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Thread: While September may be a bit early to be looking into seasonal trends for severe weather going into the cool season, there are a couple of things that stand out when looking at large scale. #1 would be the developing La Nina in the Pacific shown via SSTAs. 1/8
#2 would be that the atmosphere seems to be responding to this La Nina, with a standing wave becoming established in the 850 hPa U-wind west of the dateline, with strong trades to the east. This favours further growth of this La Nina. September NMME agrees generally. 2/8
#3 would be the widespread drought across the SW United States and the forecast (at least for this month) to either maintain or worsen over most areas. 3/8
Just from the presence of these two features alone, I'm growing a bit concerned regarding regions east of the Plains for cold season (January - April) severe wx activity. La Nina conditions favouring a strong polar jet and west Atlantic/SE ridging (moisture return)... 4/8
...and the hot/dry antecedent conditions in the SW contributing to greater elevated mixed layer potency (read: steeper lapse rates), which is often associated with significant severe wx east of the Plains. 5/8
Of course, there are caveats, namely that two indicators alone does not make a forecast. There are a copious amount of factors on all time and spatial scales that can modulate the eventual intensity/scope of severe weather events, this is mainly addressing background states. 6/8
Bottom line, residents living east of the Plains should be aware that some seasonal signals are lining up to possibly favour an active cool season severe weather pattern ahead, especially in light of the deadly tornadoes earlier this year in March/April. 7/8
For more on the influence of ENSO on early season severe weather (not to mention the caveats associated with such analyses), see these two papers by @arcook75 et al. and @ScienceByMaria, @JTAllen3, and @gensiniwx.
1: https://journals.ametsoc.org/jamc/article/56/9/2455/21724
2: https://journals.ametsoc.org/jamc/article/57/10/2439/68232
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1: https://journals.ametsoc.org/jamc/article/56/9/2455/21724
2: https://journals.ametsoc.org/jamc/article/57/10/2439/68232
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