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.

- Temperatures on the lower end of seasonable as chances for prolonged rainfall return through Wed (possibly into Thurs). Monitoring for increasing chances of localized heavy rainfall and potentially flash flooding.

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...Temperatures on the lower end of seasonable as chances for prolonged rainfall return through Wed (possibly into Thurs). Monitoring for increasing chances of localized heavy rainfall and potentially flash flooding.

Guidance continues to hone in on a low pressure system pushing into New England by Wednesday, but a mid-level trough accompanied by several embedded shortwaves will brings chances for prolonged rainfall later Tuesday into Wednesday... possibly lingering into Thursday. There continues to be a strong signal for PWATs 120-170 percent of normal to advect into the region by Tuesday, with deterministic guidance on the extreme high end of that range. The ECMWF and it's ensemble continue to be the most aggressive of the guidance suite. Not only is it favoring a slower progression of the low as well as favoring more of the near-shore/coastal track, but it's also indicating the low could deepen much further than the GFS or CMC ensembles are indicating. Soundings continue to exhibit the textbook environment for efficient rain processes with deep warm cloud depths aligned with instability and slower mid-level wind speeds, which would create an environment for training storms. ECMWF ensemble probabilities show 50-80% chance for 1.0"+/24 hours Tue into Wed across southern New England and 10-30% for 2.0"+/24 hours across the CT River Valley, and the deterministic ECMWF mirrors those locations with around 2.0"/24 hours as well. Combined with the enhanced forcing of the low, the higher PWATs and potential for training, the threats for localized heavy rainfall and potentially flash flooding are continuing to be monitored. Unfortunately, there still remains some uncertainty in the guidance regarding where exactly the low could track, with the Canadian and GFS (and their ensembles) indicating the low moving further south near NYC and/or off our southern waters. Another large question that remains is where the heaviest rain bands will set up given the timeframe is still too far out to utilize the hi-res guidance.

Otherwise, high temperatures are expected to remain on the lower end of seasonable, even dipping to slightly below normal at times given 850mb temperatures hovering around +12C to +14C through mid- week.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

00z TAF Update:

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.