Urgent freeze warning issued for 18 million Americans in three unusual states TONIGHT

An urgent weather warning has been issued for 18 million Americans as temperatures plunge below freezing through Wednesday. 

Residents across portions of southeast Alabama, the Florida Panhandle and Big Bend, north and northeast Florida, and south-central, southwest and southeast Georgia will see temperatures as low as 28 to 30°F overnight.

The National Weather Service (NWS) said: 'Appropriate action should be taken to ensure tender vegetation and outdoor pets have adequate protection from the cold temperatures. 

Young children, the elderly and the homeless are especially vulnerable to the cold. Take measures to protect them.'

The freeze is caused by a combination of strong high-pressure ridging, clear skies, light winds, and dry air behind a cold front that swept through the region late Monday into Tuesday.

A powerful continental high pressure centered over the Tennessee Valley and southern Appalachians is sinking southward, marking one of the strongest early-season high-pressure systems on record for the Southeast, bringing a deep pool of Arctic-modified air. 

'A frigid cold airmass will continue to bring colder weather across the eastern two-thirds of the country early this week,' the NWS said.

'Record low temperatures are expected to be tied or broken across the Southeast and Florida.'

The warnings are in effect until 8am ET on Thursday.  AccuWeather meteorologists said: 'Cold air of Arctic origin is plunging southward from Canada into the eastern half of the United States, reaching as far as the Gulf Coast and Florida Panhandle early this week. 

Widespread lows in the 20s and 30s will dominate much of the East, with some locations 20 to 25°F degrees below historical averages for early November. Jacksonville, Florida, may approach freezing temperatures Tuesday night, its coldest reading since February.'

The abrupt drop in temperatures is set to send power demand soaring, disrupt supply chains, and threaten unprotected infrastructure and plumbing.

Forecasters warned it could be the first major test of the season for energy and utility systems as cold air lingers into early December.

The states are still recovering from the Arctic blast that hit overnight, dropping temperatures as low as 28°F.

Parts of Florida saw record lows, with cities as far south as Orlando and Tampa potentially tying lows of 39 and 40 degrees, respectively.

Floridians have already seen local iguanas start to fall out of trees, paralyzed by the premature winter blast.

However, the NWS released its forecast for next week, showing temperatures should rise in the East, South and Midwest.

A dome of warmer high pressure is expected to build in the Midwest and East, flipping the weather from freezing to mild like a seesaw in just days.

Wild temperature swings in the Southwest are a common occurrence as the region sits at the jet stream's southern edge, with different weather systems sending lows into the mid-40s or as high as the upper 70s during this time of year.

Illinois and Indiana have a 50 to 60 percent chance of seeing temperatures warm beyond their normal November average of 44 degrees, just days after a major storm crippled travel and blanketed some areas with up to 12 inches of snow.

Tuesday morning, millions of Americans were still under official freeze warnings in the Southeast, with states including Louisiana, Mississippi, Alabama, Georgia, Florida, and the Carolinas all bracing for temperatures in the high 20s.

 Atlanta's record low for Tuesday is 26 degrees, and the meteorologists predicted it could reach around this point, meaning residents may need to drip their faucets to prevent damaged pipes.

The NWS's forecast shows the cold air shifting to the west, where temperatures will dramatically drop across more than 10 states, including California, Nevada, Arizona, New Mexico, Utah, Colorado, Idaho and Wyoming. 

In Southern California, Southern Nevada, and most of Arizona, there's a 60 percent chance temperatures will sharply drop below their normal averages for mid-November, which typically sits between 58 and 64 degrees Fahrenheit.

Meteorologists at AccuWeather also warned that this Arctic shift could bring heavy snow to parts of California as soon as Thursday.

'Temperatures are expected to drop sharply over the Sierra Nevada, allowing snow to accumulate quickly above the passes in northern areas,' the AccuWeather team warned in a statement.

Areas including Lake Tahoe, Donner Pass, and the Sierra Nevada mountain range could all see at least three inches of snowfall, with higher elevations getting up to two feet by Thursday night.