Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 282032 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 332 PM EST SAT FEB 28 2015 .SYNOPSIS... A FAST MOVING LOW PRESSURE SYSTEM WILL BRING ACCUMULATING SNOW TO THE REGION LATE SUNDAY INTO SUNDAY NIGHT...WITH THE HIGHEST AMOUNTS EXPECTED SOUTH OF THE MASSACHUSETTS TURNPIKE AND AWAY FROM THE IMMEDIATE SOUTH COAST. ANOTHER STORM IS LIKELY TUESDAY NIGHT INTO WEDNESDAY...WITH SNOW AND MIXED PRECIPITATION LIKELY CHANGING TO RAIN. COLD AND DRY CONDITIONS RETURN FOR THE END OF NEXT WEEK. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... INTO THIS EVENING... HIGH PRESSURE PUSHING E INTO THE REGION. TEMPERATURES EXPECTED TO QUICKLY DROP WITH MOSTLY CLEAR CONDITIONS AND LIGHT WINDS. TONIGHT... COLD. CONDITIONS WILL BE JUST RIGHT TO ALLOW FOR GOOD RADIATIONAL COOLING...ESPECIALLY TO THE E WHERE THE IMPACT OF INCREASING MID- TO HIGH-CLOUDS LATE IN THE PERIOD WILL BE LESS. LOOKING AT SUB-ZERO LOWS FOR LOW-LYING AREAS WHERE COLD-AIR DRAINAGE EFFECTS OFF THE HIGH TERRAIN ARE MAXIMIZED. HIGH-TERRAIN SHOULD REMAIN AT OR ABOVE ZERO. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... */ SUNDAY MORNING INTO MIDDAY... QUIET WEATHER. CLOUDS THICKENING AS WINDS INCREASE OUT OF THE S. WILL SEE TEMPERATURES GRADUALLY MODERATE INTO THE MID- TO UPPER-20S. */ SUNDAY AFTERNOON INTO EARLY MONDAY MORNING... 1) HIGHLIGHTS... - SNOW SUNDAY AFTERNOON INTO MONDAY MORNING - GREATEST INTENSITY SUNDAY EVENING AND OVERNIGHT - 6+ INCH SNOWFALLS EXPECTED BUT EXACTLY WHERE REMAINS UNCERTAIN - ANNOYINGLY...FORECAST GUIDANCE CONTINUES TO WOBBLE 1) OVERVIEW... BROAD ISENTROPIC OVER-RUNNING EVENT OF GULF MOISTURE ALONG THE 275- 295K SURFACES AMPLIFIED BY A N-STREAM IMPULSE DURING THE TIMEFRAME POSSIBLY INVOKING AN AREA OF LOW PRESSURE THAT SKIRTS ALONG THE S- COAST SUBSEQUENTLY ENHANCING LOW-LEVEL WIND PROFILE. LIGHT-MODERATE SNOWFALL IS ANTICIPATED WITH LOCALLY HEAVIER AMOUNTS. THE INTENSE PORTION OF SNOWFALL IS EXPECTED SUNDAY EVENING AND OVERNIGHT ENDING EARLY MONDAY MORNING. MODEL PREFERENCE GIVEN TO THE ECMWF/GFS/REGIONAL CANADIAN WHICH WILL SERVE AS THE CONSENSUS OF GUIDANCE FOR THIS DISCUSSION. THE 28.12Z NAM WAS CONSIDERED AN OUTLIER WITH ITS AGGRESSIVE-MODE AND SECONDARY LOW DEVELOPMENT JUST OFFSHORE. 2) PRECIPITATION-TYPE... EVALUATING THERMAL FIELDS...BLEND OF FORECAST CONSENSUS PUTS THE -2C H925 ISOTHERM W-E ACROSS S-TIER OF CT/RI AND ACROSS SE MA WITH THE 0C H85 ISOTHERM JUST S OF LONG-ISLAND AND NANTUCKET COASTLINES. THE CONSENSUS OF 1000-850 MB CRITICAL THICKNESSES LAYS RIGHT ALONG THE IMMEDIATE S-COAST OF LONG-ISLAND AND CAPE COD. AN ALL SNOW-EVENT WHEN CONSIDERING DYNAMIC-COOLING/WETBULBING OF THE LOW-LEVELS AHEAD OF THE EVENT. THERE IS PERHAPS A CHANCE OF A BRIEF MIX-OVER TO RAIN/FREEZING RAIN ACROSS THE CAPE AND ISLANDS AS S-FLOW AT THE SURFACE IS POSSIBLY ABLE TO NUDGE ABOVE-FREEZING TEMPERATURES N. YET AS WE ARE IN A FROZEN TUNDRA...WITH ANY RAIN IT WOULD LIKELY FREEZE ON SURFACES WHICH ARE QUITE COLD. THUS FREEZING RAIN PREVAILS OVER RAIN IN THE FORECAST. 3) SNOWFALL... FORECAST CONSENSUS OF A 0.3-0.6 N-S PRECIPITATION EVENT WITH A N-S SNOW-TO-LIQUID RATIO GRADIENT RANGING FROM 15:1 TO 10:1 YIELDS HIGH- END SNOW ADVISORY CRITERIA WITH SNOWFALL AMOUNTS RANGING ON AVERAGE AROUND 3-6 INCHES. AREAS OF 6+ INCHES ARE EXPECTED...ESPECIALLY FROM CENTRAL-CT E INTO SE-MA /INCLUDING THE UPPER-CAPE/. A CHALLENGE AND A LOT OF UNCERTAINTY WITH RESPECT TO HEADLINES AND DECISION-MAKING. EVALUATING LOW-LEVEL THERMAL PROFILES TOWARDS SNOW-TO-LIQUID RATIOS DERIVED VIA A COBB-TECHNIQUE...WITH THE GREATEST LIFT OCCURRING JUST BENEATH OR WITHIN THE DENDRITIC-GROWTH ZONE BASED ON A CONSENSUS OF BUFKIT GFS PROFILES...ESPECIALLY IN HEAVIER- PRECIP AREAS...BELIEVE THE SNOW WILL RANGE SOMEWHERE BETWEEN A WET AND FLUFFY SNOW. SNOW PROPERTIES FROM FLUFFY TO WET LIKELY SIMILAR TO THE N-S GRADIENT AS OUTLINED ABOVE. 4) HEADLINES... HATE A WOBBLING FORECAST WHICH IS TYPICAL WITH SUCH SMALL EVENTS. UNCERTAINTY DERIVES FROM LOW-LEVEL THERMAL PROFILES AND PRECIP AMOUNTS. SURELY EXPECTING SNOW...BUT WITH ANY SHIFT N-S OF COLDER AIR AND/OR PRECIPITATION AMOUNTS RESULTS IN AREAS SHIFTING FROM ADVISORY TO WARNING...OR VICE VERSA. ENOUGH TO JUST PULL YOUR HAIR OUT. THEN THERE IS THE TIMING OF THE SYSTEM THAT HAS SPED UP SLIGHTLY. A SHORTER RESIDENCE TIME OF COURSE WOULD YIELD LESSER SNOWFALL AMOUNTS. QUITE A CHALLENGING FORECAST RIGHT ON THE CUSP OF HEADLINE THRESHOLDS. THE EVENT DOES TAKE PLACE MAINLY FROM SUNDAY EVENING INTO THE EARLY MORNING HOURS ON MONDAY DURING WHICH TIME THERE IS NOT MUCH TRAFFIC ON THE ROAD. IS THIS A HIGH IMPACT EVENT THAT WARRANTS WARNINGS? A QUESTION TO STRUGGLE WITH AS WARNINGS ARE NECESSARY WHEN WE ARE CONFIDENT IN MEETING OR EXCEEDING 6+ INCH CRITERIA. YET THE QUESTION IN FRONT OF US IS WHERE WILL THE HIGHER AMOUNTS OF SNOW FOCUS? HAVE TO MAKE SOME SORT OF CALL...THE NEEDS OF THE MANY OUTWEIGH THE NEEDS OF THE FEW. ADVISORIES POSTED WHERE THERE IS HIGH CONFIDENCE OF SNOW BUT LIKELY TO REMAIN BELOW THE 6-INCH THRESHOLD FOR WARNING. WATCHES CONTINUE AND HAVE BEEN EXTENDED TO THE S-COAST TO COVER FOR THE UNCERTAINTY OF THE AREA EITHER FALLING WITHIN OR IN EXCESS OF ADVISORY-LEVEL SNOWS. AGAIN...6+ INCH AMOUNTS ARE ANTICIPATED BUT EXACTLY WHERE IS UNCERTAIN. 5) CLOSING NOTES... SOME CONCERN FOR THE EARLY MONDAY MORNING COMMUTE FOR E/SE MA AND RI. WHILE MOST OF THE SNOW IS EXPECTED TO BE IMMEDIATELY OFFSHORE WITH SUNRISE MONDAY...COMMUTERS COULD ENCOUNTER SLICK AND SLIPPERY ROAD CONDITIONS. SNOW COULD POSSIBLY LINGER OVER SE MA SO THERE IS THE POTENTIAL FOR REDUCED VISIBILITIES. && .LONG TERM /MONDAY THROUGH SATURDAY/... HIGHLIGHTS... * CONDITIONS IMPROVE MONDAY * ANOTHER STORM POSSIBLE TUE NIGHT INTO WED...WITH SNOW OR MIXED PRECIP LIKELY CHANGING TO RAIN * COLD AND DRY TOWARD THE END OF THE WEEK OVERVIEW AND MODEL PREFERENCES... ONE THING IS FOR CERTAIN...WITH LITTLE CHANGE IN THE NAO/AO AND THE PNA EXPECTED TO REBOUND FROM NEGATIVE VALUES BACK TOWARD A MORE NEUTRAL SOLUTION...THE COLD AIR AND LONGWAVE TROF LOOK TO REMAIN THROUGH THE LONG TERM FORECAST. THE ONLY CAVEAT WILL BE IN THE LOW LVLS ON WED...WHEN A ROBUST SW LLJ WILL ALLOW LOW-MID LVL TEMPS TO WARM CLOSER TO ABOVE CLIMATOLOGICAL NORMALS. HOWEVER... THE COLD AIR INITIALLY IN PLACE WILL LEAD TO P-TYPE ISSUES FOR THE MID WEEK SYSTEM. ECMWF DETERMINISTIC REMAINS THE MOST ROBUST OF THE OPERATIONAL MODELS...WITH HIGHER AMPLIFICATION IN THE FLOW PATTERN THAN THE GFS AND UKMET. IT STARTS MUCH COLDER...BUT ENDS MUCH WARMER. HOWEVER...DO NOTE THAT SOME OF THE GEFS AND ECENS ENSEMBLES AGREE WITH THE ECMWF. THEREFORE...WILL BE USING THE ENSEMBLE BLEND AS A BASELINE AS THIS TAKES INTO ACCOUNT BOTH THE MORE PROGRESSIVE /AND ULTIMATELY LESS ROBUST WITH THE THERMAL DIFFERENCES/ AND THE POTENTIAL FOR A MORE DYNAMIC SYSTEM. WILL USE THIS BLEND THROUGH THE REMAINDER OF THE FORECAST AS WELL. DETAILS... MONDAY... MORNING SNOWS COME TO AN END EARLY ACROSS E MA...AND FLURRIES DISSIPATE THROUGH THE BERKSHIRES AND LITCHFIELD HILLS AS DRY NW FLOW MOVES IN. BLUSTERY CONDITIONS EXPECTED...AND EVEN THROUGH HIGHS WILL STILL BE MAKING IT INTO THE LOW-UPPER 30S...NW WINDS WITH GUSTS 25-35 MPH AT TIMES WILL YIELD WIND CHILLS STRUGGLING INTO THE LOW 20S. MONDAY NIGHT INTO TUESDAY... HIGH PRES WILL MOVE ACROSS THE REGION. DRY CONDITIONS DOMINATE ALTHOUGH WITH TEMPS REMAINING BELOW NORMAL. TUESDAY NIGHT INTO THURSDAY... MODEL CONSENSUS IS NOW GENERALLY IN AGREEMENT THAT BOTH THE NRN AND SRN STREAMS PHASE IN SOME FASHION ACROSS THE CENTRAL CONUS WITH THE FORMATION OF LOW PRES WHICH IS EXPECTED TO SLIDE ACROSS SRN CANADA. WARM FRONT WILL APPROACH...BUT GIVEN DEEP SNOWPACK THE LOW LVL FRONT MAY ACTUAL STALL NEAR THE S COAST AS THE LOW MOVES OVER AS THE UPPER WARM FRONT SLIDES INTO CENTRAL NEW ENGLAND. THIS PRESENTS A GOOD OVERRUNNING SITUATION WITH STRONG 50-60 KT H92 LLJ OUT OF THE SW. GIVEN THAT THIS SYSTEM IS ASSOCIATED WITH THE SRN STREAM...IT ALSO CARRIES WITH IT A FAIR AMOUNT OF MOISTURE AS PWATS LOOK TO REACH 2-3 STD DEVIATIONS ABOVE NORMAL SO ALL IN ALL..PLENTY OF INGREDIENTS FOR HEAVY PRECIP ACROSS THE AREA. THE QUESTION WILL BE TYPE...ALL GUIDANCE BEGIN COLD ENOUGH FOR SNOW AT ONSET TUE NIGHT INTO EARLY WED. ECMWF WOULD SUGGEST MUCH MORE AND BETTER LIFT IN THE SNOW GROWTH REGION...SO THERE IS THAT TO CONSIDER. IN ANY CASE...WILL NEED TO AT LEAST WATCH FOR POTENTIAL WARNING LEVEL SNOWFALL BEFORE THE WARM AIR BEGINS TO BUILD IN FROM S TO N THROUGH THE DAY ON WED. EVEN THOUGH THE ECMWF STARTS THE COLDEST...IT ENDS AS THE WARMEST SOLUTION...BRINGING THE H92 +8C LINE ALL THE WAY INTO CENTRAL MA. SO WE DO EXPECT A GRADUAL SHIFT TO A WINTRY MIX INCLUDING SLEET AND FREEZING RAIN FOR A TIME GIVEN THE DEEP SNOWPACK ACROSS THE REGION. SOME AREAS OF WRN MA...AND THE HILLS OF NRN CT MAY ACTUALLY STAY AS A MIX OF SLEET FREEZING RAIN AND EVEN MAYBE SOME SNOW GIVEN THAT THE SFC WARM FRONT MAY STALL. WILL HAVE TO WATCH FOR ICY CONDITIONS WITH AS MUCH AS A QUARTER INCH OF ICE NOT OUT OF THE QUESTION WHERE IT DOESN/T SWITCH TO RAIN WITH MELTING /BEST CHANCE OF RAIN/MELT IS IN RI/CT AND PORTIONS OF MASS E OF I-495 AND S OF THE MASS PIKE/. HEAVY RAIN A THREAT TOO GIVEN UPPER LVL INSTABILITY AND ROBUST LLJ...SO WHERE MELTING AND HEAVY RAIN ARE OBSERVED...POOR DRAINAGE FLOODING IS A POSSIBILITY WHERE SNOW CLOGS AREA DRAINS. THIS HEAVY MIXED PRECIP EVENT COULD LINGER INTO THU...THE MORE AMPLIFIED ECMWF AND SEVERAL ECENS/GEFS MEMBERS SUGGEST THAT THE LEFTOVER COLD FRONT STALLS WITH YET ANOTHER WEAK WAVE OF LOW PRES MOVING THROUGH THU. OTHER SOLUTIONS ARE MORE PROGRESSIVE AND HAVE LESS QPF /SNOW POTENTIALLY/ ON THE BACK END. HOWEVER...BY THU AFTERNOON/EVENING...CONDITIONS SHOULD BE IMPROVING /ALTHOUGH WITH RAPID TEMPERATURE DROP/. LATE NEXT WEEK... EVEN THOUGH THERE IS UNCERTAINTY WITH HOW AMPLIFIED THE PATTERN IS BY LATE WEEK. IT APPEARS A PERIOD OF DRY BUT COLD WX IS LIKELY AS ARCTIC AIR ONCE AGAIN SHIFTS IN BEHIND THE EXITING MID WEEK SYSTEM. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH MONDAY MORNING/... 21Z UPDATE... TONIGHT THROUGH SUNDAY MIDDAY...HIGH CONFIDENCE. VFR. LOWERING AND THICKENING CLOUDS SUNDAY MORNING INTO MIDDAY. LOW-END VFR TO W-TERMINALS BY END OF PERIOD. SUNDAY AFTERNOON INTO MONDAY MORNING...MODERATE CONFIDENCE. SNOW OVERSPREADS THE REGION W-E. CIGS/VSBYS DETERIORATE TO MVFR- IFR BY EARLY EVENING...WIDESPREAD IFR-LIFR SUNDAY NIGHT. SN TO MIX WITH OR CHANGE TO FZRA ACROSS OFFSHORE ISLAND TERMINALS OVERNIGHT. PRECIP ENDS W-E TOWARD DAYBREAK MONDAY....CONDITIONS IMPROVE. KBOS TAF...MODERATE CONFIDENCE. TRENDS CERTAIN BUT NOT SO MUCH THE ACTUAL TIMING OF IMPACTS. KBDL TAF...MODERATE CONFIDENCE. TRENDS CERTAIN BUT NOT SO MUCH THE ACTUAL TIMING OF IMPACTS. OUTLOOK...MONDAY THROUGH WEDNESDAY... MONDAY...HIGH CONFIDENCE. CONDITIONS IMPROVE EARLY FROM IFR/MVFR IN SNOWS AND LOW CIGS... WITH VFR EXPECTED EVERYWHERE BY AFTERNOON. NW WINDS IN THE AFTERNOON WITH GUSTS 25-30 KT. MON NIGHT INTO TUE...HIGH CONFIDENCE. MAINLY VFR. GRADUALLY LOWERING CIGS ON TUE. TUE NIGHT INTO WED...MODERATE CONFIDENCE. MIXED WINTRY PRECIP LIKELY STARTS AS ALL SNOW...TRANSITIONS TO A MIX OF SLEET AND FREEZING RAIN...THEN TO ALL RAIN DURING THE DAY ON WED. SOME SNOW AND ICE ACCUMULATIONS ARE LIKELY WITH IFR OR POTENTIALLY EVEN LIFR CONDITIONS AT TIMES. && .MARINE... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT. MODERATE...30 TO 60 PERCENT. HIGH...GREATER THAN 60 PERCENT. SHORT TERM /THROUGH MONDAY MORNING/...HIGH CONFIDENCE. HIGH PRESSURE NEARBY WILL KEEP WINDS/SEAS WELL BELOW SMALL CRAFT ADVISORY THRESHOLDS INTO SUNDAY MORNING. A WEAK WAVE OF LOW PRESSURE MOVES ACROSS THE WATERS LATE SUNDAY INTO SUNDAY NIGHT... BUT WEAK PRESSURE GRADIENT SHOULD KEEP WINDS/SEAS BELOW SMALL CRAFT THRESHOLDS. OUTLOOK...MONDAY THROUGH WEDNESDAY... MONDAY INTO MONDAY NIGHT...HIGH CONFIDENCE. SW WINDS SHIFT TO THE NW DURING THE DAYLIGHT HOURS AS A COLD FRONT SWEEPS ACROSS THE WATERS. ONCE WINDS SHIFT NW IN THE AFTERNOON EXPECT GALE FORCE GUSTS 35-40KT. COLDER AIR TUE NIGHT COULD LEAD TO LIGHT FREEZING SPRAY. TUESDAY...MODERATE CONFIDENCE. WINDS GRADUALLY DIMINISH OUT OF THE W-NW BUT SEAS ARE LIKELY TO REMAIN ELEVATED 5-8FT ON THE OCEAN WATERS. SMALL CRAFT ADVISORIES WILL LIKELY BE NEEDED. TUESDAY NIGHT INTO WEDNESDAY...MODERATE CONFIDENCE. SNOW CHANGES TO RAIN TUE NIGHT WITH LOW VSBYS OVER THE WATERS. WINDS OUT OF THE S MAINLY AROUND 25-30 KT AT MOST. SEAS BUILD 12-15 FT BY DAYTIME ON WED ON SE OCEAN WATERS. SMALL CRAFT ADVISORIES WILL LIKELY BE NEEDED. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY MORNING FOR CTZ002>004. MA...WINTER WEATHER ADVISORY FROM NOON SUNDAY TO 7 AM EST MONDAY FOR MAZ002>008-010-014-023-024-026. WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY MORNING FOR MAZ009-011>013-015>022. RI...WINTER WEATHER ADVISORY FROM NOON SUNDAY TO 7 AM EST MONDAY FOR RIZ008. WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY MORNING FOR RIZ001>007. MARINE...NONE. && $$ SYNOPSIS...DOODY/SIPPRELL NEAR TERM...SIPPRELL SHORT TERM...SIPPRELL LONG TERM...DOODY AVIATION...DOODY/SIPPRELL MARINE...DOODY/SIPPRELL

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