97L Sweeps Toward Lesser Antilles; Nida Approaching Philippines

As it hustles westward at 25-30 mph in the western tropical Atlantic, Invest 97L is continuing to organize. In its tropical weather outlook issued at 2:00 PM EDT Saturday, the National Hurricane Center placed the tropical wave about 550 miles east of the Lesser Antilles. The wave’s envelope of shower and thunderstorm activity (convection) increased notably on Saturday, with strong convection now extending across the Leeward Islands. Upper-level wind shear is strong to 97L’s north, but light to moderate (5 - 15 knots) over the core of the wave. In addition, 97L is moving over sea-surface temperatures (SSTs) of about 28°C (82°F), which is about 0.5°C above average and more than warm enough to favor development.

Despite these supportive factors, 97L is fighting some unfavorable elements. The system is more organized aloft than at the surface, with healthy upper-level divergence but no sign yet of a closed surface circulation (see Figure 2). In addition, it’s typically more difficult for waves moving as quickly as 97L to organize. As it plows westward, 97L will be moving toward a region of moderate to strong shear (20 - 30 knots) associated with a weak upper-level trough across the central Caribbean, although it appears the shear will relax considerably as the trough shifts westward. [Update: The 0Z Sunday run of the SHIPS model keeps wind shear for 97L below 10 knots through Wednesday.] There is also a modest amount of dry air from the Sarahan Air Layer extending across the region of 97L’s track. Both the ECMWF and GFS model runs from 12Z Saturday keep 97L as an open wave during its trek across the eastern Caribbean. Ensemble guidance suggests only a modest chance of 97L traversing the eastern Caribbean as a tropical depression, based on the 12Z Saturday runs of the GFS and ECMWF ensemble models. Ensemble forecasts are produced by taking the operational high-resolution version of the model and running it at lower resolution with slight perturbations to the initial conditions in order to generate a range of possible outcomes. Only a small fraction of ensemble members develop 97L over the next several days.


Figure 1. Infrared satellite image of Invest 97L in the western tropical Atlantic as of 2115Z (5:15 pm EDT) Saturday, July 30, 2016. Image credit: NOAA/NESDIS.

In its 2 PM outlook, NHC gave 97L a 30% chance of developing into a tropical depression by Monday. Given the system’s vigorous growth today, and the mix of positives and negatives noted above, I’ll lay 50% odds on the chance that 97L will be at least a tropical depression by Monday, though it would probably struggle to maintain that status over the next several days. Squally weather, with bursts of heavy rain and wind gusts of 30-40 mph, can be expected through Sunday across the Lesser Antilles and beyond as 97L whips through the area. A flash flood watch is in effect for Sunday across Puerto Rico and the Virgin Islands, with rains of 2-4” predicted and higher amounts possible.

The long-range outlook for 97L
If 97L manages to organize even modestly over the next 2-3 days, we’ll have to keep a close eye on it. Model trends have been to route 97L on a fairly direct west to west-northwest path across the northern Caribbean, perhaps missing Hispaniola and most likely staying south of Cuba. Assuming that 97L forms a center of circulation and avoids major interaction with the high terrain of these islands, it will be well situated to strengthen--perhaps significantly--by late next week. A convectively coupled Kelvin wave (CCKW) has tended to suppress upward motion across the tropical Atlantic this week, but by later next week 97L may enter a region where the CCKW pattern favors upward motion. In addition, SSTs across the northwest Caribbean are very warm (29°C or 84°F, about 1°C above average), and there is a near-record amount of heat in the upper ocean to support rapid development if atmospheric conditions turn out to be favorable.

The SHIPS statistical intensity model is increasingly bullish on 97L, with the 18Z Saturday run of SHIPS bringing 97L to a Category 3 strength by Thursday. The last several runs of the HWRF model, which has shown increasing skill over the last several years, also project 97L to reach hurricane strength in the Caribbean (although the 12Z Saturday run appears to have had initialization problems, as noted by WU member Levi Cowan). The 12Z Saturday runs of the GFS and ECMWF models, two of the other more-trustworthy dynamical models, suggest that 97L could begin organizing just before crossing the Yucatan Peninsula and then develop further in the Bay of Campeche by late next week. It is far too soon to assign any confidence to model projections in this time range, but the available guidance indicates that 97L is well worth watching. NHC gives 97L a 60% chance of development by Thursday, August 4.


Figure 2. Surface winds across the tropical Atlantic at 1800Z (2:00 pm EDT) Saturday, July 30, 2016. The surface circulation is less organized with Invest 97L (far left) with Invest 96L (right), as 96L features weaker winds overall but a more evident surface circulation. Image credit: earth.nullschool.net.


Figure 3. Infrared satellite image of Invest 96L in the eastern tropical Atlantic as of 2045 (5:15 pm EDT) Saturday, July 30, 2016. Image credit: NOAA/NESDIS.

96L is struggling
Although the circulation of Invest 96L in the eastern tropical Atlantic is more organized than that of 97L, there isn’t much meat on the bones. There was only scattered convection on Saturday around the core of 96L, which is located about 400 miles southwest of the Cape Verdes. Although SSTs of around 28°C are more than adequate for development, wind shear of 5-10 knots along 96L’s path will increase to moderate levels (15 - 25 knots) over the next 2-4 days, and there is a large region of Sarahan dust lying ahead of 96L. Only a small fraction of ECMWF and GFS ensemble members have 96L as a depression in the central Atlantic by the middle of next week, and NHC gives 96L a 20% chance of development through Thursday.

Eighth Northeast Pacific storm of the month?
We may yet set a record for the largest number of named storms on record to develop in the eastern Pacific Ocean in the month of July. Invest 91E was gathering its forces on Saturday about 800 miles southwest of the southern tip of the Baja California peninsula, moving west-northwest at 10-15 mph. On this track, 91E should steer well clear of any major land areas, but in its tropical weather outlook issued at 2 PM EDT Saturday, NHC gave the system a 70% chance of development by Monday and a 90% chance by Thursday. If it manages to become Tropical Storm Howard before Monday, it will be the record-breaking eighth named storm this month. In the longer range, it’s possible that 91E will take a leftward-arcing path that would carry it or its remnants toward the Hawaiian Islands in about a week’s time.


Figure 4. Tropical Storm Nida as of 2130Z (5:30 pm EDT) Saturday, July 30, 2016. Image credit: RAMMB/CSU/NOAA.

Nida on track to reach the southeast China coast
Tropical Storm Nida will be sweeping across the northernmost part of the Philippines island of Luzon late Sunday. Nida’s surface winds were 50 mph as of 1800Z (2:00 pm EDT) Saturday, according to the Joint Typhoon Warning Center (JTWC). Nida has little time to strengthen before it slides across Luzon, and most areas will be on its weaker south side, but a large swath of convection will bring very heavy rains to the northern half of the island. Impacts could be considerably greater when Nida reaches the southeast China coast early next week. Models are in strong agreement on a straightforward west-northwest track for Nida, and very warm SSTs combined with only modest upper-level shear (10-20 knots) are quite favorable for strengthening. Model consensus brings Nida very close to Hong Kong by early Tuesday local time. Although many typhoons pass close enough to Hong Kong to bring high winds and heavy rain, a direct strike from a typhoon is less common. The 2100Z Saturday outlook from JTWC projects that Nida will pass directly over Hong Kong as a minimal typhoon.

We’ll be back with an update by Monday morning, or sooner if 97L develops into a tropical depression.

Bob Henson


Figure 5. WU tracking map showing the track and strength of Tropical Storm Nida projected by the Joint Typhoon Warning Center on Saturday, July 29, 2016.