textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for the evening update. Just blended in the latest observations of the sensible weather fields to the going forecast. Also freshened up the aviation forecast.

KEY MESSAGES

1. High pressure remains in control of the weather pattern into Friday.

2. A strong coastal storm will track near southern New England to end the week and into this weekend with soaking rain, gusty winds, and high surf increasingly likely. Minor coastal flooding and splashover are also possible through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High pressure peaks across northern New England through the day on Thursday, keeping a seasonably cool and dry airmass in place. Another radiational cooling night is expected tonight, with a frost advisory issued for more interior zones and into the MidCoast tonight. Overall tonight looks slightly cooler than last night through the sheltered valleys, while a breeze keeps hilltops warmer. South of these areas along the coastal plain, a northeasterly breeze overnight keeps any frost isolated and patchy. During the daytime, mainly sunny skies with highs in the 60s continue.

While the high brings cool and fair weather to northern New England, a developing storm system off the Carolinas continues to organize to our south. The surf continues to build over the next few days as a long easterly fetch strengthens across the North Atlantic between the high overhead and the developing storm system. These waves bring increasingly large breaking surf and rip currents through the end of the week.

KEY MESSAGE 2 DESCRIPTION...

While there are still details to iron out, models have overall come into better agreement in the strong coastal low lifting northward across the western Atlantic on Friday before slowing down and lingering near southern New England through the weekend. Given the decreasing spread in the guidance, confidence is increasing that this system will likely bring high surf, gusty winds, and a soaking/beneficial rain. With astronomical tides on the increase, storm surge may also push water levels close to minor flood stage near the times of high tide along with splash over.

Confidence remains highest in surf becoming dangerous by Friday as incoming swells and a long fetch of northeast winds continue to build seas...with large breaking waves and a high risk of rip currents along the coast. These conditions will likely last through the weekend, potentially requiring a High Surf Advisory.

The pressure gradient will continue to tighten between the incoming low and high pressure to the north and west. So regardless of the low track, brisk north to northeast winds are likely, most notably along the coast. These should start become more noticeable during the day Friday, but most guidance has the period of the stronger winds spanning from Friday night through Sunday, although the ECMWF is a little later as it has slowed lifting the low north. Based on latest guidance, 30-40 mph gusts along the coast look reasonable, and these could exceed 40 mph at times, especially from Cape Elizabeth southward. This could carry a limited power outage threat with leaves still on trees. Unless the low tracks farther north, winds across the interior should be more in the 20-30 mph range.

A soaking and beneficial rain is looking more likely as the latest NBM PoPs bring 60-70% south of the mountains and 70-80% along the coast and parts of SE NH. As far as timing, Friday looks to stay mostly dry, but rain chances will increase from south to north Friday night through Saturday. As far as amounts, ensemble means from the ECMWF/GFS (including the ECMWF AIFS/AIGEFS) favor a widespread 1-2" south of the mountains with increasing probabilities (60-80%) for 2+" along the coast from roughly Portland extending south to the Seacoast. Low-level lapse rates will be steepening, perhaps enough to allow a small amount of instability, and this could produce some convective elements and higher rain rates at times. As the previous shift mentioned, some enhancement is possible from the Seacoast extending into coastal Cumberland, with support from the ECMWF ensemble having 10-20% potential for 4 inches of rain on the high end

Lastly, astronomical tides will be trending higher this weekend, and models are advertising surge approaching 1.5 to 2 feet, which could push water levels close to flood stage along the coast around the times of high tide. Splashover is also possible.

Improving conditions are possible later in the day Monday as the low begins to pull away to the east, but the drier conditions are more likely by Tuesday of next week, which look to continue into Wednesday.

AVIATION /00Z THURSDAY THROUGH MONDAY/

VFR prevails through Friday morning, with nighttime valley fog likely at LEB and HIE tonight and Thursday night.

Outlook:

Friday: Mainly VFR with northeast wind gusts 20-25 kt possible, especially near the coast.

Friday night-Monday: MVFR ceilings likely with IFR possible, especially along the coast. Periods of rain and drizzle will also bring potential for MVFR to IFR visibilities. There is potential for north to northeast wind gusts of 30-35 kt for coastal sites and 20- 30 kt inland. Improvement is possible later in the day Monday.

MARINE

An easterly swell continues to build over the next few days while high pressure remains over the waters. SCA conditions continue to spread north through Thursday, and continue to build Thursday night while a storm organizes off the Carolinas.

Friday-Wednesday...A strong coastal low pressure is expected to approach southern New England late this week and linger through the weekend before potentially lifting northeast across the Gulf of Maine early next week. For this weekend, northeasterly gales are increasingly likely over the nearshore and outer waters with some potential in the Bays as well. A few gusts to Storm force can't be discounted over the outer waters as well. Improvement is possible on Monday but more likely Tuesday-Wednesday as the low pressure pulls away to the east, allowing winds and seas to subside.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Frost Advisory from midnight tonight to 7 AM EDT Thursday for MEZ012>014-022-028-033. NH...Frost Advisory from midnight tonight to 7 AM EDT Thursday for NHZ003>006. MARINE...Small Craft Advisory from 2 PM Thursday to 8 AM EDT Friday for ANZ150-152. Gale Watch from Friday afternoon through Sunday evening for ANZ150-152. Small Craft Advisory from 8 PM Thursday to 8 AM EDT Friday for ANZ153. Small Craft Advisory until 8 AM EDT Friday for ANZ154. Gale Watch from Friday morning through Sunday evening for ANZ154-180-182-184.