textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. Cool Canadian high pressure keeps weather mostly dry through the middle part of the work week. Some patchy frost overnight may be possible into the foothills.

2. A coastal storm will develop off southern New England late this week and track towards the Gulf of Maine by the weekend. Soaking rain, gusty winds, and high surf all remain possible.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Strong high pressure is forecast to remain anchored over southern Quebec thru midweek. Ensemble guidance is forecasting sea level pressure on the order of once in a decade or more in and around the St. Lawrence Seaway. This will keep the weather locally dry, with a breezy northeast wind. It will also help to keep mixing shallow and temps on the cool side for the latter half of September. Overnight I anticipate that we will get some good radiational cooling and the usual valley fog that accompanies it. We will have to be mindful of frost however, as the growing season remains active in the foothills.

KEY MESSAGE 2 DESCRIPTION...

Potentially greater weather impacts are possible from a storm developing off the Mid Atlantic coast which is forecast to track northeast off the New England coast by the weekend.

Now deterministic models have two very distinct camps, the GFS out to sea and the ECMWF with an early season nor'easter hit. However the ensemble guidance is rather on the same page. Both the GEFS and ECMWF Ens have hits and misses at roughly equal rates. Eyeballing it as slightly more than 50 percent hits for coastal precip. NBM guidance is sitting at low likely PoP for the coast and parts of southern NH which seems entirely reasonable for this range of the forecast. Some of the higher end ECMWF Ens members (top 10 percent), as evidenced by the EFI tool, do show that the QPF and wind gusts could be significant for this time of year. Also looking at ensemble sensitivity and they point towards heights over the upper Midwest into the Great Lakes over the next few days as being critical. Already 22.00z raobs show heights are higher than the previous GFS forecast, so I would not be surprised to see the 22.00z GFS run tick closer to the coast and closer the ECMWF solution.

The storm could bring a myriad of hazards though. Heavy rain is a possibility with the right track. Northeast winds this time of year already tend to be gusty, with cold advection over warm ocean waters, but the pressure gradient will be strong regardless of storm track. A low end wind advisory is not out of the question for coastal areas, and a reasonable worst case scenario (10 percent chance) could be a strong wind advisory and further inland. This will also help to build seas on the coastal waters in excess of 10 ft. While the weather will not be stellar, it is still high surf season and a high surf advisory (including a higher risk for rip currents) may be necessary for the weekend as well. With persistent NE flow and long wind fetch across the Gulf of Maine, water levels should build. This means that surge may need to be factored into a period encompassing the highest tides of the month.

AVIATION /10Z TUESDAY THROUGH SATURDAY/

Through 12z Wednesday: VFR today with NNE winds and gusts generally less than 20 kts. These subside this evening, with most locations becoming VRB. Mostly clear skies tonight could promote valley fog and local LIFR conditions tonight.

Outlook:

Wednesday-Thursday: VFR conditions expected during the day. Valley fog and local LIFR conditions possible at night.

Friday-Friday Night: Becoming cloudy and showers possible. Patchy MVFR conditions possible, especially near the coast.

Saturday - Saturday Night: Best chance for rain and gusty northeast winds. Areas of MVFR or lower possible.

MARINE

Winds stay northeast for much of the rest of the week. They will be breezy but generally below 25 kt thru tonight. Some 25 kt gusts are possible late Wed well outside the bays. By Fri into Sat gale force wind gusts are possible for the coastal waters and likely for the outer waters. As coastal storm slowly churns off New England seas will build in excess of 10 ft on the coastal waters and only slowly diminish thru the weekend.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.