Labor Day Weather Map: States Facing Extreme Heat, Severe Storms and Flash Flood Risk

The National Weather Service (NWS) and its Weather Prediction Center (WPC) have issued heat advisories and warnings, severe-weather forecasts and excessive-rainfall outlooks covering multiple parts of the country for the Sept. 5-7 Labor Day weekend.

The main threats are persistent heat across the central U.S., strong thunderstorms from the Upper Midwest into the Mid-Atlantic and localized flash flooding in the Southwest.

The WPC’s short-range forecast issued Sept. 4 said, “Rounds of strong to severe thunderstorms and locally heavy rain will slowly shift southward across the lower Upper Midwest and Mid-Atlantic during the Labor Day weekend.” It also warned that “Monsoonal moisture could trigger scattered flash flooding in parts of Arizona/New Mexico for the next few days.”

Extreme Heat Covers Parts of Central U.S.

Heat remains a major concern across the central and southern United States as the holiday weekend begins. The WPC said heat would persist across the middle of the country while cooler air expands into parts of the West Coast and Northeast.

The National Weather Service in Fort Worth said Saturday that “Very hot heat indices up to 106 are expected this weekend for portions of Northeast Texas.” The office also said above-normal temperatures and dry conditions are expected across much of North and Central Texas through next week.

The NWS heat advisory for Northeast Texas indicated that heat-index temperatures may reach 106 Saturday afternoon. The advisory is in effect from noon until 7 p.m. Saturday and is for portions of Fannin, Lamar, Hunt, Delta and Hopkins counties. The heat reaches the Mississippi Valley and Southeast farther north and east.

Active warnings from various NWS sites compiled by the NWS indicated heat-index temperatures in excess of 105 to 110 degrees in portions of Arkansas, Oklahoma, Texas, Tennessee, Mississippi, Georgia and South Carolina.

The NWS St. Louis office also had an Extreme Heat Warning in effect Saturday for portions of Missouri and Illinois, illustrating the breadth of the heat threat across the central United States.

Severe Storms Shift South

Another major threat for severe weather this holiday weekend is associated with a frontal boundary from the Great Lakes and Upper Midwest to the Mid-Atlantic.

The WPC said a deepening upper-level trough over southeastern Canada would sustain “a corridor of strong to severe thunderstorms stretching from the upper Midwest to the Mid-Atlantic.” The agency forecast the corridor to gradually shift south during the weekend.

The WPC’s Sept. 4 discussion said the thunderstorms could bring locally heavy rainfall as the system moves through the region. The greatest concern extends from the lower Great Lakes and Upper Ohio Valley toward the Mid-Atlantic, before the threat shifts toward the central Appalachians and Carolinas.

The NWS Twin Cities office summarized the national setup as follows, “Widespread strong thunderstorms with damaging winds and locally heavy rain are expected from the Upper Ohio Valley into the southern Mid-Atlantic today and portions of the central Appalachians and Carolinas on Saturday.”

As the upper-level system and associated front move south, different parts of the Ohio Valley, Appalachians and Mid-Atlantic could experience strong storms at different times through the weekend.

Flash-Flood Threat in Southwest

The Southwest faces a separate rainfall threat caused by monsoonal moisture moving into an unstable atmosphere.

The WPC issued a Slight Risk of excessive rainfall for parts of the Southwest, including portions of Arizona and New Mexico. Its Sept. 4 excessive-rainfall discussion said “scattered instances of flash flooding” were possible across portions of Arizona and New Mexico.

Moisture from the tropical Pacific and Gulf of California was helping to create precipitable-water levels from 1.0 to 1.5 inches and surface instability in the range of 1,000 to 2,000 joules per kilogram, the agency said. Forecast models showed scattered thunderstorms, with precipitation typically between 0.25 and 1-2 inches, locally.

The risk is important in areas where terrain, burn scars, or drainage channels can accelerate runoff. According to the NWS Flagstaff office, flash floods can occur even when the risk for thunderstorms is low for the day, especially in burn scar areas.

Florida Faces Heat and Thunderstorms

Florida also faces the threat of more thunderstorms during the Labor Day holiday, and heat remains a concern.

The NWS Tallahassee office said Saturday that “Showers and thunderstorms will develop each day through the Labor Day holiday.” It added that storms on Sunday and Monday “will be strong and possibly severe, with isolated damaging wind gusts possible.”

The same forecast indicated above-normal temperatures will persist for the next week. It estimated that Sunday and Monday’s heat index values will range between medium and high risk, especially over the Florida Panhandle.

Air mass across the region continues to be highly moist, with precipitable-water values in the vicinity of 2.4 inches over portions of the region, the Tallahassee office said. The moisture is expected to support additional thunderstorms and heavy rainfall during the holiday weekend.

Cooler Weather Reaches Northeast

The same frontal system responsible for the central and eastern storm threat is expected to bring cooler air into parts of the Northeast.

The WPC said, “Heat persists across the midsection of the country while cooler air expands further into the West Coast and the Northeast.”

The transition will occur as the upper-level trough over southeastern Canada and its associated cold front move east and south. Parts of the Northeast and Mid-Atlantic should therefore see a shift away from the higher temperatures affecting the central and southern states.

Weather Risks for Labor Day Travelers

In several areas, the heat and thunderstorms, along with localized flooding, may impact road travel and outdoor Labor Day events.

Individuals passing through heat advisory zones should reduce outdoor activities, consume water frequently and seek shade or air-conditioned environments. The NWS recommends that individuals listen to local alerts and warnings, as weather conditions can change dramatically within state boundaries.

Drivers shouldn’t attempt to cross a flooded roadway. In areas affected by thunderstorms, travelers should monitor NWS updates for changing warnings and be prepared for sudden heavy rain, strong winds and reduced visibility.

The forecast will continue to evolve through the holiday weekend, with the most significant differences expected to occur at the local level as individual storm systems develop and move through affected areas.