textproduct: Boston / Norton

WHAT HAS CHANGED

Issued a coastal flood advisory for tonight's high tide for the Cape and Islands, and could need to be extended into the Sunday evening high tide.

KEY MESSAGES

- Mostly cloudy with periods of off-and-on light showers tonight into early Sunday, before trending mostly sunny by Sunday afternoon. High risk for rip currents along the eastern coast Sunday, with minor coastal flooding around the evening high tide tonight and Sunday night for the Cape and Islands.

- Dry conditions Labor Day through much of Wednesday. Pattern becomes more unsettled Wednesday night into late-week with periods of showers possible and gusty winds possible Thursday.

DISCUSSION

KEY MESSAGE 1...Mostly cloudy with periods of off-and-on light showers tonight into early Sunday, before trending mostly sunny by Sunday afternoon. High risk for rip currents along the eastern coast, with minor coastal flooding around the evening high tide tonight and Sunday night for the Cape and Islands.

Slow-moving, closed mid/upper low pressure over the Canada Maritimes is helping to send wrap- around moisture in northeast flow across much of New England, within a broad cyclonic mid- level flow. This is leading to mostly cloudy skies this afternoon with scattered hit-or- mostly-miss light showers, with northeast winds around 10 mph. Expect these conditions to generally hold steady through the afternoon, with the best shot at light showers being in interior Southern New England. However later tonight and overnight, a compact but pretty robust shortwave evident in afternoon water vapor imagery well north of Montreal digs south- southeastward through New England, supporting at least a continuation of mostly cloudy conditions, but also support a risk for showers spreading back eastward overnight into early Sunday. By mid to late Sunday morning, subsidence in the wake of this disturbance brings decreased cloud cover with more sunshine and lighter northerly winds into Sunday afternoon. Any showers this afternoon into early Sunday won't amount to much in terms of impacts, but there will be some showers to dodge at times.

Northerly to northeast flow this weekend combined with rising astronomical tides support a risk for very minor coastal flooding around the evening high tides tonight and again on Sunday night for the Cape and Islands. A coastal flood advisory has been issued for tonight's high tide, and another advisory or coastal flood statement could be needed again for Sunday night's high tide. Overall minor impacts anticipated to the typically more vulnerable spots (Easy Street on Nantucket and Sesuit Harbor in Dennis). There is also an high risk for rip currents along east-facing shorelines in eastern MA, as offshore seas rise to around 4-6 ft overnight into Sunday.

Cloudy conditions and northeast breezes will help offset modest cool advection tonight, leading to lows in the mid to upper 50s. Highs Sunday top out in the upper 60s near the eastern coastline with the onshore flow, but should reach into the low to mid 70s under increasing sunshine by afternoon elsewhere.

KEY MESSAGE 2...Dry conditions Labor Day through much of Wednesday. Pattern becomes more unsettled Wednesday night into late-week with periods of showers possible and gusty winds possible Thursday.

The deeper mid-level trough shifts further eastward through the day Monday while higher heights advect in as a mid-level ridge builds to the west. This will bring in warmer temperatures aloft with 925mb temperatures +15-17C. NW flow and a sufficiently mixed boundary layer accompanied mainly clear skies should help yield high temperatures in the the mid to upper 70s for most. High pressure will support mainly dry conditions Monday with similar conditions and temperatures likely for Tuesday under the same pattern.

Ensemble guidance shows a signal for a transition to a more unsettled pattern mid to late week. Flow aloft becomes more zonal with a few embedded shortwave troughs moving through late Wednesday through late-week. This should be accompanied by a plume of above normal moisture (120-150% of normal) and a jet positioned to supply lift across the region. This should be enough to support periods of showers later Wednesday/Wednesday night into Friday. There is spread across guidance in the finer details of the timing of the waves which will influence timing of the showers. Overall, there isn't a strong signal for flash flooding at this time. Marginal instability in place may owe to a storm or downpour depending on the timing.

Another thing we will need to monitor is the potential for a period of gusty winds Thursday. An 850mb 35-45kt LLJ positions across southern New England into Thursday. If winds mix down efficiently, this would support gusty winds. There is a signal for anomalously gusty winds on the ECMWF EFI (still fairly far out). Flow looks to become more westerly behind the passage of the first wave with a signal for CAA. Both things that could facilitate the mixing down of the higher winds aloft (to some extent at least). As mentioned, timing of these waves are lower confidence and this could have influence on when we get the better mixing and thus the gusty winds. So plenty of details to be worked out. If things align, ensembles show potential for gusts in the 30-40 mph range on Thursday across the land and coastal waters. Still plenty of details to work out in the pattern before we come into better agreement about the gusts, but something to watch for now.

Temperatures Wednesday/Thursday lean near to slightly above normal with temperatures in the mid to upper 70s. A cooler airmass pushes in sometime later Thursday into Friday with cooler temperatures arriving by Friday (upper 60s/low 70s).

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

00z TAF Update:

Tonight: Moderate confidence.

Mainly BKN VFR/MVFR ceilings with periodic light -SHRA. A disturbance aloft moves ESE from Quebec bringing a more uniform batch of showers from N to S between 06Z and 15Z. NE winds around 5-10 kt.

Sunday: High confidence.

BKN VFR/MVFR ceilings early Sunday, with any MVFR ceilings improving to VFR mid to late-morning. NE winds 5-10 kt early shift to NNE by late morning/early afternoon at 6-12 kts. Cape/Islands terminals may see occasional gusts up to 20 kts. BOS may hold onto NE/ENE winds after clouds clear.

Sunday Night: High Confidence.

VFR. NW winds 5-10 kts.

KBOS Terminal...High confidence in TAF. Batch of -SHRA possible overnight. Most likely between 08-13Z. Brief MVFR cigs possible in that similar timeframe.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Thursday/...

Labor Day through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

Wednesday Night: VFR. Windy with gusts up to 30 kt. Chance SHRA.

Thursday: VFR. Windy with local gusts up to 30 kt.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Through Sunday Night: High confidence.

SCAs remain in effect for the eastern outer waters late tonight through Sunday as seas build to around 4-6 ft. NE gusts around 15-20 kt tonight into early Sunday, before becoming northerly the rest of Sunday around 10-15 kt. Off-and-on light showers this afternoon, but a better shot for showers exists overnight to early Sunday; but only minor reductions to visibility, at worst to 4 miles, are expected.

Outlook /Sunday Night through Thursday/...

Labor Day: Winds less than 25 kt. Areas of seas approaching 5 ft.

Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday through Tuesday Night: Winds less than 25 kt.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft.

Wednesday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Chance of rain showers.

Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents from 2 AM EDT Sunday through Sunday evening for MAZ007-014>016-019-022>024. RI...None. MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Sunday for ANZ250- 254-255.