A lengthy period of troughing in the Intermountain West, as well as ridging in the Southeast, continues to have an impact on the continental United States.
Cold Weather with Wind Chills in the North
Cold temperatures are most prevalent under the troughing pattern in the West, with dangerously low wind chills prevailing in the Northern Plains and the Rocky Mountains.
By the end of Thursday, a strong cold front will be pushing the freezing air mass southward into the Central Plains, where nightly lows may fall below zero by the beginning of Friday.
The chilly weather will be joined by increasing snowfall in mountain ranges throughout the West, with large accumulations forecast in certain areas of the Sierra Nevada.
The southern Sierra Nevada and Transverse range of southern California, as well as the Wasatch and the central Rockies, are expected to get the greatest snow totals today, according to forecasts.
In some places, snow accumulations on the highest peaks will be measured in feet, which, when coupled with whiteout conditions in these areas, will result in dangerous and often impossible travel conditions.
As a result of high snow accumulations across the Transverse Range and southern Nevada today, the latest World Weather Center Winter Storm Severity Index (WSSI) indicates moderate to locally catastrophic possible winter storm consequences.
Getting closer to southern California, a storm system that's circling the base of the upper trough in the West is funneling moist air from the Pacific into the Los Angeles and San Diego metropolitan regions.
The possibility for excessive rainfall is significant enough to justify a Slight Risk (threat level 2/5), which means that there is a risk of flash floods today and Thursday, as well as on Friday.
Finally, on Thursday, High Wind Warnings were issued for parts of southern Wyoming due to the possibility of severe crosswinds creating dangerous driving conditions for lightweight and high-profile vehicles traveling in the area.
Mild Temperatures in the South and East-Central Areas of the Country
Further east, the anchored ridge above the southeastern United States shows no signs of losing its spring-like regime in the South and the East-Central United States of America.
From the Southeast coast to the Rio Grande in South Texas, several record-breaking lows and highs are expected today.
In the Southeast, temperatures will remain in the 70s and lows will be 20 to 40 degrees above average on Thursday, according to the National Weather Service.
Beginning Friday, the unseasonably warm weather is likely to move into the Mid-Atlantic and areas of the Northeast, where current predicted daytime highs and mild low temperatures may challenge several records throughout the region, just in time for the new year.
According to the Storm Prediction Center, as a result of the high temperatures paired with windy conditions, there is an Elevated Risk (threat level 1/3) of fire weather for sections of the southern High Plains today and tomorrow.
Precipitation will be favorably influenced by a moist southerly flow and growing instability levels, creating a more favorable setting for severe weather.
In the Lower Mississippi Valley, the Storm Prediction Center has issued an Enhanced Risk (threat level 3/5), while a Slight Risk (threat level 2/5) has been issued for a region stretching from Northeast Louisiana to eastward the southern Appalachians, where tornadoes, damaging wind gusts, and isolated large hail are all possible today, according to the center.
The danger of Excessive Rainfall is exacerbated by the high anomalous moisture content, which has resulted in tthe issue of a Slight Risk (threat level 2/5) for the Tennessee Valley today owing to a greater likelihood of localized flash floods.
While scattered showers and thunderstorms will likely be probable throughout large sections of the South on Thursday, the chance of severe weather and flash flooding will be reduced when compared to today's threat.
Severe Weather on Friday
On Friday, a more concentrated danger of severe weather and heavy rainfall will return to the Lower Tennessee Valley, with a Slight Risk in force for both hazards in the region.
Meanwhile, light rain and snow showers are still likely throughout the Lower Great Lakes on Wednesday evening, a bit farther north.
In addition to a dusting of 2 inches of snowfall likely from southern Iowa to northern Illinois and Indiana, the possibility of light but a dangerous coating of ice from northeast Missouri to central Illinois and Indiana is also probable.
As the aforementioned strong cold front marches south from the Northern Plains, a more widespread danger of winter weather across the Plains and Midwest are expected to develop this weekend and into the following week.
As the storm approaches, accumulations of snow and mixed precipitation are still likely along and north of the front.
However, there is still some uncertainty about the particular accumulations of snow and mixed precipitation that will occur.