER THE COASTLINE FOR A NUMBER OF CONSECUTIVE DAYS. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/...
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FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH SUNDAY/... 7 PM UPDATE... INITIAL FRONT MOVING ACROSS SOUTHEAST MA AT 7 PM. A FEW SHOWERS ARE ACCOMPANYING THIS FRONT AND WILL MOVE OFFSHORE 8-830 PM. BEHIND THE FRONT A WIND SHIFT TO THE WNW. FOR THE REMAINDER OF THE NIGHT MAINLY DRY WITH JUST A SLIGHT CHANCE OF AN ISOLATED SHOWER ALONG WITH VFR CONDITIONS PREVAILING. THUS NO MAJOR CHANGES FROM 18Z TAFS. EARLIER DISCUSSION BELOW. ================================================================== TONIGHT...MODERATE TO HIGH CONFIDENCE. GENERALLY VFR WITH BRIEF MVFR CIGS IN SCATTERED TO ISOLATED SHOWERS OVERNIGHT. SUNDAY...HIGH CONFIDENCE. VFR. MORNING CLOUDS BREAK TO MOSTLY CLEAR IN THE AFTERNOON. NORTHWEST WINDS WILL GUST TO 25 KT FROM LATE MORNING THROUGH THE EVENING. SCATTERED SHOWERS ARE POSSIBLE SUNDAY MORNING. SUNDAY NIGHT...HIGH CONFIDENCE. VFR. MOSTLY CLEAR SKIES AND WINDS BECOME LIGHT. TEMPERATURES AWAY FROM THE COAST WILL FALL TO NEAR FREEZING OR A LITTLE BELOW...TURNING ANY WET SPOTS ON THE GROUND INTO PATCHES OF ICE. KBOS...HIGH CONFIDENCE IN TAF. KBDL...HIGH CONFIDENCE IN TAF. OUTLOOK...MONDAY THROUGH THURSDAY... MON...HIGH CONFIDENCE. VFR/DRY RUNWAYS AND LIGHT WINDS. TUESDAY/WEDNESDAY/THU...HIGH CONFIDENCE ON OVERALL THEME...WITH CONDITIONS LOWERING TO MVFR/IFR IN DEVELOPING RAIN BUT LOW CONFIDENCE ON EXACT TIMING OF DETAILS. STRONG EAST TO NORTHEAST WINDS POSSIBLE WED INTO EARLY THU ACROSS NE MA AND S NH.
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&& .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /TONIGHT THROUGH SUNDAY/...HIGH CONFIDENCE. TONIGHT...SOUTHEAST SWELL FROM GONZALO CONTINUES TO AFFECT THE NEW ENGLAND WATERS WITH A 14 SECOND PERIOD AND HEIGHTS OF 5 TO 8 FEET. THIS WILL CONTINUE TONIGHT ON ALL EXPOSED WATERS. SOUTHWEST WINDS GUSTING TO 20 KNOTS WILL BECOME WEST OVERNIGHT AS A COLD FRONT MOVES THROUGH. SMALL CRAFT HEADLINES CONTINUE FOR ALL WATERS EXCEPT BOS HARBOR/NARRAGANSETT BAY/MASSACHUSETTS BAY. SUNDAY...NORTHWEST WINDS OVER THE WATERS WILL BRING GUSTS OF 25 TO 30 KNOTS TO THE SURFACE. SOUTHEAST SWELL WILL CONTINUE ALTHOUGH DIMINISHED A LITTLE FROM SATURDAY. SMALL CRAFT ADVISORY IS IN PLACE FOR ALL WATERS. SUNDAY NIGHT...HIGH PRESSURE APPROACHES FROM THE WEST. NORTHWEST WINDS WILL DIMINISH OVERNIGHT AS THE HIGH GETS CLOSER. SOUTHEAST SWELL FROM GONZALO WILL DIMINISH OVERNIGHT. SMALL CRAFTS WILL END ON THE NEARSHORE WATERS BUT LINGER ON THE OUTER WATERS. OUTLOOK...MONDAY THROUGH THURSDAY... *** POWERFUL/DANGEROUS EARLY SEASON COASTAL STORM TO MARINERS LIKELY WED/THU *** MONDAY...WINDS SLACKEN AS HIGH PRES SLIDES SOUTH OF NEW ENGLAND. THUS SUBSIDING SEAS. GOOD VSBY IN DRY WEATHER. TUE/WED/THU...COASTAL STORM DEVELOPS OFF THE DE COAST YIELDING INCREASING EAST WINDS BEGINNING TUE WITH STRONGEST E-NE WINDS LIKELY WED INTO THU AS LOW PRES APPROACHES BETWEEN NANTUCKET AND THE 40N/70W BENCHMARK. E-NE GALES LIKELY WITH GREATEST RISK NORTHERN WATERS ALONG WITH INCREASING SEAS OF 15+...AGAIN OCEAN WATERS NORTHEAST OF BOSTON TO CAPE ANN. PERIODS OF HEAVY RAIN WILL REDUCE VSBY. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...FREEZE WATCH FROM LATE SUNDAY NIGHT THROUGH MONDAY MORNING FOR CTZ002>004. MA...FREEZE WATCH FROM LATE SUNDAY NIGHT THROUGH MONDAY MORNING FOR MAZ005-006-009>014-017-018. NH...NONE. RI...FREEZE WATCH FROM LATE SUNDAY NIGHT THROUGH MONDAY MORNING FOR RIZ001-003. MARINE...SMALL CRAFT ADVISORY UNTIL 2 AM EDT MONDAY FOR ANZ231>235-237. SMALL CRAFT ADVISORY FROM 8 AM SUNDAY TO 2 AM EDT MONDAY FOR ANZ230-236-251. SMALL CRAFT ADVISORY UNTIL 8 AM EDT MONDAY FOR ANZ250-254>256. && $$ SYNOPSIS...WTB/NOCERA NEAR TERM...WTB/NOCERA SHORT TERM...WTB LONG TERM...NOCERA AVIATION...WTB/NOCERA MARINE...WTB/NOCERA

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