A persistent storm system lingering off the northwest Pacific coast will better organize Thursday as upper-level energy approaches and moisture tapped deep from the Pacific streams in bringing yet another Atmospheric River event to the West Coast in what has already been an active season.
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After persistent light to moderate showers at lower/coastal elevations and snow into the mountains, temperatures will warm Thursday raising snow levels significantly and bringing the heavy rain threat up into potions of the higher terrain as well.
Moderate Risks (level 3/4) of Excessive Rainfall are in effect on Thursday and Friday over central coastal California and the central interior valleys, particularly in favorable upslope regions along the terrain of the coastal ranges and the Sierra, where several inches of rain are expected.
In addition, rainfall at higher-elevations may contribute to rapid snowmelt, which may further exacerbate the potential for scattered to numerous flooding instances downstream.
Warmer temperatures will likely push the threat for flooding over to the east slope of the Sierra in the Great Basin as well.
Moderate to locally heavy rainfall will also spread over the Pacific Northwest but without the flooding concerns seen further south as the area will not tap into the tropical moisture.
Additionally, heavy snowfall will spread across higher mountain elevations in the Cascades, Sierra, and interior ranges of the northern Great Basin Thursday into portions of the northern/central Rockies by Friday.
Snow totals over a foot are likely, with locally much higher totals of multiple feet possible, especially for portions of the Sierra.
Interior valley locations will likely see a mix of rain and snow, keeping any snow accumulations low.
Numerous and widespread wind-related advisories are in effect for northern/central California and portions of the northern/central Great Basin as gusts may reach upwards of 40-50 mph at lower elevations and up to 70 mph at higher elevations.
High temperatures Thursday will remain unseasonably cool for most of the West, with widespread 30s and 40s for interior locations and the Pacific Northwest with 50s for northern/central California.
Temperatures will warm a bit for some Friday given the incoming Pacific air, especially across the Great Basin, where highs will be into the 40s and 50s.
The Desert Southwest and Southern Rockies will remain warmer than the rest of the region, with highs in the 60s and 70s on Thursday reaching into the low 80s for some locations Friday.
Further east, an upper level wave pushing across the Plains and into the Midwest will aid the organization of an eastward moving frontal system and continue to spread snow from the Northern/Central Plains into the Great Lakes Thursday and portions of the central/northern Appalachians by Friday.
The highest chances for locally heavier snow of 6"+ will be from southern Minnesota and northern Iowa into southern Wisconsin on Thursday, and favorable lake-effect locations off Lake Erie and into higher elevations of the central/northern Appalachians in Pennsylvania into Friday.
Snow rates may exceed an 1" per hour at times, leading to difficult travel conditions.
The system over the West will eventually move into the Northern Plains Friday, bringing renewed snow chances to portions of Montana and the Dakotas with some locally heavier totals possible near the Canadian border.
High temperatures will generally be seasonable across the Midwest, with 30s and 40s for Thursday and Friday.
Highs will be a bit chillier further west, with highs only into the 20s for much of the Northern Plains.
The chance for showers and thunderstorms will linger across much of the southern tier of the country as a stagnant quasi-stationary boundary that has brought heavy rain to the region remains for Thursday.
Some locally heavy rainfall will be possible across portions of eastern Texas and into the Lower Mississippi Valley, though any flash flood risk is more marginal compared to previous days.
The incoming upper-level shortwave will help to finally push the boundary southward as a cold front late Thursday night into Friday, bringing an end to storm chances for the Plains and Lower Mississippi Valley while chances increase for the Southeast and Florida.
Highs will once again be unseasonably warm for many locations across Texas and the Lower Mississippi Valley Thursday with 70s and 80s in place, and even some low 90s along the Rio Grande.
The cold front pushing through will cool things down to more seasonable temperatures Friday, with generally 60s for highs outside of the immediate Gulf Coast and far South Texas.
Precipitation chances will also increase for the Mid-Atlantic and Carolinas as the frontal system approaches on Friday.
Light to moderate rain showers can be expected from Virginia south, while some snow may mix in to the north along the I-95 urban corridor Friday night, though very little to no accumulations are currently expected.
High temperatures will generally be in the 30s and 40s for New England, while temperatures will cool from the 50s to low 60s for the coastal Mid-Atlantic and Carolinas Thursday into the 40s and low 50s Friday. ■