textproduct: Boston / Norton

WHAT HAS CHANGED

Wind Advisories were upgraded to High Wind Warnings for coastal Rhode Island and eastern Massachusetts, with Gale Warnings upgraded to Storm Warnings on all eastern waters.

KEY MESSAGES

- Calm conditions this afternoon and evening, becoming unsettled by early Friday morning through the afternoon. Period of moderate to at times heavy rainfall and strong to damaging southerly wind gusts.

- Coastal flooding risk along south coast Friday has ended.

- Improving conditions by Friday evening and through the overnight, but windy conditions from the west remain.

- Shortwave brings a period of gusty winds and much colder air Sunday night into Monday.

- Weak disturbance may bring a period of light snow Tuesday afternoon.

DISCUSSION

KEY MESSAGE 1...Calm conditions this afternoon and evening, becoming unsettled by early Friday morning through the afternoon. Period of moderate to at times heavy rainfall and strong to damaging southerly wind gusts.

A 1030mb high-pressure east of Cape Cod provides tranquil conditions for the remainder of Thursday and the first half of the night. Will have declining conditions by early Friday morning as a robust rain and wind system pushes across the northeastern CONUS. The system is very dynamic with a deep mid-level trough, high moisture content and an anomalous 850mb jet. Good news, it is a progressive system, let's discuss the threats.

Rainfall: Rain overspreads the region from west to east during the morning, thus leading to a slow commute for some, especially by midmorning. High moisture content advects northward due to the strong low-level jet, PWATs surpassing an inch with localized higher values near the south coast of 1.2". Strong forcing from a robust mid-level trough and shortwave, will have periods of moderate and heavy rain, and cannot rule out rumbles of thunder with limited instability. Areas that could experience a thunderstorm are the southern coast of RI and south coast of MA, as the best instability is mainly over the southern waters. High-res guidance also hints at the chance of a fineline to develop, which could locally enhance rainfall rates and wind damage. Otherwise, rainfall rates aren't too impressive, plus the progressive nature, not expecting significant flooding. However, areas that normally experience nuisance flooding could see ponding. Expected amounts of rain are between 0.5 and 1.0 inches, with locally higher values in the higher terrain of northwestern CT and western MA, where terrain may help to enhance rainfall totals. Rain moves out by midafternoon and drying out late afternoon into the evening hours. A rouge orographic snow shower or two is possible overnight into Saturday morning for northwestern areas of MA, otherwise dry.

Wind: We continue to monitor the threat for strong to damaging winds ahead of an approaching cold front. As we've previously mentioned an anomalous 850mb jet with a core of 90 to 100 knots of wind are a few thousand feet above our heads by early afternoon. Highlighting this abnormality is the ensemble situational awareness table, showing the winds at 850mb are 4-5 standard deviations above normal. Now, not all this wind will mix down to the surface, a thermal inversion at the surface will keep the strongest winds aloft. Looking slightly lower at the 925mb winds, winds are muddled ever so slightly 50 to 80 knots over much of the region, with 70 to 90 knots over the east coast of MA and extreme coastal RI. If 60 percent of that wind field were to mix to the surface, would have gusts at 60 mph which was the basis for converting the Wind Advisory to a High Wind Warning for those areas. Elsewhere the forecast remains on track with potential gusts of 40 to 55 mph. One thing to keep in mind for tomorrow are the temperatures, because the warmer we get, the better chance the thermal inversion eroding. For an example, forecast sounding at KPVD at 18z shows a neutral to weak inversion, with a surface temperature of 57F mixing would only extend to 950mb where winds are 50 knots, but if the temperature were to be 60F the boundary layer would extend to 925mb where winds are 70 knots. As you can imagine, there is a 2 to 4 degree make or break when it comes to the wind forecast, especially the higher end wind gusts.

KEY MESSAGE 2...Coastal flooding risk has ended.

Storm surge forecasts via Stevens Institute and P-ETSS continue to show a 2 to 3.5 ft storm surge at our south coastal sites accompanying the period of strong southerly winds Fri. Despite this rather large storm surge no coastal flooding is expected as this storm surge looks to peak 3-4 hrs after the mid-morning high tide, with high confidence.

KEY MESSAGE 3...Improving conditions by Friday evening and through the overnight, but windy conditions from the west remain.

Cold front moves through, strong cold advection results with a well mixed boundary layer, support strong west wind gusts. The strongest gusts between 40 and 50 mph likely be confined to the Berkshires, Worcester Hills and the Islands... and a low risk for a few 50+ mph gusts. Elsewhere, potential for a period of 35 to 45 mph gusts. The strong cold advection results in temperatures falling into and through the 20s overnight.

KEY MESSAGE 4... Shortwave brings a period of gusty winds and much colder air Sunday night into Monday

A mid-level shortwave trough moves through late Saturday into Sunday bringing scattered rain and snow showers mainly across far northern Massachusetts. The larger concern will be for another period of elevated winds especially Sunday night into Monday. Continuing to see two periods of strong low level winds Sunday and Monday. First will be early Sunday morning as a southwest low level strengthens to 40-50 mph as warm air advection increases. Limiting factor will timing as the core of the jet passes through between 06-12z. There will still be locally better mixing (lapse rates between 4-6 C/km) at coastal locations and across the elevated terrain. These areas could winds gust as high as 40-45 mph thru 12z Sun. Another round of more widespread gusty winds arrives later Sunday behind a strong cold front. Strong mid and low level cold air advection will help to mix a 40-55 mph low level jet to the surface. Expecting widespread gusts of 40-45 mph from 00z-15z Monday.

Additional concern for Monday will be a significantly colder airmass Monday. Temps at 850mb plunge to around -15C which should translate to low temperatures ranging from the mid teens in the interior to the mid 20s for the Cape and Islands. These cold temps paired with strong NW winds push wind chill temperatures down into the lower single digits in elevated interior areas and lower to middle teens elsewhere.

KEY MESSAGE 5... Weak disturbance may bring a period of light snow Tuesday afternoon.

By Tuesday an upper level disturbance over northeast Quebec helps form a weak surface low near the international border. Guidance shows a broad area of warm advection on the southern side of the low as it moves southeast. Forcing from the warm advection could be enough for widespread snow showers/light snow Tuesday into Tuesday afternoon. Latest NBM shows a light event with probabilites of greater than 1 inch of accumulation Tuesday generally below 30 percent and confined to the higher elevations of Massachusetts.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

00Z TAF Update: Unsettled conditions expected to arrive overnight into Friday morning, worst of conditions peaking early afternoon, with improvements later in the day. Confidence continues to increase for southerly gusts of 40 and 50 knots across much of the region along with periods of moderate and heavy rainfall.

Today...High Confidence.

VFR. South winds between 5 and 10 knots.

Tonight through Friday...High Confidence in trends.

MVFR/IFR ceilings develop 02-06z with widespread IFR late tonight into Friday as widespread rain, locally heavy develops from west to east. Gradually improving conditions after 18z/21z after FROPA and wind shift to west.

Increasing south winds tonight with max winds during Fri 12-18z with gusts 35 to 45 knots well inland and 40 to 50 knots in the coastal plain, peaking over the Cape/Islands around 18z where an isolated gusts of 55 knots cannot be ruled out. Wind shift to west between 18z and 21z, with gusts between 25 and 35 knots.

Areas of LLWS develop tonight as low level winds increase, peaking Fri in the coastal plain with a robust LLJ 70 an 80 knots around 2,000 feet.

Friday Night...High Confidence in trends.

Improving to VFR and dry, though a low risk (less than 30 percent) for upslope snow showers in northwestern Massachusetts overnight. Westerly winds, gusting 25 to 35 knots.

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

VFR through the rest of today, then MVFR/IFR between 03z-06z and windshear possible overnight and into mid-Friday morning with rain moving in by early morning. Improving conditions after 21z Friday.

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

VFR through the rest of today, then MVFR/IFR between 02z-05z and windshear possible overnight and into mid-Friday morning with rain moving in by early morning. Improving conditions after 19z Friday.

Outlook /Saturday through Tuesday/...

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

Saturday Night: VFR. Windy with areas of gusts up to 30 kt.

Sunday through Monday: VFR. Windy with local gusts up to 30 kt.

Monday Night: VFR. Breezy.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Chance SN, chance RA.

MARINE

Forecaster Confidence Levels...

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

Tonight through Friday...High Confidence.

Southerly winds will begin to increase tonight, but especially toward early morning Friday with onset of gale to storm force gusts. Gusts to 50 kt now looks to be the case for much of the eastern outer waters Fri morning to mid-/late-afternoon, which has prompted upgrading existing gale warnings over to storm warnings. Seas will build to around 13-16 ft on the outer waters, leading to potentially dangerous marine conditions. Besides the winds and seas, rounds of rain showers, some locally enhanced late in the day as a cold front moves through.

Passage of cold front late in the day will bring WNW winds to gale force into early Sat, which then decrease as we move through Sat. Seas will gradually decrease later Fri night to around 9-12 ft, with a more substantial decrease expected into Sat.

Outlook /Saturday through Tuesday/...

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.

Sunday Night: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft.

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

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas.

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of rain, chance of snow.

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>006-008>014-017-018-026. High Wind Warning from 6 AM to 3 PM EST Friday for MAZ007-015- 016-019>024. RI...Wind Advisory from 4 AM Friday to 1 AM EST Saturday for RIZ001>003. High Wind Warning from 6 AM to 3 PM EST Friday for RIZ004>008. MARINE...Storm Warning from 6 AM to 3 PM EST Friday for ANZ230>234-250- 251-254-255. 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.