Atmospheric Clog Bringing Cool Temperatures to the East; Could Continue Through the First Half of May

All About 'Heart Attack' Snow

An atmospheric clog – better known as an "omega block" pattern – set up over the United States during the weekend, bringing a big change in weather conditions to parts of the East.

Much cooler and more unsettled conditions have taken over the Northeast due to the omega block – a sharp contrast to one of the warmest Aprils on record in many of this region's cities.

Temperatures during the next 6 to 10 days in the East will be cooler than what was experienced in April, and some wet snow is even possible in the higher terrain of upstate New York, Vermont and New Hampshire through Monday.

Friday into Sunday, the weather pattern allowed snow to accumulate in the highest elevations of the southern and central Appalachians, including Tennessee's Mount Leconte and West Virginia's Snowshoe Mountain.

Chilly air also surged into parts of Florida allowing record lows to be broken Sunday morning in Orlando (48 degrees) and Vero Beach (50 degrees).

Meanwhile, the central U.S. received a bit of a breather from the recent onslaught of severe storms and flooding, as high pressure is dominating the Plains states.

An omega block is a weather pattern in which the jet stream takes the shape of the Greek letter omega: Ω.

The pattern into early this 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 is clogged. Storm systems in the East and West have been stuck over the same areas for multiple days. 

The omega block will break down by midweek as the western storm system pushes east bringing rain thunderstorms to parts of the Midwest and South.

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The NAO is an index based on the surface sea-level pressure difference between a subtropical 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 plunge southward across the eastern half of the United States, resulting in a period of cooler-than-average temperatures. 

In this case, cooler-than-average temperatures may dominate parts of the East into mid-May, according to the latest 6- to 10-day outlook from NOAA's Climate Prediction Center.

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).

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)

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