Tropical Storm Bertha (2020)

Storm Active: May 27-28

Around May 25, a weak low pressure center formed near southern Florida along a trough extending westward into the Gulf of Mexico and eastward into the Atlantic. Over the next few days, the rains associated with the system inundated portions of south Florida. More than five inches of rainfall fell in just a few hours in a few locations that day. On May 26, the low emerged over water east of northern Florida, but upper level winds were quite strong, hampering tropical development. The situation changed drastically in a short time, however, and wind shear relaxed. During the morning of May 27, thunderstorm activity blossomed near the center of circulation and the low suddenly became a tropical cyclone. Radar and buoy data indicated it already had gale force winds, so the system was classified Tropical Storm Bertha.

When it was named, Bertha was merely a couple dozen miles off the coast of South Carolina, moving northwest. It reached its peak intensity of 50 mph winds and a minimum pressure of 1004 mb at the time of landfall that same morning. Pushing inland quickly, the storm weakened to a tropical depression that afternoon. Localized flooding occurred along the storm's path over inland North Carolina into the southern Appalachians. Bertha was downgraded to a remnant low early on May 28 and dissipated later that day near western Pennsylvania.

This image shows Bertha near the time of its landfall in South Carolina.

Because it formed so close to land, Bertha spent only one day as a tropical cyclone. Nevertheless, with its formation, 2020 became the first season since 2012 to feature two named storms in the month of May.

Tropical Storm Arthur (2020)

Storm Active: May 16-19

During the second week of May, a cold front stalled over the western Atlantic, its southwestern end threading the Florida Straits. Over the next few days, wet weather prevailed in that region and a broad circulation became evident around May 14. The same day, torrential rains soaked extreme south Florida and the Keys, with lesser impacts in Cuba and the Bahamas. The system moved northeast but lacked deep convection until May 16, when the center of circulation became better defined east of the Florida peninsula. That afternoon, it was classified Tropical Depression One.

The depression became rather asymmetric that evening, with nearly all thunderstorm activity in a semicircular band east of the center. Nevertheless, winds increased enough for it to become Tropical Storm Arthur, the first named storm of the 2020 season. 2020 therefore marked the sixth consecutive year in which a storm formed before the official start of hurricane season on June 1, a new record streak. Arthur churned steadily north-northeast through May 17. Its circulation became better defined as shear lessened, but cooler ocean waters limited convective activity, leading to just a bit of strengthening that day. Rainfall began that evening in eastern North Carolina as Arthur approached.

Near the coast, the storm encountered a deep pool of warmer water in the Gulf stream and strengthened some more. On May 18, Arthur passed just east of Cape Hatteras with peak sustained winds of 50 mph. By that time, the thunderstorm activity in the western semicircle had improved significantly, leading to heavy rains over a large swath of the coast and sustained tropical storm force winds in at least the easternmost barrier islands. The mid-latitude westerlies grabbed hold of Arthur, however, and accelerated it northeastward away from land that evening. Baroclinic processes strengthened the storm a bit more as it underwent extratropical transition overnight, bringing Arthur to its peak intensity of 60 mph sustained winds and a minimum pressure of 991 mb. It turned toward the east and became post-tropical during the morning of May 19. The remnant system turned sharply south later that day and angled toward Bermuda, but weakened to an extent that it brought only intermittent showers and gusty winds as the center passed the island on May 20. It dissipated shortly afterward.

The above image shows Tropical Storm Arthur just off the coast of North Carolina.

Arthur did not quite make landfall, but it brought heavy rain to south Florida as a tropical disturbance (before formation) and the Cape Hatteras region.

Professor Quibb's Picks – 2020

My personal prediction for the 2020 North Atlantic hurricane season (written May 13, 2020) is as follows:

20 cyclones attaining tropical depression status,
18 cyclones attaining tropical storm status,
9 cyclones attaining hurricane status, and
5 cyclones attaining major hurricane status.

I predict that the 2020 Atlantic hurricane season will continue the active trend of the last few years and likely feature a well above-average number of cyclones, although uncertainty in my forecast is higher than average. Note that the average Atlantic hurricane season (1981-2010 average) has 12.1 tropical storms, 6.4 hurricanes, and 2.7 major hurricanes. The main factor in support of this claim is the potential for a neutral to negative El Niño Southern Oscillation (ENSO) index this summer and autumn. This index, a measure of equatorial sea temperature anomalies in the Pacific ocean, has an inverse correlation with Atlantic hurricane activity: negative indices (corresponding to a La Niña) tend to favor more active seasons.

Unlike the last two years, there is a great deal of spread in the model forecasts for this year's ENSO. Some models have the index remain positive, while others show a strong La Niña event developing by season's end. The dynamical models (whose average is represented by the thick red line) lean toward La Niña more than the statistical models (green), which favor using historical data for prediction over simply modeling future changes in weather. Due to a rapid plunge of Pacific equatorial sea surface temperature anomalies in the last few weeks, I'm putting more weight on the dynamical solution. This would suggest a very active season, especially since 2018 and 2019 were active despite neutral to slightly positive ENSO index conditions.

Meanwhile, Atlantic water temperatures are running uniformly warmer than average for this time of year, and this is expected to continue through the summer. No particular regions stand out though: the Gulf of Mexico, Caribbean, and subtropical Atlantic will all have moderately high anomalies, and the tropical Atlantic just a little bit less so. Expect tropical cyclone formation in many areas this year, and not quite as much emphasis on the subtropical Atlantic.

The above image shows the strength of the Saharan Air Layer (SAL) of the atmosphere as it appeared on May 12, 2020. Tropical trade winds carry dust particles and dry air from the Sahara desert over water throughout the year, but this has particular relevance to the development of tropical waves emerging off of the west African coastline: the more dry air and dust, the more thunderstorm activity in the region is suppressed. The SAL is quite strong as of now (though there is a typical decline throughout spring), so expect a late start for long-track hurricanes this year. Come September, however, other factors indicate high risk for Cape Verde hurricanes.

I'll discuss a few more smaller-scale factors in association with assigning risks to different parts of the Atlantic basin. My estimates are on a scale from 1 (least risk) to 5 (most risk):

U.S. East Coast: 4
The position of the Bermuda/Azores high pressure system correlates with the ENSO index and has a great influence on tropical cyclone tracks. I predict that this high will remain further east than typical La Niña events, increasing the risk to the east coast but lowering it for the Gulf of Mexico. The threat to the east coast will start early to the season, in July and August.

Yucatan Peninsula and Central America: 4
The western Caribbean is at higher risk for tropical cyclones than in any of the last 3 seasons, particularly when early fall rolls around. Some of the Atlantic's highest ocean temperatures will be in this region, and wind shear will be below normal. Expect some low-latitude cyclones, possibly affecting Nicaragua and Honduras.

Caribbean Islands: 4
The Caribbean, too, looks to be at greater-than-average risk this year. Once the tropical wave train gets going in earnest, look for long-track storms approaching from the east (primarily in September). The east Caribbean should be less of a hurricane graveyard than the last few seasons, so cyclones following tracks similar to Hurricane Matthew and approaching Hispaniola and Puerto Rico from the south are likelier than average.

Gulf of Mexico: 3
Waters in the Gulf are once again very warm, and some "home-grown" cyclone development is likely. Nevertheless, I forecast that the strongest cyclones will track elsewhere for most of the season. Come October, however, there is still a moderate chance for storms moving south to north from near the Yucatan Peninsula toward the United States gulf coast, similar to Hurricane Michael.

Overall, I expect the 2020 Atlantic hurricane season to feature well above-average activity. Nevertheless, this is just an amateur forecast. Individuals in hurricane-prone areas should always have emergency measures in place. For more on hurricane safety sources, see here. Remember, devastating storms can occur even in otherwise quiet seasons.

Sources: https://www.tropicaltidbits.com/analysis/models/, https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf, http://trackthetropics.com/saharan-air-layer-sal-tracking/

Hurricane Names List – 2020

The name list for tropical cyclones forming in the North Atlantic basin for the year 2020 is as follows:

Arthur
Bertha
Cristobal
Dolly
Edouard
Fay
Gonzalez
Hanna
Isaias
Josephine
Kyle
Laura
Marco
Nana
Omar
Paulette
Rene
Sally
Teddy
Vicky
Wilfred

This list is the same as the list for the 2014 season because no names were retired that year.