A multi-day heavy rain and flash flood threat is expected to develop over portions of the Southern Plains Tuesday night and expand further eastward into the Lower Mississippi and Tennessee Valleys Wednesday.
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A quasi-stationary surface boundary beneath southwesterly flow aloft as moisture increases from both the Gulf of Mexico as well as a stream sourced from the Pacific will combine to trigger the development of multiple rounds of nightly thunderstorms beginning Tuesday night over Oklahoma.
There is a Slight Risk (level 2/4) of Excessive Rainfall as anomalously high moisture leading to rain rates upwards of 1"+ per hour as well as the tendency for storms to back-build over the same regions given generally unidirectional southwesterly flow aloft will lead to the risk of locally heavy rainfall totals of 2-4" and the potential for scattered instances of flash flooding.
Another round of storms is expected to develop late Wednesday night in the same general vicinity over Oklahoma and Arkansas, leading to a greater concern for more numerous instances of flooding given the prior day's rainfall and therefore the issuance of a Moderate Risk (level 3/4) of Excessive Rainfall.
The expectation is that more of these storms will progress further downstream into the Lower Mississippi and Tennessee Valleys late Wednesday night/early Thursday morning, expanding the flash flood threat.
There will be quite a stark contrast in temperatures north and south of this boundary, with highs generally in the 80s to the south over Texas and the Lower Mississippi Valley, with even some low to mid-90s possible along the Rio Grande.
On the other hand, highs will be in the 40s and 50s to the north over portions of the Central/Southern High Plains, Central Plains, and Middle-Mississippi Valley.
On a broader scale, conditions will be a bit more unsettled and cooler over the West and the Plains compared to the East as upper-level troughing remains in place to the west with downstream ridging to the east.
A persistent series of lows off the northwest Pacific coast and upper-level energy rotating around the trough in place will help to encourage nearly continuous lower elevation valley/coastal rain and higher elevation mountain snow chances for the Pacific Northwest and northern California.
Heavy snow is forecast for the northern coastal ranges and Sierra in California Tuesday where higher elevations may see totals well over a foot.
Some thunderstorms will be possible along the coast, and light snow may mix in for valley locations especially Wednesday night along the I-5 urban corridor of the Northwest.
Additional snow showers are possible for the Northern/Central Rockies Wednesday as a lingering surface boundary remains draped across the region and the upper-level energy passes through.
Temperatures Tuesday and Wednesday will be unseasonably cool over most of the West.
Highs will be in the 20s and 30s for the Northern/Central Rockies and Great Basin, 40s for the Pacific Northwest, and 50s for California.
Some of these highs in portions of the northern Great Basin into central/northern California are at or at least close to record high minimums for the date.
Portions of the Southwest and Southern Rockies will be the one warm spot for the West, with highs in the 60s and 70s.
A couple waves of energy rotating around the western trough will also lead to rounds of moderate to locally heavy snow across portions of the Northern and Central Plains.
The first round will continue through the day Tuesday across portions of the Dakotas, where 3-6", locally higher, will be possible through the evening.
Another round will begin later Wednesday and continue into Thursday for portions of the Northern and Central Plains as a more significant shortwave moves out from the West.
Rain showers and a wintry mix will be possible further south into the Central Plains.
Further east, some light snow showers will be possible for the Interior Northeast, but otherwise conditions will be mostly dry for the Midwest and East Coast.
High temperatures will generally be seasonable for New England and the northern Mid-Atlantic, with 30s and 40s forecast.
Highs will be closer to early Spring for the southern Mid-Atlantic and Carolinas Tuesday with 60s and 70s forecast, and more late-Spring like closer to the Gulf Coast and Florida with low to mid-80s forecast.
A number of these highs in the Southeast will be close to record-tying/breaking levels.
A cold front passing through will cool most locations back to more seasonable conditions Wednesday except along the immediate Gulf coast and into southern Florida where 80s will persist. ■