Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 220719 AFDGYX Area Forecast Discussion National Weather Service Gray ME 319 AM EDT Sun Oct 22 2017 .SYNOPSIS...
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High pressure will gradually shift off to the east today. A slow moving cold front will approach from the west Monday night and Tuesday and will very slowly move across the region through Thursday delivering beneficial rainfall. High pressure will build in from the southwest on Friday and provide fair weather over the weekend.
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&& .NEAR TERM /TODAY/...
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High pressure slowly slinks off to the east today with a light southeast flow developing over coastal areas today. There is a pool of greater moisture which has developed over the Gulf of Maine with dewpoints in the 50s noted. This will be pulled inland today on the southeast flow. Temperatures today will be similar to yesterday, generally in the 70s. The biggest question on temperatures will be how much of an influence the southeast flow has on temperatures over the coastal plain of Maine. The MET guidance suggests high temperatures around 60 due to the southeast flow and some cloud cover. But this seems a bit too drastic. At any rate, the greatest cooling influence will be felt right along the coastline. It may be a bit too early to expect marine cloud cover to develop which would be necessary to keep temperatures that cool.
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&& .SHORT TERM /TONIGHT AND MONDAY/...
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As noted before, a pool of moisture has developed over the Gulf of Maine and will remain in place there and across the coastal plain of Maine and New Hampshire tonight. This raises suspicions of low cloud development as significant moisture over the cool waters often leads to low clouds and fog. However, the moisture seems quite limited to the lowest levels right near the surface. Just above the surface at 925 MB it is much drier which suggests it may be difficult to get a marine stratus cloud layer to form. Instead it is more likely that radiational cooling will cause fog formation over both the water and the land areas, with valley locations the most favored. However, with the rapid drying just above the surface it is possible that a good deal of this moisture gets deposited as dew as temperatures cool tonight with fog possibly limited to patch ground fog. Temperatures tonight likely stay in the 40s for most of the area, but the inland cool spots could cool into the 30s in spite of the moist start to the night. Temperatures Monday will again be warm, but perhaps not quite as warm, only in the upper 60s and low 70s. With time, the moist air mass in place will likely lead to low clouds developing over the ocean and moving into the coastal plain, but this may hold off until Monday evening.
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&& .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
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The long term period starts out with a marked pattern change where deep troughing approaches from the west, likely giving us some much needed rainfall. Not too much has changed in the overall picture here - trough moves in Tuesday with heaviest rain likely Tuesday night through Wednesday night. However, there are still big questions about how amplified the trough gets and therefore how long the forecast area remains in a strong southeasterly inflow regime. This will determine how much rain we eventually get. Either way, the prefrontal air mass will be characterized by anomalously high PWATS streaming in on a strong S/SE low level jet. We`ll get our rain - it`s just a matter of how much. 00z deterministic model runs are in agreement that 1-3" of rain will be common at a minimum. However, there are some - including the GFS and ECMWF - that bring a widespread 3-6" with locally higher amounts - especially in favorable upslope areas of the Whites and western ME mountains. While it has been very dry lately, some of these higher end QPF outputs would likely pose a flooding risk for smaller steams and other poor drainage areas. This outcome wouldn`t be all that surprising given the prolonged southeasterly inflow with high PWATS. Again, this will depend on how amplified the approaching trough gets. In the dailies, we think Monday night will be the night that low clouds come in and precip chances increase due to increasing moist southerly inflow. Precipitation shouldn`t be heavy though. This general idea continues into Tuesday - PoPs increasing during the day but heavy rain holds off. Forcing for ascent really increases Tuesday night and Wednesday as the cold front and dynamics with upper trough moves in. This is when we should see rain, heavy at times, along with gusty winds. Speaking of winds, the southerly low level jet will be strongest Tuesday night and Wednesday. While there will be some degree of a low level inversion, gusts up to 40 MPH cannot be ruled out - especially in convective showers. The weather Wednesday night and Thursday will depend on how amplified the upper trough gets. Heavy rain could continue Wednesday night into Thursday if the trof takes its time moving east. We`ll be able to iron this out with time, but again, there could be a substantial amount of rain by the time the system departs. It`s much needed. But there is a low probability that it could be too much at once for small streams. Fair weather finally moves in for Friday and the weekend.
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&& .AVIATION /07Z SUNDAY THROUGH THURSDAY/...
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Short Term...VFR conditions today with light winds. Could see some ground fog developing tonight, with fog most likely in the valley locations. VFR again on Monday but there will likely be a developing layer of maritime stratus just offshore waiting to move in. It seems likely this will hold off until Monday evening. Long Term...IFR/LIFR ceilings Monday night through Wednesday. MVFR ceilings Thursday.
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&& .MARINE...
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Short Term...High pressure slowly moves east with south or southeasterly flow gradually increasing with time. Long Term...SCA`s may be needed Monday night. Gales possible on Tuesday into Wednesday.
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&& .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Kimble SHORT TERM...Kimble LONG TERM...Ekster

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