Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 252348 AFDGYX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GRAY ME 748 PM EDT TUE AUG 25 2015 .SYNOPSIS... A SLOW MOVING FRONT WILL GRADUALLY PUSH EAST ACROSS THE AREA THROUGH TONIGHT BRINGING SHOWERS AND THUNDERSTORMS WITH LOCALLY HEAVY RAINFALL POSSIBLE. ADDITIONAL SHOWERS AND THUNDERSTORMS ARE POSSIBLE ON WEDNESDAY BEFORE DRIER AIR MOVES INTO THE REGION THROUGH THE END THE WEEK. SOMEWHAT MORE HUMID WEATHER WILL RETURN THIS WEEKEND AHEAD OF A COLD FRONT THAT MAY BRING SOME SHOWERS AND THUNDERSTORMS TO THE REGION BY SUNDAY. && 23Z UPDATE... QUICK UPDATE TO ADJUST POPS AND SKY GRIDS BASED ON CURRENT RADAR TRENDS/SAT PICS. SHOWERS AND THUNDERSTORMS CONTINUING ACROSS THE SOUTHERN PORTIONS OF NEW HAMPSHIRE BUT ACTIVITY WEAKENING WITH LOSS OF HEATING. FLASH FLOOD WARNING STILL IN EFFECT TILL 10 PM FOR CHESHIRE AND HILLSBOROUGH COUNTIES AND WITH MORE CELLS DOWN STREAM WILL LIKELY LET IT GO THROUGH 10 PM. EXPECT SHOWERS AND ANY THUNDERSTORMS TO GRADUALLY DIMINISH THROUGH MIDNIGHT WITH WIDESPREAD AREAS OF FOG DEVELOPING THERE AFTER. OTHER THAN A FEW TEMP/TD TWEAKS NO OTHER CHANGES PLANNED ATTM. PREVIOUS DISCUSSION... .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... INITIAL AREA OF CONVECTION AND SPILLOVER SHOWERS TO THE NORTH OF IT HAS MOVED EAST INTO MAINE. THIS WILL CONTINUE TO WEAKEN AS IT MOVES EAST. ADDITIONAL DEVELOPMENT NOW APPEARS LIKELY OVER NEW HAMPSHIRE... VERMONT... AND NEW YORK... WITH THIS ACTIVITY TRACKING EASTWARD INTO NEW HAMPSHIRE WITH TIME. WIND SHEAR REMAINS FAVORABLE FOR STORM ORGANIZATION INCLUDING SUPERCELLS. SO STORMS WHICH ARE ABLE TO TAKE THE BEST ADVANTAGE OF THE AVAILABLE INSTABILITY MAY TAKE ON SUPERCELL STRUCTURE WITH THE THREAT FOR DAMAGING WINDS... HAIL... AND POTENTIALLY A TORNADO. THIS THREAT SHOULD DIMINISH THIS EVENING AND FURTHER TOWARD THE EAST. ACROSS MAINE AND COASTAL NEW HAMPSHIRE... A FRESH BATCH OF MARINE CLOUDS AND EMBEDDED FOG/DRIZZLE HAS ALREADY MOVED ONSHORE. THIS WILL LIKELY PERSIST THROUGH THE NIGHT AND SHOULD SERVE TO LIMIT THE INSTABILITY OVER THIS AREA. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... BY WEDNESDAY MORNING THE SURFACE FRONTAL BOUNDARY WILL BE POSITIONED NEAR THE MAINE AND NEW HAMPSHIRE STATE LINE WITH A WEAK SURFACE LOW FORMING ALONG THE FRONT. TO THE EAST OF THE FRONT LOW CLOUDS... FOG... AND DRIZZLE WILL BE THE RULE IN THE MORNING... WITH A THREAT FOR LINGERING SHOWERS OR A THUNDERSTORM AS WELL. BEHIND THE FRONT TO THE WEST... EXPECT DRYING CONDITIONS WITH DEWPOINTS FINALLY FALLING THROUGH THE 60S INTO THE 50S DURING THE DAY. THE FRONT WILL GRADUALLY MOVE EAST THROUGH THE DAY... PUSHING OUT THE LOW LEVEL MOISTURE... CLOUDS... FOG... AND INSTABILITY. CANNOT RULE OUT A SHOWER OR TWO IN THE MOUNTAINS DURING THE AFTERNOON EVEN BEHIND THE FRONT AS JUST ENOUGH LOW LEVEL MOISTURE REMAINS TO POSSIBLY TRIGGER A SHOWER OR TWO. OTHERWISE IT WILL BE MAINLY DRY BEHIND THE FRONT WITH TEMPERATURES IN THE 70S TO 80S. EXPECT A COOLER NIGHT WEDNESDAY NIGHT AS DRIER AIR WILL HAVE FINALLY ARRIVED ACROSS THE ENTIRE REGION. LOWS DROP INTO THE 50S TO LOW 60S. && .LONG TERM /THURSDAY THROUGH TUESDAY/... HIGH IMPACT WEATHER POTENTIAL: LOW. PATTERN SUMMARY: THE LONG TERM PERIOD BEGINS WITH A PERSISTENT TROUGH/RIDGE/TROUGH PATTERN EXTENDING FROM THE EASTERN PACIFIC THROUGH THE EASTERN PART OF NORTH AMERICA. THERE IS MODEST AGREEMENT THAT THE EASTERN PACIFIC TROUGH WILL DEEPEN THROUGH THE END OF THIS WEEK...WITH BUILDING DOWNSTREAM HEIGHTS CAUSING THE DOWNSTREAM TROUGHING OVER THE NORTHEASTERN UNITED STATES TO FLATTEN. HEADING THROUGH THIS WEEKEND INTO EARLY NEXT WEEK...THE EASTERN PACIFIC TROUGH BECOMES MORE MOBILE...WITH DOWNSTREAM RIDGING FURTHER STRENGTHENING ACROSS EASTERN NORTH AMERICA. THIS PATTERN EVOLUTION ARGUES FOR A RELATIVELY QUIET START TO THE FORECAST PERIOD...WITH A DRIER AIRMASS IN PLACE COURTESY OF NEARBY TROUGH. BEYOND THIS...AS THE TROUGH LIFTS OUT AND WESTERLY FLOW ALOFT STRENGTHENS...ATTENTION WILL TURN TO WEAK DISTURBANCES IN THE MID LEVEL FLOW INTERACTING WITH AN INCREASINGLY WARM AND MOIST BOUNDARY LAYER. THIS WILL INCREASE THE THREAT OF SHOWER/THUNDERSTORM ACTIVITY...WITH MODEST MID LEVEL FLOW SUGGESTING AT LEAST SOME SEVERE WEATHER RISK. GUIDANCE TRENDS/CONFIDENCE: MODEL AGREEMENT IS STRONG THROUGH THE END OF THE WEEK...WITH PRETTY SUBSTANTIAL DIFFERENCES APPEARING THIS WEEKEND INTO EARLY NEXT WEEK...EVEN WITH THE LOCATION OF THE LARGE SCALE PATTERNS. MOST OF THESE DIFFERENCES ARE WITH THE PLACEMENT AND DEPTH OF THE EASTERN PACIFIC TROUGH...WITH THE ECMWF MUCH FASTER TO MOVE THIS FEATURE EAST THAN THE GEFS/GGEM. WHILE THIS LOWERS CONFIDENCE FOR THE SECOND HALF OF THE FORECAST...THE OVERALL IDEA OF BUILDING HEIGHTS IN THE EAST IS WELL AGREED UPON. THE DETAILS... MID LEVEL TROUGHING STILL IN PLACE AS THE LONG TERM FORECAST PERIOD OPENS ON THURSDAY. WHILE THE COLUMN WILL HAVE DRIED A DECENT BIT FROM 24 HOURS PREVIOUS...COOL TEMPS ALOFT /T5S AROUND -16C/ SHOULD BE ABLE TO COAX A FEW SHOWERS TO DEVELOP DURING THE AFTERNOON HOURS...PARTICULARLY IN THE MOUNTAINS...WHICH IS CAPTURED WELL BY RECENT GEFS/SREF ENSEMBLE PRECIPITATION PERCENTILES. THUS...WILL CONTINUE WITH CHANCE POPS IN THIS AREA. GENERALLY QUIET WEATHER FOR THE FRIDAY-SATURDAY PERIOD AS SURFACE RIDGING BUILDS INTO THE REGION ALONG WITH SUBSTANTIAL DRYING OF THE COLUMN AS PWATS FALL BACK BELOW ONE INCH FOR THURSDAY NIGHT THROUGH EARLY SATURDAY. FRONT NORTH OF THE REGION WILL NEAR THE CANADIAN BORDER TOWARDS EVENING...JUST CLOSE ENOUGH TO WARRANT SOME LOW POPS FROM THE MOUNTAINS NORTH...WITH MOST LOCATIONS LIKELY REMAINING DRY. AFOREMENTIONED COLD FRONT LOOKS TO PUSH THROUGH THE REGION ON SUNDAY...BUT REALLY WASHES OUT AS IT DOES SO. THIS RESULTS IN A VERY WEAK SIGNAL FOR PRECIPITATION CHANCES WITH NO MORE THAN SLIGHT CHANCE TO CHANCE POPS FOR A FEW SHOWERS AND THUNDERSTORMS WARRANTED PRIMARILY FROM THE MOUNTAINS NORTH. FOR THE END OF THE LONG TERM FORECAST PERIOD MONDAY AND TUESDAY... MODEL AGREEMENT DECREASES...BUT IN GENERAL...HEIGHTS ALOFT CONTINUE TO BUILD AS THE WEAK FRONTAL BOUNDARY FROM SUNDAY PUSHES OFF THE COAST. THERE CONTINUE TO BE SOME VERY WEAK SIGNALS FOR ISOLATED SHOWER/STORM ACTIVITY /WITH SOME RIPPLES IN THE AMPLIFYING FLOW/...BUT GIVEN PATTERN RECOGNITION...WILL LEAN MORE TOWARDS A DRY FORECAST AT THIS RANGE GIVEN LITTLE COMPELLING SUPPORT FOR PRECIPITATION FROM THE GUIDANCE. TEMPERATURES: THURSDAY AND FRIDAY EXPECT TEMPERATURES CLOSE TO SEASONAL NORMALS AS TROUGH LIFTS OUT OF THE REGION. WITH BUILDING HEIGHTS...EXPECT TEMPERATURES TO WARM AS WE REACH THIS WEEKEND AND MOVE INTO EARLY NEXT WEEK...REACHING 5-7F ABOVE SEASONAL NORMALS. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... SHORT TERM...ONSHORE FLOW CONTINUES TO BRING IN ROUNDS OF LOW LEVEL CLOUDS AND FOG FROM PORTSMOUTH TO ROCKLAND. EXPECT THIS TO PRIMARILY BE THE CASE THROUGH WEDNESDAY MORNING WITH IFR OR LIFR CONDITIONS BEING THE RULE. THUNDERSTORM DEVELOPMENT THIS AFTERNOON REMAINS POSSIBLE ACROSS MOST OF NEW HAMPSHIRE... THOUGH CONFIDENCE IN AFFECTING ANY ONE TAF SITE IS TOO LOW TO MENTION IN 18Z TAF ISSUANCE. EXPECT IMPROVING CONDITIONS FROM WEST TO EAST DURING THE DAY WEDNESDAY. LONG TERM...LONG TERM PERIOD SHOULD BE LARGELY VFR...WITH MORNING FOG CONCERNS THURSDAY THROUGH SATURDAY MORNINGS FOR LEB-HIE...WITH ANY POTENTIAL RESTRICTIONS FROM SHOWERS/THUNDERSTORMS LIKELY NOT RETURNING TO THE FORECAST UNTIL SUNDAY. && .MARINE... SHORT TERM...MOIST SOUTHEAST FLOW PROMOTES THE DEVELOPMENT OF FOG THROUGH TONIGHT. FRONT SLOWLY PUSHES ACROSS THE GULF OF MAINE ON WEDNESDAY AND BRINGS DRIER AIR IN TO GET RID OF THE FOG. LONG TERM...WINDS AND WAVES LOOK TO REMAIN WELL BELOW SCA LEVELS THROUGH THE LONG TERM FORECAST PERIOD. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...NONE. NH...NONE. MARINE...NONE. && $$

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