textproduct: Boston / Norton

SYNOPSIS

An arctic cold front crosses the region this afternoon into early tonight with a few snow showers and a localized snow squall or two, particularly for western and northern MA. Windy conditions and bitterly cold temperatures tonight. The winds quickly diminish by Friday, but it remains unseasonably cold. Scattered snow or rain showers into Saturday with a low pressure system passing well to our south. Another arctic cold front crosses the region Sunday followed by well below normal temperatures Sunday night into Tuesday. A low pressure system approaching from the west may bring some more rain/snow showers by Tuesday night and/or Wednesday.

NEAR TERM /UNTIL 6 AM FRIDAY MORNING/

Key Messages:

* Arctic cold front moves through later this afternoon into early tonight.

* Brief (< 60 min duration) heavy snow showers or possible snow squalls with Arctic front, best chance along the Route 2 corridor but possible as far south as the Mass Pike between 2 and 6 PM. Reduced visibility to 1 mile and light coatings of snow on grassy surfaces in snow squalls.

* Surge of Arctic cold air moves in for all areas after frontal passage, with gusty NW winds (locally as high as 50 mph, especially northern and western MA) thru late evening which then decrease toward midnight.

* Wind chills as low as 10 below zero in the Berkshires and northern Worcester County hills, with wind chills 0 to 10 above zero elsewhere in Southern New England.

Details:

Active, changeable weather pattern over the next roughly 6-12 hours as a strong Arctic cold front moves through Southern New England late this afternoon into early tonight. This frontal boundary is marked by a narrow line of snow showers with visbys reduced to as low as one-half mile trailing from central NH SWwd to near Albany NY. These snow showers/embedded squalls are associated with shallow instability driven by the MUCH colder air which awaits along and behind the front. Expectation is for some scattered snow showers or possible snow squalls mainly for northern and western MA between 2 and 6 PM although the linear structure currently shown in radar mosaic probably fractures/splits. BUFKIT cross sections show some RH in a falling dendrite snow growth region between 19-23z, which could favor reduced visbys possibly impacting the PM commute for these northern and western MA locations in these snow showers/squalls. Temps are currently in the mid 30s to low 40s, so it may be difficult for snow to stick on pavement but in some localized instances it may look like someone shook a snowglobe for a brief (60 min duration or less) period of time.

Post frontal, we get a rapid surge in sea-level pressure rises and strong and deep mixing as surge of much colder air advects in. In the immediate 1-3 hrs after the frontal passage, NW wind gusts could punch into the 40-50 mph range due to the combo of strong pressure rises and the strong mixing/steeper lapse rates with model soundings showing winds at top of the mixed layer in the 45-55 kt range! Because of its short duration which precludes issuing a longer-fused wind headline, instead we opted to treat with a special weather statement in case we do get some reports of minor tree/powerline damage.

While some localized adverse impacts are possible from the above, the main story for all of Southern New England for tonight is a surge of Arctic cold air. 925 mb temps will be falling steadily through the night to values as low as -15C. The combo of sharply falling temps and the northwest wind gusts could support will add a biting cold wind chill through midnight, with wind chills as low as 10 below zero in the Berkshires and northern Worcester County, zero to 5 below zero in northern and western MA, and the single digits to teens above zero elsewhere. Again this is quite a change in airmass so bundle up in layers if you have plans outdoors tonight! The NW winds ease up overnight which may end up easing the windchills too; but this will permit strong radiational cooling to take place leading to very cold ambient temps, especially in those areas in northern, central and western MA that are still under a fresh snowpack from Tuesday's snow event. Even though winds will have eased up, lows by daybreak should be in the single digits above zero in most areas (low teens Cape and Islands), with lows 0 to 5 below zero in northern/northwest MA.

SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/

Key Messages:

* Cold Fri with highs in the 20s, around freezing Cape and Islands with winds turning light southerly.

* Ocean effect cloud cover and possible flurries Outer Cape/Islands Fri thru early aftn.

* Increasing clouds Fri night, early lows before warming toward daybreak Sat.

Details:

Friday:

Despite full sun for most, Friday is nonetheless a cold and dry day, coming out of a frigid overnight.

With a favorable wind trajectory for ocean-enhanced cloud cover developing in the pre-dawn hours, along with the unseasonably cold airmass in place driving some shallow instability off the milder ocean, we should see bands of ocean effect clouds and perhaps some scattered flurries at times underneath them across the Outer Cape and Nantucket. NWly winds then shut off by early afternoon to more of a light southerly direction, which will shut off the ocean-effect generation over the Outer Cape. But we will see cloud cover increase late Fri, mainly along the south coast.

Highs Fri are only in the 20s, with readings around 30-33 for the Cape and Islands.

Friday Night:

Modifying warm/moist advection pattern takes place Fri night, with weak ridging remaining in place as low pressure passes well south and east of the mid-Atlantic waters. Weak southerly flow will allow for 925 mb temps to rise to around -3 to -6C, with an advancing shallow layer of lower clouds northward from the southern coastal waters to the Mass Pike. Some models show some very spotty light precip in this return flow; I'm skeptical this will amount to anything, but if it is light enough to stay in liquid we could see some slick spots develop along the south coast, Cape and Islands coming out of the frigid airmass. Early lows in the mid teens to lower 20s, before temps rise into the 20s to near freezing along the south coast.

LONG TERM /SATURDAY THROUGH THURSDAY/

Key Messages...

* A few scattered rain/snow showers possible Saturday.

* More arctic air arrives Sunday night into the early part of next week.

* A few chances for precip first late Sunday/Sunday night then again around mid week.

Overall a benign stretch of weather coming up, the main thing of note being another shot of cold air early next week. The synoptic pattern features quasi-zonal flow at the mid levels much of the period punctuated by periodic weak shortwaves moving through the flow until a deeper disturbance arrives middle of next week.

To start the weekend high pressure exits east and SNE will be sandwiched between a trough of low pressure passing through northern New England and a surface low passing well to the south. The best forcing with both features is removed from our region and moisture is marginal so while we could see some light flurries/showers on the Cape islands early and passing rain/snow showers later in the day, do not expect these to be widespread or impactful. The exception could be if any lingering moisture on the roads freezes up Saturday night with the drop in temperatures making for slick roads. Another arctic front swings through Sunday night and while guidance disagrees on how much moisture will accompany it, the forecast leans toward a drier frontal passage. If we have showers around, neither Saturday or Sunday night are expected to be very wet; ensemble guidance indicates a 40-70% chance of a hundredth of an inch of QPF each day, with a near zero chance of a tenth of an inch. The fly in the ointment is the GFS which is quite a bit wetter and indicates snow Sunday night. Something to keep an eye on but for now it is the only source showing anything like this. Confidence decreases with regards to how the pattern evolves after Monday but a more active pattern looks likely with several potential shortwave disturbances Tue-Thu.

Temperature-wise, we'll moderate quite a bit on Saturday and Sunday from Friday, with highs back into the upper 30s to low 40s. Then, the arctic front Sunday night brings the return of highs in the 20s and 30s and lows in the single digits and low teens Monday night. This will be a progressive (short lived) cold airmass, though.

AVIATION /19Z 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 00z Friday: High confidence in TAF/trends but moderate on coverage of snow showers/squalls.

VFR for most of this period. An Arctic cold front could bring brief convective snow showers or possible snow squalls from BAF to BOS north and west mainly 19-23z. Indicated with VCSH and will proactively AMD for tempo SHSN & IFR visbys. If any visbys were to be reduced it would mainly be for an hour or less, with any accumulation limited to coatings due to their brief nature. We then get a gusty NW windshift upon frontal passage, with speeds 15-20 kt and gusts 32-38 kt.

Tonight: High confidence.

VFR. Gusty NW winds 30-35 kt thru 04z, then a slow decrease in gusts to around 20-25 kt overnight. After 08z, could see some MVFR ceilings over the Outer Cape and Nantucket (mainly HYA/ACK) with possible flurries at times due to ocean effect cloud cover/narrow bands of ocean effect precip.

Friday: High confidence.

VFR with ocean effect VFR/MVFR ceilings HYA/ACK and the Outer Cape thru about 16-18z as winds remain NW around 5-10 kt. Windshift to S during the afternoon will shut off ocean effect cloudiness, but will begin to bring a layer of VFR/MVFR bases from the ocean waters to BID and perhaps immediate south coast, Cape and Islands by sundown.

Friday Night: Moderate to high confidence.

VFR initially, though a shallow layer of VFR/MVFR cloud decks advance northward from the southern waters. Low prob of SHSN or possible very spotty FZDZ south of PVD underneath this as layer aloft is dry. Light south winds.

KBOS Terminal...High confidence in TAF. Low for SHSN chances.

KBDL Terminal...High confidence in TAF. Moderate for SHSN chances.

Outlook /Saturday through Tuesday/...

Friday Night: Mainly VFR, with local MVFR possible. Slight chance SN.

Saturday: Mainly VFR, with local MVFR possible. Slight chance RA, slight chance FZRA, slight chance SN.

Saturday Night through Sunday: VFR.

Sunday Night through Monday: VFR. Breezy. Slight chance SN.

MARINE

Forecaster Confidence Levels...

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

* Gale Warnings in effect for all waters tonight

A strong cold front will push over the waters tonight, bringing with it a surge in NW wind gusts to around 35-40 kt along with much colder air. Wouldn't rule out occasional instances of light freezing spray but the risk tends to not be significant given that water temps are still in the mid 40s. NW gusts will begin to subside overnight tonight, with SCAs likely to be needed into Fri AM. Gusts and seas should subside to below SCA levels into later Fri AM.

Dry weather should generally predominate, but there may be some ocean effect clouds with possible flurries or ocean effect snow showers that could briefly/locally reduce visibility overnight tonight into midday Fri. Winds turn southerly around 10-15 kt Fri night.

Outlook /Saturday through Tuesday/...

Friday Night: Winds less than 25 kt. Slight chance of rain.

Saturday: Winds less than 25 kt. Chance of rain.

Saturday Night: Winds less than 25 kt. Slight chance of rain.

Sunday: Winds less than 25 kt.

Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of snow, slight chance of rain.

Monday: Strong winds with gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain, slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Gale Warning until 1 AM EST Friday for ANZ230-236. Gale Warning until 4 AM EST Friday for ANZ231>235-237-250-251- 254>256.