Atmospheric Clog Will Bring Cool Temperatures to the East Through the First Half of May

Brian Donegan
Published: May 3,2017
An atmospheric clog, better known as an "omega block" pattern, is forecast to set up over the United States this weekend into next week and will bring a big change in weather conditions to the eastern states.
Much cooler and more unsettled conditions are expected in the eastern third of the nation as the omega block takes shape – a sharp contrast to one of the warmest Aprils on record in many of this region's cities.
Temperatures during the next 10 days in the East could be cooler than what was experienced in April in some areas, and some wet snow is even possible in parts of the Appalachians and eastern Great Lakes.
(FORECAST: Rain, Wind, Even Wet Snow in Store for Eastern U.S. Late This Week)
Meanwhile, the central U.S. will get a break from the recent onslaught of severe storms and flooding as high pressure dominates the Plains states.

An omega block weather pattern will set up over the United States this weekend into next week.

An omega block is a weather pattern in which the jet stream takes the shape of the Greek letter omega: Ω.
The pattern this weekend into next week is shown in the graphic above. The omega block is clearly illustrated by the large jet stream bulge over the central U.S. sandwiched between southward dips in the jet stream over the East and West.
Essentially, the atmosphere will be clogged. Storm systems in the East and West will be stuck over the same areas for multiple days, bringing occasional showers, clouds and cooler-than-average temperatures.
This pattern could persist for the next two weeks, according to the 8- to 14-day temperature outlook from NOAA's Climate Prediction Center.
(MAPS: 7-Day Forecast Highs and Weather)
In addition, the North Atlantic Oscillation (NAO) is expected to go negative into next week – the most negative it's been so far in 2017.
The NAO is an index based on the surface sea-level pressure difference between the Subtropical (Azores) High – a large, semi-permanent area of high pressure typically found south of the Azores in the Atlantic Ocean – and the Subpolar (Icelandic) Low – a large, semi-permanent area of low pressure located between Iceland and southern Greenland in the Atlantic.
When the NAO is in its negative phase, there is high pressure in the upper atmosphere near Greenland. This causes the jet stream over North America to dip southward across the eastern half of the United States, resulting in a period of cooler-than-average temperatures.

How Warm Was April?

Many cities in the mid-Atlantic just experienced one of their warmest Aprils on record, according to statistics compiled by the Southeast Regional Climate Center (SERCC).

The number in each box represents the rank of April's warmth, where a value of "1" indicates April 2017 was the warmest on record.
(Southeast Regional Climate Center)

Philadelphia saw its warmest April, with an average temperature of 59.5 degrees. This was nearly 7 degrees above average for the month. Records in Philly date back to 1874.
The warmest April was also recorded in Washington D.C., where the average temperature was 63.8 degrees. This was more than 8 degrees above average for the month. Records in the nation's capital date back to 1872.
Other cities that reported a record-warmest April include Cleveland, Harrisburg, Pennsylvania, Louisville, Kentucky, Richmond, Virginia, and Raleigh, North Carolina, the SERCC data showed.
(MORE: May Temperature Outlook)

MORE: Tornado Risk By Month

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.