| Above: MODIS visible satellite image of Tropical Storm Florence on Saturday morning, September 1, 2018, in the waters west of the Cabo Verde Islands. Image credit: NASA Worldview. |
Tropical Storm Florence formed on Saturday morning in the waters of the eastern Atlantic, just west of the Cabo Verde Islands. Florence’s formation date of September 1 comes a week before the average September 8 date for development of the season’s sixth named storm. This year’s tally of 6 named storms, 2 hurricanes, and 0 intense hurricanes in the Atlantic is pretty close to average for this point of the season, which reaches its halfway point around September 10 – 11. However, according to Colorado State University, the Atlantic season has produced only about 55% of the accumulated cyclone energy (ACE) one would expect at this point in the year. This is because the year's named storms have not been especially strong or prolonged.
Sea surface temperatures (SSTs) underneath Florence are a marginal 26.5°C (80°F), but the dry air and dust of the Saharan Air Layer (SAL) are confined to the north of the storm, giving Florence a moist atmosphere with a mid-level relative humidity near 70%. Wind shear is light, less than 5 knots, favoring development. Satellite images on Saturday afternoon showed that Florence now had a well-defined surface circulation, with an increasing amount of heavy thunderstorms that were becoming well-organized.
Florence’s brush with the Cabo Verde Islands may well be the only encounter the storm has with land. Steering currents favor a west-northwesterly to northwesterly track over the next week, which will take Florence out into the central subtropical Atlantic. On this track, the storm will be very unlikely to affect the Caribbean islands. However, if Florence fails to intensify much, the storm will tend to be steered by the atmospheric flow at lower levels of the atmosphere, which favors a more westerly track. In this scenario, endorsed by the 0Z Saturday run of the European model, Florence would avoid recurvature to the north and northeast, and the storm might progress far enough to the west to represent a legitimate threat to the U.S. East Coast or Canadian maritime provinces 1 – 2 weeks from now.
How much Florence will intensify is difficult to predict. SSTs will cool through Monday and the surrounding atmosphere will grow drier, discouraging development. However, wind shear will be very low, encouraging development. Conversely, beginning on Tuesday SSTs will warm up, but wind shear will rise. The net result of these conflicting influences is that Florence will likely intensify some but remain below hurricane strength over the next five days.
 |
| Figure 1. A tropical wave centered near the Turks and Caicos Islands, as seen at 10:45 am EDT September 1, 2018. Image credit: NOAA/RAMMB. |
Tropical wave near the Turks and Caicos Islands may develop in the Gulf of Mexico as early as Monday
A tropical wave located on Saturday near the Turks and Caicos Islands, Haiti, and eastern Cuba was headed west-northwest at 10 – 15 mph. This system has the potential to develop into a tropical depression when it enters the Gulf of Mexico on Monday. The Hurricane Hunters are on call to fly into the system on Monday, if necessary. Satellite images on Saturday showed that the wave was generating a moderate amount of heavy thunderstorms, but had little spin. There was plenty of moisture associated with the system, and portions of the Virgin Islands and southern Puerto Rico received 4 - 5 inches of rain Thursday through Friday. Two-day precipitation amounts as high as 5.35” were recorded at La Romana in the Dominican Republic; the capital, Santo Domingo, received 2.60”. High wind shear of 15 - 25 knots was keeping the system’s heavy thunderstorm activity disorganized, though.
The wave is expected to continue to battle high shear through Sunday as it moves over eastern Cuba and The Bahamas, and development is unlikely during this period. On Monday, the wave is expected to cross Florida and enter the eastern Gulf of Mexico, where a modest drop in wind shear may allow organization into a tropical depression to occur. The 0Z Saturday European model forecast predicted a tropical depression would form on Monday and make landfall in Louisiana on Tuesday night, as did about 40% of the model’s ensemble members. The 0Z, 6Z , and 12Z Saturday GFS model runs did not show development, though about 20% of 0Z GFS ensemble model members did. The 0Z and 12Z UKMET model runs predicted development would occur on Wednesday or Thursday, off the coast of Texas.
 |
| Figure 2. Predicted seven-day rainfall amounts for the period ending at 8 am EDT Saturday, September 8, 2018. A tropical wave that will enter the Gulf of Mexico is expected to bring widespread rainfall amounts in excess of 4" to the U.S. Gulf Coast. Image credit: NOAA/NWS. |
If the system does develop, it doesn’t have much time to intensify, and none of the ensemble forecasts had the storm reaching hurricane strength. Even if development does not occur, though, heavy rains in excess of four inches can be expected along much of the U.S. Gulf Coast beginning on Monday. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 10% and 40%, respectively. These 5-day development odds are double the 20% odds given by NHC on Friday.
 |
| Figure 3. A day-night visible image of Super Typhoon Jebi from the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi NPP satellite, collected at 1535Z (11:35am EDT) Saturday, September 1. Image credit: RAMMB/CIRA/CSU. |
Japan still in the crosshairs of Super Typhoon Jebi
An infusion of dry air and an eyewall replacement cycle have weakened Typhoon Jebi, but the storm remained a fearsome Category 4-strength super typhoon with top sustained winds of 155 mph as of 11 am EDT Saturday, September 1. The track forecast for Jebi by the Joint Typhoon Warning Center brings Jebi across central Honshu as a Category 2-strength typhoon on Tuesday, and the GFS and HWRF models suggest Cat 3 strength at landfall is still possible. A variety of major impacts, from hurricane-strength winds to torrential rains, are possible with Jebi.
Super Typhoon #Jebi via #SuomiNPP IR & #NOAA20 moonlit DNB (note mesovortices) on 8/31, 'super' impressive via any satellite or wavelength. pic.twitter.com/eASRSxiana
— UW-Madison CIMSS (@UWCIMSS) September 1, 2018
Bob Henson contributed the Jebi section of this post.
