Erika Nears Puerto Rico with High Wind, Heavy Rain

Still poorly organized--but already deadly, and growing in size--Tropical Storm Erika is likely to cause problems in Puerto Rico on Thursday night into Friday. At least four people were killed by mudslides in the wake of Erika’s passage over Dominica. The capital of Roseau, on the island’s southwest coast, was hard-hit with major river and street flooding. According to weather.com, Canefield Airport on Dominica received 12.64” of rain between 2:00 am and 2:00 pm EDT Thursday, as the island was struck by an intense blow-up of thunderstorms (convection) on the south edge of Erika’s ill-defined circulation. In a similar fashion, the typical nighttime intensification of convection could bring torrential rain to Puerto Rico and the Virgin Islands late Thursday night and Friday. In a local statement issued at 5:32 pm AST (4:32 pm EDT) on Thursday, the National Weather Service in San Juan called for widespread 4-8” totals across Puerto Rico and nearby islands, with up to 12” possible. Although the islands are in desperate need of rain to assuage an intense drought, Erika may deliver too much of a good thing.


Figure 1. NWS radar at San Juan, Puerto Rico, showed heavy rain from Erika just southeast of the island at 10:25 pm EDT Thursday, August 27. Overlaid is the location of Erika’s poorly defined center, as located by the National Hurricane Center at 8:00 pm EDT, together with previous locations (right) and the NHC-projected path (left). Image from the tropical tracking feature on WU’s Storm app.


As expected, persistent northwesterly shear kept Erika from intensifying on Thursday, with most of the storm’s convection shunted south and east of Erika’s center of circulation. At one point, a center of circulation was clearly visible beyond the northwest edge of the canopy of upper-level clouds associated with the convection--not a sign of a healthy tropical storm. Convection was redeveloping near this center on Thursday evening, though, and Erika’s overall circulation continued to expand in size, with outflow evident in most directions away from the northwesterly shear.


Figure 2. Enhanced infrared image of Tropical Storm Erika from the GOES-East floater satellite, collected at 0115 GMT Friday, August 27 (9:15 pm EDT Thursday). Image credit: NOAA/NESDIS.

At 11:00 pm EDT, the National Hurricane Center placed Erika’s center (perhaps a large center with several smaller swirls) at 16.6°N, 65.3°W, or about 135 miles south-southeast of San Juan, Puerto Rico. Erika was moving west at about 17 mph, a bit to the left of what had been predicted. If Erika takes the west-northwest tack still predicted for later tonight and tomorrow by NHC, its poorly defined center will nick the southwest corner of Puerto Rico early Friday and graze the northeast coast of Hispaniola later Friday. Such a track might result in only minor disruption to Erika’s intensity: the storm is not too tightly organized to begin with, and its large structure could help it recover fairly quickly once back over open water. However, if Erika continues on a more westward trajectory, it could spend much more time over the higher terrain of Haiti and the Dominican Republic, which could cause it to weaken into a tropical depression or open wave. Such a track could also produce devastating rainfall over these two nations.

Erika’s future
If Erika makes it into the Bahamas relatively intact--and that remains a very big "if"--we could be dealing with it for days to come. Conditions over the Bahamas will be quite favorable for strengthening, with sea-surface temperatures above 30°C (86°F) toward the northwest Bahamas. Wind shear is also projected to drop to the 10 - 20 mph range over the weekend. Erika’s fairly large size will tend to reduce its ability to spike or plummet in intensity within hours, the way Hurricane Danny did last week. However, depending on its eventual track, Erika may have at least a couple of days to gather strength over toasty waters with relatively light wind shear.


Figure 3. The NHC outlook for Erika issued at 11:00 pm EDT on Thursday, August 27.

Erika’s strength and track are increasingly difficult to predict beyond this weekend. If anything, computer models on Thursday offered an even broader long-term palette of possibilities for Erika than they did on Wednesday. The official NHC forecast from 11:00 pm EDT Thursday is consistent with the model blend from 1800 GMT Thursday, which was toward a position near the Florida coast (either just offshore or just inland) by Monday, with a general north-northwest motion thereafter. A weaker Erika would be steered more by lower-level easterly flow, perhaps moving over or near Hispaniola and on toward South Florida and up the peninsula as little more than a tropical storm. A stronger system would be more inclined to stay offshore, perhaps heading north through the Bahamas and toward the Carolinas as a Category 1 or stronger hurricane. The “early cycle” model guidance from 0000 GMT Friday, which adjusts model forecasts that were issued six hours earlier to account for more recent storm behavior, leans more toward the weaker, more westerly solutions. The 0000 GMT Friday run of the GFS model will incorporate extensive data on the steering flow around Erika collected on Thursday by the NOAA Gulfstream IV surveilliance aircraft.

It appears unlikely Erika will recurve sharply northeast, as upper-level ridging will become more pronounced across the eastern United States and the western North Atlantic as we move into early next week (see Figure 4 below). This means Erika could be a slow-moving, rain-dumping system wherever it ends up beyond the five-day forecast window. It’s important to keep in mind that roughly a third of all tropical cyclones tracked by NHC lie outside the five-day position shown in the forecast “cone”, which is based on average errors over time. Models may swing to the west or east and back again with every six-hour set of runs over the next couple of days, but the official NHC outlooks do an excellent job of smoothing out the inevitable swings from model to model and run to run. Tracking these outlooks over time will give you a very solid sense of how the model forecasts as a whole are trending. The bottom line for now: residents from Florida to North Carolina should be ready for the potential of tropical storm or hurricane impacts at some point during the next 5 to 7 days.


Figure 4. Evolution of the flow at 500 millibars, or about 19,000 feet (black contours) and that flow’s departure from the seasonal average (blue and orange tints) from the ECWMF model’s ensemble average produced at 1200 GMT on Thursday, August 27. Left panel is the starting point (1200 GMT Thursday); right panel is the 120-hour forecast for 1200 GMT on Tuesday, September 1, by which point the trough now aligned along the U.S. East Coast (blue tint in left panel) has been replaced by a sprawling ridge (orange tint in right panel) that would block any rapid northward motion of Tropical Storm Erika or its remnants. Image credit: Levi Cowan, www.tropicaltidbits.com.


Figure 5. A multispectral (RGB) satellite image of Hurricane Storm Ignacio, collected by the GOES-West floater satellite at 0130 GMT on Friday, August 28. Image credit: NOAA/NESDIS.


Figure 6. The NHC outlook for Ignacio issued at 11:00 pm EDT on Thursday, August 27.

Ignacio may threaten Hawaii early next week
Residents of Hawaii need to keep close tabs on Hurricane Ignacio, which was packing top sustained winds of 90 mph as of 5:00 pm HST (11:00 pm EDT) Thursday. Now located just under 1000 miles east-southeast of Hilo, Ignacio is on a fairly straightforward west-northwest course that should continue over the next several days. Simply extrapolating Ignacio’s track would bring the hurricane very close to Hawaii by Monday or Tuesday. Hurricanes on such a course typically weaken before they reach the islands, traveling over surface waters near the threshold for tropical development of around 26°C (79°F). In this case, Ignacio is getting a boost from favorable upper-level conditions as well as unusually warm sea-surface temperatures, roughly 2°C (3.8°F) above average for this time of year. NHC intensifies Ignacio to just short of Category 3 strength by this weekend, then gradually weakens it starting on Monday. The official forecast calls for Hurricane Ignacio to pass about 100 miles north of Hilo on Tuesday as a Category 1 storm with sustained winds of 80 - 90 mph. No hurricane has ever been officially recorded on the Big Island of Hawaii--the strongest event on record was Tropical Storm Iselle in 2014--so today’s forecast is noteworthy in itself. Beyond the five-day period, models are tending to bend Ignacio’s track slightly leftward, as upper-level ridging strengths to the north. The predicted steering currents and the very warm SSTs make Ignacio a force to be reckoned with, and all of the Hawaiian islands should take this hurricane seriously. Large swells will become a near-certainty over the next few days.


Figure 7. An infrared satellite image of Tropical Storm Jimena, collected by the GOES-West floater satellite at 0130 GMT on Friday, August 28. Image credit: NOAA/NESDIS

Jimena may become a Category 4 powerhouse
On the heels of Ignacio, Tropical Storm Jimena is quickly strengthening, with sustained winds up to 70 mph as of the 11:00 pm EDT advisory on Thursday. Located almost 1000 miles west of Cabo San Lucas, Jimena should be a hurricane by Friday morning as it moves steadily westward. Conditions are even more favorable for Jimena than for Ignacio, and satellite images already show a well-structured storm, with stout, far-reaching spiral bands and a tight core of intense convection already in place. The SHIPS Rapid Intensification Index, which quantifies the odds that a hurricane will grow quickly, shows a 50-50 chance that Jimena’s top winds will increase by at least 30 knots (35 mph) over the next 24 hours. The official NHC forecast rolls with this scenario, making Jimena a Category 4 hurricane by Sunday. It’s too soon to know whether Jimena will pose a threat to Hawaii, although it will remain well east of the islands for at least the next five days.

Jeff Masters will have our next update on Friday morning. See also Steve Gregory’s excellent overview of Erika, posted earlier this afternoon. Another WU contributor, Weather Channel hurricane specialist Bryan Norcross, weighed in today on why it’s crucial not to take any particular forecast for Erika as gospel at this point.

Bob Henson