May started with a blast of thunderstorms moving through the Midwest, MidAtlantic and Northeast. A strengthening low pressure system and a cold front was the culprit for helping the storms along. As they moved east relatively chilly air sank in from Canada on the backside of the low. Temperatures that should be in the 60s and 70s this time of year took a downward spiral into the 40s and 50s.
That cool air has be in place for almost a week now in some portions across the eastern portion of the country. There has even been some recent snow in the high peaks of the Appalachian Mountains.
The image below shows current temperatures late Sunday morning. Notice the surge of mild air across the middle of America. Much cooler weather is stationed across the East and throughout the West Coast. Why is this?
The weather pattern is kind of stuck right now due to an atmospheric roadblock of sorts. This pattern is known as an Omega Block. The Greek letter Omega is shaped much like what this atmospheric pattern looks like on certain upper level weather maps. The main map that is used to identify such phenomena is the 500mb heights and vorticity. This shows a profile of the winds and energy at about 18,000ft aloft in the atmosphere. The map below shows the two areas of low pressure on the Atlantic and Pacific sides of the US. A large area of ridging or high pressure is in the middle. This blocks the weather patterns from going anywhere fast.
The weather in the blocking high is normally warm and sunny while on each side the weather is normally cloudy and rainy. It gets even worse because the trough of low pressure is a big dip in the jet stream allowing for unseasonably cool air to filter in. This is why places like Detroit, Indianapolis, Cleveland and State College have been in this pattern for half of a week.
Part of the reason why the weather stays so stagnant in these types of patterns is because the low pressure systems often cutoff from the jet stream. The jet stream is what pushes weather systems along like a leaf in a river. The jet stream is around 30,000-40,000ft aloft in the atmosphere and is known as the 300mb level. The map below shows how there is no atmospheric river to push the low pressure out of the East.
The weather pattern will be moving slightly over the next several days, but not really too much. The Omega Block generally holds strong to start the week meaning more showers and clouds for the Northeast. The high pressure has been able to slither further eastward, which is allowing the Great Lakes and Ohio Valley to tap into some sunshine. The cool air is still in charge. The low pressure system in the East will swing around in Southeastern Canada over the next several days keeping the cool May going. The high pressure across the middle country will break down as the Omega Block pattern ends.
After a record warm April for much of the East it’s been a chilly May so far. This pattern won’t fully reverse itself till the middle or end of the month. Below is a forecast model temperature forecast for New York City. You can see the temperatures stay below average until the last 10 days of the month or so.