Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 260143 AFDGYX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GRAY ME 843 PM EST TUE NOV 25 2014 .SYNOPSIS... A WEAK AREA OF HIGH PRESSURE WILL BUILD OVER NEW ENGLAND TONIGHT. LOW PRESSURE WILL DEVELOP OFF THE MID ATLANTIC COAST ON WEDNESDAY AND TRACK NORTHEAST TO JUST SOUTH OF CAPE COD BY WEDNESDAY EVENING. THE COASTAL LOW WILL CONTINUE NORTHEAST INTO THE MARITIMES WEDNESDAY NIGHT. A TROUGH OF LOW PRESSURE WILL LINGER OVER THE REGION ON THURSDAY. HIGH PRESSURE WILL BUILD IN FROM THE WEST ON FRIDAY. A WARM FRONT LIFTS INTO THE REGION FROM THE SOUTHWEST ON SATURDAY...FOLLOWED BY A COLD FRONTAL PASSAGE ON SUNDAY NIGHT AND EARLY MONDAY. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 840 PM UPDATE: VERY QUIET EVENING. UPDATED LATEST OBS DATA WITH A FEW MINOR TWEAKS. NO OTHER CHANGES ATTM. PREV DISC: NOT MUCH TO TALK ABOUT TONIGHT...AS FOCUS REMAINS ON THE NEXT COUPLE OF DAYS. CLOUDS WILL GRADUALLY BE ON THE INCREASE TONIGHT AS A COLD FNT CONTINUES TO OOZE SWD. WILL SEE LITTLE TEMP ADVECTION WITH THIS FEATURE...BUT IMPORTANTLY FOR WED WE WILL SEE DEW POINT ADVECTION. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... ***SIGNIFICANT COASTAL STORM TO AFFECT HOLIDAY TRAVEL*** MODEL GUIDANCE IS NOW IN EXCELLENT AGREEMENT ON STORM TRACK...WITH THE ECMWF...GFS...AND NAM...ALL MOVING JUST SLIGHTLY NW OF THE 40N/70W BENCHMARK. THE CMC REMAINS THE ERN OUTLIER...SO HAVE TROUBLE PUTTING MUCH STOCK IN ITS OUTPUT ATTM. THIS LEADS TO A FAIRLY CONSISTENT QPF FORECAST AS WELL. THE GFS AND ECMWF ARE CLOSE TO OR JUST OVER 1 INCH LIQUID FOR COASTAL SWRN ME. IF YOU TAKE INTO ACCOUNT THE KNOWN BIAS FOR THE NAM TO GIVE TOO MUCH QPF ON THE COLD SIDE OF LOW PRES SYSTEMS...AND TAKE 2/3 OF ITS QPF...YOU ALSO GET JUST OVER 1 INCH QPF FOR THOSE SAME AREAS. DIFFERENCES ARISE THE FARTHER NW ONE HEADS FROM THE COAST. FOR THIS REASON THERE REMAINS A BIT OF UNCERTAINTY IN THE MTNS ON AMOUNTS. PCPN HAS SPED UP ON MODEL GUIDANCE...REACHING ALL OF THE SRN HALF OF THE CWFA BY 18Z. SO THIS FORECAST REPRESENTS A SLIGHTLY BUMP UP IN TIMING FROM THE PREVIOUS SHIFT. THIS MAKES FOR A TRICKY TEMP FORECAST. CURRENT THINKING IS THAT HIGHS OCCUR VERY EARLY IN THE DAY...POSSIBLY EVEN NEAR DAWN FOR SOME SRN NH AREAS. FROM THAT POINT FORWARD...TEMPS LIKELY FALL DURING THE DAY AS PCPN MOVES NWD. THIS IS MOSTLY DUE TO THE AFOREMENTIONED DRY ADVECTION OCCURRING OVERNIGHT TONIGHT. AS PCPN FALLS TEMPS WILL WET BULB DOWN TO NEAR FREEZING QUICKLY. HI-RES GUIDANCE...INCLUDING THE NAM12...ARW AND NMM WRF MODELS SHOW THIS IN THEIR 2 M TEMP OUTPUT. FAVORED A BLEND OF 2 M TEMPS RATHER THAN MOS FOR HOURLY TEMP TREND. AS THINGS STAND THIS AFTERNOON...GUIDANCE ALSO INDICATES MAINLY N TO NNE WINDS AT THE SFC. THIS IS COUPLED WITH NWLY AGEOSTROPHIC FLOW...WHICH WILL ACT TO KEEP WINDS MAINLY OFFSHORE THRU THE EVENT. FOR THIS REASON I FEEL THAT COASTAL FNT WILL SET UP OFFSHORE AS WELL...AND MARINE BOUNDARY LAYER WILL NOT BE A SIGNIFICANT ISSUE. THE BIGGEST DIFFERENTIATOR BETWEEN RA AND SN WILL BE INITIAL WET BULBING. SO ATTM THE FORECAST IS FOR SIGNIFICANT SNWFL RIGHT TO THE COAST...ENOUGH TO IMPACT TRAVEL. WITH MARGINAL TEMPS IT MAY ALSO BE WET AND HEAVY FOR A TIME...AND SCATTERED POWER OUTAGES ARE POSSIBLE. THIS LEADS TO ACTUAL FORECAST SNWFL AMOUNTS. IT IS STILL ROUGHLY 24 HOURS AWAY FROM THE BULK OF THE PCPN...SO THERE REMAINS SOME NUANCES THAT COULD AFFECT SNWFL. FOR SEVERAL DAYS NOW...MODEL GUIDANCE HAS BEEN CONSISTENT WITH TRACKING MID LEVEL LOW CENTERS VERY CLOSE TO THE NEW ENGLAND COAST. AT THE SAME TIME...700 MB HEIGHTS TRY AND CLOSE OFF NEARLY OVERHEAD. THIS FAVORS A STRONG BAND OF LIFT JUST ON THE COOL SIDE OF THE MID LEVEL LOW TRACK. DEFORMATION AND FRONTOGENESIS PROGS ON THE ECMWF...GFS...AND NAM AGREE WITH THIS PLACEMENT...FROM SRN NH TO JUST INLAND FROM THE COAST OF WRN ME. AS THE MID LEVEL LOWS TAKE THIS TRACK...IT WILL ALLOW SOME WARMTH ALOFT TO SNEAK NW. IT MAY NOT EVER GET ABOVE FREEZING AT THESE LEVELS...BUT WARMING TO WITH A COUPLE OF DEGREES OF FREEZING COULD LEAD TO POOR SN GROWTH FOR A TIME ALONG THE COAST. SN RATIOS WERE ADJUSTED TO REFLECT THESE EXPECTED OUTCOMES...LOWERED NEAR THE COAST...AND RAISED JUST INLAND WHERE STRONGEST DEFORMATION BANDING IS LOCATED. IN ADDITION...WE HAVE ALSO HAD VERY WARM TEMPS RECENTLY AND WILL START OFF WITH MARGINAL TEMPS EARLY WED. THIS MAY LEAD TO A FALSE SENSE OF SECURITY HOWEVER. WHEN STRONGEST LIFT COMES IN...DURING WED AFTERNOON AND EVENING...FORCING AND SOUNDINGS CONFIRM THAT SN MAY FALL HEAVILY AT TIMES. WITH RATES POSSIBLY APPROACHING 1 INCH/HR IT WILL NOT MATTER HOW WARM IT HAS BEEN...AND ACCUMULATION WILL OCCUR ON ROADWAYS. ONCE AGAIN...THE CONDITIONS WILL BE QUICKLY DETERIORATING AND EXPECTED TO BE THE WORST WED AFTERNOON AND EVENING...COINCIDING WITH THE BUSIEST TRAVEL TIMES. TRAVELERS SHOULD BE AT THEIR DESTINATIONS BY WED AFTERNOON IF AT ALL POSSIBLE. && .LONG TERM /THURSDAY THROUGH TUESDAY/... A SURFACE RIDGE OF HIGH PRESSURE WILL BEGIN TO BUILD IN FROM THE WEST ON THANKSGIVING AS THE STORM DEPARTS INTO THE MARITIMES. HOWEVER....A LARGE SCALE...UPPER LEVEL TROUGH WILL REMAIN TO OUR WEST. 12Z MODEL SUITE IS IN GOOD AGREEMENT AT THIS TIME WITH THESE LARGE SCALE FEATURES. HOWEVER...THE NEW EURO RUN DEPICTS A NEARLY STATIONARY INVERTED TROUGH ALONG THE SOUTHWEST COAST OF MAINE THURSDAY INTO FRIDAY. THE SCENARIO WOULD KEEP SCATTERED SNOW SHOWERS IN THE PORTLAND AREA FOR THE END OF THE WORKWEEK. OTHER MODELS ARE NOT PICKING UP ON THIS NORLUN TROUGH TYPE FEATURE. WILL SEE IF THIS FEATURE REMAINS ON THE FOLLOWUP MODEL RUNS. THEREAFTER...VERY QUIET WITH A BRIEF WARM UP ON SUNDAY FOLLOWED BY A STRONG COLD FRONT ON MONDAY. MODELS DIVERGE AS TO WHETHER A WAVE OF LOW PRESSURE WILL FORM ALONG THIS FRONT...AFFECTING NEW ENGLAND. A LARGE AND CHILLY CANADIAN HIGH PRESSURE SYSTEM WILL FOLLOW. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... SHORT TERM...VFR CONDITIONS PREVAIL THRU EARLY WED. CONDITIONS RAPIDLY DETERIORATE FROM S TO N THRU WED AFTERNOON. CONDITIONS WILL QUICKLY DROP TO IFR AND LOWER BY EVENING. COULD SEE SOME BRIEF MIXING WITH RA AT KPSM AND KPWM...BEFORE A TRANSITION TO ALL SNWFL. WINDS ALONG THE COAST WILL BE GUSTY FROM THE NNE...AROUND 25-30 KTS. CONDITIONS QUICKLY IMPROVE TO MVFR OR VFR THU AS LOW PRES PULLS AWAY. LONG TERM...MAINLY VFR CONDITIONS ACROSS THE REGION IN THE LONG TERM. THERE MAY BE LOCALIZED MVFR CONDITIONS IN SCT SNOW SHOWERS ALONG THE COAST THURSDAY INTO FRIDAY. MVFR CONDITIONS WITH A FRONTAL PASSAGE NEXT MONDAY. && .MARINE... SHORT TERM...GALE WARNINGS IN EFFECT STARTING WED AFTERNOON. COASTAL STORM WILL QUICKLY MOVE ACROSS THE GULF OF ME...WITH A BRIEF PERIOD OF GALE FORCE GUSTS. SCA WILL LIKELY BE NEEDED FOLLOWING THE LOW PRES. LONG TERM...WINDS WILL LIKELY REACH SCA CRITERIA LATE IN THE WEEKEND AND INTO EARLY NEXT WEEK. A SW WIND WILL VEER TO THE NW. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...WINTER STORM WARNING FROM 10 AM WEDNESDAY TO 7 AM EST THURSDAY FOR MEZ012>014-018>028. WINTER WEATHER ADVISORY FROM 1 PM WEDNESDAY TO 7 AM EST THURSDAY FOR MEZ007>009. NH...WINTER STORM WARNING FROM 10 AM WEDNESDAY TO 7 AM EST THURSDAY FOR NHZ002>010-013-014. WINTER WEATHER ADVISORY FROM 1 PM WEDNESDAY TO 7 AM EST THURSDAY FOR NHZ001. MARINE...GALE WARNING FROM 5 PM WEDNESDAY TO 7 AM EST THURSDAY FOR ANZ150>154. && $$

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