July 2012: Earth's 4th warmest; update on 94L--a threat to the Antilles

July 2012 was the globe's 4th warmest July on record, according to the National Oceanic and Atmospheric Administration's National Climatic Data Center (NCDC). NASA rated it the 12th warmest. July 2012 global land temperatures were the 3rd warmest on record, breaking a streak of three months (April, May, and June) when global land temperatures were the warmest on record. July 2012 global ocean temperatures were the 7th warmest on record, and it was the 329th consecutive month with global temperatures warmer than the 20th century average. The last time global temperatures were below average was February 1985. Global satellite-measured temperatures in July for the lowest 8 km of the atmosphere were 7th or 5th warmest in the 34-year record, according to Remote Sensing Systems and the University of Alabama Huntsville (UAH). Wunderground's weather historian, Christopher C. Burt, has a comprehensive post on the notable weather events of July in his July 2012 Global Weather Extremes Summary.

Figure 1. Departure of temperature from average for July 2012. Most areas of the world experienced much higher-than-average monthly temperatures, including most of the United States and Canada. Meanwhile, Australia, northern and western Europe, eastern Russia, Alaska, and southern South America were notably cooler than average. Image credit: National Climatic Data Center (NCDC) .

El Niño watch continues
Sea surface temperatures increased to 0.8°C above average as of August 13 in the equatorial Pacific off the coast of South America. Ocean temperatures have been near or above the 0.5°C above average threshold needed for a weak El Niño event since the beginning of July. However, winds, pressures, and cloud cover over the region have not responded in the fashion typically associated with an El Niño, and NOAA's Climate Prediction Center (CPC) said that "The lack of a clear atmospheric response to the positive oceanic anomalies indicates ongoing ENSO-neutral conditions," in their August 9 El Niño discussion. They have issued an El Niño watch, and give a 71% chance that an El Niño event will be in place by September. El Niño conditions tend to decrease Atlantic hurricane activity, by increasing wind shear over the tropical Atlantic. Wind shear has been close to average over the tropical Atlantic since the beginning of hurricane season in June, though.


Figure 2. Arctic sea ice extent in 2012 (black line) compared to the previous record low year of 2007 (blue line) shows that 2012 is fast approaching all-time record territory. A big Arctic storm with a central pressure of 963 mb affected the ice during the first two weeks of August, causing a temporary downward spike in sea ice extent. Image credit: Danish Meteorological Institute.


Figure 3. View of the North Pole on August 17, 2012 from the North Pole Environmental Observatory shows plenty of melt water pools from the warm summer the North Pole has had.

Arctic sea ice falls to 2nd lowest extent in July, nears all-time record low during August
July 2012 Arctic sea ice extent reached its 2nd lowest July extent in the 35-year satellite record, according to the National Snow and Ice Data Center (NSIDC). During the first half of August, sea ice has undergone a spectacular decline, and we are on pace to break the all-time lowest sea ice extent record set in September 2007. As of August 17, the University of Bremen was showing that sea ice extent has already broken the all-time record; the Danish Meteorological Institute put the ice loss in 2nd place behind September 2007; and the National Snow and Ice Data Center put Arctic ice loss in 3rd place behind September of 2007 and 2011.

Update on 94L
A large tropical wave (Invest 94L) located a few hundred miles southwest of the Cape Verde Islands off the coast of Africa is headed west at 15 - 20 mph. This storm is a threat to develop into a tropical storm that will affect the Lesser Antilles Islands as early as Wednesday. The storm is under moderate wind shear of 10 - 20 knots, and is over waters of 28°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis. Satellite loops show that 94L has increased in organization this afternoon, with a growing amount of heavy thunderstorm activity and spin at middle levels of the atmosphere.


Figure 4. Afternoon satellite image of Invest 94L.

Forecast for 94L
The latest 2 pm EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, and ocean temperatures will fluctuate around 28°C over the next five days, as 94L tracks westwards towards the Lesser Antilles. As is typical with storms making the crossing from Africa to the Antilles, dry air to the north will likely interfere with development, and the SHIPS model predicts increased dry air as 94L approaches the Lesser Antilles. However, with shear expected to be low, dry air may be less of an issue for 94L than it was for Ernesto or TD 7. The storm should maintain a nearly due west track through Monday night, to a point near 50°W, about 700 miles east of the Lesser Antilles. At that point, a trough of low pressure passing to the north of 94L may be able to pull the storm to the northwest well to the northeast of the Lesser Antilles, as suggested by the latest 12Z (8 am EDT) run of the NOGAPS model. The 12Z UKMET model shows a more west-northwesterly motion resulting in a near miss of the Lesser Antilles on Thursday. Our two best performing models--the GFS and ECMWF--have both been taking 94L through the Lesser Antilles with every run for the past 24 hours, though. The latest 12Z run of both models now agree on the timing, with 94L arriving Wednesday night or Thursday morning. The BAMM model, which performed as well as the ECMWF and GFS at 5-day forecasts in 2011, is also showing a track through the Lesser Antilles. Given this agreement among our top three models for long-range forecasts, I give a 60% chance that 94L will pass through the Lesser Antilles. In their 2 pm EDT Tropical Weather Outlook, NHC gave 94L a 50% chance of developing into a tropical depression by Monday afternoon.

Jeff Masters