U.S.: Severe thunderstorms forecast for portions of Plains, Mississippi Valley, and mid south
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This will lead to subsequent reinforcement/organization of a warm front pushing northward over the Central Plains and Midwest, a Pacific front moving east over the Rockies/High Plains, and a dryline extending southward through the Central/Southern Plains.
A warm, moist airmass will surge northward over the Plains Thursday evening as a strong low level jet develops.
Some moderate to locally heavy rainfall is possible with storms developing along the warm front, particularly as storms may have the tendency to repeat over the same areas as motions remain oriented along the front.
A Marginal Risk of Excessive Rainfall (level 1/4) is in effect for portions of the Upper Mississippi Valley into the Upper Great Lakes as a few isolated instances of flash flooding are possible.
A Marginal Risk of Severe Weather (level 1/5) has also been issued by the Storm Prediction Center over portions of the Central/Southern Plains for the chance of a couple isolated supercells ahead of the dryline that could produce large hail, damaging winds, and a tornado.
More widespread, impactful weather is expected Friday as a vigorous upper-level jet moves out over the Mississippi Valley and higher moisture from the Gulf has had a chance to flow northward.
Numerous and widespread storms are expected to develop ahead of and along the eastward moving cold front.
Moderate to strong buoyancy in the presence of intense upper-level and low-level shear has prompted two Moderate Risk outlooks for Severe Weather (level 4/5) from the Storm Prediction Center.
One risk area centers on the Upper Mississippi Valley, where storms will have the potential to produce widespread, significant damaging winds along with large hail and tornadoes.
Another focus area will be on the Mid-South, where very strong low level shear will increase the risk for strong, long-track tornadoes in addition to large hail and damaging winds.
A lower, but still significant risk of severe thunderstorms will exist outside these regions across the Mississippi Valley.
While the front is expected to remain rather progressive, anomalously high moisture leading to very heavy rainfall rates and storm motions roughly parallel to the front resulting in repeated rounds of rainfall may also lead to some flash flooding concerns.
A Slight Risk of Excessive Rainfall (level 2/4) has been introduced from the Mid-South eastward into the Ohio and Tennessee Valleys.
Further west behind the dryline, a tightening pressure gradient with the deepening surface cyclone ahead of the Pacific cold front pushing eastward will lead to very strong, gusty downsloping winds over the Central/Southern High Plains.
The Storm Prediction Center has issued a Critical Risk of Fire Weather for both Thursday and Friday over the region as temperatures soar into the upper 70s and low 80s, relative humidity decreases, and gusts upwards of 50 mph will promote the risk for wildfires.
High temperatures overall will be unseasonably warm over much of the central/eastern U.S.
as the warm front lifts northward.
Highs in the 60s and 70s, and even some low 80s, over the Central/Southern High Plains, Mississippi Valley, and Southeast Thursday will spread northeastward into the Ohio Valley, Lower Great Lakes, and Mid-Atlantic Friday.
Meanwhile, further north, a secondary cold front pushing south from Canada will help to reinforce the cold air over the region as moisture streams northward, leading to areas of heavy snow from the Northern Plains into the Upper Midwest beginning late Thursday night.
Winter weather-related advisories are in effect from South Dakota east into Minnesota and Wisconsin for the potential of snow totals generally between 3-6", with locally heavier totals of 10"+ possible.
In addition, gusty winds behind the front upwards of 50 mph will lead to areas of blowing snow and blizzard conditions, making travel treacherous to impossible.
A wintry mix is forecast along the southern end of the snow axis, with the potential for some impactful ice accumulations.
Temperatures will remain unseasonably chilly with highs Thursday and Friday in the 20s and 30s.
Lows may dip into the single digits for some locations Saturday morning.
The cold front sliding southward over the High Plains Friday will drop highs into the 30s and 40s for the Central High Plains.
There will be quite the stark contrast in temperatures between the frontal boundaries draped across the region, with highs on Friday in Iowa forecast to reach the low 70s and highs just to the northwest over South Dakota in the upper 20s and low 30s.
Snow will continue over the Intermountain West under the influence of the deep upper-level trough and a frontal system pushing through the region.
Additional locally heavy accumulations of 6-12"+ are possible for higher elevations in the mountain ranges of the eastern Great Basin and Northern/Central Rockies Thursday into early Friday.
Little to no snow accumulations are expected for most lower elevation valley locations, however some light accumulations will be possible over the Northern High Plains.
Another Pacific storm system will approach the Pacific Northwest early Friday, bringing moderate to locally heavy lower elevation/coastal rain and heavy snow accumulations to higher elevations of the Cascades.
Moisture will spread inland over the northern Great Basin/Northern Rockies by Friday evening, with accumulating snowfall forecast for higher elevation mountain ranges here as well.
High temperatures will remain well below average across the West under the influence of the upper-level trough and behind the initial Pacific cold front pushing through the region.
Highs will generally be in the 30s and 40s for the Northern/Central Rockies and Great Basin Thursday-Friday, with highs in the 50s over the Pacific Northwest Thursday also dropping into the 40s Friday.
Highs in California will only be in the 50s for most locations Thursday, with the potential for some record-tying/breaking low maximum temperatures over southern California.
The Desert Southwest will be in the 60s Thursday, with some low 70s possible Friday. ■