CLIMATE DATA FROM PREVIOUS El Niño YEARS:
El Niño developed in March of 2015 and continues to strengthen. Forecasters say it could be one of the strongest on record and last through the spring of 2016. A strong El Niño can result in more frequent and more intense storms that can produce heavy rainfall and snowfall in some parts of the country. The term El Niño refers to a periodic warming in the sea-surface temperatures across the central and east-central equatorial Pacific Ocean. During an El Niño year, tropical rains usually centered over Indonesia shift eastward, influencing atmospheric wind patterns worldwide. Possible impacts include: a shifting of the jet stream, storm tracks and monsoons, producing unseasonable weather over many regions of the globe.
The South most typically experiences a wetter and cooler than normal winter during strong El Niño episodes. I thought it would be great to gather data from previous moderate, strong and very strong El Niño episodes and see on average how our fall, winter and spring seasons have been impacted. Not only did I examine the strength of El Niño but also two other oscillations that impact the weather in the United States.
Teleconnection patterns reflect large-scale changes in the atmosphere wave and jet stream patterns and influence temperature, rainfall, storm tracks and jet stream location/intensity over vast areas. One of the teleconnection patterns that can impact winter weather is called the North Atlantic Oscillation otherwise known as the NAO.
The phase of the NAO depends on the strength of two pressure systems: A low pressure system across Iceland and a high pressure system located near the Azores. When NAO is positive, the pressure systems are strong, resulting in a strong westerly flow across the U.S. which can mean an increase in wetness.
When NAO is in a negative face, the pressure systems are weaker and cold air is allowed to plunge southward which can result in enhanced snowfall across the eastern United States.
The Atlantic Oscillation, the AO is a climate pattern characterized by winds circulating counterclockwise around the Arctic. When the AO is in a positive phase; a ring of strong winds circulating around the North Pole acts to confine colder air across Polar Regions. This belt of winds becomes weaker and more distorted in the negative phase of AO, which allows an easier southward penetration of colder, arctic air masses and increased storminess into the mid-latitudes.
EXAMINING THE FALL MONTHS
FALL FINDINGS: There is no strong correlation between Teleconnection phases and the number of tornadoes in the fall. We have had quiet fall weather during El Niño episodes and we have had a few active fall seasons, like before Doppler in 1957 with 19 tornadoes in Alabama in November and 17 in November in 2002. Alabama averages 3 tornadoes in October and 6 in November. Above normal tornadoes numbers in the fall occurred during the moderate El Niño in 2002, very strong El Niño of 1997 and the strong El Niño in 1957. I did not see much fall snowfall either.
EXAMINING WINTER MONTHS:
WINTER FINDINGS: Most of the past moderate to very strong El Niño episodes, NAO and AO on average were negative, especially the AO. Temperatures most El Niño winters averaged -2.7 below normal. We have experienced both very wet and drier than normal winters during El Niño episodes. If we were to examine only very strong El Niño episodes then we experienced above normal winter precipitation, near normal temperatures, and around normal snowfall. We have had some above average snowfall during an El Niño episode; 8.4” in the winter of 1963-64 and 4.4” during the winter of 1991-92. So there is no strong signal towards a guaranteed heavy snowfall during an El Niño episode but it certainly can happen. All it takes is one storm.
EXAMINING WINTER MONTH TORNADOES
There is no strong correlation between the teleconnections in the winter months and the number of tornadoes. We have seen slightly above normal winter tornado counts but nothing too extreme that stands out other than 8 that occurred in Alabama during December of 2002.
EXAMINING SPRING MONTH TORNADOES
SPRING FINDINGS: We have had some very active and some not so active springs during El Niño phases. There could be a correlation between the numbers detected because of the implication of Doppler radar and also due to El Niño in the more recent episodes. Climate change could also influence our weather patterns and the number of tornado reports have likely gone up because there are more spotting and chasing storms. It’s hard to point the finger directly at any one of those for the active springs we had during the past 3 El Niño’s. What’s interesting in the first chart is during active springs with more tornado reports than normal; precipitation has also been way above normal.
I have never done a long range forecast like this before but figured I would give it a whirl. I think we will have wetter than normal conditions through the spring and the potential for flooding rains. With more moisture, I am also leaning towards slightly higher tornado counts each month across the state this fall and especially this spring. I think we will see slightly above normal temperatures through late fall and early winter and then a shift to slightly below normal temperatures. At this time, I am forecasting at least 1 above normal snowfall this winter.
Most winter forecasts are issued in November, but I thought I would reveal some of my findings ahead of schedule and then revisit this forecast and tweak as necessary in early December.
With the possibility of multiple hazards this fall, winter and spring, now is the time to prepare for the worst and hope for the best. Maybe we will luck out with quiet weather during this strengthening El Niño.
Jill Gilardi WBRC Fox 6 News Meteorologist
LINKS WITH DATA:
El Niño ONI oceanic nino index: warm periods, cool periods: http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml
El Niño STRENGTH: http://ggweather.com/enso/oni.htm
MONTHLY/YEARLY NAO STRENGTH: https://climatedataguide.ucar.edu/sites/default/files/climate_index_files/nao_station_monthly.ascii
MONTHLY ALABAMA TORNADOES: http://www.srh.noaa.gov/bmx/?n=tornadodb_main
Average # tornadoes a month per state: http://www.spc.noaa.gov/wcm/permonth_by_state/