Following a record for the earliest formation of four named tropical systems in the Atlantic this hurricane season, there was a lull in tropical activity during July.
So far this hurricane season, with the formation of Ernesto Thursday, there have been four named tropical storms and one hurricane.
Both Tropical Storm Alberto and Tropical Storm Beryl formed before the official start of the hurricane season, June 1. Hurricane Chris and Tropical Storm Debby formed during June. When Debby formed on June 23, it marked the earliest development of four named storms in the Atlantic.
A waning La Niña pattern, which is more favorable for tropical storm development due to less frequent occurrences of high wind shear in development regions of the Atlantic, was present early in the season. La Niña is a phenomenon characterized by below-normal water temperatures in the central and eastern equatorial Pacific.
After an unusually fast start to the season, there were no named storms during July. However, it is not that unusual for no storms to be named in July. The average number of storms in the Atlantic in July over all the years of record keeping is 0.7.
"There are small factors in July that create a more hostile environment," AccuWeather Expert Senior Meteorologist Dan Kottlowski said. "July water temperatures are not consistently warm across the Atlantic basin. You still have lagging wind shear problems as well."
Compared to the past two years, water temperatures have been cooler and surface pressure is higher in the main development regions of the Atlantic. Both of these factors have been less favorable for tropical development in July.
Tropical systems thrive with warm water, so near- to below-normal temperatures in the Atlantic have not been presenting the best conditions for tropical formation.
Flooding and damage from Tropical Storm Debby in Tampa, Fla., late in June 2012. This photo was tweeted by @CarsonChambers.
With surface pressure above normal, it has been harder for tropical systems to spin up.
"It takes more time for pressure to lower [when the ambient surface pressure is higher], so it slows the development process down," Kottlowski explained. "The higher pressure has also been creating areas of stronger trade winds across the Atlantic and Caribbean, which in turn causes upwelling water."
Upwelling water occurs when the wind drives cooler water to the ocean surface. Both strong winds and cool water are unfavorable for tropical development, so this cycle has been reducing the opportunity for tropical development over the past month in the Atlantic.
Abundant dust in the atmosphere is another factor in the quiet July. Strong trade winds off of Africa stirred an abundance of Saharan dust into the atmosphere during June and July. Dust in the atmosphere is associated with large, dry air masses, and the tiny dust particles can remain suspended 25,000-35,000 feet above the surface.
The dust prevents intense thunderstorms from forming, which in turn does not allow the surface pressure to lower enough for tropical storms to wrap up.
It is important not to gain a false sense of security because of the slow July in the Atlantic. The worst is yet to come with a near-normal number of storms forecast for the 2012 Atlantic Hurricane Season.
Repeating and slow-moving storms will raise the risk of flash flooding and damaging winds over the northern and central High Plains into Thursday night.
Thunderstorms that have already brought the risk of severe weather to a portion of the mid-Atlantic states will continue track into the Northeast through Thursday night.
As July draws to a close, a storm system swinging up from the Deep South will bring downpours to the northeastern U.S. and break the back of an extended heat wave.
Rounds of showers and thunderstorms moving westward off the coast of Africa may pave the way for future tropical systems over the Atlantic Ocean in the weeks ahead.
Highs will run between 10 and 15 degrees Fahrenheit above average across much of the interior western United States into the upcoming weekend.
A budding tropical system threatens to bring flooding rain to the Philippines into this weekend with potential future impacts on China and Taiwan.
Hurricane Bertha formed 450 miles east of Jacksonville, FL. Maximum sustained winds of 75 mph with gusts to 90 mph.
Western Pacific (1990)
Typhoon Steve east of Iwo Jimo. Peak winds of 125 mph sustained gusts to 155 mph.
5-12" of rain north of Denver led to serious flash flooding (28th-29th). 108 mobile homes were destroyed and 481 others were damaged in Ft. Collins. 5 people were killed and 40 others injured.