The month of July last year averaged 1.5 degrees warmer than normal. This July averaged 1.6 degrees above average, except at Hartsfield Airport where the thermometer was often 4 degrees above anyone else in the Atlanta area on any given day. This is likely an effect of Jet engine exhaust. Rainfall for July was officially 33% below normal at Hartsfield.

As the MJO (Madden Julian Oscillation) phase is shifting from phase 6,7,8 through phase 1 into Phase 2 the upper-level Sonoran heat ridge backs up toward the West U.S. and troughiness develops from the Great Lakes and New England down into the SE U.S. between now and the 14-18th of August. I STILL see no reason for the 100s to return this summer.

This new pattern favors more opportunities for good rain and cooler temperatures between now and then. Afterward higher 500mb heights may again expand east around or after the 388 hour forecast projection (day 16) by about the 20th or so. For example, the NAEFS from Environment Canada shows only a 50-60% chance of above normal temperatures August 12-19th. This is the lowest I've seen that far in advance over Georgia in over a month. This means the little break in the heat that started Saturday will continue a spell, as they say in these parts.

However, while we may continue to have some days below 90 the high humidity will keep the discomfort factor elevated. But it's the high humidity (high precipitable water values i.e. evaporated moisture content of the air mass) that allows for scattered showers and thunderstorms, some heavy enough to produce localized flooding and wet microburst wind damage. As I explained in previous posts, "Random thunderstorm season" runs from June to September, peaking in August. It is that time of year that is most difficult to forecast rain chances as they can vary so greatly from day to day without the weather charts changing in any discernible way. Since even the models cannot predict individual thunderstorms, a daily thunderstorm forecast needs to be updated every 4-6 hours.

In August, we continue to lose daylight as days grow shorter and the average temperature starts to decline (climatology).  Another aspect of August weather is that severe weather declines some, the tornado threat is nil, and the flash flooding threat goes up. The flood threat goes up because the air is about as moist as it will get all year in absolute quantitative terms and the rain-bearing storm clouds have no jet stream to push them around at a good speed like in springtime, so they tend to lolly-gag, dumping their 1-2 inch or more per hour rain over the same small area for hours at a time. We had an example of this last week in a small portion of Fulton and Dekalb.

Also in August the odds of "nocturnal", or overnight-early morning flooding or strong storms is elevated compared to the rest of the year. This is because the air aloft is at its warmest, meaning it takes longer for the atmosphere to destabilize from below (near surface). So sometimes the air mass will destabilize from above instead thanks to cooling at and above the cloud tops, which takes place at night. This often leads to "Derechoes" (de-RAY-choh) i.e. long-track strait line wind storms or development of an MCC/MCS Meso-scale Convective Complex/Mesoscale Convective System (think in-land tropical storm without an eye) as low-level jet streams form at night near or below 5,000 feet in part in response to the cooling cloud tops aloft and subsequent rising air motions.