Tropical Storm Erika

1-4 September 2009

Luis Rosa

Orlando Bermudez

Jeffrey Cupo
National Weather Service San Juan
30 October 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.

 

a.                   Synoptic History

 

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 4. A depiction of the first Hurricane Hunter mission flown on 1 September 2009 into Tropical Storm Erika. The Stepped Frequency Microwave Radiometer (SFMR) data overlaid on Google Earth imagery shows Tropical Storm Erika having a broad circulation with possible multiple centers and the strongest winds confined to the northeast of the center.

 

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 6. The 0000 UTC 2 September 2009 Guadeloupe (TFFR) sounding depicting strong vertical shear with 30-kt winds out of the south at 200 mb or about 36,000 feet.   Note the very dry air at mid levels of the atmosphere. Image courtesy of the University of Wyoming Weather Web.

 

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.

Figure 9. A visible satellite loop on 3 September 2009 depicting the center of Tropical Storm Erika as an exposed low level swirl moving out ahead of the mid-level circulation and area of deepest convection.
 
 

b.                   Rainfall

 

 
Erika behaved like a mesoscale convective system (MCS) throughout its lifetime with the convection lasting several hours, cycling up and down depending on the time of day. Figures 8 and 9 above show the round shape typically observed with an MCS. An MCS tends to develop at night when radiative cooling takes place on the cloud tops, allowing them to become more unstable. As the sun comes up in the morning, it warms the cloud tops and caps them. Outflows then become more dominant, cutting off the influx of moisture resulting in weakening of the convection.

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 10. An 18-hr forecast from the 1800 UTC 3 September 2009 run from the Ensemble Tropical Rainfall Potential (eTRaP) for Erika suggested the associated rainfall would hold off until Friday afternoon on 4 September 2009.
The Remnants of Erika produced torrential rainfall that led to minor to moderate flash flooding across the Rio Grande de Loiza River basin on 5 September 2009. Erika produced widespread rainfall totals (Figure 11) of 4 to 8 inches over south central and southeast Puerto Rico with a maximum of 10.26 inches (Figure 12) in Las Piedras.

 

Figure 11. Rainfall map for Tropical Storm Erika shows widespread totals of 3 to 5 inches over the Rio Grande de Loiza River basin.

Figure 12. Storm Total Precipitation as estimated from WSR-88D San Juan radar shows 10+ inches of rain over Las Piedras.
 

c.                   Damage Statistics

 

 
Minor to moderate flash flooding occurred along two main tributaries of the Rio Grande de Loiza River basin, the Caguitas and Gurabo rivers, and along the Rio Grande de Loiza River itself. Several homes were flooded across Caguas, Carolina, Cayey and Fajardo in southeast Puerto Rico with a report of a mudslide, resulting in the collapse of a house in Caguas. No injuries or deaths were reported.
 

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

 
Genesis
The genesis of Erika was well anticipated. NHC gave the system a low chance of developing into a tropical cyclone as it emerged off the coast of Africa and was then upgraded to a medium, and eventually to a high chance about a day before it developed.
Track
The track forecast for Erika was one with much higher than average uncertainty as a series of TUTTs and weak migratory highs across the Atlantic resulted in very weak and ill-defined steering currents. This, in addition to the weak organization of the storm, allowed Erika to make random jumps as the storm tried to reform near the heaviest thunderstorm activity. This higher-than-average uncertainty and erratic behavior were well anticipated by NHC and the NWS San Juan. This erratic behavior set the stage for a difficult storm track forecast, even as the storm was only a day away from land.
Intensity
The intensity forecast for Erika was also one with much higher than average uncertainty. The dynamical models, Geophysical Fluid Dynamics Laboratory (GFDL) and Hurricane Weather Research and Forecasting (HWRF) consistently predicted that the storm would attain hurricane status as they tried to develop an anticyclone on top of the storm. On the other hand, the global models Global Forecast System (GFS), European Center for Medium range Weather Forecasting (ECMWF), and Canadian models forecast the storm to maintain minimal tropical storm status. The National Hurricane Center (NHC) official guidance initially had the storm attaining strong tropical storm status, but with the unfavorable environmental conditions the guidance was adjusted for a minimal tropical storm. The storm ended up dissipating on 3 September 2009 south of Ponce, Puerto Rico.
Watches and Warnings
A tropical storm watch was first issued for both Puerto Rico (PR) and the U.S. Virgin Islands (USVI) at 5 pm on 2 September 2009 or about 18 hours before the expected arrival of tropical storm force winds. A tropical storm warning was issued for both PR and the USVI at 11 am on 3 September 2009, only to be dropped six hours later, as the system showed further weakening and reconnaissance aircraft no longer showed 34-kt winds anywhere near the storm.
 
The issuance of a tropical storm watch was appropriate as the system still maintained tropical storm force winds and was expected to persist through the forecast period. A tropical storm warning was issued on 3 September 2009 as Erika’s winds were 24 hours away from PR and the USVI. This warning, however, was cancelled six hours later as visible imagery (Figure 9) showed the low-level center decoupling from the mid-level center. 

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.

 

Conclusion

 

Tropical Storm Erika presented forecasters at both the National Hurricane Center (NHC) and NWS San Juan with an extremely challenging forecast regarding every aspect of the storm, including its track, intensity, and event impact timing. NWS San Juan began alerting customers, including the media, through Hurricane Local Statements (HLS). The first hurricane local statement was issued four days ahead of the storm and hours before the National Hurricane Center (NHC) started issuing advisories on the storm. The HLS emphasized the high degree of uncertainty involved in the intensity, impact timing, and mentioned rain and flash flooding as the primary hazard from the storm. Forecasts from NWS San Juan were consistent in predicting rainfall amounts of 4 to 8 inches, but the timing of the rain ended up being delayed by over 24 hours.

 

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