textproduct: Boston / Norton

WHAT HAS CHANGED

Expanded the Wind Advisory into Cape Cod, Martha's Vineyard and Nantucket for today, while also upgrading marine headlines to Gale Warnings for all waters.

KEY MESSAGES

- Variable amounts of cloud cover, cooler than normal with strong west wind gusts 35-50 mph today.

- A period of heavy rainfall Mon into Mon night may lead to some river and small stream flooding. A period of strong to damaging wind gusts possible Mon night.

- Mainly dry weather and trending colder through midweek then moderating toward the end of next week.

DISCUSSION

Key Message 1...Variable amounts of cloud cover, cooler than normal with strong west wind gusts 35-50 mph today.

Strong Clipper low pressure system is expected to pass to our north and into the Canadian Maritimes by tonight. Pretty tight pressure gradient combined with efficient if still shallow mixing from a strong cold advection pattern should help to promote gusty to strong westerly wind gusts today. This system has had a history of strong westerly winds yesterday (e.g. several instances of 50 to 60 mph gusts) in the eastern Gt Lakes region, but the westerly lower-level jet doesn't look nearly as robust as it did yesterday, although is still considerable at around 45-50 kt at 850 mb.

Boosted wind gusts upward some for today, and most areas should end up gusting in the 35-45 mph range; however gusts could punch as high as 50 mph across the Berkshires, northern hills of Worcester County and down across the Cape and Islands where the existing Wind Advisory has now been expanded further into. This is supported by BUFKIT momemtum transfer progs as well as the 00z HRRR 10-m wind gust speeds, and the HREF ensemble minimum 10-m gust is also around 40-45 kt in the Advisory areas. Westerly gusts should really pick up once the sun comes out this morning but should peak into the afternoon as the core of the low-level jet crosses Southern New England. Gusts should drop below 40 mph by late afternoon, with a steadier decrease into the night as winds turn more northwesterly and lighter.

It wouldn't take much more of an increase in gusts to warrant expanding the Wind Advisory further into Southern New England, especially given continued early-morning observations of 35-45 kt gusts across western NY and the Gt Lakes. It does look to be at least a variably-cloudy day (more clouds north and fewer south) as moisture continues to wrap-around from the Clipper system, which casts some uncertainty in the mixing depth and that was a factor in deciding not to expand the Advisory into remainder of Southern New England. Other than the cloud cover, there may also be some scattered flurries or sprinkles dotting the area as this is a favorable setup for Gt Lakes moisture to be advected quite a distance downwind given the enhanced westerly jet, but this should be fewer and far betweeen and not impactful. Besides the windy conditions and the variable amts of cloud cover, today is otherwise a cooler day with highs in the upper 30s to low 40s, a couple degrees cooler than normal but the wind will add a bit of chill.

KEY MESSAGE 2...A period of heavy rainfall Mon into Mon night may lead to some river and small stream flooding. A period of strong to damaging wind gusts possible Mon night.

Ensemble guidance showing a strong signal for a heavy rainfall and strong wind event Mon into Mon night. PWAT and low level wind anomalies 3-4 SD above normal which is near the maximum of the CFSR climatology. This occurs ahead of a deep and amplified upper trough over the Gt Lakes which sends anomalous moisture northward into New Eng. Expect numerous showers Mon as anomalous PWAT plume overspreads the region within the low level jet axis. There is also some elevated instability so potential exists for heavy convective showers and a few t-storms as well. These locally heavy showers and isolated thunder will continue into Mon night. Given the strong forcing along the boundary along with some elevated instability a fine line of convection may develop along the front as it moves through during Mon night. The front should be moving off the coast by Tue morning with rapid drying moving from west to east. We are still expecting average rainfall of 1-2 inches with localized amounts up to 3 inches. 90th percentile of the ensemble QPF distribution continues to show rainfall amounts up to 3 inches which likely represents a high end potential from convection. MMEFS ensemble guidance continues to target RI rivers (Pawcatuck, Wood and Pawtuxet) and the lower reaches of the CT River from Hartford to Middle Haddam with the highest probabilities for (50-80%) for minor flooding during Tue-Wed timeframe.

While winds will be ramping up during Mon, the worst of the wind will likely be focused during the Mon night period just ahead of the cold front. This is when the maximum of the low level jet lifts across the region. NAM is indicating 85-90 kt LLJ at 925 mb moving across SE New Eng. Even the global guidance is showing 75-80 kt. Soundings do show a low level inversion which is normal with these southerly LLJ events so the challenge is determining how much wind can mix down. The key will be how mild it gets Mon evening ahead of the cold front and whether a fine line of convection can develop along the front which would increase potential for mixing strong winds to the surface. These are factors that should become more clear as we get within the hi-res window later this weekend. A quick look at the soundings suggest temps near 60 would increase damaging wind potential and there is certainly a chance we see temps get to 60 in some locations. But just taking 50 percent of the LLJ as an initial estimate with a strong inversion in place would support 45- 55 mph gusts, while a weakened inversion with milder temps would increase potential for 55-65 mph gusts.

KEY MESSAGE 3...Mainly dry weather and trending colder through midweek then moderating toward the end of next week.

It appears the front will have moved east of New Eng before 12z Tue with much drier air moving in from the west so expecting mainly dry weather Tue. The dry weather will continue Wed & Thu and possibly into Fri as well but we may have to contend with a shortwave and cold front which could bring a low risk for showers on Fri. It will be trending colder with the coldest day on Wed as 850 mb temps bottom out around -15 to -18C at 12z Wed. Highs Wed expected to be mostly in the 30s to near 40. Then temps gradually moderate Thu into Fri getting back to seasonable normals by Fri.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through 12z Saturday: High confidence.

Areas of MVFR/temporary IFR-visby SHSN to move offshore by 07-08z, then rapid improvement to VFR. Winds become SW/WSW and increasing to around 13-16 kt with gusts to 25 kt.

Today: High confidence.

SCT-BKN VFR ceilings (035-050) should predominate, though some MVFR ceilings possible across northern MA. Outside chance at passing rain/snow showers in Gusty WSW to W winds around 15-25 kt, with gusts 35-45 kt highest at ORH, on Cape Cod/ACK and in the Berkshires. Gusts should slowly ease around 25-35 kt late Sat while becoming W/WNW.

Saturday Night: High confidence.

VFR. Easing WNW winds to around 5-10 kt thru evening/overnight.

KBOS Terminal...High confidence in TAF. MVFR-visby SHSN should exit by 07z. VFR but with gusty WSW to W winds Sat, easing trends into Sat early evening.

KBDL Terminal...High confidence in TAF. VFR, gusty WSW to W winds thru Sat, easing trends into Sat late-afternoon.

Outlook /Sunday through Wednesday/...

Sunday: VFR. Breezy.

Sunday Night: Mainly VFR, with local IFR possible. Windy with local gusts up to 30 kt. Chance RA, chance FZRA.

Monday: Mainly MVFR, with areas IFR possible. Strong winds with areas of gusts up to 40 kt. RA, slight chance TSRA.

Monday Night: Mainly MVFR, with local IFR possible. Strong winds with gusts up to 50 kt. RA, isolated TSRA.

Tuesday: VFR. Windy with gusts up to 35 kt.

Tuesday Night: VFR. Windy with local gusts up to 30 kt.

Wednesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

High confidence.

Have upgraded SCAs/Gale Watch to Gale Warnings, which now cover all waters for today into early tonight. Gusts look to punch a little higher than prior expectations, with westerly gusts in the 35-45 kt range, but on the higher end of that range over the southern/southeast waters. Expect rough seas especially eastern/southern outer waters. Gradual decrease in winds and seas through SCA range into tonight and early Sunday AM.

Winds and seas then decrease below SCA criterion for Sunday.

However increasing southerly winds ahead of a strong low pressure should take place starting Sunday night. Gale/Storm Watches could become necessary very late Sunday night/early Monday.

Outlook /Sunday through Wednesday/...

Sunday: Winds less than 25 kt.

Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of rain.

Monday: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Rain, slight chance of thunderstorms.

Monday Night: gale force winds with gusts up to 45 kt. Rough seas up to 19 ft. Rain, isolated thunderstorms.

Tuesday: Strong winds with gusts up to 35 kt. Rough seas up to 15 ft. Slight chance of rain.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 14 ft.

Wednesday: Winds less than 25 kt. Areas of rough seas.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Wind Advisory from 10 AM this morning to 7 PM EDT this evening for MAZ002-004-008-009-022>024. RI...None. MARINE...Gale Warning until 11 PM EDT this evening for ANZ230>237-250- 251-254. Gale Warning until 11 PM EDT this evening for ANZ255-256.