textproduct: Boston / Norton

WHAT HAS CHANGED

The winter storm watch for Essex county has been upgraded to a winter storm warning. Winter weather advisories continue for the remainder of eastern MA into the Worcester Hills and down into RI.

KEY MESSAGES

- 3-6 inches of snow expected across central and eastern MA into RI, with localized bands of heavier snow, possibly up to 8 inches in Essex county. Less snow westward into CT and western MA.

- Dangerous cold tonight into Sunday. Gusty winds supporting wind chills that fall 10 to 30 degrees below zero. High confidence in hazardous freezing spray over the waters.

DISCUSSION

KEY MESSAGE 1...3-6 inches of snow expected across central and eastern MA into RI, with localized bands of heavier snow, possibly up to 8 inches in Essex county. Less snow westward into CT and western MA.

Very dynamic system moving through today as deep amplifying trough becomes negatively tilted and closes off south of New Eng with -38C pocket at 500 mb. Strong height falls will yield cyclogenesis well offshore but deep upstream mid level trough axis will result in inverted/Norlun trough extending NW from the offshore low into SNE Sat. Meanwhile, arctic front will be moving through SNE and eventually merging with inverted trough.

Increasing forcing for ascent combined with deep saturated layer through the mid levels has resulted in snow developing across SNE early this morning and this will continue through the day, although the focus will shift to central/E MA and RI late morning through the afternoon. We may see some brief enhancement to the snow along the arctic front after 12z in western New Eng, then snow should taper off here by mid-late morning behind the front. Otherwise, we will have to watch for some heavier bands of snow developing across RI and possibly into central MA this morning along an area of low level convergence. Then very interesting and complex evolution late morning through the afternoon as hi-res guidance indicating snow bands moving westward across NE MA within convergent NE flow then rotating southward across eastern MA through the afternoon as it merges with inverted trough. These inverted troughs are notorious for localized heavy snow bands and it is likely that this will be the case today as there are strong model signals for potentially heavy snowfall rates. Steep 0-3km lapse rates combined with strong low level convergence/frontogenesis and marginal low level CAPE are strong indicators of heavy snowfall rates. The snow squall parameter really lights up mid-late afternoon across eastern and SE MA where arctic front merges with inverted trough enhancing low level convergence. GFS and NAM soundings also showing strong omega, in some cases 20-30 ub/s, extending through the DGZ. This will likely result in a 2-4 hour period of heavy snow during the afternoon across portions of eastern MA with localized snowfall rates of 1-2"/hr.

Forecasting the exact placement of these mesoscale features with these inverted trough events and where heavy bands will set up is quite challenging and often offers low predictability until it begins to develop. Additionally there also can be areas of subsidence between bands leading to high variability in snowfall amounts over short distances.

Because mesoscale factors will be a big factor in snowfall we leaned on hi-res guidance for snow accum forecast. A lot of the guidance is suggesting a max QPF axis across Essex county where there is low level convergence from land sea interface, while a secondary max is possible in RI from low level convergence which sets up this morning. Overall, we are looking at accums of 3-6 inches across much of the advisory area from central and eastern MA down across RI, with locally up to 8 inches across Essex county where the winter storm watch will be upgraded to a warning. There is a low risk that localized amounts could exceed 8 inches in Essex county. We do have some concern that some of the heavier snow could extend down the coast into Boston and Plymouth county where some 6+ inch amounts are possible but confidence in exceeding 6 inches here is low. And can't rule out localized 6 inch amounts in RI if the heavier snow bands here come to fruition. Further west amounts will drop off with generally 1-3 inches across western MA into the CT valley. We want to emphasize that forecast confidence is moderate as the placement of these heavy snow bands can shift.

The snow in RI and eastern MA should taper off and end between 4-7 pm. Then ocean effect snow showers will linger on Cape Cod and Nantucket through the evening with additional accum bringing total accums to 2-4 inches here.

KEY MESSAGE 2...Dangerous cold for Saturday night into Sunday. Gusty winds supporting wind chills that fall 10 to 30 degrees below zero. High confidence in hazardous freezing spray over the waters.

Confidence remains high for the dangerously cold conditions that will follow the arctic frontal passage. The front moves through the region during the afternoon hours Saturday, and an arctic airmass follows behind it. Lower level temperatures around -10C (at 925 mb) fall to around -20C and lower across the region, particularly the interior by Sunday morning. Some guidance has 925 mb temperatures over eastern MA between -15C and -12C, but the bottom line is that the environment will be highly supportive of dangerously cold temperatures. Wind chill values will be the greatest concern. A 925 mb jet develops heading into Saturday night/Sunday morning with wind speeds around 40 to 45 knots. Forecast soundings continue to indicate that theses winds are also at the top of the mixed layer, so with these winds mixing down to the surface post-front, gusts between 30-40 mph are highly probable through the night. This will bring wind chills down to 30 below 0F in parts of the Berkshires as air temperatures on their own are already expected to be in the negatives. The rest of the interior could see wind chill values down to 25 below 0F and eastern MA can expect wind chills generally around 10 below 0F, with the outer Cape and Islands the warmest in the positive single digits.

Heavy freezing spray is another major concern through Monday morning with these very cold temperatures combined with strong, gusty winds. Ice accretion rates between 0.5-1" per hour are possible within the Heavy Freezing Spray Warning area, which includes all waters except Boston Harbor. Though, light freezing spray in Boston Harbor is not out of the realm of possibility; ice accretion rates are not expected to get that high there.

The distant offshore low to our southeast continues its way out to sea for the start of the week and high pressure over the eastern Great Lakes builds in. Gusty winds from 20 to 30 mph persist through Sunday before the pressure gradient starts to weaken, and high pressure takes control of the pattern over southern New England Monday into Tuesday. Dry conditions make a return with continued below-normal temperatures, though Wednesday could see highs in the mid 30s. These are still below normal, but may feel much warmer than what we've dealt with lately.

We are continuing to monitor another possible system that could impact southern New England sometime midweek. Latest guidance still doesn't have a solid grasp on track or exact timing, which is not expected this far out. But generally, 925 mb temperatures could climb to around -5C and even 0C by Wednesday morning. A low from the NW may cross over the northeast that could lead to showers across southern New England. But again, it is too far out to know for certain and much can and will change between now and then.

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...

Today...Moderate confidence.

MVFR-IFR conditions in areas of snow with localized heavier snow bands and LIFR possible, mainly across central/E MA into RI. Increasing N-NW winds late morning through the afternoon from W to E with gusts to 25-35 kt developing.

Tonight...Moderate confidence.

MVFR becoming VFR during the evening, but lower cigs/vsbys in ocean effect snow showers lingering on Cape Cod later into the evening. NW wind gusts 25-35 kt.

Outlook /Sunday through Wednesday/...

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

Sunday Night through Monday Night: VFR. Breezy.

Tuesday: VFR.

Tuesday Night: Mainly VFR, with local MVFR possible. Chance SN.

Wednesday: Mainly VFR, with local MVFR possible. Slight chance SN.

MARINE

Forecaster Confidence Levels...

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

Headlines:

* Gale Warnings from Sat afternoon into Sunday.

* Heavy Freezing Spray warnings for Saturday afternoon through early Mon morning.

Increasing NE winds on eastern MA waters becoming N later today, with increasing N-NW winds on the south coastal waters. Gusts increasing to 30-40 kt mid to later afternoon through tonight behind the arctic front. Areas of heavy freezing spray will develop tonight into Sunday as gusty gusty winds combine with arctic air moving into the region. Heavy freezing spray produces ice accretion rates up to 1 inch/hr creating very dangerous conditions for vessels.

Outlook /Sunday through Wednesday/...

Sunday: Strong winds with gusts up to 30 kt. Rough seas up to 12 ft. Freezing spray.

Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Freezing spray.

Monday: Winds less than 25 kt. Areas of rough seas. Freezing spray.

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Freezing spray.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday Night: Winds less than 25 kt. Chance of snow.

Wednesday: Winds less than 25 kt. Chance of rain, chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Extreme Cold Warning from 6 PM this evening to 1 PM EST Sunday for CTZ002>004. MA...Extreme Cold Warning from 6 PM this evening to 1 PM EST Sunday for MAZ002>004-008>012. Winter Weather Advisory until 7 PM EST this evening for MAZ004- 005-012>021-026. Cold Weather Advisory from 6 PM this evening to 1 PM EST Sunday for MAZ005>007-013>023-026. Winter Storm Warning until 7 PM EST this evening for MAZ006- 007. Winter Weather Advisory until 1 AM EST Sunday for MAZ022>024. RI...Winter Weather Advisory until 7 PM EST this evening for RIZ001>008. Cold Weather Advisory from 6 PM this evening to 1 PM EST Sunday for RIZ001>008. MARINE...Gale Warning from 4 PM this afternoon to 7 AM EST Sunday for ANZ230-236. Freezing Spray Advisory from 4 PM this afternoon to 7 PM EST Sunday for ANZ230. Gale Warning from 4 PM this afternoon to 1 PM EST Sunday for ANZ231-232-250-251-254. Heavy Freezing Spray Warning from 4 PM this afternoon to 7 AM EST Monday for ANZ231>237-250-251-254>256. Gale Warning from 1 PM this afternoon to 10 AM EST Sunday for ANZ233>235-237. Gale Warning from 1 PM this afternoon to 1 PM EST Sunday for ANZ255-256.