textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes. Much below normal temperatures are expected this week, peaking Thursday night into Friday night which could prompt alerts for dangerous cold. Still monitoring a coastal storm around the Sunday timeframe, but the storm's track is still uncertain.

KEY MESSAGES

- Mainly dry weather but with well below normal temperatures this week. Potentially dangerous cold/wind chills develop Thursday night into Friday.

- Still monitoring a possible late-weekend winter coastal storm, but there remains large uncertainty on its track.

DISCUSSION

Key Message 1...Mainly dry weather but with well below normal temperatures this week. Potentially dangerous cold/wind chills develop Thursday night into Friday.

Southern New England will continue to be governed by an dry and cold WNW flow, despite an otherwise fairly active northern stream in the mid-troposphere forced by an upper low over Hudson Bay. Weak shortwave disturbances in this active flow will lead to periodic increases in cloud cover today and Thurs, but its main influence will be to maintain below normal temperatures. Highs range from the mid teens to the lower 20s, with lows in the single digits to near zero given the existing cold snowpack. Although this is the climatologically coldest part of the year, it's still abnormally cold.

An even stronger shot of Arctic air then advects into Southern New England Thurs night and Fri, with 850 mb temps -18 to -21C. This level of cold low-level air combined with the snowpack seems likely to yield one or potentially two nights of widespread sub-zero air temperatures for Thurs and Fri nights. While the northwest winds won't be as strong as we've seen at various points in the winter, it will add to the degree of cold; wind chills in the -10 to -25 degree range seem likely for Thurs night and will very likely be the coldest night seen to this point in winter. Alerts for dangerous cold probably will be needed at least Thurs night and could be needed in some interior/high terrain locations Fri night as well.

Key Message 2...Still monitoring a possible late-weekend winter coastal storm, but there remains large uncertainty on its track.

Still monitoring developments pertaining to a coastal storm which has the potential to threaten the U.S. East Coast this weekend, which could impact Southern New England Sunday. Although there is growing confidence in a storm somewhere along or offshore the southeast/mid-Atlantic seaboard, its track and how close it may get to Southern New England are still quite uncertain. All outcomes remain on the table, ranging from a dry pass offshore and no impact at all to a high-ceiling, slow-moving winter storm with several impacts.

Unfortunately there still is no clarity on where the storm tracks. This is visualized by a large spread in ECMWF/GEM/GFS ensemble member low tracks, which produces this elongated west- east bend in the model-ensemble mean sea level pressure field product. Although some solutions earlier today started to adjust slightly offshore, some of the overnight guidance inched back closer to the coast. As mentioned yesterday and continues to be the case today, ensemble cluster analysis again keys on the strength details associated with a complex upper low/PV anomaly which is now meandering around Hudson Bay. Due to an amplifying western North American ridge, this feature eventually digs southward out of Canada around Fri and settles either into the central Appalachians or into southern Georgia Sat, which helps get cyclogenesis going somewhere near or offshore the Carolinas early this weekend. Exactly when this occurs and how soon does this mid/upper feature close off will be critical in the eventual track and potential impacts to the eastern seaboard, including us in Southern New England. The sooner this process occurs, similar to the 00z/28th GFS which is the soonest to close off the upper low once it crosses the US/Canada border, the worse the outcome gets - as it would draw the surface cyclone closest to the east coast. We probably won't have a good handle on how that shakes out until Thurs at earliest, and there will likely be back- and-forth waffling of storm tracks played out in the model world until that becomes more clear.

While the storm's track is far more uncertain, most models/ensembles depict a surface cyclone anticipated to rapidly deepen as it meanders northeast. The airmass associated with the closed upper low features an exceptionally-cold airmass given the southern latitude (-14C at 850 mb over Jacksonville FL). Once that air gets near the warm Gulfstream current, strong air- sea baroclinicity should really help to facilitate rapid deepening. There are several model ensemble members showing sea- level pressures in the 960-980 mb range east of the Carolinas/mid-Atlantic waters. Provided such a storm tracks close enough to Southern New England late this weekend, a storm that strong would certainly elevate the potential for strong easterly winds, large waves/hazardous marine conditions and coastal flooding, in addition to heavy snow. It's too soon to know with certainty if that's realistic, but that is the ceiling it could have.

The bottom line is that the storm's eventual track - either close to the coastline, or well offshore - is still a key unknown. Expect there to be continued changes until ensembles begin to converge on a more likely scenario, and it's hoped there is more clarity on that by mid to late this week. The important thing for now is to not single out any one possibility as being more likely than another.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

12Z TAFs: High confidence.

VFR with W/WNW winds under 10 knots. Bit of an increase in midlevel clouds late aftn/tonight as a weak disturbance moves through. Winds remain westerly on Thursday but become breezy at 10-15 knots.

KBOS Terminal...High Confidence in TAF.

VFR. Westerly winds at 10-15 knots

KBDL Terminal...High Confidence in TAF.

VFR. Westerly winds under 10 knots.

Outlook /Thursday through Sunday/...

Thursday: VFR. Breezy.

Thursday Night through Friday: VFR. Windy with gusts up to 35 kt.

Friday Night: VFR. Breezy.

Saturday: VFR.

Saturday Night: Mainly VFR, with local MVFR possible. Strong winds with local gusts up to 45 kt. Slight chance SN.

Sunday: Mainly VFR, with local IFR possible. Strong winds with gusts up to 50 kt. Chance SN.

MARINE

Forecaster Confidence Levels...

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

Decreasing westerly winds tonight, remaining under 20 knots through Thursday afternoon. Seas gradually diminish to under 5 feet. Freezing spray advisory remains in effect through this afternoon as very cold inland air in the single digits makes its way over the waters.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Freezing spray, slight chance of freezing rain, slight chance of snow.

Thursday Night: Strong winds with gusts up to 30 kt. Local rough seas. Freezing spray, slight chance of snow.

Friday: Strong winds with areas of gusts up to 30 kt. Local rough seas. Freezing spray.

Friday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft. Freezing spray.

Saturday: Winds less than 25 kt.

Saturday Night: Moderate risk for gale force winds with gusts up to 45 kt. Areas of rough seas. Chance of snow.

Sunday: gale force winds with gusts up to 45 kt. Rough seas up to 21 ft. Chance of snow. Visibility 1 nm or less.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST this morning for ANZ231>235-237-251. Freezing Spray Advisory until 1 PM EST this afternoon for ANZ231>235-237-250-251-254>256. Small Craft Advisory until 10 AM EST this morning for ANZ250. Small Craft Advisory until 4 PM EST this afternoon for ANZ254>256.