The following post is from one of our blog contributors – Fred Gossage.  The follow is his analysis regarding Spring severe weather season.

 

 

The big global weather story for the last several months has been the return and strengthening of El Niño. El Niño is a reverse of the trade winds in the equatorial Pacific that causes warmer than normal water temperatures to build up across the area. Our El Niño got off to a slow start, but once November rolled around, it rapidly intensified, with sea-surface temperature anomalies of 1.7C+ above normal found before Thanksgiving. The El Niño maintained that intensity through late January, peaking at 1.9C during the middle of the month, with the official NDJ reading from CPC as 1.8C, making this El Niño phase officially classified as the lower end of “strong”. Since then, however, the warm anomalies across the equator have rapidly cooled and have also decreased in aerial coverage. Below is a sea-surface anomaly loop for the past several weeks.

As you can see, during the last couple of weeks, the anomalies have grown much less intense, and during the past two weeks, have decreased dramatically in aerial coverage. Cooler waters off the South American coast are also growing colder and expanding northwestward toward the Equator. Given this, the fact that overall oceanic heat content across the equatorial Pacific is decreasing markedly as well, and that it’s climatologically the right time for the El Niño to pass its peak intensity, it is likely that we are on the downhill climb. The rate of cooling and shrinking of the warm anomalies, when compared to some potential analog years, suggests that we may be witness a rapid weakening of the El Niño, and it wouldn’t completely surprise me if we aren’t firmly into neutral ENSO territory or even borderline weak La Niña by the end of spring.

One of the major calling cards of an El Niño phase, in the United States, is a persistent, intense subtropical jet stream; and this winter has been no exception. The subtropical jet has been one of the main culprits in the amount of rain and number of storm systems we have had since fall. The subtropical jet stream also provides a highway for storm systems to travel and helps to increase vertical wind shear across the area. Once an El Niño or La Niña, especially a significant one, starts to weaken, it will often take several weeks or even a few months before that weakening really starts to significantly impact the global weather pattern. For this reason, it is a great concern that the active, intense subtropical jet will remain with us as we head into March and April, slowly lifting northward as spring progresses, while at the same time the sun angle increases… the days get longer… and it’s much easier to get increasing instability with each storm system. Having such intense upper-level winds and multiple storm systems juxtaposed with increasing instability is usually a bad combination once spring starts to roll around.

Another thing we are watching, is the presence of drier than normal conditions across the Southwestern United States and northern areas of Mexico. Although there have been a few storm systems in the past several weeks to help with drought conditions there, those areas are still significantly drier than normal. These areas are at a much higher elevation than the Southeastern United States. When the strong upper level winds from the subtropical jet blow across this region, they move plumes of this warm, dry air east or northeastward with them, and due to warm, dry air being more buoyant than moist air because it is less dense (although we get more clouds and precipitation out of warm, moist air, because of more moisture content to condense into clouds and precipitation), this warm, dry air mass will ride up and over moisture returning northward from the Gulf of Mexico. These plumes of warm, dry air in the mid-levels is known as the elevated mixed layer… and is a major ingredient in severe weather outbreaks from the Plains eastward. Dry air in the mid-levels of a warm sector environment will aid in convective instability, help to weed out smaller, weaker updrafts, and also aid in wind gust potential with thunderstorms. Having drier than normal conditions over this area of the Southwest and northern portions of Mexico helps increase the frequency and quality of EML plumes getting advected overtop returning Gulf moisture during the spring. Also shown by the map, is a complete lack of any dry conditions in the Southeast. This is easily apparent by our abundant rainfall since spring, and honestly, starting back in 2008. Studies have shown that, while not as important as in the Plains, having increased soil moisture in the Deep South definitely isn’t detrimental to severe weather activity, and may actually be a helpful factor.

Finally, in terms of looking at previous similar periods to get a guess at what our spring may look like (analog forecasting), it also increases confidence in an active spring severe weather season in Dixie Alley. Considering this was a strong El Niño episode, we can go back and look at the springs when a moderate or strong El Niño were dissipating. Such springs include 1973, 1977, 1995, 1998, and 2007.

All of those had significant tornado activity in Dixie Alley states, including several F2-F5 tornadoes. In fact, Alabama’s only two F5 tornadoes to occur outside of the Super Outbreak of April 3-4, 1974 happened during two such instances… the Smithfield F5 of 4/4/77 and the Oak Grove F5 of 4/8/98. Our longest tracked tornado in state history, the Brent-Centreville AL F4, also happened coming out of a strong El Niño on May 27, 1973. 1995 was the near of the Anderson Hills, AL F4 in Madison County, AL; and we all know of the deadly Miller’s Ferry and Enterprise EF4s on March 1, 2007. When comparing this winter, in terms of El Nino, North Pacific jet/water temperature structure, the water temperature structure of the North Atlantic, and the overall evolution of the large scale weather pattern through the winter… to previous moderate and strong El Niño episodes, we are strikingly close to the 1972-1973 El Niño phase (although its max anomalies were a little warmer), down to our snow event Friday being very similar to the February 9-10, 1973 snowstorm in the Southeast…

as little as 2-3 days off the calendar from the “analog”. While even an almost perfect analog will never give the exact same results over again, similar conditions will breed similar outcomes.

With all of this in mind, below is my personal thinking on how this spring will break down in terms of severe weather activity, for the state of Alabama.

As stated, this will likely be a fairly active severe weather season for the Southeast. History with similar large scale conditions show that “Dixie Alley” gets several F2+ tornadoes during the March-May period in these conditions, and that the spring severe weather season remains active through much of May, often with a smaller resurgence of activity in May (that have included tornadoes like the Brent-Centreville F4 and the Anderson Hills F4) in the past. It is likely that we will have several storm systems affecting the area because of the active subtropical jet, at least through March and early to mid April, providing numerous opportunities for at least low-end severe weather days. Because of the enhanced vertical shear as a result of the active subtropical jet, there is the opportunity for a number of these to be tornado days. With this increased vertical shear getting juxtaposed with increasing instability as we head later into March and into April, history has shown us many times over in similar larger scale conditions, that at least the potential is greater for one or more violent tornadoes to occur in northern and central Alabama under such conditions, and this year may be no exception.

Obviously, we don’t know what the future really holds, but this is a good first guess at the spring season based on large scale conditions we have now and historical conditions under similar circumstances. We will obviously know more as we get closer to the spring severe weather season. It is best that you and your family go ahead and review or develop your severe weather safety plan, and make sure you have a reliable way of getting severe thunderstorm and tornado warnings 24/7, now… before the weather turns active.

Fred Gossage

FOX 6 Weather Blog Contributor

 

Filed under: Birmingham, Rain, Regions, Thunderstorms, Tornadoes