A broad upper-level ridge anchored over the South will keep dangerous heat in place this weekend and into early next week.
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Widespread heat-related advisories and warnings are in effect from Florida north to the Carolinas and west to Texas/Oklahoma. The most excessive heat is expected to focus across a large portion of Texas into the Lower Mississippi Valley.
Daily high temperatures are forecast to range from the mid-90s in Florida, the upper 90s into portions of the Southeast, the low 100s in Oklahoma and the Lower Mississippi Valley, and the mid 100s to 110 into portions of Texas.
These temperatures when combined with high humidity ill allow heat indices to rise as much as 110 to 120F for many locations.
Numerous record-tying/breaking highs are forecast both days, with similarly record-level warm low temperatures in the upper 70s to low 80s providing little relief from the heat overnight.
This heat streak will persist into the coming week with little to no reprieve.
Additionally, a new heatwave will begin to develop Sunday across the Pacific Northwest thanks to a strong ridge of high pressure that will settle in overhead.
The more excessive temperatures will initially be focused along the interior valleys west of the Cascades, with highs soaring into the upper 90s to mid-100s.
This heat wave will also continue to intensify and expand across the region next week.
Energetic flow with embedded shortwaves north of the ridge and the passage of multiple meandering frontal systems will convection potential to the Northeast/Midwest/Plains this weekend.
The leading front tracking eastward across the Great Lakes/Midwest into the Northeast through Saturday evening focusing showers and thunderstorms from the Ohio Valley to the Northeast.
The Storm Prediction Center has issued an Enhanced Risk of Severe Weather (level 3/5) as strong flow at low levels and aloft will provide enough shear for storms to produce damaging winds, along with some hail and the threat of a few tornadoes.
Some locally heavy downpours are also possible, with a Slight Risk of Excessive Rainfall (level 2/4) over portions of southern New England where the greatest threat of some scattered instances of flash flooding will exist.
A shortwave dropping south over the Northern Plains from Canada will help to better organize a southward moving frontal system late Saturday into early Sunday with storm chances shifting southeastward into the Central/Southern Plains and the Middle Mississippi Valley through the day Sunday.
High surface moisture pooling between this frontal system and another front to the south will help storms to produce some locally very heavy downpours.
There is a Slight Risk of Excessive Rainfall in effect for the risk of flash flooding, particularly if storms grow upscale into a more organized convective system Sunday night, enhancing rainfall totals and coverage.
In addition, strong buoyancy and sufficient deep layer shear will also lead to the threat of some severe weather, with a Slight Risk in place for the threat of damaging winds, large hail, and a few tornadoes.
Forecast highs will range from the upper 70s to low 80s in New England to the mid-80s to low 90s for the Mid-Atlantic and Ohio Valley west into portions of the Midwest/Mississippi Valley.
Areas of showers and thunderstorms are also expected on Saturday over portions of the central and southern High Plains as a cold front sags southward across the region and moist, upslope flow settles in place.
An additional Slight Risk of Excessive Rainfall is in effect here as well as upscale growth of storms later in the evening may lead to some more widespread, higher rainfall totals and a few instances of flash flooding.
Daily maximum temperatures are forecast to be near or just above seasonal normal for the Central and Northern Plains region with upper 80s to lower 90s.
Once the cold front passes much cooler air will filter in resulting in maximum temperatures in the upper 60s to low 70s.
Monsoonal moisture in the Four Corners region will lead to daily rounds of afternoon thunderstorms with a few locally heavy downpours possible, and some isolated concerns for flash flooding.
This will also help to keep temperatures a bit below average, with highs only into the upper 70s for some locations.
Areas of the Central/Southern Rockies and Great Basin will be a bit closer to average, seeing highs in the mid-80s to low 90s, with upper-90s to mid-100s for the Desert Southwest.
A lingering upper-level low will keep temperatures unseasonably cool over portions of southern California, as highs are expected to be in the upper 60s to mid-70s for the coast.
Highs will warm up through the weekend further north, with 70s along the coast and 90s for the interior valleys.
Some locally heavy showers will be possible over the Sierra. ■