Below Average Temperatures Expected Second Half of April

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.

Finally a less talked about but very important signal to keep an eye on across Oceania is the Madden Julian Oscillation (MJO). Upward motion in the earth’s atmosphere caused by convective thunderstorms near Australia/Oceania has significant impacts on the weather patterns across the Pacific and into the United States.

The MJO is forecast to move out of phase six and into phase seven (image below, courtesy NOAA).

The next image (courtesy NOAA) shows temperature anomalies (departure from average) across the United States during the eight various phases of the MJO.

Notice that while phase six favors warmer than average temperatures in the eastern United States, phase seven favors cooler than average temperatures across much of the Nation outside of the southwest and southeast.

In conclusion both the negative NAO and phase seven of the MJO increase the probability that cooler than average temperatures can be expected across the DC Region the 15th through 25th of April.