Weather Impacts – Including Tropical Storm Arthur

Today - Weekend

July 2, 2014

Afternoon Update

Primary Concerns • Potential for severe thunderstorms in MA, southern NH, and northern CT

this afternoon/evening. Local flash flooding, large hail, or damaging winds are possible. • Thursday: Showers and thunderstorms with heavy rain over much of

southern New England. Localized flooding possible. Although the link to Arthur may actually shift into NY and N New England. • Arthur makes its closest approach around 2 AM Saturday. - Heavy rain axis shifts to Rhode Island/SE Massachusetts - Building southerly swell expected: * Mariners should secure vessels, but probably no need to remove from water. * Strong swell may lead to increase risk for high seas and strong rip

currents. * Rip current risk is likely to linger into the weekend • Main potential concerns: FLOODING, SEAS/RIP CURRENTS.

Uncertainty • FLOODING - Strong thunderstorms this afternoon/evening could produce flash flooding if multiple storms move over same region. - Axis of heavy rain may actually shift from central S New England on Thu, to the W by Thu night. Another axis may approach ahead of Arthur by Fri for RI and SE MA. • SEAS/SWELL & RIP CURRENTS - Current track does support some swell with potential for rip currents Fri and Sat. Concerned with beachgoers who may be caught unaware – even on Saturday. • WINDS - Could see gusts to tropical storm force over Nantucket or outer

Cape Cod. Some gusts over the waters as well.

Tropical Storm Arthur

Model guidance has gotten into better agreement on a track near the 40N 70W “benchmark.” Cape Cod and Islands are now just out of the cone of uncertainty. Strongest winds are east of the track with usually a sharp cutoff west of track. Therefore, still not expecting winds to be a major player.

This afternoon/evening

Probabilities of Winds 58+ mph

Probabilities of (1” + hail)

Probabilities of Tornadoes

Rainfall Total

Total Rainfall Today-Friday

Note the two potential areas of heavy rain. Total rainfall amounts are likely to vary quite a bit. Some areas may see more rain than this (3-5”); some areas may see an inch or less.

Marine/Shore Impacts Friday-Saturday Seas/Winds: • Building swell as Arthur makes its closest pass could lead to

10-15 ft seas southeast of Nantucket. • With the latest track guidance, we expect that only a few

gusts to 30-40 mph with an isolated gust to 50 mph is possible in extreme SE MA late Fri night into Sat morning. Sustained winds will remain below tropical storm force.

Rip Currents/High Surf: • The building swell could lead to

a high risk for rip currents across area beaches. This threat begins late Friday, but is likely to linger into Saturday at least, even though the weather will be pleasant by then.

• High surf could accompany this rip current risk, especially if the track is closer to shore.

• Little if any coastal flooding is expected.

Wind Gusts (MPH)

Seas ft

In Summary • Key is not to focus directly on Arthur, as this is unlikely to be a direct hit. Never say

never, but it has not occurred in Southern New England in July. • At this point, wind impact looks minimal as the strongest gusts would occur Friday

night into Saturday morning. May see gale force wind gusts most waters, with low potential for tropical storm force winds southeast of Nantucket.

• Heavy rain and potential for localized flash flooding mainly northwest interior (today),

and all of Southern New England (Thursday), this axis may shift back to the interior. • Additional heavy rain and localized flash flooding possible Friday, but again,

dependent on cold front timing and phasing. Current track would RI and SE Mass.

• Note that the flood risk is likely to be localized.

• Seas/swell may require action to protect boats. But not necessarily to bring them on shore.

• Swell may lead to high risk for rip currents, mainly for southeastern beaches. This is likely to linger into the holiday weekend, even when weather is quiet otherwise.