Posted: May 14, 2014 10:04 AM by Meteorologist Josh Eachus
Updated: May 15, 2014 1:06 PM
The winter of 2013 - 14 was notably cool for Southeastern Louisiana. Highlights included the 8th coldest January on record, 3 winter precipitation events, including freezing rain for a late-season Mardi Gras on March 4. However, invariably, summers along the Gulf Coast are hot and humid with frequent afternoon showers and thunderstorms. An analysis was performed to determine if whether or not an unusually cool winter provokes any alteration in summertime or succeeding annual patterns. Moreover, global wind and water circulations were analyzed to overlay Baton Rouge weather data as potential modifiers to the trends uncovered.
Making the Prediction: An analysis of over 80 years of Baton Rouge climate data was performed to return a statistically driven analysis of expected trends for the upcoming summer and remainder of the year. Data retrieved and tabulated from the Baton Rouge Airport from January 1930 to present day included the average monthly maximum temperatures, average monthly minimum temperature and monthly precipitation totals. Historical El Niño/La Niña (ENSO) and North Atlantic Oscillation (NAO) indicies from 1950 to present day were also entered into the database. Finally, winter 2013 - 2014 statistics were collected, along with their departures from average. A formula was then used to determine years for comparison that realized similar temperatures, precipitation and global indicies.
Backing the Prediction: Winter 2013 - 14 in Baton Rouge was more than 3 degrees cooler than the historical average, which is a significant number over a long period of time. The winter was also drier than usual by more than 5 inches of liquid precipitation. In a highly variable region for precipitation, this number is less significant. 9 years were identified to have similar winters: 1940, 1958, 1960, 1968, 1970, 1981, 1984, 1994 and 2001.
Temperature analysis: In the 9 similar years with a cold winter, all finished the year with below average annual temperatures. For the summer months of June, July and August, all but 3 of those years had below average summer temperatures. Overall, years with cold winter seasons tend to have an annual temperature significantly below average. Since summertime temperatures are fairly consistent for the Baton Rouge area, the lower annual temperature is likely due to the unusually cool winters lowering the average annual temperature.
Precipitation analysis: In the 9 similar years with a cold winter, 6 of those 9 finished with below average annual precipitation. In 1 of the 3 non-instances, 2001, Tropical Storm Allison accounted for 16.00" of rainfall. Extreme events such as tropical precipitation inflate statistics and the data is adjusted accordingly. 7 of the 9 following summers had below average precipitation and one of the non-instances was again 2001. Eliminating the impact of tropical systems, there is a 77% chance that both the year and summer will end with slightly below average precipitation.
El Niño/La Niña analysis: In the presence of an El Niño, South Louisiana tends to see a more active weather pattern, especially during the winter and spring months. This occurs because the jet stream, which steers weather systems, is often driven across the Gulf South in this scenario. El Niño also has implications for tropical activity. Due to that more southern jet stream, wind shear is present which tends to inhibit tropical cyclone formation. For much more on El Niño, see this fact sheet.
Winter 2013 - 2014 featured a weakly negative El Niño Southern Oscillation (ENSO), or weak La Niña. During spring 2014, ENSO was trending neutral to positive (weak El Niño). In the 9 similar years with a cold winter, the winter ENSO was positive 2 times, negative 2 times and neutral 5 times and was neutral for each summer. But ONLY 3 times was ENSO trending positive, or towards an El Niño as it is this year. Clearly, ENSO trends during "like" years yielded no significant results.
Data selection was therefore expanded to consider outcomes during years of similar trends in ENSO. But again, no significant pattern arose. Neither temperature nor precipitation varied far from average during the summer months in years where ENSO was trending positive. 71% of ENSO trending positive years concluded with above average precipitation. However, of particular interest to South Louisiana, would be that the expanded data selection to include ENSO trending positive years, yielded much more significant tropical activity then the core group of 9 comparable years or any full-on El Niño years.
NAO analysis: An analysis of the North Atlantic Oscillation (NAO) also revealed sporadic results. That noted, the NAO has a lesser impact on Louisiana summers (compared to El Niño) due to less interaction with the Polar Front Jet Stream. Climate Prediction Center correlations show only a weak relationship between the NAO and summer precipitation-which is already highly variable in Louisiana. There is some research to suggest that a negative NAO allows tropical cyclones to curve out to sea while a positive NAO steers them towards the United States and into the Gulf of Mexico. Regardless, NAO predictability is hardly reliable more than a month out.
THE 5 Predictions:
1. Given the relatively habitual nature of Louisiana summertime temperatures, winter temperatures seem to have strong impact on annual temperatures. Following a cold winter, there is some evidence to suggest that summer temperatures will be slightly cooler than usual. HIGH CONFIDENCE PREDICTION: 2014 will be a cooler than average year.
MODERATELY LOW CONFIDENCE PREDICTION: 2014 will be a slightly cooler than average summer.
2. Though highly variable in Louisiana, there does appear to be a precipitation trend for years and summers followed by cooler than average winters. Eliminating tropical impacts, cool winters lead to drier years and slightly drier summers. But, ENSO trending positive years have yielded a few VERY wet summers. Still, more summers saw below average precipitation. Given the fringe consideration of the added ENSO trending positive years in an effort to strengthen previous findings, it will not be weighted too heavily in the prediction.
MODERATELY HIGH CONFIDENCE PREDICTION: 2014 will be a drier than average year.
MODERATE CONFIDENCE PREDICTION: 2014 will be a slightly drier than average summer.
3. The saying in Louisiana is it only takes one. While the development of a positive ENSO favors depressed Atlantic Tropical Cyclone activity, years with ENSO trending positive have produced some significant events (Andrew, Katrina, Rita). By comparison, the originally considered "like" years found ZERO hurricanes with weakly neutral or weakly positive ENSO. The only significant tropical event was the heavy rain producing Tropical Storm Allison in 2001. Furthermore, a full-on, tropics quelling El Niño is not likely until at least fall, leaving several summer months for the tropics to become active.
MODERATELY HIGH CONFIDENCE PREDICTION (with a BIG disclaimer mentioned above): 2014 will find low tropical activity for Louisiana.
Please note that this is a prediction based on statistics and historic climatological patterns. In Southern Louisiana, summer temperature variability is quite low, even in abnormal years while precipitation totals can be highly variable. Not only can the tropics greatly impact rainfall amounts, but also the erratic nature of pop-up summer thunderstorms. Storms could fire at different measuring stations around the region or strike the same place many days in a row. While entered with hard evidential support, a long-range prediction has much greater margin for error than the near-term forecasts you are accustomed to on a daily basis. For that reason, stick with the WBRZ weather team for timely and accurate updates throughout this upcoming summer.
If you have any questions or feedback regarding this research, please contact Meteorologist Josh Eachus via email at: firstname.lastname@example.org.
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