Area Forecast Discussion
Issued by NWS Portland, ME

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743 FXUS61 KGYX 191658 AFDGYX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GRAY ME 1258 PM EDT TUE MAY 19 2015 .SYNOPSIS... SHOWERS ASSOCIATED WITH A WARM FRONT WILL SPREAD ACROSS THE REGION THIS MORNING. SHOWERS AND THUNDERSTORMS WILL DEVELOP THIS AFTERNOON AND EVENING AHEAD OF A COLD FRONT. A NORTHWEST FLOW WILL DEVELOP BEHIND THE FRONT TONIGHT AND PROVIDE COOL BUT MOSTLY QUIET WEATHER INTO THE WEEKEND. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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1 PM UPDATE...FORECAST REMAINS ON TRACK. SHOWERS WITH EMBEDDED THUNDER DEVELOPING ACROSS UPSTATE NY IN ADVANCE OF COLD FRONT. THESE WILL MOVE EASTWARD AND INTO THE CT VALLEY AROUND OR AFTER 4 PM. HEATING IS BEGINNING TO OCCUR THERE AND SHOULD CONTRIBUTE TO ENOUGH CAPE TO SUSTAIN A FEW STRONG TO LOCALLY SEVERE STORMS LATE THIS AFTERNOON. 950 AM UPDATE...HAVE UPDATED THE FORECAST TO FRESHEN UP THE TIMING OF POPS TODAY AS WELL AS TEMPERATURES. OVERALL...FORECAST REMAINS ON TRACK. WIDESPREAD RAIN THIS MORNING WILL PUSH OFF THE COAST IN A FEW HOURS AND SHOULD ALLOW FOR AT LEAST PARTIAL SUNSHINE THIS AFTERNOON...ESPECIALLY ACROSS THE INTERIOR. THIS WILL LEAD TO INCREASING INSTABILITY. AS A SHORT WAVE TROUGH APPROACHES THIS AFTERNOON...THUNDERSTORMS SHOULD DEVELOP IN UPSTATE NY AND VT AND REACH THE CT VALLEY BY 4 PM. THE STORMS SHOULD THEN PROGRESS EASTWARD AND EXIT THE COAST BETWEEN 8 AND 9 PM. A FEW SEVERE STORMS WILL BE POSSIBLE...ESPECIALLY ACROSS NH. PREVIOUSLY... 530AM UPDATE... HAVE UPDATED HOURLY PRECIPITATION CHANCES BASED ON RECENT RADAR TRENDS. MUCH OF THE AREA IS SEEING SHOWERS NOW EXCEPT THE IMMEDIATE COAST. THIS WILL CHANGE SOON. BACK EDGE OF PRECIPITATION MOVING INTO VERMONT NOW AND WILL REACH WESTERN NEW HAMPSHIRE WITHIN THE NEXT 2 TO 3 HOURS. ORIGINAL DISCUSSION... AREA OF SHOWERS AND A FEW EMBEDDED THUNDERSTORMS IS MOVING THROUGH THE HUDSON VALLEY EARLY THIS MORNING AND SHOULD ARRIVE IN NEW HAMPSHIRE IN THE NEXT TWO HOURS BEFORE SPREADING EASTWARD ACROSS THE REST OF NEW HAMPSHIRE AND WESTERN MAINE THIS MORNING. THIS AREA OF PRECIPITATION IS ASSOCIATED WITH A WARM FRONT-LIKE FEATURE IN THAT BEHIND IT DEWPOINTS ARE SURGING INTO THE MID 60S WHILE THE COOL MARITIME AIR MASS AHEAD OF IT IS COOLER AND WITH LOWER DEWPOINTS. BUT YOU WILL BE HARD PRESSED TO FIND A NOTICEABLE WIND SHIFT OR PRESSURE TROUGH WITH IT. SUCH IS LIFE IN NORTHERN NEW ENGLAND. AREA OF RAIN WILL MOVE INTO THE MIDCOAST REGION OF MAINE LATE IN THE MORNING AND PERSIST INTO THE AFTERNOON. WHILE NEW HAMPSHIRE WILL SEE THE RAIN ENDING EARLIER IN THE MORNING. BREAKS IN THE CLOUDS SHOULD ALLOW FOR PRETTY GOOD WARMING INTO THE MID TO UPPER 70S OVER WESTERN NEW HAMPSHIRE. THE COMBINATION OF THIS WARMING... THE INCREASING LOW LEVEL MOISTURE... AND COOLING TEMPERATURES ALOFT ASSOCIATED WITH A SHORTWAVE TROUGH WILL PROVIDE SUBSTANTIAL INSTABILITY THIS AFTERNOON... PERHAPS THE BEST SEEN YET THIS SEASON. CAPE APPROACHES OR EXCEEDS 1000 BY LATE THIS AFTERNOON AND EARLY EVENING OVER VERMONT AND NEW HAMPSHIRE. JUST AS INSTABILITY IS INCREASING... A COLD FRONT WILL BEGIN MOVING ACROSS NORTHERN NEW ENGLAND FROM WEST TO EAST. THIS WILL PROVIDE LOW LEVEL CONVERGENCE TO ADD LIFT TO THE EQUATION... AIDING IN THE DEVELOPMENT OF THUNDERSTORMS. BEST INSTABILITY AND LIFT DOES NOT ARRIVE UNTIL LATE IN THE DAY... SO THUNDERSTORM POTENTIAL SEEMS TO BE MORE IN THE 4PM-8PM TIME PERIOD... WITH REMNANT STORMS LIKELY MAINTAINING THEMSELVES THROUGH ELEVATED INSTABILITY THROUGH MIDNIGHT TONIGHT. MESOSCALE MODELS INCLUDING THE NAM AND HRRR INDICATING POTENTIALLY TWO SEPARATE AREAS OF MORE WIDESPREAD THUNDERSTORM ACTIVITY LATE THIS AFTERNOON. THE FIRST IS OVER THE NORTHERN ADIRONDACKS AND THE SECOND OVER THE CATSKILLS... BOTH AREAS FIRING AS THE FRONT MOVES THROUGH TO PROVIDE LIFT AND MAY BE ENHANCED DUE TO TERRAIN-BASED INSTABILITY. IF THIS SCENARIO UNFOLDS... THEN THE NORTHERN AREA OF THUNDERSTORMS WOULD MOVE EAST ACROSS VERMONT AND THE NORTHERN HALF OF NEW HAMPSHIRE AND INTO NORTHWEST MAINE IN THE EVENING. THE SOUTHERN CLUSTER WOULD MOVE EAST MAINLY INTO MASSACHUSETTS. HAVE ORIENTED AFTERNOON/EVENING THUNDERSTORM CHANCES AS BEING GREATEST IN THE NORTH WHERE THE MORE NORTHERN CLUSTER OF STORMS WOULD MOVE ACROSS BUT HAVE MAINTAINED AT LEAST A CHANCE OF THUNDERSTORMS OVER ALL OF NEW HAMPSHIRE AND EXTENDING INTO FAR WESTERN/NORTHERN MAINE IN THE EVENING. ONE FINAL INGREDIENT TO ADD TO THE MIX TODAY IS INCREASING WIND SHEAR. BY EVENING... AROUND THE TIME THE FRONT WILL BE MOVING INTO NEW HAMPSHIRE... WINDS ALOFT APPROACH 50 KT AT 700 MB WHILE SURFACE WINDS ARE BROADLY LIGHT OUT OF THE SOUTH. THIS WILL PROVIDE ENOUGH SPEED SHEAR TO PROVIDE STORM ORGANIZATION AND SUPERCELL TYPE STRUCTURES. THIS MAKES SEVERE THUNDERSTORMS MORE LIKELY... WITH DAMAGING WINDS AND HAIL BEING THE PRIMARY THREATS. HOWEVER... GIVEN LOW LEVEL WINDS OUT OF THE SOUTH... CANNOT RULE OUT THE POSSIBILITY OF AN ISOLATED TORNADO PARTICULARLY IN AREAS WHERE LOW LEVEL WINDS ARE CHANNELED BY TERRAIN AND PRODUCE GREATER LOCALIZED DIRECTIONAL SHEAR. HAVE MENTIONED THE THREAT OF DAMAGING WINDS AND LARGE HAIL IN THE PUBLIC FORECAST FOR LATE THIS AFTERNOON. HAVE ALSO ORIENTED HOURLY PRECIPITATION CHANCES TO BETTER REFLECT THE IDEA THAT EARLY MORNING SHOWERS WILL MOVE EAST BEFORE THE CHANCE OF THUNDERSTORMS ARRIVES LATE.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... COLD FRONT WILL PUSH EAST ACROSS THE REGION TONIGHT WITH THUNDERSTORM ACTIVITY LIKELY LINGERING INTO THE EVENING... POSSIBLY TO AROUND MIDNIGHT IN NORTHERN MAINE. COOLER AND DRIER AIR WILL BE MOVING IN BEHIND THE COLD FRONT. COULD SEE SOME FOG NEAR THE COAST BEFORE THE FRONT MOVES THROUGH AND BEGINS TO MIX/DRY THINGS OUT. TEMPERATURES FALL INTO THE UPPER 40S AND LOW 50S. WILL BE A COOLER DAY ON WEDNESDAY FOR MOST OF THE REGION. BUT ON THE COAST IT MAY ACTUALLY BE A FEW DEGREES WARMER... A TYPICAL RESULT OF A PASSING COLD FRONT AND A SHIFT TO OFFSHORE WINDS. HIGH TEMPERATURES RANGE FROM THE MID 50S IN THE NORTH TO THE MID 60S NEAR THE COAST. DEWPOINTS WILL BE FALLING INTO THE 30S WHICH WILL MAKE IT DIFFICULT FOR ANY SHOWERS TO DEVELOP IN SPITE OF SOME LIFT MOVING THROUGH WITH THE PASSING OF THE UPPER LEVEL TROUGH. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... A LARGE UPPER LEVEL TROUGH WILL DOMINATE THE FLOW ACROSS NEW ENGLAND THROUGH THE WEEKEND. SEVERAL DISTURBANCES WILL ROTATE AROUND THE TROUGH, HOWEVER, COOL DRY WEATHER WILL RULE FOR MUCH OF THE PERIOD. A MOISTURE-STARVED FRONT DROPPING SOUTHWARD FROM CANADA COULD BRING A FEW SHOWERS TO THE REGION FRIDAY. BEHIND THE FRONT...TEMPERATURES WILL BE MAINLY IN THE 60S SATURDAY BUT SHOULD START TO WARM INTO THE 70S MOST AREAS FOR SUNDAY AND MONDAY AS THE TROUGH AXIS SLIDES EAST OF THE REGION. MODELS NOW DELAY NEXT SIGNIFICANT PRECIPITATION UNTIL TUESDAY, ALLOWING FOR A VERY PLEASANT HOLIDAY WEEKEND. NOTE: THE FREEZE/FROST PROGRAM STARTS IN NORTHERN ZONES THURSDAY MAY 21. LOW TEMPERATURES THURSDAY AND SATURDAY MORNINGS WILL DROP NEAR OR BELOW FREEZING ACROSS NORTHERN ZONES AND MAY REQUIRE FREEZE WARNINGS AND/OR FROST ADVISORIES. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... SHORT TERM...EARLY MORNING IFR/LIFR CONDITIONS WILL BEGIN IMPROVING THIS MORNING EVEN AS AN AREA OF RAIN MOVES ACROSS THE AREA. COULD SEE MVFR CEILINGS LINGER THROUGH MUCH OF THE DAY BUT THERE IS A BETTER CHANCE THAT CONDITIONS WILL BREAK OUT TO VFR FOR AT LEAST A FEW HOURS. THERE IS A CHANCE OF THUNDERSTORMS ACROSS MUCH OF NEW HAMPSHIRE LATE THIS AFTERNOON INTO EARLY THIS EVENING. LOW CLOUDS MAY MAKE A RETURN VISIT TO COASTAL MAINE EARLY THIS EVENING BUT THE PASSING OF A COLD FRONT DURING THE NIGHT WILL LIKELY IMPROVE CONDITIONS TO VFR EVENTUALLY. LONG TERM...MAINLY VFR CONDITIONS EXPECTED. && .MARINE... SHORT TERM...SOUTHERLY FLOW INCREASES TODAY THEN SHIFTS TO THE WEST TONIGHT BEHIND A COLD FRONT. WEST WINDS COULD GUST TO AROUND 20 KT ON WEDNESDAY. LONG TERM...WITH MAINLY OFFSHORE FLOW...WINDS AND SEAS EXPECTED TO REMAIN BELOW SCA CRITERIA. STRONGEST WINDS AND HIGHEST SEAS SHOULD BE WELL OFFSHORE. && .FIRE WEATHER... MOIST AIR AND WIDESPREAD WETTING RAIN TODAY WILL KEEP FIRE DANGER CONDITIONS LOW. DRY AIR MOVES IN BEHIND A COLD FRONT TONIGHT... LEADING TO MUCH DRIER CONDITIONS WEDNESDAY INTO THE WEEKEND. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...NONE. NH...NONE. MARINE...NONE. && $$ NEAR TERM...EKSTER

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