textproduct: Boston / Norton

WHAT HAS CHANGED

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 this morning with perhaps an isolated t-storm or two.

- Expecting a very sharp temperature gradient Tues with a secondary round of showers in the PM.

- Milder but unsettled pattern later this week brings showers from time to time.

- Watching mainstem Connecticut River for possible minor river flooding from Hartford through Middletown this weekend.

DISCUSSION

KEY MESSAGE 1...Showers with brief heavy downpours this morning with perhaps an isolated t-storm or two.

Continuing to eye a quick moving round of rain showers in the early morning hours of Tuesday as a mid-level shortwave moves through. Latest model guidance seems to indicate a delayed arrival compared to earlier in the evening... thinking maybe more after sunrise instead of just before... especially for eastern MA. Still looks to be just enough instability to possibly pop a rumble of thunder or two, especially later in the morning, but probs are still quite low... HREF indicating around 10 percent or so. Not everywhere will see showers this morning, but some areas could see brief heavy downpours, and the Tuesday morning commute is likely to be impacted.

KEY MESSAGE 2...Expecting a very sharp temperature gradient Tues with a secondary round of showers in the PM.

Already starting to see a backdoor front hovering over northern New England, and expecting that to drop south into southern New England later Tues. Overnight model guidance has solidified in indicating NE MA/Merrimack Valley could see that front drop through as early as mid-day or very early afternoon. Still some uncertainty in how far south/east the front will drop which makes the temperature forecast tricky, with a 25-30 degree temperature difference possible along the boundary. Still thinking NE MA has a change at seeing the low 50s, and confidence in the lower CT Valley reaching into the lower 70s is high. Once the front moves in, temps will likely drop quickly... into the low 40s, possible the upper 30s.

Expecting to see a few hours of drying out after this morning's quick push of showers, but confidence is increasing in a secondary round of showers (and possible t-storms) developing later in the afternoon/evening, especially across the interior with storms initiating across upstate NY. The latest guidance seems split with some models continuing to indicate a low-end severe possibility lingering into southern New England and others keeping the convection in VT and NH. Regardless, instability remains rather unimpressive with only pockets of CAPE near 500 J/kg scattered across interior SNE. We'll continue to monitor throughout the morning, but the most likely scenario continues to be a few spots of weakening convection moving into the region.

KEY MESSAGE 3...Milder but unsettled pattern later this week brings showers from time to time.

Subtropical ridge is expected to build off East Coast later this week and into weekend, while mean trough sets up near Great Lakes, resulting in broad and moist SW flow aloft. Presence of frontal boundary nearby gives an unsettled look to much of period from Thursday through the weekend as it lifts north and sags southward from time to time, but forecast will probably appear more pessimistic with chances of rain each day, while in reality showers will be tied to passage of several weak short waves and position of front itself, meaning it won't rain all of the time.

Depending upon the exact location of front, it at least appears right now that some warmer days are ahead, especially over the weekend when temperatures possibly reach the 70s per ensemble probabilities. Forecast reflects a bit of a compromise right now given uncertainty in position of front and will show temperatures that are a little cooler.

KEY MESSAGE 4...Watching mainstem Connecticut River for possible minor river flooding from Hartford through Middletown this weekend.

The combination of rainfall and upstream snowmelt is expected to lead to rises along the mainstem Connecticut River over the next several days. Ensemble river forecasts show low chances (30-40%) of reaching minor flood along lower Connecticut River from Hartford to Middletown this weekend, but that appears to be a higher-end scenario right now. It is more likely that the river will remain within its banks but we will need to assess rainfall and snowmelt over next couple of days to get a better sense of potential impacts.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

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

12z TAF Update...Moderate confidence except low confidence in timing of any precipitation.

Mainly VFR this morning...but clusters of showers with downpours with even perhaps a rumble of thunder will result in briefly lower localized conditions. Otherwise...SW winds gust 20 to 30 knots with the strongest near the south coast this morning.

SW winds prevail until a backdoor cold front crosses eastern MA in the 17z to 20z time frame. This should reach central MA after 21z/22z. It will then stall and gradually begin to move back as a warm front later tonight. Then the warm front flips back to a backdoor cold front and sweeps through the entire region during the day Wednesday. VFR conditions will dominate southwest of the backdoor cold front...but low clouds will result in MVFR- IFR ceilings to the northeast of the backdoor front.

We have one cluster of showers moving through this morning. Models really struggling with timing of additional showers...but perhaps another round somewhere around mid-afternoon with the greatest risk overnight. These additional showers may contain a few thunderstorms too along with brief downpours.

KBOS Terminal...Moderate confidence in TAF except low confidence in timing of any showers/isolated t-storm risk. We think the backdoor cold front crosses the region in the 18z/19z time frame...which will be followed by MVFR-IFR ceilings.

KBDL Terminal...Moderate confidence in TAF but low confidence in timing of any showers/isolated t-storm risk.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: MVFR/IFR conditions possible. Breezy. SHRA likely.

Thursday through Thursday Night: MVFR/IFR conditions possible. Breezy. Chance SHRA.

Friday: Mainly VFR, with local IFR possible. Breezy.

Friday Night through Saturday: Breezy. Slight chance RA.

MARINE

Forecaster Confidence Levels...

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

Expecting another round of 20-30kt SW wind gusts along south coastal waters but also ahead of backdoor front into Cape Cod Bay and on outer E MA waters today. Could see some brief marginal Gale force gusts as well. Maintaining Small Craft Advisories for these waters today and tonight, except through Wednesday on outer waters where seas will take longer to subside.

Front drops south and brings rapid wind shift to N/NE, starting near Merrimack River around midday and probably settling near Providence and Plymouth by late afternoon. It returns northward tonight before dropping back south Wednesday.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Winds less than 25 kt. Seas up to 5 ft. Rain showers likely. Local visibility 1 nm or less.

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. Areas of seas approaching 5 ft. Chance of rain showers.

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

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

Saturday: Winds less than 25 kt. Areas of seas approaching 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 ANZ231- 250. Small Craft Advisory until 7 PM EDT Wednesday for ANZ232>235- 237. Small Craft Advisory until 2 PM EDT Wednesday for ANZ236. Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256.