Area Forecast Discussion
Issued by NWS Portland, ME

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256 FXUS61 KGYX 210624 AFDGYX Area Forecast Discussion National Weather Service Gray ME 124 AM EST Sat Jan 21 2017 .SYNOPSIS... High pressure moves east tonight while a dissipating wave moves into the region from the southwest. This will primarily just bring some clouds to the area, though some light rain or freezing rain is possible. Expect warm temperatures on Saturday as an upper level ridge crosses the region. A cold front will drop south out of Canada Saturday night into Sunday morning, bringing cooler weather again. A storm system tracking up the coast will bring wintry precipitation New England Monday and Tuesday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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120 AM Update... Only minor changes to the forecast to account for current mesoscale trends. 640 PM Update... Only a few changes to temperatures and winds for this update. Sky cover remains entrenched with calm winds in place. Downsloping winds on the north side of the Whites has kept clouds out of portions of our northern counties but has since relaxed. May be adjusted temperatures slightly upward here depending on if cloud cover fills in shortly. Previous discussion... Cloud cover lingered much longer than initially expected today. High pressure axis crossing the area saw a shift in the light wind flow to a southeasterly return flow. This is allowing clouds to dissipate to the northwest of the mountains (the downslope side), while clouds pile up to the south and east of the terrain. This cloud structure will play a role in temperatures overnight, as areas in the clear will cool quite a bit more than areas stuck in the clouds. Have decided to go above guidance values for southern New Hampshire and most of the coastal plain of Maine where temperatures may struggle to drop much below freezing. To the north and northeast where clearing is expected, temperatures will be colder, in the 20s to upper teens. A sharp shortwave trough is moving northeast into New York state this afternoon. This has been producing rain as it moved through Pennsylvania, but as it arrives in New England the trough will get lost in the ridge over New England. This area of rain has already been breaking up a bit as the day has gone on and expect this to continue this evening. By the time it arrives in New Hampshire after midnight, it should be not much more than clouds. But some light rain or freezing rain is possible, depending on surface temperatures. Considering confidence in precipitation occurrence is quite low, we do not plan to highlight the freezing rain threat tonight unless/until it gets closer to arriving.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... As the remnant wave moves east across the area expect a west to northwest flow behind it with warmer air spilling in. Temperatures should reach the 40s across the entire area. Downslope warming could allow temperatures to reach the 50s in southern and coastal areas. Models often under-forecast warm temperatures in January in part due to the expectation that snow cover will have a greater negative effect on high temperatures (an old snow pack often negates this effect) and also due to extra mixing from the downslope winds. Therefore went a little above guidance for high temperatures in the south and the coastal plain on Saturday. A trough dropping south through the Gulf of Saint Lawrence will send a cold front south through Maine and New Hampshire Saturday night into Sunday morning. This will bring colder temperatures into the area, but may only reach the northeast part of the area by Sunday morning. Temperatures generally expected to be in the 30s, dropping into the 20s behind the front. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Extended forecast focused mainly on the significant coastal storm set to affect the region Mon thru Tue. Outside of that window I stayed close to the multi-model consensus. Some of the things we do know about the storm system include...that it will be a slower mover. With a well develop closed low...we should see precip occurring over parts of the forecast area for at least 24 hours. I am also not particularly concerned with lack of moisture or QPF in this event. Ensemble guidance is showing significantly anomalous H8 u wind into the region...in excess of -5 standard deviations. That is typically a strong signal for higher QPF events...though such strong flow will also likely have a terrain/upslope dependence to it as well. Couple that Ely flow with a +3 standard deviation PWAT anomaly...and the ingredients are there for a high QPF system. This strong Ely flow will also develop in a well mixed boundary layer initially. Winds...especially near the coast could become quite gusty for a period Mon night. It is still a long way out...but this type of Ely flow could produce advisory level gusts or higher near the coast. Confidence is also fairly high that a surface high pressure will be building SEwd into the Canadian Maritimes ahead of the approaching system. This type of evolution is very favorable for cold air damming and temps below guidance. Given that we are still 5 or so days away from the event...guidance is still struggling with handling that damming...but the 20.12z ECMWF seemed to have the best handle on it. The forecast leans heavily in this direction. With all that said...there is plenty we are less confident in. Both the GEFS and ECMWF EPS sensitivity argue that a key feature in downstream evolution is the upper low pressure still sitting off the Pacific Northwest coast. It still could take another 24 to 36 hours before this feature is onshore and can be well sampled by the raob network. When this happens...models could shift more strongly in favor of a warmer or colder scenario for our region. At this time ensemble sensitivity points towards higher heights across the Wrn CONUS and into Canada favoring a colder solution for the Northeast. This is no real surprise...given that a stronger high to our N would result in a colder temp profile overall. However we will not know if models are handling those heights well for another day or so. Mid level temps will be critical in this forecast as they regard to ptype. Given how mesoscale in nature that can be...confidence is quite low in these temps. I leaned again towards the 20.12z ECMWF mid level temps...which are cooler than the GFS...but still do warm well above 0C from the foothills and S. This brings into play all winter hazards...rain...snow...sleet...and freezing rain. Overall I feel the high is in a strong position and deep...so colder solutions are most likely. This could still mean a cold rain however. But in general...snow at onset transitioning quickly to rain near the coast. Interior will hold onto low level cold the longest...with accumulating snow followed by sleet and freezing rain. The best chance to remain all or mostly snow will be the Wrn ME mtns. && .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/... Short Term...Persistent MVFR cloud layer will remain in place for much of the night to the south and east of the mountains. To the north, it should be fairly clear. By morning a dissipating wave will bring more clouds and an outside chance of some light freezing rain to southern and western New Hampshire. Generally expecting MVFR to possibly IFR conditions as this moves through during the day Saturday, followed by VFR conditions again Saturday night. Long Term...VFR generally expected thru Sun. A cold front dropping Swd may bring widely scattered SHSN along it...but confidence is low on coverage and timing. Ely onshore flow Mon may bring MVFR CIGs into Srn and Wrn areas...especially MHT...PSM...CON...and LEB. Conditions rapidly deteriorate Mon night into Tue. IFR and LIFR will be widespread...except for maybe HIE where downsloping winds keep CIGs higher. Interior terminals will see SN changing to FZRA/PL...and maybe RA...while coastal terminals will see brief SN changing to RA. Winds may also be gusty near the coast...with sfc gusts in excess of 35 kts. IFR or lower conditions linger into Wed. && .MARINE... Short Term...Light winds expected over the waters as high pressure moves by. A cold front will drop south into the Gulf of Maine Long Term...A strong Ely flow will develop ahead of approaching low pressure early next week. Winds will increase from SCA thru gale force on Mon...with the best window for storm force wind gusts on the outer waters looking to be Mon night. I have higher confidence on wind gusts than the timing of them...so for that reason no headlines have been issued. But gusts to 50 kts are in the forecast...and the HWO will highlight the storm force gust threat. && .TIDES/COASTAL FLOODING... Despite being at the low part of our tide cycle...large building waves may product some erosion and splash-over near the time of high tide Monday night and Tuesday. Preliminary storm surge values appear to be around 1.5 to 2 feet with a persistent...strong easterly wind developing. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$

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