Spring Flood Outlook 2008

Summary:
The current hydrologic situation is complex across the Southeast U.S. There are still extensive areas of D3 Extreme to D4 Exceptional drought while other areas are near or above flood stage.

March and April represent the time of the year with the highest probability of rainfall...high stream flows...enhanced basin recharge...and the resulting potential for flooding in Central Alabama.

Heavy rain events typically originate along the Gulf Coast and then spread north and northeast. Scattered areas of early spring flooding are actually "normal" for Central Alabama and occur most years. For an above-normal chance of flooding...not only would moderate flooding be possible...but the overall coverage would need to be significant.

Furthermore...once the spring "green up" occurs...from mid March into May...the chances for substantial runoff and subsequent flooding are significantly reduced.
Current Conditions:
Climate regime...A moderate to strong La Nina event has been observed and has persisted through the winter months. This pattern typically results in below-normal precipitation over Central Alabama...especially in the southern portions. While some moderation in this pattern is possible late this spring... for the most part it will continue through May with the likelihood for below-normal rainfall.

Drought Monitor...Extensive and intense drought conditions have been observed over Central Alabama through the winter...and in fact for much of the past two years. While most areas have been impacted by drought...recent rainfall has begun to ease the drought conditions somewhat...with the most intense drought area now receding into extreme northeast Alabama.

Past Rainfall...2007 was an extremely dry year across Central Alabama with many areas reporting rainfall deficits of 20 to 25 inches. In consideration of rainfall going back through this winter...most of Central Alabama has seen below to well-below-normal rainfall. An exception has been the southeast sections around Troy where rainfall has been near to a little above normal. Rainfall since January 1st has been occurring more frequently with periodic storm systems bringing substantial rainfall.

Some precipitation amounts for Central Alabama January 1st through March 6th:
Birmingham 10.33
Montgomery 9.68
Anniston 11.90
Tuscaloosa 11.21

Average precipitation expected and departure from normal from January 1st through March 6th:
Birmingham 10.77...Down 0.44
Montgomery 11.77...Down 2.09
Anniston 11.31...Up 0.59
Tuscaloosa 11.99...Down 0.78

Stream Flow...Stream flow over Central Alabama has been averaging well below normal since the onset of the drought. Daily record lows have been set in many locations. Since the beginning of the peak recharge period...there has been some modest improvement in many areas. Tthis improvement has generally been limited...resulting in raising conditions into the normal range.

Reservoirs...Most reservoirs across Central Alabama have seen improvement during the past several weeks due to periodic rainfall and increased runoff that normally occurs during the winter months. Reservoir operators have been able to increase many pool levels to near or above normal winter pool levels.
Soil Moisture... Recent rainfall over Central Alabama has improved upper soil moisture...as would be expected this time of the year. Overall ...however...with the Southeast drought lasting two years...lower soil moisture is still well below normal.

Key Factors Considered In This Outlook:
March and April are primary recharge periods due to increased rainfall and minimal evaporation. Limited...minor flooding would be expected as normal. Recent rainfall has generally fit into a more normal late winter/early spring pattern.

Recent rains have caused some areas to see significant improvement in surface water and upper soil moisture as well as the potential for flooding. While other areas have seen improvement...it has been modest.
The Climate Prediction Center is generally calling for below-normal rainfall through spring.
We expect La Nina conditions to persist through spring.

Most reservoirs have the capacity to hold water through the spring. This would lower the potential for downstream average flows.

Persistence would point towards a continued drought pattern.

Outlook for Spring Flooding:
The outlook for flooding this spring is for well below normal chances over the northern half of Central Alabama...and below normal chances over the southern portions. Much of the information used to compile this Flood Potential Outlook was supplied by the Southeast River Forecast Center and by the United States Geological Survey.