In Eastern Atlantic, Grace Intensifies While Fred Clings to Life

Tropical Storm Grace became the Atlantic’s seventh named storm of the 2015 tropical season on Saturday. Grace’s arrival keeps this year’s pace of named Atlantic storms close to the long-term average: during the period 1966-2009, the average date of formation for the seventh named storm was September 16. With top sustained winds of 45 mph as of 11:00 am EDT Sunday, Grace was moving just north of due west at about 14 mph through the eastern Atlantic, about 450 miles west-southwest of the Cape Verde islands. Grace took advantage of the typical nighttime surge in tropical cyclone convection to build a north-south swath of heavy thunderstorms around its center. The convection has ebbed somewhat in intensity over the last few hours, but Grace remains quite well structured in visible satellite imagery, though its convective envelope remains elongated.


Figure 1. An infrared view of Tropical Storm Grace (center) and a significant wave moving into the Atlantic from Africa (far right), taken at 1515 GMT (11:15 am EDT) on Sunday, September 6, 2015. Image credit: UW-CIMMS


Over the next couple of days, Grace will move over sea-surface temperatures of 27-28°C (80-82°F), slightly above average for this time of year. Wind shear will be relatively light beneath a weak upper-level ridge. The National Hurricane Center projects Grace to become a strong tropical storm by late Monday, and I’d put at least 50-50 odds on the likelihood of Grace making it to minimal hurricane status by Monday or Tuesday. Later in the week, as Grace moves west of 40°W, the storm will encounter an intensifying belt of southwesterly upper-level winds extending from the Caribbean into the central Atlantic. Vertical wind shear of more than 40 knots will develop, and Grace is likely to decay significantly as the shear disrupts its circulation and injects dry air into its core. NHC projects Grace to be no more than a minimal tropical storm by Thursday, as it approaches the Lesser Antilles.

Grace is the fourth storm this year to develop in the deep tropics of the eastern Atlantic--the Cape Verde development region, which can produce some of the longest-lived and most dangerous hurricanes to affect the United States. Getting so many Cape Verde storms this year is a bit surprising given the hostile conditions engendered by this year’s intensifying El Niño event, which is at record strength for early September. As noted by wunderground member Webberweather53, this is the first time that at least four named systems have developed east of latitude 60°W during a strong El Niño event. El Niño is responsible for hurricane-snuffing wind shear that has been at record levels for most of the summer over the Caribbean and nearby waters. It’s no coincidence that this year’s only two Atlantic hurricanes to date, Danny and Fred, both peaked in strength well east of the Lesser Antilles. Although Danny briefly reached Category 3 strength, the Atlantic has seen a total of only about 78 hours of hurricane activity between Danny and Fred. As a result, the Atlantic’s accumulated cyclone energy, or ACE--a measure of the longevity and strength of each year’s tropical cyclones--has reached a mere 20% of the typical end-of-season total. By comparison, the Northeast Pacific has already produced 32% more ACE than a typical season. El Niño typically favors the Northeast Pacific over the Atlantic in tropical cyclone activity.



Figure 2. Disorganized convection associated with Tropical Depression Fred is shown in this enhanced infrared image from the GOES-East satellite at 1615 GMT (12:15 pm EDT) on Sunday, September 6, 2015. Image credit: NOAA/NESDIS.

Fred refuses to quit
Though it was given a terminal diagnosis days ago, Tropical Depression Fred has shown it isn’t quite ready to leave this mortal coil. Now in its seventh day as a tropical cyclone, Fred’s entire life has unfolded east of longitude 45°W. It became the first hurricane in modern records to pass through the Cape Verde islands, and over the last several days it’s survived in the face of persistent wind shear by generating new convection each time a batch of thunderstorms is sheared away from its center. Downgraded to a tropical depression on Saturday evening, Fred is beginning to interact with an upper-level trough to its northwest and should begin moving more rapidly to the northeast over the next several days, then begin curving back toward the southeast as the trough passes it by. Although the ECMWF model merges Fred with the trough, most other models indicate that Fred will get yet another lease on life over the next several days, as wind shear lessens somewhat. Fred will continue to travel over SSTs that are around 1°C (1.8°F) above average for at least the next five days, so it’s possible Fred could linger as a tropical depression or weak tropical storm throughout this week.

Another healthy wave coming off Africa
African continues to generate an impressive supply of easterly waves that have at least a chance of developing into tropical cyclones as they move into the Atlantic. Our next tropical wave is in the process of moving from West Africa into the Atlantic at a fairly low latitude (see Figure 1 above). SSTs remain unusually high in this region, and I expect we will see this wave emerge into a tropical depression and perhaps become Tropical Storm Henri late this week. Once again, the tendency for high wind shear from the Caribbean northeastward will greatly reduce the odds that any tropical cyclone from the deep Atlantic tropics might approach North America relatively intact.


Figure 3. Sea-surface temperatures (left) and departures from average for this time of year (right), both in degrees C, across the North Atlantic for the week ending on August 29, 2015.

Tropical Storm Linda forms in the Northeast Pacific
Tropical Storm Linda became the latest named system in the Northeast Pacific at 11:00 am EDT Sunday. With winds of 45 mph, Linda is located about 610 miles south of the southern tip of Baja California. Satellite imagery now shows Linda organizing very quickly, and it has a window of a couple of days where it could attain at least Category 1 hurricane strength before it reaches cooler waters and more stable air. Late this week, Linda or its remnants should curve westward before making a run at Baja California. Fortunately, there is no sign at present that Linda will resemble Category 5 Hurricane Linda of 1997. That Linda was the strongest hurricane on record in the eastern Pacific, with top sustained winds of 185 mph. With damage from Linda relatively minor, its name was not retired, but the storm gave a brief scare to southern California. An NHC report notes that some model runs showed a weakening Linda heading toward Baja California or southern California, and special weather statements were issued by the Oxnard, CA, NWS office. (Because lists of hurricane names for the Northeast Pacific and Atlantic are rotated every six years, the same alphabet is being used in 2015 as in 1997--which happens to be the last year that a strong El Niño was in place during the Atlantic/Pacific hurricane season.)


Figure 4. Tropical Storm Linda congeals in the Northeast Pacific, as shown by this infrared image from the GOES-West satellite at 1630 GMT (12:30 pm EDT) on Sunday, September 6, 2015. Image credit: NOAA/NESDIS.

Elsewhere in the Pacific
Downgraded on Saturday night after more than a week as a hurricane, Tropical Storm Jimena is now feeling the effects of marginal SSTs and southwesterly wind shear as it carries out a gradual cyclonic loop north of Hawaii. Jimena’s top sustained winds are down to 60 mph, and they should continue to decrease today and Monday. The GFDL and HWRF models indicate that Jimena could make a comeback late in the week before recurvature into the midlatitudes, but SSTs would be barely supportive of strengthening. The official NHC forecast puts little stock in any late-week revival scenario for Jimena, weakening it to a post-tropical low by Thursday.

Further west, we still have Typhoon Kilo in the open Northwest Pacific. If we include its stint in the Northeast Pacific, Kilo is now in its ninth day of packing hurricane-force winds and its 18th day classified as a tropical cyclone. The latest outlook from the Joint Typhoon Warning Center gives Kilo a final surge of strength to Category 3 status later this week as it begins recurving well east of Japan. Kilo may have 6-7 days left to go as a tropical cyclone, which would leave it well short of the longevity record of 31 days set by Hurricane/Typhoon John (1994).

Bob Henson