Tropical update; 2nd warmest July on record

Heavy thunderstorms continue over the waters east of Florida this morning along a broad area of low pressure that has developed from the remains of an old cold front. Wind shear is 10-15 knots in a narrow band along this old front, which is low enough to allow some development to occur. Visible satellite imagery from this morning shows that this disturbance is poorly organized, with no signs of a surface circulation and only a moderate amount of heavy thunderstorm activity. I expect the Hurricane Hunter mission scheduled for today into this system will be canceled.

Development along old fronts is usually slow, so the earliest we should expect a tropical depression to form in this area is Wednesday. There is a significant amount of wind shear on either side, plus plenty of dry air to the north, so anything that develops will likely struggle to intensify.

The computer models are forecasting that any low that forms along the old front will begin moving northwestwards towards the Carolinas over the next three days, in response to a trough of low pressure swinging across the eastern U.S. When the trough moves out to sea on Thursday, high pressure is forecast to build back in, forcing the system back towards the west, or leaving it nearly stationary off the East Coast. None of the computer models forecast that the storm will grow to anything stronger than a 40-mph tropical storm.


Figure 1. Current satellite of the Florida region.


Figure 2. Preliminary models tracks for the East Coast disturbance.

Wave off of Africa
The tropical wave that moved off the coast of Africa yesterday is near the Cape Verde Islands today. The wave is looking much less organized this morning, with lower thunderstorm activity. It is crossing into cooler waters of only 26C, which may be partly responsible for this loss of thunderstorm activity. Wind shear is about 10-15 knots, which could allow some slow development, but I don't expect that this wave will develop. Water temperatures in front of it are even cooler, and wind shear is forecast to be marginal. None of the computer models develop this system.

Second hottest July on record in the U.S.
July 2006 was hotter than the scorching summer of 1988 across the U.S., and was the second warmest July on record, according to data released by the National Climatic Data Center yesterday. The average temperature of 77.2�F was second only to the Dust Bowl year of 1936, which had an average temperature of 77.5�F. January to July has been the warmest such year-to-date period on record. The nationally averaged year-to-date temperature was 55.3�F, or 3.2�F above the 1901-2000 average. The previous record of 54.8�F was set in 1934. The July heat wave broke more than 2,300 daily temperature records for the month and eclipsed more than 50 records for the highest temperatures in any July. The heat was most extreme in South Dakota, where temperatures averaged 10�F above normal in some areas. The all-time state temperature record of 120�F set in 1936 was matched on July 15. July of 2006 ranked as the 26th driest July in the 112-year record, and severe drought conditions from Texas northwards to the Dakotas have been responsible for billions of dollars in agricultural losses.


Figure 4. Rank by county of temperatures for July 2006.

Is it Global Warming?
Is this year's heat wave due to global warming? Well, you can't blame one hot summer in one country on global warming. When one plots up the average U.S. July temperature from 1895-2006 (Figure 5), it is apparent that there has been about a 1�F warming in July temperatures since the late 1800's. However, there is a lot of natural variability. While it has been very hot in July during the past 20 years, there was a period in the 1930s was was equally hot. Nevertheless, the globe has warmed about 1.4�F in the past since 1970, and this trend has continued this year. Globally, July 2006 was the third warmest July on record since records began in 1880 (1.01�F/0.56�C above the 20th century mean) and the sixth warmest year-to-date (January-July) (0.92�F/0.51�C).

An overwhelming majority of climate scientists believe that a significant portion of the warming since 1970 is due to anthropogenic (human) causes, and is likely to accelerate over the next few decades. Assuming that the forecasts of these scientists are correct, we should expect summers like the Summer of 2006 to be commonplace 10-15 years from now. I know I'm planning on doing a lot more swimming in Lake Superior in coming summers, something I've never been able to do until this year.


Figure 5. Average July temperature for the U.S. from 1895-2006. Image credit: National Climatic Data Center.

Sea ice continues to shrink
Santa's shop on the North Pole Ice Cap continues to grow more endangered. Northern Hemisphere sea ice extent, as measured from passive microwave instruments onboard NOAA satellites, was the lowest for any July on record in 2006 at 8.7 million square kilometers (Figure 6). The previous July low extent record was set in 2005 with 9.1 million square kilometers.


Figure 6.Departure from normal of sea ice extent over the North Pole for July, 1979 - 2006. Image credit: National Climatic Data Center.

I'll be back Wednesday with my Atlantic tropical outlook for the rest of August.

Jeff Masters