Florida Disturbance 98L and Central Atlantic Wave 99L Both Threats to Develop

An area of low pressure (98L) centered just offshore from the southeast coast of Florida on Friday morning is likely to develop into a tropical depression by early next week. This system had a good deal of spin and was producing a moderate-sized area of disorganized heavy thunderstorm activity over the waters to its northeast, as seen on satellite images.

Steering currents will likely take the center of 98L inland over Florida Friday evening through much of Saturday, limiting the chances of development during that time. However, by Sunday, when the center is expected to move back over water off the coast of central Florida, conditions appear quite favorable for development, with low wind shear (less than 10 knots), warm SSTs near 30°C (86°F), and a moist atmosphere with a mid-level relative humidity of 60 - 70%.

The Friday morning runs of our top models for predicting tropical cyclone genesis—the European, GFS, and UKMET models—all predicted that 98L would develop into a tropical depression sometime Sunday through Tuesday, as it takes a track roughly parallel to and very close to the Southeast U.S. coast. 98L will likely bring heavy rains of 2 - 4” to the northwest Bahamas and south and central Florida over the weekend. By Monday, 98L will likely spread heavy rains northwards along the coast as far as the Outer Banks of North Carolina.

In their 8 am EDT Friday Tropical Weather Outlook, NHC gave 98L 2-day and 5-day odds of development of 40% and 70%, respectively. The next name on the Atlantic list of storms is Dorian.

Invest #99L has officially been designated out in the MDR. Its structure remains healthy during the diurnal min w/ a developing surface vortex.

Increasing likelihood that our first MDR #TC of the NATL season comes from this system. pic.twitter.com/qHzKCHMT9r

— Philippe Papin (@pppapin) August 23, 2019

99L in the central Atlantic headed towards Lesser Antilles

At 8 am EDT Friday, a tropical wave located in the central Atlantic near 10°N, 42°W, about 1400 miles east-southeast of the Lesser Antilles Islands, was headed west at about 15 mph. This system was designated 99L by NHC on Friday. Satellite images of 99L on Friday morning showed that the system had a high amount of spin and modest-sized area of heavy thunderstorms that were steadily growing more organized. A Friday morning pass by the ASCAT satellite showed that 99L may even have a closed surface circulation (see tweet above).

According to the 12Z Friday run of the SHIPS model, dry air will be the main challenge for 99L to overcome for development. The system was embedded in a dry atmosphere with a mid-level relative humidity of 50%, and was predicted to experience a mid-level relative humidity between 40 – 50% over the next five days. This presents challenging conditions for development, even though wind shear is predicted to be mostly a moderate 10 – 20 knots and SSTs will be warm, near 28.5°C (83°F).

99L is expected to slow down to a forward speed of about 10 mph this weekend, with a track that will vary between west and northwest. This could bring the system into the Lesser Antilles Islands as early as Wednesday evening. Out of the Friday morning (0Z) runs of our top models for predicting tropical cyclone genesis—the European, GFS, and UKMET models—one of them, the GFS, predicted that 99L would development into a tropical depression next week. 99L also had support for development from about 20% of the 51 members of the 0Z Friday European model ensemble. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day odds of development of 10% and 20%, respectively. Given the appearance of 99L on satellite imagery since the time this advisory was issued, we think a bump-up in 2-day and 5-day odds of development to 30% and 40% is warranted.

Although 99L will likely struggle with dry air during the coming week, it may help set the stage for the next tropical wave behind it to struggle less, by moistening the atmosphere across the tropical Atlantic.

Chantal’s demise is near

You know a tropical cyclone is in trouble when NHC uses the word “meager” to describe its heavy thunderstorm activity, which is how the 5 am NHC discussion for Tropical Depression Chantal described it. Chantal, which was located in the water of the far north Atlantic to the southeast of Newfoundland, Canada on Friday morning, is expected to degenerate into a remnant low by Friday night, due to the large amounts of dry air the system has encountered.

Figure 1. Visible-wavelength image of Tropical Storm Ivo at 1445Z (10:45 am EDT) Friday, August 23, 2019. Image credit: tropicaltidbits.com.

Tropical Storm Ivo soon to decay in Northwest Pacific

After peaking with top sustained winds of 65 mph on Thursday, Tropical Storm Ivo will soon be on the downswing as it heads north-northwest parallel to the Mexican coast. At 11 am EDT Friday, Ivo was about 435 miles southwest of Cabo San Lucas, Mexico, passing over virtually uninhabited Clarion Island with top winds of 60 mph. Northeasterly wind shear of around 20 knots is evident in the storm’s southwestward-shunted envelope of showers and thunderstorms.

The SHIPS analysis on Friday morning shows that the wind shear affecting Ivo will slacken to 5-10 knots by the weekend. However, by that point Ivo’s track will be taking it over dramatically cooler water (below 26°C or 79°F), and drier midlevel air will be intruding. NHC predicts that Ivo will be a tropical depression by Sunday and a remnant low by Monday, if not sooner.

#Taiwan is bracing for the arrival of Tropical Storm #Bailu. It's due to make landfall on Saturday night (local time) and will bring half a month's worth of rain and strong winds. pic.twitter.com/FY7zBeDhb3

— BBC Weather (@bbcweather) August 23, 2019

Bailu to be a big rainmaker for Taiwan

Tropical Storm Bailu is on a steady track to sweep across the southernmost part of Taiwan on Saturday local time. Hauling a large swath of moisture, Bailu is traveling over warm SSTs (29-30°C or 84-86°F) amid light to moderate wind shear (5-10 knots). The storm could reach typhoon strength (Category 1 equivalent) before making landfall, as predicted by multiple runs of the high-resolution HWRF model. The winds on Bailu’s stronger right-hand side will slam head on into the higher terrain of eastern Taiwan, so widespread heavy rains are a sure bet. Taiwan’s Central Weather Bureau has issued advisories for torrential rain across east-central and southeast parts of the country. Widespread totals of 5-10” can be expected, with amounts of 10-15” or more possible on favored upslope terrain, where mudslides will be a threat.

Bob Henson co-wrote this post.