What was once a tranquil expanse of sun-splashed seas only two weeks ago has rapidly become overwhelmed by the shattering roars of wind-swept waves across the entire Atlantic basin. Three distinct tropical systems are now dominating the Atlantic with the potential for two more systems to emerge later this week.
It’s hurricane season, and this is your Sunday Storm.
Hurricane Florence leads the pack in the western Atlantic, followed by Tropical Storms Isaac and Hurricane Helene. Each storm is set to make their presence known across all corners of the western Atlantic. Isaac is steadfast on a track toward the Caribbean, possibly making landfall as a Category 1 Hurricane somewhere in the Lesser Antilles Wednesday before continuing towards Central America or the Yucatan Peninsula of Mexico. Isaac is trailed by Helene, who will shake up seas for the fish in the open waters of the North Atlantic. However, by far the most ferocious of the trio is Florence, an exceptionally dangerous storm which quite literally has its eye set on the Eastern US.
Florence has recently undergone a dramatic period of intensification. As of 11am September 9, Florence was a Category 1 Hurricane located roughly 750 miles southeast of Bermuda with maximum sustained wind speeds of 75 mph and minimum central pressure of 985 mb. Below is recent satellite imagery from Sunday afternoon. The high cloud tops are denoted by the bright red colors near Florence’s center of circulation, signaling convection that is continuing to intensify.
Late last week Florence encountered harsh environmental conditions that weakened her from a 125 mph Category 3 Hurricane to a 65 mph Tropical Storm. She was still battling the swath of wind shear and dry air as early as Saturday morning. However, upon emerging a victor from the wind shear, Florence has gradually re-intensified. A period of rapid intensification is expected over the next 24-48 hours, which will likely result in Florence strengthening into an extremely dangerous Category 4 hurricane prior to landfall.
Conditions are certainly ripe for tropical cyclone intensification. Florence is currently basking in a deep pool of very warm water as indicated by the most recent run of the Reynolds Sea Surface Temperature Analysis below. Sea Surface Temperatures (SSTs) between 28°C and 30°C (82.4°F and 86°F) now occupy the entire ocean ahead of Florence. The minimum SST necessary for cyclone intensification is 26°C (79°F), so there is an abundance of potential energy available for the hurricane to absorb in the form of latent heat. The warmer the water, the more that will evaporate and later condense. The latent heat released during condensation fuels the warm core of tropical cyclones.
Florence is still several days away from potential US impacts, so the ultimate track, intensity, and impacts from the storm are still highly uncertain. An overview of the various solutions is presented in the spaghetti plots below. These plots depict the various possibilities for the future track of Florence from the 12 UTC Sunday September 9 GEFS and EPS . These are only two ensemble suites, but dozens of other ensembles and deterministic global and hurricane models illustrate a similar degree of uncertainty. Combined, these two ensemble suites represent 71 possible tracks, from crossing northern Florida to recurving out to sea, leaving the Eastern US mostly unscathed. Notably, the various forecast tracks over the next two days (48 hours) are in strong agreement in both direction and storm intensity, with the tracks quickly spreading apart thereafter. In order to produce a reasonable forecast, it is imperative that this forecast uncertainty be diagnosed and thoroughly analyzed for clues about which scenarios are the most likely.
Data is generally the primary source of uncertainty in all numerical forecasts. Oceans are the most data-sparse regions of the Earth’s surface, so tropical cyclone prediction is even more susceptible to forecast changes than other types of weather originating over land. This is why the National Hurricane Center sends aircraft through the eye of the storm to collect reconnaissance data. The observations will be ingested into the numerical models, which will ideally reduce uncertainty. The first reconnaissance missions were initiated Saturday, so forecast uncertainty will gradually improve over the coming days as recurring missions incorporate this data into successive models runs.
While the impacts of Florence as the storm approaches the US are still uncertain, specific details related to the forecast are reasonably irrefutable. There is strong model agreement that the westward progression of the storm through the steam bath of the Western Atlantic will continue through at least mid-week. Since Florence was only just recently a Tropical Storm, she was not strong enough at the upper levels to recurve northward against the dominant ridge in time to miss a swipe at North America. Additionally, model guidance unanimously depicts that vertical wind shear and dry air intrusion will remain minimal over the next several days. Hence, intensification of Florence into a major hurricane is unquestionable, and it is expected that the storm reaches this strength by Monday.
With the short-term track and the near-guarantee of ideal environmental conditions for Florence to intensify, the only source of uncertainty remaining is the position of the steering ridge to the north. If the ridge is located further east, then its clockwise circulation will help pull Florence to the north, and eventually, out to sea. If the ridge is further west, Florence will continue to be directed to the west toward the mainland US. Thus, in order to solidify the track and intensity of Florence, the forecast of interest is actually that of the ridge. Ridges are upper-level regions of high pressure. Beneath them the weather is generally tranquil. How hard could it be to forecast a sunny day? If only it were that simple.
More important than the weather underneath the ridge is actually the weather upstream, to the west of the ridge. The weather to the west of the ridge will influence its position. The ridge’s position will influence Florence’s eventual track. Florence’s eventual track will influence its intensity, which will feedback into its track. How frustrating; an entire chain of events must be understood to nail just one forecast detail. This is why forecasting the track of hurricanes is so arduous. A complete picture of the atmosphere is necessary to be confident about one detail within one relatively small area.
With the uncertainty hinged on the position of the ridge, it would be useful to determine the upstream features that influence the strength and location of the ridge. It is not merely enough to acknowledge the differences in the ridge positioning and cyclone tracks depicted by model guidance. It would instead be more productive to discern why numerical guidance depicts such differences in the first place. Such a scrupulous analysis of features forecast by numerical guidance can yield clues about the likelihood of various model solutions.
Before delving into detail within the models, it is important to note that the ridge will ultimately encompass the entire eastern two-thirds of the US by Thursday. Two upper level highs will actually be present. The one of importance is over the western Atlantic Ocean. It’s the clockwise circulation about this high that will dictate the track of Florence. Below are Sunday morning’s 00 UTC 500 mb fields for 5pm EDT Thursday courtesy of weathermodels.us as predicted by the ECMWF and the GFS, respectively. There are some subtle differences highlighted that contribute to the stark differences between the two tracks.
It appears that the differences in timing and intensity of two upper-level troughs have direct influences on the position of the ridge over the western Atlantic Ocean. The first feature is a shortwave trough traversing western Canada. The GFS depicts this feature to be more amplified and slightly faster than the ECMWF. If this were the case, the larger, broader piece of the trough to the south would intensify due to more intense cold advection from the north. The result would be a more significant force pushing against the unseasonably strong wall of a ridge over North America, shifting it eastward. With the ridge shifting eastward, the clockwise circulation can pick up Florence and direct it toward the north, resulting in a chance for the storm to remain just offshore or making landfall in northern North Carolina or Virginia. These same upper-level features are weaker and slower on the ECMWF. Thus, the ridge remains centered further west, keeping Florence further south and west, resulting in a landfall somewhere in the Carolinas.
The two upper-level troughs have yet to develop. Guidance suggests that these features will not form until Monday. Unfortunately, this would imply that the positioning of the ridge and therefore the track of Florence cannot be solidified until at least Tuesday, when several runs of models have ingested data from the troughs.
The intensity differences in Florence and the ridge are important to recognize too. Diagnosing these differences could help shed light on which forecast camp to favor. As presented below, the 12 UTC September 9 ECMWF depicts a weaker Florence making landfall as a Category 3 Hurricane near the North Carolina/South Carolina border with a minimum central pressure of 955 mb. Meanwhile, the GFS depicts a monster Category 4 Hurricane with 932 mb minimum central pressure just missing the Outerbanks. These are two drastically different storms. In both cases Florences stalls as it becomes trapped south of the boomerang-shaped ridge. If this were to occur, several days of torrential rain, gusty winds, and coastal flooding would ensue, ravishing the Carolina and lower Mid-Atlantic coasts. But with the vast dichotomy between the two intensities, the GFS solution would be far more catastrophic than the ECMWF.
That the GFS has consistently been forecasting a stronger storm than the ECMWF would imply that the ridge over the western Atlantic would actually be stronger than what the ECMWF predicts. A stronger storm would release more latent heat, which would contribute to the intensification of downstream ridging. Remarkably, the reverse is true in the two model forecasts presented above. More head scratching is the fact that the GFS historically over-amplifies the effect of latent heating on downstream ridges due to its convective parameterization, resulting in further easterly tracks. At least one of these solutions for the ridge, and ultimately Florence, must be incorrect.
So with the confusion about the ridge intensity, which forecast should be favored? Clearly more than two model suites ought to be recruited for help. Fortunately, there are dozens of global and hurricane models to consult. Rather than scrupulously investigating each model solution, it would be efficient to assume that the ridge placement and intensity is the most important factor in storm track and intensity. Since stronger storms tend to recurve northward with ease compared to weaker storms, it would also be a fair assumption that the tracks furthest to the right in the guidance envelope will generally be the strongest. The plot below from Tropical Tidbits portrays the deterministic global and hurricane model tracks. Most tracks are to the north (to the right) of the ECMWF’s track, which suggests that the ECWMF has a weaker solution than most guidance. Hence, support for a track closer to the GFS solution is favored.
Forecasts over the weekend have trended farther north compared to last week. Based on the above discussion and the assumption that these trends will continue and eventually oscillate to the north and south over the coming days, the forecast looks quite dire but still uncertain. A Florence landfall or near-landfall somewhere between North Carolina and Virginia as a Category 3 or Category 4 Major Hurricane late Thursday is most probable at this time. Consequentially, confidence has grown that there is a high risk of several life-threatening impacts from Florence.
The most significant risk is storm surge flooding, which could result in catastrophic inundation of coastal areas between South Carolina and Virginia. With Florence potentially stalling, storm surge could build over the course of several days as winds force increasing volumes of water to plow onto the shore. Charleston, Myrtle Beach, Morehead City, Virginia Beach and Norfolk are all at risk from such a deadly storm surge.
Further inland, the prolonged rainfall will produce extreme accumulations of 12-18″ or more. This could result in widespread freshwater flooding. Low lying landscapes of the Carolinas are generously sprinkled with freshwater swamps, rivers, lakes and estuaries. After a historically wet summer for many cities in these low-lying areas, this quantity of rain would result in historic flooding as lakes, rivers, and swamps become vast inland seas.
Howling hurricane force winds will undoubtedly wreak havoc if Florence were to take a path as feared. Power will be wiped from millions of businesses and households. Downed trees, power-lines, and flying debris will make travel impossible for several days. Structural collapse should be expected near the coast where Florence makes landfall. Though not all buildings and structures will be destroyed in the hurricane’s path, enough wind damage will occur such that most communities in the vicinity of the eye will be unrecognizable by the time Florence finally departs. This is not hype. This is the reality of a Category 3 or 4 Hurricane coming into contact with major US cities on the eastern seaboard.
We do want to caution that the aforementioned impacts are still too far in the future to determine their exact timing, location and magnitude. However, the population from South Carolina to the Mid-Atlantic coast need to keep a close eye on Florence’s progress. The Governors of North and South Carolina have already declared states of emergencies in anticipation of potential impacts.
Please be sure to check back with WeatherOptics throughout the week as Hurricane Florence evolves. Also, keep an eye out for tomorrow’s 5 Things to Know where we’ll release some new updates on Florence’s track and the potential impacts.