Dorian Finally Exits Bahamas and Heads North; Two More Storms Form in the Atlantic

Hurricane Dorian sat north of Grand Bahama for 24 hour yesterday while gradually weakening from upwelling, meaning that the island went through the eyewall of a severe hurricane for over a day. Serious, catastrophic destruction has been reported in both Great Abaco and Grand Bahama, both receiving category 5 landfalls from Dorian. Many structures are reportedly destroyed and hundreds remain missing. The hurricane has finally lifted north and is currently a category 2 hurricane with winds of 110mph. It is still very significant, although it has come off of its record-breaking peak. The storm will continue to bring dangerous weather to the Florida and Georgia coastlines today and tomorrow before scrapping the Carolinas by Thursday and early Friday. In addition to Dorian, two more tropical cyclones have formed in the Atlantic – Tropical Storm Fernand in the western Gulf and Tropical Depression 08L in the eastern Atlantic. Both are expected to peak as moderate tropical storms, with Fernand making landfall in northern Mexico in about 24 hours.

Basin-wide shot of all of the Atlantic tropical activity, including Hurricane Dorian east of Florida, Tropical Storm Fernand near Mexico, and Tropical Depression 08L northwest of Cabo Verde. (College of DuPage NEXLAB)
Aerial pictures of the damage from Great Abaco. (Twitter)

We’ll begin the article discussing Hurricane Dorian, which remains an extremely dangerous and expanding hurricane. Because the storm sat in the vicinity of Grand Bahama for about 36 hours, a combination of land interaction and ocean upwelling caused the storm to weaken from a category 5 to a category 2 over the course of 24 hours. The storm has since lifted north, and has remained relatively steady throughout the course of the day, as it has traveled into a region of warmer waters within the Gulf Stream and has markedly expanded in size. The eyewall remains relatively solid, and the remnants of the previous eyewall remain within the eye. Because of yesterday’s eyewall replacement, the storm has expanded in size and tropical storm force winds now extend as far west as the Florida coastline. The outer feeder bands are traversing up the coast as the core begins to accelerate northward, bringing squally weather with heavy rains, gusty winds, and storm surge flooding up to 5′.

Base Reflectivity of Dorian’s core from the Melbourne, FL, radar site, showing a disheveled, but dangerous core. (RadarScope)

As the hurricane progresses northward through tomorrow, the intensity is expected to remain steady as it moves up the tongue of the Gulf Stream. It is possible that Dorian strengthens back to major hurricane status as it does so, which would not necessarily increase the impacts in Florida and Georgia given that the core is expected to remain offshore. However, that does not mean that dangerous weather conditions are not anticipated. Like has been said for the past several days, hurricanes are not points on a map, but rather are large sprawling weather systems. After all, Hurricane and Storm Surge Warnings are still in effect for much of the central and northern Florida coastline. In eastern Florida, tropical storm force winds are extending as far west as Lake Okeechobee, with winds approaching 60mph closer towards the shore. Embedded isolated hurricane force gusts are possible along the immediate coastline up to the Georgia border. These winds are capable of producing scattered power outages and tree damage. Storm surge flooding generally ranging from 1-3′ is forecast or imminent along portions of the coastline, especially for areas north of Cape Canaveral, with isolated inundations of up to 5′ a possibility. As far as rainfall totals are concerned, 2-5″ are expected to fall along portions of the First Coast near Jacksonville. Expect an additional 1-3″ further south, where some rainfall totals have already exceeded these numbers. Conditions should begin to improve by tonight along the Treasure Coast and throughout the day tomorrow along the Space Coast as Dorian starts to lift northward out of the region. In Georgia, impacts will be generally less severe given that Dorian is expected to track further offshore, but Tropical Storm and Storm Surge Warnings, as well as Hurricane Watches, are still in effect for the coastline. Rainfall totals of 2-4″, storm surges up to 3′, and wind gusts up to 60mph can be expected, with conditions deteriorating tonight, peaking on Wednesday, and leveling off after that.

Storm Surge Inundation Potential across northern Florida, Georgia, and South Carolina, with areas in blue indicating 1’+ of inundation, areas in yellow 3’+ of inundation, and orange areas 6’+ of inundation. (NHC)

By Wednesday Night, Dorian will take a turn towards the northeast, which will cause it to begin weakening and expanding further as the steering trough interacts with the hurricane. As for impacts in South and North Carolina, they are expected to be more widespread even though Dorian itself will likely start to weaken. The core, however, is expected to track closer and accelerate in proximity to those coastlines, so Hurricane and Storm Surge Warnings have been issued for all of South Carolina coastlines and portions of southern North Carolina, with Hurricane and Storm Surge Watches extending up towards Norfolk, Virginia. By Thursday morning, the core of the storm may only be 20mi offshore of South Carolina, meaning that effects will be more widespread. Storm surges up to 7′ or potentially higher will extend from areas near Charleston northward Myrtle Beach. Winds up to hurricane force may extend along this portion of coastline as well. As for North Carolina, the core could make landfall in the Outer Banks, where storm surge values may also exceed 7′ in the Pamlico Sound. Dorian itself should weaken to a category 1 hurricane at this time, but dangerous winds will still come regardless. Isolated tornadoes are not out of the question, although any that form will be weak and short-lived. For Virginia, the impacts will be less severe, as the coastline reverts away from Dorian’s forecast path, but tropical storm and brief hurricane conditions are also possible with storm surges of 1-3′ possible near Norfolk and the southern Chesapeake Bay. Rainfall will be the biggest issue in all 3 states, as the storm’s convective shield will also begin expanding polewards due to the trough interaction, meaning heavy rainfall will extend farther inland. Wide swaths of 6-10″ or more will extend along the South Carolina and North Carolina coasts, possibly extending relatively far inland, especially in North Carolina. This can lead to flash flooding and river flooding that could be potentially life-threatening. The flooding should not be as severe or widespread as in what was seen with Hurricane Florence a year ago, but should not be disregarded nonetheless.

[Image of WPC QPF U.S. rainfall potential]
WPC 1-5 day Quantitative Precipitation Forecast for Hurricane Dorian, showing a large swath of heavy rains exceeding 6″ along the coastal portions of the Carolinas. (NHC)

Dorian is expected to track offshore New England by Friday and Saturday, but will still cause rough seas and generally rainy weather, especially in coastal Massachusetts and Maine. The storm is then expected to impact the Canadian Maritimes this weekend as it undergoes extratropical transition. Contrary to the past few days, it will be moving very quickly to the northeast, and could bring some significant weather with it. Heavy rainfall, high winds, and coastal erosion from storm surge or rough seas is all possible, although the impacts are slightly more uncertain given that it is further out in time. Everyone in Dorian’s expected path needs to have their preparations rushed to completion, especially in the Carolinas. While impacts will not be as severe as was seen in the northern Bahamas, it will still be a dangerous hurricane as it approaches the coast. Remember that the storm does not need to make landfall in order to cause significant problems.

NHC 5 day forecast track for Hurricane Dorian.

In addition to Dorian, a new tropical cyclone formed this morning in the western Gulf, previously Invest 93L. It has since strengthened to become Tropical Storm Fernand, packing winds of 40mph and a pressure near 1004mb. Fernand’s convection is currently lopsided towards the west due to some easterly shear from an upper-level ridge located south of Louisiana, the same one that was partly responsible for Dorian’s stalling. The ridge is also helping steer the storm west along with the typical surface trade flow, and as the ridge retrogrades west into southern Texas, Fernand should track west into northern Mexico in the state of Tamaulipas by tomorrow afternoon. Given the warm waters and high moisture content of the region, Fernand should gradually intensify up to landfall, with a current expected peak of 50mph, and will dissipate over the mountains of Mexico by Thursday. Tropical Storm Warnings are in effect for this whole region of coastline. As it makes landfall, it will mainly bring heavy rains as the primary threat from Tampico to the Rio Grande Valley. Gusty winds capable of knocking out power is also a possibility for these regions. As for South Texas, expect some more rainfall near Brownsville and points north and west, but do not expect the storm itself to make landfall there.

Visible imagery of Fernand in the western Gulf. (MSFC)
cone graphic
NHC 5 day forecast track for Tropical Storm Fernand

One more tropical cyclone formed today over the far western Atlantic in the form of Tropical Depression 08L. This storm formed from a tropical wave, dubbed Invest 91L, and tracked near Cabo Verde while Dorian was ravaging the Bahamas. 08L is still rather broad, with scattered, yet uncentralized convection, but does have the qualifications of a tropical cyclone. It is expected to become a tropical storm as it travels northwest into a break in the subtropical ridge over the central Atlantic Ocean. If it does gain a name, it would be “Gabrielle”. The main issues for this system over the next several days will be dry Saharan air and low sea surface temperatures. This is why the NHC only forecasts 08L to be a moderate tropical storm at peak. It may try to get stronger as it approaches the subtropics, but it should not be a threat to land areas regardless.

Water Vapor Imagery of 08L northwest of the Cabo Verde Islands, with a large swath of dry Saharan air to the west of the outflow shield.
NHC 5-day forecast cone for Tropical Depression 08L

Two more systems, neither of which are tropical cyclones, are active, including Invest 92L, which is currently near Bermuda. It is bringing locally inclement weather to the island. Chances of development are moderate, but it should curve out to sea as a trough, the same one curving Dorian northeast, tracks east and takes both systems with it. The other system is a tropical wave that should emerge off the African Coast by tomorrow that has a higher chance of development (near 70%). Models are in decent agreement that it will track west through the tropical Atlantic as some sort of tropical tropical entity, although there is high uncertainty with the eventual outcome given that it is a long-range system to watch.

NHC 5-day Graphical Tropical Weather Outlook for the Atlantic Basin

The tropics remain very busy right now, as was expected. The climatological peak of the season is a week away, and indications are that activity should remain high through October. This is a good time, if you are not being impacted by Dorian or other storms, to get your hurricane plan ready just in case something else comes along and threatens your location. Please adhere to your local NWS office and local officials for the latest and most important information to you. Another article will be released tomorrow or Thursday.

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