A less-than-impressive Tropical Storm Fiona
continues to work its way across the central tropical Atlantic, while another system--Invest 99L
—is drawing more interest from tropical weather watchers. It appears 99L could make a break for the Caribbean early next week, perhaps as a significant tropical cyclone (see below).
As of 11 am EDT Friday
, Fiona was located at 17.8°N, 43.5°SW, or about 1300 miles west of the Cape Verdes, with top sustained winds holding at 45 mph. Fiona is moving west-northwest at about 10 mph and poses no threat to land. Never a massive storm, Fiona was looking rather spindly on satellite imagery Friday morning, with several fragmented bands feeding into a somewhat disheveled core of convection (showers and thunderstorms). Sea surface temperatures are more than adequate for development, at around 27°C (81°F)—about 0.5°C above the seasonal average—and the storm will be passing over progressively warmer waters. However, Fiona’s overall convective pattern has weakened over the last few hours, and it appears the decline will continue for the next day or two, as vertical wind shear increases to moderate levels (20-25 knots) and dry Saharan air works its way into the circulation (see Figure 2 below). Figure 1.
Latest visible satellite image of Tropical Storm Fiona.Figure 2.
A large mass of dry air from the Sahara Desert (yellows and reds) was encircling Tropical Storm Fiona, located near 45°W, as of 1200Z (8:00 am EDT) Friday, August 19, 2016. Invest 99L, centered to the southeast around 35°W, is expected to track well south of the Saharan air layer for at least the next couple of days. Image credit: University of Wisconsin-Madison/CIMMS/NOAAThe outlook for Fiona
Models are in near-unanimous agreement that Fiona will continue on its west-northwest track for the next 3 to 4 days, heading toward a weakness in the sprawling ridge that extends across the subtropical Atlantic. Uncertainty increases somewhat beyond that point, as there are signs that ridging will begin to fill in the weakness, slowing Fiona’s progress. Both dynamical and statistical models agree that Fiona should remain below hurricane strength throughout the next five days, and some members of the ECMWF and GFS ensembles bring Fiona back to depression status by this weekend or early next week. The National Hurricane Center
expects Fiona to weaken into a tropical depression by Sunday.
Assuming that Fiona holds together as a tropical cyclone, it could end up linger around the latitude of 30°N for at least a day or two next week, perhaps longer. A belt of unusually warm SSTs is straddling that latitude, but if Fiona tarries too long in one area, it may churn up cooler waters that could have a weakening effect. Climatology would suggest that eventual recurvature toward the north and northeast remains the most likely destiny for Fiona, if it survives that long.Keeping an eye on 99L
Located about 500 miles southwest of the Cape Verde Islands, 99L is a large but disorganized tropical wave, with a broad zone of scattered convection. The wave is located close to 10°-15°N and 30°W and is heading westward. 99L’s low latitude, and the lack of northward component to its motion, means that it is well positioned to head toward the Caribbean. An upper-level ridge to the northwest of 99L should keep it on a low-latitude path for the next several days, and models are in strong agreement that 99L will continue on its general westward trek, which could put it in the vicinity of the Lesser Antilles by around the middle of next week. The track forecast becomes more uncertain by that point, and it’s too soon to tell which parts of the island chain might be affected.
Wind shear is expected to remain fairly light along 99L’s path (10-15 knots), and SSTs should remain in the 28-29°C range over the next several days. Both of these factors favor development of 99L. In its 8 am EDT tropical weather discussion
, NHC gave 99L only a 10% chance of development by Sunday, but a 50% chance by Wednesday. The discussion noted another wave coming off Africa over the weekend that has a 30% chance of development by Wednesday.Figure 3.
Infrared image of Invest 99L from 1145Z (7:45 am EDT) Friday, August 19, 2016. Image credit: NOAA/NESDIS
Steering flow at the 200-mb level, about 40,000 feet above the surface, will help keep Invest 99L moving generally westward for the next several days. Wind speeds are colored in knots; multiply by 1.15 to obtain wind speed in mph. 99L and Fiona are indicated as low-pressure centers (“L”) around 30°W and 45°W, respectively. Image credit: tropicaltdibits.com
Nearly all of the ECMWF and GFS ensemble members keep 99L below tropical storm strength for at least the next several days, with the GFS ensemble suggesting that 99L could intensify in the 4- to 5-day period. A word of caution, however: although the ECMWF and GFS are among the top-performing models for tropical cyclone track
forecasts, they are much less skilled at predicting intensity
, especially for periods less than 5 days. The best-performing model for intensity over the last several years has been the HWRF, as we discussed in our May post on hurricane intensity prediction
. Both the 00Z and 06Z Friday runs of the HWRF model intensify 99L to tropical storm strength over the weekend and to hurricane strength as soon as Tuesday or Wednesday. The IVCN
, a blend of several high-performing models, also projects that 99L could reach hurricane strength by early next week.
Climatology favors the development of long-track hurricanes in the deep tropical Atlantic during late August. Given the favorable conditions at hand, we will need to watch 99L very closely. Figure 5.
The latest WU hurricane tracking map
shows a plethora of systems across the Northern Hemisphere tropics.Elsewhere in the (busy) tropics
It’s looking a lot like late August across the northern tropics, with no fewer than seven systems showing up on WU’s hurricane tracking map on Friday morning (see Figure 5 above). Fortunately, none of the current systems were at hurricane/typhoon strength. In the East Pacific, Tropical Storm Kay
, christened on Thursday, is paralleling the coast of Mexico several hundred miles south of Baja California. Only a minimal tropical storm, with 40-mph sustained winds, Kay has only a short window of potential modest strengthening before it moves over progressively cooler SSTs. The storm is expected to remain offshore as it gradually weakens.Tropical Storm Dianmu
made landfall on the far north coast of Vietnam around 06Z (2 am EDT) Friday morning. A giant burst of convection is dumping torrential rains on the mountains of northern Vietnam and Laos, and the heavy rains will spread into northern Thailand
on Friday and Saturday. Rainfall amounts of up to 16 inches (400 mm) have been reported in parts of northern Vietnam, according to vietnamnet.vn
. Further east, Tropical Storm Mindulle
is about to accelerate northward while strengthening
, which could bring it onto the coast of Japan’s Honshu island near Tokyo as a minimal hurricane by Monday.
Jeff Masters and I will be monitoring the tropics throughout the weekend. We’ll have our next update on Saturday.
Bob HensonFigure 6.
Infrared image of Tropical Storm Kay off the southwest coast of Mexico as of 1500Z (11 am EDT) Friday, August 19, 2016. Image credit: RAMMB/CIRA/CSU