A split flow pattern is going to affect the United States Sunday through much of the upcoming week. After a cold front pushes through the Northeast Saturday evening, cooler air will be able to move in with a disturbance coming in from Northern Canada. Temperatures will be 5 to 15 degrees below normal, especially Sunday and Monday. A warm front will move into the region for Tuesday so temperatures will be slightly warmer.
Another part of this split flow pattern will be a low pressure system moving into the Western United States from the Pacific, which will dive south by the middle of the week. This will allow the Polar jet stream to dive south and bring daytime temperatures into an area from the Plains to Mississippi and Ohio Valleys that are as much as 10 to 25 degrees below normal. At the same time, the subtropical jet stream will be able to interact and allow for areas of large scale lift during the week and result in huge amounts of precipitation. When jet streaks are next to each other, their resultant areas of large scale lift, which occur in at the left side of the end of the jet streak and right side of the beginning of the jet streak, can overlap each other and create an even faster jet stream and result in huge amounts of lift. Within these areas of huge large scale lift, plenty of rain usually falls as a result of daytime thunderstorms, which give way to decaying nighttime thunderstorms becoming mesoscale convective systems (MCSs). Over the next week, this trade off between daytime popping thunderstorms and nighttime decaying storm systems will allow for huge amounts of rain. Forecast totals reach 5 to possibly near 10 inches of rain in Southern Texas over the next 7 days, especially in storms that go over the same area multiple times, slow moving storms and just general thunderstorms with heavy rain.