Winter is here. Snowy, soggy, and frigid temperatures settle in for the long haul

(CNN)Heavy snow, blustery winds, and bitterly cold temperatures make for a very frozen start to February for those in the Midwest and northeastern US.

Just days after many in the Midwest and Northeast were digging out from record snow, even more flakes will be flying as we head into the weekend.
Winter weather alerts are posted for more than 25 million people Friday as heavy snow slides through the Great Lakes region early in the day. By the afternoon, that system will begin to shift into the Northeast.
    Most areas along the Great Lakes will pick up an additional 2-4 inches, with some isolated higher amounts thanks to the added benefit of lake effect snow.
      Rain will be the main focus from Virginia down through Florida. Thunderstorms will also be likely for much of the Florida Peninsula on Saturday.
        A low-pressure system that will develop off the coast of the Carolinas will slide up the East Coast Sunday. The system's proximity to the New England coast will determine whether many northeastern cities get rain or snow.
        "The system will intensify as it tracks off the Eastern Seaboard, and has the potential to bring another round of significant snow across the mid-Atlantic and parts of the Northeast," says Dave Hennen, CNN Meteorologist.
          "There is currently a lot of uncertainty in where/who will get the most snow and whether the major metro areas in the Northeast get hit."

          Chicago closes Covid-19 testing sites

          Due to the anticipated "inclement weather," city-operated Covid-19 testing sites in the Chicago area will close for five days, Chicago's Office of Emergency Management announced Thursday.
          The closures, which will run through February 10, will impact mobile-testing operations as well as static sites at Midway Airport, Douglass Park, Prosser Career Academy, and Gately Park. Each site services roughly 1,000 patients per day when operational, the Chicago Department of Public Health told CNN.
          Chicago is under a winter weather advisory until Friday at noon; and while flurries of snowfall are expected, the main concern is wind. Gusts are expected to reach as high as 45 mph.
          The area is also at risk for excessive cold, with wind chills well below zero during the weekend and into next week
          Residents in need of Covid-19 testing during that time are asked to contact their primary care provider or seek an alternate testing location.

          You can't have snow without cold

          "While it can be too warm to snow, it cannot be too cold to snow," according to the National Snow and Ice Data Center (NSIDC) website. "Snow can occur even at incredibly low temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur when there is relatively warm air near the ground -- typically -9 degrees C (15 degrees F) or warmer -- since warmer air can hold more water vapor."
          The coldest air will start pushing through the upper Midwest on Saturday. Waking up Saturday morning, at least half a dozen states will see temperatures below zero. Even high temperatures are expected to be 25-30 degrees below normal in Minnesota, Iowa, Wisconsin, Illinois, and the Dakotas.
          Temperatures Sunday morning
          From there, the cold air spreads south and east in the following days.
          "Another reinforcing cold front will move through the region Sunday morning," says the National Weather Service Office in Cleveland. "Temperatures will be the coldest so far this season this weekend into early next week with below-average temperatures lingering through next week."
          From Sunday through Thursday of next week, Cleveland will see high temperatures 15-20 degrees below normal, keeping them below freezing for almost a full week.
          It's not just the Midwest that will be seeing this deep freeze. Southern states like Texas, Louisiana and Mississippi will be 15 to 30 degrees below normal Tuesday and Wednesday.
          Another concern is how the bitter cold air will affect the flow of water and traffic on the major rivers in the Midwest.
          "Areas to our south -- the lower portions of the upper Mississippi and the Illinois River -- are seeing relatively decent flows and levels are good for navigation, especially around St. Louis, but extreme cold outbreaks give us a concern for ice bites, which takes flow out of the river," said Justin Palmer, a hydrologist with the North Central River Forecast Center in Chanhassen, Minnesota.
          "Ice gets created in the pools, we drop a lot of flow, which drops stages and becomes an issue for potential groundings. On the Illinois River, however, with this extreme push of cold air, we could potentially see some ice form, but overall, we're seeing good traffic in St. Louis and believe this year the rivers have been more open with this warmer year."
          Ice bite events are associated with frigid cold weather. Ice formation in rivers often causes reductions in flow downstream, which drops water levels, making them unnavigable. This can have a significant impact at St. Louis, leading to low water levels that ground navigation and expose water intakes.

          The polar vortex is on the move

            This frigid cold is compliments of the polar vortex, a large area of low pressure located near the poles. During the winter months, it breaks down, sending cold air that has been bottled up. It can be responsible for extremely frigid temperatures and huge plunges of cold air.
            The polar vortex has been weakened even more so this year by the Sudden Stratospheric Warming events that occur about six times a decade. This will cause the polar vortex to weaken even further, making for bitterly cold conditions.