More than 240 million people across the U.S. will be getting some relief from the recent bitter blast of arctic air, but for those living and working in the Northeast, that increase in temperatures coupled with rounds of heavy rain expected to push through the region this week has forecasters concerned about flooding due to rapid snowmelt.HOW TO WATCH FOX WEATHERIt’s been a cold start to December, but for millions of people, the temperatures will flip starting Sunday and Monday, with forecast highs rising as much as 10-15 degrees above average instead of the recent below-average temperatures.By Monday, about 245 million people from coast to coast will be feeling the milder temperatures, but the warmup will be brief.DOWNLOAD THE FREE FOX WEATHER APPThese are by no means summer temperatures, so don’t go running to the beach just yet.Chicago’s high temperature is expected to be in the lower 50s on Sunday, while the New York City metro area is expected to be in the mid-50s.As we approach the start of the new workweek, temperatures will rise into the 40s and 50s in the Northeast, while northern New England will stay on the cooler side.But as temperatures rise, the snowmelt will increase, and that could lead to some localized flooding and runoff, especially in areas that just picked up feet of snow from the first significant lake-effect snowstorms of the season.Warmer temperatures won’t last, however, as another strong cold front pushes through the region on Wednesday. That front will send temperatures plunging once again, reminding millions of people that we haven’t jumped from winter to spring.As millions of people in the Northeast and New England enjoy the briefly warmer temperatures, rounds of beneficial rain are also expected across the region, including the Interstate 95 corridor from New York City through Providence, Rhode Island, and Boston.Some areas could receive several inches of rain this week, which will undoubtedly help to alleviate the monthslong drought that helped fuel wildfires across the region, though it won’t entirely erase the drought.The first round of rain is expected Monday afternoon, but it won’t be long-duration like what’s expected later this week.The FOX Forecast Center said an area of low pressure moving along the U.S.-Canada border in the Midwest will combine with a surge of moisture that’s also fueling a flood threat in the South.Rain on Monday morning will mainly affect areas to the west of the I-95 corridor.Cities like Pittsburgh, Pennsylvania, Columbus, Ohio, and Charleston, West Virginia, will likely need the umbrellas to start the day.By the afternoon, heavier rain will move into the I-95 corridor just in time for the Monday evening rush. And to the west, in communities near Lake Erie and Lake Ontario, rapid snowmelt from the warmer temperatures and rain could lead to flooding.The second round of rain is expected by midweek and will have a much larger impact across the Northeast.Wednesday is expected to be a washout, and NOAA’s Weather Prediction Center (WPC) has placed millions of people from Connecticut to Maine, including Boston, in a Level 2 out of 4 risk for flooding.As colder air rushes back into the region behind the cold front fueling Wednesday’s rainfall, rain could mix with or change over to snow over parts of the interior Northeast by Wednesday night. However, it’s too soon to predict where snow will fall and how much could accumulate due to the wet ground and warmer temperatures before the winter weather arrives.