Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 270718 AFDBOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TAUNTON MA 318 AM EDT WED AUG 27 2014 .SYNOPSIS...
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HIGH PRESSURE OFFSHORE WILL DRAW WARM AIR ACROSS NEW ENGLAND TODAY. A COLD FRONT WILL THEN SWEEP THROUGH DURING THE LATE AFTERNOON AND EARLY NIGHT. DRY AND COOLER CONDITIONS RETURN THURSDAY AND FRIDAY BEHIND THE FRONT. HURRICANE CRISTOBAL WILL PASS WELL OFFSHORE THURSDAY...BUT WILL DELIVER ROUGH SURF AND RIP CURRENTS TO SOUTH COAST OCEAN BEACHES. ANOTHER FRONT MAY BRING UNSETTLED CONDITIONS SUNDAY AND MONDAY.
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&& .NEAR TERM /THROUGH TODAY/...
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THROUGH 8 AM... HIGH PRESSURE CONTINUES TO SHIFT OFFSHORE WITH A LIGHT SOUTHWEST FLOW MOVING INTO NEW ENGLAND. INCREASING CIRRUS SKY COVER WILL MOVE ACROSS FROM NEW YORK. LOWER THICKER CLOUDS OVER CENTRAL NY EXTRAPOLATE TO THE CT VALLEY AFTER 8 AM. THE FAIR SKIES AND LIGHT FLOW WILL ALLOW SOME FOG TO FORM ESPECIALLY ALONG THE SOUTH COAST WHERE THE SOUTHWEST FLOW IS OFF THE OCEAN. TODAY... COLD FRONT APPROACHES FROM THE ST LAWRENCE VALLEY WITH A BAND OF THICKER CLOUDS. MEANWHILE SUN THIS MORNING AND MIDDAY WILL GENERATE SUFFICIENT MIXING TO REACH 850 MB...WHERE TEMPS ARE 16-17C. IF WE ATTAIN FULL MIXING THEN MAX TEMPS SHOULD REACH NEAR 90. INCREASING SKY COVER AS THE FRONT APPROACHES MAY LIMIT MAX TEMPS A FEW DEGREES BELOW THIS. MAIN QUESTION WILL BE THE POTENTIAL FOR CONVECTION AND ITS SEVERITY. INSTABILITY FIELDS FROM THE ECMWF ARE SOMEWHAT SUPPORTIVE OF CONVECTION WITH TOTALS IN THE MID 40S AND CAPE 500-700 J/KG...WHILE THE GFS HAS SIMILAR MID 40S TOTALS BUT IS MORE GUNG- HO WITH CAPE 1500-2000 J/KG. EVEN WITH THE MILDER ECMWF VALUES...GENERAL THUNDER IS REASONABLE. WE WILL CONTINUE TO ADVERTISE CHANCE POPS FOR SHOWERS/TSTMS. SEVERE STORMS ARE LESS LIKELY. MOST OF THE FAVORABLE DYNAMICS WILL BE NORTH AND WEST THROUGH THE AFTERNOON/EVENING. WINDS AT 850 MB WILL BE MOSTLY BELOW 20 KNOTS WHILE 500 MB WINDS OF 35 KNOTS OR HIGHER ARE AT BEST REACHING OUR EXTREME NORTHWEST AREAS AROUND 00Z...NEAR SUNSET. PRECIP WATER VALUES INCREASE DURING THE AFTERNOON WITH GFS AND NAM SHOWING 1.75 TO 2.0 INCHES. EVEN IF WE STAY NEARER TO 1.5 INCHES...THIS WILL BE AROUND 1.5-1.9 STD DEVIATIONS ABOVE NORMAL. PUTTING IT TOGETHER...CAN/T RULE OUT ISOLATED SEVERE WIND GUSTS BUT BETTER CHANCE OF A FEW LOCALLY HEAVY DOWNPOURS. OTHER CONCERN WILL BE INCREASING SWELL FROM HURRICANE CRISTOBAL. FIVE FOOT SWELL DOESN/T ARRIVE IN THE SOUTHERN WATERS UNTIL AFTER 00Z. WHILE CONDITIONS MAY DETERIORATE A LITTLE DURING THE DAY...EXPECT THE SIGNIFICANT CHANGES TO HOLD OFF UNTIL AFTER SUNSET.
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&& .SHORT TERM /TONIGHT THROUGH 6 PM THURSDAY/...
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TONIGHT... CONSENSUS OF THE MODEL SOLUTIONS BRINGS THE COLD FRONT TO THE COASTAL PLAIN AROUND SUNSET AND OFFSHORE EARLY TONIGHT. EXPECT LINGERING SHOWER/TSTM THROUGH 10 PM IN RI AND EASTERN MASS...AND THROUGH MIDNIGHT CAPE AND ISLANDS. WINDS TURN NORTHWEST BEHIND THE COLD FRONT AND DRAW COOLER DRIER AIR DOWN FROM CANADA. THIS WILL MEAN CLEARING SKIES AND DEW POINTS IN THE 50S AND LOWER 60S. THURSDAY... UPPER SHORTWAVE MOVES ACROSS NEW ENGLAND WITH BRIEF COOLING ALOFT. MOISTURE FIELDS SHOW LINGERING 80 PCT RH VALUES BETWEEN 800 AND 900 MB. THIS SUGGESTS DIURNAL CUMULUS WILL FORM DURING THE DAY. MIXING WILL REACH TO 850 MB WHERE TEMPS OF 10-12C WOULD SUPPORT MAX SFC TEMPS IN THE UPPER 70S AND LOW 80S. WINDS ALOFT WITHIN THE MIXED LAYER ARE FORECAST AT 20 KNOTS...WHICH MAY GET MIXED TO THE SURFACE IN GUSTS.
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&& .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
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*/ HIGHLIGHTS... - COOL AND DRY WEATHER UNDER HIGH PRESSURE FRIDAY - HOT AND HUMID CONDITIONS INTO THE WEEKEND - POSSIBLE THUNDERSTORMS SUNDAY...WITH HEAVY RAIN INTO MONDAY */ OVERVIEW AND MODEL PREFERENCE... CONTINUED DECENT SIGNAL OF A WARMER AND PERHAPS WETTER PATTERN INTO EARLY SEPTEMBER. DESPITE A NEARLY-EQUAL AMPLITUDE NEGATIVE NAO/PNA FORECAST...DOWNSTREAM RIDGING PER NEGATIVE PNA DOMINATES ESPECIALLY DURING THE SUMMER-MONTHS. NO SURPRISE TO SEE THE GEFS/NAEFS/CPC 6-10 AND 8-14 DAY ANOMALY FORECASTS HIGHLIGHTING NEAR- TO ABOVE-NORMAL CONDITIONS ACROSS THE REGION. CPC ALSO HINTS AT ABOVE-AVERAGE WETTER CONDITIONS LIKELY AS LOW-LEVEL HUMIDITY / PRECIPITABLE WATER VALUES ARE FORECAST TO BE ABOVE-AVERAGE PER ENSEMBLE FORECASTS PARENT WITH RIDGING AND ACCOMPANYING S-FLOW. EVALUATING DETERMINISTIC/ENSEMBLE GUIDANCE...THERE IS GOOD AGREEMENT WITHIN THE BROAD-SCALE PATTERN. BUT FINER DETAILS VARY...ESPECIALLY WITH REGARDS CONFLUENT SETUP INTO THE WEEKEND AND POTENTIAL WEAK- WAVE LOW INTO EARLY NEXT WEEK. MODELS APPEAR TO STRUGGLE WITH BOTH THE STRENGTH AND EVOLUTION OF THE W-ATLANTIC RIDGE / BERMUDA HIGH AND LOW OVER NE CANADA...BOTH OF WHICH INFLUENCE THE CONFLUENT SETUP AND ATTENDANT STALLED FRONTAL BOUNDARY. WOULD EXPECT A STRONGER BERMUDA HIGH TO HAVE GREATER INFLUENCE OVER THE REGIONS WEATHER AGAINST THE TROUGHING PATTERN WELL N. THIS COULD RESULT IN THE COMBINATION OF BOTH MOISTURE CONFLUENCE AND MID- TO UPPER-LEVEL DYNAMICS REMAINING N OF S NEW ENGLAND PRIOR TO A WEAK- WAVE LOW KICKER INTO EARLY NEXT WEEK. WITH THE MODEL VARIABILITY OF FRONTAL POSITION AND EVOLUTION OVER THE WEEKEND INTO EARLY NEXT WEEK...NO MODEL SOLUTION IS CERTAIN AND THUS WILL TREND WITH A CONSENSUS FORECAST. HIGHLIGHTS/CONFIDENCE ARE BROKEN DOWN IN THE DAILY DISCUSSION BELOW. */ DAY-TO-DAY DETAILS... THURSDAY NIGHT... DRY AND QUIET OVERNIGHT UNDER LIGHT WINDS AS SURFACE HIGH PRESSURE EDGES CLOSER FROM THE W. EXPECTING AREAS OF EFFECTIVE RADIATIONAL COOLING...PERHAPS PATCHY DENSE FOG. AVERAGE LOWS AROUND THE REGION IN THE LOW- TO MID-50S...BUT WOULD NOT BE SURPRISED TO SEE SEVERAL AREAS IN THE UPPER-40S...ESPECIALLY THOSE AREAS WHICH RADIATE WELL. FRIDAY... HIGH PRESSURE IN CONTROL WITH LIGHT AND VARIABLE WINDS. WITH LITTLE CHANGE TO THE AIRMASS ALOFT /H85 +8-10C/...ANTICIPATE ANOTHER DAY OF SEASONABLE CONDITIONS UNDER MOSTLY CLEAR CONDITIONS. HIGHS AROUND THE MID- TO UPPER-70S...COOLER ALONG THE SHORES WITH EXPECTED SEA- BREEZES. OVERNIGHT...MAY SEE SOME MID- TO HIGH-LEVEL CLOUDS STREAM INTO THE REGION LATE AHEAD OF THE WEEKENDS WET-WEATHER. OTHERWISE LOWS AROUND THE LOW- TO MID-50S AS WINDS TURN S AND ONSHORE AS THE SURFACE HIGH PRESSURE SHIFTS OFFSHORE. SATURDAY INTO MONDAY... A CONFLUENT PATTERN BETWEEN AIRMASSES SETS UP ACROSS THE NE-CONUS PARENT TO WHICH A FRONTAL BOUNDARY STALLS AND BECOMES THE FOCUS FOR LOW- TO MID-LEVEL MOISTURE CONFLUENCE. STRETCHED MID-LEVEL ENERGY TRANSLATES ALONG THE BOUNDARY PROVIDING ASCENT AND YIELDING PERIODS OF RAIN...POSSIBLE THUNDERSTORMS. ACTIVITY APPEARS TO CONCLUDE AS A WEAK-WAVE LOW TRANSLATES ALONG THE FRONT PUSHING ACTIVITY OFFSHORE AROUND MONDAY. NO ONE MODEL SOLUTION IS PREFERRED. BELIEVE THE BERMUDA HIGH WILL HAVE A GREATER INFLUENCE OVER THE AREAS WEATHER INITIALLY SATURDAY INTO SUNDAY. DAILY OCCURRENCES OF SCATTERED SHOWERS / THUNDERSTORMS ARE POSSIBLE /THE GREATER PROBABILITY ON SUNDAY ACROSS INTERIOR NEW ENGLAND/...BUT WILL HOLD OFF HEAVIER RAIN TILL A WEAK-WAVE LOW ALONG THE FRONT ACTS AS A KICKER PUSHING ACTIVITY THROUGH THE REGION AND OFFSHORE AROUND MONDAY. CONSIDERING THE CONFLUENCE OF HIGH THETA-E MOIST TONGUE WITHIN THE LOW-LEVELS ALONG AND S OF THE STALLED FRONT YIELDING PWATS AROUND 1.75 AND 2.00 INCHES...GREATEST CONCERN IS FOR HEAVY RAIN AND THE POSSIBILITY OF FLOODING. UNCERTAIN AS TO THUNDERSTORM AND SEVERE WEATHER POTENTIAL. A CONDITIONALLY UNSTABLE PROFILE S OF THE FRONT IS MODELED IN A REGION OF FALLING HEIGHTS AND PERHAPS UPPER-LEVEL SUPPORT WITHIN THE RRQ OF THE UPPER-LEVEL JET. SOME INDICATIONS OF DECENT INSTABILITY WITHIN MODEST ROUGHLY UNI-DIRECTIONAL W-SHEAR. FREEZING LEVEL HEIGHTS ARE AROUND 14 KFT...SO IF ANY SEVERE THREATS WOULD LEAN TOWARDS DAMAGING WINDS AS 0-6 KM BULK SHEAR AND MEAN WIND ARE NEARLY PARALLEL. LOW CONFIDENCE AT THIS TIME. WE ARE STILL 5-6 DAYS OUT FROM THE FORECAST PERIOD. CAN ONLY SPECULATE FOR NOW AS THE RIDGE-TROUGH INTERSECT YIELDING THE CONFLUENT FLOW REGIME IS CRITICAL IN DETERMINING THE COLLOCATED FRONTAL BOUNDARY. EXPECTING WOBBLES IN THE FRONTAL POSITION WITH LATER FORECASTS RESULTING IN VARYING OUTCOMES. THIS IS WHY CHANCE OVER LIKELY POPS CONTINUE. WILL PREVAIL WITH CONDITIONS THAT ARE SLIGHTLY ABOVE-AVERAGE. INTO MIDWEEK... A BRIEF PERIOD OF QUIET WEATHER IS ANTICIPATED BEHIND THE WEAK-WAVE LOW. A FOLLOW-UP COLD FRONT IS POSSIBLE TOWARDS THE END OF THE WEEK.
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&& .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
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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 THROUGH THURSDAY/...MODERATE CONFIDENCE. TODAY...MOSTLY VFR. PATCHY MVFR/IFR IN FOG ESPECIALLY ALONG THE SOUTH COAST THIS MORNING. SCATTERED SHOWERS/TSTMS SHOULD DEVELOP THIS AFTERNOON...INITIALLY INLAND FROM AROUND MHT TO BAF AND BDL BUT THE POTENTIAL SPREADS EAST TO BOS-PVD LATER IN THE DAY. LOW CONFIDENCE IN SPECIFIC SITES THAT WOULD BE AFFECTED SO NO MENTION IN THE TAFS AT THIS TIME. THIS WILL CHANGE AS WE GET LATER INTO THE DAY. BRIEF MVFR/IFR UNDER THE CORE OF ANY SHOWERS/STORMS...ALSO BRIEF HEAVY RAIN AND MAYBE A STRONG WIND GUST. TONIGHT...MOSTLY VFR. ANY LINGERING SHOWERS/TSTMS IN EASTERN MASS/RI WILL MOVE OFFSHORE OR DISSIPATE DURING THE FIRST PART OF THE NIGHT. WINDS THEN BECOME NORTHWEST OVERNIGHT AS THE COLD FRONT MOVES OFFSHORE. THURSDAY...VFR. NORTHWEST WINDS MAY GUST TO 20 KNOTS WITH SCATTERED TO BROKEN SKY COVER AROUND 5000 FEET FROM MIDDAY TO EVENING. KBOS TERMINAL...MODERATE CONFIDENCE IN TAF. SW-WINDS PREVAILING. SCT SHRA/TSRA POSSIBLE AROUND 21Z...LOW CONFIDENCE ON DIRECT HIT FOR TERMINAL SO NO MENTION AT THIS TIME. KBDL TERMINAL...MODERATE CONFIDENCE IN TAF. SW-WINDS PREVAILING. SCT SHRA/TSRA POSSIBLE AROUND 19Z...LOW CONFIDENCE ON DIRECT HIT FOR TERMINAL SO NO MENTION AT THIS TIME. OUTLOOK...THURSDAY NIGHT THROUGH SUNDAY... THURSDAY NIGHT INTO FRIDAY...HIGH CONFIDENCE. VFR WITH SKC THURSDAY NIGHT INTO FRIDAY. LIGHT WINDS. PATCHY MVFR- IFR FOG POSSIBLE DURING THE OVERNIGHT PERIOD. SEA- BREEZES FRIDAY. SATURDAY INTO SUNDAY...MODERATE CONFIDENCE. VFR. INCREASING S-WINDS. BETTER CHANCES FOR SHRA/TSRA SUNDAY ACROSS THE INTERIOR.
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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 INTO WEDNESDAY NIGHT/...HIGH CONFIDENCE. TONIGHT...TRANQUIL WEATHER REGARDING WINDS AND SEAS. HOWEVER LOW CLOUDS...FOG AND POSSIBLY PATCHY DRIZZLE WILL LIKELY IMPACT THE SOUTHERN WATERS LATER TONIGHT. WEDNESDAY...LOW CLOUDS/FOG MAY LINGER INTO EARLY AFTERNOON ACROSS THE SOUTHERN WATERS. ELSEWHERE GOOD VSBY. LOW RISK OF A FEW SHOWERS/TSTMS TOWARD SUNSET. SW WINDS 10-15 KT POSSIBLY GUSTING TO 20 NEAR SHORE. 5 FT SOUTH SWELLS FROM HURRICANE CRISTOBAL ARRIVE IN THE SOUTHERN WATERS LATE IN THE DAY. WEDNESDAY NIGHT...SOUTH SWELLS FROM CRISTOBAL INCREASE RAPIDLY TO 6-9 FT BY DAYBREAK THU ACROSS THE SOUTHERN WATERS. A FEW TSTMS POSSIBLE WED EVENING. WIND SHIFT FROM SSW TO NW WITH THE FRONTAL PASSAGE. OUTLOOK...THURSDAY THROUGH SUNDAY... THURSDAY...HIGH CONFIDENCE. S-SWELL ASSOCIATED WITH CHRISTOBAL CONTINUES...BEGINNING TO DIMINISH INTO THE EVENING PERIOD. WITH BREEZY NW-FLOW WITH GUSTS UP TO 25 KTS POSSIBLE...AN OPPOSING WIND WILL MAKE FOR SOME CHOPPY CONDITIONS. WAVE HEIGHTS OF 8-10 FEET ARE FORECAST ACROSS THE ATLANTIC-EXPOSED S/SE WATERS. S-FACING BAYS AND HARBORS BEING SHELTERED FROM THE SWELL WILL HAVE LOWER WAVE HEIGHTS...BUT STILL HAVE THE NW-WIND THREAT. FRIDAY...HIGH CONFIDENCE. HIGH PRESSURE IN CONTROL. LIGHT WINDS. DIMINISHING SEAS WITH ONLY THE S/SE OUTER WATERS EXPERIENCING WAVE HEIGHTS IN EXCESS OF 5-FEET BY MORNING. GOOD BOATING WEATHER. SATURDAY INTO SUNDAY...MODERATE CONFIDENCE. INCREASING S-WINDS. FEEL MAJORITY OF WATERS WILL REMAIN DRY AS SHOWER AND THUNDERSTORM ACTIVITY IS ONGOING ACROSS INTERIOR NEW ENGLAND...MAINLY SUNDAY. SOME OF THE WET-WEATHER MAY DRIFT ACROSS THE E-WATERS POSING THREATS TO MARINERS...YET LOW CONFIDENCE AT THIS TIME. PROLONGED S-FETCH LENDING TO WAVE HEIGHTS IN EXCESS OF 5-FEET ACROSS THE S/SE WATERS. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...AIR QUALITY ALERT FROM 1 PM THIS AFTERNOON TO 9 PM EDT THIS EVENING FOR CTZ002>004. MA...NONE. NH...NONE. RI...AIR QUALITY ALERT FROM 11 AM THIS MORNING TO 11 PM EDT THIS EVENING FOR RIZ001>005. MARINE...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS FROM 5 PM THIS AFTERNOON TO 6 PM EDT THURSDAY FOR ANZ235-237-254>256.
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&& $$ SYNOPSIS...WTB/SIPPRELL NEAR TERM...WTB SHORT TERM...WTB LONG TERM...SIPPRELL AVIATION...WTB/SIPPRELL MARINE...NOCERA/SIPPRELL

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