textproduct: Boston / Norton

WHAT HAS CHANGED

KEY MESSAGES

- Mainly dry conditions expected today. Increased risk for rip currents today, mainly for Cape Cod and Nantucket.

- 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...Mainly dry conditions expected today. Increased risk for rip currents today, mainly for Cape Cod and Nantucket.

Mainly dry conditions expected today with weak high pressure overhead. Aloft, heights continue to fall today as a mid-level trough works into the region while a low pressure system passes well offshore. The only "impact" this system will have the region is bringing a stream of moisture brushing across mainly the southeast portions of MA. This has been prevalent in the low level moisture fields with easterly flow to reinforce it. Model soundings indicate a shallow layer of moisture at the surface with dry air above. Indicator for low clouds and perhaps some patchy drizzle in the highest coverage spots. The area that has the greater risk for seeing this is Nantucket and perhaps the outer most portion of the Cape. Aside from the clouds in some areas, it will be a mainly dry day with a northeasterly breeze for the eastern coastal areas. Temperatures lean slightly below normal in the upper 70s to low 80s for the interior and mid to upper 70s in the eastern and coastal areas. Areas such as Nantucket may struggle to get out of the low 70s today if the clouds stay persistent.

Prolonged easterly component flow will lead to an increased risk for rip currents today. The greatest risk will be for the east-facing beaches on Cape Cod and Nantucket; however, there is also an elevated risk for the south coast and remaining east-facing beaches. Nantucket was added to the current Rip Current Statement.

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.

The first embedded shortwave likely moves through late Monday into Tuesday. WAA and moist advection occur ahead of the shortwave during the day Monday resulting in a warmer day in the low to mid 80s and upper 70s for coastal areas. There is still potential for scattered showers and thunderstorms late afternoon/evening though guidance is still lacking agreement on the coverage and convective potential. This keeps us at probabilities less than 40% still with west/central southern New England being more favored.

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 /08Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

06z TAF Update...

Through 12Z...Moderate confidence.

VFR for most. Very low chance of some marginal MVFR conditions across Cape and Island terminals after 08Z with the higher risk for ACK. Light northerly to variable winds.

Sunday...High confidence overall (Moderate for cigs in east).

VFR for most. A stream of moisture brushes across the Cape and Nantucket today resulting increased cloud cover. The potential for MVFR ceilings is still lower confidence, though the higher risk would be for ACK with periods of MVFR/IFR cigs possible. Also can't rule out a deck of lower clouds near the BOS terminal around 12Z, but this is very low probability at this time.

E winds for the interior 5-10 kts. NE winds for east coast terminals 8-12 kts few gusts to 20 kts possible for Cape/Islands.

Tonight...High confidence overall (Moderate for cigs for Cape/Islands).

If ceilings ended up lowering today for the Cape/Islands, expect MVFR/IFR cigs to continue overnight for those areas. Greatest risk for IFR cigs at ACK with patchy FG possible. VFR elsewhere. Light northerly to VRB winds.

KBOS Terminal...Moderate Confidence in TAF.

Only moderate confidence due to the low end potential for a patch of lower MVFR cigs to develop offshore of the BOS terminal 11-14Z this morning. This is very low probability as it is not showing up on satellite yet. Will have to keep an eye out for development this morning.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/...

Monday: VFR.

Monday Night: VFR. Slight chance SHRA.

Tuesday: Breezy. Chance SHRA, slight chance TSRA.

Tuesday Night through Wednesday: Breezy. SHRA, chance TSRA.

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

Thursday: Mainly VFR, with local MVFR possible. Breezy. 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 /Monday

Monday through Monday Night: Winds less than 25 kt.

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

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

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

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents through Monday morning for MAZ022-024. RI...None. MARINE...None.