And they’re off…

Following a record-breaking 2017, Tropical Storm Alberto has just kicked off this year's cyclone season by drenching Alabama and the Florida Panhandle. After last year's successful forecast, NASA's Tim Hall has dusted off his ouija board to find out what we can expect for the rest of 2018.

Despite Alberto, projections for sea-surface temperatures (SST) in the North Atlantic Main Development Region (MDR) are near neutral, and the tropical Pacific will be neutral or in a weak El Nino state. Neither of these factors suggest a large deviation above or below the climatological mean. To verify this, we used the stochastic tropical cyclone model of Hall and Yonekura (2013) to simulate 30,000 years at fixed values of the 2018 projected North Atlantic SST and Nino34 (a measure of El Nino/Southern Oscillation) and computed hurricane incidence rates state-by-state on the US Gulf and East Coasts.

The results shown in the figure above bear out the expectation: the rate is everywhere slightly below the 1944-2015 mean, but nowhere is the difference significant. The full-coast reduction is 4%, which is considerably smaller than the threshold for significance. (We’d be irresponsible if we didn’t add the usual qualifier: low-activity years are perfectly capable of producing damaging hurricane landfalls.)

There are more than a few seasonal forecasts to choose from. How does our average-season forecast compare to others? It’s hard to compare numbers directly, since different forecasters use different metrics of activity and different historical baselines for comparison, and few make regional landfall forecasts. But generally, recent forecasts agree with ours, and for the same reasons, North Atlantic SST and El Nino. The Table shows some other forecast statements.

Organization Forecast date Statement
NOAA May 24 “A near normal season is most likely (40%)” (see note)
CFAN April 5 “60% chance of a normal season and a 40% chance of a below normal season”
TSR April 5 12 named, 6 hurricanes, 2 majors (compared to 12.1/6.4/2.7 over 1981-2010)
TWC April 19 13 named, 7 hurricanes, 2 majors

Note: Below/above normal is defined by 71%/120% of the 1981-2010 seasonal accumulated cyclone energy.

The chart below compares my forecast accumulated hurricane counts thru the season for 2017 (yellow) and for 2018 (light blue). Also shown is historical average (dark blue) and the actual 2017 hurricanes.

Dr Hall is a climate scientist at the NASA Goddard Institute for Space Studies in New York City. He specialises in hurricanes and their relationship to climate.  He develops statistical hurricane and weather hazard models, and has worked as a consultant to RMS and reinsurance companies. Dr Hall can be reached at www.linkedin.com/pub/timothy-hall/a2/709/b27/

Posted: Monday, May 28th, 2018