Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 182258
AFDBOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TAUNTON MA
658 PM EDT FRI JUL 18 2014

.SYNOPSIS...
HIGH PRESSURE OVER THE NORTHEAST USA WILL MOVE OFFSHORE OVER THE
WEEKEND. LOW PRESSURE DEVELOPING OFF THE CAROLINAS WILL PASS
SOUTHEAST OF NANTUCKET SUNDAY. HIGH PRESSURE MAINTAINS DRY WEATHER
THROUGH THE EARLY WEEK. A COLD FRONT FROM CANADA MOVES OVER NEW
ENGLAND MIDWEEK.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...

7 PM UPDATE...
UPDATED FORECAST TO ACCOUNT FOR CURRENT TRENDS...INCLUDING SKY
GRIDS TO ACCOUNT FOR PERSISTENT BAND OF CIRRUS ALONG SOUTH COAST.

PREVIOUS DISCUSSION...

TONIGHT...
HIGH PRESSURE OVERHEAD WITH LIGHT FLOW AND MOSTLY CLEAR SKIES.
ANTICIPATE SOME RADIATION FOG IN THE USUAL SPOTS. BEST CHANCE AS
WITH LAST NIGHT WILL BE IN THE CT VALLEY ESPECIALLY IN NORTHERN
MASS AND SOUTHWEST NH. DEWPOINTS IN THE 50S TO NEAR 60...AND SO
WITH RADIATIONAL COOLING WILL EXPECT MIN TEMPS CLOSE TO DEWPOINT
INLAND AND LOW 60S ALONG THE COAST.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
SATURDAY...HIGH PRESSURE MOVES OFFSHORE TO THE EAST BY AFTERNOON.
EXPECT DRY WEATHER WITH LIGHT WINDS SHIFTING TO EAST-SOUTHEAST BY
AFTERNOON. SIMILAR MIXING DEPTHS TO TODAY ARE FORECAST. BUT THE
ONSHORE FLOW WILL BUFFER THE WARMTH ALONG THE COAST. EXPECT A
RANGE FROM MID 70S TO LOW 80S.

SATURDAY NIGHT...
OFFSHORE HIGH PRESSURE WILL KEEP THE WEATHER DRY MOST PLACES. WE
ARE WATCHING A SOUTHERN STREAM SHORTWAVE THAT IS EXPECTED TO
GENERATE A COASTAL LOW OFF THE CAROLINAS SATURDAY. WITH SOUTHWEST
FLOW ALOFT...WE EXPECT THIS LOW WILL MOVE UP THE COAST BRINGING
INCREASING HIGH CLOUDS TO ALL AREAS DURING THE NIGHT. IF THE LOW
COMES CLOSE ENOUGH THEN IT COULD SPREAD SOME SHOWERS INTO THE
SOUTH COAST AREA AND POSSIBLY INTO THE REMAINDER OF RI AND
SOUTHEAST MASS. CURRENTLY...ONLY THE GFS BRINGS MEASUREABLE RAIN
TO OUR AREA BY 12Z SUNDAY. WE WILL BRING SLIGHT CHANCE POPS UP TO
A PVD-PYM LINE WITH CHANCE POPS ON NANTUCKET...WITH TIMING AFTER
04Z/MIDNIGHT. CLOUDS MAY KEEP TEMPS A LITTLE MILDER...WE USED A
RANGE FROM UPPER 50S TO MID 60S.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
HIGHLIGHTS...

* CHANCE OF SHOWERS OVER THE SOUTHEAST W/COOL ONSHORE FLOW SUNDAY.
* FAIR WEATHER W/SEASONABLE TEMPS AND INCREASING HUMIDITY MONDAY.
* TEMPERATURES IN THE UPPER 80S TO LOWER 90S TUESDAY...WEDNESDAY
  AND POSSIBLY THURSDAY...WITH DEWPOINTS IN THE MID 60S.
* BEST CHANCE OF SHOWERS AND THUNDERSTORMS IN THE WEDNESDAY AND
  THURSDAY TIMEFRAME WITH THE APPROACH OF A COLD FRONT.

OVERVIEW AND MODEL PREFERENCES...

MID-LEVEL TROUGH EJECTS OUT OF THE NORTHEAST US AND INTO THE
CANADIAN MARITIMES ON MONDAY. A RIDGE BUILDS OVER THE REGION
TUESDAY AND WEDNESDAY AND THE NEXT TROUGH APPROACHES THURSDAY.

THERE IS DISAGREEMENT IN THE MODELS EARLY IN THE PERIOD WITH
THE SPEED OF AN OFFSHORE LOW IN THE SUNDAY - MONDAY PERIOD...
WITH THE ECMWF PLACING THE LOW SOUTH OF THE BENCHMARK AT 12Z
SUNDAY...MEANWHILE THE GFS PLACES IT NEAR CAPE HATTERAS ATTM.
THIS IMPACTS JUST HOW QUICKLY SKIES CLEAR ON SUNDAY AND WENT
WITH A COMPROMISE SOLUTION FOR NOW THAT CLEARS SKIES OUT SUNDAY
NIGHT. THE GFS ALSO APPEARS A BIT OVERDONE ON IT/S NORTHWEST
EXTENT OF QPF ON SUNDAY...ESPECIALLY WITH HIGH PRESSURE DRAPED
ACROSS THE REGION...SO KEPT CHANCE POPS ON THE CAPE/ISLANDS.

LATER IN THE PERIOD...THE GFS IS MORE PROGRESSIVE WITH A FRONTAL
PASSAGE ON THURSDAY WHILE THE ECMWF STALLS THE FRONT ACROSS THE
REGION INTO FRIDAY.

DETAILS...

SUNDAY...BEST CHANCE OF SHOWERS WILL BE ACROSS THE CAPE/ISLANDS WITH
A MOIST ONSHORE FLOW...WHILE A RIDGE OF HIGH PRESSURE SHOULD CURTAIL
THE PRECIP FROM MAKING FURTHER NORTH/WEST. HOWEVER...THIS WILL HAVE
TO BE CLOSELY MONITORED. THERE WILL BE CONSIDERABLE CLOUDINESS...WITH
A GRADUAL CLEARING TREND GETTING UNDERWAY...ESPECIALLY LATER IN
THE DAY. AN ONSHORE FLOW WILL KEEP COASTAL AREAS IN PARTICULAR
COOLER.

MONDAY...HIGH PRESSURE BUILDS IN WITH PARTLY SUNNY SKIES AND DRY WX
EXPECTED. AN ONSHORE FLOW WILL PERSIST BEHIND THE DEPARTING LOW...
AND AS THE PRESSURE GRADIENT RELAXES...A SEA-BREEZE COMPONENT TOO.
THEREFORE EXPECT COASTAL AREAS COOLER WITH SEASONABLE TEMPS OVER THE
INTERIOR. IT WILL BECOME NOTICEABLY MORE HUMID LATER MONDAY...AND
THERE MAY BE FOG/LOW CLOUDS ALONG THE SOUTH COAST MONDAY NIGHT WITH
THE APPROACH OF A WARM FRONT.

TUESDAY...WARM FRONT MOVES THROUGH THE REGION. ALTHOUGH SOME MODELS
INDICATE LIGHT QPF /ESPECIALLY NORTHWEST/...COLUMN IS DRY AND THERE
IS LITTLE SUPPORT FOR THIS...SO LEFT FORECAST DRY. SUN AND MXG TO
850 HPA SUPPORTS HIGHS 85 TO 90...EVEN ALONG THE COAST...AS THE
PRESSURE GRADIENT LOOKS TO SUPPRESS THE SEA-BREEZE.

WEDNESDAY...TROUGH APPROACHES FROM THE WEST. AT THIS POINT...THERE
APPEARS TO BE MORE THAN ADEQUATE INSTABILITY FOR CONVECTION...BUT
SHEAR IS MINIMAL. THERE MAY ALSO BE SOME SLIGHT RIDGING AT MID-
LEVELS WHICH COULD KEEP THE ATMOSPHERE CAPPED AND WHETHER OR NOT
THERE IS SUFFICIENT MOISTURE IN THE COLUMN IS QUESTIONABLE. THE BEST
CHANCE OF CONVECTION MAY HOLD OFF UNTIL WEDNESDAY NIGHT. ANOTHER HOT
DAY IS IN STORE WITH 850 HPA TEMPS SUPPORTING HIGHS IN THE UPPER 80S
TO LOWER 90S...AND ONCE AGAIN...THE PRESSURE GRADIENT APPEARS TO
INHIBIT THE SEA-BREEZE.

THURSDAY AND FRIDAY...FRONT MOVES THROUGH THE REGION SOMETIME DURING
THIS TIMEFRAME WITH THE BEST CHANCE OF SHOWERS AND THUNDERSTORMS ALONG
WITH TEMPERATURES RETURNING TO MORE SEASONABLE LEVELS BEHIND THE
FRONT.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
FORECASTER CONFIDENCE LEVELS...

LOW...LESS THAN 30 PERCENT.
MODERATE...30 TO 60 PERCENT.
HIGH...GREATER THAN 60 PERCENT.

SHORT TERM /THROUGH THURSDAY NIGHT/...

TONIGHT...HIGH CONFIDENCE.
VFR. PATCHY FOG/LOW CLOUDS IN NORTHWEST MASS/SOUTHWEST NH WITH
IFR/LIFR CIGS AND VSBYS.

SATURDAY...HIGH CONFIDENCE. VFR. SEA BREEZES DEVELOP BY LATE AM.

SATURDAY NIGHT...HIGH CONFIDENCE.
VFR. SHOWERS MAY BRING BRIEF MVFR VSBYS TO THE ISLANDS LATE AT
NIGHT.

KBOS TAF...HIGH CONFIDENCE IN TAF. SEA BREEZE DEVELOPS AROUND
14-15Z SATURDAY.

KBDL TAF...HIGH CONFIDENCE IN TAF.

OUTLOOK...SUNDAY THROUGH WEDNESDAY...

SUNDAY...MODERATE CONFIDENCE. MVFR/IFR POSSIBLE IN LOW CEILINGS AND
SHRA MAINLY CONFINED TO CAPE COD AND THE ISLANDS. VFR EXPECTED
ELSEWHERE.

MONDAY THROUGH WEDNESDAY...LOW CONFIDENCE. MVFR/IFR POSSIBLE IN FOG
AND LOW CLOUDS ALONG THE SOUTH COAST...ESPECIALLY MONDAY NIGHT INTO
TUESDAY MORNING. MVFR/IFR POSSIBLE NORTHWEST IN SHRA/TSRA LATE WED.

&&

.MARINE...
FORECASTER CONFIDENCE LEVELS...

LOW...LESS THAN 30 PERCENT.
MODERATE...30 TO 60 PERCENT.
HIGH...GREATER THAN 60 PERCENT.

THIS EVENING THROUGH SATURDAY NIGHT...HIGH CONFIDENCE.

WAVE MODELS CONTINUE TO OVERFORECAST WAVE HEIGHTS OFF OUR COAST.
FORECAST VALUES ON THE OUTER WATERS FOR 18Z WERE A FULL FOOT
ABOVE OBSERVED VALUES. WE USED 80 PERCENT OF THE WAVEWATCH VALUES
THROUGH THE FORECAST PERIOD.

TRANQUIL BOATING WEATHER EXPECTED WITH HIGH PRESSURE OVERHEAD.
LOCAL SEA BREEZES DIMINISH THIS EVENING. LIGHT WINDS TREND TO
SOUTHEAST SATURDAY AS THE HIGH MOVES OFFSHORE...THEN TO EAST
SATURDAY NIGHT. WINDS AND SEAS WILL REMAIN BELOW SMALL CRAFT
THRESHOLDS THROUGH SATURDAY NIGHT.

COASTAL LOW PRESSURE DEVELOPING OFF THE CAROLINAS SATURDAY WILL
MOVE UP THE COAST SATURDAY NIGHT. CURRENT FORECAST TRACK WOULD
KEEP MOST OF THE STORM WELL OFFSHORE. BUT FORECAST MODELS AGREE ON
LIGHT SHOWERS INTO OUR SOUTHERN WATERS AFTER MIDNIGHT SATURDAY
NIGHT.

OUTLOOK...SUNDAY THROUGH WEDNESDAY...

MODERATE CONFIDENCE. LOW PRESSURE MOVES SOUTH OF THE WATERS SUNDAY
FOLLOWED BY HIGH PRESSURE BUILDING IN ON MONDAY. A WARM FRONT MOVES
THROUGH ON TUESDAY AND A COLD FRONT WILL APPROACH ON THURSDAY.

SCA FOR SEAS POSSIBLE...ESPECIALLY OVER THE OUTER WATERS...DURING
THE SUNDAY-MONDAY TIMEFRAME.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...NONE.
MA...NONE.
NH...NONE.
RI...NONE.
MARINE...NONE.

&&

$$
SYNOPSIS...WTB
NEAR TERM...WTB/LF
SHORT TERM...WTB
LONG TERM...LF
AVIATION...WTB/LF
MARINE...WTB/LF



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