574  
FXUS61 KBOX 201832  
AFDBOX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BOSTON/NORTON MA  
232 PM EDT THU SEP 20 2018  
   
SYNOPSIS
 
 
HIGH PRESSURE BUILDS INTO NEW ENGLAND TONIGHT BRINGING CLOUDS  
BUT DRIER WEATHER. THE HIGH MOVES OFF TO THE EAST FRIDAY,  
BRINGING GUSTY SOUTHERLY WINDS AND WARMER TEMPERATURES. A COLD  
FRONT SWINGS THROUGH THE REGION FRIDAY NIGHT WITH SHOWERS AND  
SCATTERED THUNDERSTORMS. LARGE HIGH PRESSURE WILL THEN BRING  
DRY BUT COOL CONDITIONS THROUGH THE WEEKEND INTO EARLY NEXT  
WEEK. ANOTHER WARM FRONT MAY APPROACH LATE TUESDAY INTO  
WEDNESDAY WITH SCATTERED SHOWERS AND POSSIBLY A FEW  
THUNDERSTORMS.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
HIGH PRESSURE OVER THE MARITIMES BUILDS SOUTH OVER NEW ENGLAND.  
ONSHORE FLOW BECOMES LIGHT AND VARIABLE DURING THE NIGHT. MODELS  
SHOW INCREASING LOW LEVEL MOISTURE, WHICH WOULD SUPPORT  
DEVELOPING PATCHES OF FOG. OTHERWISE DRY WEATHER. OBSERVED DEW  
POINTS IN THE 50S SHOULD PROVIDE A LOWER BOUND FOR TEMPERATURES,  
SO MIN TEMPS IN THE 50S.  
 

 
   
SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/
 
 
HIGH PRESSURE MOVES OFFSHORE FRIDAY MORNING. THIS SHOULD BRING  
WINDS AROUND OUT OF THE SOUTH, WITH MILDER AIR DRAWN NORTH OVER  
SOUTHERN NEW ENGLAND. LOW LEVEL WINDS ALOFT INCREASE TO AROUND  
25 KNOTS, SO EXPECT GUSTY WINDS NEAR THAT SPEED LATE IN THE DAY.  
INSTABILITY REMAINS WEST OF US IN NY THROUGH THE DAY, SO EXPECT  
A RAIN-FREE DAY. TEMPS ALOFT REACH AN EQUIV OF 7-9C AT THE TOP  
OF THE MIXED LAYER, SO MAX TEMPS SHOULD BE IN THE 70S.  
 
COLD FRONT MOVES ACROSS SOUTHERN NEW ENGLAND FRIDAY NIGHT.  
THETA-E RIDGE IS PROJECTED TO ENTER WESTERN MA/CT AROUND  
MIDNIGHT AND MOVE OFF CAPE COD TOWARD 8 AM/12Z. SOME STABILITY  
PARAMETERS ARE FAVORABLE FOR THUNDER, OTHERS ARE MEDIOCRE. MOST  
FAVORABLE WOULD BE K INDICES AS HIGH AS THE MID 30S. WE WILL  
CONTINUE TO MENTION SHOWERS AND A CHANCE OF THUNDER. THE SOUTH  
TO SOUTHWEST FLOW SUPPORTS INCREASING DEW POINTS WITH VALUES IN  
THE 60S, SO MIN TEMPS SHOULD ALSO BE IN THE 60S. STRONG SOUTH TO  
SOUTHWEST WINDS ALOFT WILL SUPPORT GUSTS 25 TO 30 KNOTS UNTIL  
THE COLD FRONT MOVES THROUGH.  
 

 
   
LONG TERM /SATURDAY THROUGH THURSDAY/
 
 
HIGHLIGHTS...  
 
* A LARGE HIGH BUILDS ACROSS FROM CENTRAL CANADA SATURDAY  
AFTERNOON INTO MONDAY WITH COOL BUT DRY CONDITIONS  
* ANOTHER FRONTAL SYSTEM MAY APPROACH LATE MONDAY NIGHT OR  
TUESDAY WITH SHOWERS AND A FEW THUNDERSTORMS, WHICH MAY LINGER  
INTO MID WEEK  
 
OVERVIEW...  
 
00Z MODEL GUIDANCE CONTINUES TO SIGNAL A CHANGEABLE WEATHER  
PATTERN ACROSS THE NORTHEAST U.S. FOR MOST IF NOT THE ENTIRE  
LONG TERM PERIOD. A GENERAL, BROAD STEERING MID LEVEL FLOW  
ACROSS THE NORTHERN TIER STATES WILL KEEP WEATHER SYSTEMS MOVING  
ALONG INTO EARLY NEXT WEEK. BEYOND THAT, IT APPEARS THE H5 MASS  
FIELDS START TO AMPLIFY AS A LONG WAVE TROUGH DIGS ACROSS THE  
WESTERN PLAINS STATES WHILE RIDGING BUILDS ACROSS EASTERN CANADA  
BY AROUND MONDAY NIGHT OR TUESDAY, THOUGH NOTING SOME SOLUTION  
SPREAD AMONGST THE MODEL MEMBERS.  
 
LARGE HIGH MOVES IN BEHIND THE FRONT THROUGH THE REMAINDER OF  
THE WEEKEND INTO THE START OF THE WORK WEEK. RATHER COOL  
CONDITIONS, ESPECIALLY MOVING INTO MONDAY MORNING AS ANOTHER  
REINFORCING HIGH BRINGS OVERNIGHT LOWS IN THE LOWER-MID 40S  
ACROSS THE HIGHER INLAND TERRAIN, RANGING TO THE MID-UPPER 50S  
ALONG THE S COAST.  
 
ANOTHER COMPLEX SYSTEM APPROACHES MONDAY NIGHT OR TUESDAY, BUT  
MODEL TIMING WITH THE EXIT OF HIGH IN QUESTION, WHICH IS LENDING  
TO MODEL SOLUTION SPREAD. MAY SEE ANOTHER ROUND OF SHOWERS AND  
EVEN A FEW THUNDERSTORMS, BUT NOTING ONLY MARGINAL INSTABILITY  
WITH THIS NEXT SYSTEM.  
 

 
   
AVIATION /18Z THURSDAY THROUGH TUESDAY/
 
 
FORECASTER CONFIDENCE LEVELS...  
 
LOW - LESS THAN 30 PERCENT.  
MODERATE - 30 TO 60 PERCENT.  
HIGH - GREATER THAN 60 PERCENT.  
 
SHORT TERM /THROUGH FRIDAY NIGHT/...  
 
THROUGH TONIGHT...MODERATE CONFIDENCE.  
 
SURFACE FLOW FROM THE NORTHEAST WILL DIMINISH AND BECOME  
LIGHT/VARIABLE EARLY TONIGHT. LINGERING MOISTURE BOTH IN THE  
SURFACE LAYER AND ABOUT 5000 FEET ABOVE SFC. WE WILL CONTINUE TO  
INDICATE CLOUDS DURING THE NIGHT. ALSO POTENTIAL FOR MVFR/IFR  
CIGS/VSBYS IN FOG AFTER MIDNIGHT, ESPECIALLY IN THE INTERIOR.  
 
FRIDAY...MODERATE CONFIDENCE.  
 
LINGERING MVFR/IFR CLOUDS EARLY, BUT ALL AREAS SHOULD TREND TO  
VFR DURING THE MORNING AS WINDS TURN FROM THE SOUTH. WINDS AT  
2000 TO 4000 FEET WILL REACH 25 KNOTS MID TO LATE AFTERNOON, AND  
WILL GENERATE GUSTS AT THOSE SPEEDS AT THE SURFACE.  
 
FRIDAY NIGHT...MODERATE-HIGH CONFIDENCE.  
 
COLD FRONT MOVES EAST FROM NY AND CROSSES SOUTHERN NEW ENGLAND  
BETWEEN MIDNIGHT AND SUNRISE. VFR MUCH OF THE TIME, BUT AREAS OF  
MVFR CIGS/IFR VSBYS IN SHOWERS AND SCATTERED THUNDERSTORMS  
BETWEEN MIDNIGHT AND SUNRISE.  
 
KBOS TERMINAL...HIGH CONFIDENCE IN TRENDS. LOWER CONFIDENCE IN  
TIMING.  
 
KBDL TERMINAL...HIGH CONFIDENCE IN TRENDS. LOWER CONFIDENCE IN  
TIMING.  
 
OUTLOOK /SATURDAY THROUGH TUESDAY/...  
 
SATURDAY: VFR. BREEZY. SLIGHT CHANCE SHRA.  
 
SATURDAY NIGHT THROUGH SUNDAY NIGHT: VFR.  
 
MONDAY: VFR. BREEZY.  
 

 
   
MARINE
 
 
FORECASTER CONFIDENCE LEVELS...  
 
LOW - LESS THAN 30 PERCENT.  
MODERATE - 30 TO 60 PERCENT.  
HIGH - GREATER THAN 60 PERCENT.  
 
SHORT TERM /THROUGH FRIDAY NIGHT/...HIGH CONFIDENCE.  
 
TONIGHT...NORTHEAST WINDS DIMINISH AND BECOME LIGHT AS HIGH  
PRESSURE BUILDS OVER THE WATERS. SEAS LINGER NEAR 5 FEET ON THE  
EASTERN OUTER WATERS THROUGH THE NIGHT. SMALL CRAFT ADVISORY  
CONTINUES ON THOSE EASTERN WATERS.  
 
FRIDAY...INCREASING SOUTHERLY WINDS WITH GUSTS TO 25-30 KT  
DEVELOPING BY LATE AFTERNOON, ESPECIALLY EASTERN MA WATERS.  
SEAS REMAIN NEAR 5 FEET ON THE EASTERN OUTER WATERS.  
 
FRIDAY NIGHT...WINDS FROM THE SOUTH AND SOUTHWEST MUCH OF THE  
NIGHT. WINDS AT 2000 FEET REACH 35-40 KNOTS, AND SOME OF THIS  
MAY BE BROUGHT TO THE SURFACE IN GUSTS. SEAS BUILD TO 5-7 FEET.  
A COLD FRONT CROSSES THE WATERS LATE AT NIGHT AND SHOULD PRODUCE  
A FEW SHOWERS, POSSIBLY A THUNDERSTORM. A GALE WATCH WILL BE  
ISSUED FOR OUR EASTERN OUTER WATERS AND FOR MASSACHUSETTS/IPSWICH  
BAYS.  
 
OUTLOOK /SATURDAY THROUGH TUESDAY/...  
 
SATURDAY: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH GUSTS UP  
TO 30 KT. AREAS OF SEAS APPROACHING 5 FT. SLIGHT CHANCE OF RAIN  
SHOWERS.  
 
SATURDAY NIGHT THROUGH SUNDAY: WINDS LESS THAN 25 KT.  
 
SUNDAY NIGHT: WINDS LESS THAN 25 KT. SEAS LOCALLY APPROACHING  
5 FT.  
 
MONDAY: LOW RISK FOR SMALL CRAFT ADVISORY WINDS WITH GUSTS UP  
TO 25 KT. SEAS UP TO 5 FT.  
 

 
   
BOX WATCHES/WARNINGS/ADVISORIES
 
 
CT...NONE.  
MA...NONE.  
RI...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 8 AM EDT FRIDAY FOR ANZ250-254.  
SMALL CRAFT ADVISORY UNTIL 8 PM EDT THIS EVENING FOR ANZ251.  
 

 
 

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