Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 232302 AFDGYX Area Forecast Discussion National Weather Service Gray ME 702 PM EDT Fri Jun 23 2017 .SYNOPSIS... Tropical moisture from the remnants of Tropical Storm Cindy... and an approaching frontal system will result in showers and thunderstorms overnight. A series of surface troughs and weak cold fronts will cross the area this weekend into early next week bringing us the chance of showers from time to time...especially in central and northern areas. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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7 PM Update...Have updated the forecast a bit, mainly for PoPs over the next several hours based on latest trends in radar and mesoscale model data. For the most part, have relegated PoPs to the far west and north for the rest of the evening as points east remain relatively stable and lack forcing for ascent. Chances will once again increase after midnight. At this time it appears the severe potential is done, but heavy rain later tonight will have to be monitored. Previously... High Impact Weather Potential: Isolated severe thunderstorms possible through this evening. Localized flooding also possible. Very minor coastal flood issues at the time of high tide tonight. Current Pattern: Early afternoon water vapor imagery reveals deep...most southwesterly flow entrenched over the northeastern United States with blended total precipitable water product showing a warm sector featuring PWATS of 1.5 to 2 inches /+3 to 4 sigma/ having overspread all of the northeast except far northern Maine. First band of shower activity along the surface warm front has pushed north with southern portions of the GYX forecast area having broken into the warm sector with temperatures over southern areas racing through the 80s /and even lower 90s/ with dewpoints rising towards 70F. Surface cold front is gradually pushing into the eastern Great Lakes region with areas to the east of this lacking much in the way of organized forcing. The result has been isolated/scattered clusters of showers and thunderstorms given modest isentropic ascent of the warm/moist airmass in the warm sector. There are a few forecast challenges through the near term period. First...is whether we can see any significant convection through the evening. Then...the focus turns to Cindy/s remnants that will stream north and east along the approaching cold front...with at least some potential for a period of heavy rain later tonight. Through this evening...Regional radar mosaic continues to paint a band of showers along the international border close to the low level warm front. South of this...activity is much more isolated...but feel that as the cold front begins to approach from the west...showers/storms will become more numerous...particularly over western areas /CT Valley/. SPC mesoanalysis shows a blossoming area of 1000 J/kg of MLCAPE which...along with the aforementioned good PWATS should favor precip loading and the potential for gusty winds...with a few severe storms not out of the question. Otherwise...a steamy late afternoon as temps over northern areas continue to climb. Tonight...Expect scattered to numerous showers/thunderstorms over western areas to gradually progress east...but with the thunder aspect dwindling some as instability diminishes. High resolution guidance /HRRR and the High Res window WRF runs/ all keying in on taking Cindy moisture /over the Ohio Valley/ and having it make a run at at least the southern half of the forecast area well after midnight and towards daybreak Saturday. PWATS will increase to well over 200% of normal...with warm rain processes likely becoming dominant. Thus...see some potential for heavy rain after midnight as this band of showers /and some embedded thunder/ pushes in. Expect marine stratus/fog to continue to expand inland and west as the night goes on. Temperatures will be very mild ahead of the front...likely remaining in the 70s over southern NH and upper 60s to around 70 elsewhere. See tides/coastal flooding section for more details on high tides tonight.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... High Impact Weather Potential: Locally heavy rain through the mid morning hours over coastal areas. There is also a moderate risk of rip currents along the coastline on Saturday. Saturday: 12Z Guidance trend has favored a slower arrival of the surface cold front with the front bisecting the forecast area at 12Z Saturday. South and east of this feature...showers /with a few embedded rumbles/ will likely continue through at least mid morning before the shortwave associated with Cindy/s remnants shifts east. Will have to keep an eye on heavy rain potential...but confidence in any significant issues is not high given that it looks to be a pretty quick hit...and due to poor run to run model continuity. The story of the afternoon will be robust dry advection which should take morning dewpoints that will be near 70 over southern/coastal areas back into the upper 50s by late in the day. T8s remain +12-13C through the afternoon...which will allow temperatures to reach well into the 80s south and east of the mountains...with mid/upper 70s to the north. Saturday Night: Cold advection continues under northwesterly flow aloft. Secondary cold front nears the international border during the evening...and may very well see some evening shower activity along and ahead of this feature. With a digging shortwave over the Great Lakes region...however...expect heights to actually begin to rise after midnight. This...combined with the llevel drying should slowly bring mountain shower activity to an end /as the front stalls to our north. Expect temperatures able to fall back into the lower 50s in the mountains /decent gradient aloft should preclude all but patchy fog/ with upper 50s to just above 60 to the south and east. && .LONG TERM /SUNDAY THROUGH FRIDAY/... An approaching upper level trough will trigger scattered showers on Sunday. A few locations over central and northern areas may be sufficiently destabilized to trigger and thunderstorm or two, mainly during the afternoon and early evening hours. A broad long wave trough will remain over the region on Monday, allowing for more clouds and the chance for a shower across the region, again mainly during the afternoon and early evening hours. Lapse rates will not be overly steep during the day, which may minimize the threat for thunderstorms over the region. On Tuesday, the long wave trough progresses eastward, allowing for more in the way of showers and thunderstorms across the region. The wind flow will shift to the west on Wednesday with drier air entering the region, allowing for some clearing over downslope regions. Upslope areas likely to have yet another shower, mainly diurnally driven. Towards the end of next week, the 12Z model suite diverges somewhat with low pressure passing through southern New England, central New England or to the north of New England depending on the model of choice. This will have large ramifications on temperatures and weather conditions late in the week next week. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Short Term... Summary: A cold front will move through the region tonight with scattered showers and thunderstorms...becoming more numerous after midnight. Drier air moves into the region on Saturday and continues through Saturday night. Restrictions: VFR conditions should dominate into this evening all sites except KRKD where marine stratus will bring a prolonged period of IFR cigs. Approaching cold front and very moist low level airmass will allow for restrictions to expand...with MVFR/IFR conditions likely after midnight as lingering Cindy moisture streams up ahead of approaching cold front. Expect improvement to VFR on Saturday as cold front moves south of the region. VFR expected Saturday night. Winds: Southerly winds strengthen to 10g15kts for this afternoon...remaining near 10kts tonight before shifting west northwest 12g18kts for the day on Saturday and diminishing to less than 10kts Saturday night. LLWS: 1kft winds to 35kts from the south tonight at RKD above stable marine layer...causing a period of LLWS. Thunder: Can/t completely rule out a stray thunderstorm at any of the terminals through this evening...although the best chances will be at HIE/LEB. Tonight...a few thunderstorms may be embedded with showers that move through the region associated with the cold front. Long Term...A dry west to northwest flow out of Canada will keep conditions VFR late this weekend and much of next week. There will be a chance of afternoon showers and possibly isolated thunderstorms, with the best chance being across the interior. && .MARINE... Short Term...SCAs continue outside the bays for southerly winds ahead of an approaching cold front. Winds will shift westerly and diminish some on Saturday with waves gradually subsiding. Winds/waves continue to subside on Saturday night. Long Term...Winds will be fairly light through the coming week in the absence of any strong pressure systems, with the daily flow being dominated by the sea breeze. && .TIDES/COASTAL FLOODING... Astronomical high tides will continue through the weekend with the possibility of minor flooding particularly with the tide tonight which is 11:11 pm at Portland. The astronomical high tide will be 11.8 feet with a storm tide overnight likely coming in at around 12.2 feet. With little in the way of wave action, expect only very minor issues along the coastline. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory until noon EDT Saturday for ANZ150-152- 154. && $$ NEAR TERM UPDATE...Ekster SHORT TERM...Arnott LONG TERM...Cannon AVIATION...Arnott

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