textproduct: Boston / Norton

WHAT HAS CHANGED

Confidence continues to increase for a prolonged Arctic outbreak beginning at the end of this week into this weekend. Confidence remains low in the late weekend coastal storm.

KEY MESSAGES

- Cold and dry tonight through Tuesday night. Wind chill index near or below zero tonight through Wednesday

- Near-normal to mild temperatures midweek. A weak clipper system brings low chances of higher terrain snow showers and light rain for the coastal plain.

- Increasing confidence in an Arctic outbreak beginning Friday night with dangerous cold and wind chills, while low confidence remains in a late- weekend coastal storm.

DISCUSSION

KEY MESSAGE 1...Cold and dry tonight through Tuesday night. Wind chill index near or below zero tonight through Wednesday

A cold front will bring an arctic air mass to the region tonight and last through Tuesday night. Strong CAA will bring 850mb temps down to -20C by Tuesday morning, resulting in an extended period of sub- freezing temperatures starting at sunset tonight. Any melted snow from today will quickly refreeze tonight, and untreated surfaces could become slippery. Temperatures take a dive off the deep end tonight, dropping into the single digits to low teens across the region. With winds picking up to 20-30mph from CAA, the wind chill index/feels-like temperatures will drop to near or slightly below zero across the region. CAA continues into tomorrow, and temperatures only warm into the mid-teens in the west and mid-20s in the east. Gusty winds also continue into Tuesday with gusts of 15- 25mph. Wind chill index will make it feel like the single digits to low teens outside. Winds drop off Tuesday night as high pressure builds in just to the south. With clear skies, light winds, and fresh snowpack, overnight temperatures will drop into the single digits across the region, with low teens near the coasts.

KEY MESSAGE 2...Near-normal to mild temperatures midweek. A weak clipper system brings low chances of higher terrain snow showers and light rain for the coastal plain.

After a brief shot of Arctic air temperatures moderate for Wednesday and Thursday, this period features a chance of precipitation mainly between Wednesday evening and Thursday. Driving the weather midweek are a mid-level low over Hudson Bay with embedded shortwave energy rotating around the cyclonic flow. This supports development of a weak Alberta clipper tracking southeast from the Canadian Rockies towards the northeast. Ahead of it, a developing southerly LLJ with wind speeds 30 to 50 knots, which advects a somewhat warmer air mass into southern New England. Near-normal temperatures Wednesday and an increasing S-SSW wind. Warm advection rain and/or higher terrain snow showers are possible, 20-35 POP, from late Wednesday through the first-half of Thursday. As typical with most clippers, moisture content is limited, as PWATs are between 0.2" and 0.4". With little downstream blocking, the system is progressive. Ensembles guidance remains unimpressive, with probabilities for snowfall exceeding one inch generally less than 20% range in interior southern New England. The LLJ weakens Thursday, still fairly mild for mid-January as highs return to the upper 30s and low 40s.

KEY MESSAGE 3...Increasing confidence in an Arctic outbreak beginning Friday night with dangerous cold and wind chills, while low confidence remains in a late-weekend coastal storm.

While still several days away, confidence continues to increase in a significant Arctic outbreak impacting southern New England beginning late Friday night and persisting through next weekend. As noted in prior discussions, a lobe of the Polar Vortex is forecast to break off and drive much colder air into the region. This cold air mass does not appear to be short-lived, with the Climate Prediction Center (CPC) highlighting much of the Northeast, Upper Midwest, and Great Lakes under a moderate risk for much below normal temperatures into the middle of next week. For the upcoming weekend, overnight low temperatures and wind chills may fall to around -20F in the higher terrain and -10F across the coastal plain. Raising the potential for Cold Weather Advisories. The Magnitude of this cold is notable, with the forecast 925mb temperatures on the order of -25C to -30C, more typical of 500mb. Those with outdoor plans this weekend should continue to monitor the forecast closely. Given these conditions, mariners should be prepared for a high likelihood of freezing spray, though the extent and severity remain uncertain at this time.

In addition, there are indications of a potential coastal system late next weekend. While some guidance suggest a system may pass offshore Sunday into Sunday night, confidence remains low. Recent AI-based guidance favors greater impacts across southern New England; however, QPF may be overstated and may not adequately resolve a sharp northern gradient. For now, maintained low-end PoPs, reflecting only a slight chance for snow showers.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

Today: High Confidence.

VFR. West winds gusting 20-25 knots, becoming light after sunset.

Tonight: High Confidence.

VFR. West winds 5-10 knots.

Wednesday: High Confidence.

VFR. Southwest winds 5-10 knots. KBOS Terminal...

VFR. Gusty W winds 23-27 knots through this evening.

KBDL Terminal...

VFR. Increasing W winds through this evening with gusts of 20-25 knots.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance RA, slight chance SN.

Thursday through Friday Night: VFR. Breezy.

Saturday: VFR. Breezy. Slight chance SN.

MARINE

Forecaster Confidence Levels...

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

Rest of Today through Tuesday Night...

Cold front will bring a period of strong westerly winds to the waters tonight through Tuesday afternoon with gusts of 30-35 knots. Seas increase tonight highest in the southern waters at 10-13 feet, and 7-10 feet in the northern waters. Winds decrease quickly Tuesday night, with gusts dropping below 20 knots. Seas also gradually diminish to 4-6 feet. With cold arctic air moving offshore, expecting light freezing spray both tonight and Tuesday night as air temps drop into the 20s.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Strong winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain.

Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain.

Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas.

Friday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Seas up to 5 ft.

Friday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. Freezing spray likely, slight chance of snow.

Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Freezing spray likely, slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ230- 236. Gale Warning until 9 AM EST this morning for ANZ231>235-237- 250-251-254>256.