The 2021 North Atlantic Hurricane Season: Penn State ESSC Forecast

By NASA, Public Domain, https://commons.wikimedia.org/w/index.php?curid=78491891

ESSC scientists Dr. Michael E. Mann and Daniel J. Brouillette and alumnus Dr. Michael Kozar have released their seasonal prediction for the 2021 North Atlantic hurricane season, which officially starts on 1 June and runs through 30 November.

The prediction is for 11.9 +/- 3.4 total named tropical cyclones, which corresponds to a range between 9 and 15 storms, with a best estimate of 12 named storms. This prediction was made using the statistical model of Kozar et al. (2012, see PDF here). This statistical model builds upon the past work of Sabbatelli and Mann (2007, see PDF here) by considering a larger number of climate predictors and including corrections for the historical undercount of events (see footnotes).

The assumptions behind this forecast are (a) the persistence of current North Atlantic Main Development Region (MDR) sea surface temperature (SST) anomalies (+0.4 °C in late March and early April 2021 from NOAA’s Coral Reef Watch) throughout the 2021 hurricane season, (b) the presence of neutral El Niño-Southern Oscillation (ENSO)-negative conditions by boreal late summer and early fall 2021 (ENSO forecasts here; we used early April 2021), and (c) climatological mean conditions for the North Atlantic Oscillation in boreal fall/winter 2021-2022.

If a weak La Niña develops again by late summer and fall 2021, then the prediction will be slightly higher: 12.9 +/- 3.6 storms (range of 9-17 storms, with a best guess of 13).

Using an alternative model that uses "relative" MDR SST (MDR SST with the average tropical mean SST subtracted) in place of MDR SST yields a lower prediction (10.4 +/- 3.2 total named storms). This alternative model also includes neutral ENSO conditions.

Previous Forecasts:

Year (click to see forecast)
Prediction
Best Guess
Range
Actual Count
2020 19.8 +/- 4.4 20 15-24 30
2019 10.1 +/- 3.2 10 7-13 18
2018 10.2 +/- 3.2 10 7-13 15
2017 15.3 +/- 3.9 15 11-20 17
2016 18.9 +/- 4.4 19 14-24 15
2015 6.9 +/- 2.6 7 4-10 11
2014 9.3 +/- 3.0 9 6-12 8
2013 16.0 +/- 4.0 16 12-20 14
2012 11.2 +/- 3.3 11 8-15 19
2011 16.25 +/- 4.0 16 12-20 19
2010 23.4 +/- 4.8 23 19-28 19
2009 11.5 +/- 3.4 12 8-15 (6-13 if El Niño) 9
2007 n/a 15 n/a 15

References:       
Kozar, M.E., Mann, M.E., Camargo, S.J., Kossin, J.P., Evans, J.L., 2012: Stratified statistical models of North Atlantic basin-wide and regional tropical cyclone counts, J. Geophys. Res., 117, D18103, doi:10.1029/2011JD017170.

Mann, M.E., Sabbatelli, T.A., Neu, U., 2007: Evidence for a Modest Undercount Bias in Early Historical Atlantic Tropical Cyclone Counts, Geophys. Res. Lett., 34, L22707, doi:10.1029/2007GL031781.

Sabbatelli, T.A., Mann, M.E., 2007: The Influence of Climate State Variables on Atlantic Tropical Cyclone Occurrence Rates, J. Geophys. Res., 112, D17114, doi: 10.1029/2007JD008385.

Vecchi, G.A., Knutson, T.R., 2008: On Estimates of Historical North Atlantic Tropical Cyclone Activity, J.Climate, 21, 3580-3600, doi:10.1175/2008JCLI2178.

Footnotes:

The tropical cyclone series was corrected based on an estimated historical undercount taken from Vecchi and Knutson (2008).

Prediction made: 08 April 2021

This webpage last updated: 12 April 2021