A multi-day heavy rain and flash flood threat is expected to continue over portions of the Southern Plains and Lower Mississippi Valley Wednesday and Thursday after an initial round of heavy rain late Tuesday night.
Article continues below
A quasi-stationary surface boundary sits beneath southwesterly flow aloft with ample moisture flowing in from both the Gulf of Mexico and an additional stream sourced from the Pacific.
A shortwave passing over the region is forecast to trigger another round of thunderstorms Wednesday afternoon and evening.
Finally, a cold front will help to initiate a final round of thunderstorms late Thursday.
Marginal Risks (level 1/4) of Excessive Rainfall have been issued for both Wednesday and Thursday from southern Oklahoma/northern Texas into southern Arkansas as a chance of isolated regions of flash flooding will be possible as a result of repeated instances of thunderstorms.
There will be quite a stark contrast in temperatures north and south of this stationary boundary, with highs generally in the upper 70s and 80s to the south over Texas and the Lower Mississippi Valley.
Some highs may reach the low to mid-90s along and near the Rio Grande.
On the other hand, highs will be in the 40s and 50s to the north over portions of the Central Plains and Middle-Mississippi Valley.
Temperatures in the Southern High Plains appear more likely to recover Wednesday as the boundary lifts north of the area, bringing warm air into portions of the Southern Plains.
A cold frontal passage late Thursday evening will bring more seasonable temperatures across the South Plains on Friday.
On a broader scale, conditions across the West and Plains will be cooler and more unsettled than the East as this winter's dominant pattern of upper-level troughing over the West with downstream ridging to the east continues.
A persistent storm system lingering off the northwest Pacific coast and upper-level energy rotating around the trough in place will result in nearly continuous lower elevation valley/coastal rain and higher elevation mountain snow chances for the Pacific Northwest and northern California before an even more impactful Atmospheric River approaches Thursday.
Some thunderstorms will be possible along the coast, with chances of light snow for some 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.
Forecast precipitation markedly increases on Thursday with the arrival of the Atmospheric River, raising snow levels and bringing the threat for heavy rainfall and flooding portions of the West.
A Moderate Risk (level 3/4) of Excessive Rainfall is in effect on Thursday and Friday over central coastal California and the central interior valleys as the threat of several inches of heavy rain, including some areas of higher terrain with a snowpack, will impact the area.
Rainfall at higher-elevations may contribute to rapid snowmelt, which may lead to numerous flooding instances downstream.
At higher elevations in the mountains, 12"+ of snow will be possible for portions of the Cascades and Sierra, with additional heavy snowfall also forecast for the Blue Mountains of Oregon and Snake River Ranges in Idaho.
Temperatures on Wednesday and Thursday will be unseasonably cool over most of the West.
High temperatures will be in the 20s and 30s for the Northern/Central Rockies, 30s and 40s for the Great Basin, 40s for the Pacific Northwest, and 50s and 60s for much of California.
Portions of the Southwest and Southern Rockies will be comparatively warmer, with temperatures maximizing in the 60s and 70s for much of the region.
Light snow showers over the Northern Plains/Upper Midwest and a wintry mix/rain for the Central Plains will linger through the day Wednesday before a more potent shortwave approaches from the west, helping to better organize the frontal system currently over the Plains and increase precipitation chances overall.
The chances for locally heavier snow are highest for the western Dakotas late Wednesday night and into the day Thursday, which will stretch into the Upper Midwest on Thursday.
Some gusty winds may lead to a few areas of blowing snow and difficult travel conditions.
Further east, some light snow showers will be possible for the Interior Northeast, but otherwise conditions will be mostly dry for the East Coast through Thursday.
High temperatures during the latter half of the week will generally be seasonable with temperatures in the 30s and 40s for New England/the northern Mid-Atlantic, 50s and 60s for the southern Mid-Atlantic and the Carolinas, and 60s and 70s for the Southeast and Florida.
The system over the Midwest will begin to push towards the Appalachians/East Coast late Thursday night/early Friday morning, increasing precipitation chances towards the end of the current forecast period.
The initial burst of precipitation may fall as snow along the I-95 corridor, but accumulations will be light the mode of precipitation changes to rain.
There also may be some light ice accumulations at higher elevations, particularly in the northern Appalachians. ■