April 2015 Tornado Forecast
After a very persistent pattern through much of the winter, featuring a large ridge across the West Coast of the U.S. and troughing in the East, the pattern is undergoing changes. After record low tornado activity-to-date for 2015 through most of March, a somewhat anomalous severe weather event struck portions of Oklahoma with a handful of early season tornadoes. Despite those few tornadoes to close out March, the number of tornadoes-to-date, 37, is among the lowest since records began.
With all of this said, the severe weather season has been waking up in general as we finish March. While tornadoes specifically have not been numerous, severe wind and hail events are ramping up. An expected pattern shift will likely come in steps before we see a large-scale regime change. This, of course, is assuming there is such a change.
The upper level analysis for March 31st shows a ridge axis over the Rockies, which is dampening out. While troughing continues from eastern Canada into the northeastern U.S., seasonality takes over as the trough has been relaxing north. As we move forward, there are signs of a more zonal flow taking shape, but with some eastward development of ridging.
For the first third of April, the guidance is fairly unanimous in two waves of changes. The initial wave, pun intended, pushes a short-lived ridge toward the East Coast for Thursday, April 2nd and Friday, April 3rd. Although some anomalously negative height anomalies are prevalent on the models from Hudson Bay to New England for the period from April 4-5, that Arctic “blast” should also be relatively brief. The Euro ensembles/weeklies and CFS show a more dominant and slower-moving ridge setting up across the East from roughly April 7-10.
While the setup favors a multi-day severe weather risk through the first couple of days of April across the Plains to lower Midwest/mid-South, severe hail and wind should be greater risks than tornadoes. Keep in mind that while the daily tornado probabilities are increasing rapidly through April, tornado incidence is much more frequent during the second half of the month. Although details are a bit more unclear, a ridge axis across the East and one or more troughs digging east from the Rockies between April 6 and 10th could favor enhanced tornado threats from the Plains to mid-South.
- April 1-10 forecast: Near average tornado counts in the U.S. Focus on the southern/central Plains to mid-South.
Into the middle third of April, there may actually be another lull in tornado and overall severe weather activity. As the pattern dampens out and attempts to realign, the setup is somewhat complex. Weather ensembles diverge, but the general theme favors somewhat above average heights for the eastern half of the country for mid-April. There are also signs that the ridge axis may actually retrograde slightly west, allowing for some modest troughing in New England, or in other words, the dreaded early spring backdoor cold front setup. In terms of severe weather, while some smaller scale events would be possible, tornado activity overall may be suppressed.
- April 11-20 forecast: Moderately below average tornado counts. (Perhaps on the order of only 50% to average)
Does the pattern finally change for the end of April? Even some of the weakest tornado seasons have tended to ramp up around this time with some notable events in the final days of April. Even without a larger-scale regime change, one would tend to expect at least a few significant tornado events to close out April. For storm chasers, perhaps the beginning of the “official” tornado season of 2015 will have more going for it than the beginning of 2014 did. (Recall that April 27-29 was the most significant tornado outbreak of 2014, but most of the tornadoes were in the tough chasing terrain of Dixie Alley)
The Euro weeklies have been consistently showing eastern U.S. ridging in late April, coinciding with digging troughs across the center of the country. Even among the CFS ensembles, as of 12z 3/31, about half of the ensembles favor a ridge in the East for the latter third of April. The remaining half are fairly zonal and show some subtle troughiness digging into the Plains to Mississippi Valley. I think this means that not only does tornado activity increase as climo states, but that there could be multiple tornado outbreaks (not necessarily major, though) toward the end of April. If the “Southeast Ridge” is prominent enough, I could see a scenario with early season tornadoes as far northwest as Nebraska and Iowa.
- April 21-30 forecast: Somewhat above average tornado counts. Focused on central Oklahoma to southeastern Kansas, northwestern Arkansas and much of Missouri.
- April 2015 tornado estimate: 165 tornadoes in the U.S.
The 1991-2010 average number of tornadoes in April is 155. The 20 year period above featured anywhere from 100 to just over 200 tornadoes in most of the seasons. 1992 and 1993 were exceptions with less than 100 tornadoes in April of each year. 2011 was an extreme anomaly with over 700 April tornadoes. There has been more volatility lately with another active April in 2012 with 206 tornadoes, but only 86 in April 2013. April 2014 was relatively quiet outside of the April 27-29 outbreak, which featured 82, or 82/129, or 64% of the entire months worth of tornadoes is just 10% of the days of April.
Recent history tells me to be skeptical, but there are at least a few encouraging signs to consider. Although drought conditions continue across much of Kansas, western Oklahoma and north Texas, March featured above average rainfall from eastern Texas into Arkansas and southern Missouri. Given the combination of climo and model guidance, a good fetch of quality moisture from the Gulf and adjacent moist soil of Texarkana should be able to have no problem advecting north into the mid-South and adjacent eastern portions of the Plains tornado alley. If Mother Nature cooperates, I think tornado activity will be at least near average through April, if not somewhat above average, especially late in the month.
Beyond that, who knows if the drought across the Texas/Oklahoma panhandles and Kansas will ultimately limit May tornadoes there, as it did for much of last May. We’ll save that discussion for the next monthly tornado outlook at the end of April.
