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WeatherTiger Potential Tropical Cyclone 9 Forecast Discussion for July 29th
Potential Tropical Cyclone 9 is a potential threat to Florida. It's a tricky, uncertain outlook, and Dr. Ryan Truchelut of WeatherTiger is breaking down the three keys to the weekend forecast for the storm.
Posted by Tallahassee Democrat on Wednesday, July 29, 2020
Our latest analysis video and tropical analysis column are linked below. A public version of our June seasonal outlook, detailing our reasoning behind a forecast for a very active season ahead, is now available here.
WeatherTiger Tropical Update for July 29, 2020, issued 4:00PM
As the 2020 hurricane season tallies its record ninth tropical system, Florida finds itself inside cone number one for the year. And while Florida has faced far more formidable cones, this week’s potential tropical threat may be among the more complex and baffling.
The National Hurricane Center’s 5 p.m. Wednesday advisory for Potential Tropical Cyclone 9 finds it with sustained winds of 45 mph, moving west-northwest at a brisk 23 mph to the southeast of Puerto Rico.
PTC 9 remains an elongated tropical wave, with several prospective centers of circulation wrapped in a broad area of rotation extending across much of the northeastern Caribbean. Lacking a single dominant center, it has not yet become Tropical Storm Isaias. This transition may occur at any time in the next 12 to 24 hours. With gale-force winds already observed, tropical storm watches and warnings extend west to the southeastern Bahamas.
Because the internal structure of the system remains poorly defined, there is more than the typical uncertainty regarding its future track and intensity, even over the next 24 to 48 hours. There are three major issues at hand that will determine the extent of eventual impacts on Florida.
The first issue: where will a single center of circulation consolidate? Amazingly, this is still a relevant question after watching this wave for nearly a week. Intense convection has been split into southwest and northeast camps for the past three days.
Today, storm activity is more concentrated around a prospective vortex, but given the loose windfield, center reformations north or south are certainly possible through Friday. The slower-than-anticipated development of a single center has caused intensity forecasts to decline and the expected track to shift west.
The second issue: to what extent will PTC 9 interact with land? Hispaniola and eastern Cuba are surprisingly mountainous, including the 10,000’-plus summit of Pico Duarte in the Dominican Republic. These ranges can easily disrupt or destroy the low- and mid-level circulations of tropical systems and have crippled erstwhile Florida threats like Debby (2000), Ernesto (2006), and Erika (2015) on similar trajectories.
While PTC 9 will not avoid Hispaniola and eastern Cuba entirely, it remains unclear whether the circulation center (such as it is) will traverse the length of the islands or merely scrape past their northern coasts. Should the former occur, reverting to an open tropical wave is possible, and PTC 9 staying a sprawling and disorganized system into the weekend is likely. A weaker system would be more likely to continue moving more westward into the Florida Straits or far eastern Gulf, as shown by the NHC track and UK’s weather model.
If there is a northward adjustment and less land interaction, land-induced weakening may be more temporary. A stronger storm would also be more likely to make an earlier turn north in response to an upper-level low moving across the Ohio Valley this weekend, resulting in a course perhaps threatening Florida’s East Coast or points north. Both the American and European models and their ensembles have swung east towards this scenario in the last 12 hours; on the other hand, both models have been consistently biased too far north and too strong with track and intensity forecasts for PTC 9 this week.
The third issue is how favorable an environment PTC 9 finds itself in on the other side of the Greater Antilles, which is highly dependent on the first two outcomes. Should the system track south and reach the Florida Straits or Gulf by the weekend, it will likely do so against unfavorable southerly wind shear that would keep it weak as it then turns north.
This shear will be present, but lighter, if the storm is instead located near the northern Bahamas in two or three days. Shear interactions are sometimes idiosyncratic, and chances of modest re-intensification in 3-5 days should the storm make it further west are non-zero. However, the odds of PTC 9 making a run at high-end tropical storm or hurricane strength are better should it zag north earlier.
So, what’s going to happen? The fair, honest answer is that I simply don’t know. The significant uncertainties in position, structure, and land interaction through Friday preclude a satisfying answer to this question.
Clearly, there are many obstacles in the path of this system, and my hunch is that PTC 9/Isaias will be mostly a rainmaker. Enhanced precipitation is likely this weekend over much of the Florida peninsula, particularly southeastern sections, and possible in the Panhandle starting Sunday or Monday. Forecast clarity, including for impacts on Florida, should improve greatly once PTC 9 clears Hispaniola early on Friday. I hope.
Elsewhere, there are no organized tropical cyclones. A wave in the far eastern Atlantic has a slight chance of development over the next two or three days, but an unfavorable environment further west means this has no chance of threatening land.
With the landfall of potent Category 1 Hurricane Hanna over the weekend, the U.S. coastline has notched a record four tropical cyclone landfalls through July. Time will tell if Florida’s first cone of 2020 will translate into a fifth landfall. With uncertainty unusually high, there’s little to do but keep watching the skies.
Dr. Ryan Truchelut is co-founder and chief meteorologist at WeatherTiger, a Tallahassee-based start-up providing advanced weather and climate analytics, consulting, and forecasting solutions to enterprises large and small. Get in touch at ryan@weathertiger.com or follow along on Twitter (@wx_tiger) or Facebook. Get this forecast first by registering for our Hurricane Forecasting Subscription package.