Erika Hangs on, With Big Uncertainties Ahead

After going through a rather sickly phase during the day on Wednesday, Tropical Storm Erika began to rally after sunset, a sign that it may yet survive--and perhaps eventually thrive--en route to a possible U.S. East Coast landfall. As of 8:00 pm EDT Tuesday, Erika was located at 16.7°N, 59.5°W, or about 150 miles east of Antigua. Erika remains a relatively weak tropical storm, with top sustained winds of around 45 mph. An Air Force hurricane hunter measured peak sustained winds at flight level off41 knots (47 mph) at 2339 GMT (7:30 pm EDT). Tropical storm watches and warnings are in effect throughout the Leeward Islands, but the main effect of Erika--heavy rains--is largely a blessing for this drought-parched region.


Figure 1. Infrared imagery from NOAA’s GOES-East floater satellite shows the intensification of Erika’s core showers and thunderstorms between 1815 GMT Wednesday, August 26, and 0045 GMT Thursday, August 27. Convection over tropical cyclones often intensifies during the night. Image credit: NOAA/NESDIS.

Northwesterly wind shear and pockets of mid-level dry air kept Erika from intensifying on Wednesday. The storm’s main low-level center of circulation was displaced well northwest of the showers and thunderstorms (convection), a major hindrance to any strengthening. The typical nighttime development of convection will provide Erika another chance to either refocus convection over its main center of circulation or to develop a new center. Either option could give Erika an infusion of strength and help it to survive another day’s battle with wind shear on Thursday night. On its current track, Erika should pass just northeast of Puerto Rico on Thursday night. Wednesday’s models edged northward with Erika’s track, so it looks increasingly unlikely that Erika’s future strength will be dented too much by interaction with the mountains of Hispaniola or Cuba. However, the unrelenting wind shear will actually intensify from Thursday into Friday, according to several models. Erika has a fairly large, well-structured circulation that should give it a fighting chance, although it’s still possible that the storm will degenerate into an open wave by the weekend.


Figure 2. The NHC forecast for Erika as of 2100 GMT (5:00 pm EDT) on Wednesday, August 26.

Erika and the East Coast: A big question mark
The next big question is how much strength Erika will gain over the Bahamas, assuming it survives to that point. Sea-surface temperatures are about 1°C (1.8°F) warmer than average from the Bahamas to South Florida, running in the 86°F to 90°F range, and Erika should encounter weaker wind shear by this point. The official NHC forecast from 5:00 pm EDT Wednesday (Figure 2) brings Erika into eastern Florida as a Category 1 hurricane on Monday. This is a classic case where the intensity prediction (75 mph) may not be the most likely outcome in itself, but rather a split-the-difference compromise. As NHC notes in its forecast discussion, this outlook incorporates the chance that Erika could arrive at the Bahamas too disrupted to strengthen (a real possibility) or could intensify more than predicted (another real possibility). One significant change on Wednesday was that the most reliable long-range models are now consistently calling for Erika to approach U.S. shores as a hurricane. Both the 1200 GMT ECMWF run and the 1800 GMT GFS run bring Erika north and northeast just off the Florida and Carolina coastlines as an intensifying hurricane. The now-bullish GFS has joined the chorus of the HWRF and GFDL models, which have insisted for more than a day that Erika could intensify dramatically while over the Bahamas. We need to keep in mind that if Erika were to become a north- or northeastward-moving hurricane along the East Coast, only a slight change in track angle could have a major influence on coastal impacts. It also remains possible that the upper-level trough existing the East Coast this weekend will not be strong enough to pull Erika with it, which would leave behind a slower-moving, more erratic storm that could stall offshore or cause multiple days of trouble inland (see Figure 3 below). And this all presumes that Erika will even make it to the Bahamas in solid enough shape to strengthen.

We shouldn’t put too much stock in any individual model run, as there is plenty of variety in the outcome even within the ECMWF and GFS ensembles (which simulate the weather a number of times, with slight variations in the starting point to account for observational uncertainty). The overarching message at this point is that residents from Florida to North Carolina should be aware of the possibility for a significant hurricane during the early to middle part of next week, and plan and prepare accordingly.

Jeff Masters will be back with a full update on Thursday. See also Steve Gregory’s update from Wednesday afternoon.

Bob Henson



Figure 3. Upper-atmospheric flow at the 500-mb level (about 18,000 feet) projected by the GFS model run from 1800 GMT on Wednesday, August 26, for Tuesday afternoon, September 1. In this scenario, a strengthening upper-level ridge over the eastern U.S. and a weak upper low center over Louisiana (a left-behind remnant of the upper-level trough that now extends from the northeast U.S. to the Gulf states) would put Erika in a weak steering pattern just off the southeast U.S. coast.