textproduct: Boston / Norton

WHAT HAS CHANGED

Coastal storm for Sunday continues to trend further offshore. Winter Storm Watches converted to Winter Weather Advisories for the Cape and Islands for Sunday. Expectations for minor inundation for Sunday morning, evening and Monday morning tidal cycles led to converting Coastal Flood Watch for the eastern MA coast to Coastal Flood Advisories, but the Coastal Flood Watch was maintained for the Cape and Islands.

KEY MESSAGES

- Frigid temperatures continue today, and while sub- zero wind chills are again expected tonight, wind chills may not drop as low as the past couple nights.

- A powerful coastal storm passing offshore of Southern New England brings accumulating snow, gusty winds, hazardous marine conditions and minor to pockets of moderate coastal flooding to eastern and southeast New England.

- Dry weather and not as cold into the middle of next week, though turns colder and more active late next week/early weekend.

DISCUSSION

ey Message 1...Frigid temperatures continue today, and while sub- zero wind chills are again expected tonight, wind chills may not drop as low as the past couple nights.

Cold Weather Advisories continue into the morning today. Although a layer of stratus has developed across western New England, much of Southern New England is cloud-free and that's allowed for temps to really bottom out. Wind chills have been more in the -5 to -15 range in most areas. Today ends up still bring another frigid day, with highs in the upper teens to lower 20s. More mixed signals if another cold weather advisory is warranted for tonight; northeast winds do pick up which will lower the wind chills, but the airmass also starts to modify enough and increasing cloudiness could temper how low temperatures may drop. Forecast wind chills tonight in interior Southern New England drop to as low as 5 to 10 below zero, which is still on the marginal side.

Key Message 2...A coastal storm passing offshore of Southern New England brings accumulating snow, gusty winds, hazardous marine conditions and minor to pockets of moderate coastal flooding to eastern and southeast New England.

The 00z suite of guidance continues to favor an offshore pass of a powerful coastal storm expected to develop near the NC/SC coast today. While the 00z ECMWF and to an extent the 00z NAM continues to be the most moisture-laden and on the western envelope of the storm's precipitation shield (and by western, no further west than the eastern portion of Narragansett Bay), both these models have also struggled the most run-to-run. Across the suite of 00z guidance, the consensus track seemed slightly further to the south/east (offshore) than earlier forecasts. This has led to slight downward reductions in anticipated snow totals, northerly wind gusts, as well as significant wave heights. The latter two elements also influence coastal flooding expectations.

We made several headline changes overnight based on the above trends, while being cautious of additonal southeastward adjustments in the storm's track. The Winter Storm Watch was converted to a Winter Weather Advisory; we also added a Winter Weather Advisory for the eastern coast of Plymouth County due to ocean effect snowbands being modeled by higher-resolution guidance, with just enough QPF to justify one off road impact along Route 3, although the greater accumulation should be more along the southern portion of eastern Plymouth County (roughly Kingston southward to the Cape Cod Canal). Over the waters, we converted gale watches over to warnings, with storm warnings on the eastern/southeast waters adjacent to Nantucket and the Outer Cape. Finally on the coastal flooding front, headline decisions there remained challenging due to the offshore trend and the potential for lower storm surge values. The Coastal Flood Watch was converted to Coastal Flood Advisories for the eastern coast of MA from the mouth of the Merrimack River to the Cape Cod Canal; we opted to hold the Coastal Flood Watch for the Cape and Islands as there still is the potential for the bay side of Cape Cod and Nantucket to experience areas of moderate (2-3 ft inundation) coastal flooding, but the continued offshore trend left open the possibility of minor/1-2 ft inundation.

Snow/Wind: Spotty bands of ocean-effect snow in the initial NE flow could begin as soon as the pre-dawn hours tonight in eastern Plymouth County and northern portion of Cape Cod, while tending to fill in from the south as the northwest periphery of the synoptic precipitation band begins to brush the southeast coast by Sunday morning. Roads probably have been treated, but the snow should should have no trouble sticking immediately when it first develops given the cold temps we've had. Snow then continues at a generally light to at times moderate clip through the day, then begins to taper off around early Sunday evening in Plymouth and Bristol Counties, and closer to late Sunday night over the Cape and Islands as the storm pulls away and drier NWly flow entrenched over the interior Southern New England advects in. Snow growth parameters in the Advisory area are really middling at best, even across the Cape; which is reflected in probabilities of 1" per hour snow rates being essentially nil. So rates should be more manageable across the Advisory area, with visby reductions at least as much from blowing snow off the standing mounds of snow versus what falls from the sky. As mentioned, the 00z ECMWF remains moister and has about 50% probabilities of a half inch of QPF across Nantucket; but we view this as a moist outlier. We opted for snow totals in the 2 to 4 inch range in eastern Plymouth County, but the higher end of those totals should be in the southern part of the County where there's just enough terrain enhancement in northeast flow ocean effect setups to give totals a modest boost. Across the Cape and Islands, looking for a general 3-5 inches with perhaps near a half-foot over the Outer Cape and Nantucket. As snow rates should be manageable, think this fits more closely as a winter weather advisory setup. The snow ends up being wind-driven with gusts in the 40-45 mph range over land; that could make the snow really difficult to measure. We don't expect the wind to lead to any damage.

Coastal Flooding: The three tides of concern specific to coastal flooding are the Sunday morning and evening high tides, as well as the Monday morning high tide. Although it still looks like the storm surge peaks Sunday afternoon which coincides with low tide, the prospect of lower storm surge values (1 - 2.5 ft, highest near Nantucket) with the storm continuing to pass further southeast could lead to lesser total water levels corresponding to more minor inundation. Storm surge guidance from Stevens Institute also is down from previous runs, now more in the 1 to 2 ft range. Because of that, we sided towards coastal flood advisories for minor inundation and some beach erosion for the eastern MA coast excluding Cape Cod and the Islands. Didn't have enough confidence to convert the Cape and Islands over to an Advisory or Warning, wanting to wait another surge model cycle before making that final determination. It could very well turn into a minor with pockets of moderate flooding, particularly for Nantucket.

As a reminder:

Minor flooding refers to shallow flooding up to 1 foot deep and can result in temporary road closures on more vulnerable coastal roads.

Moderate flooding refers to flooding 1 to 3 feet deep and can cause more widespread road closures, damage to shorefront property, and debris on coastal roads from large waves. Vehicles left in flood prone areas can also be flooded.

Key Message 3: Dry weather and not as cold into the middle of next week, though turns colder and more active late next week/early weekend.

Drier weather with modifying temperatures from the frigid airmass we've been stuck in much of this week, to something more closely resembling seasonable cold. Highs could return to around the freezing mark through midweek. Some modest snowmelt should take place but those mounds of snow will still be around. Latest guidance seems to indicate our next chance for snow comes from an energetic Clipper system in WNW flow. Odds for significant snow accums from that Clipper system seems long, as is typical for fast-moving Clippers. Could be a period of breezy conditions in its passage, but that also will usher in another shot of below normal temps as we move into next weekend.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Through Today: High confidence.

VFR. Light WNW winds under 10 knots today

Tonight: High confidence thru 06z, then moderate for the eastern terminals.

VFR, though layer of MVFR stratus starts to turn landward from the ocean waters ~06-09z. Periodic bands of ocean effect snow showers develop with MVFR visbys around 09-11z, with steadier light snow starting to break out for the Cape and Islands. Winds turn NE and increase in speed to 10-15 kt with gusts 20-25 kt.

VFR and dry weather, but MVFR/VFR cigs arrive toward morning in eastern MA.

Sunday/Sunday Night: High confidence western terminals, moderate confidence eastern terminals.

MVFR-IFR with steady light snow (1-3 SM visby) across the Cape terminals and ACK. A few inches of accumulating snow likely limited to those terminals. Intermittent ocean effect snow at BOS with 4SM -SN indicated; given cold pavement, some runway accum here but snow totals an inch or less. Ocean effect precip for BOS starts to taper off around 20z as winds start to turn offshore.

VFR inland. NE winds 15-25 kt with gusts 25-40 kt, highest gusts at ACK; winds turn NW with a decrease in speed 21z Sun-00z Mon from west to east. This could cause areas of blowing/drifting snow off existing snowmounds.

KBOS Terminal...High Confidence in TAF thru Saturday.

Light WNW winds today around 10 knots. MVFR develops around 09z, with periodic/intermittent ocean effect snow showers by 11z. Limited accum (less than an inch) but pavement is cold so it will stick. NE winds increase Sunday morning to around 15 kt with gusts 28-30 kt.

KBDL Terminal...High Confidence in TAF.

VFR. Light WNW winds today under 10 knots, becoming north early Sunday and increasing to around 10-13 kt with gusts 20-23 kt.

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR. Strong winds with local gusts up to 45 kt.

Monday through Monday Night: VFR. Breezy.

Tuesday through Wednesday: VFR.

MARINE

Forecaster Confidence Levels...

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

Diminishing winds and seas today as high pressure builds in.

A powerful ocean storm which tracks well south and east of the waters on Sunday. Potential for high end gales to low end storm force NE gusts to 40-50 kt over the waters starting Sunday morning. Seas expected to build to 15-20 ft on the waters east of Cape Cod resulting in dangerous conditions for mariners. In additions, areas of light to moderate freezing spray will develop Sat night/Sun morning and again Sun night.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 19 ft. Freezing spray likely, chance of snow.

Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 16 ft. Freezing spray likely.

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

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

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

Wednesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Cold Weather Advisory until 10 AM EST this morning for CTZ002>004. MA...Cold Weather Advisory until 10 AM EST this morning for MAZ002>019-026. Coastal Flood Advisory from 8 AM Sunday to 2 PM EST Monday for MAZ007-015-016-019. Winter Weather Advisory from 4 AM Sunday to 4 AM EST Monday for MAZ019-022>024. Coastal Flood Watch from Sunday morning through Monday afternoon for MAZ022>024. RI...Cold Weather Advisory until 10 AM EST this morning for RIZ001>004. MARINE...Gale Warning from 7 AM Sunday to 1 AM EST Monday for ANZ230. Gale Warning from 4 AM Sunday to 4 AM EST Monday for ANZ231- 232. Freezing Spray Advisory from 10 PM this evening to 1 PM EST Sunday for ANZ233>237-256. Gale Warning from 6 AM Sunday to 1 AM EST Monday for ANZ233- 234. Gale Warning from 3 AM Sunday to 1 AM EST Monday for ANZ235- 237-256. Gale Warning from 5 AM to 11 PM EST Sunday for ANZ236. Gale Warning from 7 AM Sunday to 4 AM EST Monday for ANZ250. Freezing Spray Advisory from 4 AM to 1 PM EST Sunday for ANZ251. Gale Warning from 6 AM Sunday to 4 AM EST Monday for ANZ251. Storm Warning from 7 AM Sunday to 4 AM EST Monday for ANZ254- 255.