textproduct: Boston / Norton

WHAT HAS CHANGED

High confidence that cold weather headlines will be needed tonight for portions of the interior.

KEY MESSAGES

- Dangerous cold this morning will moderate a bit this afternoon. Another very cold night tonight with wind chills down to 5 to 15 degrees below zero. Dry weather through Monday.

- Dry and calm to start the week before another disturbance arrives midweek that could bring more unsettled weather.

DISCUSSION

Key Message 1...Dangerous cold this morning will moderate a bit this afternoon. Another very cold night tonight with wind chills down to 5 to 15 degrees below zero. Dry weather through Monday.

Classic shallow arctic airmass with with mixed boundary layer confined to lowest 1k ft with sharp inversion above. Wind chills will bottom out through 8 am at 10-20 below zero in eastern MA, RI and CT and 20-30 below zero across central and western MA. The boundary layer remains quite shallow today but deepens to about 2k ft. We get some moderation today as low level temps increase a bit allowing temps to recover through the teens. But strong pressure gradient will result in gusty winds to 20-30 mph at times. Wind chills will only top out in the single numbers in the afternoon but remaining below zero over the higher elevations.

It will be another very cold night with lows back down into the single numbers and below zero in portions of western MA. With a steady NW wind up to 15 mph, wind chills will bottom out at 5-15 below zero and cold weather headlines will likely be needed for portions of the interior. Airmass recovery continues Monday with highs ranging through the 20s but gusty winds will continue.

Dry weather is expected through Monday. Upper low and cold pool will drift SE and offshore with heights steadily rising through Monday as broad ridging aloft moves east from the Gt Lakes. Surface high pres will only move slightly east from the Gt Lakes keeping a relatively strong gradient across New Eng which will keep winds elevated. Expect mostly sunny days and cleat skies tonight, but ocean effect clouds will linger on Cape Cod and Nantucket through this evening before clearing.

Key Message 2...Dry and calm to start the week before another disturbance arrives midweek that could bring more unsettled weather.

High pressure remains in control Monday night into the daytime hours Tuesday before a clipper system approaches from the NW. Some snow/rain showers are not out of the question sometime Tuesday evening through Wednesday as the center of the surface low passes over northern New England. More southerly flow kicks in sometime Tuesday night as the low approaches, advecting in slightly warmer and more moist air. Ensemble guidance has PWAT values just under 0.5" Tuesday evening and 925 mb temperatures climb closer to 0C, especially closer to the south coast. Overnight lows get closer to the low to mid 20s. This warmer air and southerly flow is short-lived, though, as the system continues its track SE across northern New England. By the daytime hours Wednesday, winds shift back to the NW. 925 mb temperatures settle to around -5C, supportive of daytime highs up into the mid 30s, which would feel much warmer than what we have been dealing with. This system is still multiple days out, so finer details regarding precipitation type, timing, and track are not clear at this time and will very likely change between now and then.

Following this system, high pressure builds in from the Great Lakes, drying the region out for the second half of the week. Ensemble guidance has temperatures moderating a bit and climbing back closer to climatological norms as well as a more climatologically normal airmass settles overhead.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

06z TAF Update...

Through Monday...High confidence.

VFR through the period, but patchy MVFR cigs at times today over the outer Cape and Nantucket where scattered ocean effect showers will linger into this morning before ending. NW gusts 25-35 kt decreasing to 20-25 kt today. NW winds 7-15 kt tonight with 20+ kt gusts along the immediate coast and higher elevations. NW gusts to 20-25 kt again Monday.

Outlook /Monday Night through Thursday/...

Monday Night: VFR. Breezy.

Tuesday: VFR. Chance SN.

Tuesday Night: Mainly VFR, with local IFR possible. Chance SN.

Wednesday: VFR. Breezy. Slight chance SN.

Wednesday Night: VFR. Slight chance SN.

Thursday: Mainly VFR, with local MVFR possible.

MARINE

Forecaster Confidence Levels...

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

Headlines:

* Gale Warnings continue this morning

* Heavy Freezing Spray warnings for Saturday afternoon through tonight.

N-NW gusts to 35-40 kt will gradually diminish below 30 kt this afternoon. SCA gusts to 25-30 kt will continue tonight into Monday morning. Rough and hazardous seas today slowly subsiding tonight into Monday. Areas of moderate to heavy freezing spray will continue through tonight as arctic air combines with rough seas. Heavy freezing spray produces ice accretion rates up to 1 inch/hr creating very dangerous conditions for vessels.

Outlook /Monday Night through Thursday/...

Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Freezing spray.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of snow.

Tuesday Night: Winds less than 25 kt. Chance of snow, chance of rain. Local visibility 1 nm or less.

Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain, slight chance of snow.

Wednesday Night through Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Extreme Cold Warning until 1 PM EST this afternoon for CTZ002>004. MA...Extreme Cold Warning until 1 PM EST this afternoon for MAZ002>004-008>012. Cold Weather Advisory until 1 PM EST this afternoon for MAZ005>007-013>023-026. RI...Cold Weather Advisory until 1 PM EST this afternoon for RIZ001>008. MARINE...Gale Warning until 7 AM EST this morning for ANZ230-233>237. Freezing Spray Advisory until 7 PM EST this evening for ANZ230. Gale Warning until 1 PM EST this afternoon for ANZ231-232-250- 251-254>256. Heavy Freezing Spray Warning until 7 AM EST Monday for ANZ231>237-250-251-254>256.