textproduct: Boston / Norton

WHAT HAS CHANGED

A Flood Watch has been issued for central and eastern Massachusetts as well as Rhode Island. Periods of rain will continue, potentially moderate to heavy at times. Storm totals of 2-4" with localized 5-7" are possible. Issued Flood Watch in areas that may potentially see those higher amounts with potential for impacts to small rivers/streams and urban/poor drainage areas.

KEY MESSAGES

- Strong/long duration Nor'easter will continue to bring damaging wind gusts, coastal flooding/beach erosion, and periods of heavy rain through the weekend. Rain and gusty winds persist into Monday. - Marked improvement Tuesday, followed by mainly dry and warmer weather for the remainder of the week.

DISCUSSION

KEY MESSAGE 1...Strong/long duration Nor'easter will continue to bring damaging wind gusts, coastal flooding/beach erosion, and periods of heavy rain through the weekend. Rain and gusty winds persist into Monday.

Impacts from this Noreaster are well underway, with satellite imagery showing a distinct, rotating low-pressure center positioned just to our south. Evident bands of higher cloud tops indicate robust convection, which is being reflected on radar as convective showers and even a few embedded thunderstorms. This afternoon, these bands have pivoted into the South Coast and southeast MA. Expect these conditions to persist through the evening, bringing periods of moderate to heavy rainfall rates. Given the banded and scattered nature of the convection, impacts will be localized, but areas across southern MA, RI, and CT face the highest risk through tonight. While we will continue to monitor for isolated flash flooding under the heaviest downpours, the primary hydro threats remain urban ponding and poor-drainage flooding. Strong NE winds persist into tonight. Peak gusts of 4560 mph were observed along the coast early this morning, with localized gusts of 6070 mph across Block Island and the Cape and Islands. Expect 4555 mph gusts, occasionally up to 60 mph to continue this afternoon, highest along the immediate coast.

Strong to Damaging Wind Gusts:

Tonight, the low is forecast to slowly track northward, but west of the 40/70 benchmark before taking a sharp westward turn toward the NJ coast. As the low pivots, a potent 925mb low-level jet (LLJ) oriented SW-NE across southern New England will strengthen to 6070 kts. This introduces a renewed risk for a period of consistently stronger surface gusts, particularly across the South Coast, Block Island, and portions of CT if these winds can efficiently mix down. Gusts of 4560 mph are possible in these areas, and isolated higher gusts along the immediate coast cannot be ruled out. As the low shifts further west toward NJ on Sunday, the 925mb LLJ will weaken. In response, winds will veer to the E/ENE, with surface gusts decreasing into the 3045 mph range. The pressure gradient and associated jet will continue to gradually weaken as the low eventually loops back to the northeast and pushes offshore. Winds will follow suit, with gusts finally dropping below 30 mph by Monday night.

Prolonged Period of Steady Rain, Heavy at Times:

Abundant moisture, optimal dynamics, and marginal instability will maintain widespread rain impacts through the remainder of this event. Added low-level convergence along the higher terrain of the Worcester Hills and Berkshires will continue to support periods of moderate to heavy rain. High-resolution and global guidance highlights a broad footprint of 24 of rain, with localized maximums of 56 possible before the system and rain exit late Monday. Notably, the 48-hour HREF Probability Matched Mean (PMM) highlights areas of Rhode Island and eastern Massachusetts for potential 57 totals. Because this is primarily a prolonged rainfall event rather than a widespread torrential downpour, the overall flash flood threat is somewhat limited by the lack of widespread extreme rain rates. However, the sheer volume of water raises significant concerns for urban and poor-drainage flooding, as well as rapid rises on small rivers and streams. To address this threat, a Flood Watch has been issued for Rhode Island and eastern Massachusetts, aligning with the highest-confidence footprint for the 57 storm totals.

Coastal Flooding:

The prolonged period of strong northeast winds and high offshore seas will continue to result in beach erosion and coastal flooding over multiple high tide cycles through Sunday. Storm surge values of 1 to 2 feet, perhaps briefly approaching 2.5 feet, are expected along east-facing, ocean-exposed shorelines. Coastal Flood Warnings remain in effect along the eastern Massachusetts coast, including the Cape and Islands, through the Sunday night high tide cycle. Consideration is being given to extending these headlines into the Monday morning high tide cycle.

The combination of elevated water levels and 15 to 25 foot offshore seas will result in minor coastal flooding, with a few pockets of low-end moderate flooding possible during each high tide cycle. However, the most significant impact from this long-duration event is expected to be severe beach erosion given the persistent strong northeast winds and large waves. Astronomical tides peak this weekend, so coastal impacts will remain a concern even as winds gradually diminish. In addition, water may struggle to fully drain between successive high tide cycles, while heavy rainfall could further exacerbate drainage and flooding issues.

Coastal Flood Advisories remain in effect for the south coasts of Rhode Island and Massachusetts. While northeast winds are somewhat of a mitigating factor for coastal flooding in these areas, very large swell associated with the nor'easter to our south will propagate northward toward the coast. Pressure falls will also contribute to elevated water levels. Therefore, pockets of splashover and minor coastal flooding are expected, along with minor beach erosion through the weekend.

KEY MESSAGE 2...Marked improvement Tuesday, followed by mainly dry and warmer weather for the remainder of the week.

The weekend's long-duration nor'easter exits east-northeast as the mid-level flow becomes less amplified and blocking diminishes. Throughout the coming week, the mid-level flow becomes fairly zonal, with a few weaker impulses passing through during the mid to late week. These may bring a low probability of a light shower or two.

Expect marked improvements Tuesday as the decaying nor'easter exits the region and winds shift to the northwest. With northwest flow, anticipate lower PWATs advecting into southern New England, bringing any lingering early morning showers to an end. While PWATs remain somewhat elevated into the afternoon, there is little forcing to support additional precipitation. Latest guidance has slowed the clearing trend compared to previous runs, so clouds may hang tough through much of the afternoon before some breaks develop later in the day. Northwest flow will also keep temperatures a few degrees below climatological normals, though it will be noticeably warmer than this past weekend. Highs Tuesday reach the upper 60s to around 70 degrees.

A mid-level ridge builds across the southeastern CONUS this week, while surface high pressure builds south of the region Wednesday through Friday, leading to a stretch of quieter and warmer weather. Guidance hints at a weak northern stream shortwave Wednesday night into early Thursday, followed by a secondary shortwave Thursday night into Friday. PWATs Wednesday night into Thursday remain below 1.2 inches and don't really align with the best forcing. Outside of a brief light shower, would anticipate more clouds than anything. PWATs increase to around 1.5 inches Thursday night with the passage of the secondary weak shortwave, when a few light showers could brush through the region. No significant impacts are anticipated with either disturbance. Temperatures trend warmer through the middle and latter half of the week, though latest guidance has trended cooler compared to previous runs. Highs generally return to the lower and middle 70s, with Thursday potentially the warmest day as highs reach the middle to upper 70s. As for overnight temperatures, there is no notable cold expected, with lows generally falling into the 50s.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

Today through Sunday...High confidence in trends, moderate confidence in timing.

Periods of windswept rain will impact the region through Sunday. MVFR with IFR ceilings spreading north and west through the evening. Potential for borderline LIFR ceilings tonight, though lower in confidence as we do have windy conditions that typically limit low ceilings. Low risk for a thunderstorm this afternoon with the higher risk for southern terminals (BDL/PVD). TEMPOs have been included for a few hours this afternoon for TSRA although as convective showers shift through. NE wind gusts of 30 to 50 knots expected with the strongest of those winds near the coast tonight. Wind gusts will diminish a bit on Sunday...But still expect NE wind gusts of 25 to 40 knots.

KBOS Terminal...High confidence in trends, moderate confidence in timing.

Confidence lower in ceiling heights overnight tonight. There is potential for borderline LIFR conditions. This is lower confidence as higher winds typically limit the potential for very low ceilings.

KBDL Terminal...High confidence in trends, moderate confidence in timing.

Convective showers this afternoon. There is a risk for a brief thunderstorm or pulse or two of lightning this afternoon which has resulted in the TEMPO TSRA.

Outlook /Sunday Night through Thursday/...

Monday: MVFR/IFR conditions possible. Windy with local gusts up to 30 kt. SHRA.

Monday Night: MVFR/IFR conditions possible. Chance SHRA.

Tuesday through Tuesday Night: Mainly VFR, with local IFR possible.

Wednesday through Thursday: VFR.

MARINE

Forecaster Confidence Levels...

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

Through the Weekend...

* Dangerous conditions for Mariners through the weekend with NE wind gusts of 45 to 60 knots and seas offshore of 15-25 feet.

The strengthening early season Nor'easter will bring strong NE wind gusts of 45-60 knots and seas offshore between 10 and 20 feet. Seas will remain quite high Sun into Mon along with NE wind gusts of 25 to 40 knots persisting. Main update was to extend Storm Warnings for the waters through late morning Sunday. Once these expire, these will be replaced by Gale Warnings for Sunday then SCAs for Monday more likely.

Outlook /Sunday Night through Thursday/...

Monday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Chance of rain showers.

Monday Night: Winds less than 25 kt. Rough seas up to 8 ft. Chance of rain showers.

Tuesday through Tuesday Night: Winds less than 25 kt. Seas up to 5 ft.

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

Wednesday Night through Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Wind Advisory until noon EDT Sunday for CTZ002>004. MA...Wind Advisory until noon EDT Sunday for MAZ002>006-008>014-017- 018-026. Flood Watch through late Monday night for MAZ004>007-012>024- 026. High Wind Warning until 8 AM EDT Sunday for MAZ007-015-016- 019>024. Coastal Flood Warning until 8 PM EDT Sunday for MAZ007-015-016- 019-022>024. High Surf Advisory until 8 PM EDT Sunday for MAZ007-015-016- 019-022>024. Coastal Flood Advisory until 8 PM EDT Sunday for MAZ020-021. RI...Wind Advisory until noon EDT Sunday for RIZ001. Flood Watch through late Monday night for RIZ001>008. High Wind Warning until 8 AM EDT Sunday for RIZ002>008. Coastal Flood Advisory until 8 PM EDT Sunday for RIZ002- 004>008. MARINE...Storm Warning until 10 AM EDT Sunday for ANZ230>237-250-251- 254>256-280>283.