Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 010617 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 115 AM EST SUN MAR 1 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. HIGH PRESSURE BRINGS DRY COLD AIR FOR THE EARLY WEEK. LOW PRESSURE FROM NORTHERN CANADA AND ASSOCIATED LOW FORMING OVER THE PLAINS MONDAY WILL MERGE...THEN PASS WEST AND NORTH OF NEW ENGLAND. THIS WILL SWING A COLD FRONT ACROSS THE REGION ON WEDNESDAY. DRY AND COLD WEATHER RETURNS FOR THE END OF THE WEEK. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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115 AM UPDATE... CLEAR SKIES...LIGHT WINDS AND DEEP SNOW PACK WAS RESULTING IN ANOTHER EXCELLENT NIGHT OF RADIATIONAL COOLING. TEMPS ALREADY BELOW ZERO IN SOME OF THE NORMALLY COLDEST OUTLYING LOCATIONS...WHILE THE URBAN HEAT ISLAND OF BOSTON STILL HAD TEMPS IN THE LOWER 20S! LOW TEMPS WILL BOTTOM OUT NEAR 10 BELOW ZERO ACROSS THE NORMALLY COLDEST OUTLYING LOCATIONS. OTHER AREAS THAT DO NOT RADIATE AS WELL SUCH AS THE HIGHER TERRAIN/URBAN CENTERS WILL SEE LOWS MAINLY IN THE SINGLE DIGITS...TO LOWER TEENS. THESE LOW TEMPS WILL LIKELY OCCUR BY 4 OR 5 AM AND THEN LEVEL OFF OR RISE BY DAYBREAK...AS SOME MID/HIGH LEVEL CLOUDINESS MOVES IN FROM THE WEST.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... */ 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 NIGHT THROUGH SATURDAY/... BIG PICTURE... THE BROAD UPPER FLOW SHOWS A RIDGE IN THE EAST AND TROUGH IN THE WEST WITH A WEST SOUTHWEST FLOW FROM THE PLAINS TO THE EASTERN USA COAST. WITHIN THIS FLOW ARE BOTH NORTHERN AND SOUTHERN STREAMS AT THE START OF THE WEEK. THE WESTERN TROUGH MIGRATES EAST DURING THE MIDWEEK AT WHICH TIME THE TWO STREAMS MERGE TO FORM A 140- 160 KT UPPER JET FEEDING NORTHEAST INTO THE GREAT LAKES AND ST LAWRENCE VALLEY. THIS JET LINGERS OVER OR NEAR NEW ENGLAND THURSDAY- FRIDAY BEFORE MOVING OFF. THE WESTERN TROUGH MOVES ACROSS NEW ENGLAND BY THE END OF THE WEEK. DAILIES... MONDAY... SUNDAY STORM MOVES OFF PAST NOVA SCOTIA WHILE THE SUPPORTING UPPER SHORTWAVE AND COLD POOL SWING ACROSS NEW ENGLAND. THE COOLEST AIR ALOFT IS IN NORTHERN NEW ENGLAND WHILE LEFTOVER MOISTURE LINGERS BELOW 800 MB THROUGH THE DAY. THE GFS IS FASTER THAN THE ECMWF IN BRINGING RISING PRESSURE TO THE REGION BUT HAVE 1 MB/HOUR RISES HERE BY EVENING. PUT TOGETHER...EXPECT A CLEARING TREND BUT WITH LEFTOVER CLOUDS ESPECIALLY NORTH OF THE MASS PIKE. SUSTAINED NORTHWEST WINDS SHOULD RISE TO 15-20 KTS. MIXING REACHES TO AT LEAST 850 MB AND POSSIBLY A LITTLE HIGHER...WHICH SUPPORTS GUSTS TO AT LEAST 30 KNOTS AND POSSIBLY 35 KNOTS. TEMPS AT 850 MB WILL BE -11C TO -14C...MIXING THIS TO THE SURFACE WOULD SUPPORT MAX SFC TEMPS IN THE 30S. MIN TEMPERATURES UPSTREAM THIS MORNING WERE IN THE SINGLE NUMBERS WITH A FEW SPOTS BELOW ZERO. WITH LIGHT PRESSURE GRADIENT ESP IN THE WEST...WE TENDED A FEW DEGREES COOLER THAN CONSENSUS. TUESDAY-WEDNESDAY... HIGH PRESSURE IN CONTROL MUCH OF TUESDAY BUT MOVING OFFSHORE DURING THE AFTERNOON. CROSS SECTIONS SHOW INCREASING MOISTURE AT MID AND HIGH CLOUD LEVEL...SO INCREASING HIGH CLOUDS. SYSTEM MOVING INTO THE GREAT LAKES SPREADS PCPN INTO NEW YORK...BUT WARM ADVECTION LIFT REMAINS MOSTLY TO OUR WEST THROUGH THE AFTERNOON. SOME LIFT FORECAST AT 290K /ABOUT 730 MB AT THE TIME/ BUT NONE BELOW THAT. MEANWHILE DEWPOINTS IN THE SINGLE NUMBERS PROGRESS INTO THE TEENS DURING THE DAY...BUT ENOUGH OF A TEMP/DEWPOINT SPREAD TO SLOW THE ADVANCE OF ANY PCPN FROM THE CLOUDS BY A FEW HOURS. WARM ADVECTION LIFT MUCH OF THE NIGHT SUPPORTS THE IDEA OF PCPN. TEMPERATURES START COLD DURING THE EVENING BUT SHOULD RISE OVERNIGHT. THE QUESTION REMAINS AS TO HOW HIGH THEY WILL CLIMB. EVEN WITH THE SURFACE SYSTEM TRACKING UP THE ST LAWRENCE VALLEY THERE IS COLD GROUND AND A COLD SNOWPACK. THIS WOULD ALLOW THE WARMER TEMPS TO SCREAM OVERHEAD WHILE THE IMMEDIATE SURFACE REMAINS SEVERAL DEGREES COLDER. WE TRIMMED TEMPS BACK A LITTLE FROM MODEL VALUES...BUT LOW CONFIDENCE AT THIS TIME DUE TO THE POTENTIAL THAT SURFACE TEMPS COULD REMAIN IN THE 30S. QPF FORECASTS RANGE FROM 0.50 TO 1.00 INCHES WITH THE HIGHER AMOUNTS TENDING TOWARD THE SOUTH COAST. THIS POINTS TO A MESSY STORM WITH INITIAL SNOW ACCUMS OF SEVERAL INCHES FOLLOWED BY A CHANGE TO MEASURABLE ICE ACCUMULATION AND THEN A CHANGE TO RAIN WITH POTENTIAL FOR POOR DRAINAGE FLOODING IN BETWEEN THE SNOW BANKS. THE COLD FRONT SHOULD MOVE THROUGH DURING THE AFTERNOON/EVENING WITH A SHIFT TO NORTHWEST WINDS. THIS WILL CHANGE THE PCPN BACK TO SNOW BEFORE TAPERING OFF. THURSDAY...DIFFERENCES IN THE GFS AND ECMWF IN HANDING THE UPPER JET. THE ECMWF MOVES THE JET OFFSHORE AND PUTS SOUTHERN NEW ENGLAND INTO SUBSIDENCE. THE GFS MAINTAINS THE JET OVERHEAD WITH OUR AREA IN THE RIGHT ENTRANCE REGION. THE FIRST SCENARIO ENDS THE PCPN AND COOLS THE AIR. THE SECOND SCENARIO WOULD SUPPORT LIFT AND GENERATE WAVES ALONG THE DEPARTING COLD FRONT...MAINTAINING A CHANCE OF SNOW OVER AT LEAST SOUTHERN PARTS OF OUR AREA. OUR THURSDAY FORECAST TENDS TOWARD THE LATTER FOR NOW. FRIDAY-SATURDAY... HIGH PRESSURE BUILDS OVER THE REGION THURSDAY NIGHT AND FRIDAY WITH CLEARING SKIES AND A BRIEF PERIOD OF COLD AIR. TEMPS ALOFT TREND UP ABOUT 10C ON SATURDAY SUGGESTING LESS CHILL AT THE SURFACE. ANOTHER COLD FRONT APPROACHES THE ST LAWRENCE VALLEY AND COULD SPREAD SOME CLOUDS INTO OUR AREA SATURDAY AFTERNOON/EVENING. BUT PCPN CHANCES LOOK SMALL. && .AVIATION /06Z 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 /TODAY AND TONIGHT/... TODAY AND TONIGHT...MODERATE CONFIDENCE. VFR CONDITIONS THROUGH AT LEAST MID MORNING. SOME MVFR TO IFR CONDITIONS DEVELOP FROM WEST TO EAST LATE THIS MORNING AND THIS AFTERNOON IN AREAS OF LIGHT SNOW. HOWEVER...THE BULK OF THE MODERATE TO EVEN BRIEFLY HEAVY SNOW WILL OCCUR THIS EVENING WHEN WIDESPREAD IFR TO LIFR CONDITIONS OCCUR. THE LOWER CONDITIONS MOST LIKELY SOUTH OF THE MA TURNPIKE. PTYPE PRETTY MUCH ALL SNOW...EXCEPT FOR THE VERY IMMEDIATE SOUTH COAST/CAPE WHERE SNOW MAY MIX WITH OR CHANGE TO RAIN FOR A TIME. WHETHER OR NOT THAT HAPPENS IS UNCERTAIN...BUT WE DO THINK THAT AT LEAST THE ISLANDS WILL CHANGE TO RAIN. BULK OF SNOW ENDS BY 8 OR 9Z TONIGHT AND MUCH OF THE THE REGION SHOULD ALREADY HAVE VFR CONDITIONS BY 12Z MONDAY. KBOS TAF...CONFIDENT IN OVERALL TRENDS BUT EXACT TIMING IS MORE UNCERTAIN. WHILE SOME LIGHT SNOW IS POSSIBLE THIS AFTERNOON...BULK OF ACCUMULATING SNOW WITH WIDESPREAD IFR TO LIFR CONDITIONS EXPECTED THIS EVENING. KBDL TAF...CONFIDENT IN OVERALL TRENDS BUT EXACT TIMING IS MORE UNCERTAIN. WHILE SOME LIGHT SNOW IS POSSIBLE LATE THIS MORNING AND AFTERNOON...BULK OF ACCUMULATING SNOW WITH WIDESPREAD IFR TO LIFR CONDITIONS EXPECTED AFTER 3 OR 4 PM. OUTLOOK...MONDAY THROUGH THURSDAY... MONDAY... HIGH CONFIDENCE. SOME IFR/MVFR IN SNOW EARLY MORNING BUT OTHERWISE CONDITIONS WILL IMPROVE TO VFR DURING THE MORNING. NORTHWEST WINDS WILL INCREASE WITH GUSTS AROUND 30 KNOTS BY MIDDAY. VFR AND DIMINISHING WIND MONDAY NIGHT. TUESDAY...MODERATE CONFIDENCE. VFR DURING THE DAY WITH INCREASING SKY COVER. MVFR TUESDAY EVENING BECOMING IFR/LIFR OVERNIGHT IN DEVELOPING SNOW AND FREEZING RAIN. WEDNESDAY...MODERATE CONFIDENCE. IFR/LIFR IN RAIN AND FREEZING RAIN. THURSDAY...LOW CONFIDENCE. VFR NORTH OF A HFD-BOS LINE. MVFR CIGS AND AREAS OF MVFR/IFR VSBYS IN SNOW.
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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 /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 THURSDAY..MODERATE CONFIDENCE. MONDAY...WEST TO NORTHWEST WINDS INCREASE WITH FREQUENT GUSTS TO GALE FORCE ESPECIALLY AWAY FROM THE NEARSHORE. THIS COULD GENERATE SOME LIGHT FREEZING SPRAY ESPECIALLY EARLY MONDAY NIGHT. SEAS 5 TO 10 FEET. WINDS AND SEAS DIMINISH MONDAY NIGHT. A GALE WARNING MAY BE NEEDED. TUESDAY... LINGERING 5 FOOT SEAS ON THE OUTER WATERS BUT DIMINISHING DURING THE MORNING. WINDS AND SEAS INCREASE AGAIN TUESDAY NIGHT AFTER MIDNIGHT AHEAD OF A GREAT LAKES COLD FRONT. SOUTH WINDS WILL APPROACH 25 KNOTS WITH SEAS BUILDING TO 5 TO 8 FEET. VSBYS WILL BE REDUCED IN SNOW CHANGING TO RAIN. WEDNESDAY...SOUTHWEST WINDS WITH GUSTS 25-30 KNOTS. SEAS BUILDING TO 6 TO 12 FEET. REDUCED VSBYS IN RAIN. A SMALL CRAFT ADVISORY MAY BE NEEDED. THURSDAY...NORTHWEST WINDS WILL GUST TO 25 KNOTS...AND SEAS 5 TO 8 FEET. BOTH SHOULD DIMINISH TOWARD EVENING. A SMALL CRAFT ADVISORY MAY BE NEEDED. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...WINTER STORM WATCH FROM NOON EST TODAY THROUGH MONDAY MORNING FOR CTZ002>004. MA...WINTER WEATHER ADVISORY FROM NOON TODAY TO 7 AM EST MONDAY FOR MAZ002>008-010-014-023-024-026. WINTER STORM WATCH FROM NOON EST TODAY THROUGH MONDAY MORNING FOR MAZ009-011>013-015>022. RI...WINTER WEATHER ADVISORY FROM NOON TODAY TO 7 AM EST MONDAY FOR RIZ008. WINTER STORM WATCH FROM NOON EST TODAY THROUGH MONDAY MORNING FOR RIZ001>007. MARINE...NONE.
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&& $$ SYNOPSIS...WTB/SIPPRELL NEAR TERM...FRANK SHORT TERM...SIPPRELL LONG TERM...WTB AVIATION...FRANK/WTB MARINE...WTB/SIPPRELL

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