Area Forecast Discussion
Issued by NWS Portland, ME

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427 FXUS61 KGYX 232353 AFDGYX Area Forecast Discussion National Weather Service Gray ME 753 PM EDT Fri Sep 23 2016 .SYNOPSIS... Showers will continue along a cold front through this evening over southern New Hampshire and far southwestern Maine before much cooler and drier air arrives for the weekend as Canadian high pressure builds into the region. This will bring the potential for frost or freezing conditions Saturday and Sunday nights... especially over northern areas. Gradually moderating temperatures are expected Monday before another approaching cold front brings the threat for showers on Tuesday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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8 PM Update...Narrow band of rain showers are progressing across southernmost NH as forecast and should exit over the next 1-3 hours. Therfore, very little changes to the going forecast. 530 PM Update...Have updated the grids for the next few hours based on latest trends in observations and mesoscale model guidance. The result was little change to the going forecast. However, we did tighten up the PoP gradient a little bit across srn NH for this evening based on latest radar mosaic loops and HRRR guidance. Narrow stripe of showers is beginning to take shape across Upstate NY to the NW of ALB. This area of showers is expected to mainly affect Cheshire and Hillsborough counties this evening. Previous discussion... High Impact Weather Potential: Patchy frost in the far northern valleys possible. Current Pattern: Water vapor imagery early this afternoon shows an onslaught of northwesterly flow extending from Manitoba south and east to a leading shortwave now pressing through New England. This shortwave is interacting with a modest /although thinning/ moisture plume along the east coast to produce multiple bands of showers as it pushes south and east. Also of interest is an area of darkening in water vapor imagery over western Ontario which represents another upstream shortwave that will sharpen as it rapidly pushes southeast into New England during the first half of the night. Beyond this...sprawling 1030mb high pressure over northern Ontario will begin to push toward the region...bringing a clearing/drying/ cooling trend to the area. Forecast concerns are thus focused on precipitation potential through this evening...and temperatures as the continental polar airmass arrives tonight. Through Early Evening: Shower activity has dwindled markedly over the past 1-2 hours...so through 6pm expect little more than a few sprinkles/very light rain showers over southern areas with some breaks in the clouds opening up over northern areas. Tonight: High resolution near term guidance consensus has been honing in on another period of more widespread shower activity over southern NH and far southwestern ME this evening as aforementioned shortwave over Ontario sharpens and interacts with H8 frontal zone. The best window of opportunity for rain looks to be between 8pm and midnight...generally south of a LEB-PWM line. This will certainly help to boost meager rainfall totals in these areas thus far...with another few tenths of an inch of rain quite possible. Beyond this...cold and dry advection will be the rule with gradual clearing from north to south as PWATs retreat back below 0.5". Temperatures will fall rapidly...particularly with partial clearing after midnight...with consensus lows in the upper 30s north to mid/upper 40s south looking reasonable. Can/t rule out a few patches of frost over the far north...but not widespread /or confident/ enough to warrant a county-based headline.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... High Impact Weather Potential: Frost/Freeze Potential Saturday Night. Pattern: The short term forecast period will be dominated by continued northwesterly flow aloft given highly amplified trough/ridge/trough pattern across North America...with large surface high over eastern Ontario Saturday morning remaining nearly stationary through this forecast period. T8s start the period near the freezing mark...and gradually settle lower by Saturday night with these values representing -1 to -2 sigma departures from climatology. PWATs will similarly be in the -1 to -2 sigma range...promising little in the way of sensible weather through the period. Forecast concerns will center around temperatures Saturday night as the coolest night of the early fall season is expected. Saturday: Longwave trough will continue to deepen along the east coast of NOAM as sharp longwave ridging over the central portion of the continent slowly builds and moves east. This will maintain northwesterly flow through the column over our region...although any significant cool advection will be over by daybreak. BUFKIT profiles agree with CU rule suggestions from the guidance indicating some SCT cumulus development in the mountains...with very little in the way of cloudiness making it to the coast. So...expect a mostly sunny day for most spots. T8s will only be around +1-3C...so expect northern areas to remain in the 50s...with areas south of the mountains to reach the lower/middle 60s. Given the robust gradient aloft...expect northwesterly winds 10-20kts for the day...adding a bit of chill to the air. Saturday Night: High pressure remains north and west of the area with PWATs falling to around 0.25" in continental polar airmass. Expect any afternoon cumulus to clear...with a chilly night in store across the region. Primary factor potentially limiting the chill will be an ongoing breeze as 1000mb geostrophic winds remain 20-30kts overnight likely keeping many locations coupled. Honestly...this looks more like a freeze vs. no freeze situation with perhaps too much wind to support frost. Given the breeze...have kept overnight lows a bit higher than statistical guidance. Breeze will also restrict widespread fog...but given how cool temperatures will be...some CT valley locations will still probably see some patches. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Chance of High Impact Weather: Minimal. Where for the last few months 500mb blocking was hard to come by south of 50 N, we are finally starting to see sow blocks develop that will bring troughs down in the 30-40 N range. The models do not show stable blocking, but it is enough to bring temps to around normal with some fluctuations either side of normal through the long range. However, with the instability of the blocking, systems will be mobile and not very strong, so any chance for a soaking rain remains low through next week. The cold air will continue to stream in Sunday, as 500 mb trough works through in the morning. this air is very dry, but should produce some clouds in the mountains Sunday, maybe become mostly cloudy for a few hours. The coastal plain will see more sun in the downslope as N-NW winds pick up in the 10-20 mph with some higher gusts. Highs will be cool, and a few degrees below normal ranging from the low to mid 50s in the mountains to the low 60s in srn NH and along the ME coast. Sunday night will turn out to be an very good rad cooling night as sfc high builds in ands winds die off quickly with clearing skies. Lows will range from the mid-upper 30s in the mountains to the low 40s along the coast and in urban areas of the Merrimack valley. This will bring the threat of frost to almost everywhereexcept the immediate coast. Light winds shift to the SW on Monday as the high shifts offshore and it looks to be mainly a sunny day with highs a few degrees warmer than Sunday, bring temps to around normal, generally in the 60s. The next chance for rain will be in the Mon night thru Wed time frame as 500mb closed low tracks NE out of the great lakes and drags a cold front across the region. The Euro is a little progressive the fropa, ad moving the rain out by Tue night, with the GFS lingering it. However, the Euro also merges the two closed 500 lows to our north and spins the resultant low back SW and brings it just N of ME. This would result in cooler temps late in the week than the GFS. Went with Superblend, which split the difference which keeps temps around normal Wed-Fri, though if the Euro is right, they would be cooler, and if the GFS they would be warmer. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Short Term... Summary: A cold front will push south of the region this evening with high pressure building in it/s wake through Saturday and remaining over the region through Saturday night. Restrictions: Band of showers will bring MVFR restrictions northern terminals...with CON/MHT/PWM/RKD likely seeing brief drops to MVFR. Conditions improve to VFR as showers depart this evening...with LIFR/IFR fog likely HIE/LEB...and possible CON/AUG. VFR for Saturday / Saturday night with the exception again being overnight fog HIE/LEB. Winds: Northwesterly winds 10g16kts this afternoon will diminish to 5-10kts /light and variable in the valleys/ for the overnight before increasing to 12g20kts from the northwest for the day Saturday. Northwest winds will diminish to below 10kts again Saturday night. LLWS: No LLWS expected through Saturday night. Thunder: Very slight chance of thunder MHT through 21UTC...but chance not great enough to mention in TAF. No thunder beyond this through Saturday night. Long Term...Sunday could continue to some N-NW wind gusts in the 20-25 kt range, but VFR Expected Sun-Mon. Some showers are possible Tuesday and Tue night, and could see a period of flight restrictions ahead of cold front Tue afternoon and night. && .MARINE... Short Term...Strengthening northerly winds tonight will bring winds/seas close to marginal SCA values for the overnight before shifting northwest on Saturday...with winds/waves remaining near marginal SCA values through Saturday night...after a brief lull in the winds early Saturday. Long Term...Borderline SCA possible Sunday in N-NW winds, but winds diminish by Sunday evening and stay below SCA through Wed. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ150-152- 154. && $$ SYNOPSIS...Arnott NEAR TERM UPDATE...Ekster SHORT TERM...Arnott LONG TERM...Cempa AVIATION...Arnott/Cempa MARINE...Arnott/Cempa

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