textproduct: Boston / Norton

WHAT HAS CHANGED

Adjusted snowfall for tonight, although much of this should fall on grassy, non-paved surfaces. Confidence continues to grow in the likelihood for at least some wind headlines Saturday. In addition, confidence also increasing that a strong storm system will bring a period of heavy rain along with potentially strong to damaging wind gusts very late Sunday night into Monday night. River and small stream flooding is likely from this system given heavy rain and additional snow melt in northern New England.

KEY MESSAGES

- Light rain changes to a mix of rain/sleet and then to wet snow into early tonight. Reduced visibilities likely during the evening commute. Minor accumulation (up to 2", locally more possible) but pavement should be mostly wet.

- Strong westerly wind gusts of 35-55 mph on Sat...Wind Headline will likely be needed at least for parts of the region.

- Inland runner brings a period of heavy rain, strong winds, and unseasonably mild temps very late Sun night into Mon night. Some river and small stream flooding is likely too.

- Turning much colder with mainly dry weather Tue through Thu.

DISCUSSION

KEY MESSAGE 1...Light rain changes to a mix of rain/sleet and then to wet snow into early tonight. Reduced visibilities likely during the evening commute. Minor accumulations, but pavement should be mostly wet.

Colder air continues to overspread southern New England, leading to rain changing to snow into this evening. Not expecting much from this snowfall as cloud bases are rather high. Our warmth the past few days will also play a role, keeping pavement temperatures likely high enough to be just wet. The exception may be across portions of southeast MA, which could see local accumulations around 1". It's a low probability for much more than that.

Confidence in specific accumulations for specific locations is not especially high. Adjustments may become necessary in the short-term as this event unfolds. Since this is Mid March, snowfall this time of year is dependent upon time of day and precipitation rate. Not expecting much in terms of heavy precipitation, but the timing is still the largest question since that also relates to temperatures.

Even though low temperatures drop to around 25-30 degrees tonight, the risk for black ice looks minimized by strong drying/evaporation.

KEY MESSAGE 2...Strong westerly wind gusts of 35-55 mph on Sat. Wind headlines will likely be needed at least for parts of the region.

Low pressure will be lifting northeast into the Canadian Maritimes on Sat. The result will be modest cold air advection aloft and an 850 mb jet on the order of 35 to 50 knots. Very steep low level lapse rates coupled with the increasing March sun angle will result in much of this mixing down to the surface. Therefore, expect westerly wind gusts of 40 to 55 mph at times with wind Headlines likely being needed for much of the region. A fair amount of sunshine should still allow high temps to reach well into the 40s, but it will feel colder given the strong winds.

KEY MESSAGE 3...Inland runner brings a period of heavy rain, strong winds, and unseasonably mild temps very late Sun night into Mon night. Some river and small stream flooding is likely too.

The long range model guidance is in fairly good agreement in amplifying shortwave energy forming a deep anomalous upper trough over the Great Lakes. This induces a meridional upper level flow into southern New England which will result in an unseasonably mild airmass into the region. The NAEFS guidance is showing both the LLJ and Pwat plume 3-4 standard deviations above normal, which is often a signal for a potentially significant event. Given these ingredients, the model guidance is indicating the potential for a widespread 1-2" of rain with localized 3"+ amounts not out of the question especially if there are any convective elements involved. And there is some elevated instability, so certainly the risk for at least isolated thunder. The two big concerns revolve around the potential for some river and small stream flooding and a period of strong to perhaps damaging southerly wind gusts. We will discuss more below.

A lot of the MMEFS guidance indicates that this expected rainfall coupled with recent river rises from snowmelt will push several of our points into flood. In addition, there is still a fair amount of SWE left in the snowpack across the mountains of northern New England. Given strong southerly flow and anomalously high temps/dewpoints advecting all the way up there...additional snowmelt and runoff will also tend to push river levels higher. The other concern will be a period of strong to potentially damaging southerly wind gusts. These events are often difficult to forecast even in the very short term given the inversion. However, given the potential for temps to break 60 and global guidance showing a southerly LLJ 3-4 standard deviations above normal there is concern sometime Mon and/or Mon night. Certainly a decent shot at Wind Advisory criteria for parts of the region and even a low risk for High Wind Warning criteria. Something to watch in the coming days.

KEY MESSAGE 4...Turning much colder with mainly dry weather Tue through Thu.

Behind this strong system...the long range guidance is in very good agreement in much colder but mainly dry weather in the Tue through Thu time frame. Highs will mainly be in the 40s Tue and Thu, but many locations may struggle to break 40 for highs on Wed.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

This Evening: Moderate confidence.

Mainly VFR, with areas MVFR in showers. Rain transitions to snow during the evening push most likely, but doesn't linger long.

Rest of Tonight: High confidence.

Rapid improvement to VFR from west to east. By late evening, just about all of southern New England should be VFR. Could be closer to midnight for the Cape and islands. NW winds around 10 kt with occasional gusts to 20 kt. Even though temperatures will be subfreezing, rapid drying should limit any freeze-up on runways.

Friday: High confidence.

VFR, though with increasing midlevel clouds late in the day. NW winds become light and then shift to SW around 5-10 kt.

Friday Night: High confidence.

Mainly VFR. Areas MVFR in rain and snow showers across northern MA. Increasingly gusty S winds expected.

KBOS Terminal...Moderate confidence. Rain mixed with PL then brief SN. visibilities 2-4 SM in wintry precip, although runways should be wet. Rapid VFR improvement by late evening.

KBDL Terminal...Moderate confidence. Light snow developing by 21Z, but should be over by 13/00Z. Mainly VFR, expected for brief MVFR in precipitation late this afternoon and early evening.

Outlook /Saturday through Tuesday/...

Saturday: VFR. Strong winds with gusts up to 45 kt.

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

Sunday: VFR. Breezy.

Sunday Night: Windy with gusts up to 35 kt. Chance RA, chance FZRA.

Monday: Strong winds with gusts up to 45 kt. RA, slight chance TSRA.

Monday Night: Strong winds with gusts up to 45 kt. RA.

Tuesday: Breezy. Slight chance RA.

MARINE

Forecaster Confidence Levels...

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

Rough seas linger across the outer coastal waters into Friday morning. Winds expected to briefly diminish Friday afternoon, before increasing once more from the south Friday night.

Outlook /Saturday through Tuesday/...

Saturday: Moderate risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft.

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

Sunday: Winds less than 25 kt.

Sunday Night: Low risk for gale force winds with gusts up to 35 kt. Local rough seas. Chance of rain.

Monday: gale force winds with local gusts up to 45 kt. Rough seas up to 14 ft. Rain, slight chance of thunderstorms.

Monday Night: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Rain.

Tuesday: Strong winds with local gusts up to 30 kt. Rough seas up to 13 ft. Chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 AM EDT Friday for ANZ235-237-250- 256. Small Craft Advisory until 4 PM EDT Friday for ANZ254-255.