textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes needed with this early morning update. A chilly morning in the foothills has panned out with Fryeburg bottoming out at the freezing mark. The Frost Advisory will be allowed to expire on time at 7 AM. The Aviation Discussion was also updated for the 12Z TAFs.

KEY MESSAGES

1. Strong high pressure remains in place through Thursday. Areas of frost will be possible in the foothills during the overnights.

2. A strong coastal storm will track near southern New England to end the week. Soaking rain, gusty winds, high surf, and coastal flooding are possible through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A seasonably strong high pressure is forecast to remain in place across southern Quebec. This will be the case thru at least Thu. High pressure is forecast to continue strengthening and building south thru sunrise, with mid level temps warming slightly by Thu as the high starts to weaken. So overall I think this morning has the potential to be the coldest, but not by much. Persistence for low temps again tonight may be the best way to go, or at least a heavy blend of observed lows with the latest guidance.

As a result some areas of frost will be possible into the foothills and localized in some of the traditionally cooler locations. Frost Advisory remains in effect thru sunrise. Valley fog will also be possible each mostly clear night where we start to radiate.

KEY MESSAGE 2 DESCRIPTION...

By Fri a coastal storm will be organizing and deepening as it slowly moves north to a position in the vicinity of the 40N/70W benchmark. All guidance forecasts a stall or loop south of southern New England before eventually tracking east. This will bring a prolonged period of potential impacts to the local forecast area. I will try and tackle the impacts in the order of confidence below:

High surf...A long duration northeast wind with a long fetch will allow seas to build thru the first half of the weekend at least. Seas building to 10 ft looks likely, but significant wave heights of 15 to 20 ft are not out of the question if the closer/stronger tracks are realized. Despite the poor weather if that occurs, a High Surf Advisory may still be needed to highlight the dangers of large, nearshore breaking waves.

Winds...The strong pressure gradient forecast to set up between the coastal storm and retreating high pressure. Northeast winds also tend to keep a fairly well mixed boundary layer thanks to cold advection. So I anticipate it will be gusty regardless of storm track. Even the weaker guidance at this time is forecasting 24 to 48 hours of 30 to 35 mph gusts along the coast, especially from Casco Bay southward. As the low level cold conveyor belt reaches its strongest Sat into Sun, gusts could be even higher than that if the storm really tucks in towards Cape Cod. I am not ruling out the necessity for a Wind Advisory for parts of the area.

Coastal Flooding...With sustained northeast winds and building seas over several days we are primed to maximize the potential storm surge by the weekend. Thankfully astronomical tides are not peaking until next week, but they will be increasing thru the weekend. Some guidance is already indicating that surge could be on the order of 2 ft, which would place us near flood stage especially Sat night and Sun midday. Something to keep a close eye on.

Rainfall...This will be most highly dependent on storm track. The farther west the center loops and the longer it meanders off Cape Cod the better chance we have at seeing widespread soaking rains. The ECMWF Ens is much more bullish on the west/wetter storm tracks, with the GEFS showing plenty of misses in the mix still. However both ensemble systems have chance of greater than 2 inches of total QPF between 40 and 60 percent from Portland south thru the weekend. The only thing holding me back from going with categorical PoP is the uncertainty in timing, as rain may be most likely anywhere between Sat and Mon depending on which model system or ensemble member you look at. QPF will drop off rather quickly to the northwest of the storm, so much of the mtns northward are still looking relatively dry thru the event. Given the relative strength of the storm system and the lingering warm season moisture/SST profile I am wary that dynamics could squeeze out some significant rain even locally if the track plays out just right. So while right now the forecast is for a manageable soaking rain, I have my eye on the coastal areas south of Cape Elizabeth to the Mass border with a reasonable worst case scenario (~10 percent) approaching 4 inches.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Through 12Z Thursday...VFR conditions will prevail again today with steady northeast winds gusting 15-20 kts. Then we rinse and repeat for tonight. Valley fog will be possible again at LEB, but have left it out of HIE since they did not develop any last night. All other terminals should remain VFR into Thursday morning.

Outlook...

Thursday: VFR conditions expected with gusty northeasterly winds 15-20 kts once again.

Friday: VFR conditions expected. By midday northeast surface gusts of 25 kt will be possible outside of the mtns.

Friday night-Sunday night: Widespread MVFR is possible with areas of IFR developing near the coast in rainfall. Northeast surface gusts in excess of 30 kt are possible Sat into Sun outside of the mtns.

Monday: Areas of MVFR may linger with rain possible. Northeast surface gusts of 25 kt possible near the coast.

Tuesday: VFR conditions expected.

MARINE

Developing and slow moving coastal storm will drive marine weather for the foreseeable future. Easterly swell will continue to build thru the end of the work week. SCA conditions are expected to develop by the middle of the day today. This will gradually expand northward thru Thu. Wind will continue increasing as well, with northeast gales looking increasingly likely for at least the coastal and outer waters, if not all waters, by Fri. Seas will continue to build in excess of 10 ft by Sat, but 15 to 20 ft remains possible. Significant rainfall, especially in the vicinity of Jeffrey's Ledge, will reduce visibility on the waters thru at least Sun. Winds and seas will be slow to diminish into early next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Frost Advisory until 7 AM EDT this morning for MEZ012-033. NH...Frost Advisory until 7 AM EDT this morning for NHZ003-004. MARINE...Small Craft Advisory from 2 PM Thursday to 8 AM EDT Friday for ANZ150-152. Small Craft Advisory from 8 PM Thursday to 8 AM EDT Friday for ANZ153. Small Craft Advisory from 11 AM this morning to 8 AM EDT Friday for ANZ154.