textproduct: Boston / Norton

WHAT HAS CHANGED

The risk for severe weather, including damaging winds and a tornado or two, has increased across portions of western and central Massachusetts and Connecticut.

KEY MESSAGES

- Areas of smoke and haze over the region through the evening. Tracking two rounds of showers and potential severe storms this afternoon and evening. The first round impacts the area between 3 and 8 pm and the second arrives after 9pm. Storms primarily bring a damaging wind and flash flood threat with a low chance of a tornado or two.

- Turning drier and seasonable Sunday and Monday. High risk for rip currents on south-facing beaches on Sunday.

- Dry weather continues into Monday Night, but more active weather returns around midweek. Dry weather expected late next week.

DISCUSSION

KEY MESSAGE 1...Areas of smoke and haze over the region through the evening. Tracking two rounds of showers and potential severe storms this afternoon and evening. The first round impacts the area between 3 and 8 pm and the second arrives after 9pm. Storms primarily bring a damaging wind and flash flood threat with a low chance of a tornado or two

Active afternoon and evening in store as a seasonably-strong frontal system moves through Southern New England over the next 24 hours. First, on the returning smoke. Smoke progs and AQI readings show the thickest smoke over the region now through 23z before moving away from the region after 03z. Air Quality Alerts remain in effect for today for PM2.5, at request of our state environmental agency partners.

In terms of the heavy rain/severe threat, we're still seeing a notably strong wind field developing this afternoon and evening with 800mb winds increasing to 45-50kts. The strong mid-level winds will help one to perhaps two rounds of severe weather form this afternoon and overnight despite marginal instability. More details below.

First round of precipitation arrives between 12 and 2pm as a strong warm front moves in from the SW. The severe threat will be limited with this feature as instability will be quite limited as it moves through. There may be a few elevated thunderstorms with CAMs such as the HRRR showing the greatest risk of that kind of activity across northern MA. The front will serve to increase moisture and instability with PWATs surging to 2-2.25" by 18z.

Surface CAPE values won't be too impressive, with most CAMS showing values between 750-1000 J/kg, for the majority of our CWA. Greatest chance for >1000 J/kg lies across western and central Massachusetts and Connecticut with MUCAPE values perhaps approaching 1500 J/kg in areas with localized clearing. While the absolute values of instability are marginal at best, the low-level, or 0-3 km CAPE values are noteworthy. The HRRR shows these values approaching 100-150 J for most of the CWA as well as a corridor of 150-200 J for NW Massachusetts. The importance of 0-3 km shear is that it provides enough vertical motion to help tilt any horizontally rotating updrafts in a highly sheared environment.

Speaking of shear, guidance continues to show impressive values, with bulk shear approaching 50-60kts after 18z. With the aforementioned wind field, modeled hodographs are elongated and curved, especially west of the CT River Valley. HRRR BUFKIT soundings show 0-3 km SRH values >300 m2s2 and 0-1 km values between 150-250 m2s2! Looking at the parameters as a whole, the main threat with today's severe storms still appears to be areas of straight line winds. Secondary threat is for a tornado or two, with the greatest threat across western and central Massachusetts and Connecticut.

Still have a high potential for areas of torrential rain with any storms that form, fortunately, fast, progressive storm motions will help to cap a more widespread flash flood risk. Best potential for a flash flood risk from high rain rates in urbanized areas this afternoon if and where any cell training occurs. Most of the CAMs pencil in areas south of the Mass Pike as having the most concentrated area of showers and storms, and especially the Hartford- Providence-Southeast MA corridor. Opted for high Likely to Categorical PoP for these areas, then just a generic Chance PoP north of here to make the distinction. The main risk with this first wave of storms is the risk for torrential downpours capable of street flooding. Continued QPF placement uncertainties precluded a Flood Watch with this issuance, as there are still a good number of guidance sources which shift the rain/storms into the southern waters.

As this initial round of showers/storms shifts southward, the next period of showers and thunderstorms arrives late in the day into tonight (between 6 to 11 PM) with the cold front. Most models show a pretty extensive line of thunderstorms developing late this afternoon as the front moves southward across upstate NY and VT. We'll have to watch the northeast extent of this line as it slips south-southeast into the Berkshires, northern CT and central MA, with less of a risk into eastern MA. Straight- line wind damage, frequent lightning and torrential downpours would be the main risks with this second round. The risk for late- evening strong storms is more uncertain the further south one goes, since these areas will probably have been worked over pretty well with the afternoon heavy downpours/storms, with weakening trends more likely. Front clears into the southern waters overnight, with northwesterly breezes and turning much less humid by sunrise.

KEY MESSAGE 2...Turning drier and seasonable Sunday and Monday. High risk for rip currents on south-facing beaches on Sunday.

The cold front brings a stretch of pleasant summer weather Sunday and Monday as PWATs behind the front fall to >0.5". The drier air mass will bring significantly lower humidities and temperatures in the 70s to mid 80s.. At the beaches though, swell directions Sunday are still oriented from SW to NE from today's storm system, with seas in the 4 to 6 ft range. This will favor a high risk for dangerous rip currents. With Sunday being a better day to go to the beach than today, issued a rip current statement for the southern beaches.

KEY MESSAGE 3...Dry weather continues into Monday Night, but more active weather returns around midweek. Dry weather expected late next week.

Latest guidance suite was in reasonable agreement with the evolution of the overall synoptic pattern next week. That said, the usual timing and magnitude differences are present, leading to an average confidence in the forecast details.

Monday should be dry with slightly below normal temperatures. Then we enter a period of more unsettled weather between Tuesday and Wednesday Night. This is when a low pressure should move across eastern Canada and sweep a series of fronts through southern New England. It is not anticipated to be raining this entire time, but the risk for showers and a few thunderstorms will be there. The most likely time period is currently Tuesday night into Wednesday, but this timing could change with later forecasts.

A large high pressure is anticipated to arrive late Thursday into Friday with drier weather.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

Through Tonight: Moderate confidence.

VFR CIGS with MVFR visibility this afternoon and evening. Widespread showers and areas of smoke/haze. TSRA more isolated for the moment. A cold front approaching from the N will lead to a period of more widespread IFR/MVFR until this front passes by. Thinking VFR develops just before the Sunday morning push at most locations.

Sunday and Sunday Night: High confidence.

VFR. NW winds around 10-15 kt, diminishing Sunday Night.

KBOS Terminal...Moderate confidence in TAF due to timing issues. Mentioned prevailing SHRA, but not thinking it will be raining this entire time. Greatest risk for any TSRA between 18/21-19/03Z.

KBDL Terminal...Moderate confidence in TAF due to timing issues. Likely to see a lull in TSRA 18/20-22Z, but SHRA still around.

Outlook /Monday through Thursday/...

Monday through Monday Night: VFR.

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

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

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

Wednesday Night: VFR. Windy with local gusts up to 35 kt. Chance SHRA.

Thursday: VFR. Breezy. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Tonight: High confidence.

Small Craft Advisories continue. Southwest wind gusts 25-30 kt are expected with seas becoming rough. Scattered thunderstorms may produce localized stronger wind gusts as well, which may require the issuance of some Special Marine Warnings.

Sunday: High confidence.

NW winds around 15-20 kt, with seas around 4-6 ft. High risk for rip currents at the beaches/surf zone.

Sunday Night: High Confidence.

Winds and seas diminish.

Outlook /Monday through Thursday/...

Monday: Winds less than 25 kt. Seas locally 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.

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

Wednesday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Rain showers likely, slight chance of thunderstorms. Local visibility 1 to 3 nm.

Wednesday Night: Low risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft. Chance of rain showers, slight chance of thunderstorms.

Thursday: Winds less than 25 kt. Rough seas up to 11 ft. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Air Quality Alert until midnight EDT tonight for CTZ002>004. MA...Air Quality Alert until midnight EDT tonight for MAZ002>024- 026. High Risk for Rip Currents from Sunday morning through Sunday evening for MAZ020-022>024. RI...High Risk for Rip Currents from Sunday morning through Sunday evening for RIZ006>008. Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until noon EDT Sunday for ANZ230>237-250- 251. Small Craft Advisory until 8 PM EDT Sunday for ANZ254>256.