857  
FXUS61 KBOX 202021  
AFDBOX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BOSTON/NORTON MA  
421 PM EDT FRI JUL 20 2018  
   
SYNOPSIS
 
 
ANOTHER DRY AND COOL NIGHT IS IN STORE AS HIGH PRESSURE EAST OF  
NEW ENGLAND SLOWLY SLIDES EAST. LOW PRESSURE ALONG THE SOUTH  
CAROLINA COAST WILL MOVE NORTHWARD, HUGGING THE COAST SATURDAY  
AND PASSING TO OUR WEST SUNDAY. ITS ASSOCIATED WARM FRONT WILL  
MOVE ACROSS SOUTHERN NEW ENGLAND LATE SATURDAY NIGHT AND SUNDAY,  
BRINGING SHOWERS AND THUNDERSTORMS WITH TORRENTIAL DOWNPOURS  
AND GUSTY WINDS. BERMUDA HIGH PRESSURE THEN BUILDS WEST TOWARD  
SOUTHERN NEW ENGLAND NEXT WEEK BRINGING TROPICAL HUMIDITY AND  
WARM TEMPERATURES. SCATTERED SHOWERS AND THUNDERSTORMS ARE  
POSSIBLE EACH DAY.  
 

 
   
NEAR TERM /UNTIL 6 AM SATURDAY MORNING/
 
 
415 PM UPDATE...  
 
HIGH PRESSURE TO THE EAST OF SOUTHERN NEW ENGLAND WILL PROVIDE  
US WITH ONE LAST REFRESHING NIGHT. OVERNIGHT LOWS WILL BE IN THE  
50S, WITH CLEAR SKIES. MOST MODELS ARE NOT INDICATING MUCH FOG  
DEVELOPMENT OVERNIGHT, BUT THERE COULD BE LOCALLY DENSE SHALLOW  
GROUND FOG IN A FEW SPOTS. IT SHOULD BE NOTED THAT THE NAM 925  
MB RH IS THE FASTEST OF ALL MODELS IN BRINGING AN AREA OF FOG  
AND LOW CLOUDS, NOW SEEN OVER THE WATERS SOUTH OF NOVA SCOTIA,  
WESTWARD STRAIGHT INTO NORTHEASTERN MA BY DAYBREAK. ALL OTHERS  
DELAY THIS UNTIL SATURDAY AFTERNOON. WILL NEED TO WATCH ITS  
PROGRESSION.  
 

 
   
SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/
 
 
SATURDAY...  
 
SUNSHINE SATURDAY MORNING WILL BEGIN TO FADE BEHIND INCREASING  
HIGH CLOUDINESS FROM THE SOUTHWEST AND LOW LEVEL CLOUDINESS  
ENCROACHING FROM THE EAST BY MID-AFTERNOON. AREAS OF FOG ARE  
POSSIBLE ALONG THE EASTERN MA COAST BY LATE AFTERNOON. CANNOT  
RULE OUT SOME DRIZZLE DEVELOPMENT THERE TOWARD EVENING. WINDS  
WILL BE FROM THE EAST AHEAD OF A STRONG WARM FRONT MOVING  
NORTHWARD ALONG THE EASTERN SEABOARD. TEMPERATURES WILL BE A  
LITTLE COOLER THAN ON FRIDAY, WITH HIGHS MAINLY IN THE MID AND  
UPPER 70S EXCEPT SOME LOWER 80S IN THE CONNECTICUT RIVER VALLEY.  
 
SATURDAY NIGHT...  
 
* TROPICAL DOWNPOURS IN SHOWERS AND THUNDERSTORMS, WITH GUSTY  
WINDS LIKELY LATE  
* LOW PROBABILITY OF AN ISOLATED TORNADO TOWARD DAYBREAK, MAINLY  
IN NORTHERN CT, RI, AND SOUTHEAST MA  
 
A LOW PRESSURE SYSTEM, CHOCK FULL OF TROPICAL MOISTURE, WILL BE  
WORKING ITS WAY NORTHWARD AND HUGGING THE MID-ATLANTIC COASTLINE  
SATURDAY NIGHT. IT WILL THEN HEAD INLAND, WELL TO OUR WEST-  
SOUTHWEST TOWARD DAYBREAK SUNDAY. ITS VERY STRONG WARM FRONT  
WILL BE MOVING ACROSS SOUTH COASTAL NEW ENGLAND LATE AT NIGHT.  
 
A VERY STRONG LOW LEVEL SOUTHERLY JET DEVELOPS...ON THE ORDER  
OF 55-65 KNOTS AT 925-850 MB...AFTER 2 AM SUNDAY. WITH BACKED  
EASTERLY FLOW AT THE SURFACE, LOW-LEVEL (0-1 KM) HELICITY  
VALUES ARE THROUGH THE ROOF, WITH UP TO 400-600 FORECAST IN  
WESTERN CT AND 100-300 IN THE EASTERN PORTIONS OF OUR AREA  
TOWARD DAYBREAK. EXPERIENCE HAS SHOWN THAT WHERE A RAPID INFLUX  
OF CAPE INTERSECTS WITH HELICITY VALUES OVER 150, AND WITH  
DEWPOINTS SOARING TO THE LOWER 70S BY DAYBREAK, THE POTENTIAL  
EXISTS FOR AN EARLY MORNING TORNADO. THIS IS ESPECIALLY TRUE AT  
THIS TIME OF THE YEAR. THE CAPE IS FORECAST TO REACH 400-800  
J/KG IN RI AND SOUTHEAST MA, POSSIBLY ALSO NORTHEAST CT, BY  
AROUND 09Z, WHICH IS 5 AM. THUS, A THREAT EXISTS FOR A POSSIBLE  
TORNADO MAINLY FROM ABOUT 4 AM INTO THE EARLY DAYLIGHT HOURS ON  
SUNDAY.  
 
WITH PRECIPITABLE WATER VALUES RAPIDLY RISING TO 2.5 INCHES,  
ANY SHOWERS AND THUNDERSTORMS HAVE THE POTENTIAL TO PRODUCE 1-2  
INCHES OF RAIN IN AN HOUR, NOT UNLIKE A FEW DAYS AGO. THIS COULD  
PRODUCE LOCALIZED FLASH FLOODING, ESPECIALLY IN URBAN AREAS THAT  
ARE TYPICALLY KNOWN FOR POOR DRAINAGE.  
 
WIND GUSTS TO 30-50 MPH ARE POSSIBLE IN THE STRONGEST SHOWERS OR  
THUNDERSTORMS, DUE TO THE MODERATELY STRONG WINDS ABOVE THE  
SURFACE.  
 

 
   
LONG TERM /SUNDAY THROUGH FRIDAY/
 
 
BIG PICTURE...  
 
THE UPPER FLOW WILL BE DRIVEN BY HIGH PRESSURE EAST OF NEWFOUNDLAND  
AND LOW PRESSURE COVERING THE EASTERN USA. COASTAL LOW, SURFACE AND  
ALOFT, WILL BECOME TRAPPED UNDER THAT CLOSED EASTERN LOW. THIS DRAWS  
THE COASTAL SURFACE LOW FARTHER WEST, SWEEPING ACROSS NY AND THE  
EASTERN GREAT LAKES. THE LARGER SCALE GRADIENT FAVORS SOUTHERLY  
WINDS SUNDAY AND MONDAY, TRENDING FROM THE SOUTHWEST MID AND LATE  
WEEK.  
 
THE STALLED HIGH PRESSURE OVER THE OCEAN BUILDS WEST TOWARD NEW  
ENGLAND MONDAY AND TUESDAY, THEN HALTS AND NUDGES BACK TOWARD THE  
EAST WEDNESDAY THROUGH FRIDAY. UPPER LOW OVER BRITISH COLUMBIA MOVES  
EAST TO CENTRAL CANADA EARLY NEXT WEEK, AND EVENTUALLY APPROACHES  
NEW ENGLAND LATE WEEK. THIS WILL PUSH THE NORTHERN JET CLOSER TO NEW  
ENGLAND BY LATE WEEK.  
 
MODEL MASS AND THEMAL FIELDS ARE IN GOOD AGREEMENT THROUGH  
WEDNESDAY. DIFFERENCES EMERGE MID WEEK MAINLY WITH THE CANADIAN  
SHORTWAVE THAT APPROACHES NEW ENGLAND. THE GGEM BECOMES A FAST  
OUTLIER TUESDAY WHILE THE ECMWF AND GFS SHOW BETTER AGREEMENT IN  
SENDING THE LOW INTO EASTERN CANADA AND HINT AT A COLD FRONT AT THE  
END OF THE WEEK. CONFIDENCE IS HIGH THROUGH THE EARLY PART OF NEXT  
WEEK, THEN MODERATE-HIGH FOR MID TO LATE WEEK.  
 
CONTOUR HEIGHTS REMAIN ABOVE NORMAL THROUGH MUCH OF NEXT WEEK.  
EXPECT TEMPS TO BE NEAR NORMAL ON CLOUDY DAYS AND ABOVE NORMAL WHEN  
THERE IS ANY SUNSHINE. CLOUDS AND HIGH DEW POINTS SHOULD KEEP  
NIGHTTIME TEMPS ABOVE NORMAL THROUGH THE PERIOD.  
 
CONCERNS...  
 
SUNDAY... COASTAL LOW MOVES NORTHWEST ACROSS NJ/PA/NY. THIS LIFTS A  
WARM FRONT THROUGH SOUTHERN NEW ENGLAND DURING THE MORNING.  
SOUTHERLY FLOW OVER THE FRONT WILL CONTINUE TO GENERATE LIFT. THE  
AIR MASS MOVING UP THE COAST WILL BE VERY HUMID. PRECIPITABLE WATER  
VALUES ARE FORECAST TO REACH 2-2.5 INCHES, WELL ABOVE NORMAL AND  
CAPABLE OF PRODUCING HEAVY DOWNPOURS. DEW POINTS IN THE AIR MASS ARE  
EXPECTED TO BE IN THE LOW TO MID 70S.  
 
WINDS AT 850 MB ARE FORECAST AT 50-60 KNOTS, WHILE WINDS NEAR THE  
SURFACE AND IN MEDIOCRE LAPSE RATES ARE AROUND 30 KNOTS AND POSSIBLY  
35 KNOTS AT TIMES. EXPECT GUSTY SOUTH-SOUTHEAST WINDS ESPECIALLY  
ALONG THE COAST. CAPE REACHES 500-1000 JOULES/KM. HELICITY VALUES AT  
0-1KM ARE FORECAST TO BE AS HIGH AS 300 TO 600 AS THE WARM FRONT  
MOVES THROUGH, ALTHOUGH MUCH OF THIS MOVES THROUGH NY AND PA. EVEN  
SO, SOUTHERN NEW ENGLAND WILL BE IN THE NORTHEAST QUADRANT OF THE  
WEATHER SYSTEM. THIS IS A LOW CAPE/HIGH SHEAR ENVIRONMENT, WHICH  
COULD FAVOR DEVELOPMENT OF ROTATION IN THUNDERSTORMS. THIS WILL  
NEED TO BE MONITORED.  
 
CONDITIONS SHOULD IMPROVE DURING SUNDAY AFTERNOON AS THE WARM FRONT  
MOVES NORTH AND WEST OF OUR AREA.  
 
MONDAY THROUGH FRIDAY...  
 
SOUTHERLY FLOW CONTINUES THROUGH MOST OF THE WEEK, TRANSPORTING WARM  
AND VERY HUMID AIR INTO SOUTHERN NEW ENGLAND. THE ATLANTIC HIGH  
PRESSURE IS EXPECTED TO DRIFT WEST AND CLOSER TO OUR AREA TUESDAY  
AND WEDNESDAY. THIS WOULD SHIFT THE CORE OF THE TROPICAL FLOW AND  
HIGHER DEW POINT AIR FARTHER WEST...THIS WOULD IN TURN SHIFT THE  
BEST CHANCE OF SHOWERS/TSTMS INTO WESTERN NEW ENGLAND AND NEW YORK.  
 
BY THURSDAY THE NORTHERN STREAM SHORTWAVE STARTS APPROACHING THROUGH  
THE GREAT LAKES. THIS WILL PUSH THE TROPICAL FLOW AND HIGHEST DEW  
POINTS BACK OVER SOUTHERN NEW ENGLAND. EXPECT SHOWERS/THUNDER TO  
RETURN FOR THESE TWO DAYS.  
 
COLD FRONT ASSOCIATED WITH THE NORTHERN SHORTWAVE APPROACHES ON  
FRIDAY. GFS AND ECMWF SUGGEST THE FRONT MOVES THROUGH OUR AREA ON  
FRIDAY. THIS TIMING MAY WAVER OVER THE NEXT FEW MODEL RUNS AND WILL  
NEED TO BE MONITORED.  
 

 
   
AVIATION /19Z FRIDAY THROUGH WEDNESDAY/
 
 
FORECASTER CONFIDENCE LEVELS...  
 
LOW - LESS THAN 30 PERCENT.  
MODERATE - 30 TO 60 PERCENT.  
HIGH - GREATER THAN 60 PERCENT.  
 
SHORT TERM /THROUGH SATURDAY NIGHT/...HIGH CONFIDENCE.  
 
2015Z UPDATE...  
 
TONIGHT...VFR. ONLY EXCEPTION WILL BE IFR IN PATCHY DENSE FOG,  
MAINLY SCATTERED ACROSS INTERIOR SOUTHERN NEW ENGLAND.  
 
SATURDAY... VFR. E WINDS INCREASING TO 10-15 KT. MVFR CEILINGS  
POSSIBLY OVERSPREADING EASTERN MA AND RI FROM EAST TO WEST IN  
THE AFTERNOON. PATCHY FOG POSSIBLE ALONG THE EASTERN MA COAST  
TOWARD EVENING.  
 
SATURDAY NIGHT...MVFR BECOMING IFR THROUGHOUT THE REGION.  
SHOWERS AND THUNDERSTORMS WITH HEAVY RAIN AND POSSIBLE G30-40  
KT, MAINLY AFTER 06Z.  
 
KBOS TERMINAL...VFR. EASTERLY SEA BREEZE BECOMES LIGHT AND  
VARIABLE THIS EVENING AND THE RESUMES AN EASTERLY DIRECTION LATE  
TONIGHT AND ON SAT. MVFR POSSIBLE MID TO LATE SAT. AFTERNOON.  
IFR SATURDAY NIGHT WITH HEAVY SHOWERS AND THUNDERSTORMS WITH  
POSSIBLE G30-40 KT.  
 
KBDL TERMINAL...VFR. BECOMING MVFR THEN IFR SATURDAY NIGHT WITH  
HEAVY SHOWERS AND THUNDERSTORMS WITH POSSIBLE G30-40 KT.  
 
OUTLOOK /SUNDAY THROUGH WEDNESDAY/...  
 
SUNDAY: MAINLY MVFR, WITH AREAS IFR POSSIBLE. WINDY WITH GUSTS  
TO 30 KT. SHRA LIKELY, CHANCE TSRA, PATCHY BR.  
 
SUNDAY NIGHT: MVFR/IFR CONDITIONS POSSIBLE. BREEZY. CHANCE  
SHRA, SLIGHT CHANCE TSRA, PATCHY BR.  
 
MONDAY: MAINLY MVFR, WITH AREAS VFR POSSIBLE. BREEZY. CHANCE  
SHRA, SLIGHT CHANCE TSRA.  
 
MONDAY NIGHT: MAINLY MVFR, WITH LOCAL IFR POSSIBLE. CHANCE  
SHRA, SLIGHT CHANCE TSRA, PATCHY BR.  
 
TUESDAY: MAINLY MVFR, WITH AREAS VFR POSSIBLE. BREEZY. CHANCE  
SHRA, SLIGHT CHANCE TSRA.  
 
TUESDAY NIGHT: MAINLY MVFR, WITH LOCAL IFR POSSIBLE. BREEZY.  
SLIGHT CHANCE SHRA, ISOLATED TSRA, PATCHY BR.  
 
WEDNESDAY: MAINLY MVFR, WITH AREAS VFR POSSIBLE. BREEZY. CHANCE  
SHRA, SLIGHT CHANCE TSRA.  
 

 
   
MARINE
 
 
FORECASTER CONFIDENCE LEVELS...  
 
LOW - LESS THAN 30 PERCENT.  
MODERATE - 30 TO 60 PERCENT.  
HIGH - GREATER THAN 60 PERCENT.  
 
SHORT TERM /THROUGH SATURDAY NIGHT/...HIGH CONFIDENCE.  
 
HIGH PRESSURE REMAINS IN CONTROL THIS EVENING INTO SATURDAY,  
KEEPING WINDS AND SEAS BELOW SMALL CRAFT THRESHOLDS.  
 
A GALE WATCH HAS BEEN ISSUED FOR NARRAGANSETT BAY AND THE WATERS  
SOUTH OF RHODE ISLAND FOR SATURDAY NIGHT AND SUNDAY. A SMALL  
CRAFT ADVISORY HAS BEEN ISSUED FOR ALL OTHER WATERS FOR SATURDAY  
NIGHT AND SUNDAY.  
 
A STORM SYSTEM WILL BE MOVING NORTHWARD ALONG THE MID-ATLANTIC  
COAST AND PASSING WELL TO THE WEST OF THE WATERS BY SUNDAY. A  
STRONG WARM FRONT WILL BE MOVING ACROSS THE WATERS LATE SATURDAY  
NIGHT AND SUNDAY MORNING. SHOWERS AND THUNDERSTORMS, WITH  
TORRENTIAL RAIN AND GUSTY WINDS ARE EXPECTED LATE SATURDAY NIGHT  
INTO SUNDAY. GALE FORCE WIND GUSTS ARE POSSIBLE, HENCE THE GALE  
WATCH IN THE WATERS ALONG THE RI COAST. WINDS ARE FORECAST TO BE  
NEAR 30 KNOTS OVER ALL OTHER WATERS LATE SATURDAY NIGHT AND  
SUNDAY, BUT IT IS POSSIBLE THAT PORTIONS OF THESE AREAS COULD  
NEED TO BE UPGRADED TO GALES IN SUBSEQUENT FORECASTS.  
 
VISIBILITIES WILL BE REDUCED IN POSSIBLE FOG ALONG THE MA EAST  
COAST SATURDAY AFTERNOON/EVENING AND ACROSS ALL WATERS SATURDAY  
NIGHT.  
 
OUTLOOK /SUNDAY THROUGH WEDNESDAY/...  
 
SUNDAY: LOW RISK FOR GALE FORCE WINDS WITH GUSTS UP TO 30 KT.  
AREAS OF ROUGH SEAS. RAIN SHOWERS LIKELY, CHANCE OF  
THUNDERSTORMS, PATCHY FOG. AREAS OF VISIBILITY 1 TO 3 NM.  
 
SUNDAY NIGHT THROUGH MONDAY: WINDS LESS THAN 25 KT. AREAS OF  
ROUGH SEAS. CHANCE OF RAIN SHOWERS, SLIGHT CHANCE OF  
THUNDERSTORMS.  
 
MONDAY NIGHT: WINDS LESS THAN 25 KT. LOCAL ROUGH SEAS. SLIGHT  
CHANCE OF RAIN SHOWERS, ISOLATED THUNDERSTORMS.  
 
TUESDAY: WINDS LESS THAN 25 KT. SEAS UP TO 5 FT. SLIGHT CHANCE  
OF RAIN SHOWERS, ISOLATED THUNDERSTORMS.  
 
TUESDAY NIGHT THROUGH WEDNESDAY: WINDS LESS THAN 25 KT. AREAS  
OF SEAS APPROACHING 5 FT. SLIGHT CHANCE OF RAIN SHOWERS,  
ISOLATED THUNDERSTORMS.  
 

 
   
BOX WATCHES/WARNINGS/ADVISORIES
 
 
CT...NONE.  
MA...NONE.  
RI...NONE.  
MARINE...SMALL CRAFT ADVISORY FROM 2 AM TO 6 PM EDT SUNDAY FOR  
ANZ230>234-250-251-254-255.  
GALE WATCH FROM LATE SATURDAY NIGHT THROUGH SUNDAY AFTERNOON  
FOR ANZ235>237-256.  
 

 
 

 
SYNOPSIS...GAF/WTB  
NEAR TERM...GAF  
SHORT TERM...GAF  
LONG TERM...WTB  
AVIATION...GAF/WTB  
MARINE...GAF/WTB  
 
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