textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes. Confidence remains high on well below normal temperatures this week with another surge of Arctic air Thursday night into Friday. Low confidence on potential impactful winter storm Sunday.

KEY MESSAGES

- Mainly dry with well below normal temperatures this week, and potentially hazardous wind chills Thursday night into Friday.

- Monitoring a possible late-weekend coastal storm. Uncertainty remains large in the track.

DISCUSSION

KEY MESSAGE 1...Mainly dry with well below normal temperatures this week, and potentially hazardous wind chills Thursday night into Friday.

Fairly active northern stream through the end of the week as a series of moisture starved shortwaves move through the region. A few snow showers will be possible through the early evening with a shortwave passage, then we have another shortwave moving through Wed night with a low risk for a few flurries. In between the 2 shortwaves a period of shortwave ridging will provide mostly sunny skies on Wednesday.

Arctic air will remain entrenched across the region this week with 850 mb temps -15C to -20C. Highs through Thu will be in the teens to lower 20s with lows mostly in the single numbers, while overnight and morning wind chills will be below zero. A third shortwave will drop down Thu night into Fri bringing a reinforcing shot of arctic air Thu night through Fri night. In fact we may see widespread below zero temps late Fri night and early Sat. The potential exists for dangerously cold wind chills which will likely require cold weather headlines from Thu night and Fri night.

KEY MESSAGE 2...Monitoring a possible late-weekend coastal storm. Uncertainty remains large in the track.

We continue to monitor the potential for a coastal storm late next weekend. Deterministic and ensemble has remained generally consistent and agreeable with development of a coastal storm off of the East Coast this upcoming weekend. Despite this consistent signal for a coastal storm over the past runs, there is plenty of uncertainty. Very normal this far out.

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. Where model solutions start to diverge is with degree of amplification of the trough and whether the 500mb low closes off. This is present in the WPC Cluster Analysis with half of the grand ensemble with a closed 500mb low. Within that guidance, there are solutions that favor a closed low shifting across the Ohio Valley, potentially shifting as south as the Georgia/Carolinas before it tracks back northward. This amplified trough and possible low will keep cold air locked in across the northeast through the weekend. Surface coastal low development has been consistent across guidance, but with solutions even showing deep surface low development(sub-990mb) off the East Coast. The ingredients are there for deep low development, with cold air in place interacting with a warm/moist air with air-sea interactions helping to enhance. As expected, given the uncertainty in the details of the pattern, there is large variance in the track of the surface low evidence across ensemble guidance. Some solutions track around the 40N/70W benchmark, but there is still spread with how close to shore the low tracks with a chunk of solutions showing an offshore pass. This will heavily influence the amounts and impacts with this potential storm.

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 could depend on the small-scale details/thermodynamics of the system as it evolves. Another thing to keep monitoring...

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. Glancing the upper percentiles of ensemble guidance, aside from the fact the spread is large between them, the 75th/90th percentiles (worst case scenarios of the ensemble suite) indicate potential for an impactful snow storm. The Probabilistic WSSI indicates probabilites 20-50% 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 (strong winds, coastal flooding). Ensembles indicate moderate probabilities for gusts greater than 40 mph across the waters and southeast coastal areas.

We are 5-6 days out still, so there is plenty of uncertainty and we expect to see continued spread and changes from run to run over the next few days. The important thing is to not stay locked in on a solution, especially this far out. This is best seen from a view, leveraging the full range of possibilities. As we go through the week, ensembles should begin to converge on more likely solutions/scenarios which will be messaged accordingly alongside the other potential (lower probability) scenarios.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

00Z TAFs: High confidence.

Scattered moderate to heavy snow showers possible this evening through 01z for inland terminals. 03z for the Cape and Islands. These snow showers could reduce visibilities down to 1SM or less. Otherwise VFR this evening. There will be a brief period of gusty winds this evening at 20-25 knots then decreasing westerly winds overnight. West winds at 10-15 knots Wed.

KBOS Terminal...High Confidence in TAF.

Snow showers possible this evening through 01z, otherwise VFR tonight. Brief period of gusty winds possible tonight before decrease winds overnight. VFR Wed with west winds around 10-15 knots.

KBDL Terminal...High Confidence in TAF.

VFR tonight. West winds around 10-15 knots through Wed

Outlook /Thursday through Sunday/...

Thursday through Friday Night: VFR. Breezy.

Saturday: VFR.

Saturday Night: Mainly VFR, with local MVFR possible. Strong winds with local gusts up to 40 kt. Chance SN.

Sunday: Mainly VFR, with local IFR possible. Strong winds with gusts up to 45 kt. Chance SN.

MARINE

Forecaster Confidence Levels...

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

Another surge of gusty W winds late this afternoon into tonight with the passage of a shortwave. Gusts to 25-35 kt likely with strongest gusts on southern waters where Gale warnings are in effect. SCAs in effect elsewhere. Winds diminish late tonight through Wed.

Freezing spray advisory expanded to much of the nearshore waters for a period of moderate freezing spray tonight into Wed morning. Another round of widespread moderate to perhaps heavy freezing spray is possible Thu night into Fri.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Freezing spray, slight chance of snow showers.

Thursday Night through Friday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Freezing spray.

Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Freezing spray.

Saturday: Winds less than 25 kt. Slight chance of snow.

Saturday Night: Moderate risk for gale force winds with gusts up to 45 kt. Areas of rough seas. Chance of snow, chance of freezing spray. Local visibility 1 nm or less.

Sunday: gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Snow likely, chance of rain. Visibility 1 nm or less.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 4 AM EST Wednesday for ANZ230-236. Freezing Spray Advisory until 10 AM EST Wednesday for ANZ231>235-237-251-254-255. Small Craft Advisory until 7 AM EST Wednesday for ANZ231>235- 237-251. Small Craft Advisory until 10 AM EST Wednesday for ANZ250. Gale Warning until 1 AM EST Wednesday for ANZ254>256.