textproduct: Boston / Norton

WHAT HAS CHANGED

Axis of heaviest precipitation shifted slightly south. This led to slightly lower snowfall totals for portions of central and western MA. Winter Weather Advisories continue this afternoon into tonight.

Confidence has increased for a period of impactful snow and mixed precipitation Friday night into Saturday. Highest accumulations look to be along and north of the Mass Pike. Guidance has trended further southeast for a coastal storm that could impact the region Sunday into Monday though it is still quite early and confidence remains low.

KEY MESSAGES

- Some travel impacts from accumulating snow this afternoon and evening.

- Complex system brings a variety of precipitation types Friday and Friday night including snow, freezing rain, and rain. Scattered snow showers linger into Sunday.

- Watching a potential winter storm Sunday into Monday.

DISCUSSION

KEY MESSAGE 1...Some travel impacts from accumulating snow this afternoon and evening.

Very much a tricky forecast this afternoon into tonight. Much of the deterministic guidance shifted the axis of heaviest precipitation farther south, especially some of the high resolution models such as the HREF and 3-km NAM. Meanwhile, the HRRR and RAP develop a secondary axis of heavier precipitation this afternoon into this evening. For this forecast, followed the consensus trend, but remain wary of the possibility of the secondary band.

Snowfall accumulations were trended downward slightly. The shift of the precipitation toward higher temperatures means a wetter snowfall, which reduces the overall amount. Much preferred the HREF representation of precipitation types, as the NationalBlend solution had entirely too much freezing rain. This means snow for the higher elevations of central and western MA, with rain most likely elsewhere at the onset this afternoon. As temperatures lower toward this evening, there may be a brief period of freezing rain or sleet before the transition to snowfall is complete.

Continued the Winter Weather Advisory across central and western MA for 2-4" of snowfall. Contemplated expanding an advisory into northern CT and southern RI due to the possibility of icing. Decided against it as confidence in widespread icing is low as well as the window of time when icing would be possible. This may change as more details come into better focus today.

That said, confidence is a bit lower farther east, including the Boston and Providence areas, since temperatures initially may be too warm to allow snow to accumulate. We still think 1-2" snowfall totals are more likely in these areas.

Precipitation ends sometime late this evening. It is quite possible the Winter Weather Advisory can be canceled earlier than its current end timing.

KEY MESSAGE 2...Complex system brings a variety of precipitation types Friday and Friday night including snow, freezing rain, and rain. Scattered snow showers linger into Sunday.

Brief ridging Thursday gives way to an active and unsettled period of weather Friday and Friday night. Upper level low ejects over the Great Lakes pushing a surface warm front NE towards the region. Guidance has also hinted at a secondary coastal low developing just south of southern New England Friday night. More on that second detail below. Thermal profiles lean towards a mixed precipitation event as model soundings are resolving a significant warm nose associated with the warm front and subsequent strong WAA.

Models also show the potential for a front end "thump" of snow across the interior Friday afternoon with decent values of omega in the DGZ (-15-20 ubar/sec). P-type forecast looks a bit more straightforward across the south coast as marginal temps may lead to a mostly rain event while a mid level warm nose also introduced the likelihood of freezing rain for parts of the interior. The northward extent of FZRA will be determined by the magnitude and eventual location of the mid level warm nose. Details that will be ironed out in future forecast updates.

The 00z model suite shows a steady increase in the number of ensembles that show secondary low formation near or S of the Cape and Islands Friday night. The formation of even a weak area of low pressure would help to pull colder air farther south and keep precipitation as a wintry mix and snow across northern Massachusetts, mainly north of the Pike. Similar to the previous discussion, the chance for 4+ inches of snow through Saturday along and south of the MA Pike is between 15-30% and 40-55% chance to the north.

KEY MESSAGE 3...Watching a potential winter storm Sunday into Monday.

Guidance continues to show a potentially significant winter storm developing off the East Coast Sunday into Monday, but as usual at this time range, we are seeing a variety of solutions and high run to run variability even among ensembles.

Much of the uncertainty stems from how quickly a northern stream shortwave can close off as it tracks through the Great Lakes and toward the East Coast, which is dependent upon the strength of the upper ridge over the North Atlantic. A stronger ridge, as seen in previous GFS and Canadian runs, would help the low to close off more quickly and gain a negative tilt, allowing the coastal storm to track closer to southern New England with higher impacts. Conversely, a weaker ridge, as depicted by the more recent 00z guidance, would mean the upper low closes off later and maintains more of a positive tilt, keeping the coastal storm more out to sea. It's also interesting to note the AI versions of GFS and ECMWF, which have performed well this winter, are fairly similar and continue to bring the low close enough to impact at least the South Coast, Cape Cod, and Islands.

Right now odds favor at least some impact to CT, RI, and SE MA if not more of the area, but keep in mind we are several days out and probably won't see any notable trends in guidance until Thursday at the earliest. So for now, expect to see the usual run-to-run variations in the deterministic models and some showing big hits. It's just way too soon to lock into any one model solution.

One thing that we are becoming more confident in is the potential for coastal flooding and marine impacts, even with a more offshore track. We are approaching higher astronomical tides Sunday and Monday and much of eastern MA coast could be vulnerable to at least minor coastal flooding and beach erosion. On the coastal waters, there is the potential for NE gale force winds and rough seas, given fact that much of the guidance is showing a potentially strong coastal storm.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

06z TAFs: Moderate confidence (60%).

Fog bank continues to creep northward towards the BOS terminal and will likely make it perhaps as far north as BED by 07-08z. Confidence in the northward extent of fog is very low. Elsewhere VFR expected except across the SE MA and much of RI.

Some improvement this morning as fog mixes. Push of drier mid level air helps to improve CIGs to VFR/MVFR across N MA. Light rain or light snow arrives around midday and changes to light snow everywhere (except Cape Cod/Islands) by late afternoon, ending Wed night with VFR conditions to follow. Bulk of the snowfall looks to stay S of BOS.

Improvement to VFR expected from NE to SW tonight. Widespread VFR Thursday.

KBOS Terminal...Low confidence in TAF. Main question is if fog develops this morning and when we see an improvement to VFR ahead of the rain/snow. Expectation is that low clouds will stick through at least 09z. Model guidance has trended significantly drier with the mid levels. So ceilings may be primarily MVFR outside of any snowfall this afternoon and evening.

KBDL Terminal...Moderate confidence in TAF. Higher confidence that IFR ceilings persist through today.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. Chance FZRA, slight chance SN.

Friday: Mainly IFR, with local MVFR possible. Breezy. RA, chance SN, chance FZRA.

Friday Night: Mainly IFR, with local MVFR possible. Chance SN, chance RA, chance FZRA.

Saturday: Mainly VFR, with local IFR possible. Chance SN, slight chance RA.

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

Sunday: Mainly VFR, with local MVFR possible. Breezy. Chance SN.

MARINE

Forecaster Confidence Levels...

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

High confidence through the end of this week.

Weak low pressure will pass across the southern coastal waters this afternoon and evening. Leftover E swell will maintain 5 ft seas on outer waters where Small Craft Advisories remain posted. Otherwise, relatively light winds and seas expected through Thursday.

Outlook /Thursday Night through Sunday/...

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

Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain, chance of freezing rain. Local visibility 1 to 3 nm.

Friday Night: Winds less than 25 kt. Local rough seas. Chance of rain, chance of snow. Local visibility 1 to 3 nm.

Saturday: Winds less than 25 kt. Seas up to 5 ft. Chance of snow, slight chance of rain.

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain, slight chance of snow.

Sunday: Winds less than 25 kt. Seas up to 5 ft. Chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Winter Weather Advisory from 1 PM this afternoon to 4 AM EST Thursday for MAZ002>004-008>012-026. RI...None. MARINE...Small Craft Advisory until 7 AM EST Thursday for ANZ250. Small Craft Advisory until 7 PM EST Thursday for ANZ254>256.