91L Likely to Become a Gulf of Mexico Tropical Cyclone by Tuesday

September 2, 2018, 4:35 PM EDT

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Above: Tropical wave 91L over The Bahamas, as seen at 12:15pm EDT September 2, 2018. Image credit: NOAA/RAMMB.

A tropical wave located on Sunday afternoon in the central Bahamas was headed west-northwest at about 15 mph, and has the potential to develop into a tropical depression when it enters the Gulf of Mexico on Monday. The National Hurricane Center (NHC) designated this wave as 91L on Sunday morning. The Hurricane Hunters are on call to fly into the system on Monday afternoon, if necessary; in its Tropical Weather Outlook issued at 2 PM EDT, NHC said that a tropical storm watch could be issued for parts of the northern Gulf Coast as soon as Sunday night. The next name on the Atlantic list of storms is Gordon.

 Satellite images on Sunday showed that 91L was generating a moderate amount of heavy thunderstorms, and had developed a modest amount of spin. There was plenty of moisture associated with the system, with Cuban radar showing heavy rains affecting much of the north-central coast of Cuba and adjacent waters on Sunday afternoon. Jamal in eastern Cuba picked up 2.49” of rain in the 24 hours ending at 8 am EDT Sunday. The increased organization of 91L was likely due to a drop in wind shear, which fell from a high 15 – 25 knots on Saturday to a moderate 10 - 20 knots on Sunday. 91L was entangled with an upper-level low pressure system, which was helping enhance some of the heavy thunderstorm activity. Water temperatures under 91L were a very warm 29.5°C (85°F).

Sunday night through Monday morning, the wave will cross South Florida, bringing gusty winds, heavy rains of 1 – 3”, and the possibility of isolated waterspouts and weak tornadoes. When 91L is in the eastern Gulf of Mexico on Monday and Tuesday, the SHIPS model predicts that wind shear will fall to a light 5 – 10 knots, and SSTs will warm another degree, to 86°F. These conditions are favorable for development, and 91L is likely to organize into a tropical depression by Tuesday afternoon, if not sooner. Our top three models for predicting tropical cyclone genesis, the European, GFS, and UKMET models, all predicted with their 0Z Sunday runs that 91L would develop by Tuesday. The 12Z Sunday runs of the GFS and UKMET models also predicted development. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 50% and 80%, respectively. These 5-day development odds are double the 40% odds given by NHC on Saturday.

Precipitation forecast

Figure 1. Predicted seven-day rainfall amounts for the period ending at 8 am EDT Sunday, September 9, 2018. A tropical wave that will enter the Gulf of Mexico is expected to bring widespread rainfall amounts in excess of 5" to the U.S. Gulf Coast. Image credit: NOAA/NWS.

A heavy rain threat for the Gulf Coast

The 91L system is expected to continue on a west-northwesterly track through Wednesday, which would bring it ashore along the coast between Alabama and Louisiana on Tuesday night or Wednesday morning. This does not give the system much time for intensification, and it is unlikely that 91L will become Hurricane Gordon. None of the ensemble members of the 0Z Sunday runs of the European and GFS models predicted that 91L would attain sustained winds in excess of 45 mph before landfall, and wind is not the main threat of this system—heavy rain is. The latest rainfall guidance from the National Weather Service predicts that 91L will bring widespread rainfall amounts of 5" to the U.S. Gulf Coast. The coast along the Texas/Louisiana border is already experiencing heavy rains due to a separate weather system; some areas had already gotten 3" of rain by Sunday afternoon, according to the NWS in Lake Charles.  From the NWS in Houston: "Our local flash flood decision tree is indicating a high risk of flash flooding with maximum rainfall totals by Monday evening between 8-10 inches. What is worrisome is that the TT WRF is showing a boundary and training cells over Galveston/Brazoria and southern Harris counties Monday morning (09-18z). A Flash Flood Watch will be required tonight and Monday for a large part of SE TX."

Figure 2. MODIS visible satellite image of Tropical Storm Florence on Sunday morning, September 2, 2018. Wind shear and dry air had combined to remove most of the storm’s heavy thunderstorms, exposing the center to view. Image credit: NASA Worldview.

Tropical Storm Florence headed into the open Central Atlantic

Tropical Storm Florence formed on Saturday morning in the waters of the eastern Atlantic, just west of the Cabo Verde Islands, and was headed west-northwest at 18 mph into the Central Atlantic. Conditions were marginal for development on Sunday afternoon, with sea surface temperatures (SSTs) a cool 26°C (79°F) and the storm entering a region where the dry air and dust of the Saharan Air Layer (SAL) were increasing. Moderate wind shear on Sunday had injected dry air into the core of the storm and stripped away most of Florence’s heavy thunderstorms, as seen on satellite images.

Florence’s brush with the Cabo Verde Islands on Friday 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 Atlantic, far from land. 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 Sunday run of the European model, Florence would avoid recurvature to the north and northeast late this week, and might progress far enough to the west to represent a legitimate threat to the U.S. East Coast or Canadian maritime provinces 10 – 14 days from now. The steering currents that far in advance are impossible to accurately predict, and will likely depend upon the very uncertain interaction of Typhoon Jebi in the Western Pacific with the large-scale atmospheric circulation later this week.

More African waves to watch this week

The African tropical wave factory will kick out at least two more systems worth monitoring this week, with the European model showing the potential for development of new tropical waves emerging from the coast on Monday and on Thursday. It’s that time of year!

Figure 3. Infrared satellite image of Typhoon Jebi as of 1430Z (10:30 am EDT) Sunday, September 2, 2018. Image credit: CIMSS/SSEC/University of Wisconsin–Madison.

Jebi arcing toward Japan

Still a powerful Category 3-strength storm with 125-mph sustained winds as of 11 am EDT Sunday, Typhoon Jebi is on track for a Tuesday landfall on Japan’s largest and most populous island, Honshu. Jebi’s winds peaked at 175 mph on Friday, making the storm Earth’s strongest tropical cyclone of 2018 thus far. An infusion of dry air and an eyewall replacement cycle have since dented Jebi’s strength. The typhoon appeared to be reintensifying somewhat early Monday local time, though, with strong thunderstorms wrapping back around its center. Conditions should remain mostly favorable for Jebi to at least hold its own if not gain a bit of strength through Monday, with low to moderate wind shear (10 knots or less) and very warm SSTs (28-29°C or 82-84°F).

The Joint Typhoon Warning Center is predicting a straightforward recurvature track for Jebi, bringing its center inland not far from Osaka on Tuesday afternoon local time at Category 2 strength. The HWRF model, one of our top intensity models, has consistently indicated that Jebi could be close to Cat 3 strength at landfall. A variety of major impacts, from hurricane-strength winds to torrential rains, can be expected. Widespread amounts of 4” – 8”, with local totals topping 10”, can be expected from the Osaka-Kyoto area to the Tokyo region. Model output suggests the heaviest rains will likely remain east of the hard-hit Chūgoku region, where some 220 people were killed by devastating floods in July. The Japan Times reported Sunday that around 27,000 people were forced to evacuate Nagato, in Yamaguchi Prefecture, due to heavy rain on Saturday.

Jon Erdman (weather.com) notes that Japan has already been hit or brushed by at least 12 different tropical cyclones since mid-June, six of which were typhoons. Japan’s largest number of tropical cyclone landfalls in a single year is 10, set in 2004.

Bob Henson wrote the Jebi portion of this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995 while working on his Ph.D. in air pollution meteorology at the University of Michigan. He worked for the NOAA Hurricane Hunters from 1986-1990 as a flight meteorologist.


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