CSU’s Latest Hurricane Outlook: Steady As She Goes

There were no major adjustments in the August update to the 2016 CSU Atlantic hurricane outlook, issued on Thursday by Colorado State University. These outlooks are produced by Dr. Phil Klotzbach, who worked closely with CSU’s Dr. William Gray for many years prior to Dr. Gray’s passing in April. In the updated outlook, CSU projects a total of 15 named storms this year, including the four that developed before August 1 (Hurricane Alex and Tropical Storms Bonnie, Colin, and Danielle) as well as this week’s Hurricane Earl. The outlook also calls for a seasonal total of 6 hurricanes (including Alex and Earl) and 2 major hurricanes. These numbers are slightly above the long-term annual average, although CSU expects the rest of the season to produce near-normal activity. The predicted numbers of hurricane days and major hurricane days for 2016 as a whole were nudged upward slightly (from 21 to 23 and from 4 to 5, respectively).

In addition to these single-number forecasts, the CSU outlook includes uncertainty brackets similar to those used in NOAA’s seasonal hurricane outlooks. For example, CSU assigns two-thirds odds that the number of named storms developing after August 1, including Earl, will range betwen 8.7 and 13.3, or one standard deviation within the single-number prediction of 11.


Figure 1. Forecasts/hindcasts for the amount of accumulated cyclone energy (ACE) ocurring after August 1 (magenta line) compared to the actual number (blue line). The statistical scheme used by CSU for its August outlooks was updated in 2012, so the pre-2012 forecasts shown here are actually “hindcasts,” generated as if the current statistical tools had been in place then. The year-to-year ups and downs in ACE values are well predicted by this model, resulting in a strong correlation between observed and predicted values of 0.86. However, the ACE values for the last decade are generally overpredicted. According to Dr. Phil Klotzbach, this may be related to an increasing trend in July SSTs over the northeast subtropical Atlantic since the late 1970s. July SSTs in this region is one of the predictors used in creating the August outlook. Warmer SSTs here in July tend to be correlated with increased Atlantic hurricane activity, but a long-term warming trend may affect the predictive value, according to Klotzbach. Image credit: Courtesy Phil Klotzbach, CSU.

Mixed signals remain
There is still considerable uncertainty shrouding how the rest of the season will unfold, according to CSU. The variables in play include the potential development of La Niña (still a question mark in its timing and strength), as well as the unorthodox North Atlantic pattern now in place. Sea surface temperatures have been extremely warm across the western Atlantic, coupled with cooler-than-average SSTs in the eastern subtropical Atlantic and the far North Atlantic. This is roughly consistent with the negative phase of the Atlantic Multidecadal Oscillation as tracked by an AMO index developed by Klotzbach and Gray. This index has been in negative territory since late 2014, although it crept close to the zero line in July 2016, noted Klotzbach in a tweet on Wednesday. Other signals are also in some conflict: for example, upper-level wind shear has been fairly low, which favors tropical cyclone development, but the atmosphere across the tropical Atlantic has been more stable than usual over the last several months, which works against development.

CSU found that the closest analog years for hurricane-related conditions observed this past June and July are 1958, 1959, 1966, 1978, 1992, and 1998. These years produced anywhere from 7 to 14 named storms, 4 to 10 hurricanes, and 1 to 3 major hurricanes, reflecting the wide range of outcomes still possible.


Figure 2. Departures from seasonally averaged sea surface temperature (SST, shown in degrees C) in the Atlantic Ocean for the last week of July 2016. SSTs were well above average over large parts of the western Atlantic adjoining North and South America, as well as across the deep tropics of the North Atlantic that serve as the main development region for Atlantic hurricanes. Below-average SSTs prevailed over most of the northeast Atlantic subtropics and midlatitudes. Image credit: NOAA/NHC.

Are we ready for a major hurricane?
CSU is giving 51 percent odds that a major hurricane will strike somewhere along the U.S. Gulf or Atlantic coast--which is a quite unsettling prospect, given that a major landfall (Category 3 or stronger on the Saffir-Simpson scale) has not occurred in the U.S. since Hurricane Wilma in 2005. Moreover, the Gulf of Mexico has not seen a hurricane outside of the Bay of Campeche since Ingrid in 2013, and Florida has not experienced a landfalling hurricane of any strength since Wilma. All three of these “droughts” are unprecedented in length, a point whose consequences are analyzed in an excellent roundup on Thursday by Capital Weather Gang.

See our posts from May 27 and June 1 for more background on the outlooks issued this spring by CSU and other groups. NOAA will be updating its seasonal hurricane outlook on Thursday, August 11, and CSU will be issuing 14-day outlooks every two weeks from now through October. Note that the CSU outlooks are now linked from a new website, tropical.colostate.edu.



Figure 3. Number of named storms in the Atlantic basin predicted for the 2016 season by various forecast groups. Bottom axis shows the name of each group and the time frame when its forecast was issued (or updated). The zone between the orange and red horizontal lines indicates the number of named storms one might expect in a typical year. Image credit: Barcelona Supercomputing Center and Colorado State University.

A new tool for comparing and contrasting seasonal outlooks
At last, we have a one-stop virtual shop for evaluating how the seasonal hurricane outlooks issued by an increasing number of entities stack up against each other. CSU’s Phil Klotzbach teamed up with Louis-Philippe Caron at the Barcelona Supercomputing Center in Spain to produce the new site, which was designed by Iskiam Jara and supported by the Ireland-based insurance firm XL Catlin. The site formally debuted on Thursday with a CSU press release.

Colorful graphics depict each forecast and whether it was issued only as a single number or includes a probabilistic range. The site allows you to toggle between the outlooks for the number of named storms, hurricanes, and major hurricanes. At the bottom of the main forecast page, you’ll find blurbs on each of the 17 organizations that have released seasonal hurricane outlooks, including The Weather Company. Klotzbach is hoping to compile forecasts issued in past years by the various groups, which would make the site a true treasure trove for researchers and weather enthusiasts.

For a full update on current tropical activity, including the potential for multi-day heavy rains along the eastern Gulf Coast associated with a near-shore disturbance, see this morning’s post from Jeff Masters. Have a great weekend, everyone!

Bob Henson