The mid- to upper level flow pattern will become increasingly amplified over the next few days with an omega block forming from California, eastward into the Ohio Valley.
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This pattern will bring periods of unsettled weather first to portions of the eastern and southern U.S., and
eventually to the western U.S. heading into the weekend.
Southerly flow supporting a moist airmass along a cold front stretching from the Lower Great Lakes southwestward through the Ohio Valley, Middle/Lower Mississippi Valley, and southern Plains will help to focus the first area of showers and thunderstorms.
The boundary is forecast to slow through Friday and become quasi-stationary in some areas, bringing repeated rounds of storms as storms will have a tendency to move generally parallel to the boundary.
The plentiful moisture will lead to locally heavy downpours, and there is a Slight Risk of Excessive Rainfall (level 2/4) across portions of the Lower Mississippi Valley/Mid-South, ArkLaTex, and southern Plains
where scattered instances of flash flooding will be possible.
In addition, increasing upper-level winds as an upper-level trough drops southward over the very moist, buoyant airmass will lead to some more intense clusters of thunderstorms.
The Storm Prediction Center has outlined an Enhanced Risk of severe weather (level 3/5) stretching from the Ohio Valley southwestward through the Tennessee Valley, the Lower Mississippi Valley,
and into portions of Texas for the threat of damaging winds, large hail (especially in Texas), and perhaps a tornado or two.
The front will make some progress south and eastward into Saturday, bringing shower and thunderstorm chances to the Northeast, Mid-Atlantic, Appalachians, and Southeast.
Moderate to locally heavy rainfall is expected with some additional isolated instances of flash flooding possible.
To the West, some scattered showers and thunderstorms will be possible along a lingering frontal boundary through the Four Corners region Saturday before a second upper-level trough solidifying the omega block drops southward from the northeastern Pacific and over the West Coast by late Friday/early Saturday.
Precipitation chances will first pick up over the Pacific Northwest Friday evening before spreading into the Great Basin and northern Rockies Saturday.
Some snow will be possible for some of the higher mountain peaks, particularly in the Sierra Nevada.
The increasingly amplified pattern stretching west to east across the CONUS will bring large temperatures contrasts heading into the weekend.
Forecast highs from the Ohio/Tennessee Valleys east to the East Coast will remain above average Friday, with the greatest anomalies focused across portions of the Mid-Atlantic.
Forecast highs generally range in the 70s across the Northeast and Ohio Valley with 80s for the Mid-Atlantic and the Tennessee Valley/broader Southeast.
The cold front will bring cooler temperatures to the Ohio Valley and much of the Southeast Saturday as
highs fall into the 60s and 70s.
Highs along the immediate East Coast will remain well above average and into the 70s north with 80s further south.
Portions of the Great Lakes/Midwest and much of the Plains will be well below average the next couple of days under the eastern trough, with highs generally in the 50s and 60s for the Great Lakes/Midwest and 60s and 70s for the central/southern Plains outside of southern Texas.
Much of the West will be well above average Friday, especially across the northern Great Basin/Rockies, as highs rise into the 70s and 80s for most locations.
The well above average temperatures will shift eastward on Saturday as the second upper-level trough drops southward along the West Coast. Highs across the northern Plains, northern/central Rockies, and
eastern portions of the Great Basin will remain and rise into the 70s and 80s.
Much cooler temperatures will come to the West Coast under the influence of the trough with highs dropping into the 50s and 60s. ■