textproduct: Boston / Norton

WHAT HAS CHANGED

High Wind Watch for Cape and Islands upgraded to a High Wind Warning. Wind Advisories remain in effect for rest of southern New England. Storm Warnings issued for waters adjacent to Cape Cod and south of the Islands.

KEY MESSAGES

- Locally heavy rainfall and strong to damaging wind gusts late tonight into Friday

- Much colder with gusty west wind gusts Friday night

- Coastal flood risk along south coast Friday morning has ended - Next system moves through late Saturday into Sunday bringing light rain Saturday and gusty winds Sunday into Monday.

DISCUSSION

KEY MESSAGE 1...Locally heavy rainfall and strong to damaging winds.

A widespread rain event is expected late Thu night into Fri as a robust upper trough and strong cold front approaches from the west. An anomalous low level jet will transport abundant moisture northward into SNE. Strong forcing for ascent acting on this moisture will bring a period of widespread rain, developing in the west late Thu night then moving east across SNE during Friday. The wind and PWAT anomalies and marginal elevated instability support locally heavy rain with some downpours and can't rule out an isolated t-storm given strong forcing at the nose of the low level jet. However, meager instability will limit rainfall rates and duration of heavy rain will be limited so we are not looking at an excessive rainfall event. Generally widespread rainfall of 0.50 to 1 inch, with locally 1-2 inches possible where more persistent downpours occur. Western New Eng has best chance of morning commute being impacted by locally heavy rain based on timing, but significant flooding impacts are not expected.

It's a fairly fast moving system so rain will be tapering off during the afternoon from west to east after the cold front moves through and much drier air moves in from the west. The rain may end as a brief period of snow in the Berkshires as the column cools from top down. While drier air is moving in during the afternoon, risk of scattered rain or snow showers will continue late afternoon and early evening after the main rain shield exits as the robust upper trough and cold pool move in from the west.

Regarding wind potential, this looks to be a decent wind event, especially for eastern New Eng as a robust 70-80 kt develops 12-18z with the jet potentially strengthening to 90+ kts as it moves east over the Cape/Islands around 18z before moving offshore. Wind anomalies approach +4SD as the jet moves offshore and the ensemble forecast low level wind is near the maximum relative to CFSR climatology. As is the case with all southerly LLJ events it's always about the extent of mixing. Soundings do show a fairly strong inversion for the Cape/Islands but there are impressive pressure falls which often helps to accelerate winds from isallobaric contribution. Also temps for the Cape get into the mid 50s which will help to weaken the inversion somewhat. We decided to upgrade to a high wind warning for these reasons as well as the potential for a 90+ kt low level jet which just taking 50% yields gusts close to high wind criteria. Potential for 55-65 mph gusts focused from late morning into early afternoon. Now for the rest of SNE, we continued wind advisories for 40-50 mph gusts in the morning, but there is potential for an area of enhanced winds across RI and SE MA. This is where temps may get close to upper 50s which could erode inversion enough to produce some 60 mph gusts here. This will have to be monitored for possible upgrade to a warning here.

KEY MESSAGE 2...Much colder with gusty west wind gusts Friday night

After the cold front moves through strong cold advection will result in well mixed boundary layer which will support strong west wind gusts. The strongest gusts to 40-50 mph will likely be confined to the Berkshires, Worcester Hills and Islands. There is a low risk for a few 50+ gusts. Elsewhere, looking at potential for a period of 35- 45 mph gusts. Soundings support stronger gusts than the wind guidance is suggesting so adjustments to the forecast may be necessary. The strong cold advection will result in temps falling into and through the 20s overnight Fri night.

KEY MESSAGE 3...Coastal flood risk has ended

Guidance is in agreement that the core of the low level jet and strongest winds and surge will occur at least a few hours after high tide on the south coast. As a result we are not expecting any coastal flood impacts. It would take about a 3 ft surge at high tide to produce minor flooding along Narragansett and Buzzards Bay. While a 3 ft surge at Providence is possible, we have high confidence it will occur after high tide.

KEY MESSAGE 4...Next system moves through late Saturday into Sunday bringing light rain and gusty winds lingering into Monday.

A mid-level shortwave trough moves through late Saturday into Sunday bringing scattered rain showers to southern New England. Another 40 to 60 kt low level jet, quickly followed by a cold front, is expected to push north into SNE by early Sunday morning. While there is currently some uncertainty in the guidance on where exactly the core of the jet will set up, the cold front pushing through behind the jet will provide an opportunity for efficient mixing and likely bring 30 to 40 mph gusts to at least the coastal waters, possibly the Cape and Islands, Sunday into Monday.

AVIATION /06Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

06Z TAF Update:

Today...High Confidence

VFR. Light westerly winds 5-10 kts shift southerly this afternoon.

Tonight through Friday...High Confidence in trends

MVFR/IFR ceilings begin to creep in from west to east late tonight, with rain chances beginning during the late overnight/early morning hours. Southerly winds begin to increase to 10 to 15 kts with gusts 20 to 30 kts, and windshear once again becomes a concern overnight and into Friday morning with 2kft winds around 35 to 40 kts.

KBOS Terminal...High confidence in trends. Moderate confidence in timing.

VFR today, then MVFR/IFR by late tonight/early Friday morning. Windshear possible overnight and into mid-Friday morning with rain moving in by early morning.

KBDL Terminal...High confidence in trends. Moderate confidence in timing.

VFR today, then MVFR/IFR by late tonight/early Friday morning. Windshear possible overnight and into mid-Friday morning with rain possibly moving in around midnight.

Outlook /Friday Night through Monday/...

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.

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

Sunday Night through Monday: 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.

Gale and Storm Warnings in effect for Friday

Winds decrease and seas subside briefly during the day today. Winds then increase late tonight with a period of southerly gale force gusts to 45 kt during Fri morning and early afternoon. Storm force gusts to 50 kt are expected over the waters adjacent to Cape Cod as a strong low level jet moves through. Storm force winds will diminish to gales by mid-late afternoon with westerly gales continuing on all waters Fri night. Seas increase to 8-12 ft Friday over the outer waters with 12-15 ft possible for the outer southern waters.

Outlook /Friday Night through Monday/...

Friday Night: gale force winds with local gusts up to 45 kt. Rough seas up to 16 ft.

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

Saturday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas.

Sunday: Strong winds with gusts up to 35 kt. Rough seas up to 11 ft. Slight chance of rain.

Sunday Night: Strong winds with gusts up to 40 kt. Rough seas up to 12 ft.

Monday: Strong winds with gusts up to 35 kt. Areas of rough seas.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Wind Advisory from 4 AM Friday to 1 AM EST Saturday for CTZ002>004. MA...Wind Advisory from 4 AM Friday to 1 AM EST Saturday for MAZ002>021-026. High Wind Warning from 7 AM to 3 PM EST Friday for MAZ022>024. RI...Wind Advisory from 4 AM Friday to 1 AM EST Saturday for RIZ001>008. MARINE...Gale Warning from 5 AM Friday to 7 AM EST Saturday for ANZ230- 250-251. Storm Warning from 9 AM to 3 PM EST Friday for ANZ231>234-254- 255. Gale Warning from 3 AM Friday to 7 AM EST Saturday for ANZ235- 237-256. Gale Warning from 5 AM Friday to 1 AM EST Saturday for ANZ236.