Area Forecast Discussion
Issued by NWS Portland, ME
Issued by NWS Portland, ME
049 FXUS61 KGYX 181810 AFDGYX Area Forecast Discussion National Weather Service Gray ME 210 PM EDT Mon Aug 18 2025 .SYNOPSIS... Cooler weather continues through the middle of this week with a weak trough triggering some showers Wednesday, followed by high pressure building in from the west through Saturday. Temperatures will warm back into the 80s by late week and to start off the upcoming weekend. && .NEAR TERM /THROUGH TONIGHT/... Impacts and Key Messages: * No significant weather impacts expected. Visible satellite this afternoon shows plenty of high clouds streaming in from an MCS over the Great Lakes Region. These shouldn`t effect tonight`s temperatures which are expected to plummet. Continued to blend in some MAV which handles radiational cooling well and this suggests temperatures bottom out in the low to mid 40s areawide, with some upper 30s in northern New Hampshire and the western Maine mountains. Cloud cover will eventually get thicker toward day break Tuesday, which may help keep temperatures from getting even lower and may also keep fog development minimal, but do expect at least some degree of valley fog. && .SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/... Impacts and Key Messages: * No significant weather impacts expected. High pressure remains centered over the Gulf of Maine Tuesday which will continue a cool dry flow. Skies look to end up more in the partly cloudy range as low pressure approaches from the Great Lakes Region. We will see continued drying at the surface as relative humidity values once again drop to around 40-45% during peak heating. Overall, it will be another fair weather day with high temperatures similar to today (Monday); low to mid 70s areawide. Will start to see cloud cover increase Tuesday night as low pressure moves closer to the region. This keeps temperatures from plummeting like we have seen the past few nights, but it will still be cool with temperatures in the low 50s south of the mountains, and mid- to upper 40s to the north. The 12Z NAMNest and 12Z HRRR suggest showers may be see as early as around sunrise Wednesday morning, but there is still significant uncertainty between them so kept chances low during that time. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... An upper-level shortwave trough crosses the area Wednesday, bringing an increase in cloud cover as well as a chance of showers. Global models as well as ensembles depict the highest probabilities of measurable rainfall being from the foothills northward with a tenth to a quarter of an inch of rain possible. Temperatures will be mostly in the 70s for highs. Worth mentioning: the NAM and NAMnest are at odds with the global models with being much more robust with moisture and QPF by developing a a sharp inverted trough near southern areas and strong isentropic lift. If this ends up being the case, temperatures would also be much cooler than the current forecast, but this seems more of an outlier scenario at this point. However, we`re getting into that 48-hour window where additional hi-res guidance should bring more confidence to the forecast. Once the Wednesday system passes, another period of mostly dry weather is expected from Thursday through Saturday with gradually warming temperatures, although the mountains may see some diurnal showers on Thursday. This dry weather is owed to an upper ridge steadily building toward New England as the latest forecast from NHC has Tropical Cyclone Erin passing well to the south and east of the area as well as any extra moisture that would potentially bring any additional rain chances. However, long- period swells from Erin are modeled to begin arriving from mid to late week, likely bringing high surf to the coast as well as high risk of dangerous rip currents. Highs go from the 70s Thursday to the 80s Friday and Saturday. Global models are in good agreement by shifting the high to the east of the area by Sunday as an upper low digs in north of the Great Lakes and sends a frontal boundary toward New England. Things can always change, but with at this point there is a pretty good signal from ensembles for this front to bring measurable rainfall to portions of the area Sunday or Monday, but amounts are uncertain. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... Short Term...Gusty winds subside tonight which will lead to the development of valley fog effecting LEB. Otherwise, VFR will prevail overnight into Tuesday. Ceilings look like they may thicken and lower toward MVFR Tuesday night as low pressure approaches from the west. Long Term...A low pressure system will bring increasing shower chances on Wednesday into Wednesday night and the potential for IFR to MVFR restrictions. However, confidence in details is low other than it looks like the NH terminals have the higher chances. After this system passes, primarily VFR Thurs-Sat other than the possibility of nighttime valley fog at HIE and LEB. && .MARINE... Short Term...Wind gusts and seas remain below SCA criteria through Tuesday night as high pressure remains centered over the waters. Winds shift northerly tonight, than back to southerly Tuesday afternoon, with gusts falling below 10kts. Long Term...Conditions remain below SCA levels Wednesday. Winds remain below criteria through Friday, but seas will build through late week as long-period swells from Tropical Cyclone Erin (passing well to the south and east of the waters) arrive over the waters Thursday and Friday, likely requiring a Small Craft Advisory. Seas will start to come down over the weekend but could still remain above SCA levels based on the current forecast. Southerly winds may also gust to around 25 kt as the pressure gradient tightens with high pressure to the east and an approaching frontal boundary from the west. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR/SHORT TERM...Baron LONG TERM...Combs