Enjoy the above average temperatures this week because the medium range forecast models continue to suggest The Mid-Atlantic may experience below average temperatures the second half of April.
The North Atlantic Oscillation (image below, courtesy The Weather Channel) continues to trend negative, meaning that an upper-level ridge of high pressure is forecast to build across Greenland and the Canadian Maritime.
This upper-level ridge of high pressure acts as a block causing the jet stream to buckle further south into the eastern United States bringing below-average temperatures.
The next image (courtesy Pivotal Weather) shows the 500 millibar (mb) height anomalies through April 15. You’ll notice the bright red and orange colors holding strong from the eastern Hudson Bay of Canada toward Greenland, indicative of the upper-level blocking that I just mentioned.
Just as the famous Yoshino cherry trees begin to blossom along the Tidal Basin, the North Atlantic Oscillation (NAO) is forecast to dive providing an ominous and reliable warning that a blocking pattern across the northern Atlantic Ocean is setting up as we head into early April.
The image above (courtesy NOAA) shows the NAO index plunging this week. When the NAO is negative, high pressure builds over the north Atlantic Ocean causing the jet stream to dip into the eastern United States. This dip in the jet stream typically initiates east coast storms followed by a blast of cold air.