A stubborn upper-level high over portions of the eastern to south-central U.S. will generally remain in place, continuing a heat wave over the region with the focus for the most intense heat over portions of the Midwest/Ohio Valley and expanding in the Mid-Atlantic.
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Forecast temperatures generally in the mid-90s Friday will rise into the upper 90s to 100 Saturday in the Mid-Atlantic, with record-tying/breaking temperatures possible.
Heat-related Advisories and Warnings are in place as these temperatures, when combined with the humidity, will bring heat index values into the 100-105 degree range, with values as high as 110 possible through the southern Mid-Atlantic on Saturday.
Additionally, overnight low temperatures will remain in the mid-70s for many locations, at potential record-tying/breaking levels, providing little relief from the heat overnight.
The early arrival of this heat in the Summer season, persistence over multiple days, and abundant sunshine will aggravate heat stress on those without reliable air conditioning.
Temperatures more broadly from the East Coast to the central/southern Plains will be in the low 90s Friday and mid-90s Saturday.
Meanwhile, relief continues to the north of a frontal boundary draped though southern New England, with highs now mostly in the 70s and low 80s.
Meanwhile, areas of showers and thunderstorms will continue along this frontal boundary to the north of the upper-high, as moisture from the Gulf flows northward through the Plains and around eastward through the Upper Great Lakes and into New England.
The greatest threat for significant heavy rainfall will be ahead of a surface wave over portions of the Upper Midwest as overnight storm complexes help prime the soils Thursday night/Friday morning ahead of another round of storms expected later Friday.
A Moderate Risk of Excessive rainfall (level 3/4) is in effect Friday centered on southern Minnesota and adjacent west-central Wisconsin, northern Iowa, and southeastern South Dakota, where the highest potential for heavy downpours, several inches of rainfall, and scattered to numerous instances of flash flooding exists.
Additional scattered instances of flash flooding will be possible more broadly across the Upper Midwest.
A few severe thunderstorms are expected as well, as the Storm Prediction Center has included a Slight Risk of severe weather (level 2/5) for the region, extending southwestward into the central Plains and the northwestward into the central/northern High Plains.
Damaging winds, large hail, and a few tornadoes are all possible.
An upper-level shortwave approaching from the west will help to deepen the frontal wave and push the system eastward on Saturday, bringing the threat for heavy rainfall to portions of the Great Lakes southwestward into the Upper Mississippi Valley.
A Slight Risk of Excessive Rainfall (level 2/4) is in place Saturday as an organized complex of storms is expected to follow the system eastward, bringing additional locally heavy rainfall and scattered instances of flash flooding.
Some severe thunderstorms will again be possible with a Slight Risk in place mainly for the threat of damaging winds.
An upper-level trough moving over the Southwest will bring areas of showers and thunderstorms to the Four Corners Region on Saturday.
Anomalously high moisture spreading into the region from the remnants of former T.S. Alberto will help contribute to locally heavy downpours and the threat for flash flooding, with a Slight Risk of Excessive Rainfall in effect Friday.
The focus for storms should shift more into portions of the Desert Southwest on Saturday, with a lingering threat of some isolated flash flooding.
As this trough moves eastward, a ridge will begin to build over the West in its wake, bringing and expanding area of hot, above average Summer temperatures.
Forecast highs in the 90s on Friday will soar into the 100s on Saturday for portions of the Great Basin and central California Valleys.
Heat-related Advisories and Watches have been issued for the central Valleys south into interior southern California beginning Saturday.
Conditions will also be notably hot over the Desert Southwest on Friday, particularly over central Arizona where Excessive Heat Warnings are in place, with highs into the 110s.
The noted increasing moisture/storm chances will help cool temperatures down a few degrees Saturday.
Elsewhere, a couple of waves of low pressure will pass over portions of the Southeast/Florida as well as south Texas, with daily thunderstorms chances Friday and Saturday. ■