textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Warm and humid on Wednesday with isolated to scattered showers and thunderstorms possible Wednesday afternoon

- Trending cooler for the end of the week with chance of showers Thu.

- Below normal temperatures this weekend into early next week with unsettled conditions.

DISCUSSION

KEY MESSAGE 1...Warm and humid on Wednesday with isolated to scattered showers and thunderstorms possible Wednesday afternoon

An area of surface high pressure positioned to the south will continue to support southwest flow and low-level WAA into southern New England on Wednesday. 925 hPa temps rise to 20-22C which will support surface temperatures in the mid to upper 80sF on Wednesday afternoon. Dewpoints increase as well to the low 60s. This will support some diurnal instability with the latest suite of high resolution model forecasts suggesting 400-800 J/kg of SBCAPE. 0-6km shear is modest but sufficient between 30-35 knots to support organized convection as a moisture starved cold front pushes through the region Wednesday evening. Mid-level dry air and weak mid-level lapse rates will be the limiting factors for thunderstorm development, but enough ingredients are in place to warrant some risk.

The latest 12Z HREF members depict considerable spread across each of it's 5 non time-lagged members. NAMNest and FV3 members support some isolated shower/thunderstorms near the south coast while the ARW, NSSL-WRF, and HRRR keep things mostly dry. Leaned on a blend of guidance to place a 20-25 percent chance of showers or storms over CT, RI, and southeast MA between 2 and 6PM tomorrow. The cold front then ushers out any remaining showers and cloud cover to support a quiet/dry Wednesday night.

KEY MESSAGE 2...Trending cooler for the end of the week with chance of showers Thu.

Cooler temps Thu and Fri in the post frontal airmass, but near seasonable with highs upper 60s near the eastern MA coast to low-mid 70s further inland. Northern stream shortwave drops south across the region Thu with increasing moisture which may lead to scattered showers developing in the afternoon, especially across eastern half New Eng. Then on Friday there are timing differences with cutoff low moving south from eastern Canada. GFS is the quickest among the deterministic guidance and brings increasing risk of shower from the north during the afternoon while ECMWF and GGEM and slower. GEFS ensemble probs of 0.1" QPF suggest a drier/slower solution for Fri as weak shortwave ridging develops between the 2 systems so leaned toward NBM PoPs which keeps it mainly dry Fri. Then increasing shower chances during Fri night as the cutoff low approaches.

KEY MESSAGE 3...Below normal temperatures this weekend into early next week with unsettled conditions.

Timing differences continue with arrival of anomalous cutoff low which will impact sensible weather Sat. Consensus of the guidance favors the cutoff low and accompanying cold pool aloft dropping south across New Eng during Sat which should be the cooler and wetter of the weekend days. Expect at least scattered showers Sat with highs in the 60s with some improvement Sunday. However, the overall pattern features a blocking pattern into early next with upper trough persisting across the NE. Multiple shortwave passages are anticipated which will bring the risk for showers at times early next week with temps averaging a bit below normal, but a washout is not expected.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight...High Confidence

VFR. SW wind 5-10 kt. Some fog/low stratus possible at Cape/Island terminals toward daybreak with IFR conditions possible.

Wednesday...High Confidence

Mainly VFR, but IFR stratus/fog at ACK with LIFR possible later in the afternoon. Stratus may expand across Cape Cod by late morning and through the afternoon. Low risk for afternoon SHRA/TSRA mainly over CT, RI, and Cape/Islands terminals. Best chances would be after 21Z. Confidence not currently high enough to include PROB30 group in TAF. SW-W winds 10-20 kt with wind shift to NW after 21z interior.

Wednesday Night... High Confidence.

Winds flip to the north/northwest between 03-06Z Thursday behind a cold front. This will push out any fog/low stratus over The Islands terminals. VFR elsewhere with north/northwest wind 5-10 knots.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF.

Outlook /Thursday through Sunday/...

Thursday: VFR. Breezy. Chance SHRA, slight chance TSRA.

Thursday Night: Mainly VFR, with local MVFR possible.

Friday: VFR.

Friday Night: Mainly VFR, with local IFR possible. Chance SHRA.

Saturday: VFR. Breezy. Chance SHRA.

Saturday Night: VFR. Breezy. Slight chance SHRA.

Sunday: VFR.

MARINE

Forecaster Confidence Levels...

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

Tonight and Tomorrow

Southwest winds associated with high pressure to the south will persist over the coastal waters through tomorrow night. Winds stay at modest speeds around 10-15 knots with some gusts to 20 knots possible. Seas remain manageable and sub-SCY level with significant wave heights between 1-3 feet.

Tomorrow Night

A moisture-starved cold front moves over the coastal waters tomorrow night. This may be accompanied by a few showers and possibly a thunderstorm near the south coast. Winds shift to the northwest behind the front around 10 knots.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Chance of rain showers.

Thursday Night: Winds less than 25 kt. Slight chance of rain showers.

Friday: Winds less than 25 kt.

Friday Night: Winds less than 25 kt. Chance of rain showers.

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

Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers.

Sunday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

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