Friday, May 27, 2016

Tropical Storm Bonnie (2016)

Storm Active: May 27-30, June 2-4

Around May 24, a broad area of scattered showers and thunderstorms began to develop in association with a low-pressure trough situated to the north of Hispaniola. Over the next few days, convection gradually became more concentrated near the low-pressure center as it deepened and moved generally toward the west-northwest. By the afternoon of May 27, the circulation had become well-defined with a curved band of strong thunderstorms just to the north and west of the center. This resulted in the designation of the system as Tropical Depression Two.

Overnight, the system continued west-northwestward toward a tongue of warmer Gulf Stream waters off of the coastline of South Carolina. Even as it encountered higher sea surface temperatures, however, significant wind shear out of the south kept the center exposed through the day of May 28. By that morning, rain bands had begun to affect the U.S. coastline. The convection deepened somewhat that afternoon and the circulation became better defined, so the system was named Tropical Storm Bonnie. Since Bonnie was named before the official start of the season on June 1, 2016 became the first year since 2012 to have multiple preseason storms. A few hours later, the storm reached its peak intensity of 45 mph winds and hesitated slightly in its forward motion.

Shortly afterward, however, a large burst of wind shear out of the south ripped the existing convection away from Bonnie, weakening it overnight just before its landfall in South Carolina as a tropical depression during the morning of May 29. Despite the decay of the system, heavy rainfall continued for a large region of the southeast U.S. through the day. Shortly afterward, the cyclone took its anticipated turn toward the north east over land. By the morning of May 30, the system had weakened sufficiently that it no longer met the criteria of being a tropical cyclone. Bonnie's remnants continued to generate shower activity as the center moved slowly northeast back over the Atlantic Ocean on May 31.

In fact, the system became more organized back over water, slowly redeveloping convective bands and increased sustained winds. By the morning of June 2, it had regained tropical depression status near the Outer Banks. That day, the Bonnie strengthened slightly over the warm Gulf Stream waters as it moved away from land, but remained slightly below tropical storm strength. Meanwhile, it began to accelerate eastward under the influence of a subtropical ridge. Overnight, the system seemed to be on the wane as it moved over cooler water and convection diminished, but a resurgence during the afternoon of June 3 prompted once again upgrading Bonnie to a tropical storm. However, very cold water finally took its toll on the storm on June 4: it lost all significant shower activity, weakening to a tropical depression, and shortly thereafter, a remnant low. The remnants moved east-southeast for another couple of days before dissipation.

The above image shows Bonnie shortly before landfall in South Carolina.

Bonnie's slow motion near the U.S. coastline as both a tropical and a non-tropical system brought torrential rains to the Carolinas.

Wednesday, May 18, 2016

Professor Quibb's Picks – 2016

My personal prediction for the 2016 North Atlantic Hurricane Season (written May 18, 2016) is as follows:

14 cyclones attaining tropical depression status*,
13 cyclones attaining tropical storm status*,
7 cyclones attaining hurricane status*, and
3 cyclones attaining major hurricane status.
*Note: Hurricane Alex formed on January 13, long before the official start of the season on June 1 and before I made these predictions.

This prediction calls for a nearly average Atlantic hurricane season, with predictions just barely exceeding historical averages in all categories.

The picture for the 2016 Atlantic hurricane season is unusually murky, due to several uncertainties regarding significant factors that influence tropical cyclone formation. First, the 2015-16 El Niño event has continued to unfold, ranking in the top 3 historically in both intensity and duration. Positive sea surface temperature anomalies have persisted into May in the equatorial Pacific, indicating the continuation of the event. The chart below compares El Niño events since 1950.

The 2015-16 event (black line) is probably most comparable to the 1997-98 event in its qualities, so if this trend were to repeat, the 2016 season would end with the ENSO in a negative phase. However, it has occurred that El Niño events persist to the end of the second year, or that they become roughly neutral. A neutral ENSO (El Niño Southern Oscillation) index, all else held equal, would lead to an average hurricane season, and a negative index to a more active season. The latest predictions indicate that neutral conditions will in fact prevail during the season's peak in September and October, but there is a great deal of uncertainty.

Second, the Atlantic Multi-Decadal Oscillation (AMO) (an empirically observed trend in tropical cyclone activity that has decades-long period) appears to be wrapping up the positive phase that led to busier hurricane seasons during the 2000's and early 2010's. However, this trend is harder to predict than the ENSO, and while some meteorological experts believe that it is now entering its negative phase, it is difficult to know for certain. The combination of these two factors yield an expectation of an average season, but with an unusually high probability of deviance from this prediction.

Finally, we examine a few more proximate factors to cyclone formation in the Atlantic. Current mean sea surface temperatures, as with all global temperatures, are anomalously high relative to historical data. However, temperatures in the Gulf of Mexico and along the U.S. Eastern seaboard are lower relative to average than the southern Caribbean and central tropical Atlantic. These latter areas may therefore be especially favorable to cyclonogenesis. Normally, preseason wind shear tendencies would also be relevant to my forecast, but due to the possible rapid changes in the ENSO index, these observations would have little predictive power.

My estimated risks for different parts of the Atlantic basin are as follows (with 1 indicating very low risk, 5 very high, and 3 average):

U.S. East Coast: 2
Neither the jet stream nor the negative anomaly in sea surface temperatures is as pronounced in this region as in 2015. Nevertheless, wind shear may still inhibit development in this region, leading to a lower risk of landfalls.

Yucatan Peninsula and Central America: 4
The southern Caribbean has some of the most anomalously warm temperatures in the Atlantic, and could fuel tropical cyclones that traverse it. After upper-level winds subside about midway through the season, there is potential for dangerous hurricanes to develop in this region.

Caribbean Islands: 3
The Caribbean Islands are at about average risk this year, with moderately warm temperatures and a diminishing El Niño that will lead to a fair, but not exceedingly high likelihood of westward-tracking cyclones. Expect 2-3 tropical storms, at least one of which is of hurricane strength, to affect the islands.

Gulf of Mexico: 2
The Gulf remains rather safe this year, continuing the trend from the previous two seasons. Rather low temperatures will limit the potential for significantly damaging landfalls.

Overall, the 2016 season is expected to be around average, but there is an unusually low degree of confidence in this forecast due to expected shifts in climate throughout the year. Regardless, everyone should take sufficient preparedness measures, since dangerous storms can occur even in quiet seasons.


Saturday, May 14, 2016

Hurricane Names List – 2016

For the North Atlantic Basin, the list for naming tropical cyclones in 2016 is as follows:


This list is the same as that for the 2010 season, with the exception of Ian and Tobias, which replaced the retired names Igor and Tomas, respectively.