99L a Potential Threat to Hispaniola, Bahamas, U.S. East Coast

The main Atlantic tropical weather threat to populated areas continues to be Invest 99L, a large tropical wave with an increasing amount of heavy thunderstorm activity that was located about 800 miles east of the northern Lesser Antilles Islands late Monday morning. This disturbance was moving west to west-northwest at 15 - 20 mph, and will bring heavy rains and gusty winds to these islands beginning on Tuesday evening. Conditions for development will steadily improve in the coming days, and the storm could be trouble for the Bahama Islands late this week--and is a threat to make landfall along the U.S. East Coast early next week. Satellite loops on Monday morning showed that 99L had finally managed to fire up a respectable amount of heavy thunderstorm activity near its core, in defiance of the dry air from the Saharan Air Layer (SAL) that had been interfering with development over the past few days. Water vapor satellite imagery showed that there continued to be a fair amount of dry air around the storm, though the amount of dry air had decreased since Sunday. Other conditions were generally favorable for development, with wind shear a light 5 - 10 knots and sea surface temperatures (SSTs) near 27.5°C (82°F), which was close to average.


Figure 1. Latest satellite image of 99L.

Track forecast: 99L a potential threat to Hispaniola, the Bahamas, and the U.S.
A strong ridge of high pressure will keep 99L headed north of due west over the next few days, and the storm should pass through the northern Lesser Antilles Tuesday night through Wednesday, track close to Puerto Rico on Wednesday night, and affect Hispaniola and the Southeastern Bahamas by Thursday. The uncertainty about the track increases greatly thereafter, due to a potential weakness in the ridge of high pressure steering the storm caused by a trough of low pressure passing to the north of 99L. The storm should slow its forward motion to 5 - 10 mph, in response to this trough, and may turn to the north near the central Bahamas. The track of 99L may also be affected by the remnants of Tropical Storm Fiona, which could be a few hundred miles to the north or northeast. At this time, it appears that 90L near the Cabo Verde Islands off the coast of Africa, which is expected to become Tropical Storm or Hurricane Gaston late this week, will be too far from 99L next week to exert a steering influence on it. The steering situation is too complex next week to say how great a threat the storm may pose to the U.S., but 99L is a legitimate threat to make landfall along the East Coast.


Figure 2. The dry air of the Saharan Air Layer (SAL) as analyzed by satellite at 8 am EDT Sunday, August 21, 2016 (top) and Monday, August 22, 2016 (bottom). The amount of dry air from the SAL interfering with 99L has decreased since Sunday. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Intensity forecast for 99L: commentary by Jeff Masters
Heavy rains from 99L will be capable of causing flash flooding problems in the Lesser Antilles, Virgin Islands, and Puerto Rico, but wind damage should not be an issue, since 99L will likely be, at worst, a moderate-strength tropical storm with 55 mph winds once it leaves the islands. The 8 am EDT Monday run of the SHIPS model showed moderately favorable conditions for development through Thursday. Wind shear will be in the light to moderate range, 5 - 15 knots, and SSTs will increase from 27.5°C (82°F) to 29°C (84°F), accompanied by an increase in the total heat content of the ocean. Working against development of 99L will be the large size of the storm, dry air of the SAL, potential interaction with the land areas of Puerto Rico and Hispaniola, and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO). None of the Sunday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, GFS and UKMET models--showed development of 99L into a tropical depression or tropical storm over the next four days, though the UKMET model predicted it could be a tropical depression in the Bahamas in five days. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 20% and 50%, respectively. I think these odds are too low, and should be 30% and 60%, respectively. The Hurricane Hunters are scheduled to investigate 99L on Tuesday afternoon.


Figure 3. Total oceanic heat content (called the Tropical Cyclone Heat Potential, or TCHP) in kilojoules per square centimeter (kJ/cm^2), for August 21, 2016. TCHP was at near-record or record values in the waters surrounding the Bahamas. TCHP in excess of 90 kJ/cm^2 (orange colors) is commonly associated with rapid intensification of hurricanes. Image credit: NOAA/AOML.

I am concerned about the storm’s potential impacts on the Bahamas and the U.S. beyond five days, when the storm will likely be near or just north of the central Bahamas. The models are predicting a more favorable environment for development then, and the storm will be moving quite slowly, potentially allowing for some very high rainfall totals in the Bahamas, and to a lesser extent, Haiti and the Dominican Republic. While the 00Z Monday runs of the GFS and European model ensemble forecast had fewer than 10% of their members predicting that 99L would eventually become a hurricane, the storm will likely spend at least three days over a region of ocean with SSTs that are near record-warm: 29 - 29.5°C (84 - 85°F). Warm waters extend to great depth, resulting in an unusually high total ocean heat content. With some models forecasting that wind shear will be in the light to moderate range early next week, we have the potential for 99L--which does have a large circulation--to rapidly intensify into a large hurricane. Hopefully, the unfavorable phase of the MJO will help to put the brakes on such a possibility.


Figure 4. MODIS visible satellite image of 90L to the southwest of the Cabo Verde Islands, taken on Monday morning, August 22, 2016. Image credit: NASA.


90L in the Eastern Atlantic near tropical depression status
A large tropical wave a few hundred miles southwest of the Cabo Verde Islands off the coast of Africa (Invest 90L) is near tropical depression status. Satellite images on Monday morning showed a well-organized system with plenty of spin, low-level spiral bands, and an increasing amount of heavy thunderstorms. Wind shear was moderate, 10 - 15 knots, and SSTs were warm enough for development, 28°C (82°F). The Monday morning operational runs of the European, GFS and UKMET models all showed development of 90L into a tropical storm in 1 - 2 days. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 100%. The wave will head west-northwest at 15 - 20 mph through Tuesday, skirting the Cabo Verde Islands, then turn more to the northwest on a path similar to Fiona’s. The storm will be moving into a region of ocean where very few tropical cyclones ever make the long trek westwards to hit the United States or Bermuda. The next name on the list of Atlantic storms is Gaston.


Figure 5. Enhanced infrared image of Tropical Storm Fiona at 1515Z (11:15 am EDT) Monday, August 22, 2016. Image credit: NOAA/NESDIS.

Fiona hangs in for now
Weak but resilient, Tropical Depression Fiona continues to chug through the central Atlantic. Fiona’s convection has been pulsing and decaying every few hours, but it continues to be skewed toward the east side of the low-level center, prohibiting any further development. Fiona was downgraded to depression status by NHC at 11:00 pm EDT Sunday, but the change only involved a drop of 5 mph in Fiona’s top sustained winds. In its 11:00 am EDT update, NHC kept Fiona as a depression with top winds still at 35 mph. Strong vertical wind shear of about 30 knots is still plaguing Fiona, and dry Saharan air surrounds the circulation, with mid-level relative humidity at only around 40%.

Fiona continues to move west-northwest around the west end of the large upper-level ridge that’s also helping to steer 99L and 90L. Over the next day or so, shear will decrease, humidity will increase, and SSTs will remain in the very warm 29-30°C range along Fiona’s track. Most dynamical models project that Fiona will continue to slowly weaken over the next several days, and NHC predicts Fiona to be a remnant low by Tuesday evening. If Fiona manages to make it through Tuesday, a life extension isn’t out of the question, although shear will again increase by late in the week. A weakness in the ridge would allow Fiona to angle more northwestward later in the week, after which the track forecast would become much more complex, especially if there were any interaction with 99L. If Fiona becomes a remnant low, its moisture could well be entrained by 99L’s circulation toward the end of the week.

If Fiona loses its identity as a tropical cyclone before any landfall, as seems probable, it will break the Atlantic’s remarkable string of five consecutive landfalls from the first five named storms of 2016.


Figure 6. Infrared satellite image of Invest 98E as of 1500Z (11:00 am EDT) Monday, August 22, 2016. Image credit: NOAA/NESDIS.

East Pacific: Kay on its last legs as 98E begins cranking up
As expected, cooler waters along the path of Tropical Storm Kay are taking a toll. Kay is now a minimal tropical storm, with top sustained winds of 40 mph. Kay’s west-northwest path will soon take it over waters cooler than the 26°C (79°F) benchmark for tropical development. In its 11:00 am EDT Monday advisory, NHC projects Kay to be a depression by tonight and a remnant low by late Tuesday.

Even further offshore, Invest 98E--about 850 miles south of Cabo San Lucas, Mexico--has a decent shot at eventual development. 98E is now a large, poorly organized wave, but statistical and dynamical models agree on significant strengthening, especially toward the end of the week. The NHC gives 98E a 10% chance of development by Wednesday and a 50% chance by Saturday. On its consistent west-northwest track, 98E will remain far away from any land areas.


Figure 7. Visible satellite image of Tropical Storm Mindulle at 0009Z Monday, August 22, 2016 (9:09 pm Sunday EDT), several hours before Mindulle swept northward into Japan near Tokyo. Image credit: CIRA/RAMMB/CSU, courtesy wunderground member 1900hurricane.

West Pacific: Mindulle hammers Japan while Lionrock lurks to the south
At least one person has died and 29 have been injured by winds and rain associated with Tropical Storm Mindulle, according to the Japan Times. Mindulle made landfall on Japan’s Honshu island near Tateyama, about 50 miles south of Tokyo, around 12:30 pm Monday local time (11:30 pm Sunday EDT). After being briefly upgraded by the Japan Meteorological Agency to a minimal Category 1 typhoon, Mindulle passed just east of the Tokyo city center, with millions affected by cancelled or delayed flights and trains across the region. At 12Z Monday (8:00 am EDT), Mindulle was passing across northernmost Honshu, its top winds down to 40 mph. As it transitions to an extratropical cyclone, Mindulle will make its next landfall on eastern Hokkaido island, which was drenched over the weekend by torrential rain associated with Tropical Storm Kompasu. All of Hokkaido and northern Honshu islands were under warnings Monday for heavy rain and potential landslides.


Figure 8. Rainfall rates from 6:00 pm to midnight JST Monday night, August 22, 2016, averaged 30-50 mm/hr (1.2” to 2”/hr) over parts of northern Honshu island, Japan, in bands around the center of Tropical Storm Mindulle. Image credit: Japan Meteorological Agency.

The burst of tropical cyclones across the Northwest Pacific in recent days resulted in some odd interactive behavior, including the multil-day southwestward motion of Tropical Storm Lionrock. Located several hundred miles south of Honshu late Monday local time, Lionrock was drifting south-southeast at just 3 mph. Lionrock’s top sustained winds are just 40 mph, and little strengthening is expected over the next few days as the storm embarks on a leisurely clockwise loop. Further down the line, a weakness in the ridge blocking Lionrock will open up north of Japan, which should induce a northward or northeastward acceleration. Conditions by that point will favor strengthening: the GFS and ECMWF agree that Lionrock could be a significant typhoon by this weekend--possibly threatening Japan and/or the Korean peninsula, --but with a great deal of uncertainty on its eventual track. Wunderground member 1900hurricane posted a detailed analysis on Sunday of the West Pacific situation, including Lionrock’s potential future.

Baton Rouge has its wettest month in 174 years of recordkeeping
The astounding rains that led to catastrophic flooding in Louisiana led to a phenomenal monthly rainfall total in Baton Rouge, where formal weather observations began quite early, in 1843. As of Sunday, the August rainfall total in Baton Rouge was 26.97”, which crushes the previous record of 23.73” (May 1907), according to WU weather historian Christopher Burt. This monthly total includes 0.76” on Sunday and a total of 6.08” since August 14, when the rains directly associated with the flood-making upper low had already ceased. For the summer since June 1, Baton Rouge has picked up an amazing 40.95”--more rain in three months than downtown Los Angeles has recorded over the last five years (38.79”)!

Jeff Masters and Bob Henson