As we have discussed for the past several weeks, La Nina will be the dominant forcing mechanism for this Summer. La Nina will have moderate strength and may reach strong status if the SOI continues to intensify over 20. Another factor to consider is the state of the stratosphere with pockets of warm anomalies in an overall cold stratospheric environment. This factor will support the potential for additional cut-off upper-level low-pressure systems and continue an enhanced severe thunderstorm season, especially in July and August.
The 500 MB pattern will feature a strengthening western Atlantic ridge that will reach an apex of strength in late July through mid-August. The 500 MB pattern will feature a persistent trough between the western Aleutian Islands to the Gulf of Alaska, forcing a persistent ridge over the Rockies and Desert Southwest. To counter the ridge over the Rockies and the ridge over the western Atlantic, a trough will likely be from the Plains to the Tennessee River Valley and over the northwestern Atlantic. This type of 500 MB pattern will support the development of clashes with maritime air masses, Pacific/Polar air masses, and tropical air masses over the Northern Mid Atlantic, much like in the Spring. However, given the state of La Nina this summer, the potential for a powerful western Atlantic ridge driving tropical air masses into the northern Mid Atlantic from the Gulf Coast will be much stronger, especially from mid-June through August.
JUNE:
Influences from the Spring pattern will remain with below-normal heights around the northwestern Atlantic keeping the threat of back-door cold fronts and stalling warm fronts a theme. However, after the middle of June, the western Atlantic ridge will start to build with a shift to near normal and then above normal temperatures due to a greater influence of air masses from the Gulf Coast and Tennessee River Valley. Precipitation will average slightly above normal.
JULY:
A true La Nina Summer will increase the threat of heatwaves and humid air masses. I expect dynamic thunderstorms capable of reaching severe levels, possibly even a weak tornado outbreak, to start to show up. Influences from tropical low-pressure systems will have to be monitored, but mostly in the form of remnant low-pressure systems. A frequent theme will be the wavering strength of the western Atlantic ridge and the potential for either a cut-off upper-level low-pressure system or a negatively titled trough over the Tennessee River Valley. July will feature above-normal temperatures and above-normal precipitation. I would not be surprised if temperatures frequently run from the mid-80s to lower 90s for highs and lower to mid-70s for lows, forcing daily averages above normal, much like last year. Precipitation will average near to slightly above normal but could push to well above normal if tropical features are a factor.
AUGUST:
August will be very similar to July but with a high threat of heat waves for the first two weeks of the month. There will be an increasing threat from influences from tropical low-pressure systems from the Gulf Coast or from the Bahamas. A concerning pattern to watch out for is when a tropical low-pressure system around the Bahamas is caught between an upper-level trough/low in the Tennessee River Valley and the western Atlantic ridge. Temperatures will average above normal and precipitation above normal.
