textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes made to the forecast...But we did issue small craft headlines Saturday into part of Sunday for all our coastal waters.

KEY MESSAGES

- Showers and scattered thunderstorms with torrential rainfall along with a localized flash flood threat Sat afternoon and night. A few severe thunderstorms possible too with the greatest risk interior MA/CT and onto the south coast. Areas of smoke return late tonight into Sat but not as bad as what we had Wed.

- Drying out Sunday into the start of next week, but more active weather returns around midweek.

DISCUSSION

KEY MESSAGE 1...Showers and scattered thunderstorms with torrential rainfall along with a localized flash flood threat Sat afternoon and night. A few severe thunderstorms possible too with the greatest risk interior MA/CT and onto the south coast. Areas of smoke return late tonight into Sat but not as bad as what we had Wed.

The main concern with this forecast package will be a couple rounds of showers and scattered thunderstorms Sat afternoon and night. This activity will bring the potential for localized flash flooding as well a few severe thunderstorms as well. We will discuss this in more detail below.

High pressure slowly shifts east of our region tonight...But it will still remain in control of our weather. Diminishing wind and just some high clouds will result in another good night of radiational cooling. Low temps should be in the 50s across many low lying locations...while the urban heat island of Boston drops only into the middle to upper 60s. The high resolution model guidance also indicates the return of wildfire smoke overnight...But it does not look to be as significant as what we saw on Wed.

Otherwise...the focus is on the severe/localized flash flood potential for Sat afternoon and evening. This is a difficult forecast given a low instability/high shear environment. A vigorous shortwave for July standards will approach the region and induce an impressive southerly LLJ 3+ standard deviations above normal on the order of 45-50+ knots. This will combined with Pwats on the order of 1.75 to 2.25 inches with the highest of those values near the south coast. These parameters are certainly favorable for pockets of torrential rainfall with any thunderstorms and a localized flash flood threat. This risk would increase if we are able to get any severe weather...which we will discuss below.

The greatest instability will be to our west and south where the highest severe weather probabilities will exist. Nonetheless...moisture return will allow for surface instability to reach near 1000 J/KG. While this is on the lower side...given the very impressive jet dynamics for July standard there will be the risk for a few severe thunderstorms into our region as well. Greatest risk will be across interior MA into CT and towards the south coast...but can not rule them out across the rest of the region. Appears the activity may come in two main rounds...the first being in the afternoon/early evening hours followed by a sound round later in the evening into the overnight hours with probably a bit of a lull in between. The main severe weather risk would be localized straight line wind damage. However...if a secondary meso-low pressure system is able to develop and enhance low level helicity there would be a low probability for a brief tornado.

As for the localized flash flood threat...given the Pwat plume in place the potential exists for 2+ inches of rain within 30-45 minutes. This would be problematic if it falls over a vulnerable urban center. That is why this risk will increase if we are able to get severe weather in more robust updrafts...which would favor interior MA into CT. However...flash flooding parameters are most favorable near the south coast where the nose of the strong low level jet impinges on 70+ dewpoints etc. Later shifts may need to consider a Flash Flood Watch...But still felt there was too much uncertainty to do it now. While the localized flash flood threat exists across our entire region...greatest risk is across interior MA/CT and onto the south coast.

The last of the showers and thunderstorms should clear the Cape/Islands toward daybreak. It will be quite muggy Sat night with lows only dropping into the 60s to near 70 by daybreak Sun.

KEY MESSAGE 2...Drying out Sunday into the start of next week, but more active weather returns around midweek.

The low pressure system and its fronts associated with Saturday's system shifts off to the northeast through the day Sunday as the post-cold frontal airmass takes hold over southern New England. Drier, less humid conditions prevail through much of the start of next week with seasonable temperatures. High pressure takes hold Monday and starts shifting more offshore Tuesday, which will lead to the return of more southerly flow. Moisture advected in by this southerly flow may increase dewpoints back into the upper 60s/low 70s by Tuesday afternoon. NAEFS guidance also indicates anomalous PWATs for this time of year making a comeback over southern New England between the Tuesday/Wednesday period. These moist conditions combined with the arrival of another strong frontal system from the northern Plains will increase the chance for some downpours and overall widespread rain. As we are still far out from this system, details regarding timing, exact location, and severity/strength are still uncertain. But generally, guidance is in agreement regarding an unsettled midweek timeframe.

AVIATION /19Z FRIDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

Tonight through Saturday night...High confidence in trends with moderate confidence in specifics and timing.

Mainly VFR tonight into Sat morning despite the return of some smoke during the overnight hours. Conditions deteriorate Sat afternoon and night in round of showers and scattered t-storms. This will bring widespread MVFR conditions with localized IFR/LIFR conditions. A few strong to severe storms will be possible with localized brief strong wind gusts and torrential rainfall. Even outside of any thunderstorms...we expect SW wind to gust between 25 and 30 knots with a few gusts up to 35 knots Sat afternoon and evening. LLWS will be a concern as well.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday through Wednesday/...

Sunday: VFR. Breezy.

Sunday Night through Monday Night: VFR.

Tuesday: VFR. Breezy. Chance SHRA, chance TSRA.

Tuesday Night: MVFR/IFR conditions possible. Windy with local gusts up to 30 kt. SHRA, chance TSRA.

Wednesday: Mainly VFR, with areas MVFR possible. Windy with gusts up to 35 kt. SHRA likely, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Tonight through Saturday night...High confidence.

High pressure slowly shifts east of the waters tonight...but winds/seas remain below small craft advisory thresholds. Strong shortwave will induce an impressive southerly LLJ of 50+ knots Sat afternoon and night. Despite the inversion...we expect widespread southwest wind gusts of 25-30 knots with a few gusts near 35 knots. Seas will also respond and become quite rough across the waters. Therefore...have hoisted small craft headlines for all waters over this time. Scattered thunderstorms later Sat and Sat night may produce localized stronger wind gusts as well...which may require the issuance of some special marine warnings.

Outlook /Sunday through Wednesday/...

Sunday: Winds less than 25 kt. Local rough seas.

Sunday Night: Winds less than 25 kt. Seas up to 5 ft.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Monday Night: Winds less than 25 kt.

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

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain showers, chance of thunderstorms. Local visibility 1 to 3 nm.

Wednesday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 15 ft. Rain showers likely, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 11 AM Saturday to noon EDT Sunday for ANZ230>237-250-251. Small Craft Advisory from 11 AM Saturday to 5 PM EDT Sunday for ANZ254>256.