textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Heavy rain at times continues into this evening, leading to the potential street and flash flooding, especially across western New England.

* Light showers linger into Friday, with improving conditions for the upcoming weekend.

* Unsettled weather returns late Sunday night into Monday, followed by drier conditions Tuesday.

DISCUSSION

KEY MESSAGE 1...Periods of heavy rainfall continue into Wednesday evening, leading to potential street and flash flooding, especially across western portions of MA.

High amplitude vertically stacked low over southeast NYS this afternoon, with a deep Atlantic moisture plume pivoting cyclonically across New England. A very atypical system for late July, with cool season jet dynamics (negative tilt mid/upper trough) via strong cyclonically curved flow aloft, combined with 35-45kt of moist low level SE jet providing upslope enhancement, acting on a warm season moisture plume up to 150% of normal. This will continue to support the ongoing forecast of potential flash flooding, with the greatest threat across the eastern slopes of the Berkshires.

As the low level jet pivots and lifts north into NH/VT, the heaviest rainfall shifts north into NH/ME tonight. This will result in lower rainfall rates and more of a stratiform rain across MA/RI/CT tonight.

Thursday the vertically stacked low aloft drifts over SNE, with surface low over the Gulf of Maine. Moist cyclonic flow aloft continues, yielding overcast conditions along with scattered showers. -10C to -12C at 500 mb will provide some elevated instability and some of the CAMS indicate isolated to scattered heavy showers with embedded thunder across CT/RI and southeast MA. Moisture plume also across this area with PWATs up to 1.5 inches. Thus, a low risk for isolated flash flooding across this region Thu PM. As for temperatures, persistent easterly flow and thick cloudcover/rainfall will depress high temperatures into the low to mid 70s Thursday.

Drying trend Thu night as vertically stacked low lifts northeast into Maine and the Maritimes. However, low clouds, drizzle and fog may linger.

KEY MESSAGE 2...Light showers linger into Friday, with improving conditions for the upcoming weekend.

The aforementioned vertically stacked low pressure system begins to lift northeast away from southern New England Friday morning. Friday serves as a transition day as the departing low gives way to subtle mid-level ridging and weak surface high pressure heading into the weekend. While periods of light rain and drizzle remain possible Friday, the overall trend is toward improving conditions.

Even as the primary low departs, its trailing mid-level trough will linger over the region, allowing weak shortwave energy to rotate through southern New England. With PWATs remaining around 1.3 inches and the lowest several thousand feet of the atmosphere remaining saturated, scattered light showers or areas of drizzle remain possible. Winds gradually shift from onshore to northerly and eventually north-northwesterly by late Friday. Combined with abundant cloud cover, highs will remain below normal, generally in the middle 70s to around 80.

The weekend will not be as sunny as last weekend, but it should be considerably drier. A mix of clouds and sunshine is expected both Saturday and Sunday. PWATs remain near climatological normals Saturday before increasing to around 1.2 to 1.5 standard deviations above normal by Sunday afternoon ahead of the next system. While most locations should remain dry, a passing shortwave could produce an isolated afternoon shower or even a rumble of thunder. Overall, subtle mid-level ridging and developing south to southwest return flow will support a warming trend, with highs returning to seasonable levels in the lower to middle 80s both Saturday and Sunday.

KEY MESSAGE 3...Unsettled weather returns late Sunday night into Monday, followed by drier conditions Tuesday.

Attention then turns to the next system, as a mid-level trough and associated cold front approach late Sunday night into Monday. While uncertainty remains regarding the exact timing and evolution of the system, this currently appears to be the next opportunity for widespread rainfall.

Ensemble guidance indicates PWATs will be slightly above normal, with most solutions favoring a progressive system that should limit the flash flood threat. Current ensemble probabilities for 24-hour rainfall amounts exceeding one-half inch range from 20 to 40 percent. High pressure is then expected to build into the region Tuesday, bringing a return to drier conditions, although confidence in the timing of the frontal passage remains lower given the 5 to 6 day forecast horizon.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18z TAF update...

This afternoon and Tonight...High confidence on trends.

Bands of moderate to heavy rain will continue to impact terminals this afternoon. However, with dry slot moving across CT and eventually into RI and eastern MA, heavy rain bands will transition to more of stratiform light rain/drizzle later this afternoon and into tonight. Nonetheless, any remaining VFR vsbys will transition to MVFR and cigs will transition to IFR/LIFR.

East surface winds will remain gusty along the coastal terminals, up to 25 kt at times through about 03z, then becoming northeast and slackening a bit thereafter. Marginal LLWS will continue until about 00z-03z, as ESE low level jet traverses the region. Not expecting any TSRA, as this is more of a cool-season Nor'easter than a summer- time rain event.

Thursday...Moderate confidence.

Vertically stacked low is very slow to move, therefore LIFR/IFR cigs with MVFR vsbys in areas of light rain/drizzle and fog, only slowly improve to IFR/MVFR cigs and MVFR/VFR vsbys. Low risk for a few heavy showers and isolated TSRA across RI and southeast MA terminals Thu PM. Light surface winds with broad area of low pressure over the region.

Thursday Night...Moderate confidence.

Widespread IFR and lower end MVFR in lingering spotty light showers/drizzle and areas of fog. N winds remaining generally below 10 kt.

KBOS Terminal...High confidence on trends. Marginal LLWS until about 03z, with east to southeast winds up to 45 kt at 2kft. East surface winds gusting up to 25 kt at times, then slackening a bit after 03z as winds turn to the NE. MVFR/IFR cigs at 18z in SHRA will continue to trend toward IFR/LIFR this evening and overnight, along with VFR vsby trending to MVFR/IFR.

KBDL Terminal...High confidence in trends. Steady rains this Wed AM have transitioned to more showery this afternoon, providing more fluctuations in cigs and vsbys. However, overall trends will be down to MVFR vsbys and IFR cigs.

Outlook /Thursday Night through Monday/...

Friday: VFR. Slight chance SHRA.

Friday Night through Saturday Night: VFR.

Sunday: VFR. Chance SHRA, slight chance TSRA.

Sunday Night through Monday: Mainly VFR, with local IFR possible. Chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Surface low over western Long Island Wed afternoon, will continue to yield ESE winds 20 to 30 kt. As this low slowly moves east, the strongest winds will lift north into NH/ME tonight. However, seas will remain rough. Heavy showers at times will limit vsby at times. During Thu and Thu night, the low over the Gulf of ME will slowly continue to move NE. Light N-NE winds with seas slowly subsiding. Vsby 1-3NM at times in areas of fog and showers.

Outlook /Thursday Night through Monday/...

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

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

Saturday Night through Sunday: Winds less than 25 kt.

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

Monday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Flood Watch until 8 PM EDT this evening for CTZ002>004. MA...Flood Watch until 8 PM EDT this evening for MAZ002>004-008>012. RI...Flood Watch until 8 PM EDT this evening for RIZ001-002. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230>237-251. Small Craft Advisory until 8 PM EDT Thursday for ANZ250. Small Craft Advisory until 2 PM EDT Thursday for ANZ254>256.