A woman fans herself underneath a hot boiled crawfish sign outside a grocery store in New Orleans.
CNN  — 

A heat wave in parts of the South and Mississippi Valley is peaking and a new one is building in the West on Friday, putting more than 70 million people in those areas under heat alerts ahead of the holiday weekend.

More dangerous severe thunderstorms and areas of heavy rain along the northern side of the heat dome are forecast to track eastward from the Central Plains to the Ohio Valley and eventually the mid-Atlantic over the next couple of days, bringing some heat relief, but also the risk of damaging storms and additional power outages.

Oppressive heat smothers South

In the mid- to lower Mississippi River Valley and the Deep South, temperatures on Friday will again near or top 100 degrees Fahrenheit.

Memphis and Nashville, Tennessee; Birmingham, Alabama; Little Rock, Arkansas; and New Orleans are under excessive heat warnings, as heat and humidity may combine to make temperatures feel like 105 to 115 degrees.

This continues distressing, deadly heat that parts of the South have experienced for days. Eleven people died in recent days due to heat-related illnesses in Texas’ Webb County, which includes Laredo, and at least two people died from the heat in Louisiana’s Caddo Parish this month, officials said.

Parts of Texas have sweltered under triple-digit temperatures for more than two weeks – but the state is seeing signs of relief. Dallas was not under any heat alert Friday morning as temperatures there dip to more typical summer values.

“Although a few locations across Central and East Texas may reach advisory criteria, the prevalence of 105 heat indices will be far less widespread, and our protracted stretch of advisories/warnings for heat will come to an end,” the National Weather Service office in Dallas said.

Heat across the rest of the South is expected to follow a similar pattern to Dallas, with slight relief slowly occurring over the holiday weekend.

“An increase in showers and thunderstorms this weekend into next week during peak heating will bring some relief from the heat as the upper-level ridge weakens slightly across the region,” the National Weather Service office in Memphis said.

Storms will tame temps this weekend

Rounds of storms each day “will begin to erode the upper ridge (heat dome) responsible for the ongoing heat wave across the South and help bringing cooler air ever so slowly east across the Central Plains,” the Weather Prediction Center said Friday morning.

But the trade-off is rounds of dangerous storms.

Damaging winds and large hail are the main threats today as storms move around the periphery of the heat dome parked over the South. Denver, Atlanta and Charlotte, North Carolina, are in a Level 1 of 5 risk on Friday.

A level 2 of 5 slight risk of severe storms stretches from Colorado to Kentucky on Friday, including Colorado Springs, St. Louis, Indianapolis, Cincinnati and Nashville, Tennessee.

Thunderstorm complexes with heavy downpours, hail and damaging wind gusts will develop over the mid-Mississippi and Ohio Valleys Saturday, according to the Storm Prediction Center.

On Saturday, a level 3 of 5 enhanced risk of severe storms covers portions of Missouri, Illinois, Indiana and Kentucky, including St. Louis and Louisville. Damaging winds and large hail are the most likely threat with these storms. A Level 2 of 5 risk stretches farther east to the Virginias, Carolinas and Georgia.

By Sunday, storms look to target the Ohio Valley into the mid-Atlantic where a Level 2 of 5 risk is already highlighted. Philadelphia, Washington, DC, and Indianapolis are included.

Southwest bakes as new heat wave builds

As parts of the South begin to see some relief, heat in the West is building.

Millions across Las Vegas and Phoenix will be under excessive heat warnings beginning Saturday, with temperatures projected to soar above 110 degrees for both locations this weekend. And many parts of California are under excessive heat warnings or heat advisories.

Las Vegas has gone a record-breaking 293 consecutive days without reaching 100 degrees, but is forecast to top 100 degrees today, and shoot up to 110 degrees by Sunday and beyond.

“This heat should be taken seriously and precautions need to be taken to prevent heat-related illnesses,” the National Weather Service office in Las Vegas warned. “Avoid outdoor activity during the hottest parts of the day (10 a.m. - 4 p.m.), drink extra water, take breaks in cool or shady locations, and wear light colored, light weight clothing.”

More than 600 people in the US are killed by extreme heat every year, according to the US Centers for Disease Control and Prevention.

As human-induced climate change forces temperatures to rise around the globe, officials urge people to take safety precautions during heat waves, including staying hydrated, avoiding leaving pets and kids in cars unattended and finding cool, indoor spaces to wait out the high temperatures.

CNN’s Aya Elamroussi contributed to this report.