A broad area of showers and thunderstorms is expected over central and eastern portions of the country heading into the first half of the holiday weekend ahead of an approaching cold front.
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Today, storm chances will stretch from the Lower Great Lakes southwest through the Middle Mississippi Valley and into the Southern Plains/Rockies.
Plentiful moisture and instability ahead of the front will lead to some more robust thunderstorms and locally heavy rainfall, with some isolated flash flooding possible.
In addition, an accompanying upper-level trough overhead will bring some higher winds over the Great Lakes, leading to stronger shear and the potential for some severe weather.
There is a targeted Slight Risk (level 2/5) from the Storm Prediction Center over portions of eastern Michigan mainly for the risk of damaging winds.
On Saturday, the front will progress further south and eastward into the Ohio Valley and Northeast while slowing in progress and becoming nearly stationary with southwestward extent into the Southern Plains.
A broader area of severe weather is expected over portions of the northern/central Appalachians and Mid-Atlantic as the upper-level trough digs southward over the region.
Damaging winds will once again be the primary threat.
In addition, isolated flash flooding will remain possible, with the potential that a higher threat may materialize given the slowing frontal progression..
Further south, another more concentrated area of heavier rainfall is expected to continue along 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 may lead to locally higher rain totals of 2-4"+ over Friday and Saturday.
A Slight Risk of Excessive Rainfall (level 2/4) has been included for both days for the potential of some scattered flash flooding.
Daily thunderstorms are also forecast over the central/southern Florida Peninsula.
One more day of excessive heat is forecast over the Ohio/Tennessee Valleys and southern Appalachians today as temperatures soar once again into the mid- to upper 90s.
High humidity may bring head indices into the low 100s.
A few record-tying/breaking highs will also be possible.
The approaching cold front and increasing storm chances/cloud cover will finally bring some relief on Saturday as highs drop into the 80s across the Middle Mississippi and Lower Ohio Valleys, with upper 80s to low 90s for the Upper Ohio and Tennessee Valleys.
Elsewhere, highs will be particularly mild north of the frontal boundary across the northern Mid-Atlantic and New England, with 70s and even some upper 60s expected today.
Temperatures will warm up a few degrees as the boundary lifts northward as a warm front on Saturday.
Conditions will also be well below average over portions of the Southern Plains to the northwest of the frontal boundary, with low to mid-80s forecast.
Areas ahead of the front from Texas into the Southeast and the southern Mid-Atlantic will generally be in the upper 80s to low 90s.
In the wake of the departing trough to the east, an upper-level ridge over the Pacific Coast will shift inland, helping to focus hot, much above average temperatures over the northern tier of the West.
Forecast highs on Friday for the northern Great Basin and Pacific Northwest will range between the low to mid-90s for most locations.
Then, on Saturday, the heat will intensify as well as expand into the northern Rockies, with many highs in the mid- to upper 90s, 10-20 degrees above average.
Heat 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.
Elsewhere in the West, temperatures will generally be near or just above average, with 60s and 70s along the coast, 80s to low 90s for the Great Basin, mid-90s for the central California Valleys, and 100s in the Desert Southwest.
A few showers and thunderstorms will be possible along a lingering frontal boundary through the central/southern Rockies and into the Southwest. ■