Area Forecast Discussion
Issued by NWS Portland, ME

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753 FXUS61 KGYX 200539 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1239 AM EST Fri Jan 20 2017 .SYNOPSIS... The area will remain generally under the influence of high pressure...with fair weather and warmer than normal temperatures. A weak front will try and push into the area during the weekend...but is unlikely to bring us much more than very light showers or flurries. On Sunday a cold front drops through the area...bringing more cooler temperatures than precipitation. This front will set up the battle ground between rain and mixed precipitation or snow as the next weather maker moves up the coast on Monday into Tuesday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1237 AM Update... Have updated the grids based on current observations and latest trends. Have continued trend of raising temperatures for the overnight hours. Patchy fog continues to develop over southern and central New Hampshire with plenty of low level moisture remaining in place. 940 PM Update... Low level stratus as well as fog is now affecting the region with moisture trapped under an inversion evident on this evening`s KGYX 00Z sounding. NW winds have recently begun to pick up as surface trough moves away and northwest flow aloft increases in response to blossoming baroclinic leaf structure over the Ohio River Valley. Temperatures...skies...and winds were adjusted through the overnight hours for latest near term guidance and current trends. Temperature readings are still on the warm side having only fell a few degrees so far from today`s highs. 725 PM Update... Stratus is hanging tough under a low level inversion tonight with temperatures/dew points nearly stable or even creeping up. As a result have adjusted the nocturnal trend. Will be likely settling on temperatures a few degrees warmer than the previously forecasted lows which were in the teens and low 20s. We may not see much clearing with light and variable winds through the night as high pressure drifts in from the west. Previous discussion... Upper trof is crossing the area this afternoon...with s/wv ridging expected to build in quickly in its wake. Surface ridge axis will settle overhead around 12z. That points towards radiational cooling tonight...but dewpoints are also rising this afternoon rather than mixing out. Still I feel this is mainly shallow in nature...and we should trend colder than guidance tonight. But the low level moisture does lead me to think some patchy fog is likely across the area.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/... Another mild January day on Friday...with temps reaching towards 40 near the coast. High pressure will be drifting E and allowing return flow to set up. Clouds should gradually increase later in the upper ridge axis approaches. Friday night is an interesting a s/wv trof lifts out of the OH River Valley and shears out into the ridge. Low levels are forecast to remain at or near saturation...while upper levels remain dry within the ridging. SW flow with the s/wv produces some modest isentropic upglide...especially into SWrn zones before fizzling out. This low level lift combined with unsaturated mid and upper levels points towards a drizzle or freezing drizzle scenario. Forcing for ascent will be on the weaker side so it could amount to nothing...or even flurries. I opted to put flurries or patchy drizzle/freezing drizzle in the forecast...though PoP is low due to lack of confidence. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Upper level ridging over the area on Saturday gives way to a trough dropping south from Canada and another trough moving in from the southwest. Not a lot of cold air associated with this as blocking upper ridge remains to our north, limiting the cold air available. However, there will be just enough cold air to make the forecast interesting. Main period of concern is from Monday into Tuesday as the trough coming in from the southwest is likely to develop a coastal low and spread precipitation into our area. As the northern stream trough drops south through the Canadian maritimes it will send a cold front southward through northern New England Saturday night into Sunday. Expect cooler temperatures along with an easterly low level flow which should set up cold air damming conditions as time goes on. By Sunday night, temperatures across the area should fall freezing. Meanwhile, upper level low pressure tracking across the southern portion of the country will reach the Southeast coastline by Monday morning, with surface low pressure developing over the near Norfolk, VA. This low will expand and grow as it tracks northeast toward Cape Cod Monday into Monday night, spreading moisture northward into our area. Temperatures associated with this system will be somewhat tricky considering the cold low developing to our south and the blocking ridge to our north. Models are coming into better agreement that there will be enough cold air aloft for at least a period of snow across the area, with the potential for warmer air aloft to change precipitation over to sleet and freezing rain. Confidence is fairly high that temperatures in the low levels will remain cold enough to be concerned about frozen/freezing precipitation right through the day on Tuesday, as low level easterly flow promotes cold air damming at the surface. At this point it is becoming a question of snow vs freezing rain more than a question of a mix to rain. Our blend of models is still struggling with the cold air damming, so temperatures were lowered especially on Tuesday to account for this. With a northerly flow expected across the area, it is unlikely that temperatures warm above the low to maybe mid 30s. Light snow or drizzle could begin as early as Sunday as the onshore easterly flow begins. But the best precipitation chances begin arriving Sunday night into Monday when the best forcing for ascent north of the low moves into our area. This should provide a few inches of snowfall as it moves from south to north through the area. Meanwhile, the surface low will track northeast toward Cape Cod Monday night, providing another chance for more substantial precipitation. It is during this time period that more mixed precipitation is possible with warm air aloft possibly moving as far north as the Canadian border, changing precipitation to freezing rain or freezing drizzle. It is also possible that the developing low will be strong enough to keep cold air pulled in closer to the low center, keeping precipitation as snow. This is a source of significant uncertainty at this point, so the going forecast will mention just about every precipitation type. The low will move out Tuesday into Tuesday night, bringing precipitation to an end. Expect warmer temperatures again behind the system as there is a general lack of cold air available. Temperatures Wednesday and Thursday of next week are likely to be 5 to 10 degrees above normal. Another wave moving through Wednesday night could provide another chance of precipitation, mainly rain or snow depending on surface temperatures at the time. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... Short Term...MVFR CIGs remain on the NW side of the mountains. High pressure builds in overnight...and CIGs will gradually scatter out. HIE will remain MVFR the longest...with LEB scattering out this evening. With clearing and low level moisture...I do think BR or FG is possible overnight with IFR or lower conditions...but I am not confident in location to put in the TAFs. Currents signs point to interior NH being more likely than western ME however. Very late Friday night some flurries or drizzle/freezing drizzle is possible across SW NH...possibly affecting LEB...CON...and MHT. Confidence is low however. Long Term...Could see some MVFR ceilings on Saturday as a dissipating wave moves into the area. Generally VFR afterward with a front dropping down from the south early on Sunday. An easterly low level flow could pull in some moisture for some MVFR or IFR ceilings later on Sunday, with MVFR/IFR conditions lasting through midweek as a storm system tracks up the coast. Expect a wintry mix of precipitation types ranging from snow to sleet to freezing rain from Monday into Tuesday. && .MARINE... Short Term...Winds remain below SCA thresholds...though seas outside the bays remain at or above 5 ft. Model guidance favors another small increase in seas tonight after the passage of the upper trof...and so I have extended the SCA for hazardous seas thru 12z. Long Term...Light winds are expected through Sunday when an easterly low level flow begins. This flow will strengthen, likely reaching advisory levels Sunday night. As low pressure tracks up the coast, expect winds to reach into Gale force range out of the east or northeast on Monday, lasting until the low moves by Tuesday. There is a low chance that storm force gusts could occur. && .GYX WATCHES/WARNINGS/ADVISORIES...
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ME...None. NH...None. MARINE...Small Craft Advisory for hazardous seas until 7 AM EST this morning for ANZ150-152-154.
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&& $$ NEAR TERM...Cannon SHORT TERM...Cannon LONG TERM...Marine is the U.S. government's official web portal to all federal, state and local government web resources and services.