Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 271333 AFDGYX Area Forecast Discussion National Weather Service Gray ME 933 AM EDT Tue Jun 27 2017 .SYNOPSIS... A cold front will approach from the west today and will likely aid in the development of showers and thunderstorms, a few of which may be strong to severe this afternoon. The front will eventually move off the coast on Wednesday and will bring a few showers. A warm front will approach on Thursday and bring increasing chances of showers and thunderstorms during the afternoon and nighttime hours. Another low pressure system may affect the region on Saturday with more showers and thunderstorms. && .NEAR TERM /THROUGH TONIGHT/...
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Update... Products: Main product suite has been updated to include the expansion of the marginal risk area into far western Maine this afternoon and evening. Namely, the HWO, DSS slide, Social Media have been updated with updates to the grids and texts products. An email activation has been sent to HAMS operators. Details below. Have updated the grids based on current conditions and latest set of mesoscale models. Monitoring the threat for a few isolated storms containing large hail today. Instability is not overly impressive as plenty of cloud cover remains over much of the forecast area. However, low freezing levels will lower even further aloft this afternoon as a vigorous short waves passes north and west of the region. This will steepen lapse rates even further. SPC has expanded the marginal risk further to the north and east into western Maine which seams reasonable. Onshore winds may prevent strong storms along the coastline. Am working up sounding information data for Portsmouth today in the event a couple storms do make it into the marine layer. Otherwise, very little change to the main focus of the forecast. Minor adjustments made to cloud cover, temperature and dew point grids this morning. Prev Disc... A few showers are moving into New Hampshire early this morning associated with a shortwave trough aloft. These are dying showers, remnants of thunderstorms yesterday evening, and should not cause much fanfare as they drift across the area. The bigger story will be the next round arriving in the afternoon. Potent shortwave trough crosses the region this afternoon, adding synoptic scale lift to aid in the generation of afternoon showers and thunderstorms. Although temperatures will be cool (in the 70s) and dewpoints quite dry for the time of year (low 50s), instability today will be driven primarily from cold air aloft associated with the upper trough. As such, the best instability will exist further to the north and west beneath the coldest air aloft, while the best shear will be to the south and east beneath the core of the upper jet. There does look to be a sweet spot between the two which will exist over our area this afternoon, leading to the threat for some stronger, more organized thunderstorms. This may include a few rotating supercells as well as clusters of storms. Given the cold temperatures aloft, expect hail to be the primary threat from the storms today, but there may be a wind threat as well especially with any storm clusters which can combine downdrafts. Convective initiation will be most likely over Vermont and New Hampshire where heating of the terrain beneath the upper cold pool will occur near the time of the shortwave trough`s arrival. Further to the east, an onshore southerly flow across Maine will limit instability from Portland to Augusta and Bangor, but some remnant storms may move through these areas during the evening as they push eastward and weaken.
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&& .SHORT TERM /WEDNESDAY/... Expect an area of thunderstorms to be ongoing this evening, possible organized into mesoscale convective clusters. These will move east across the forecast area and diminish in intensity as the evening progresses. Overnight temperatures will drop into the 50s for most of the area. Core of the upper cold pool moves across the area early on Wednesday, and should provide enough instability for another round of showers and possible thunderstorms. These will be most likely across the north and east where the best forcing and cold air aloft will exist during the period of best heating. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Deterministic models and ensembles remain in good agreement in the long term period with two distinct rain chances centered around late Thursday/Thursday night and Saturday. The morning hours on Thursday should turn out OK, but an approaching short wave trough and SFC warm front will allow for rapidly increasing chances for rain Thursday afternoon. Elevated instability will increase Thursday night which will add to a thunder threat along with the rain. That system moves off to the east on Friday, even though there will still be a chance for a few showers and thunderstorms. The next short wave trough approaches Saturday. With a moist and unstable air mass in place, showers and thunderstorms appear likely, mainly during the afternoon hours and mainly in the mountains. We`ll have to watch for further short wave amplification Sunday which would prolong the chance of showers and thunderstorms. At this time, Monday looks to be fair with a cooler and drier air mass in place. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... Short Term...Expect VFR conditions to prevail today with a developing light southerly wind. Showers and thunderstorms are likely in the afternoon, forming in New Hampshire and progressing eastward through the evening hours. Any areas that get significant rainfall could have a potential for fog tonight. More showers are possible on Wednesday, mainly in northern areas. Long Term...Conditions deteriorate to MVFR or lower later Thursday afternoon and especially at night in low clouds, showers, and thunderstorms. VFR conditions are primarily expected Friday, but a few showers and thunderstorms may lower conditions at times, especially during the afternoon hours. More showers and thunderstorms are expected Saturday with associated locally lower conditions. && .MARINE... Short Term...Light southerly flow expected over the waters today, shifting to the west or southwest on Wednesday. Long Term...SCA conditions are likely Thursday night through Friday night. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ JC is the U.S. government's official web portal to all federal, state and local government web resources and services.