Hottest U.S. summer in 75 years; La Niña is back; Nate and Maria update

The U.S. had its hottest summer in 75 years, and Texas smashed the record for hottest summer ever experienced by a U.S. state since record keeping began in 1895, NOAA's National Climatic Data Center said yesterday. The June - August average temperatures in Texas and Oklahoma were a remarkable 1.6°F and 1.3°F warmer than the previous hottest summer for a U.S. state, the summer of 1934 in Oklahoma. Texas' summer was 2.5°F hotter than their previous hottest summer, in 1998. Louisiana had its hottest summer on record in 2011, and the 4th hottest summer for any U.S. state since record keeping began in 1895. For the U.S. as a whole, the summer of 2011 was the 2nd warmest summer on record, just 0.1°F behind the notorious Dust Bowl summer of 1936. Fifteen states had a summer average temperature ranking among their ten warmest. Washington and Oregon were the only states across the lower 48 to have below-average summer temperatures. Texas also had its driest summer on record, with rainfall 5.29 inches (134.4 mm) below the long-term average, and 1.04 inches (26.4 mm) less than the previous driest summer in 1956. New Mexico had its second driest summer, Oklahoma its third driest summer, and New Jersey and California had their wettest summers on record. The U.S. Climate Extremes Index (CEI), which is sensitive to climate extremes in temperature, rainfall, dry streaks, and drought, indicated that an area nearly four times the average value was affected by extreme climate conditions during summer 2011. This is the third largest summer value of record, and came on the heels a spring season that was the most extreme on record. The CEI goes back to 1910.


Figure 1. Average temperatures for the summer in Texas and Oklahoma, at 86.8 degrees F (30.4 degrees C) and 86.5 degrees F (30.3 degrees C), respectively, exceeded the previous seasonal statewide average temperature record for any state during any season. The previous warmest summer statewide average temperature was in Oklahoma, during 1934, at 85.2 degrees F (29.6 degrees C). Image credit: National Climatic Data Center.

More bad news for Texas: La Niña is back
La Niña, which contributed to extreme weather around the globe during the first half of 2011, has re-emerged in the tropical Pacific Ocean and is forecast to gradually strengthen and continue into winter, NOAA announced yesterday. Over the past two weeks, ocean temperatures have cooled significantly in the Equatorial Pacific off the coast of South America. Ocean temperatures in the region 120°W - 170°W and 5°S - 5°N, called the Niño 3.4 region, were 0.6°C cooler than average over the first week of September. The threshold for a weak La Niña is temperatures 0.5° cooler than average, so we are now experiencing weak La Niña conditions. Drought conditions are common over the southern tier of states during a La Niña event, since the cooling of the equatorial Pacific waters usually pushes the jet stream such that rain-bearing low pressure systems pass through the Midwest and avoid the South. It is likely that the drought gripping Texas, Oklahoma, and New Mexico will continue well into 2012, due to the emergence of La Niña. La Niña events also typically cause wetter than normal winters in the Pacific Northwest and Ohio Valley, colder winters in the Pacific Northwest and northern Plains, and warmer temperatures in the southern states.


Figure 2. Departure of Sea Surface Temperature (SST) from average on September 8, 2011. Cooler than average waters in the equatorial East Pacific signify the emergence of La Niña. In the Atlantic, SSTs remain very warm in the Main Development Region between the coast of Africa and Central America, including the Caribbean. The Gulf of Mexico is cool where Tropical Storm Lee stirred up the water, and the waters off the Mid-Atlantic and New England coasts are cool due to the passage of Hurricane Irene. Image credit: NOAA/NESDIS.

Tropical Storm Nate
Tropical Storm Nate continues to remain nearly stationary in Mexico's Bay of Campeche. Latest visible satellite loops show that Nate is having trouble with the dry air to its north, which is getting wrapped into the circulation and interfering with intensification. Since the storm is stationary, it is upwelling cooler waters from the depths that are also slowing intensification. Wind shear has fallen to the low range, near 5 knots, so once Nate manages to wall off the dry air to its north and begin moving away from the pool of cool water beneath it, steady intensification should occur. Nate probably has time to intensify to a strong Category 1 hurricane, and perhaps a Category 2 hurricane, before making landfall Sunday in Mexico. The main hazard to Mexico will probably be very heavy rains that will cause flash flooding and mudslides. Thanks to last night's dropsonde mission by the NOAA jet, the computer models have now come into much better agreement on the future path of Nate. A ridge of high pressure is expected to build in to the north of the storm, forcing it westward or southwestward to a landfall in Mexico. Nate is too far south to be turned northwards towards Louisiana, as some model runs were suggesting yesterday.

Nate is a small storm, and is not likely to bring significant rains to Texas; only extreme South Texas near Brownsville could see an inch or so of rain over the weekend from an outer spiral band of Nate. Our latest wundermap wind forecast map from the European Center model, with the fire layer turned on, shows that Nate's wind field on Saturday and Sunday will not be large enough to fan the fires burning in Texas.


Figure 3. True-color MODIS image of Tropical Storm Nate taken at 1:40 pm EDT Thursday September 8, 2011. At the time, Nate was a tropical storm with 60 mph winds. Image credit: NASA.

Tropical Storm Maria
Tropical Storm Maria is not changing much in intensity it bears down on the Lesser Antilles Islands, data from an An Air Force Reserve hurricane hunter aircraft currently investigating the storm reveal. Top winds found by the aircraft at their flight level of 5,000 feet were just 48 mph as of 10am EDT, though some stronger surface winds were observed by their SFMR surface wind instrument. Satellite loops show that Maria's heavy thunderstorms have steadily increased in areal coverage and intensity this morning. Maria has grown into a very large tropical storm, and will bring heavy rains and strong gusty winds to nearly all of the islands in the Lesser Antilles. There is still a moderate 10 - 15 knots of wind shear affecting Maria, and this is slowing down intensification. Maria passed just south of buoy 41101 this morning. Sustained winds at the buoy ranged from 22 - 37 mph this morning, and the pressure dropped to 1003 mb. Martinique radar shows heavy rains from Maria are now affecting the islands, but the thunderstorms are not well-organized into spiral bands.

The intensity forecast models predicts steady strengthening for Maria, and I think likely that Maria will be a tropical storm with 50 - 60 mph winds in the Virgin Islands and Puerto Rico on Saturday, and develop into a Category 1 hurricane shortly after pulling away from Puerto Rico on Saturday night or Sunday morning. The northeastern portion of the Dominican Republic with get heavy rains from Maria, but not tropical storm-force winds. The computer models are unified on taking Maria across the Northern Lesser Antilles, Virgin Islands, and close to eastern Puerto Rico, but then diverge on how strong the steering influence a trough of low pressure forecast to move off the U.S. East Coast early next week will have. The majority of the models predict that the Turks and Caicos Islands and the Bahamas will miss seeing the core of Maria, and the storm will curve to the northwest and then north between the U.S. East Coast and Bermuda on a track that would likely take Maria near Newfoundland, Canada. However, two models--the very reliable ECMWF, and the less reliable NOGAPS--predict that Maria will not get picked up by the trough, and instead will plow straight through the Turks and Caicos Islands and Bahamas towards Florida. Given that the ECMWF model predicts an unrealistically weak storm and the NOGAPS model was our poorest-performing major model last year, I believe a more northerly path missing the Turks and Caicos Islands and Bahamas is more likely. We need a dropsonde mission by the NOAA jet to help reduce some of the track uncertainty, but unfortunately we have only one such airplane, and it is tied up flying missions for Tropical Storm Nate in the Gulf of Mexico.

Lee's rains trigger historic flooding in New York and Pennsylvania
Rivers in New York and Pennsylvania swollen by record rains from the remains of Tropical Storm Lee have mostly crested and are on their way down this morning, but it will likely be another day before many of the 120,000 people evacuated from the historic floods can return to their homes. Flooding in many areas of Pennsylvania and New York exceeded that of Hurricane Agnes of 1972, which did $11.8 billion in damage (2010 dollars), and was the costliest hurricane in Pennsylvania's history. Binghamton, New York received 8.48" of rain in the 24 hours ending at 8 am EDT yesterday. This is nearly double the previous all-time record set just last year, when 4.68" fell on Sep 30 - Oct. 1, 2010. Binghamton has also already broken its record for rainiest year in its history; records go back to 1890. The Susquehanna River at Binghamton crested at 25.71', its highest level since records began in 1847, and spilled over the flood walls protecting the city. Rainfall amounts in Pennsylvania were even greater, with Harrisburg receiving 13.30", and a storm-maximum 15.37" falling in Elizabethtown, Pennsylvania. In Wilkes-Barre, PA, the Susquehanna River crested at 38.83' at 1:45 am this morning, just below the 41' flood wall protecting the city. The flood walls were 37' back in 1972 when Hurricane Agnes' rains pushed the Susquehanna River to 41', flooding the downtown area with 9' of water, damaging or destroying 25,000 buildings and causing $1 billion in damage. It's a good thing the flood walls were raised to 41 feet, or else a repeat disaster would have occurred. The extreme rains were due to the slow-moving remains of Tropical Storm Lee as it interacted with a stationary front draped along the Eastern U.S. Adding to the potent moisture mix was a stream of tropical moisture associated with Hurricane Katia that collided with the stationary front.


Figure 4. The Susquehanna River at Binghamton has crested this afternoon at its highest flood height on record, 25.71'. Records at this gauge go back to 1847. Image credit: NOAA/AHPS.

I'll have an update this afternoon.

Jeff Masters