Still Too Soon to Write Off Struggling 99L

By Bob Henson
Published: 15:42 GMT le 26 août 2016

After vexing and perplexing forecasters and the public for days, the strong tropical wave dubbed Invest 99L is attempting once more to organize itself. The elongated circulation associated with 99L extends from the far southeastern Bahama Islands across eastern Cuba to just west of Jamaica. As of early Friday, 99L had failed to develop a coherent circulation, with a low-level center devoid of showers and thunderstorms (convection) spinning hundreds of miles north of intense convection over parts of Hispaniola and Jamaica. The NOAA Hurricane Hunter mission that had been scheduled for Friday morning was cancelled.

The main reason for 99L’s failure to develop has been unexpectedly strong vertical wind shear, as evident in the high-level cirrus blowing northward off the tops of thunderstorms. This wind shear has injected relatively dry air into the wave while keeping a consolidated center from forming. However, 99L is now moving into a region of much lower wind shear for the next couple of days, as indicated by Friday morning (12Z) data from the SHIPS statistical model. A new burst of convection appeared on Friday morning near the low-level circulation between far eastern Cuba and the southeast Bahama Islands (see Figure 1). If this were to persist, it could make the beginning of a long-awaited growth phase for 99L. The National Hurricane Center reduced the 2-day odds that 99L would become a tropical depression to 20% in its 8 AM Tropical Weather Update. I wouldn’t be surprised to see those odds going back up at 2 pm if the convection continues to blossom near 99L’s center. In order to develop further, 99L will need to fend off dry air that continues to influence its thunderstorm activity (see Figure 2).


Figure 1. Infrared image of Invest 99L as of 1415Z (10:15 am EDT) Friday, August 26, 2016. Image credit: NOAA/NESDIS.


Figure 2. Visible satellite image of 99L taken at 11:08 am EDT August 26, 2016. A surface circulation center was trying to form over the central Bahamas. Heavy thunderstorms were building along the southeast side of the center, but were ingesting dry mid-level air. This dry air is heavier than moist air, and creates strong downdrafts within its thunderstorms that are robbing 99L of moisture. When these downdraft hit the ocean, they spread out horizontally, creating an arc-shaped band of surface cumulus clouds showing the outflow boundary of the air from the downdraft. As 99L continues to struggle with dry air, expect to see more of these arc-shaped outflow boundaries.

A victory for the GFS--but it’s too soon to write off 99L
In the latest iteration of the “ECMWF vs. GFS” battle that has raged for years, the GFS appears to have won the latest round. Several of our most reliable models, including the ECMWF and UKMET, developed 99L into a tropical storm by today, while the GFS remained insistent that no major development would occur. Kudos to the GFS for this one!

Even the more gung-ho runs of the ECMWF and UKMET earlier this week generally intensified 99L at a fairly modest pace for the period from Wednesday through Friday, with more rapid strengthening toward this weekend. Although it is not well organized enough to take full advantage of the situation, 99L is now entering a region of light to moderate wind shear (5 - 15 knots) and very warm sea-surface temperatures (29.5 - 30°C or 85 - 86°F), which would allow a more organized system to intensify. Though the odds of any rapid development are fairly low, it would be prudent to monitor Friday’s convective blow-up and make sure that 99L has no surprises up its sleeve.

Outlook for 99L this weekend and beyond
The 00Z Friday ensemble of the GFS and ECMWF are in general agreement that 99L will continue moving west-northwest on a track that would put it somewhere in or near the Florida Keys around Sunday. None of the GFS and ECMWF ensemble members bring 99L up to tropical storm strength before that point, while the HWRF--overall the best-performing intensity model of recent years--is an outlier, calling for 99L to reach tropical storm strength by Sunday. If 99L does organize today, we'll be looking closely at the next couple of rounds of models to see if any major changes occur. In any event, this morning’s burst of convection reminds us that 99L is capable of bringing very heavy rains to southern Florida over the weekend.

There remains plenty of uncertainty over 99L’s future beyond the weekend. The operational GFS and ECMWF model runs from 0Z Friday take 99L northward through the eastern Gulf and into the upper Gulf Coast of Florida. A minority of GFS ensemble members bring 99L further west, while the four ECMWF ensemble members that make up the “high probability cluster” (those that have performed the best on 99L over the last 24 hours) keep the system moving northward, very close to Florida’s west coast, as a tropical depression or weak tropical storm. The HWRF and UKMET are more aggressive on intensifying 99L further west in the open Gulf, where there would be less interaction with land. We can expect models to get a better handle on 99L if and when it develops a coherent circulation.

The bottom line: 99L remains a system well worth monitoring as it makes its way into south Florida and the Gulf of Mexico.


Figure 3. Members of the GFS ensemble modeling system (GEFS) are in close agreement that 99L will end up in or near the Florida Straits, but there is still wide disagreement on its path beyond that point later next week. These paths were generated at 12Z (8:00 am EDT) Friday, August 26, 2016, based on data from the 06Z Friday model runs.

A pair of East Pacific systems worth watching
Next week may be active in the East and Central Pacific, as two systems--Tropical Storm Lester and Invest 98E--chug westward on trajectories that could put them in the vicinity of the Hawaiian Islands late next week or beyond. Already well-organized, Lester is packing top sustained winds of 60 mph. Wind shear is injecting some dry air into Lester, but that should abate over the weekend, allowing Lester to become a hurricane as it continues on a nearly due-west path as projected by the 5:00 am EDT outlook from NHC. Further to the west, 98L is off to a healthy start, with a large and growing shield of convection. NHC gives 98L an 80% chance of development by Sunday, and models are in broad agreement that it will become Tropical Storm Madeline in the next several days. 98L will have a northward component to its motion at first, but in the longer range it should end up heading westward a few hundred miles ahead of Lester, likely peaking below hurricane strength.


Figure 4. Enhanced infrared image of Tropical Storm Lester produced with data from NASA’s MODIS (Moderate Resolution Imaging Spectroradiometer) as of 0915Z (5:15 am EDT) Friday, August 26, 2016. Image credit: CSU/RAMMB/CIRA.

A couple of factors are making 98L and Lester worth keeping an eye on:

--Although SSTs are cooler than average near the equator (with a borderline La Niña attempting to develop) and in the subtropics around 30°N, sandwiched in between is a region of SSTs that are 0.5°C to 1.0°C above average in the latitude belt around 15-20°N, which includes Hawaii. This would allow a westward-moving system approaching Hawaii to remain over waters near or just above 26°C (79°F), a standard benchmark for supporting tropical development.

--A very strong upper-level ridge in the Northeast Pacific should keep Lester and 98E from heading very far northwest or recurving northeast in the common fashion of East Pacific tropical storms and hurricanes. Such paths typically sound the death knell for these cyclones, as they end up over the cool waters north of 20°N.

Unusually high SSTs associated with El Niño and long-term warming have given Hawaii more than its historical share of tropical storm action in recent years. Two tropical cyclones moving from east to west became the first since Hawaii's statehood (1959] to make landfall in the Big Island: Tropical Storm Iselle (August 2014) and Tropical Storm Darby (July 2016). (In addition, an unnamed tropical storm hit the Big Island in 1958.) We have plenty of time to monitor both 98E and Lester. If 98E does become Madeline, it will keep us well ahead of schedule for East Pacific named storms. For the period 1971-2009, the average formation date of the “M” storm was September 28.

Jeff Masters will be back with our next update late Friday afternoon.

Bob Henson


Figure 5. Long-range projected tracks for Invest 98E (EP98) and Tropical Storm Lester, produced by the 20 members of the GFS ensemble modeling system (GEFS) as of 00Z Friday, August 26, 2016. Any encounter with the Hawaiian Islands would be no sooner than Wednesday, August 31, and the systems would most likely be at tropical storm strength at best. Image credit: NOAA/ESRL.

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About The Author
Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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