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 , 2:06 PM GMT on June 30, 2006
A new area of disturbed weather has formed in the extreme southwestern Gulf of Mexico, near the coast of Mexico. The thunderstorm activity has little organization, and strong upper-level winds from the west are creating 15-25 knots of wind shear over the system. Wind shear is expected to remain 15 knots or higher for at least the next two days, making it unlikely for this disturbance to develop. The disturbance will move slowly northwest and bring welcome heavy rains to South Texas and Northeast Mexico over the weekend. Extreme drought conditions prevail there, but flash flooding may be a concern nevertheless.
Figure 1. Model forecast tracks of the tropical disturbance in the Gulf of Mexico.
Tropical wave in the Caribbean and Bahamas
The tropical wave in the eastern Caribbean we've been watching this week is just barely visible as a line of showers moving across Hispaniola towards Cuba and the Bahama Islands. Strong upper-level winds should continue to prohibit development of this wave over the Caribbean. However, the northern portion of this wave, now located north of Hispanolia and the easternmost Bahama Islands, is kicking up an impressive area of deep thunderstorms this morning. Wind shear is a high 20 knots over this disurbance, but the disturbance is moving northwest towards an area of lower wind shear. This may allow for some slow organization today before increased wind shear over the weekend tears this system apart. The disturbance is not a threat to land, as it is expected to recurve to the north and northeast around the Bermuda High.
Are we due for a repeat of the Hurricane Season of 2005?
That's the question on everyone's minds as we hit the end of the first month of hurricane season. After all, we've already had one tropical storm (Alberto) that occurred about the same time as last year's first storm (Arlene), and we almost had a second tropical storm near North Carolina this week, which would have made June 2006 match the June 2005 total of two tropical storms. Let's take a look at the large-scale weather patterns over the Atlantic and compare June 2005 with June 2006.
A look at June 2005
In Figure 2, we see the sea surface temperature (SST) and surface pressure departure from normal for June 2005. June SSTs were 1-2 C above normal, and June pressures were much below normal over almost the entire North Atlantic. The pressure at the center of the Bermuda High was 5 mb below normal, and pressures over the Caribbean and tropical Atlantic were about 1 mb below normal. The force of the trade winds that blow across the ocean is governed by the pressure gradient between the center of the Bermuda High and the tropics. The greater the difference in pressure, the stronger the winds have to blow, in their effort to equalize the pressure. In June 2005, this pressure gradient was very weak, and thus the trade winds were much lighter than usual. Lighter winds meant less evaporation occured over the surface waters, and thus less evaporative cooling (loss of latent heat). Without the usual strong winds to cool the ocean, the waters were able to heat up to record warm levels.
A ridge of high pressure dominated the Eastern U.S. in June 2005, and this ridge of high pressure continued to dominate throughout the entire hurricane season. As a result, the jet stream stayed far to the north, keeping wind shear low over the Gulf of Mexico and Caribbean. This ridge also acted to steer hurricanes into the Gulf Coast.
Figure 2. Sea Surface Temperature departure from normal for June 2005 (left) and surface pressure departure from normal (right). The arrows show the departure of surface winds from normal. The Bermuda High had a central pressure 5 mb lower than normal, which meant that the winds circulating around the high were weaker than normal. Thus, the surface wind anomaly arrows plotted above appear to rotate counterclockwise around the Bermuda High. Image credit: "The 2005 hurricane season: An echo of the past or a harbinger of the future?" Geophysical Research Letters, 33, March 2006.
A look at June 2006
An opposite trend in surface pressures occurred in June 2006, compared to June 2005 (Figure 3). The Bermuda High has averaged 7 mb stronger than normal this month--and a full 12 mb stronger than last year's June Bermuda High. Low pressure has dominated the Eastern U.S., thanks to a persistent dip in the jet stream that has brought rain, floods, and relatively cool weather to the Eastern U.S. This dip in the jet stream has also brought much higher levels of wind shear to the Gulf of Mexico and Caribbean than we saw last year in June.
Figure 3. Surface pressure departure from normal for June 2006. Note that surface pressures at the center of the Bermuda High were 7 mb above normal, and that pressures along the East Coast of the U.S. were below normal. Image credit: NOAA Climate Prediction Center.
As a result of the stronger Bermuda High this June, the trade winds blowing across the tropical Atlantic are much stronger than they were last year in June. These stronger winds have cooled the ocean, and SSTs are only 0.5 - 1.0 degrees C warmer than average this June, compared to 1 -2 degrees above normal in June 2005. This year's SST anomaly is still quite high, but probably not enough to support major hurricanes in July, like we saw last year with Dennis and Emily.
Figure 4. Sea surface temperatures and departures from normal (anomalies) for June 2006. SSTs are still 0.5 - 1 degrees C above normal over the tropical Atlantic, but still a full degree cooler than 2005. Image credit: NOAA Climate Prediction Center.
Outlook for July 2006
The two-week forecast from the GFS model continues to show the jet stream dipping over the Eastern U.S., and a stronger than normal Bermuda High. This pattern favors a continuation of the cool, rainy weather over the Northeast U.S., and hurricane strikes on Florida and the East Coast of the U.S.--or recurvature out to sea. The Gulf Coast has lower than average odds of a strike. This jet stream pattern should act to keep wind shear high over the main breeding grounds for July tropical cyclones--the Gulf of Mexico, Bahamas, and western Caribbean. If a hurricane does manage to develop and dodge the shear, it is unlikely it will become a major hurricane, due to the relatively cool ocean waters expected this July, compared to July 2005. Thus, July 2006 will not be a repeat of July 2005, which had five named storms, three hurricanes, and two intense hurricanes. If the current upper-level jet stream pattern holds in place, I think we can expect one or two named storms in July, one of them being a hurricane (not major). Will the current jet stream pattern hold? We are not very good at anticipating when these "blocking patterns" in the large scale atmospheric flow will change. Sometimes they can last for an entire season. If the pattern breaks in the last half of July, we could see more activity than I am forecasting here. In any case, I am still anticipating that August and September will be very busy again this year, so enjoy the relatively quiet start to hurricane season!
Have a great 4th of July weekend everyone, and I'll be posting daily updates.
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