textproduct: Boston / Norton

WHAT HAS CHANGED

Extreme Cold Watch has been expanded to all of northern Connecticut and for western and central Massachusetts.

KEY MESSAGES

- Dry weather through Friday. Snow showers late Friday night, lingering through Saturday evening.

- Dangerous cold Saturday night into Sunday, along with hazardous marine candidness.

DISCUSSION

KEY MESSAGE 1...Dry weather through Friday. Snow showers late Friday night, lingering through Saturday evening.

High pressure remains in place through Friday bringing dry conditions. Low temperatures overnight will depend on how quickly the moisture/clouds move in overnight. Should be clear to start the night, allowing efficient radiational cooling. As long as clouds to arrive too early, lows should drop into the single digits and teens perhaps a few below zero readings in the higher elevations of the interior. For Friday, temperatures range in the mid to upper 20s in most areas, a little below normal for this time of year.

An arctic front is expected to sweep across the region Sat morning and afternoon. During this time, a low pressure system tracks offshore with an accompanying inverted trough shifting across portions of the region. The main result of this set-up will be light showers developing early Saturday morning, continuing through the day Saturday with showers lingering across the east coast/Cape & Islands through Saturday evening. Overall this event is looking to be minor impact with light accumulations of fluffy snow. Ensembles and the NBM show high probabilities 1-3" inches of snow for most of southern New England by Saturday night.

Where it gets a little more detailed and lower confidence will be with the positioning of the inverted trough and the ocean- effect/enhancement. These will be important details when it comes to assessing the potential for locally higher snow amounts. Ocean- effect and enhancement is likely with surface winds shifting NE to N Saturday ahead of an approaching arctic front with NW winds in its wake. The convergence alone may act as an enhancer for snowfall. This will favor the coastal areas that are typically impacted by ocean-effect (east coast/Cape & Islands). There is still uncertainty where exactly the bands/enhancement will set up, especially with respect to the east coast (e.g. Boston Logan Airport, Plymouth). Models continue to indicate an inverted trough accompanying the offshore surface low extending through southern New England. This could be another forcer for snowfall enhancement; however, the positioning and strength of the inverted trough less certain which will influence the amounts and where those more enhanced amounts could end up. Model soundings show a favorable environment overall with a period of decent lift in the snow growth region and marginal instability, more so further east. Ensembles highlight probabilities for 3"+ across central southern New England and the east coast/Cape & Islands with overall more consistency with probabilities over the east coast/Cape. This aligns with where we are seeing potential positioning for mesoscale features, so the localized higher amounts will depend on where these set up. So bottom line, 1-3" is looking most likely with potential for localized 3"+ for areas. Held off on advisories for now given the uncertainty in where these localized mesoscale enhancements will set up.

Quickly falling temperatures behind the arctic front, marginal instability, and onset of gusty winds raises concern for a few snow squalls as the front moves through Saturday. Higher risk looks to be western southern New England into central portions as well.

KEY MESSAGE 2...Dangerous cold Saturday night into Sunday, along with hazardous marine candidness.

Dangerous cold expected Saturday evening through Sunday as an Arctic front pushes through. 925mb temperatures lowering to -20C across the Berkshires and elsewhere are between 10-15C below zero. Majority of the region experiences minimum temperatures -5F to +5F, while at the coast, those temperatures are 5F-15F. Overnight, pressure gradient increases due to high pressure the north and a deepening area of low pressure to the southeast. Factoring the cold temperatures with a northwest gusts between 30-40 mph, wind chill values are as low as 25 to 30 below zero across western and central Massachusetts. Major urban centers like Boston, Providence, Hartford, and Worcester are "less" cold, but still dangerous, wind chill values are 10-20 degrees below zero. There is little improvement during Sunday, with daytime temperatures reaching the teens and sub-zero wind chills.

Have expanded the Extreme Cold Watch to include all of western and central Massachusetts, additionally, all of northern Connecticut. Areas currently not under a watch, will likely have Cold Weather Advisories issued in the next 24-36 hours. Either way you look at it, spending any duration outdoors Saturday evening through Sunday morning should be considered dangerous, wear multiple layers, and don't leave any bare skin exposed.

During this time, it's worth reminding boater there is potential for heavy freezing spray across all the waters as icing rates could reach 0.5-1.0 inch per hour. This has prompted a Heavy Freezing Spray Watch for all waters with exception of Boston Harbor.

Below normal temperatures continue into early next week, daytime highs are in the teens and 20s on Monday, near 30 on Tuesday, and trending towards the middle 30s for Wednesday and Thursday. Outside of a weak clipper system on Monday night there is fairly quiet conditions early next week. Will need to monitor a mid to late week system, though there is low confidence in this part of the forecast due model disagreement on placement and timing. Stay tuned.

AVIATION /20Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

Through Friday...High confidence.

Mainly VFR through the period. Cigs around 3-5k ft develop from E to W late tonight into Friday with some localized MVFR possible. NW winds 10-15 kt today, becoming light NNW/N tonight. NNE winds Friday 5-10 kts.

Outlook /Friday Night through Tuesday/...

Saturday: MVFR/IFR conditions possible. Windy with gusts up to 30 kt. SN.

Saturday Night: VFR. Windy with gusts up to 35 kt. Chance SN.

Sunday: VFR. Windy with areas of gusts up to 30 kt.

Sunday Night through Monday Night: VFR. Breezy.

Tuesday: VFR.

MARINE

Forecaster Confidence Levels...

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

Headlines:

* Gale Watch remains in effect 1 PM Saturday to 1 PM Sunday.

* Heavy Freezing Spray Watch in effect 4 PM Saturday to 7 AM Monday.

Tonight through Friday Night... High Confidence.

No significant weather impacts are expected tonight and Friday as an area of high pressure remains in control. A clipper system late Friday night will bring snow showers. During this time, seas are less than 3 feet, with gusts below advisory criteria, not anticipating any freezing spray for the rest of Thursday through Friday night. The threat for freezing spray returns on Saturday.

Outlook /Friday Night through Tuesday/...

Saturday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Snow likely. Visibility 1 to 3 nm.

Saturday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Freezing spray, chance of snow. Local visibility 1 nm or less.

Sunday: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft. Freezing spray, slight chance of snow.

Sunday Night: Strong winds with areas of gusts up to 30 kt. Areas of rough seas. Freezing spray.

Monday through Monday Night: Winds less than 25 kt. Seas up to 5 ft. Freezing spray.

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Extreme Cold Watch from Saturday evening through Sunday afternoon for CTZ002>004. MA...Extreme Cold Watch from Saturday evening through Sunday afternoon for MAZ002>004-008>012. RI...None. MARINE...Gale Watch from Saturday afternoon through Sunday afternoon for ANZ231>235-237-250-251-254>256. Heavy Freezing Spray Watch from Saturday afternoon through Monday morning for ANZ231>237-250-251-254>256.