Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 280425 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 1125 PM EST THU NOV 27 2014 .SYNOPSIS... A WEAK SHORTWAVE MOVING UP THE COAST MAY PRODUCE ACCUMULATING SNOW ACROSS EASTERN MA INCLUDING THE BOSTON AREA. HIGH PRESSURE MOVES OVER NEW ENGLAND FRIDAY AND SHIFTS OFFSHORE OVER THE WEEKEND. A WARM FRONT MOVES ACROSS THE REGION LATE SATURDAY NIGHT AND EARLY SUNDAY. A COLD FRONT THEN SWINGS THROUGH NEW ENGLAND MONDAY. ANOTHER WARM FRONT/COLD FRONT COMBINATION WILL MOVE THROUGH THE REGION WEDNESDAY AND WEDNESDAY NIGHT. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 11 PM UPDATE... PRECIP FROM FIRST SHORT WAVE MOVING NE...BUT NOTING SOME MIX OF SHORT WAVE ENERGY WITH SOME OCEAN EFFECT SNOW MOVING TOWARD THE COAST AT 03Z. STEADIEST PRECIP NOTED ACROSS RI/E MA...THOUGH HAS BEEN WEAKENING ESPECIALLY ACROSS CENTRAL AND WESTERN AREAS OVER THE LAST COUPLE OF HOURS. WHERE IT HAS BEEN MAINLY SNOW IT HAS BEEN LIGHT. LITTLE IF ANY ACCUMULATION REPORTED TO US SO FAR THOUGH HAVE RECEIVED ONE REPORT OF AN INCH OF SNOW IN WEST WARWICK RI AT 940 PM. ONLY A TRACE HERE AT KBOX SO FAR. HAVE UPDATED FORECAST TO REFLECT DECREASING TRENDS WELL INLAND BUT DID INCREASE POPS A BIT ALONG THE COAST MAINLY WHERE THE ADVISORY IS IN EFFECT...FOR NOW. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... *** ACCUMULATING SNOW POSSIBLE LATE TONIGHT INTO EARLY FRI AFTERNOON ACROSS EASTERN MA FROM CAPE ANN TO BOSTON TO CAPE COD *** LOW PRESSURE OFF OF NORTH CAROLINA WILL MOVE NORTHEASTWARD...PASSING WELL OUTSIDE THE 40N/70W BENCHMARK. HOWEVER...AN INVERTED TROUGH WILL DEVELOP AS THE SHORTWAVE CROSSES SOUTHERN NEW ENGLAND. THESE TWO FEATURES WILL PROVIDE ENOUGH LIFT AND INSTABILITY ALONG THE EAST COAST OF NEW ENGLAND TO PRODUCE A NARROW BAND OF RAIN/SNOW SHOWERS. THE MAIN QUESTION IS JUST WHERE DOES THIS TROUGH SET UP AND HOW CLOSE TO THE COAST DOES IT GET. THESE FEATURES LEAVE A LOT OF UNCERTAINTY AS A SHIFT OF 10 TO 20 MILES TO THE EAST WILL LEAVE MOST AREAS WITHOUT SNOW. HOWEVER... MODELS HAVE CONSISTENTLY BEEN FORECASTING UP TO HALF AN INCH OF QPF ALONG THE EAST COAST OF MASSACHUSETTS TO JUST OFF THE COAST FOR THE LAST THREE MODEL RUNS. THAT LENDS SOME LEVEL OF CERTAINTY TO THE POTENTIAL FOR A QUICK FEW INCHES OF SNOW. IN ADDITION...STEEP LOW LEVEL LAPSE RATES AND TOTAL TOTALS OVER 50 BOTH INDICATE PLENTY OF INSTABILITY THAT WILL BE REALIZED. AT THIS POINT...GENERALLY HAVE SNOWFALL TOTALS JUST UNDER ADVISORY CRITERIA /3 INCHES/. HOWEVER...THERE ARE A FEW POCKETS OF AROUND 3 INCHES RIGHT AROUND THE BOSTON AREA. DUE TO THE POTENTIAL FOR A CHANGE IN AMOUNTS IF HIGHEST QPF ENDS UP OVER LAND RATHER THAN OVER THE OCEAN...AM OPTING TO ISSUE A WINTER WEATHER ADVISORY ALONG THE IMMEDIATE EAST COAST FROM CAPE ANN SOUTH THROUGH COASTAL PLYMOUTH COUNTY. EXPECT SNOW TO BEGIN SHORTLY AFTER MIDNIGHT AND CONTINUE THROUGH THE MORNING HOURS ON FRIDAY. WHILE THERE IS QUITE A BIT OF UNCERTAINTY WITH THESE SYSTEMS...GIVEN THE NUMBER OF SHOPPERS AND TRAVELERS WHO MAY BE OUT AND ABOUT FRIDAY MORNING...FELT AN ADVISORY WOULD BE A GOOD IDEA. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... BIG PICTURE... ZONAL FLOW THIS WEEKEND WITH 500 MB HEIGHTS BUILDING OVER NEW ENGLAND WHILE A SHORTWAVE TROUGH DROPS THROUGH WESTERN CANADA INTO THE PACIFIC NORTHWEST. THE SHORTWAVE THEN RACES EAST TO CROSS NEW ENGLAND MONDAY NIGHT. ANOTHER SHORTWAVE RACES EAST ACROSS THE NORTHERN TIER DURING THE WEEK. THERE ARE DIFFERENCES BETWEEN THE ECMWF AND GFS REGARDING THE AMPLITUDE AND SPEED OF THIS SHORTWAVE WITH THE GFS FASTER/LESS AMPLIFIED. WE FAVOR A MODEL BLEND FOR MOST OF THE LONG TERM. MOVEMENT OF SHORTWAVES THROUGH ZONAL FLOW LEADS TO QUESTIONS ON TIMING SO CONFIDENCE IS AT BEST MODERATE. LATE IN THE LONG TERM WE TEND CLOSER TOWARD THE ECMWF. AS NOTED BY HPC...THE GREATER AMPLITUDE TROUGH SHOWN BY THE ECMWF IS MORE REASONABLE DUE TO DEEP LOW PRESSURE IN THE BERING SEA AND RESULTING BUILDING OF A RIDGE ALONG THE PACIFIC COAST. THE DAILIES... FRIDAY NIGHT-SATURDAY... HIGH PRESSURE BUILDS OVER NEW ENGLAND WITH DRY CHILLY WEATHER. CROSS SECTIONS SHOW INCREASING MID AND HIGH LEVEL MOISTURE SATURDAY AS THE HIGH SHIFTS EAST AND WARMER AIR STARTS MOVING IN OVER THE TOP OF THE COLDER AIR. SO WE EXPECT MOSTLY CLEAR SKIES FRIDAY NIGHT AND INCREASING SKY COVER ON SATURDAY. WE STAYED CLOSE TO A COMPOSITE OF GUIDANCE FOR TEMPS BOTH PERIODS. SATURDAY NIGHT THROUGH MONDAY... A WARM FRONT APPROACHES SATURDAY NIGHT. THIS WILL MAINTAIN CLOUDS OVER THE REGION. LOW LEVEL PROFILES LOOK RATHER DRY AND LIFT IS WEAK. SO ANY PCPN SHOULD BE LIMITED. WE SHOWED SLIGHT CHANCE POPS MAINLY ALONG AND NORTH OF THE MASS PIKE SATURDAY NIGHT AND IN SRN NH SUNDAY MORNING. THE WARM FRONT LIFTS NORTH ON SUNDAY ALLOWING FOR MILDER TEMPS AND DEWPOINTS AS WELL AS A GENERALLY DRY DAY. THE MODELS SHOW DIFFERENCES IN TIMING THE PASSAGE OF THE COLD FRONT. THIS IS NOT A SURPRISE AS WE WILL BE IN A ZONAL FLOW. THE GFS IS FASTEST WITH PASSAGE MONDAY MORNING. THE ECMWF IS SLOWER WITH PASSAGE MONDAY EVENING. WE USED A BLEND OF THIS DATA WITH CHANCE POPS IN THE WEST IN THE MORNING...SLIGHT CHANCE ALL AREAS AFTN...AND CHANCE ALL AREAS EARLY MONDAY NIGHT. AT BEST MODERATE CONFIDENCE IN THIS TIMING. TUESDAY... HIGH PRESSURE BUILDS OVER THE REGION WITH COLDER AIR. MIXING WILL BE LIMITED TO 925 OR 950 MB...AND TEMPS AT THAT LEVEL SUPPORT MAX TEMPS IN THE UPPER 20S AND 30S. WEDNESDAY-THURSDAY... SHORTWAVE MOVES OUT OF THE PACIFIC NORTHWEST AND PUSHES A COLD FRONT ACROSS THE NORTHERN TIER OF STATES TUESDAY- WEDNESDAY. THIS WILL DRAW A SOUTHWEST LOW LEVEL JET OF 40-45 KNOTS LEADING THE COLD FRONT UP TO OUR AREA BY WEDNESDAY. THIS JET WILL SUPPLY MOISTURE AND GENERATE SUFFICIENT LIFT FOR CLOUDS AND A CHANCE OF PCPN. CONFIDENCE IN TIMING IS LOW BUT AS NOTED ABOVE WE FAVORED THE TIMING SHOWN BY THE ECMWF FOR ONSET OF CLOUDS AND ANY PCPN. LOW LEVEL TEMPERATURES WILL DETERMINE PCPN TYPE...BUT TREND SHOULD BE FROM SNOW TO RAIN EARLY WEDNESDAY AND WEDNESDAY MORNING. LINGERING COLD AIR AT THE SURFACE MAY SUPPORT A PERIOD OF SLEET OR FREEZING RAIN DURING THE TRANSITION DURING THE MORNING...THEN TEMPS SHOULD BE WARM ENOUGH FOR PCPN TO FALL AS RAIN ANY OTHER TIME OF THE DAY. TIMING OF THE COLD FRONT PER THE ECMWF WOULD BE LATE WEDNESDAY OR EARLY WEDNESDAY NIGHT. HIGH PRESSURE BRINGS DRIER WEATHER ON THURSDAY. && .AVIATION /04Z FRIDAY THROUGH TUESDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. .SHORT TERM /THROUGH FRIDAY/...MODERATE CONFIDENCE. TONIGHT...MVFR CIGS REMAIN OVER SOUTHERN NEW ENGLAND THROUGH THE NIGHT. ONE AREA OF LIGHT RAIN/SNOW SHOWERS MOVES NORTHEAST ACROSS CT/RI/SOUTHEAST MASS THROUGH MIDNIGHT. REPORTED VSBYS IN THE SHOWERS HAVE BEEN 3-5 MILES OR HIGHER. COULD BE BRIEF IFR CIGS/VSBYS BUT MAINLY EXPECT 1200-1700 FOOT CIGS AND 3-5 MILE VSBYS THROUGH 11 PM. SECOND AREA OF PRECIP DEVELOPS ACROSS E MA INTO RI...POSSIBLY AS FAR W AS KORH-KIJD AFTER MIDNIGHT...AS INVERTED TROUGH TAKES SHAPE NEAR THE COAST. AREAS OF MVFR-IFR VSBYS DEVELOP IN RAIN AND/OR SNOW. VSBYS SHOULD REMAIN MAINLY VFR ACROSS THE CT VALLEY. FRIDAY...SNOW/RAIN SHOWERS WILL CONTINUE THROUGH MID-MORNING ALONG THE E COAST OF MASS...POSSIBLY AS FAR INLAND AS ORH-IJD. PRECIP COMES TO AN END LATE MORNING...SOONEST WEST AND LATEST EAST. CONDITIONS WILL IMPROVE TO VFR FRIDAY AFTERNOON. KBOS TERMINAL...MEDIUM CONFIDENCE IN TIMING OF RAIN/SNOW TONIGHT AND FRIDAY MORNING. HIGH CONFIDENCE IN IMPROVING CONDITIONS FRIDAY. KBDL TERMINAL...HIGH CONFIDENCE. OUTLOOK...FRIDAY NIGHT THROUGH TUESDAY...MODERATE CONFIDENCE FRIDAY NIGHT AND SATURDAY...VFR. INCREASING SKY COVER SATURDAY. NORTHWEST WINDS BECOME SOUTHWEST BUT REMAIN LIGHT. SATURDAY NIGHT-SUNDAY...LOWERING CLOUDS SATURDAY NIGHT WITH POTENTIAL FOR MVFR CIGS/VSBYS IN WIDELY SCATTERED PCPN LATE AT NIGHT AND SUNDAY MORNING. LOW LEVEL WINDS...AROUND 2000 FEET...WILL INCREASE OVERNIGHT AND SUNDAY TO 40-45 KNOTS AND MAY CREATE LOW LEVEL WIND SHEAR. MONDAY...VFR WITH SCATTERED SHOWERS AS A COLD FRONT MOVES THROUGH. SOUTHWEST WINDS AHEAD OF THE COLD FRONT WILL SWITCH TO NORTHWEST AFTER THE FRONT MOVES THROUGH. TUESDAY...VFR WITH NORTH TO NORTHEAST WINDS. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. .SHORT TERM /THROUGH FRIDAY/...HIGH CONFIDENCE. OVERNIGHT...STILL NOTING SEAS UP TO 7 FT ON THE EASTERN WATERS AT 02Z...BUT APPEAR TO BE SUBSIDING. HAVE KEPT SMALL CRAFTS GOING. OTHERWISE CONTINUED TRENDS FROM PREVIOUS FORECAST WITH SEAS SUBSIDING EARLY...THEN BEGINNING TO BUILD AGAIN LATE AS LOW PRES PASSES S OF THE WATERS. WILL ALSO SEE N-NW WINDS THAT MAY BECOME NE ALONG THE COAST BUT REMAIN LIGHT THOUGH GUSTS INCREASE TOWARD DAYBREAK. FRIDAY...WINDS INCREASE...GUSTING TO NEAR 25 KTS. HAVE SMALL CRAFT ADVISORIES UP FOR MOST OF THE COASTAL WATERS FOR EITHER SEAS...GUSTY WINDS OR BOTH. OUTLOOK...FRIDAY NIGHT THROUGH TUESDAY...MODERATE CONFIDENCE. FRIDAY NIGHT...DIMINISHING WINDS AND SEAS. LINGERING 5 FOOT SEAS ON THE OUTER WATERS BEFORE MIDNIGHT MAY REQUIRE A SMALL CRAFT ADVISORY. SATURDAY...WINDS AND SEAS SHOULD REMAIN BELOW SMALL CRAFT THRESHOLDS. SUNDAY...SOUTHWEST WINDS INCREASE TO 25 KNOTS AND SEAS BUILD TO 5-7 FEET. A SMALL CRAFT ADVISORY MAY BE NEEDED ON MOST WATERS. MONDAY...A COLD FRONT MOVES THROUGH DURING THE AFTERNOON OR EVENING. SOUTHWEST WINDS SHIFT TO NORTHWEST LATER IN THE DAY...WITH SPEEDS 20 TO 25 KNOTS. SEAS DIMINISH A LITTLE BUT REMAIN AT OR ABOVE 5 FEET ON THE EXPOSED WATERS. A SMALL CRAFT ADVISORY MAY STILL BE NEEDED. TUESDAY...NORTH WINDS BECOME NORTHEAST WITH GUSTS AROUND 25 KNOTS. SEAS LINGER AT 5 TO 6 FEET ON THE OUTER WATERS. A SMALL CRAFT ADVISORY MAY BE NEEDED. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...NONE. MA...WINTER WEATHER ADVISORY UNTIL NOON EST FRIDAY FOR MAZ007-015- 016-019. NH...NONE. RI...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 6 PM EST FRIDAY FOR ANZ235-237-250- 251-254>256. && $$ SYNOPSIS...WTB/RLG NEAR TERM...RLG/EVT SHORT TERM...RLG LONG TERM...WTB AVIATION...WTB/RLG MARINE...RLG/EVT

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