The Onslaught of Severe Weather
Last week I posted some general ideas of what to expect over the next few months and briefly touched on U.S. severe weather. In this update I will provide a bit more detail on 2017 severe weather and what to expect over the next few months.
As mentioned in my last blog, this year is off to a record pace in terms of severe weather reports and U.S. insured loss. In fact, if you think a major severe weather event is occurring about every week so far in 2017, you’re likely correct as there have been only four severe weather free weeks so far in 2017 according to PCS wind and thunderstorm event designations.
Historically, in terms of number of PCS wind thunderstorm events, the activity has been unprecedented with 19 events so far, which is 271% above the average number of events that have occurred since 2000.
However, in terms of insured loss it is difficult to estimate how the recent PCS events might develop. Given the wide scope of impact, it would be safe to say at least another billion dollars (or two) could still be expected to develop from existing PCS designated events that have occurred at the end of April. If this development occurs, insured loss through the end of April would be at a historically high level. However, it would be far lower than the costly severe storm year of 2011, where the loss was driven by the deadly April tornadoes in Alabama and the Joplin, Missouri tornado in May. This highlights the remarkable luck that has occurred with tornado related insured loss over the last several years and especially this year since reports of tornadoes are running above normal for the first time since 2012.
Since the U.S. has not experienced a major marquee tornado loss this year, most of the insured loss continues to be related to hail or localized wind damage with smaller tornado losses mixed in such as the East New Orleans tornado on February 7th of this year. Below is a break down how insured losses have compared to 2016 by state thru May 1st, keeping in mind further development of 2017 losses is expected. As of right now Texas, which last year saw 47% of the total reported U.S. insured loss, is reporting a lower level of loss as of May 1 compared to last year at this time.
Possible Cause of Severe Weather
The main stream media continue to put focus on El Niño–Southern Oscillation (ENSO) and its possible impacts on this year’s severe weather, but severe weather cannot be tied to just one atmosphere climate forcer. As mentioned in my previous post, several bits of research have been done around this relationship of Gulf of Mexico Sea Surface Temperatures (SST) and severe weather Convective Available Potential Energy (CAPE). Currently, the SST, are at record high levels, which appears to be helping provide extra fuel for any storm system that traverses across the U.S.
The Gulf of Mexico has recorded 324 consecutive days of above average temperatures — eclipses previous record by a whopping 134 (!!) days 🔥 pic.twitter.com/5qEsM6598P
— Michael Lowry (@MichaelRLowry) March 31, 2017
However, another hypothesis is the relationship the Rocky Mountains and Northern Plains snowpack has on severe weather and tornado occurrence. There is little research around this connection, but a quick crude analysis shows there is possibly a connection here suggesting in years when May snowpack is below normal in the Colorado basin U.S. tornado count in May is also below normal. This is not the case this year with near record snow pack across the Rocky Mountain; however, the correlation between above average snowpack years and tornadoes is not as clean. Combining this theory with warmer than normal Gulf of Mexico SSTs creating warmer, moister air and the extensive spring snowpack in the Rocky Mountains provides an interesting hypothesis that would be a great master thesis for some young inspiring meteorologist.
Forecasted Severe Weather
As shown with the severe weather losses and number of events the first few months have been active, but the first few months of the year historically make up only 19.5% of the severe weather activity on average as recorded by NOAA Local Storm Reports (Tornado, Hail, Wind). An active January thru April can quickly be superseded by a quiet May, June, & July. In attempting to understand if early activity will lead to an overall above active year, I used a trailing 17 year average to find above and below average periods. In this sample approach, over the last 17 years, 7 years were above average in the January-April period. Of the years that had an above average January – April reports of severe weather, only one year (2016) went on to have above average numbers for the remainder of the year, the remainder of those years ended quieter then normal.
In fact other researchers have done some similar studies that suggest a fast start does not necessarily mean the reminder of the season will be active.
In terms of insured loss as of May 1, historically the U.S. insured loss is developed at 42% and we know there will be further development of 2017 losses that have occurred at the end of April. Regionally, it would appear severe weather will continue to be a common occurrence across the Southern Gulf States into the Carolinas into June. More typical periods of warmth across the Northern Plains will trigger severe weather into the summer. However, the stormy periods are not expected to last long like the current persistence pattern the south has seen so far this year.
Note on Wildfire Risk
Florida will continue to see a higher risk for wildfire, but after June the risk could shift to Western states. Despite significant rain and mountain snow across California early this year, wildfires will still pose a threat this summer. Significant precipitation has led to abundant vegetation which can serve as fuel for fires. Early in the season, heat may be inconsistent across California, but temperatures are predicted to rise in July, which will likely dry out this new vegetation and increasing the chances for fires.