textproduct: Boston / Norton

WHAT HAS CHANGED

Updated Aviation section.

Possible t-storms early Tuesday morning and Wednesday. Wavy frontal boundary will lead to large swings in temperature around midweek into the weekend with several rain chances.

KEY MESSAGES

- Showers with brief heavy downpours and possible isolated t-storm late tonight and Tue morning.

- Sharp temperature gradient across SNE on Tue. Another round of scattered showers or t-storms possible late Tue and Tue evening.

- Wavy frontal boundary nearby of Southern New England leads to a temperature roller-coaster starting Wed through the weekend, to go along with several chances for rain.

- Increasing flows are anticipated on the Connecticut River midweek into the weekend as rain with some snowmelt moves downriver from northern New England. These flows could cause rises into flood stage during this timeframe.

DISCUSSION

KEY MESSAGE 1...Showers with brief heavy downpours and possible isolated t-storm late tonight and Tue morning

Mid level shortwave quickly moves through SNE late tonight into Tue morning. The shortwave passage will be accompanied by an increase in moisture and elevated instability leading to a modest instability burst as noted by SWI/KI/TT fields. Expect at least scattered convective showers developing and moving though SNE late tonight and Tue morning. While the showers are not expected to widespread and impact all areas, some brief heavy downpours are possible and can't rule out an isolated rumble of thunder. The morning commute for some will likely be impacted.

KEY MESSAGE 2...Sharp temperature gradient across SNE on Tue. Another round of scattered showers or t-storms possible late Tue and Tue evening

A backdoor front will be moving southward into SNE Tue. It is expected to move into NE MA late morning or early afternoon but there is low confidence on how far south and west it gets during the day which will make temp forecast very difficult as there will likely be up to a 30F temp gradient along the boundary. We expect temps to fall into the 40s over NE MA in the afternoon while highest confidence of temps reaching into the 70s will be across lower CT valley. Lower confidence in between but cloud cover will likely limit how warm it gets on the warm side of the boundary.

We expect a lull in shower activity in the afternoon but will have to monitor a second round of showers and t-storms during the late afternoon and evening in the interior. Convection is expected to develop across upstate NY in the afternoon and a few showers or t- storms may spill into western MA/CT late day and evening. Some of the machine learning probs are indicating a low severe wx risk in western New Eng. Mid level lapse rates are somewhat favorable and wind field and deep layer shear is robust. Instability is the limiting factor as forecast CAPES are near 500 J/kg and weakening in the evening so max updraft forecasts are underwhelming. Something to monitor tomorrow in case instability overachieves but at this time the risk for severe is low and the more likely scenario is for weakening convection moving into the region.

KEY MESSAGE 3...Wavy frontal boundary nearby of Southern New England leads to a temperature roller-coaster starting Wed through the weekend, to go along with several chances for rain.

A frontal boundary in the vicinity waffling north and south over the next few days will be the main driver of our weather from midweek into the weekend. Although we can say it is an unsettled weather pattern, variation in the front's position day-to-day renders a lower-confidence forecast on both high temperatures and the timing of the best rain chances.

Wednesday looks to be the mildest day, although the frontal boundary will be moving southward from northern New England. Temperatures could be quite mild, with widespread mid/upper 60s and a few spots in the interior could reach into the lower 70s. It looks quite a bit cooler along the coasts in northeast MA and for the Cape and Islands. Exactly how warm we get will hinge on the position of the frontal boundary from Tuesday night; if it lies further to the north there's a better chance for more areas to see highs reaching into the lower 70s. Latest guidance seems to suggest the front lies over Southern New England, which would offer slightly cooler temps. Better chances for showers with low chances for a rumble or two of thunder along the frontal boundary, though the risk for thunder is more the exception than the rule.

We then hit the trough in the temperature roller-coaster on Thursday, as the frontal boundary will have remained far enough south to allow for shallow cold air from a 1040 mb high pressure over northeast ME to advect in on a stiff onshore wind; not to mention at least mostly cloudy to overcast that will be stubborn to shake free of underneath very mild temps at 925-850 mb. Highs could struggle to reach the low 40s with narrow diurnal ranges. Periods of light rain or drizzle could be possible too, in what looks be a dreary, raw day.

For Friday into the weekend...it looks as though the frontal boundary will be lifting northward on Friday, bringing returning milder temps to Southern New England. Another good chance for rains again takes place Fri night into early on Saturday. Saturday then sees a potential return to cooler onshore flow before rebounding back again into the 60s for Sunday.

KEY MESSAGE 4...Increasing flows are anticipated on the Connecticut River midweek into the weekend as rain and some snowmelt move downriver from northern New England. These flows could cause rises into flood stage during this timeframe.

Generally mild temperatures and several periods of rain are anticipated the next 7 days. WPC's latest 7-day QPF forecast in northern New England shows at least 2 inches of QPF in northern New England. A combo of rain and snowmelt in the upper headwaters of the Connecticut River in NH/VT will lead to rises as that added flow moves downriver into the MA/CT portion of the Connecticut River. While current flows are at baseflow levels, latest hydrologic ensemble forecasts from the NAEFS suggest several gaged forecast points along the Connecticut River (Northhampton, Thompsonville, Hartford and tide-influenced Middle Haddam) should begin to experience rises mid to late this week into the weekend, potentially to over bankfull. It is still uncertain when the Connecticut River specifically will start to rise and what the crest flows will look like, as these are sensitive to both local and upstream QPF inputs as well as how much snowmelt drains into the river headwaters (at least 2-3" of SWE is noted per NOHRSC analyses). We're not in the range of NERFC river forecasts as of yet but the potential exists for rises which could prompt river flood warning(s) on portions of the CT River late this week into the weekend.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

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

00z Update...High confidence tonight, then much lower confidence Tue and Tue night due to uncertainty in location and timing of backdoor front, which directly impacts wind and ceiling forecast.

VFR tonight with diminishing SW winds, but LLWS expected near coast with 020 winds 40-60kt, strongest near Cape Cod.

Conditions lower to MVFR/IFR overnight in scattered showers, perhaps an isolated thunderstorm, before improving to MVFR or even VFR in afternoon. Additional scattered showers or thunderstorms may reform later Tue afternoon or evening, especially across interior.

SW winds prevail Tue with 25kt gusts near south coast, with wind shift to N-NE into ORE-ORH-PVD corridor by evening as backdoor front pushes southward.

KBOS Terminal...Moderate (40%) confidence in TAF. Conditions could remain VFR longer overnight into Tue if timing of front slows down.

KBDL Terminal...Moderate (40%) confidence in TAF. Conditions could remain VFR longer overnight into Tue if timing of front slows down.

Outlook /Wednesday through Saturday/...

Wednesday: Mainly MVFR, with local VFR possible. Breezy. SHRA likely, isolated TSRA.

Wednesday Night through Thursday: Mainly MVFR, with local IFR possible. Breezy. Chance SHRA.

Thursday Night: Breezy. Chance RA, chance FZRA.

Friday: Breezy. Slight chance RA.

Friday Night: Breezy. Chance RA.

Saturday: Breezy. Slight chance RA.

MARINE

Forecaster Confidence Levels...

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

Persistent gusty SW winds will continue today. Could see some gusts approach gale force across the waters south and east of Nantucket, as well as the outer RI coastal waters, but think a Small Craft Advisory is more appropriate. Winds and seas briefly subside tonight into Tuesday, but still expecting rough seas across the southern coastal waters. Extended the Small Craft Advisory into Wednesday where confidence is highest.

Outlook /Wednesday through Saturday/...

Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft. Chance of rain showers.

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

Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers.

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain.

Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain.

Saturday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230- 236. Small Craft Advisory until 11 PM EDT this evening for ANZ231- 250-251. Small Craft Advisory until 2 AM EDT Tuesday for ANZ232>234. Small Craft Advisory until 7 PM EDT Wednesday for ANZ235-237- 254>256.