textproduct: Boston / Norton

WHAT HAS CHANGED

Coastal Flood Statement issued for the Rhode Island and Massachusetts south coast for the 7-8 AM Monday high tide, with splashover looking likely. Coastal Flood Advisory for the Massachusetts east coast continues for the late-morning high tide. Generally uneventful workweek otherwse, but we are monitoring the potential for a significant Arctic air outbreak this weekend. Dangerously low wind chills, gusty winds and freezing spray for the waters are all possible this weekend.

KEY MESSAGES

- Minor coastal flooding along the eastern Massachusetts coastline for the late-morning/noontime high tide, with splashover to very minor coastal flooding for the Rhode Island/Massachusetts south coast for the early morning high tide.

- Generally tranquil through the workweek. Less-below- normal temperatures through midweek, although turns a bit blustery Wed and Thu.

- More active late-week and especially this weekend. Minor accumulating snow from a passing Clipper system Fri/Fri night. Then a period of gusty winds and deep Arctic air favoring the potential for dangerous wind chills this weekend, with gale force winds and freezing spray over the waters. Ocean effect snow could be possible, too.

DISCUSSION

Key Message 1...Minor coastal flooding along the eastern Massachusetts coastline for the late-morning/noontime high tide, with splashover to very minor coastal flooding for the Rhode Island/Massachusetts south coast for the early morning high tide.

One last high tide cycle to go, and coastal flooding expectations for the eastern Massachusetts coast remain unchanged. Storm surge values should clock in lower than yesterday, at around 1 to 1.5 ft, but the morning high tide is about a half-foot higher than the one that took place Sunday evening. Minor coastal flooding (inundation about 1 ft deep) seems most likely, with beach erosion still a concern given the continued built-up offshore waves and impact over the past two high tide cycles. There is also the continued potential for some of the storm surge to freeze on the typically vulnerable shoreline roads and become icy. The Coastal Flood Advisory remains valid for the east coast. Looking ahead, decreasing astro tides for the evening high tide, in lockstep with easing storm surge and lowering seas, should then end the threat for coastal flooding.

However we did add a Coastal Flood Statement for the Rhode Island and Massachusetts South Coast, including Narragansett and Buzzards Bays for the early morning high tide around 7-8 AM. Looking back at tidal gages at Fox Point and New Bedford from yesterday, the observed surge was lower than forecast because of the strong northerly winds. Now that those have subsided, observed storm surge values are now higher than they were yesterday, at around 1.6-1.7 ft. Thinking that storm surge values will be gradually coming down to around 1-1.25 ft due to rising sea-level pressures, but that decrease from a total-water-level perspective could be offset by the higher astro tide values associated with the morning high tide (see above). Splashover seems more likely with total water levels at Fox Point clocking in just under 7 ft minor flooding benchmarks, but there is the potential for minor coastal flooding as a worst- case scenario if surge is slower to subside.

Key Message 2...Generally tranquil period through the workweek. Less-below-normal temperatures through midweek, although turns a bit blustery Wed and Thu.

Otherwise, a generally uneventful workweek looks be the case with prevailing dry weather, despite Southern New England still being governed by northwest flow. Temperatures modify somewhat toward less- below-normal levels, with highs in the upper 20s to the mid 30s. For context, climo highs are in the upper 30s. Couple weak disturbances in that flow could bring a bit more cloud cover and blustery conditions Wed and Thu, with highs around the mid to upper 20s.

Key Message 3: More active late-week and especially this weekend. Minor accumulating snow from a passing Clipper system Fri/Fri night. Then a period of gusty winds and deep Arctic air favoring the potential for dangerous wind chills this weekend, with gale force winds and freezing spray over the waters. Ocean effect snow could be possible, too.

More active weather pattern starting late Fri night/Sat, but especially for this weekend. Pretty strong mid-level disturbance in the northwest flow and associated surface Clipper low pressure system digs southeastward into the Northeast later Fri/Fri night. Pretty energetic system overall but likely lacking of moisture and its progressive nature could limit snow accums as it moves offshore. At first look potential for hazardous accums looks low, with minor/nuisance snow accums more favored.

The more significant and potentially hazardous period however looks to develop this weekend. The potent mid-level shortwave closes off into a wound-up mid-level circulation, with the surface Clipper low pressure system slowing and really strengthening as it nears the Canadian Maritimes. Meanwhile a strong 1038+ mb high develops into the Gt Lakes, creating a tight northerly pressure gradient and advects southward what is easily the coldest air thus far this winter (e.g. multi- model ensembles suggest 850 mb temps as low as 30 below zero in the Berkshires, and in the 20 below zero range all the way to the southern coast!). We've had Arctic outbreaks with periods of low wind chills in the recent past, but the modeled temperatures are a different level of frigid air. This cold has been advertised recently in CPC Hazards Outlooks; what also adds confidence to potentially dangerously- low wind chills is that MEX MOS guidance wind speeds Saturday and Sunday are as high as 35 knots, perhaps in response to the deepening offshore cyclone. Very impressive signal for a Day-6 to Day-7 MOS forecast as MOS often trends toward climatology at that timeframe. Though still some details to iron out, there is somewhat higher confidence on dangerous wind chills Sat and Sun, to go along with accretion of freezing spray and gale- or- stronger-force NW winds over the waters. What's less certain and lower confidence is if we can generate ocean effect bands of snow for the eastern coast. Although this sounds like an especially harsh period of significant cold, a very early outlook into the early part of the following week suggests this level of cold could be short-lived, with multi- model support for a pattern change toward modifying southwest flow.

AVIATION /07Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Rest of Tonight: High Confidence

VFR. Decreasing NNW winds around 10 knots, remaining gusty near the waters around 20 knots

Monday: High Confidence

VFR. Winds will be gusty again in the afternoon around 18 to 20 knots from the NW before becoming calm after sunset

Monday night: High Confidence.

VFR. Calm winds.

Tuesday: High Confidence

VFR. Light SW winds under 10 knots.

KBOS Terminal...Moderate Confidence in TAF.

VFR. Gusty NW winds gradually diminishing through Monday.

KBDL Terminal...High Confidence in TAF.

VFR. NW winds around 10 knots through Monday.

Outlook /Tuesday through Friday/...

Tuesday through Tuesday Night: VFR.

Wednesday through Thursday: VFR. Breezy.

Thursday Night: VFR.

Friday: Mainly VFR, with local MVFR possible. Chance SN.

MARINE

Forecaster Confidence Levels...

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

Gradually improving marine conditions today with high pressure starting to build in. Winds diminish to under 20 knots from the NW today, however seas will be slower to diminish, staying around 6 to 9 feet today. Seas finally drop under 5 feet early Tuesday morning. Freezing spray remains possible this morning before temperatures inland warm this afternoon.

Outlook /Tuesday through Friday/...

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

Tuesday Night through Wednesday: Winds less than 25 kt.

Wednesday Night: Winds less than 25 kt. Freezing spray.

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

Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Coastal Flood Advisory until 2 PM EST this afternoon for MAZ007-015-016-019-022>024. RI...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ231- 232-235-237. Freezing Spray Advisory until 10 AM EST this morning for ANZ231>234-236-251-255. Freezing Spray Advisory until 7 AM EST this morning for ANZ235- 237. Small Craft Advisory until 7 AM EST Tuesday for ANZ250-254>256. Small Craft Advisory until 1 AM EST Tuesday for ANZ251.