textproduct: Boston / Norton

WHAT HAS CHANGED

Refined shower chances and flash flood potential through the upcoming week.

KEY MESSAGES

- Low-impact weather through Monday night. A few thunderstorms are possible Monday afternoon across western portions of southern New England.

- Periodic chances of scattered showers and thunderstorms early to mid week next week, potentially lingering into Thursday. Increasing potential for heavy rain late Tuesday through Wednesday with at least localized flash flooding possible.

DISCUSSION

KEY MESSAGE 1...Low-impact weather through Monday night. A few thunderstorms are possible Monday afternoon across western portions of southern New England.

A weak area of low pressure continues to spin southeast of Nantucket, maintaining low clouds and occasional light precipitation across the outer Cape and Islands. While not the best beach day there, a High Risk for rip currents continues through this evening for east-facing beaches of Cape Cod and Nantucket. Elsewhere, most of southern New England enjoys sunshine and comfortable conditions. The remainder of today through tonight will be relatively quiet. Lingering low-level moisture from northeasterly flow combined with light winds and mostly clear skies overnight may allow for patchy fog to develop in the typical low-lying and river valley locations of eastern Massachusetts. Overnight lows fall into the 50s inland and the lower 60s along the immediate coast, which is seasonably cool for late July.

Monday will feature seasonable temperatures, with highs in the lower to middle 80s inland and upper 70s along the coast. A shortwave trough dropping into the Northeast, along with an embedded 850-700mb disturbance, will provide enough forcing for isolated to scattered showers and thunderstorms during the mid to late afternoon, primarily west of the Connecticut River Valley. PWATs increase to around 1.3 inches, near to slightly above climatological normals but not especially anomalous. While a few thunderstorms may produce locally heavy downpours, rainfall amounts should generally remain modest, with isolated localized totals approaching one-half inch. Flash flooding is not anticipated, as 3-hour flash flood guidance remains around 2.5 to 3.0 inches. Instability increases to around 800-1200 J/kg Monday afternoon, but weak deep-layer shear (generally less than 30 kt) should limit storm organization. As a result, convection is expected to remain isolated and pulse-like, with many locations remaining dry. The highest PoPs therefore remain focused west of the Connecticut River. CSU machine-learning guidance also keeps the better severe-weather signal west of eastern New York. While severe weather appears unlikely, the primary hazards with any storms will be cloud-to-ground lightning and brief, locally heavy rainfall.

Monday night, any lingering convection will diminish during the evening. Southwesterly flow aloft then increases moisture, with PWATs climbing toward 1.5 inches. Low clouds become more widespread overnight, and scattered light showers may begin to develop ahead of a more unsettled pattern on Tuesday. Overnight temperatures remain mild, holding in the 60s.

KEY MESSAGE 2...Periodic chances of scattered showers and thunderstorms early to mid week next week, potentially lingering into Thursday. Increasing potential for heavy rain late Tuesday through Wednesday with at least localized flash flooding possible.

A mid-level trough positions across the region through much of the week. A series of shortwaves embedded in the flow are expected to progress across the region through the week supporting periodic chances for showers and thunderstorms.

Model guidance has remained generally consistent with a deepening trough into Tue/Wed with a strong signal for above normal moisture (120-170% of normal) advecting into the region by Tuesday. Most deterministic and ensemble solutions have now come on board with the development of a low pressure system. In many solutions now, the low pressure is quite anomalous in magnitude for late-July. Combined with the enhanced forcing from the low, this is a set-up for heavy rain potential. However, some model guidance has been more aggressive than others. The ECMWF and its ensemble members have shown the strongest signal for heavy rain/flash flooding potential with a stronger low pressure system tracking in the vicinity of Long Island, toward eastern MA/Cape Cod in many of its solutions. It also leans on a slower progression of the trough as well. Soundings exhibit the textbook environment for efficient rain processes with deep warm cloud depths aligned with instability. ECMWF ens probs show 40-75% probability for 1+ of rainfall later Tue into Wed and 10- 40% for 2"+ across a good portion of southern New England. The deterministic guidance over the last 24 hours of runs has showed a large footprint of 2"+ amounts across the region in that same timeframe. With weak steering flow this support slow storm motions of embedded heavy rain and thunderstorms. This combined with potential for training for training will increase the risk for flash flooding with more "scattered" occurrences as opposed to an isolated case or two. While the recent deterministic runs of the Canadian and GFS have gone more on board with the heavy rain potential, their ensemble guidance still have shown solutions leaning toward a weaker low and differing track resulting leading to lesser rainfall amounts. There is still a signal for localized heavy rain/flash flooding within these solutions; however, they are more isolated or localized. There is still uncertainty; however, we are at least seeing increasing confidence for widespread rain with at least localized heavy downpours/flash flooding possible in spots, particularly later Tue through Wed. Depending on the track of the low, showers may linger through Thurs.

Temperatures lean seasonably cool to slightly below normal at times through the week ranging in the mid 70s to low 80s.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

18z TAF Update:

This Afternoon & Tonight...High confidence, except moderate confidence across Cape Cod and the Islands.

Mainly VFR conditions with ENE to NE winds, becoming light and variable or northerly overnight. The exception is across Cape Cod and the Islands, where a distant offshore low continues to advect MVFR stratus and reduced visibilities into the region. There remains a low to moderate potential for this stratus to expand north and northwest toward KPVD and KOWD after 08z before retreating back toward the coast around sunrise.

Monday...High confidence, except moderate confidence across Cape Cod and the Islands.

Predominantly VFR. Areas of MVFR stratus may linger across Cape Cod and the Islands through around 15z before improving to VFR by 18z. E to ESE winds remain below 10 kt. A weak front approaches western southern New England after 18z, bringing a slight chance of isolated showers or thunderstorms to western terminals, including KBDL. Confidence remains too low to include in the current TAF package, though later forecasts may need to introduce VCSH or VCTS if confidence increases.

Monday Night...Moderate confidence.

VFR conditions at the start of the period should gradually deteriorate to MVFR, with localized IFR/LIFR ceilings developing overnight. Confidence in the timing of these restrictions remains lower. Scattered showers are also possible overnight, with more widespread precipitation expected by Tuesday. Light S winds.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, slight chance TSRA, patchy BR.

Tuesday Night: Mainly MVFR, with local IFR possible. Breezy. SHRA likely, slight chance TSRA, patchy BR.

Wednesday: Mainly MVFR, with areas IFR possible. Breezy. SHRA, slight chance TSRA, patchy BR.

Wednesday Night: Mainly VFR, with areas MVFR possible. Breezy. SHRA likely, chance TSRA.

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

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

Friday: VFR. Slight 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 Tuesday...High confidence.

Light winds and seas 1-4 ft through Monday. Increasing winds and seas for Sunday; however, below SCA. NE winds 10-15 kts with gusts to 20 kts. Light NE to SE winds Monday.

Seas increase late Tuesday into Wednesday with SCA conditions becoming more likely from winds and increased seas.

Increased risk for rip currents today with the greatest risk for the east-facing beaches/coastline.

Outlook /Tuesday through Friday/...

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

Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms, patchy fog.

Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Rain showers likely, slight chance of thunderstorms, patchy fog.

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain showers likely, slight chance of thunderstorms.

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms.

Thursday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of rain showers, slight chance of thunderstorms.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents until 8 PM EDT this evening for MAZ022- 024. RI...None. MARINE...None.