1-4 September 2009
Tropical Storm Erika was a short-lived (1-4 September, 2009) and slow-moving tropical storm that produced torrential rainfall on the islands of the northeast Caribbean (Figure 1). Erika reached peak intensity as a 50-kt tropical storm about 315 nm east of Guadeloupe. Rainfall associated with Erika caused torrential rainfall in the Leeward Islands, U.S. Virgin Islands, and Puerto Rico. These heavy rains led to minor to moderate flash flooding across the eastern third of Puerto Rico.
Figure 1(above): Track of Tropical Storm Erika as it moved across the tropical Atlantic Ocean into the eastern Caribbean Sea over the five day period covering 1 September 2009 through 5 September 2009.
Tropical Storm Erika originated from an African easterly wave on 25 August 2009. The wave was initially accompanied by deep convection and plenty of vorticity as depicted in satellite imagery (Figure 2). The wave slowly became better organized as it moved across the Atlantic Ocean where it encountered warm waters and a low-shear environment. By 28 August 2009, a QuikSCAT pass showed the system had a complete circulation, elongated in the east-west direction, with top winds of 30 mph (Figure 3). The first Air Force Reserve Reconnaissance aircraft mission was flown on 1 September 2009 and found a minimum central pressure of 1007 mb and a closed, albeit broad, center with peak flight level winds of 52-kts and Stepped Frequency Microwave Radiometer (SFMR) winds of 45-kts (Figure 4). The National Hurricane Center initiated advisories on this tropical system at 5 pm on 1 September 2009 as Tropical Storm Erika, the fifth named storm of the 2009 Atlantic Hurricane Season. Erika reached its peak intensity 6 hours later as a 50-kt tropical storm about 315 nm to the east of Guadeloupe. However, soon thereafter, Erika began encountering mid-level dry air and moderate vertical shear that eventually led to its demise. Figure 5 is a Meteostat-9/GOES-11 combined image that shows the dry air as indicated by the orange and yellow colors to the west of Tropical Storm Erika. The 0000 UTC 2 September 2009 soundings from Guadeloupe (Figure 6) and St. Martin (Figure 7) show vertical wind shear with 30 knot winds out of the south to southwest at 200 mb. Also, a layer of very dry air between 700-600 mb also likely contributed to the rapid weakening of the storm. A visible satellite image on 2 September 2009 shows a disorganized storm with what appears to be the center of circulation of the storm just to the west of the island of Guadeloupe and the deepest convection well to the east of the islands (Figure 8). Erika continued to move rather slowly and erratically across the Leeward Islands producing torrential rainfall along its path. On 3 September 2009, visible satellite imagery showed the center of Erika as an exposed low level swirl moving out ahead of the mid level circulation and the area of heaviest rainfall (Figure 9).
Figure 2. An infrared satellite image from 25 August 2009 showing an African Easterly Wave exiting the west coast of Africa.
Figure 3. A 0800 UTC 28 August 2009 QuikSCAT image depicting a complete circulation elongated in a west to east direction with top winds around 30 MPH.
Figure 5. A Meteostat-9/GOES-11 combined image on 1 September 2009 depicting drier air, denoted by the orange and yellow colors representing Saharan Air Layer strength, to the west of Tropical Storm Erika.
Figure 7. The 0000 UTC 2 September 2009 St. Martin (TNCM) sounding depicting strong vertical shear with 30-kt winds out of the south at 200 mb or about 36,000 feet. Note the dry air at mid levels of the atmosphere. Image courtesy of the University of Wyoming Weather Web.
Figure 8. A visible satellite image on 2 September 2009 depicting a disorganized Tropical Storm Erika with the surface circulation to the west of the island of Guadeloupe and the deepest convection to the east of the Leeward Islands. Image courtesy of the U.S. Naval Research Laboratory.
The forecast called for Erika to impact the US Virgin Islands and Puerto Rico with torrential rains to arrive on the island Thursday evening. Due to the erratic movement and intensity changes, Erika dissipated before reaching our area, although, the rain arrived early Saturday morning. Most of the numerical guidance available had a fast bias with the storm. Only the Ensemble Tropical Rainfall Potential (eTRaP) (Figure 10) product, issued by the Satellite Analysis Branch (SAB), had a good handle on the timing of the precipitation indicating the rains would not arrive until Friday morning at the earliest.
Figure 11. Rainfall map for Tropical Storm Erika shows widespread totals of 3 to 5 inches over the Rio Grande de Loiza River basin.
c. Damage Statistics
Preventive action taken by the PR Aqueducts and Sewer Authority releasing water at the Carraizo dam in Trujillo Alto two days before the onset of heavy rains prevented additional flooding downstream along the Rio Grande de Loiza. The U.S. Virgin Islands fared much better with rainfall that averaged between 1 to 3 inches.
d. Forecast and Warning Critique
A flash flood watch was also issued simultaneously with the tropical storm warning. The local forecast called for 4 to 8 inches of rain beginning Thursday afternoon, but in reality, the bulk of the rain did not arrive until early Saturday morning on 5 September 2009, due to the slow and erratic movement of the system and the extreme difficulty forecasting the timing of the event.