Weak El Nino Forms; Elevated Risk for Flooding in Southeast This Spring

Ocean temperatures in the central Pacific have been warmer than average for nearly a year. Only recently, though, have the anomalies reached a threshold and duration that NOAA can officially declare that El Niño conditions are being observed.  As of their March 5 report, the El Niño is considered “borderline” or weak, and generally expected to stay that way through the summer months of 2015.  While widespread or significant impacts are usually not anticipated with weak El Niño conditions on a global scale, some influences from this weather pattern may be felt in North America this spring, especially in the Southeast as it relates to rainfall.

Snapshot of sea surface temperature anomalies on March 16, 2015 (courtesy: NOAA). The yellow and orange colors represent water that is warmer than normal for this time of year.

Snapshot of sea surface temperature anomalies on March 16, 2015 (courtesy: NOAA). The yellow and orange colors represent water that is warmer than normal for this time of year.

When waters in the central Pacific are unusually warm, a stronger current of wind develops in the upper atmosphere out of the west or southwest and moves toward the southern tier of the United States.  Sometimes referred to as the subtropical jet stream, these higher winds can carry in deeper amounts of moisture from both the Pacific Ocean and Gulf of Mexico.  Spring is already an active time of year for the jet stream, as several storm systems move along the boundaries of retreating winter and advancing summer air masses.  When there is more moisture for these storm systems to work with, they can produce an abundance of rain.  For this reason, the chances of higher-than-normal rainfall in the next three months are greater than 40% for many areas of the Southeast US, including parts of Florida.

El Niño was officially observed by NOAA in early March. Map shows potential impacts across the nation this spring.

El Niño was officially observed by NOAA in early March. Map shows potential impacts across the nation this spring.

According to the National Weather Service, many rivers and streams in Georgia, Alabama, and the Florida panhandle are already experiencing above normal stream flows for this time of year due to a surplus of rain over the winter months.  The Suwannee and Santa Fe Rivers are not immediately at risk to flood, but in a Hydrologic Outlook issued earlier this month, it was noted that normal to above normal rainfall in their basins will lead to an increased flood risk moving into hurricane season in June.  The potential for heavy rain typically diminishes by late April and May in North Florida, largely because frontal systems have less of an impact.  So it remains to be seen how a weak El Niño may influence trends this season.  The latest long range forecast models suggests heavy rain is most probable in the next 14 days across portions of Texas, Louisiana, Alabama, Mississippi and Georgia where a persistent storm track could yield several significant rainfall events. Confidence is much lower on whether or not heavy rain will fall across the Florida peninsula, since it would be more removed from the aforementioned prevalent storm track.

Other potential influences from a weak El Niño across the country include warmer and drier trends in the western third of the nation, fewer tornadoes in the dubbed “tornado alley”, and a late-season blast or two of unusually cold air for the Great Lakes and New England.  The UF Weather team will continue to monitor the latest trends of the ocean temperatures over the Pacific, along with the day-to-day tracking of potential weather systems that might impact river levels.  We are also watching for clues on how this current weather pattern may (or may not) influence the upcoming 2015 hurricane season, which is now only 75 days away.