August 2020

Monthly Diagnostics of Climate Events for the RCC-Washington Region

(i) Temperature During the month of August, mean maximum temperatures (Tmax) were near average across the islands of the Caribbean (Fig. 1). A small portion of Hispaniola observed 1-2°C positive anomalies. Minimum temperatures (Tmin) were generally slightly warmer than average in Cuba, which observed positive anomalies of 1-2°C (Fig. 2). Outside of a small portion of Hispaniola, the rest of the Caribbean region was near average. Northern parts of Mexico experienced much-above-normal Tmax. An area centered over Chihuahua State registered positive anomalies as large as 4-6°C (Fig. 1). Southern and eastern Mexico observed mostly near-normal Tmax. A small area of Jalisco and Guanajuato states observed negative Tmax anomalies (1-4°C). In Central America, positive anomalies reached 2-4°C in eastern Guatemala, Honduras, western El Salvador, and northern Nicaragua. Lesser positive anomalies (1-2°C) were present in eastern Honduras, northeast Nicaragua, and Costa Rica. Tmin was well-above normal in northwestern Mexico. The largest anomalies (4-6°C) were observed in Sonora State (Fig. 2). Most of the rest of the country was also warmer than normal. The northern half of Central America observed very warm Tmin. The largest positive anomalies (2-6°C) were located from southern Guatemala through northwest Nicaragua. Conversely, southern Nicaragua and northern Costa Rica exhibited 1-4°C negative anomalies.

(ii) Precipitation In the Caribbean, rainfall was enhanced for many of the Lesser Antilles during August. Monthly totals exceed 100mm in some cases and reached 200mm for several of the southern Islands (Fig. 3). Heavy rain also impacted Hispaniola, Cuba and the Bahamas. More than 200mm, and even 300mm of rainfall was reported. While this led to positive monthly anomalies (>100mm) in the Bahamas and southern Hispaniola, northern Hispaniola and Cuba still registered deficits of as much as 100mm or more (Fig. 4). More moderate, but still mostly above-average, rains were received in the windward Antilles. Very heavy rain was observed along the west coast of mainland Mexico during August. Monthly totals exceeded 300mm, and even 500mm (Fig. 3). Parts of southern Mexico and the Yucatán Peninsula received similar totals. The Baja California peninsula and north-central Mexico received 25mm or less of rainfall. The wettest areas along Mexico’s west coast had surpluses greater than 100mm (Fig. 4). Chihuahua, Durango, and Sinaloa states registered the largest deficits of more than 100mm. In Central America, the heaviest rainfall, exceeding 500mm, occurred in El Salvador, eastern Honduras, parts of Costa Rica and Panama. Besides small portions of northern Belize and northwestern Honduras, the rest of the region observed at least 100mm of rainfall. The resulting pattern was positive anomalies of as much as 100+mm for much of the region, but 100mm negative anomalies or more in localized parts of southern Nicaragua, eastern Costa Rica, western Panama, Belize, and the Gulf of Honduras region.

For more information, visit: http://www.cpc.ncep.noaa.gov/products/international/usrcc/carib_cm.shtml

(iii) Notable Events

Hurricane Laura impacted the Caribbean with damaging winds and flooding rains during late August (Fig. 5). Laura formed in the Atlantic main development region and passed over the Lesser Antilles as a tropical storm causing minor flooding and power outages. It made landfall in southern Dominican Republic. Up to 300mm of rainfall was recorded in southern Dominican Republic along with damaged homes, power outages, and fatalities across the island. Next, still as a tropical storm, Laura brought similar condition, but no fatalities, to southeastern Cuba. Wind gusts reached 146 km/hr there.

Another tropical system, tropical storm Marco, affected southern Mexico on August 20. Veracruz, Oaxaca and Chiapas states were the worst affected. Hundreds of homes were flooded and water rescues took place. Escuintla in Chiapas recorded 186.4mm of rain in 24 hours from Marco. Other places experiencing detrimental impacts from heavy rain include Trinidad and Tobago where 120mm of rain in 9 hours on August 8 triggered floods and landslides. Flash floods in Panama on August 8-9 killed 11 people. Also, heavy rainfall from August 8-10 caused flooding and landslides in Veracruz, Mexico.

(iv) Sea Surface Temperature and Circulation

During August, sea surface temperatures (SST) in the equatorial East and Central Pacific remained below average and continued to cool. Negative mean monthly anomalies of 0.5°C to 2.0°C were observed along the equator. The Niño3.4 index resided at a minimum value of -0.9°C by the end of the month. With a response in atmospheric circulation now evident, NOAA’s Climate Prediction Center declared a La Niña Advisory on September 10 indicating the presence of an ongoing La Niña. The La Niña is favored to persist through northern hemisphere winter (~75% chance).

Narrowing the focus to the RCC region, above-normal SST was widespread throughout

the Gulf of Mexico, Caribbean, and Atlantic during August. Positive SST anomalies of 0.5-1.5°C were ubiquitous in those regions (Fig. 6). Larger anomalies (1.0-2.0°C) were observed in the Atlantic along 30°N lat. The East Pacific exhibited a swath of positive anomalies (0.5-1.5°C) between 10°N and 15°N lat, negative anomalies along the equator, and near-average SST in-between.

August’s 850mb circulation pattern featured westerly wind anomalies over southern

Mexico and Central America (Fig. 7). This pattern likely helped to enhance rainfall along the Pacific coast in those regions. Easterly wind anomalies were present in central Mexico and the Baja California peninsula. Wind anomalies were small and variable across the Caribbean.

At the 200mb level, a large anomalous trough was present over northern Mexico and

the Gulf of Mexico (Fig. 8). This may have helped suppress precipitation in north-central and northeast Mexico. Upper-level easterly wind anomalies were present across the Caribbean and Central America.

Figure 1. Gridded mean maximum temperature anomaly (°C) during the month of August

2020. Anomalies are computed with respect to the 1981-2010 base period.

Figure 2. Gridded mean minimum temperature anomaly (°C) during the month of August 2020. Anomalies are computed with respect to the 1981-2010 base period.

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

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

Figure 5. Tropical Storm and Hurricane force wind swath history for hurricane Laura (Aug 20-29) Image and analysis from the National Weather Service – National Hurricane Center.

Figure 6. Average sea surface temperature (SST) anomalies (°C) for the month of August 2020. Anomalies are computed with respect to the 1981-2010.

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.

11 September 2020

Update prepared by Climate Prediction Center / NCEP