textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes needed to the forecast from the previous update.

KEY MESSAGES

- Rain develops late this afternoon for most. Light- accumulating icing in northern and western interior sections of Southern New England, resulting in slippery roads/sidewalks. Precipitation ends by the pre-dawn Sunday hours for most, but rain lingers over Cape and Islands thru Sunday morning.

- Turning windy and much colder Sun night into Mon, with hit- or-miss snow squalls early Sun night and increasing wind gusts into the 35- 50 mph range late Sun night into early Mon.

- Dry weather with temperatures returning to above normal levels through midweek. Monitoring a potential coastal storm system late next week.

DISCUSSION

KEY MESSAGE 1...Rain develops late this afternoon for most. Light- accumulating icing in northern and western interior sections of Southern New England, resulting in slippery roads/sidewalks overnight. Precipitation ends by the pre-dawn Sunday hours for most, but rain lingers over Cape and Islands thru Sunday morning.

A quasi-stationary frontal boundary draped eastward across the mid- Atlc region is associated with a shield of rain from just south of NYC westward through the Alleghany Mtns in PA. This feature returns northward as a warm front late this afternoon to early tonight, roughly between 4 to 8 PM from the south coastal waters to northern MA, spreading its precipitation shield northward.

Temperatures should remain warm enough for most of Southern New England for a period of steady light rain, to at times moderate rains in RI and SE MA. Further north and west into the interior higher terrain areas (Berkshires, northwest Middlesex and northern Worcester Counties), the combination of a cold air damming/drainage signature indicated in most model sea level pressure/sfc temp fields and initial wet-bulb cooling, temperatures hovering around the 30 to 32F range support a period of light-accreting freezing rain tonight into the overnight hours. Warm nose between 850 mb as much as 6 to +10C advecting in. Following the FRAM methodology, rather light winds and light precip rates should help to accrete ice readily, although sfc temps are rather borderline and this area is more removed from where the heavier QPF will be. Winter Weather Advisories remain posted for the east slopes of the Berkshires and northern Worcester and northwest Middlesex Counties; overall not much change to forecast flat ice accumulations of around a tenth to two tenths of an inch, resulting in slippery untreated sidewalks and primary/secondary roads. Will also mention that due to the presence of a shallow layer of sub-freezing air around 925 mb (as low as -2C) below the warm nose likely from the initial wet-bulb cooling, areas in the Merrimack Valley, North Shore and perhaps into the Assabet Valley and part of Metrowest could see rain mixed with re-frozen ice pellets with sfc air temps in the 35-37F range, before turning to plain rain. This wouldn't be of any impact though, but it wouldn't shock to hear of reports of sleet mixed with rain in those locations until the shallow sub-freezing layer underneath the warm nose modifies. Temps to be slowly warming to above-freezing levels by Sun morning.

Although rain lingers along the Cape and Islands into Sun morning, the rest of Sun trends dry with slowly modifying temps amid weaker cool advection. The real punch of colder air arrives later Sun/Sun evening, to be discussed below.

KEY MESSAGE 2...Turning windy and much colder Sun night into Mon, with hit-or-miss snow squalls early Sun night and increasing wind gusts into the 35-50 mph range late Sun night into early Mon.

Rather potent 500 mb shortwave trough and associated 925 mb cold front races through Southern New England late Sunday night, thinking timing from around 7-9 PM into the Berkshires and around 9 PM to midnight toward the eastern MA coast. Even though temps will be steadily falling, steep lapse rates from sfc to nearly 800 mb generate enough shallow instability to support a risk for scattered, but brief, passing snow showers and possible snow squalls with the cold front's passage. Snow Squall Parameter values are rather high in the 1-3 unit range largely from the steeper lapse rates, although moisture is lacking. For context, I don't view this as a higher-end snow squall scenario like what happened on the morning of New Years' Day which should still be somewhat fresh in peoples' minds, but there could be some reduced visbys with coatings of snow.

West-northwest wind gusts then pick up substantially post-cold front, with efficient mixing due to the colder air working in aloft. BUFKIT profiles still show speeds at top of the mixed layer in the 40 to 45-kt range, with gusts developing in most areas late Sunday night to around midnight. Peak of the gusts take place overnight to early Monday, with speeds in the 35-45 mph range with occasional gusts around 50 mph around the higher terrain. Wouldn't rule out the need for wind advisories for the higher terrain but it still looks too borderline for those as yet; depending on how much freezing rain develops tonight/early Sunday, that is a factor which could tilt a decision towards issuing one. Nonetheless, Sunday night turns windy and cold, with apparent temps around 5-15 degrees above zero.

KEY MESSAGE 3...Dry weather with temperatures returning to above normal levels through midweek. Monitoring a potential coastal storm system late next week.

Our weather pattern then turns quieter as we move into early next week. It looks as though the period of colder air that settles in Sunday night into Monday is short-lived. Dry weather, easing winds and temperatures recovering back toward above normal levels (upper 30s to lower-mid 40s) Tue/Wed.

Will be monitoring developments pertaining to a possible coastal storm system around the Thu/Fri timeframe. Weather pattern across CONUS becomes amplified late in the week with a deep upper level ridge over the Pac NW and a downstream amplified trough from eastern Canada southward to the TN Valley/Gulf region. Teleconnections have been keying on this late-week period as a potentially favorable one for possible coastal storm development and is one we'll have our eyes on. While there are some deterministic solutions which show a more impactful storm and some others which are shutouts, ensemble means still depict a very broad area of possible development, ranging from offshore the Carolinas to as far north as the 40/70 benchmark. There is still too much uncertainty in the specific details regarding this potential storm's track, location and strength, and those details will in part hinge on the position and tilt of the deep upper trough and if an embedded shortwave in the OH/TN Valleys can close off early enough to pull any offshore storm closer to the coast. Because of the uncertainty I didn't make many changes to NBM at this point, and will let more coherent trends in the guidance dictate adjustments. Still, this is a period we're going to need to keep close watch of.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

TAF Update: 18z

Rest Today... High confidence

VFR through the afternoon. Rain begins to move in from SW to NE tonight. Lower IFR/LIFR CIGS will lag behind the rain with dry low level air. Winds turn light easterly.

Tonight... High confidence

Moderate to heavy rain will help saturate the low levels between 02-04z tonight, allowing CIGS to turn IFR/LIFR. CIGS remain IFR/LIFR overnight after the rain ends due to light ENE flow. Rain ends between 06-08z tonight with showers lingering into Sunday morning. Significant LLWS concerns for the Cape and Islands tonight as 2kft winds approach 60 to 70 knots from the SW while a steep low level inversion could keep surface winds under 20 knots from the SE.

Sunday... High confidence in trends, moderate confidence in timing.

Winds turn WNW Sunday morning, but CIGS may be slow to improve until Sunday afternoon when drier low level air begins to work in behind the low. Rain showers and low CIGS will linger for the Cape and the Islands for much of the day.

KBOS Terminal... Moderate Confidence.

VFR with winds shifting east this afternoon. Rain will reach the terminal between 23-00z, however IFR/LIFR CIGS will lag behind the rain due to low level dry air. Moderate rain should reach the terminal around 03z along with the IFR/LIFR CIGS. CIGS remain IFR/LIFR through mid Sunday morning with lingering showers and drizzle. Only moderate confidence on when CIGS improve on Sunday. Current expectation is CIGS wont improve until late Sunday morning to early afternoon.

KBDL Terminal... Moderate Confidence.

VFR with light and variable winds this afternoon. Rain moves in between 21-22z this evening. IFR CIGS will lag behind the rain until closer to 02z when moderate rain move in. Rain ends between 06-07z, however showers could linger until day break. CIGS improve to MVFR/VFR mid to late Sunday morning.

Outlook /Sunday Night through Thursday/...

Monday: VFR. Strong winds with local gusts up to 40 kt.

Monday Night through Tuesday Night: VFR. Breezy.

Wednesday: Mainly VFR, with local IFR possible. Breezy. Slight chance RA.

Wednesday Night: Mainly VFR, with local MVFR possible. Chance RA, chance SN.

Thursday: VFR. Slight chance RA, slight chance SN.

MARINE

Forecaster Confidence Levels...

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

Tonight through Sunday night: High confidence.

A stationary frontal boundary will bring a rain to the coastal waters this afternoon through much of Sunday. Winds tonight will be out of the ESE at 10-20 knots with gusts up to 30 knots possible in the southern waters. Seas increase to 4-7 feet. Winds turn West on Sunday and increase to 30-40 knots by Sunday evening. Gale watch has be converted to a gale warning for Sunday night into Monday morning. Seas increase to 7-10 feet Sunday night.

Outlook /Sunday Night through Thursday/...

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas.

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft.

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

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas.

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

Wednesday Night: Winds less than 25 kt. Local rough seas. Chance of rain.

Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Winter Weather Advisory from 5 PM this afternoon to 10 AM EST Sunday for MAZ002-004-008-009-026. RI...None. MARINE...Gale Warning from 7 PM Sunday to 7 AM EST Monday for ANZ230>237-251. Gale Warning from 7 PM Sunday to 10 AM EST Monday for ANZ250- 254>256.