textproduct: Boston / Norton

WHAT HAS CHANGED

High Wind and Coastal Flood Watches in effect for the coastline Friday and Saturday.

KEY MESSAGES

- Dry, cool, and breezy weather continues through Thursday.

- An anomalous early season strong coastal storm will impact the region with strong winds and coastal flooding/erosion Friday and Saturday, especially eastern MA.

DISCUSSION

KEY MESSAGE 1...Dry, cool, and breezy weather continues through Thursday.

Not much change to our weather through Thursday. A sprawling high pressure remains over western Canada. Expecting gusty east to northeast winds to continue Thursday.

KEY MESSAGE 2...An anomalous early season strong coastal storm will impact the region with strong winds and coastal flooding/erosion Friday and Saturday, especially eastern MA.

Unfortunately at 48-72 hours out, models still displaying large a spread in track of ocean low along with its accompanying strong wind fields. The EC/UKMET and EC-AIFS are on the western side of the envelope of solutions, with a surface low track westward toward NJ coast. Meanwhile, the NAM, GFS and its AI member (AIGFS) are farther east toward a Cape Cod track. Given the time range is still 48+ hours out, a blend/probabilistic approach is more sound than a deterministic path. With that said, here is our info regarding each hazard:

Strong Winds...forecast based on following the 90 percentile of the NBM, with 45+ mph wind gusts begin to overspread coastal areas of RI and eastern MA early Friday, then peaking late Friday into Saturday. This appears to be the window of greatest wind impacts. Given it's only late September and trees are fully leaved, the wind threshold to bring down large branches and limbs is noticeably lower than winter. Past experience supports G40-45 mph is sufficient this time of year for wind impacts, including disruptions of power. Therefore, we will hoist a high wind watch for the entire coastline of MA/RI, with the highest probs of impact, including power interruption is across the Cape and Islands, including BID and Cape Ann. We realize winds may fall short of the watch/warning threshold of 58+ mph, but again, the threshold is lower this time of year (40-45mph) given fully leaved trees. Hence, basing this watch on impacts, not hard core wind thresholds. Timing, strongest winds somewhere in the Friday/Saturday time frame. Winds don't completely drop off Sunday/Monday, just not as strong as the low weakens.

Ferry travelers to Boston, Nantucket, Marthas Vineyard and Block Islands, should check in advance with ferries as service interruptions are likely, especially Friday and Saturday given timing of peak winds.

Coastal Flooding...

Long duration of 40-50 kt northeast flow (possibly gusts up 60 kt over coastal waters) directed towards the coastline Friday and Saturday, will pile up lots of water along the coastline and given the duration of the wind event, water will struggle to evacuate seaward, hence, adding to surge values, which may be somewhat underdone due to this affect. Our surge forecast was based on the ETSS and Steven's Institute, which offers 1.5-2.0 ft for high tides Fri and Sat. Verification could turn out to be up to 2.5 ft based on reasoning above. Given rising astro tides, (peaking early next week) combined with this surge, high likely for at least minor coastal flooding along the entire MA coastline from MVY/ACK/CC/Plymouth/BOS to NH border. If more westward solutions of UKMET/EC and its AI models, could be some inundation and erosion for RI coastline. Stay tuned. Greatest risk for moderate flooding is south of Boston to Cape Cod and Islands during the late Fri and Sat high tide cycles. However, given the duration of large waves pounding the coastline, seawalls and eroding sand dunes over multiple tide cycles, coastal erosion could tip the scales for impacts to be moderate or higher. Hence, we think this warrants a coastal flood watch.

Rainfall...12z ensembles from EC/GFS and Canadian all have low probs of 2-4" of rain across eastern MA. However, this is spread out over 3 days. Thus, not expecting any flooding, other than nuisance type roadway ponding. Given the blocky pattern, this coastal/ocean storm will likely linger into Monday with wet impacts.

Temperatures...cool/moisture onshore flow Fri/Sat/Sun will limit daytime highs in the upper 50s to lower/mid 60s. Solid 10 degs cooler than normal.

AVIATION /20Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

Through Tonight...High confidence.

VFR. NE winds of 10 to 20 kt with gusts of 20 to 30 kt, strongest towards the southeast New England coast. Gusts around 35 kt for Nantucket.

Thursday and Thursday Night...High confidence.

VFR, outside of MVFR cigs over ACK and some of the outer Cape late Thursday into the night. NE winds of 10 to 20 kt with gusts of 20 to 30 kt, strongest towards the southeast New England coast. Gusts near 35 kt for Nantucket.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Friday: Mainly VFR, with local MVFR possible. Strong winds with gusts up to 50 kt. Chance RA.

Friday Night: Mainly MVFR, with areas VFR possible. Strong winds with gusts up to 50 kt. RA likely.

Saturday: Mainly MVFR, with areas VFR possible. Strong winds with gusts up to 45 kt. RA likely.

Saturday Night: Mainly MVFR, with areas VFR possible. Strong winds with areas of gusts up to 40 kt. SHRA, slight chance TSRA.

Sunday: Mainly MVFR, with areas VFR possible. Strong winds with local gusts up to 40 kt. SHRA likely.

Sunday Night: Mainly VFR, with local IFR possible. Windy with gusts up to 35 kt. Chance SHRA.

Monday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Thursday Night...High Confidence.

NE winds 20-30 kt with gusts up to 35 kt at times briefly subside around sunset. Gusty winds resume Thursday and continue through Thursday night. This northeast fetch will yield rough seas on the ocean waters, 6-12 ft. Dry weather and good visibility prevail. Rain could arrive from the SE as early as Thursday afternoon for the waters south and east of Nantucket. It is more likely this rain arrives Thursday night.

Outlook /Friday through Monday/...

*** Dangerous sea and wind conditions from an Ocean Storm Friday/Saturday and Sunday. Storm Force wind gusts (48+ kt) possible during this time. ***

Friday: gale force winds with gusts up to 45 kt. Rough seas up to 25 ft. Rain.

Friday Night: gale force winds with gusts up to 45 kt. Rough seas up to 24 ft. Rain. Local visibility 1 to 3 nm.

Saturday: gale force winds with gusts up to 45 kt. Rough seas up to 23 ft. Rain, slight chance of thunderstorms.

Saturday Night: Strong winds with local gusts up to 45 kt. Rough seas up to 19 ft. Rain showers, chance of thunderstorms.

Sunday: Strong winds with gusts up to 40 kt. Rough seas up to 16 ft. Rain showers likely.

Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft. Chance of rain showers.

Monday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Wind Watch from Friday morning through Saturday evening for MAZ007-015-016-019>024. Coastal Flood Watch from Friday afternoon through Saturday afternoon for MAZ007-015-016-019-022>024. RI...High Wind Watch from Friday morning through Saturday evening for RIZ005>008. MARINE...Small Craft Advisory until 8 AM EDT Friday for ANZ230-236. Gale Warning from 8 AM Friday to 8 PM EDT Saturday for ANZ230- 236. Small Craft Advisory until 8 PM EDT this evening for ANZ231- 254. Gale Warning from 8 PM this evening to 8 AM EDT Friday for ANZ231-254. Storm Watch from Friday morning through Saturday evening for ANZ231-254-281. Gale Warning until 2 AM EDT Friday for ANZ232>235-237-255-256- 282-283. Storm Watch from late Thursday night through Saturday evening for ANZ232>235-237-255-256-282-283. Small Craft Advisory until 2 AM EDT Friday for ANZ250-251-280. Gale Warning from 2 AM to 11 AM EDT Friday for ANZ250-251-280. Storm Watch from Friday morning through Saturday evening for ANZ250-251-280. Gale Warning until 8 AM EDT Friday for ANZ281.