textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Elevated heat and humidity continues through the weekend. Scattered showers and thunderstorms possible tonight and Saturday afternoon & evening.

- Summer warmth and humidity continues along with a daily chance of a pop-up shower or thunderstorm.

DISCUSSION

KEY MESSAGE 1...- Elevated heat and humidity continues through the weekend. Scattered showers and thunderstorms possible tonight and Saturday afternoon & evening.

Day Cloud Phase satellite imagery shows development of diurnal cumulus across portions of southern New England this afternoon. SHowers and thunderstorms continue to develop across PA and NY along the periphery of a mid-level ridge shifted offshore of southern New England. For the rest of the afternoon, expect mainly dry conditions. With a few cells beginning to pop near southern NY, can't rule out an isolated thunderstorm in western MA/CT after 4 PM. Daytime heating has allowed instability values to rise into marginal ranges this afternoon to support pop-up showers/storms. An isolated shower or storm may possibly initiate of the sea breeze boundary along the south coast as well late this afternoon. Hi- res guidance continues to bring a more consistent batch of remnant scattered showers and a few storms after 8 PM tonight. The best chance for seeing a storm will be in western southern New England. With precipitable water values approaching 2 inches, brief heavy downpours are possible. This batch of showers/storms are likely to weaken and diminish as they push east overnight. With increased cloud cover for much of the overnight and the warm airmass, overnight temperatures likely remain in the upper 60s and low 70s tonight.

Saturday, the mid-level ridge pushes further southeast of the region with flow becoming more zonal. The warm/moist airmass will stay in place tomorrow with SW flow supplying advection of higher dewpoints in the low 70s. Expect another day of elevated humidity and slightly above normal temperatures in the mid to upper 80s. With the added humidity it may feel closer to low to mid 90s. There still is potential for scattered showers and thunderstorms in the late afternoon/evening on Saturday. Model guidance indicated a weak embedded disturbance moving through the flow which should help provide additional support for thunderstorms accompanied by the diurnal heating and moist/unstable airmass. However, there the CAMs continue to show a spread in solutions with regards to potential and location across southern New England. Some solutions indicate more isolated potential, keeping the higher activity south of the region. Others are more robust, simulating a few clusters of thunderstorms pushing across southern New England Sat evening. The primary concern with any storms will be localized heavy downpours supported by PWATs approaching 1.8". Mid- level flow is sufficient to keep storms moving from west to east which minimizes the concern for more widespread flash flooding issues. Can't rule out localized flash flooding if a storm or two were to push over a more vulnerable area (urban/poor drainage, area of recent heavy rainfall).

KEY MESSAGE 2...Summer warmth and humidity continues along with a daily chance of a pop-up shower or thunderstorm.

There haven't been many notable changes to the extended portion of the forecast, which encompasses Sunday through the upcoming week. In general, this timeframe features continued summer warmth with manageable humidity and daily chances for afternoon showers or thunderstorms. As mentioned previously, the synoptic pattern trends toward a flattening mid-level ridge, with nearly zonal flow across the CONUS and the subtropical ridge expanding from the western Atlantic into the lower third of the CONUS. This will allow westerly flow with embedded shortwave disturbances to move through the region.

Temperatures aloft remain quite warm, with 925mb temperatures around +23C to +25C from Sunday through Tuesday. BUFKIT forecast soundings depict a well-mixed boundary layer, supporting afternoon temperatures well into the upper 80s to around 90F. Dew points will remain elevated, though should fall short of oppressive levels, generally in the 65F to 70F range. Nevertheless, this will contribute to heat indices in the lower 90s Sunday through Tuesday. A potential break from the summer warmth arrives during the middle portion of next week as a front moves through, though there remains a fair amount of spread among the guidance regarding the exact evolution of this feature. Given the uncertainty, stayed close to NBM guidance for Wednesday and beyond, with highs expected to be in the lower to middle 80s. Overnight temperatures will remain seasonably warm, with lows generally in the 60s.

Regarding thunderstorm chances, embedded shortwave disturbances moving through the zonal flow should provide sufficient lift for isolated to scattered thunderstorms each day. Storms should remain largely pulse-type and disorganized rather than evolving into organized severe convection. PWATs up to around 2.0 inches will further support the potential for localized downpours. Gusty winds and frequent lightning will be the primary hazards. That said, thunderstorms will be highly localized, and not every community will see one, with many locations remaining dry. As such, there is no reason to cancel outdoor plans.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight: Moderate confidence.

VFR at least initially, but a few things to monitor. One is SCT TSRA mainly for BDL-PVD and perhaps into the Cape airports as line of (currently) strong to severe storms in vicinity of NYC hubs moves ENE. VCTS shown for BDL at 00z and PVD at 03z where confidence was high enough for a thunder mention; also included VCSH for BOS and the Cape after 04z to delineate where storms could weaken and/or be a lesser risk. Meanwhile stratus/fog at IFR-LIFR levels should again develop near ACK by 01z then gradually expand northward to the south coast, but the risk for storms casts more uncertainty with that than usual. W to SW winds around 5-8 kt, winds locally stronger in/near TSRA.

Saturday through Saturday Night: Overall high confidence, but lower on TSRA timing.

South-coastal sub-MVFR stratus/fog from overnight dissipates to VFR. Main question mark is timing of TSRA, which again varies between 18-20z at earliest to as late as 01z, then moving eastward. Risk increases into the afternoon but this will need refinement; for now have indicated PROB30s for a 4 hour window starting around the 21z timeframe. TSRA dissipates late Saturday evening, with south-coastal fog/stratus returning with VFR elsewhere.

Winds on Saturday are SW around 5-10 kt with very low to nil risk for a seabreeze, then shifting to W/WNW overnight.

KBOS Terminal...Moderate confidence. Thinking TSRA stays enough south for just a VCSH mention after 04z. W winds initally will shift to SW around 5 kt by 04z. Seabreeze not likely Sat. Monitoring for TSRA Sat aftn/evening but not likely any sooner than 21z if they get close.

KBDL Terminal...Moderate confidence. VCTS 00-03z for ongoing complex of storms SW of HFD, moving ENE then VCSH 03-05z. VFR otherwise, though another risk for TSRA Sat aftn/evening but timing is still uncertain.

Outlook /Saturday Night through Wednesday/...

Sunday: VFR. Breezy. Isolated SHRA, isolated TSRA.

Sunday Night through Monday: VFR.

Monday Night: VFR. Slight chance SHRA.

Tuesday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

Tuesday Night: VFR. Slight chance SHRA.

Wednesday: VFR. Chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Through Saturday... High Confidence.

High pressure remains positioned off of the east coast. This will continue to produce a SW flow across the entire region into the weekend. Gusts to 20 kts possible over the southern waters Saturday, but under SCA.

Outlook /Saturday Night through Wednesday/...

Sunday through Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Monday through Monday Night: Winds less than 25 kt.

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

Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory until 8 PM EDT this evening for CTZ002>004. MA...Heat Advisory until 8 PM EDT this evening for MAZ003>007- 010>021-026. RI...Heat Advisory until 8 PM EDT this evening for RIZ001>007. MARINE...None.