Florence: Clues from the Past
NHC 5 p.m. Friday advisory
Tropical Storm Florence is tracking west toward the United States, but its westward movement alone is not the most interesting aspect of this storm. In fact, most tropical systems move from east to west across the tropical Atlantic, particularly in September, near the peak of the Atlantic hurricane season. Many storms do eventually make landfall in the United States, but a much greater number of storms curve to the north and east, safely away from the U.S. mainland. Two things that make this storm particularly intriguing are, first, that the upper air pattern may continue to allow for Florence to move westward, potentially making it close to the East Coast. Secondly, storms at the latitude of Florence, about 25° N, rarely approach the East Coast, especially when they’re this far north, but also east of Bermuda, or roughly 65° W.
Climatology is helpful when it comes to the tropics. Systems in specific areas tend to track a certain way, given similar weather patterns from the past. During the summer, a blocking ridge of high pressure near Bermuda, commonly referred to as a “Bermuda high,” is generally in place over the open Atlantic. Generally speaking, hurricanes that track near the Bahamas tend to turn northward toward the East Coast around the western fringes of this high. If a tropical system is sufficiently far west before the turn happens, the result is a landfall before such a system has time to make a hard turn toward the east.
Data from https://coast.noaa.gov/hurricanes/
As far as climatology is concerned, very few systems near the location of Florence have made a U.S. landfall. In fact, 91 tropical systems have been located within 150 nautical miles of the Friday afternoon position of Florence, near 24.8° N, 52.5° W, yet only one of these systems has made landfall as a tropical entity on the East Coast. Part of the reason is the anomalous current position of a >590dm area of high pressure, located from the mid-Atlantic coast to a couple of hundred miles north of Bermuda. For at least the next few days, there is strong support from computer models that Florence will continue to track west to west-northwest underneath this high. After all, the high is literally blocking any sudden northward turn from Florence.
Data from https://coast.noaa.gov/hurricanes/
Assuming a general west-northwest track continues into early next week, which is supported by most computer models and the official National Hurricane Center forecast, Florence should be located between 25° and 30° N, as it reaches the longitude of Bermuda. To get a better climatological look at such a scenario, let’s consider all hurricanes (Florence is forecast to become a major hurricane by this time) that have passed within 150 nautical miles of 27.5°N, 65° W in August and September since 1948. Using 1948 as a cutoff allows for a look at reasonable analyses of upper air patterns and ignores systems before the satellite era, that may have not been very well-documented. There are a total of 26 matches and of these hurricanes, three eventually made a U.S. landfall (Diane 1955, Dora 1964 and Ginger 1971) and Emily in 1993 came close to making a landfall. Hurricane Kyle in 2002 is an odd case of a tropical system impacting the East Coast later in the season (October), but it was a relatively weak system that did not reach hurricane status west of 65° W.
Mean 500mb pattern for hurricanes Diane, Dora, Ginger and Emily
The mean 500mb pattern at those times shows a >590dm high between the mid-Atlantic coast and Bermuda, which matches up reasonably well to the forecast pattern for early next week. The important takeaway is that a tropical system underneath such a high is going to continue moving on a west-northwest trajectory until either the high shifts east, and/or a significant trough of low pressure swinging across the continental U.S. has the ability to pull the system north, or otherwise force the ridge eastward.
As Hurricane Diane passed south of Bermuda in 1955 as a major, category 3 hurricane, high pressure was already retreating to the east and a cutoff low was deepening across the eastern U.S. Diane was pulled northeast as the lows phased and resulted in weakening of the system, as it made landfall as a category 1 hurricane in North Carolina.
The upper air charts featured more of a zonal pattern of the U.S. in 1964 with Hurricane Dora, as that storm continued west-northwest, making landfall as a major hurricane in North Florida. With the hurricane passing over the Gulf Stream with no significant wind shear in the upper levels of the atmosphere, it was able to more or less maintain intensity as it approached the coast.
The case of Hurricane Ginger in 1971 is a bit different than Diane and Dora, as well as the current upper air pattern. Well before its eventual landfall, Ginger meandered around the open Atlantic, east of the Bahamas, as it was trapped under high pressure to the north. High pressure, originally situated near the Carolinas, eventually weakened, allowing for Ginger to turn back northwest. This turn was probably, in part, influenced by an approaching trough over the Great Lakes, which also caused the hurricane to get squashed, weakening rapidly after making landfall in North Carolina as a minimal category 1 hurricane. During this interaction, the archived track shows Ginger turned east to east-southeast as a dissipating tropical depression over the days following landfall.
Wikipedia Commons
In 1993, high pressure was situated near Bermuda as Hurricane Emily approached the East Coast. Without strong steering currents, the hurricane slowly moved northwest toward the Carolinas. Emily intensified to become a major, category 3 hurricane as it brushed the Outer Banks of North Carolina. At this time, high pressure gradually retreated eastward, allowing for Emily to eventually turn to the northeast and back out to sea.
Of these four cases, there are certainly similarities, as well as differences, to Florence. Unlike with Diane, no notable low pressure system or trough is expected to form across the East Coast, which suggests there will be less wind shear in play this time around. Assuming that’s correct, there is evidence to believe that Florence could maintain major hurricane status as it moves west of 65° W, assuming it re-intensifies to category 3 intensity. In the case of Dora, high pressure was situated several hundred miles farther south than what forecasts show for next week, suggesting that Florence will not be forced to continue westward into Florida. While Ginger was forced to turn southwest of Bermuda, Florence is expected to generally continue a track west-northwest under high pressure. While the general pattern with Florence does look similar to 1993’s case with Emily, model data suggests anomalously strong high pressure this go-around will probably prevent a sharp curve out to sea with Florence.
Of course, comparisons to these four cases are based on assumptions from medium-range model data, but it is useful to consider what took place in those setups and how it lines up with Florence’s potential track/intensity.
Mean 500mb pattern for hurricanes near 27.5 N, 65 W that curved out to sea
There is wide variability with the other 22 cases that featured a hurricane curving out to sea after passing near 27.5° N, 65° W, but there was one common theme. Most of those cases involved a significant trough over eastern North America, which pulled the hurricane away from the East Coast. Just a few of those systems in recent memory include Hurricane Katia in 2011 (initially hyped by the media as it fell on the heels of Hurricane Irene), Hurricane Bill in 2009 and interestingly enough, Hurricane Florence in 2006.
Let’s consider other climatology with East Coast landfalling hurricanes. The most typical track for a landfalling hurricane north of Florida on the East Coast starts with a hurricane passing just to the north of Puerto Rico, before turning northwest and (usually) north. Noteworthy examples of such hurricanes that turned north include hurricanes Gloria (1985), Floyd (1999) and Irene (2011). Those storms took the smooth curve northwest, to north and eventually northeast. Mean 500mb pattern for eight hurricanes that made an East Coast landfall, but tracked well south of the forecast track for Florence:
There were a few hurricanes that were still tracking west of due north at landfall and those include Hugo (major hurricane in South Carolina in 1989), Fran (1996 in North Carolina) and Isabel (2003 in North Carolina). There have been some mentions of the apparent similarities between Isabel and 2018’s Florence. While both setups featured an anomalously strong area of high pressure north of Bermuda in September, high pressure in Isabel’s case appeared to be centered a bit farther northeast than what’s expected in the coming days ahead of Florence. Note that high pressure is forecast to be a bit more east-west elongated next week, based on the European ensemble mean forecast below, which would support a more westward track with Florence, at least in comparison to Isabel.
It is still several days out and things could change, but the European ensembles and operational runs over the past few days have been fairly consistent with a track that takes Florence generally west-northwest toward the Carolinas in the coming days. Earlier data, most notably from the GFS ensembles, that curved Florence north and northeast out to sea seems to be coming into better line with most of a westward track. Given the anomalous strength and position of high pressure, based on model data, trends and review of climatology suggest that Florence is more likely to continue west into early next week, than to turn out to sea, away from the East Coast. Assuming that Florence does intensify this weekend, as most forecasts suggests, there is also reason to believe that Florence will at least maintain hurricane (if not major hurricane) intensity as it approaches the East Coast.
Keep in mind that Florence is more than five days away from land and forecasts will continue to evolve.
