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Weather forecasting and analysis, space and historic events, climate information

1:45 PM | An "omega-shaped" blocking pattern in the upper atmosphere to cause an extended period of chilly, wet weather next week in the Mid-Atlantic region

Paul Dorian



Tonight, tomorrow and Sunday Rain will move into the I-95 corridor late today and it will come down hard at times this evening and into the overnight hours with more than half an inch likely by early tomorrow morning. While much of tomorrow will be rain-free, another round of showers is likely to move into the Mid-Atlantic region during the afternoon as a frontal system passes through the area. Each of these two systems can also feature an isolated thunderstorm or two mixed in with the overall rainfall. Sunday then promises to be a breezy and cool day with some sunshine returning to the Mid-Atlantic region.

Next week Next week is looking downright ugly for much of the central and eastern U.S. with an extended period of wet and chilly weather. The main reason for the dismal outlook is an unfolding “omega-shaped” blocking pattern in the upper part of the atmosphere that is likely to set up next week across the eastern half of the nation. An “omega-shaped” blocking pattern – not too unusual for the month of April - is one in which upper level winds at around 500 millibars (~18,000 feet) actually follow the shape of the Greek letter “omega”. In this type of blocking pattern, surface weather systems (fronts, highs and lows) move very slowly which generally result in extended periods of similar weather conditions for a given location. Also, the upper level features are stuck in place and will change very little between Monday and Wednesday (see 500 mb forecast maps above for Monday and below for Wednesday showing very little change).

In this particular case, it looks like the Mid-Atlantic region will suffer through an extended period of chilly and wet weather with the threat for significant rain likely lasting from later Monday night right into Friday of next week and perhaps even into the weekend. Temperatures for much of this time period will be well below normal in the Mid-Atlantic region for this time of year and also for much of the eastern half of the nation. In fact, this pattern could be cold enough that some late season snow falls in parts of the Upper Midwest and northern Plains later next week. By the time we get to the end of next week, the combination of the rainfall expected during the next 36 hours and the significant rainfall expected next week in the developing blocking pattern, will likely result in at least a few inches of total rainfall over large portions of the central and eastern U.S. For more on “The Omega Block” check out video #15 at the “Meteorology 101” section of the weather web site.