August 2021

Monthly Diagnostics of Climate Events for the RCC-Washington Region

(i) Temperature

During the month of August, maximum temperatures (Tmax) were near average across the entire Caribbean (Fig. 1). Mean minimum temperatures (Tmin) were near average throughout the Caribbean as well, except for Jamaica which exhibited 1-2°C negative anomalies and eastern Puerto Rico which exhibited 1-2°C positive anomalies (Fig. 2).

In Mexico, Tmax was close to normal, with only a few small scattered locations with deviations of 1-2°C from average. (Fig. 1). In Central America, Tmax averaged near or above normal. Portions of eastern Guatemala, western El Salvador, and Honduras recorded positive Tmax anomalies of 1-4°C. In Mexico, Tmin was near average in the majority of the country. In the northwest, Tmin along either side of the Gulf of California peninsula were above normal by 1-4°C (Fig. 2). There were several other patches throughout the country with 1-2°C positive anomalies. Southern Guatemala and El Salvador registered positive Tmin anomalies of 1-4°C. Conversely, portions of Nicaragua and northern Costa Rica observed 1-4°C negative Tmin anomaly, while eastern Panama and small parts of southern Honduras observed 1-2°C anomaly.

(ii) Precipitation

Rainfall performance was poor across northern portions of the Caribbean, with a swath of larger tropical cyclone enhanced totals just south of the big islands. The largest monthly totals (200-500mm) were observed along the southern coasts of Hispaniola and Cuba, and over Jamaica (Fig. 3). Lighter amounts less than 100mm were observed along northern Hispaniola and Cuba as well as most of the Bahamas and Turks and Caicos. Robust totals of 75-150mm were observed across the Lesser Antilles. These amounts widely exceeded averages by as much as 100mm (Fig. 4). Significant rainfall deficits resulted from the pattern over Cuba and the northern two thirds of Hispaniola. Negative anomalies were as much as 100-200+mm. The Bahamas and Turks and Caicos registered 25-100mm deficits as well.

In Central America, heavy rainfall was widespread along the Pacific coast, but totals were suppressed in many inland and Atlantic-facing areas. As much as 500mm of rainfall was observed in southern Guatemala, the Gulf of Belize region, the Gulf of Fonseca region, and in parts of Panama (Fig. 3). Parts of central Honduras, Nicaragua, and northern Belize received less than 75mm. These amounts were below average in central Nicaragua, northern Guatemala, Belize, northern El Salvador and Honduras, with negative anomalies of 25-100+mm (Fig. 4). Rainfall was above average by more than 100mm in southern Nicaragua and Costa Rica. In Mexico, the heaviest rainfall (more than 300mm and locally more than 500mm) was recorded along the Pacific Rim and in parts of Oaxaca and Veracruz States. Monthly total amounts of well in excess of 100 overspread much of the southern half of Mexico. Many areas in southern Mexico and the Yucatan Peninsula received less rainfall than normal, registering negative anomalies of 100-300mm. Similar deficits emerged in northwest Mexico, including parts of Sonora, Sinaloa, Durango, and Chihuahua States. Smaller deficits were observed in the Northeast, while significant positive anomalies emerged along the southwest coast. (iii) Notable Events

Three tropical cyclone affected the RCC region during August (Fig. 5). The first was Fred, which became a named tropical storm on August 11. Fred brought rains and breezy conditions to the Leeward Antilles before tracking across the Caribbean and making landfall in Hispaniola late on August 11. It brought heavy rain to the southern part of the island and its remnants also impacted Cuba. Following closely on Fred’s heals was Grace. The storm was first named on August 14, but varied in strength throughout its lifetime, topping out as a category 1 hurricane. Grace traversed the northern Caribbean and made landfall on Hispaniola on August 16 - only two days after the powerful earthquake in Haiti. Grace continued westward - twice making landfall in Mexico. The first was in Tulum, along the Yucatan Peninsula, and the second was near Tecolutla in Veracruz State as a much stronger Category 3 storm. In addition to coastal power outages, very heavy rain in that region, up to 180mm in 24 hours, caused flooding and mudslides which killed seven people. Finally, Hurricane Ida impacted western Cuba. The storm made landfall there on August 27 as a category 1 hurricane with winds of 130 km/hr. The Havana Times reported no fatalities, but did report crop losses, power outages, and damages to homes and properties. Damage was not ‘highly serious’ according to local officials.

(iv) Sea Surface Temperature and Circulation

During August, sea surface temperatures (SST) in much of the equatorial East and Central Pacific were slightly below average, but were slightly warmer than average in the far- eastern Pacific. SST anomalies generally remained between -1.0°C and 0.0°C with an area of 0.0-1.0°C positive anomaly near the coast of Ecuador. The Niño3.4 index dipped in the middle of the month to -0.9°C, but ended August with a value of -0.3°C. ENSO Neutral conditions are present, but a La Nina watch is in place according to NOAA’s Climate Prediction Center, indicating that La Nina conditions are favored to emerge during the next few months. There is a 60% chance for ENSO neutral conditions persisting through the end of the July-September season and a 70% chance for La Nina by November-January.

Narrowing the focus to the RCC region, SST was mostly near normal values for another month across the greater Caribbean region. However slightly warmer than average SST was observed in the western Half of the Gulf of Mexico. (Fig. 6). Positive SST anomalies also surrounded the Baha California Peninsula. 1-2°C anomalies were observed.

August’s 850mb circulation pattern featured a region of modestly anomalous cyclonic circulation covering central Mexico. Westerly wind anomalies were present across southern Central America. Meanwhile, general easterly anomalies were present over the northern Caribbean. Along and to the north of Feed and Grace’s tracks. A region of lower-level difluence may have contributed to suppress rainfall over Central America. (Fig. 7).

At the 200mb level, a robust anomalous cyclonic circulation was observed over and to the north of Greater Antilles (Fig. 8). This may have contributed to suppressed precipitation over Hispaniola, Mexico, and the Bahamas. Westerly anomalies were observed over northern Mexico.

Figure 1. Gridded mean maximum temperature anomaly (°C) during the month of August. Anomalies are computed with respect to the 1991-2020 base period.

Figure 2. Gridded mean minimum temperature anomaly (°C) during the month of August 2021. Anomalies are computed with respect to the 1991-2020 base period.

Figure 3. Satellite-estimated rainfall total (mm) during the month of August, 2021.

Figure 4. Satellite-estimated rainfall anomaly (mm) during the month of August, 2021. Anomalies are computed with respect to the 1998-2012 base period.

Figure 5. Plot of the tracks of the 3 tropical cyclones that impacted the RCC region during August. Size of the circle indicates relative strength of the storm. Data form National Hurricane Center best track archive.

Figure 6. Average sea surface temperature (SST) anomalies (°C) for the month of August 2021. Anomalies are computed with respect to the 1991-2020 base period.

Figure 7. 850mb mean vector wind anomalies for the month of August. Anomalies are computed with respect to the 1981-2010.

Figure 8. 200mb wind vector anomaly for the month of August. Anomalies are computed with respect to the 1981-2010.

9 September 2021 Update prepared by Climate Prediction Center / NCEP