textproduct: Boston / Norton

WHAT HAS CHANGED

Still monitoring a major coastal storm around the Sunday timeframe, but the storm's track is still uncertain. The best chance for significant snow, wind, and coastal flooding impacts is across the Cape and Islands.

KEY MESSAGES

- Still monitoring a possible late-weekend winter coastal storm. There is still a large amount of uncertainty in the track of the storm resulting in a wide range of possible impacts.

- Mainly dry weather through the end of the week. Potentially dangerous cold/wind chills Thursday night into Friday night.

DISCUSSION

KEY MESSAGE 1...Mainly dry weather through the end of the week. Potentially dangerous cold/wind chills Thursday night into Friday night.

Very cold pattern will persist through the end of the week as SNE remains under influence of northern stream with a few shortwave disturbances reinforcing the arctic air across the region. The coldest of the arctic air will be Thu night into Fri night as an elongated northern stream trough axis drops south through New England. 925 mb temps bottom out around -18C to -22C at 12z Fri. Expect low temps of -5F to +5F Thu night with highs Fri in the teens. Not expecting very windy conditions but enough wind to drive wind chills down to -10 to -20F across much of the region late Thu night into Fri morning, with -20 to -25 wind chills over the higher terrain. This will likely require cold weather advisories for much of SNE. Even colder air temps Fri night with lows zero to -10F away from the immediate coastline but with a bit less wind so expect similar wind chills to Thu night, generally -10 to -20F late Fri night into Sat morning.

Generally dry weather expected through Sat. Will see more cloud cover develop later this afternoon and evening with perhaps a few flurries as a weak shortwave moves through. More diurnal cu develops Thu, then clearing Thu night with mostly clear skies Fri into Sat.

KEY MESSAGE 2....Still monitoring a possible late-weekend winter coastal storm. There is still a large amount of uncertainty in the track of the storm resulting in a wide range of possible impacts.

We are maintaining a close watch on a potential coastal storm late this weekend. Deterministic and ensemble guidance remain in general agreement regarding the development of a low-pressure system off the East Coast. Despite this consistent signal, models still diverge on the storm track, a critical detail that will dictate the both the magnitude of impacts as well as the overall footprint of the storm.

There is good consensus for an amplified ridge setting up over western CONUS/Canada, helping a trough dig southward from Canada later in the week. Model guidance continues to lack agreement on the upper level pattern including the degree of amplification of the upper trough, whether a 500mb low closes off, and how it tracks. This is present in the WPC Cluster Analysis with half of the grand ensemble showing a closed off low. There are solutions that favor a closed low shifting across the Ohio Valley region, potentially shifting as south as the Georgia/Carolinas before it tracks north. As the upper low tracks up the East Coast it will support cyclogenesis offshore of the Carolinas. The track of the upper low will be important as it would influence the resultant surface low development and later track.

This amplified trough and possible low will keep cold air locked in across the east. Surface coastal low development has been consistent across guidance with solutions showing deep surface low development(sub-980mb) off the East Coast. There is a favorable set- up for rapid/deep cyclogenesis with cold air interacting with the warmer maritime waters, helping establish a sharp baroclinic zone. Given the uncertainty in the details of the pattern still, there is a large variance in the track of the surface low across ensemble guidance which will ultimately translate to impacts and the footprint of storm impacts. Tracks range from a hit to a full miss with some in between solutions showing a "graze" hit to the Cape/Islands in regards to impacts.

Cluster analysis also signals at differences in the amplification of the ridge to the east (offshore) which may influence the low track. This may likely be a more complex detail as the strength of this ridge would depend on the small-scale details/thermodynamics of the system as it evolves. Convection may act to enhance the downstream ridge keeping the surface low track more inland.

Plenty to be said on the pattern, low tracks, model QPF with large amounts of variance, but what prompts the close monitoring are the solutions that indicate an impactful storm. The upper percentiles of ensemble guidance show a large spread in QPF/snow amounts relative to the lower displaying the "hit or miss" type outlook of the storm. The 75th/90th percentiles (worst case scenarios of the ensemble suite) continue to indicate a potentially moderate to even high impact winter storm in terms of snow amounts alone. Across ensembles, the highest probabilities are further southeast, especially across the Cape/Islands as even a further offshore track may still impact those areas. The lower probabilities as you go further west are a sign of the uncertainty in the track which is what leads to the question of the footprint of the storm overall. The Probabilistic WSSI indicates probabilites 20-60% for moderate winter storm impacts with the highest probabilities for eastern and southeast MA. This realistically displays potential for a storm while leveraging the large amount of uncertainty in the track, amounts with the storm.

Snow would not only the the concern in the case of a strong coastal low pressure with a closer to shore track. A strong jet/pressure gradient could support a large strong wind footprint with the storm. Ensembles indicate high probabilities for gusts greater than 40 mph for the Cape/Islands and the coastal waters. Across ensemble guidance, there are probabilities between 40-80 percent for gusts greater than 58 mph across the outer Cape, Nantucket, and even higher across the coastal waters. Potential for coastal flooding and erosion impacts will also need to be monitored due to the potential for strong winds, large waves, and higher astronomical tides. It's still too soon to know the expected impacts, but this shows that the ceiling is high for an impactful storm if everything comes together.

Bottom line, we are five days out still. It is expected to see shifts in track from model run to run and continued spread among guidance. The track remains uncertain which means we still have a wide range of scenarios, some pointing to an impactful coastal storm across portions of New England and others a miss. It remains important to not lock in on a solution and look at this from a probabilistic standpoint while noting trends in guidance. The hope is that we gain more clarity in the next day as solutions come into better agreement.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

00Z TAFs: High confidence.

VFR. Light westerly winds at 5-10 knots tonight and Thursday.

KBOS Terminal...High Confidence in TAF.

VFR. Light west winds around 10 knots through Thursday

KBDL Terminal...High Confidence in TAF.

VFR. Light west winds through Thursday. Could be some LLWS towards Thursday evening as winds aloft increase faster then surface winds.

Outlook /Friday through Monday/...

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

Friday Night: VFR. Breezy.

Saturday: VFR.

Saturday Night: VFR. Strong winds with local gusts up to 45 kt. Chance SN.

Sunday: Mainly VFR, with local IFR possible. Strong winds with gusts up to 55 kt. Chance SN.

Sunday Night: VFR. Strong winds with gusts up to 55 kt. Chance SN.

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

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 /Friday through Monday/...

Friday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Freezing spray.

Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 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. Local rough seas. Chance of snow, chance of freezing spray. Local visibility 1 nm or less.

Sunday: storm force winds with gusts up to 60 kt. Rough seas up to 22 ft. Snow likely, chance of rain, chance of freezing spray. Visibility 1 nm or less.

Sunday Night: gale force winds with local gusts up to 45 kt. Rough seas up to 24 ft. Chance of snow, chance of freezing spray. Areas of visibility 1 nm or less.

Monday: Strong winds with gusts up to 35 kt. Rough seas up to 21 ft. Chance of freezing spray, slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.