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04/05/2019, 11:01 am EDT

Southern Company Winter 2018-19 SYSTEM Review

Executive summary: The meteorological winter 2018-19 season was characterized by extremes and extreme events. Meteorological winter averaged very cold across the North-central U.S. while the Southeast States including Southern Company was exceptionally warm. The national precipitation pattern was historic with each state (except Montana) containing areas of above normal precipitation. Southern Company observed a very warm winter season featuring heavy precipitation early to mid-winter followed by a drier trend as spring approached. Leading forecast factors included a slowly developing El Nino climate and a historically intense polar vortex which emerged in February. Fig. 1-2: U.S. temperature and precipitation anomalies for winter 2018-19. Meteorological Winter 2018-19 State Temperature/Precipitation Rankings Fig. 3-4: Based on the 1895-2019 climatology the state rankings for temperature and precipitation for meteorological winter 2018-19 are indicated. The winter 2018-19 review/discussion: Heading into winter 2018-19 the consensus of most climate forecasts was anomalous warmth in the Northwest U.S., favoring wet weather risk if El Nino developed in the Gulf region and a potential cold scenario for the Central U.S. However, many surprises greeted energy markets and certainly Southern Company during the 2018-19 cold season. November started with an unexpected bang! Un-forecast anomalous chill registered in the top 10 all-time coldest for the month in the East-central U.S. and some of that chill poked into the Southern Company (system temperature -3.52°F). Natural gas prices spiked to the highest level in 4 years (and highest of winter). November was also very stormy in the East including Southern Company where the system average percent of normal precipitation was 137%. With the market set for early season chill the month of December reversed the climate pattern sharply warmer and the first month of meteorological winter ranked of 104 of 125 years where 125 is warmest. The Southern Company System Average also reversed sharply to +2.92°F. Of equal interest to Southern Company in December was heavy rainfall. The system average of precipitation in December was a whopping 222% of normal including an all-time record wet month (for December) in Georgia. The November/December climate combination was a classic example of the new climate regime haunting the U.S. (and globe) in recent years producing extremes. The November cold outbreak was caused by an early season stratospheric warming event over the polar region coupled with historic early season North America snow cover to generate the widespread cold. In December the cold retreated to Canada as Pacific warmth spawned by El Nino-related climate occurrences formed. After a turbulent start to the cold season the month of January stayed mostly warmer than normal (nationally) with the storm track focused on the Mid-South to Northeast U.S. A weak El Nino influence was sustaining both the storm track and mild mid-winter pattern. Across Southern Company a weaker version of the December climate was observed as the system average temperature was still quite warm (+1.86°F) and precipitation regime wetter than normal (118%). February brought disastrous weather to the U.S. spawned by an extremely cold polar vortex over the high altitude far northern latitudes and finally a tropical atmosphere able to couple with persistent equatorial East Pacific warming and initiate a full El Nino climate. A piece of the high level polar vortex broke off from the stratosphere and descended to the weather atmosphere (troposphere) over central North America. The wet fetch of Pacific moisture driven by the new full-throttle El Nino climate into the polar vortex caused more thermal and precipitation extremes which initiated the great flood of 2019 in the Midwest U.S. As cold as the Central and West U.S. was in February, Southern Company was quite warm with a system average of +8.02°F ranking 3rd warmest in history. After a wet start to the cold season causing extreme wet Palmer Drought Severity index (PDSI) into Southern Company service territory a drier pattern emerged in Georgia during February while northern Alabama was historic wet. The system average for precipitation in February was 79% of normal which ignores just how wet the northern Alabama climate was realized. In March the dry trend initiated during late winter continued and strengthened as the system average was a very dry 48% of normal. The month of March was temperate with a system average of +0.20°F. Of course, nationally the big story in March was the persistent historic flooding across the Great Plains. Snow melt was contributing to the rising rivers in March while precipitation events remained intense across the central Great Plains. The great flood of 2019 was caused by the combination of extreme cold causing a frozen ground and iced over river areas followed by heavy precipitation and eventual snow melt. The Climate Impact Company (CIC) forecast verification for the 2018-19 cold season across Southern Company missed the extreme events. The November chill was unexpected although the precipitation forecast was reasonable. The December warmth and wetter than normal pattern was forecast by CIC but not to the extreme which occurred. Southern Company SYSTEM Temperature & % of Normal Precipitation Observations for The 2018-19 Cold Season Fig. 5-6: The Southern Company SYSTEM temperature anomalies and percent of normal precipitation for each month of the 2018-19 cold season using 5 primary Southern Company locations. Southern Company SYSTEM Cold Season 2018-19 Forecast Versus Observations Fig. 7-8: The Southern Company SYSTEM monthly temperature anomaly and percent of normal precipitation forecast for the 2018-19 cold season versus the verified values. Southern Company Gas Population Weight Heating Degree Days Fig. 9-10: Gas population weight heating degree days for each month of the 2018-19 cold season for Alabama and Georgia compared with the 30-year normal and last 3 cold seasons. A Comparison of Sub-freezing Morning Temperatures Past 2 Winter Seasons Fig. 11: A comparison between number of days when morning low temperature was less than 32F for the past 2 meteorological winter seasons.