textproduct: Boston / Norton

WHAT HAS CHANGED

Risk for excessive rainfall and potential street flooding has increased Saturday and Saturday night for the South Coast. The severe weather risk level has also increased for Saturday and Saturday night.

KEY MESSAGES

- Drier and seasonable with better air quality today, but smoke could return again tonight/overnight.

- Very muggy Saturday with showers and thunderstorms. Torrential rainfall is possible Saturday late afternoon to overnight mainly along the RI/MA South Coast, with a risk for severe weather too.

- Drying out with much less humidity Sunday into early next week, but our weather turns active again by mid next week.

DISCUSSION

KEY MESSAGE 1...Drier and seasonable with better air quality today, but smoke could return again tonight/overnight.

Cyclonic flow aloft governs Southern New England today and brings a seasonably cooler but more refreshing airmass to our area today. As of early Friday morning, there were still some areas reporting smoke or haze in CT - although air quality indices, while still in the unhealthy for sensitive groups range, were slowly improving as northerly flow takes hold. Latest smoke guidance from the HRRR-Smoke and RRFS shows the more concentrated area of smoke gradually progresses to our south into southern NY and PA. More on that to follow, though. Today's probably the pick of the workweek with better visibilities and generally good air quality, to go along with full sun and comfortable humidity levels as higher dewpoints last couple days mix out. Potential risk for seabreezes late in the day as NW sfc winds ease and PBL flow decreases to around 10 kt. Highs in the lower to mid 80s.

For tonight, we get into a lower-level southerly return flow. As that occurs, we'll be starting to moisten up again in modest SW flow but also start to get into the smoke layer that had been to our southwest again per near-surface HRRR-Smoke/REFS progs. BUFKIT profiles show a pretty good inversion developing. If we do get the smoke layer to return, with the nature of inversions limiting mixing, those outside later tonight and overnight could smell the smoke and/or lead to degraded air quality again. May not be as extensive as it was over the last couple days though.

KEY MESSAGE 2...Very muggy Saturday with showers and thunderstorms. Torrential rainfall is possible Saturday late afternoon to overnight mainly along the RI/MA South Coast, with a risk for severe weather too.

Active weather returns for Saturday, and especially late in the day into the overnight. After a day of comfortable humidities Friday, we then turn absolutely tropical Saturday. This occurs as a plume of rich moisture sourced from the northeast Gulf and Carolina coast advects northeastward, along/ahead of a seasonably-strong frontal system working its way across the eastern Great Lakes with rather strong low to mid-level windfields for mid-July (upwards of 45 kt at 925 mb by Sat night), enhancing low-level and deep layer shear profiles. Main potential hazards include thunderstorms capable of heavy downpours and possible street and flash flooding where repeated storms develop, as well as a lesser risk for severe weather. We'll talk about each below.

On the hydro front...Strong integrated water vapor transport fields are progged especially for southeast New England Saturday evening, as precipitable water values rise to around 2-2.25", about two standard deviations above mid-July climatology. Warm- cloud- depths also push upwards of 13,000 ft. These big-picture factors favor highly efficient downpours. Placement of maximum QPF is still uncertain and pretty scattershot, but the best potential for excessive rainfall and possible street and urbanized flooding seems poised for CT-RI-southeast MA during the evening/overnight, where the above described ingredients are maximized. Cloud-layer flow aloft is pretty strong so individual storms should be moving along vs sitting in one place. We're also not totally in the range of the mesoscale guidance either; that really precludes a Flood Watch, but later shifts could consider one for the southern coast once there's better agreement and the mesoscale guidance is reviewed. Some of the larger cities in RI (Providence, Cranston, Warwick) and southeast MA (New Bedford, Fall River) as we know tend to be vulnerable to flash flooding when rain rates turn excessive. That said, to bring more awareness, coordinated with WPC's Excessive Rainfall desk to increase the excessive rainfall threat level to Slight (Level 2 of 5) along the southern third of Southern New England; the risk looks lower further north away from the southern coast.

Severe weather risk was also increased into the Marginal to Slight risk levels. With weak lapse rates, instability values are pretty meager with low normalized CAPE values typical of tropical-like settings. In fact the higher values take place during the nighttime hours as dewpoints rise. But the low-level and deep layer shear profiles are quite strong, and 0-1 km SRH values nearly to 300 units are notable. 0-3 km CAPE values can aid in these really muggy environments, which are around 50-100 J/kg, not especially off the charts but notable. It's probably more a secondary risk to the heavy rain/possible flooding but wet downbursts and/or a spin-up tornado could develop if enough low level instability and low- level shear can match up.

KEY MESSAGE 3...Drying out with much less humidity Sunday into early next week, but our weather turns active again by mid next week.

Drier and much less humid weather returns again for Sunday into early next week, with seasonable temperatures. We then look to turn active again around midweek, with the global guidance offering another strong frontal system coming out of the northern Plains and interacting with Gulf moisture.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Today: Overall high confidence, but moderate on seabreeze potential.

VFR. NW winds around 8-12 kt, though they weaken by early afternoon. A risk for seabreezes at BOS and PVD but it would be later in the day (after 19z) if the NWlys can slacken off enough to bring the seabreeze onshore.

Tonight: Moderate confidence.

VFR conditions should generally prevail, but FU looks to return tonight in light SW flow, earliest south and west and latest north and east. Kept visbys at 6 SM. Light SW winds.

Saturday: Moderate confidence.

VFR initially, some areas of FU around in the morning but coverage should decrease. Categories decrease to MVFR-IFR levels in showers and thunderstorms. Heavy downpours could reduce visibility, especially later into Saturday from HFD-PVD- PYM line south. SW winds increase to 10-15 kt with gusts 25-30 kt.

KBOS Terminal...Moderate confidence in TAF. VFR today with decreasing NW winds; risk for late-day seabreeze. Smoke could return again tonight but may not restrict visibility much.

KBDL Terminal...High confidence in TAF. VFR today. Smoke could return again tonight but may not restrict visibility much.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Mainly VFR, with local IFR possible. Windy with local gusts up to 30 kt. SHRA, chance TSRA.

Sunday: VFR. Breezy.

Sunday Night through Monday Night: VFR.

Tuesday: VFR. Breezy. Chance SHRA, 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.

Winds and seas remain below small craft advisory thresholds through tonight. NW winds around 10-15 kt, easing and becoming light SW tonight. Seas 3 ft or less all waters. Visbys locally reduced along the far southern offshore waters but generally good visibility elsewhere.

SW gusts increase to around 25-30 kt Saturday, with a low chance of Gale force gusts along the eastern/southeast waters Saturday evening. Marine headlines likely. Showers and thunderstorms, some of which could reduce visibility.

Outlook /Saturday Night through Tuesday/...

Saturday 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.

Sunday: Winds less than 25 kt. Areas of rough seas.

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

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

Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.