Several developing waves of low pressure acting on a stalling frontal boundary will continue to be the focus for the threat of life-threatening flash flooding and significant severe weather from the Ohio Valley west-southwest through the Middle/Lower Mississippi Valley, Mid-South, and ArkLaTex the next couple of days.
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Deep moisture pooling along the front combined with impressive dynamics will set the stage for persistent rounds of intense thunderstorms capable of producing torrential downpours throughout the region.
This will especially be the case for the Lower Ohio Valley into the Mid-South and Lower Mississippi Valley, where the necessary ingredients of moisture, instability, forcing, and wind shear best overlap.
Storm initiation and movement roughly parallel to the boundary will lead to repeated rounds of heavy rainfall over the same areas, leading to significant to extreme, potentially historic, rainfall totals over increasingly saturated soils.
High Risks of Excessive Rainfall (level 4/4) remain in effect through Saturday, highlighting an increasingly dangerous and life-threatening situation.
Outside of the High Risk, Moderate and Slight Risks also continue to be in place over broader areas of the Ohio Valley into the south-central U.S. through Saturday where scattered to numerous instances of flash flooding can be expected.
As has been said repeatedly for the last several days, this has the makings of a catastrophic, potentially historic heavy rainfall and flash flood event, with some locations potentially seeing rainfall amounts as
high as 10-15"+ through the weekend.
The repeating rounds of heavy rainfall each day will also raise the risk of widespread significant river
flooding, including potentially catastrophic major flooding, in the Ozarks, portions of the Middle Mississippi Valley, and much of the Ohio Valley.
Thankfully, the heaviest rainfall should ease somewhat for the hardest hit areas on Sunday as the frontal boundary finally begins to push eastward into the Southeast and Mid-Atlantic; however, major river
flooding is likely to continue into early next week.
With the front on the move, the focus for heavy rainfall shifts eastward into parts of the Carolinas southward to the Central Gulf Coast to round out the weekend.
A Slight Risk (level 2/4) remains in effect as a result.
In addition to the flash flood threat, additional bouts of significant severe weather will also be in play for many of the same locations through the weekend.
The same ingredients of moisture, instability, forcing, and wind shear that are conducive to intense downpour producing thunderstorms are also supportive of supercells and severe weather.
The Storm Prediction Center has issued a Moderate Risk (level 4/5) for Friday from the ArkLaTex
into southeast Missouri for strong tornadoes, very large hail, and damaging winds.
A broader Slight Risk is in place from Texas through the Lower Ohio Valley.
On Saturday, Slight to Enhanced Risks are delineated for the Lower Mississippi Valley through the Mid-South and Ohio Valley, with severe thunderstorms capable of producing strong tornadoes, very large hail, and damaging wind gusts above 70 mph.
Similar to the flash flood threat, the severe weather risk shifts eastward into parts of the Mid-Atlantic, Southeast, and southern Appalachians on Sunday, with the Storm Prediction Center issuing a Slight Risk for the potential for damaging winds and possibly a few tornadoes.
Farther west, a passing cold front will help support snow showers over the Rockies as well as portions of the High Plains the next couple of days.
Accumulations are expected to remain light with snows over the Plains, though a corridor of a few inches of snow could focus in North Dakota.
To the south, an area of low pressure in the lee of the Rockies will help to focus upslope flow along the mountains, resulting in moderate to locally heavy snows for the regional mountain ranges on Friday, with the potential for some snow for portions of the High Plains along the Front Range.
The cold front to the north will also pass southward by late Friday and into the day Saturday, bringing renewed post-frontal upslope flow and additional heavy snow for the mountains of the Front Range.
There is also an increasing potential for accumulating snow to spread across portions of New Mexico and into the southern High Plains later Friday into Saturday.
Drier weather returns on Sunday as high pressure takes control.
The highly amplified flow pattern will lead to well above average temperatures across the Southeast as an upper-level ridge dominates the region.
Meanwhile, temperatures will remain much cooler and below average under the upper-level trough over most of the western/central U.S. Highs the next couple days across the Southeast are forecast to be in the 80s to low 90s, with numerous record-tying/breaking highs possible.
Farther north, the Mid-Atlantic/Northeast will cool to the 50s and 60s for highs on Friday, and a cool surface high will take hold in the Northeast Saturday for below normal highs in the 40s.
Some wintry weather can't be ruled out across northern New England on Sunday either given persistently
cool temperatures on the north side of the front.
Meanwhile, forecast highs across the Plains into the Interior West will be mostly well below average.
Northern and central areas of the Rockies and Plains should see high temperatures in the 30s and 40s on Friday, and southern Rockies/High Plains can expect highs that are below average by 20-25 degrees by
Saturday with actual highs falling into the 40s.
The West Coast can expect warmer, above average temperatures compared to the rest of the West as a
ridge builds northward over the region, with highs in the 60s and 70s.
The West will continue to warm up on Sunday while cooler air across the Plains spreads eastward into the Ohio Valley, the Mid-South, and the Lower Mississippi Valley. ■