La Nina to Persist thru Winter: Implications for Flooding

Sep 20, 2010 - Both the NWS/Climate Prediction Center (CPC) and the International Research Institute for Climate & Society (IRI) call for rather high probabilities for a "dry" winter and spring across south Louisiana and SW Mississippi.

The IRI posts an 80% to 90% chance (IRI_ENSO.gif) that the current La Nina (cooler-than-normal waters in the central equatorial Pacific) will remain in place through the winter ... and better than a 60% likelihood that it will persist into and through the spring (March-April-May).

Some Thoughts on the Outlook:

Why is this important? In general, flood threats are reduced along the Gulf Coast when the La Nina phase of ENSO (El Nino/Southern Oscillation) is in place during the winter and spring.

While everyone knows the flood potential resulting from tropical systems, many are unaware that the vast majority of Louisiana's flooding comes in the winter and spring rather than during Hurricane Season. As an example, of the last 50+ times when the Amite River (at Denham Springs) rose above flood stage (Amite_DS_Floods.png), fewer than a half-dozen were the result of tropical rains.

Note, however, that the winter/spring flood threat is reduced -- but not eliminated! -- during La Nina winter/springs.  Averaged across south Louisiana, 6-month rainfall (Nov-May) was near-normal to below-normal for roughly 80% of past La Nina winter/springs (since the 1950s).  

Individual storm events during winter/spring -- generally associated with slow-moving or stalled fronts and "winter lows" developing in the western Gulf -- can occur in any ENSO phase.  So a big rain event is still possible regardless of ENSO, but there tends to be fewer of these big events during La Ninas.

Since La Nina winter/springs tend to be a little "drier," the environment tends to be less frequently at a state of "saturation."  A drier environment suggests a reduced chance of flooding, and greatly decreases the likelihood of multiple floods in the same season.

At saturation, not only is the ground soaked, but ditches, canals and basins (lakes, retention/detention ponds, etc.) and every other water-holding site (low-lying swampy areas) are often "at capacity."  In this state, ANY additional rain goes immediately into runoff, and channels (natural and man-made) can experience rapid rises.  

Antecedent conditions -- the pre-storm environmental status -- can be as important, if not more important, in determining a local/regional flood threat during a rain event than the amount of rain that falls.  Just a modest rain event (3" to 4", something that occurs more than once-each-year, on average) can produce a notable flood event if the pre-storm environment is at capacity.  This is precisely why we can't make the assumption that a rain event translates into an equivalent flood event: i.e., a 10-year rainstorm (a rain event expected to occur once every ten years, on average) does not mean a 10-year flood.  

The Bottom Line:

Flooding is virtually an ever-present threat for the nation's "wettest" state, and as we head toward the winter, it is time to assess the readiness of communities that are frequently affected by rising waters. But current environmental signals suggest that the potential for serious flooding in south Louisiana and SW Mississippi during the coming 6-8 months -- short of a late season tropical system -- appears to be lower than average ... good news for flood-prone areas.