Let’s start off by reviewing normal August rainfall. Interior sections of the Southeast U.S. typically receive around 3 or 4 inches of rain in August. Higher amounts, 5 inches or more, occur over higher elevations of the Appalachians (due to differential heating), along the coast (due to land/sea interactions), and over Florida.
You might find it interesting to see the pattern of rainfall from August 2011. Most of the region experienced below-normal rainfall last year (red-shaded areas). An exception was along the North Carolina and Virginia coast where Hurricane Irene brought over 15 inches of rain (blue-shaded spot).
Here is a comparison of normal rainfall between July and August. Note the modest decrease in rainfall over interior sections of Mississippi, Alabama, and Georgia from July to August.
Evapotranspiration (ET) rates remain high thoughout the month. Here is an annual plot of reference evapotranspiration for a location in North Carolina. While not quite as high as in June or July, ET is still quite significant
Tropical activity starts the annual ramp up in August, especially the latter half of the month. Starting out low on August 1st, tropical activity really heats up by month’s end. (Note: tropical activity has picked up a bit in the Atlantic, right on schedule, over the past few days).
The NOAA/NWS Climate Prediction Center is calling for “Equal Chances” for either “above” or “below-normal” rainfall in August. Temperatures are expected to continue the trend of “above-normal” for most areas, but not as high probabilities as over the Midwest..
Here is my outlook for runoff in August. In consideration of many water management issues, runoff is just as important as rainfall as it is a key driver for stream flow and reservoir inflows (and potentially flooding). I am calling for a swatch of below-normal runoff across interior portions of the Southeast U.S., and normal runoff along the coast, in Florida, and across higher elevations of the Southeast U.S..
Following are some of my forecaster notes for this runoff outlook:
- This outlook does not forecast specific inland-moving tropical systems.
- CPC’s precipitation outlook is utilized.
- Normal, or above-normal rainfall, over the area of prolonged drought will foster soil moisture recharge – with reduced runoff potential.
- A lingering soil moisture surplus from T.S. Debby will enhance runoff should heavy rainfall develop over a small portion of North Florida.
- Thereare indications of a lingering (weak) frontal boundary over southern sections of Alabama and Georgia which could enhance daily diurnal rainfall.
- Coastal forcings will (typically) enhance daily rainfall.
- Longer-term climate models (both GFS and CAS) indicate continuing soil moisture deficits over a large part of the region.
- The role of persistence would support a continued reduction in runoff. .
- Recent excessive rainfall near Birmingham (green circle) has enhanced short-term runoff prospects (over a limited area).