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| A Monarch (Danaus plexippus) on Blazing Star (Liatris spicata) |
Will there be a surprise Atlantic hurricane this July?
Probably not.
It is mid-July, and we have yet to see a named storm form in the Atlantic Basin. The computer weather forecast models are not predicting any development through at least July 20, and if we make it to the end of the month without a named storm forming, it will be the first July since 2009 without a named July storm. |
| A pair of Great Spangled Fritillary (Speyeria cybele) Butterflies on Blazing Star |
Since the current active hurricane period began in 1995, 13 of 17 years (76%) have had a named storm form during July. The busiest July occurred in 2005, when five named storms and two major hurricanes formed. These included Hurricane Dennis and Hurricane Emily—the strongest hurricanes ever observed so early in the season.
Few Major Hurricanes Form in July
Only eight major hurricanes have formed in July since record keeping began in 1851. Most of the last half of July activity occurs in the Gulf of Mexico and waters off the Southeast U.S. coast. These type of storms form when a cold front moves off the U.S. coast and stalls, with the old frontal boundary serving as a focal point for development of a tropical disturbance (as happened with Alberto, Beryl, Chris, and Debby earlier this year).
There will be at least two cold fronts moving off the U.S. Mid-Atlantic coast over the next two weeks. The first of these fronts will push offshore around July 20, and weather forecasters will be watching the waters offshore of North Carolina for development then. Formation potential will be aided by ocean temperatures that are about 0.7° C (1° F) above average along the U.S. East Coast.
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| A Giant Swallowtail (Papilio cresphontes) on Butterfly Bush (Buddleja davidii) |
El Niño forming,
Atlantic Hurricane Activity Supressed
For two consecutive weeks, ocean temperatures of 0.5 - 0.6° C above average have been present in the tropical Eastern Pacific, which is the threshold for a weak El Niño episode. NOAA's Climate Prediction Center has issued an El Niño Watch, and gives a 61% chance that El Niño conditions will be present during the August - September - October peak of the Atlantic hurricane season. The likely development of a full-fledged El Niño episode means that Atlantic hurricane activity will likely be suppressed in 2012, due to the strong upper-level winds and high wind shear these events typically bring to the tropical Atlantic.
Wind shear: above average
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream has two bands of strong high-altitude winds that are currently bringing high wind shear to the Atlantic.
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| A pair of wasps mating, perhaps. . . piggyback, for sure. |
The southern branch (subtropical jet stream) is bringing high wind shear to the Caribbean, and the northern branch (polar jet stream) is bringing high wind shear to the waters offshore of New England. This configuration often leaves a "hole" of low shear between the two branches, off the Southeast U.S. coast and over the Gulf of Mexico.
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| Eastern Tiger Swallowtail (Papilio glaucus) on Butterfly Bush |
The jet stream is forecast to maintain this two-branch pattern over the coming two weeks. Wind shear has been about 10 - 20% higher than average over the first two weeks of July, and is predicted to be mostly above average for the coming two weeks. This will lower the odds of a July tropical storm forming.
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| Wasp eyes. |
Dry air: above average
There has been an unusual amount of dry, stable air in the Caribbean this year creating low levels of vertical instability. This has occurred due to a combination of dry air from Africa, and upper-atmosphere dynamics creating large areas of sinking air that dry as they warm and approach the surface. The Gulf of Mexico and tropical Atlantic between the coast of Africa and the Lesser Antilles have also seen low vertical instability this summer. June and July are the peak months for dry air and dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past two weeks. Expect dry air to be a major deterrent to any storms that try to form in the tropical Atlantic during July.
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| Twelve Spotted Skimmer Dragonfly (Libellula pulchella) |
Steering currents: average
The predicted steering current pattern for the next two weeks is a typical one for July. We have an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs are frequent enough and strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea.
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| An Arrow-Head Orb Weaver Spider (Verrucosa arenata) |
Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. There is no reliable forecast for what might happen during the peak months of August, September, and October—we might experience a repeat of the favorable 2010 and 2011 steering current pattern, which recurved most storms out to sea—or the unfavorable 2008 pattern, which steered Hurricanes Ike and Gustav into the Gulf of Mexico.
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| Banded Pennant Dragonfly (Celithemis fasciata) |
A below average chance of a July tropical storm
Given that none of the computer models are forecasting tropical storm formation in the coming seven days, sea surface temperatures are only slightly above average, and wind shear and vertical stability are above average. . . the chance of a named tropical storm forming is likely 30% or less for the remainder of July.