Thursday, May 14, 2015

Professor Quibb's Picks – 2015

My personal prediction for the 2015 North Atlantic Hurricane Season (written May 13, 2015) is as follows:

9 cyclones attaining tropical depression status*,
7 cyclones attaining tropical storm status*,
3 cyclones attaining hurricane status, and
1 cyclone attaining major hurricane status.

*Note: Tropical Storm Ana formed on May 8, almost a month before official start of the hurricane season and before I published my predictions.

The above prediction anticipates a significantly below-average hurricane season, particularly relative to the past few decades. It calls for just over half the typical number of tropical storms, and less than half of the usual hurricanes and major hurricanes. If this prediction were to come true, 2015 would be the least active season since 1994.

Several factors are stacked against tropical cyclone formation this season. First, a weak El Nino event that developed in 2014 has persisted so far this year. In addition, predictions indicate that it will strengthen over the coming months. Already, the anomalously high sea surface temperatures in the Pacific associated with an El Nino have caused a stronger jet stream and stronger wind shear across the United States and the neighboring Atlantic.

The relative scarcity of Atlantic hurricanes over the 2013 and 2014 seasons also suggests that we finally may be entering the "cool phase" of the Atlantic Multi-Decadal Oscillation (AMO), an apparent cycle of above-normal and below-normal hurricane activity whose period is measured in decades. We do not completely understand this cycle and cannot predict it, but the elevated activity for the 15 years ending in 2012 and the subsequent lull could indicate a reversal. Currently, cool water temperatures abound throughout the tropical Atlantic, supporting the hypothesis that we are entering a "cool" period. Regardless, the low Atlantic sea surface temperatures will help to suppress cyclone formation.

Finally, sea-level pressures over areas of the north central Atlantic have been higher than normal over the past few months. Though I wrote elsewhere that a stronger high-pressure system over the central Atlantic tends to steer Atlantic hurricanes towards land and that the strong high appears with the La Nina, and not the El Nino, the truth is more complicated. Both its position and strength influence hurricane activity in different ways and the latter may not correlate with the ENSO cycle. This year, there is both an El Nino and a higher intensity Bermuda high, and both of this factors tend to reduce hurricane activity.

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 Coat: 1
A combination of a strong jet stream and cool ocean waters will result in a very low risk of landfall for the U.S. East Coast. Ironically, Tropical Storm Ana made landfall in South Carolina before the season even officially began. However, this was due to a blocking pattern that should occur only infrequently, particularly if the El Nino strengthens.

U.S. Gulf Coast/Northern Mexico: 3
The anomalies in water temperatures are less pronounced in the Gulf of Mexico, so this region is at greater risk for landfalling cyclones. The Bay in Campeche in particular may be a birthplace of one or more short-lived tropical storms this season.

Yucatan Peninsula and Central America: 3
The high sea-level pressures this season will enhance trade winds, causing the potential for westward-tracking, fast-moving systems through the central Caribbean and into the Central American states. Despite the expected quiet season, this region will have about average risk.

Caribbean Islands: 2
The Caribbean can expect some activity, but El Nino-related dry air may lead to the demise of some tropical systems before they can undergo much strengthening. This region probably has little to fear from Cape Verde-type strong hurricanes this year.

Overall, the 2015 season is expected to be quiet, and possibly historically quiet. The precise strength of the developing El Nino is the main uncertainty in predicting the level of activity. Regardless, even quiet years can have devastating storms, so be sure to always practice hurricane preparedness!


Tuesday, May 12, 2015

Hurricane Names List – 2015

For the North Atlantic Basin, the list for naming tropical cyclones in 2015 is as follows:

Ana (used)
Bill (used)
Claudette (used)
Danny (used)
Erika (used)
Fred (used)
Grace (used)
Henri (used)
Ida (used)
Joaquin (used)
Kate (used)

This list is the same as the list for the 2009 season because no names were retired that year.

Friday, May 8, 2015

Tropical Storm Ana (2015)

Storm Active: May 8-11

During the first few days of May, a stationary front stalled over the northwest Caribbean sea, extending through the Bahamas. By May 5, an upper-level trough had formed over the central Bahamas, generating a widespread area of showers and thunderstorms extending into eastern Florida. A weak surface low formed in association with the trough the next day and moved close to the Floridian coast. Due to a blocking pattern situated over the mid-Atlantic states, the low moved little over the next few days, drifting only slowly northward. Meanwhile, the low deepened over the warm Gulf stream waters southeast of the Carolinas. By May 8, the system was exhibiting gale force winds in a broad region encompassing its center as it meandered a few hundred miles off of the South Carolinian coastline. Late that night, the low acquired enough convective organization to initiate advisories, but the large radius of maximum winds led to its classification as Subtropical Storm Ana.

Despite relatively favorable sea surface temperatures, dry air impeded development of convection near the center of Ana over the next day. Through most of the day May 8, almost all thunderstorm activity was confined to the southern and eastern portions of the system's circulation and the center remained isolated. This also contributed to Ana remaining a subtropical cyclone that day as it moved little. During the night, significant cloud cover finally developed near the center and the broader structure disappeared, indicating that Ana had transitioned to a tropical storm early on May 9. By this time, it had acquired a slow north-northwestward motion towards the U.S. coastline. During the day, convective banding structure improved, but Ana simultaneously began to move over the cooler water nearest to the coastline. As a result, cloud tops warmed, though the system maintained its approximate peak intensity of 60 mph winds and a minimum pressure of about 998 mb. Meanwhile, bands of rain began to move into parts of the Carolinas. Overnight, Ana weakened as it interacted with the aforementioned cooler water and land, but made landfall just south of the border between the Carolinas at 6:00 am EDT, May 10, still as a tropical storm.

The cyclone quickly weakened to a tropical depression over land and began to move toward the north and north-northeast as the ridge over the mid-Atlantic shifted eastward and diminished. By May 11, Ana was well-inland over North Carolina but still maintained its identity as a weak tropical depression. Late that night, Ana degenerated into a remnant low over eastern Virginia. The low was absorbed soon after. Ana became the second-earliest cyclone to make a landfall in the United States when it did so on May 10, behind only a cyclone of February 1952.

The above image shows Ana approaching the United States coastline on May 9, just after transitioning to a tropical storm.

Ana stalled southeast of the Carolinas due to a blocking ridge situated to its north.