Monday, May 15, 2017

Professor Quibb's Picks – 2017

My personal prediction for the 2017 North Atlantic Hurricane season (written May 15, 2017) is as follows:

15 cyclones attaining tropical depression status*,
15 cyclones attaining tropical storm status*,
6 cyclones attaining hurricane status, and
3 cyclones attaining major hurricane status.
*Note: Tropical Storm Arlene formed on April 19, 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 slightly exceeding historical averages in all categories.

In contrast to 2016, the conditions for the 2017 season are fairly common and uncertainty is relatively low. The first condition taken into account is the state of the El Niño Southern Oscillation Index (or ENSO index), a measure of sea surface temperature anomalies in the Pacific Ocean that has a tendency to affect Atlantic hurricane activity. After the index took a brief dip into negative territory this past winter, the index has returned to nearly zero, or "neutral." As shown in the figure below from the International Research Institute for Climate and Society, a modest increase is expected over the coming months.



As a result, the conditions prevailing for the hurricane season are likely to be neutral or weakly El Niño. Since El Niño tends to suppress Atlantic activity and cause cyclones to, on average, take more easterly tracks, this factor would suggest a quieter hurricane season.

Sea surface temperatures, meanwhile, are a bit above average across the Atlantic basin, but the anomalies are not as great in magnitude as they have been over the past few years. The warmest areas are currently the Caribbean and the tropical portion of the Atlantic farther east. Parts of the Gulf of Mexico, meanwhile, are slightly cooler than average. Further warming of the current higher-than-normal areas is likely over the next few months, so these might be conducive to cyclonogensis. The tropical Atlantic has also been quite moist, as has the Caribbean, supporting the development of hurricanes. The Gulf of Mexico, in contrast, has been persistently dry. Finally, with the developing El Niño, increasing wind shear is likely across the Atlantic, especially at higher latitudes and near the United States. Such shear is hostile to tropical systems, so I predict limited activity near the U.S. east coast, despite a pocket of warm water there.

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: 3
The presence of an El Niño would tend to reduce risk, as stated above. However, seasonal forecasts indicate that high temperatures will prevail near the coast for most of the summer, resulting in higher oceanic heat content. Look for quick forming and quick hitting systems - long-lived hurricanes are likely to miss the coast this year.

Yucatan Peninsula and Central America: 3
Signs in the adjacent Caribbean Sea point to elevated tropical activity this year: warm waters, moist air, and limited wind shear. However, steering ridges will have a difficult time setting up along the Caribbean Islands to the north, preventing developing storms from tracking due westward for the most part and instead allowing them to gain latitude. A combination of these two opposing factors leads to the "average" designation for this region.

Caribbean Islands: 4
Complementing the previous point, the Caribbean Islands will be in the more likely path of tropical cyclones. Coupled with the fact that the tropical Atlantic is warm, there is significant risk for landfalling tropical storms and hurricanes this year.

Gulf of Mexico: 1
Atmospheric conditions about the Gulf are already dry and strong upper-level winds are moving across the region. With an increasing ENSO index, this state of affairs is likely to continue indefinitely. Combined with the slightly cooler waters, these signs indicate a very low risk for the Gulf coast.

Overall, the 2017 season is expected to be near or just slightly above average, but with a lower than average risk to landmasses (most storms should curve out to sea). While the confidence in this forecast is somewhat higher than last year, everyone in hurricane-prone areas should still take due precautions as hurricane season approaches. Dangerous storms may still occur in quiet seasons. Sources: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf, http://www.ssd.noaa.gov/PS/TROP/TCFP/atlantic.html

Saturday, May 13, 2017

Tropical Storm Arlene (2017)

Storm Active: April 19-21

During mid-April, a non-tropical low over the central Atlantic well east of Bermuda was producing a large area of tropical storm force winds and scattered thunderstorm activity. As the system drifted eastward, it more more organized, and began to show signs of subtropical development by April 18. Though convection remained mainly confined to the southeast quadrant by the next morning, the low had acquired enough organization to be classified Subtropical Depression One. At that time, the cyclone was moving north-northeast at a moderate clip as it interacted with an extratropical low.

Any cover the center of circulation had managed to develop that day was quickly stripped away by increasing wind shear by early on April 20. The system made a comeback later that morning, however, and in fact became more symmetric, resulting in its reclassification as a tropical depression. It turned toward west-northwest that afternoon and unexpectedly strengthened into Tropical Storm Arlene, only the second known tropical storm to form in April in the Atlantic. Further, its central pressure dropped to 993 mb, the lowest for a tropical system ever recorded in the month of April. Arlene's unusual run ended the next day as it became extratropical and was quickly absorbed by a larger system.



The above image shows Tropical Storm Arlene near its peak intensity over the open Atlantic.


Arlene did not approach any landmasses during its short lifetime. However, it was notable in that it was only the second Atlantic tropical storm known to form in April, after Ana in 2003.

Friday, May 12, 2017

Hurricane Names List – 2017

The name list for tropical cyclones forming the North Atlantic basin for the year 2017 is as follows:

Arlene
Bret
Cindy
Don
Emily
Franklin
Gert
Harvey
Irma
Jose
Katia
Lee
Maria
Nate
Ophelia
Philippe
Rina
Sean
Tammy
Vince
Whitney

This list is the same as used in the 2011 season, with the exception of Irma, which replaced the retired name Irene.