textproduct: Boston / Norton

WHAT HAS CHANGED

There is potential for very minor splashover at Nantucket due to the higher than normal astronomical tide tonight.

KEY MESSAGES

- Isolated to scattered showers and thunderstorms possible this afternoon with the greatest risk for western southern New England.

- A low pressure system will bring a prolonged period of rain late Tuesday through Thursday, with light scattered showers potentially lingering into Friday. Periods of heavy rain with scattered instances of flash flooding are possible.

DISCUSSION

KEY MESSAGE 1...Isolated to scattered showers and thunderstorms possible this afternoon with the greatest risk for western southern New England.

Today will be of a transition day between the dry weekend and the upcoming wet pattern. Much of the day will feature dry conditions and mainly sunny skies with highs in the low to mid 80s for the in interior and mid to upper 70s for the coastal areas. Low clouds will continue to scatter of the Cape and Islands this morning as an offshore low pressure system continues to pull further away. A mid- level trough positions over the region ahead of an embedded shortwave tracking across the Great Lakes region. Flow becomes more southerly later in the day with moisture advecting in. Instability builds across west and central southern New England ahead of the wave with the help of daytime heating with MLCAPE values 500-1200 J/kg. This should support development of afternoon/evening showers and thunderstorms. Mid-level lapse rates and shear are in little to marginal values which will favor more pulse storms rather than severe. With the stronger forcing further west, the coverage will be more limited as well (isolated to scattered). The greatest risk of a shower or thunderstorms today will be for western southern New England. Due to the lower coverage nature probabilities are on the lower end (15-30%) for these areas. There is a low chance (< 10%) for a stray shower/storm to make into central MA/CT. With precipitable water values rising to 1.0-1.25" in the afternoon, can't rule out a brief downpour with any storms alongside the lightning risk.

Any remaining showers or storms diminish tonight. Moisture continues to increase overnight into Tuesday with PWATs continuing to slowly increase closer to 1.5". Low clouds spread from west to east with some scattered light showers possible.

KEY MESSAGE 2...A low pressure system will bring a prolonged period of rain late Tuesday through Thursday, with light scattered showers potentially lingering into Friday. Periods of heavy rain with scattered instances of flash flooding are possible.

Model guidance has stayed fairly consistent with a strong signal for a prolonged period of substantial rainfall accompanied by a risk for flash flooding. There is strong agreement across model guidance with a deepening mid-level trough across the Northeast Tuesday into Wednesday enhancing large-scale ascent over the region. Southerly flow will help advect in a plume of well-above normal moisture (120- 170% of normal) into the region. Precipitable water values may approach 1.75-2.0" by Tuesday night. A 500 mb low closes off generally west of the southern New England, slowly tracking eastward into late-week. Guidance has also stayed consistent with the development of a surface low pressure which will be vital with the magnitude and potential impacts of this robust system. Deterministic and ensemble guidance continues to indicate disagreement on the low strength and more importantly track. The tracking of the low center and features will likely have a strong influence on where the most favorable forcing is for banded heavy rain (LLJ position, frontogenesis). The notable spread/uncertainty across model solutions for the low details has translated directly to rain totals as well rate rates and location of bands. This creates a complex forecast where leveraging the full range of solutions is key instead of honing in on one solution.

What is more certain is that an anomalous low pressure for late July tracks in the vicinity of southern New England early Wednesday into late-week. The cooler-season like forcing with this system combined with the moist, unstable warm season airmass will provide a favorable set-up for banded rainfall, heavy at times. Soundings exhibit deep moisture with sufficient warm cloud depths accompanied by instability. All in support of efficient rain processes and thus heavier rates as well. Track will also influence timing of this to some extent as well. Generally the rain increases from the west/southwest on Tue/Tue night with the greater risk of western locations seeing the first true push of rain in this timeframe. Probabilities continue to increase overnight into early Wednesday when the better forcing arrives. Early Wednesday through early Thursday is where we will see majority of the rain and potentially heavy rainfall/rates. As the low center rotates through it will wrap around additional precipitation as it very slowly moves through the region. This would continue rain through Thursday, potentially even Friday.

Across ensemble guidance rain totals range from 1.5 to 3 inches for most areas. The ECMWF ens leaned higher with totals compared to the GEFS. This has been consistent with previous runs. The placement of the heavier bands will be vital in where the higher totals end up. A more predictable mechanism that may act to enhance amounts is the upslope. Surface flow turns to the SE to E on Wednesday which may enhance amounts across the east slope of the Berkshires and even to some extent the Worcester Hills. This has been evident in QPF fields across models guidance with the higher end amounts in those areas. The ECMWF ens highlighted low probs (10-30%) for 4"+ along the east slope. Details on the rain rates will become more clear as models develop a better consensus on the track of the low and its features as well. Given the environment localized heavier rates will be favored thus increasing the potential for flash flooding. Urban/poor drainage areas will be most vulnerable. WPC has placed us in a slight risk for a good portion of southern New England to message the potential for scattered instances of flash flooding in the Wed- early Thu timeframe.

Breezy southerly to easterly winds are possible across the region with magnitude dependent on the strength and track of the low. Given the full moon/slightly higher tides, stronger easterly flow may bring some splashover to locations like Nantucket for the Wed night high tide.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Lingering lower clouds likely scatter/exit the Cape/Islands 12-15Z. Light and variable winds for interior and N/NNE less than 8 kts for east terminals.

Monday...High confidence. (moderate for timing of VFR for Cape terminals)

Lower IFR ceilings should be scattered out for Cape and Islands by 15Z otherwise VFR today. ESE to SE winds 5-10 kts. Low chance (< 25%) for an isolated shower or thunderstorm after 18Z for western terminals (BDL). Too low in coverage to have anything more than a VCSH in there right now.

Monday Night...Moderate confidence.

VFR conditions to start with the general trend being deteriorating ceilings toward low end VFR/MVFR, with localized IFR/LIFR ceilings possible. Still lower confidence in the timing of the lower ceilings. Widely scattered showers possible overnight. Potential for widespread precipitation increases for Tuesday. Light S winds.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with areas MVFR possible. Breezy. Scattered SHRA, isolated TSRA.

Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Scattered SHRA, isolated TSRA, patchy BR.

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

Wednesday Night: MVFR/IFR conditions possible. Breezy. SHRA likely, chance TSRA.

Thursday: Mainly VFR, with areas IFR possible. Breezy. SHRA likely, 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 today. Light easterly winds becoming southerly this evening. Winds increase late Tuesday into Wednesday with seas rising to 5 ft. SCAs will likely be needed Wednesday.

A low pressure system may track across the coastal waters Wednesday; however, there still is uncertainty in the exact track and overall strength of the low. This will influence wind magnitude. There is a low probability (< 15%) for isolated gusts near Gale force across the southern coastal waters Wednesday. Due to the low probabilities, no Gale Watch has been issued.

Outlook /Tuesday through Friday/...

Tuesday: Winds less than 25 kt. Chance of rain showers, chance of thunderstorms, patchy fog. Local visibility 1 to 3 nm.

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Scattered rain showers.

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

Wednesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms.

Thursday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, chance of thunderstorms.

Thursday Night: Winds less than 25 kt. Rough seas up to 7 ft. Chance of rain showers.

Friday: 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.