textproduct: Boston / Norton

WHAT HAS CHANGED

Updated Aviation section.

KEY MESSAGES

- Dry conditions through tonight. Unsettled weather returns Thursday with periods of showers.

- Another round of unsettled weather possible for this weekend.

DISCUSSION

KEY MESSAGE 1...Dry conditions through tonight. Unsettled weather returns Thursday with periods of showers.

An upper level low slowly tracks across eastern Canada well to our north today and into Thursday. As it tracks eastward, it will induce troughing across the region accompanied by increased SW flow this afternoon. Despite the pattern transition, expect mainly dry conditions for the rest of the afternoon and tonight. A weak boundary moves through overnight, but would at worst case result in an isolated shower tonight.

The higher risk for seeing showers will likely arrive Thursday a few embedded shortwaves move through the southwesterly flow. A plume of above above normal advects into the region with ascent further supported by a 500mb jet. This will support scattered showers developing from west to east in the afternoon/evening. An additional wave moves through overnight producing another push of showers. There is still spread across guidance with regards to the track of these shortwaves. This will influence rain probabilities and where the higher risk is.

Some guidance indicates a more southerly track of the waves with the higher risk for widespread showers further south or clipping the south coast and Cape/Islands. Across both GFS and ECMWF ensembles, there are probabilities 50 to 80% for at least 0.01" draped across southern New England with the higher probabilities for the south coast/Cape and Islands. The second wave that moves through overnight is further south in track across most guidance, so the better shot will be south coast, Cape and Islands getting brushed with a round of showers. With very marginal instability, mainly in the west and S/SE southern New England there is a low probability of a few isolated thunderstorms or embedded convection.

In terms of amounts, global ensembles range amounts anywhere from a 0.01 to 0.50" with with the greater potential for the higher end of that range confined south and southeast. Models like the GFS trend a little drier and track the better forcing south which has led to lower QPF overall (< 0.25"). On the other hand, HREF PMM totals indicate potential for higher amounts 0.25-0.75" in spots across the south coast/southeast MA likely indicating at some convection leading to isolated downpours. So all in all, no major concern for impacts, but could see periods of showers later tomorrow into the overnight period.

KEY MESSAGE 2...Another round of unsettled weather possible for this weekend.

Looking like an extended period with a risk for rainfall from Friday into Sunday due to a stalled front somewhere along the coast. The greatest risk for rainfall is towards southeastern New England. The details are still not certain, since much will depend on exactly where this front stalls. The most likely location is across the coastal plain of RI and southeast MA, which would steer a low pressure right through our region. The most likely time for that would be sometime Saturday into Sunday. As humid as we are, it is possible we could see 1-2 inches of total rainfall in spots near the front. Farther northwest of this front, total amounts are most likely less than one half inch. Localized thunderstorms are also possible, especially during the afternoon and evening hours this weekend.

Once this front finally moves offshore some time Monday, the high pressure that follows should provide dry weather with more comfortable humidity for the first half of next week.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

Tonight...High confidence.

Mainly VFR tonight. May see patchy MVFR cigs and BR for Cape and Islands after midnight. Low chance of an isolated shower overnight. Light WSW winds.

Thursday...High confidence.

VFR. Scattered showers arrive from west to east after 18Z with a few thunderstorms possible. Confidence not high enough to include TSRA in TAF, but SHRA included as PROB30 for now.

KBOS Terminal...Moderate confidence with respect to the sea breeze. Sea breeze remains just off the terminal at BOS this afternoon. Kept prevailing as SW for remainder of afternoon. There is a low chance (<15%) for the sea breeze to make it in 18-20Z.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Friday through Friday Night: VFR. Chance SHRA.

Saturday: Mainly VFR, with areas MVFR possible. Chance SHRA.

Saturday Night: Mainly VFR, with local IFR possible. Chance SHRA, slight chance TSRA.

Sunday: VFR. Chance SHRA, chance TSRA.

Sunday Night: VFR. Chance SHRA.

Monday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Thursday Night...High confidence.

Southwest winds 8-12 kts with gusts up to 20 kts this afternoon. Light WSW flow overnight with light southerly flow on Thursday. Seas 1-3 ft. Patch fog possible overnight tonight.

Outlook /Friday through Monday/...

Friday through Friday Night: Winds less than 25 kt. Chance of rain showers.

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, slight chance of thunderstorms.

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain showers likely, slight chance of thunderstorms.

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

Monday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.