textproduct: Boston / Norton

WHAT HAS CHANGED

High wind watch issued for Cape Cod and the Islands for Friday. The risk for minor coastal flooding for the south coast during the Friday morning high tide has diminished somewhat.

KEY MESSAGES

- Dry weather with mild temperatures continue through Thursday. Gusty winds 25-35 mph arrive this morning and linger through the evening.

- Widespread rain with strong to possibly damaging winds late Thursday night into Friday.

- The risk for minor coastal flooding for south-facing shores during the Friday morning high tide has diminished.

DISCUSSION

KEY MESSAGE 1...Dry and milder through Thursday. Gusty winds today.

SW-ly LLJ has already starting to build in, and surface winds are beginning to respond... especially over the Cape/Islands and waters. Will start to see those gusty winds 25-35 mph by mid-morning as well. Will be a rather chilly start to the day with wind chills in the teens to low 20s by daybreak. However, warmer temperatures will begin to make their appearance today with afternoon highs in the low to mid 40s. Expect to see winds drop off overnight tonight into Thursday morning, with even warmer temperatures in the mid 40s to low 50s.

KEY MESSAGE 2...Locally heavy rainfall and strong to possibly damaging winds

Deep trough moves across the Gt Lakes and towards New England Thursday night into Friday. Ahead of the trough an anomalous low level jet 3-4SD above normal develops which advects PWATs 1-1.25" northward into SNE. This is a strong signal for locally heavy rainfall as a period of strong low level convergence acts on anomalous moisture. However, what will likely prevent an excessive rainfall event is it's a fast moving system with bulk of heavy rain likely limited to about a 6 hr period late Thu night into Fri from west to east. Also, while a few t-storms can't be ruled out instability appears to be meager which should keep rainfall rates in check. Overall, looking at widespread rainfall 0.50-1 inch with locally 1-2 inches possible where more persistent downpours, which is consistent with the 90th percentile of the global ensemble guidance. The heavy rain may impact the morning commute Friday, especially across CT valley as timing of low level jet appears to favor western New Eng. Minor urban and poor drainage street flooding is possible but not expecting significant impacts. The heavier rain is expected to impact eastern New Eng later in the morning into early afternoon. The cold front moves through during the afternoon followed by strong drying so rain will taper off after the wind shift, but scattered showers should develop in the afternoon/evening as robust upper trough approaches from the west.

Regarding strong wind potential, core of the southerly low level jet 70-80 kt develops across SE New Eng Fri morning, potentially peaking around 85 kt as it reaches the outer Cape/Islands by early afternoon before moving offshore. The challenge with these southerly low level jet events is always determining how much wind will mix down to the surface. An examination of the forecast soundings reveal a neutral temp profile for areas away from the immediate coast, while a fairly strong inversion is present over the Cape/Islands. It looks like we would need temps to reach 58-59 degrees to efficiently mix the strongest winds to the surface, and it will be close as current forecast has temps mid-upper 50s away from immediate south coast as the low level jet is moving through. One factor which could help enhance strong gusts is the strong pressure falls which develop across eastern MA and adjacent waters. Using a conservative 50-60 percent of the low level jet would yield gusts to 45-55 mph, strongest across SE MA, mostly during the morning into early afternoon on the Cape/Islands. If the milder temps are realized, gusts could reach 60+. This will have to be monitored closely, but we decided to issue a high wind watch for Cape/Islands where 925 mb winds peak around 85 kt with 850 mb winds exceeding 90 kt. Wind advisories will likely eventually be needed for at least RI and eastern MA.

Strong cold advection in the post-frontal environment will yield strong west wind gusts Fri night. Soundings are well mixed boundary layer with potential for 40-50 mph gusts across all SNE.

KEY MESSAGE 3...Diminishing risk for minor coastal flooding

Latest guidance suggests the timing of the low level jet and highest surge will occur a few hours after high tide Fri morning which would likely preclude minor coastal flood impacts along the south coast. The greatest risk would be for Narragansett Bay and Buzzards Bay, but it would take a storm surge of 2.5 to 3 ft during high tide to produce minor flooding. Latest guidance from the Stevens Institute indicated a 2-2.5 ft surge for Providence but occurring after high tide. The 95th percentile which represents a 5 percent exceedance probability has Providence slightly exceeding the 7 ft flood stage but this is a very low probability at this time. It would take more favorable timing of the strongest winds at high tide and a worst case wind forecast to realize this potential, which would yield just minor flooding impacts along Narragansett and Buzzards Bay.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18Z TAF Update: High confidence.

Tonight and Tomorrow ...High Confidence VFR. SW wind gusts from 20 to 30 knots continue through late this afternoon. Winds shift to W this evening becoming light overnight, then S around 10 kt Thu afternoon.

Tomorrow Night...High Confidence in trends

MVFR/IFR ceilings overspread the region from west to east tomorrow night with RA. Winds increase out of the south with sustained speeds of 15 to 20 knots with gusts approaching 25 to 35 knots by Friday morning.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night: MVFR/IFR conditions possible. Strong winds with local gusts up to 40 kt. Chance RA.

Friday: Mainly MVFR, with local IFR possible. Strong winds with gusts up to 45 kt. RA.

Friday Night: VFR. Strong winds with gusts up to 45 kt.

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

Saturday Night: VFR. Windy with areas of gusts up to 30 kt. Slight chance RA, slight chance SN.

Sunday: VFR. Windy with local gusts up to 30 kt. Slight chance RA.

MARINE

Forecaster Confidence Levels...

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

Today Through Thursday...High confidence.

Gale force wind gusts likely through Wednesday evening as a southwesterly low level jet settles overhead. Seas increase to 8-10 feet in the southern waters, and 6-8 feet in the eastern waters today. Winds decrease and seas subside briefly during the day Thursday before ramping back up Thursday night.

Outlook /Thursday Night through Sunday/...

Thursday Night: Low risk for gale force winds with gusts up to 40 kt. Areas of rough seas. Chance of rain.

Friday: gale force winds with gusts up to 45 kt. Rough seas up to 14 ft. Rain.

Friday Night: gale force winds with areas of gusts up to 45 kt. Rough seas up to 14 ft. Chance of rain.

Saturday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft.

Saturday Night: Low risk for gale force winds with gusts up to 35 kt. Seas up to 5 ft. Slight chance of rain.

Sunday: Strong winds with gusts up to 35 kt. Areas of rough seas. Slight chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Wind Watch from Friday morning through Friday afternoon for MAZ022>024. RI...None. MARINE...Gale Warning until 7 PM EST this evening for ANZ230>237-251. Gale Warning until 1 AM EST Thursday for ANZ250-254>256.