Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 11:37 AM GMT on August 01, 2006
It's August in the tropics, the first of the peak months of hurricane season. Befitting the arrival of August comes the arrival of Tropical Storm Chris, which formed this morning just east of the Leeward Islands. The formation of Chris came in defiance of significant adversity--wind shear was 20-25 knots last night when the storm formed into a tropical depression, and is still a rather hefty 15-20 knots. Considerable dry air lies to Chris' north, and strong upper-level winds from the north are acting to push this dry air into Chris' core, keeping the storm from intensifying much. Radar out of Martinique shows a decent band of thunderstorm to the storm's southeast and east, but no thunderstorm activity on the northwest side where shear and dry air are impacting the storm. We don't have a recent QuikSCAT pass to gauge the winds, but two passes by a satellite equipped with a microwave sensor came up with an estimate of 40 mph surface winds. The Hurricane Hunters are scheduled to visit at 2pm EDT this afternoon, so we'll know more then.
This shear and dry air will continue to affect Chris over the next two days. The shear is forecast to gradually weaken, which may allow some slow intensification. Chris must tightrope walk a very narrow path between two upper-level cold lows in order to strengthen significantly. One of these cold lows is just north of the Bahamas, and the other is northeast of Chris. These lows are forecast to move slowly west-northwest in tandem with Chris, and if Chris can stay exactly between them, low enough wind shear exists to potentially allow some strengthening. Any deviation from this scenario will put Chris under hostile wind shear, which will act to limit intensification or even dissipate the storm.
Last night's computer model runs did not start out with a very good initial picture of the current strength of Chris, and dissipated the storm within 72 hours. We need to wait until the next set of model runs based on this morning's 8am EDT (12Z) data are available before taking much stock in both the track and intensity forecasts of the models. The NOAA jet is scheduled to fly the storm tonight, so the best model data for Chris will be available Wednesday morning.
Chris will bring heavy rains and high winds to the Leeward Islands today, primarily to those islands lying to the south of the storm's center, where dry and and wind shear are less of a problem. Puerto Rico should get a good soaking on Wednesday, and after that, the prognosis is very uncertain. Chris could become a hurricane late in the week, but I put the chances of this at 10%. Dissipation is a more likely scenario, since there is so much wind shear around. The most likely scenario of all is that Chris will remain a tropical storm over the next five days.
August hurricane outlook
As we've seen repeatedly, sea surface temperatures are important for hurricane formation and intensification, but nothing happens unless the wind shear is low. When we look at wind shear, the standard measure is the wind at the upper atmosphere (200 mb, usually around 40,000 feet in altitude) minus the wind just above the surface (850 mb, or about 5,000 feet altitude). This difference in wind speed is plotted in Figure 1 for July 2006. The key feature to look at is the anomalies in the bottom portion of the plot. The entire tropical Atlantic was under higher than normal wind shear during July, with wind shear up to 6-8 meters per second over the Caribbean (12-16 knots). The only region that experienced below normal wind shear was off the coast of North Carolina. Not coincidentally, this is where July's only named storm formed (Beryl).
Figure 1. Observed wind shear for July 2006. The top portion shows the difference in wind between 200 mb and 850 mb pressure levels, and the bottom image shows the departure from normal.Image credit: NOAA's Climate Prediction Center.
What, then can we expect for wind shear in August? The long range shear forecast from NOAA (Figure 2) shows near normal wind shear over the Atlantic for the remainder of hurricane season. The latest 2-week outlook from the GFS model agrees, calling for near normal wind shear during the first half of August. Since wind shear is expected to be near normal, and SSTs should be near normal, we should expect a near normal level of hurricane activity for August. "Normal" should be placed in context with the above-normal level of hurricane activity we've been seeing since 1995. In the ten years since 1995, not including the El Nino year of 1997, the Atlantic has averaged 4.4 named storms, 2 hurricanes, and one intense hurricane in August. My prediction for August follows a similar line: 4-5 named storms, two hurricanes, and one intense hurricane. All this assumes that El Nino doesn't rear its head; the recent warming of ocean waters in the Equatorial Pacific along the coast of South America could be the prelude to an El Nino event. These events create atmospheric circulation patterns that greatly increase the wind shear over the Atlantic, significantly cutting down on hurricane activity. However, the El Nino forecast models are predicting a continuation of the current neutral El Nino condition through August and September, and it is uncommon to have an El Nino event begin at this time of year. I doubt El Nino will be a factor in this year's hurricane season.
Figure 2. Forecast wind shear July August through October 2006. Image credit: NOAA's Climate Prediction Center.
What will be the steering pattern for August?
For much of June and July, the jet stream made a dip over the eastern U.S., creating a persistent trough of low pressure. In concert with the jet stream, the Bermuda High has stayed further east than it did in 2005. The resulting steering pattern has been taking tropical waves through the Bahamas, then north along the East Coast and out to sea. Tropical Storm Beryl took this path as well. The long range forecast from NOAA's Climate Prediction Center calls for not as strong a trough of low pressure for the remainder of hurricane season. Instead, we should expect a near normal steering pattern, with all regions of the Atlantic under their usual risk of hurricane strikes. However, the latest 2-week GFS model forecast is calling for a continuation of the June and July steering pattern, but with a somewhat weaker trough over the Eastern U.S. Thus, I am forecasting that the entire East Coast of the U.S. will have a higher than average risk of hurricane strikes in August, and the Gulf Coast will have a lower than average risk. The highest risk area of the East Coast will probably be North Carolina and South Carolina. As far as the actual percentage risks, I'll leave that up to Dr. Bill Gray's forecast team at Colorado State, who will be putting out their updated Atlantic hurricane season forecast on Thursday, August 3.
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