textproduct: Gray - Portland

WHAT HAS CHANGED

A High Surf Advisory has been issued from Friday through the weekend to highlight the danger of large breaking waves and high risk for rip currents.

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. Soaking rain, gusty winds, high surf, and coastal flooding are possible through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

We will remain under the influence of cool high pressure at the surface through at least early on Friday. The airmass overhead is very dry as sampled by the 00z GYX upper air sounding. The sampled PWATs were at 0.31 inches which is close to the minimum for this time of year. Thus, with clear skies and light winds, conditions will be favorable for radiational cooling and frost formation in the sheltered valleys this morning. Winds will be a bit breezier on the hilltops and areas along and south of the coastal plain which may limit the frost potential here. Lows are forecast to mainly be in the 40s but some mid to upper 30s are likely in the northern valleys and portions of the interior. Frost Advisory remains in effect for portions of the Interior and Midcoast until 11z.

While the high brings cool and fair weather to northern New England, a developing storm system off the Carolina coast 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...

It would not be a coastal party without some uncertainty bringing confidence down in the medium range. While all NWP systems agree on a strong early season coastal storm developing off the Mid Atlantic and drifting north before doing a loop, what they do not agree on is where that stall and capture takes place. For instance the deterministic GFS favors Cape Cod while the ECMWF tucks into NYC. Even their respective ensemble systems have little overlap between low locations. The ensembles are fairly agnostic on strength however, all featuring similar spread. So given how similar the different model suites with the upper air pattern, that leads to somewhat lower confidence on the outcomes locally. A farther northeast storm will bring greater impacts vs. one hanging off the Jersey Shore. Much like yesterday the impacts in order of confidence are as follows:

High surf...It will be several days of northeast winds along the longest possible fetch. Seas will build thru Sat before the weakening allows waves to diminish slowly thru the beginning of next week. Seas building to around 10 ft remains likely, but significant wave heights up to 15 ft are not out of the question if the closer/stronger tracks are realized. This will especially be true towards Jeffrey's Ledge where the fetch will be the longest.

Winds...This may be a 2/2A with coastal flooding below. A strong pressure gradient forecast to set up between the coastal storm and retreating high pressure. Both pressure systems are anomalous for this time of year. Northeast winds also tend to keep a fairly well mixed boundary layer thanks to cold advection. As a result those winds should be fairly consistently gusty beginning as early as Fri. It could be a greater than 48 hour period with wind gusts frequently in excess of 35 mph along the coast. The greatest area of concern will be Casco Bay south to the Seacoast. This is where strongest winds aloft are forecast as well as the greatest onshore component of those winds. The more extreme wind forecasts approaching 60 mph are becoming more unlikely. However a low end Wind Advisory remains possible, especially with leaves still on the trees for now. The window for the best possible mixing and gusts pushing 45 mph will be during the closest approach Sun.

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. Because the astronomical tides are not super high there is lower confidence that we reach flood stage, but the combination of high water and pounding surf should lead to significant splashover at the very least. The Stevens Inst Flood Advisory System is below flood stage at Portland at this time Sat into Sun, but surge forecasts also remain less than 2 ft. Given the very favorable multi-cycle set up I tend to think we will come in closer to that number than 1 ft. This should be enough for water levels to get right up to flood stage.

Rainfall...This remains the lowest confidence impact of the bunch because it will be so dependent on storm track and there is such a wide range of possibilities at this point in time. The forecast stall and capture, with a looping surface low, is a favorable set up for fairly widespread soaking rains, even the farther southwest ECMWF cases. The late warm season moisture content and strong synoptic forcing does mean that some significant rainfall remains possible, and the ECMWF Ens especially continues to put a 20 to 30 percent chance of 4 inches or more rainfall across parts of southwestern Maine and the Seacoast. However when you drill down to 24 hour QPF, the probabilities of more than 2 inches in that period is 10 percent or less. So this really looks like a long duration light to moderate rain. I would like to see the CAM guidance as the event gets closer, as there could definitely be convective elements that train and lead to some locally higher rainfall amounts. Now that models have settled into some agreement on timing, the NBM bumping up to categorical PoP and lingering that thru Mon morning seems reasonable from a messaging standpoint.

AVIATION /06Z THURSDAY THROUGH TUESDAY/

VFR prevails through Friday morning, with valley fog likely at LEB and HIE this morning and possibly again tonight.

Outlook:

Friday: Areas of MVFR developing as rain moves towards the area. Northeast surface gusts of 25 kt or more possible near the coast.

Saturday-Sunday: Widespread MVFR likely, areas of IFR or lower possible near the coast in rain. Coastal terminals may see northeast surface gusts of 30 kt or more.

Monday: Areas of MVFR or lower during the first half of the day as rain ends. North northeast surface gusts of 25 kt or more possible.

Tuesday: VFR conditions expected.

MARINE

SCA conditions continue to spread north today, and continue to build tonight as a storm organizes off the Carolina coast.

Steadily increasing northeast winds are expected thru the weekend. Building seas in excess of 10 ft are likely by Fri afternoon. Early Sat seas may reach at least 15 ft in the vicinity of Jeffrey's Ledge. Northeast gales may develop as soon as Fri, and storm conditions are not out of the question if the storm tracks closer to the Gulf of Maine. Moderate to locally heavy rainfall over the waters will also reduce visibility to below 3 miles at times thru Mon morning. Winds and seas will be slow to diminish by early next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Frost Advisory until 7 AM EDT this morning for MEZ012>014-022- 028-033. High Surf Advisory from 8 AM Friday to 8 PM EDT Sunday for MEZ023>027. NH...Frost Advisory until 7 AM EDT this morning for NHZ003>006. High Surf Advisory from 8 AM Friday to 8 PM EDT Sunday for NHZ014. MARINE...Small Craft Advisory from 2 PM this afternoon to 8 AM EDT Friday for ANZ150-152. Gale Warning from 2 AM Saturday to 8 PM EDT Sunday for ANZ150- 152. Small Craft Advisory from 2 AM Saturday to 8 PM EDT Sunday for ANZ151-153. Small Craft Advisory from 8 PM this evening to 8 AM EDT Friday for ANZ153. Small Craft Advisory until 8 AM EDT Friday for ANZ154. Gale Warning from 8 PM Friday to 8 PM EDT Sunday for ANZ154. Gale Warning from 11 AM Friday to 8 PM EDT Sunday for ANZ180- 182-184.