Thunderstorms, heat wave incoming: Weather could upend Labor Day weekend plans

Thunderstorms and intense heat could disrupt Labor Day weekend plans across the country as an expected record number of people travel domestically by plane for the unofficial last weekend of summer.

From Friday to Monday, swaths of the country will face torrential downpours and possible flooding, especially across the Midwest, Plains, Northeast and south-central U.S. Meanwhile, portions of the Northwest are projected to endure a scorching heat blast that will send temperatures 10-20 degrees above their average this time of year.

Away from the continental U.S., the remnants of former tropical storm Gilma are expected to bring heavy rain and winds to the islands of Hawaii over the weekend. On Labor Day and into early next week, developing tropical systems in the Atlantic could spread showers and thunderstorms across the U.S. Virgin Islands and Puerto Rico.

Unseemly weather isn’t the only threat to an enjoyable holiday weekend. Bacteria and algae and other hazards have closed beaches across the country, including more than 60 in Massachusetts alone.

Over 17 million people are expected to pass through airport security from Thursday, Aug. 29 through Wednesday, Sept. 4, potentially the busiest Labor Day travel period on record, according to the Transportation Security Administration.

Here is what you need to know about the weekend forecast:

Storm tracker: National Hurricane Center watching 2 disturbances in Atlantic

Storms to start the holiday weekend across central, eastern US

On Friday, a vast swath of the central and eastern U.S. could see possible thunderstorms, from New Mexico, Texas and Louisiana to Michigan, western New York, central Pennsylvania and the Florida peninsula, according to AccuWeather .

The broad zone stretches across 20 states, and though the rain will not fall consistently it still raises a risk of "potentially severe conditions," AccuWeather said. Storms expected to drench the lower Great Lakes, Mississippi Valley and the southern Plains could, for example, stir up "thunderstorms and locally heavy rainfall, with some isolated flash flooding possible," according to the weather service .

Another trough could unleash higher winds over the Great Lakes region, triggering a 2/5 risk level from the Storm Prediction Center for damaging winds for eastern Michigan.

With intermittent rain, parts of the Northeast could see flooding

Threat of robust storms will extend further into the Ohio Valley and Northeast on Saturday and Sunday, with "areas of severe weather" expected to lash the Appalachians and the Mid-Atlantic region, the weather service said.

The major cities at risk of stormy weather include Philadelphia, Baltimore, Washington D.C., Pittsburgh and Arlington, according to the Storm Prediction Center .

"Damaging winds will once again be the primary threat," the weather service said . "In addition, isolated flash flooding will remain possible, with the potential that a higher threat may materialize given the slowing frontal progression."

A cold front sweeping across the Midwest and Plains is expected to provide relief from the intense heat that baked portions of the Mid-Atlantic this week.

Storms forecast to break out across Gulf Coast states

While some parts of the Southeast will see little rain through the weekend, the Gulf Coast and other portions of the region could see flooding rain.

Through the weekend, heavier rainfall is expected to drench portions of the western Louisiana and upper Texas Gulf Coasts as an area of low pressure lingers in the vicinity. Higher rain rates and repeated rounds of storms raises the "potential of some scattered flash flooding," the weather service said. Daily thunderstorms are also forecast over the central and southern Florida Peninsula.

Heavy rain in expected on Labor Day from Texas to the Carolinas, Georgia and Florida, where localized flash flooding can occur, AccuWeather said.

Dozens of Massachusetts beaches closed due to bacteria levels

Over 60 beaches across Massachusetts are closed because of bacteria or other hazards such as potentially harmful chemicals or rip tides.

The Massachusetts Department of Public Health (MDPH) said in a statement that swimming in the water at these closed beaches poses a threat of illness due to high levels of bacteria. Symptoms can range from nausea and vomiting to a sore throat and fever.

The number of shuttered beaches this year saw a sizable drop from a year ago, when more than 90 beaches across the state’s Atlantic coast were closed.

In other regions, officials warned beachgoers to avoid certain areas because of bacteria.

In Michigan, authorities told people to avoid five beaches because of “high bacteria levels from an unknown source.” In Wisconsin, four of the City of Lake Geneva's beaches remained closed due to blue-green algae , a bloom that can produce toxins that can make humans and animals sick, or even cause them to die in some cases.

In Florida, the Duval County Health Department issued a health alert for toxic blue-green algae in the St. Johns River, the state’s longest river. The cause of the bloom has been attributed to excess nutrients in the river, primarily nitrogen.

Heat to bake the Northwest through the holiday weekend

A late summer heat wave has triggered advisories across the Pacific Northwest as forecasters expect temperatures to spike well above their seasonal average through the weekend.

Afternoon highs on Friday across the northern Great Basin and Pacific Northwest will peak around the mid-90s, according to AccuWeather. On Saturday, the heat will worsen as it expands into the northern Rockies, sending temperatures into the upper-90s.

The weather service office in Spokane, Washington , scheduled a heat advisory to take effect Saturday morning and expire Monday night. Throughout central and northeast Oregon, as well as southeast Washington, forecasters expected "hot conditions with temperatures up to 95 to 100 degrees," according to the weather service in Pendleton, Oregon .

"Advisories have been issued for much of the northern Great Basin given the heightened risk of heat-related illnesses, and extra care should be taken to seek shaded areas and remain hydrated this holiday weekend," the weather service warned.

Contributing: Kathleen Wong, USA TODAY

This article originally appeared on USA TODAY: Thunderstorms, heat wave incoming: Weather could upend Labor Day weekend plans