Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 230125 AFDGYX Area Forecast Discussion National Weather Service Gray ME 925 PM EDT Thu Sep 22 2016 .SYNOPSIS... Mild conditions will continue through tonight south of a strong frontal boundary draped north of the region. This boundary will move south as a cold front tonight and Friday...spreading some showers across the region. Canadian high pressure will arrive Friday night Saturday...remaining in control of our weather through the weekend and into early next week. This will bring a much cooler airmass to the region...with a frost possible for northern areas both Saturday and Sunday nights. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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925 PM Update...Have adjusted timing of PoPs a little bit overnight as well as sky cover. Nice night out there for the most part as skies are still mostly clear through some cirrus, even in a large portion of the north. Clouds will eventually fill in from N to S overnight with rain showers arriving in the mountains around or after midnight. 615 PM Update...Little change to the going forecast. Have adjusted PoP timing and little bit for tonight and backed off on cloud cover a little bit for this evening. Otherwise, forecast remains on track. Previous discussion... High Impact Weather Potential: Minimal. Current Pattern: Early afternoon water vapor imagery showing a fairly healthy shortwave over western Ontario sharpening somewhat as it digs south and east. Ahead of this feature is a region of brisk zonal flow atop a surface frontal zone that extends from the Great Lakes eastward into far northern Maine. Closer to home...south of this boundary temperatures aloft and at the surface are well above normal...with T8s +1-2 sigma yielding temperatures in the 80s over my southern zones. Changes are in the works...however...as the aforementioned shortwave interacts with the downstream frontal system...with a wave of low pressure forming along it and pushing east. This weak low will move just north of the forecast area overnight...with primary forecast concerns centered on precipitation potential just ahead of the surface boundary as it slowly edges south. Through Early Evening: Very little weather to speak of through this evening...with some SCT/BKN mid level cloudiness in WAA regime south of the quasi-stationary surface boundary. Very warm temperatures will continue into the evening with 60s and lower 70s the rule even through 8pm. No precipitation expected through at least 7-8pm. Tonight: With developing wave of low pressure upstream... southwesterly jet at H8 strengthens overhead this evening...likely keeping frontal boundary north of the forecast area through at least the first half of the night. Can/t rule out a shower in the moistening area of warm advection over far northern areas...but expect that we/ll have to wait until after midnight for appreciable shower activity to arrive. Once the weak wave of low pressure passes east of our longitude /around 6Z/ ...the boundary will be able to sink south...with height falls ahead of the upstream shortwave spreading south and east coincident with the right entrance region of a 120kt jet streak on the 1.5 PVU surface. This should help bring showers further south through daybreak Friday...but still likely confined to the northern half of the forecast area. With only modest dynamics...and residing south of the surface low...expect QPF values to remain relatively light...generally less than a quarter inch...perhaps a bit higher over extreme northwestern ME and Coos county, NH. Lows temperatures should remain mild...with modest gradient south of the front as well as increasing cloud cover. Thus...expect most sites to remain in the upper 50s.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... High Impact Weather Potential: Very low risk of a rumble of thunder far southern NH Friday afternoon. Elevated rip current risk over coastal areas as long period swell builds. Pattern: The period starts with shortwave north of the region making its closest approach...with flow through the column turning increasingly northwesterly through the short term forecast period. This will drive the cold front and it/s associated moisture plume south and out of the region by Friday evening...with a change to a cooler/drier than normal airmass Friday night. Forecast concerns center around just how much precipitation will be experienced along the front Friday...turning to just how chilly it will be /particularly in northern areas/ as the cool/dry airmass arrives Friday night. Friday: Cold front over the northern zones to start the day will take much of the day to press south. Am not too excited about precipitation potential as right entrance region jet forcing exits along with surface low...with weakening llevel convergence along the southward sinking front. Thus...expect a line of morning showers to become more scattered in nature as it pushes south...and thus QPF amounts will /unfortunately/ lessen as you head to our most drought- stricken areas. Do expect a mostly cloudy to cloudy day for just about everyone...with some clearing arriving over northern areas towards evening behind the departing front. With the front bisecting the area for much of the day...expect a pretty large range in temperatures. Southern NH could again make a run at 80 with T8s near what they are today...but with a good bit less in the way of sun. Behind the boundary with robust CAA developing...areas from the mountains north will likely not reach much above the lower 60s. QPF: Total QPF through this rain event looks to reach the quarter to half inch range north of the mountains...with amounts likely closer to one tenth of an inch to the south of this. Thunder: Can/t completely rule out a rumble of thunder with any showers that survive into far southern New Hampshire during the afternoon as MUCAPE plume of 200-400 J/kg is present /better values to the south of the CWA/ Have included a slight chance of thunder in this area. Friday Night: Surface front will be south of the region to start the period Friday evening...sinking south overnight as northwesterly flow aloft strengthens...with surface high pressure building into the region. Fairly robust cold and dry advection is expected through the overnight with PWATs falling below seasonal norms /-1 to -2 sigma/ after midnight. Certainly a dry period...although some lingering cloud cover is possible beneath developing subsidence inversion. 1000 mb geostrophic wind remains 15-20kts overnight...so most locations will likely remain coupled. This should keep temperatures and fog from getting too out of hand...but some patchy river fog in the CT valley is certainly possible. Consensus lows weighted more toward MOS appear appropriate...with some upper 30s likely over the far north...and upper 40s to around 50 along the coast and in southern NH. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Models are in disagreement over the long term period. Canadian and GFS in one camp and ECMWF in another. All models do agree that high pressure will be in control Saturday through Monday. Cold high pressure at that with a chance of frost in some areas Sunday morning and again Monday morning. The models diverge for Monday and the remainder of the week. The GFS closes off an upper level system well to our west and never does drive a cold front through the region as high pressure aloft keeps the system at bay. The ECMWF brings a cold front through the region Monday night with about a third of an inch of precipitation. For the now will introduce a low chance POP over the forecast area Monday night and will continue low chance POP through Wednesday. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... Short Term... Summary: A cold front will slowly settle through the region tonight and on Friday bringing deteriorating flying conditions. High pressure and drier air will arrive from the north Friday night with conditions improving. Restrictions: VFR at the moment all sites. As the front arrives from the north tonight...expect deteriorating conditions /some IFR fog possible LEB ahead of the front overnight/...with shras possible HIE/AUG/LEB/RKD by daybreak Friday...spreading south during the day. This will likely bring a period of MVFR restrictions as early as late tonight HIE...spreading to LEB/AUG through early Friday. Confidence decreases further south...where restrictions may be more brief as shower activity weakens. Improvement to VFR is likely all sites Friday night...with additional restrictions possible HIE/LEB in fog development. Winds: Southerly winds less than 10kts today /with sea breezes developing at all coastal sites/. Winds will go light/variable tonight before gradually shifting northwesterly and increasing to 12g18kts for the day on Friday before decreasing to less than 10kts from the northwest Friday night. LLWS: West/southwesterly winds ahead of the front will strengthen tonight...with 1-2kft winds increasing to around 30kts overnight with a period of LLWS. No LLWS beyond this. Thunder: Very slight chance of a thunderstorm Friday afternoon over far southern NH /MHT/...otherwise no thunder is expected. Long Term...Expect VFR Saturday through Monday. Except chance MVFR/IFR in patchy valley fog Saturday night and again Sunday night at HIE and LEB. MVFR is possible in Showers Monday night and Tuesday. && .MARINE... Short Term /through Friday Night/... Winds/waves look to remain below SCA levels through Friday...with strengthening north/northeasterly winds Friday night potentially bringing seas to marginal SCA levels. Long Term...SCA criteria winds and seas are possible Saturday into Sunday behind the exiting cold front. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM UPDATE...Ekster SHORT TERM...Arnott LONG TERM...Hawley AVIATION...Arnott/Hawley MARINE...Arnott/Hawley

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