A block in the upper level flow has prevented any movement by a cold front in the Nation’s midsection to eastern portion of the United States. That is about to change as another upper level low pressure system moving into the Western United States from the Pacific Ocean helps to break down the block. This allows the stalled upper low over the Southwestern United States to move northeast into the Northern Plains. As it does, cold air, which already resided over portions of the Northern Plains, will allow for snowfall from Monday into Tuesday. Models have come into better agreement that an area from eastern South Dakota to Southwestern Minnesota to eastern Nebraska could see upwards of 6 inches as the upper level low has its strongest area of energy rotate around and allow winds from the southeast to come in and provide a better supply of moisture within the already cold air. Overall, the snow swath of 1 inch or more should fall north of the upper level low’s track. That would designate an area from Colorado, east into Western Nebraska, north to south-central North Dakota, east into Northeast Minnesota and western portions of the Upper Peninsula of Michigan and from Wisconsin back to Western Nebraska.
To the upper level low pressure system’s southeast, the aforementioned cold front will continue to provide plenty of moisture in the Ohio River Basin of the Mississippi River as wave after wave of low pressure continues to move through that region. Over 5 inches of rainfall is forecast in that region over the next 5 days. Thankfully, the cold front will be pushed slowly east away from the region by Wednesday as the upper level low pressure system in the Northern Plains intensifies and moves northeast into Canada.
At the same time, a high pressure system moves southeast out of Canada into the Northeast on Monday with plenty of cold, dense and dry air. Warmer and lighter air from the south will slowly move north into the Northeast, but some of the leftover cold may allow for some freezing on roadway surfaces as the rain comes into an area east of the Appalachians where cold air sagged south on Monday and will be stuck there (called cold air damming). The lighter warm air runs over the denser cold air and, when rain falls, it freezes on the surface if it’s at or below freezing. The other area worth watching will be New England overnight Monday night as the cooler air from Monday may allow for freezing on roadways especially in higher elevations and valleys. As is common in these situations, dry air puts a hold on rain coming north until it can moisten the lower levels of the atmosphere. This may push the rainfall start time until later.
A tool that meteorologists use to better understand the pattern is analog guidance, which looks at situations where the setup was similar and takes their observed outcomes like wind damage or snowfall amounts. It can be used in this case to gauge the outcome of the onset of precipitation into New England. Based on projections of the short range North American Model, it indicates an appreciable chance of freezing precipitation especially from New York into Southern New Hampshire, where the highest percent of similar setups have any freezing rain. While this doesn’t necessarily mean that the freezing rain will occur in these areas, it does at least indicate freezing rain is a possibility in the Northeast.