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 , 4:10 PM GMT on September 17, 2005
We are now in the third week of September, when water temperatures in the Atlantic are at their yearly peak and historically the strongest hurricanes develop. Given that this year's Atlantic water temperatures are the highest on record, we have in place the best fuel source ever seen for making intense hurricanes. Combined with the low levels of wind shear developing and forecast to remain low over the tropical Atlantic, we have a high potential for the formation of major and potentially destructive hurricanes. A newly formed tropical depression approaching the Windward Islands and a developing system north of the Dominican Republic both have the potential to develop into serious hurricanes, and will need to be watched closely this week.
TD 17 (Phillippe?)
TD 17 is here, and will likely be the first major hurricane of September. This storm will be with us for the next two weeks, since it is moving slowly and has a large area of ocean ahead of it. The storm is in a an environment favorable for intensification, and should be Tropical Storm Phillippe Sunday and Hurricane Phillippe by Tuesday. The shear over the storm is 10 knots, and forecast to decrease. The waters beneath it are a warm 30C (86F). Some impressive lowel-level spiral bands have formed. Upper level outflow is good on the southeast and north sides, and an upper-level anticyclone overhead should provide a very favorable ouflow environment for intensification. All indications are that Phillipe will be a hurricane, and probably a major hurricane.
Fortunately, initial computer model forecasts do not show this storm striking any land areas. A large trough in the mid-Atlantic is pulling TD 17 northwest, and the storm should pass east of the Lesser Antilles Islands. Tropical storm conditions may affect some of the northernmost islands, though. Once the storm moves north of the islands, long-range computer models indicate the possibility it will continue northwestwards and threaten Bermuda. However, remember how wrong these long-range forecast were for Ophelia, forecasting a landfall in Georgia early on! It is impossible to say where TD 17 may go five days from now.
Blob northeast of the Dominican Republic (Rita?)
A disturbance northeast of Puerto Rico continues to generate some impressive clusters of thunderstorms, and definitely has the look of a system organizing into a tropical depression. A mid and possibly low-level circulation has developed near 22N 69W, about 250 miles east of the Turks and Caicos Islands. The main convection is well east of this circulation center, and would have to build over the center before a tropical depression could form. There is about 10 knots of westerly shear over the disturbance, keeping the convection blown over to its east side. This shear is expected to decrease over the next 24 hours, which should allow TD 18 to form later today or tomorrow. The upper-level winds look favorable--an upper-level anticyclone has developed on top, and should provide good outflow for the storm once more deep convection establishes itself. Shear is expected to remain low, water temperatures are very warm--30 to 31C. The chances for this system to become Tropical Storm Rita--and possibly Hurricane Rita--are high.
This system is expected to move west-northwestward through the Bahama Islands and towards South Florida the next few days. Several of the long-range computer models have been consistently forecasting that a strong ridge of high pressure will build across Florida about five days from now, forcing the system on a more westerly or even west-southwesterly path across Cuba towards Mexico's Yucatan Peninsula.
Figure 1. Early track model forecasts for the Puerto Rico blob.
The Intertropical Convergence Zone (ITCZ), remains active. However, the main activity is at about 8N latitude, which is probably too far south to generate a tropical cyclone. If some of this activity works its way to 9N or 10N, we may have a better chance of development. Most of the global computer models indicate that a new tropical storm will form from the ITCZ sometime during the next week.
There is some good news in the tropics today--Ophelia is weakening rapidly, and has given Massachusetts a wide enough berth so that tropical storm conditions will not occur there. Nova Scotia, though, will likely get a direct hit from Ophelia and receive 45 mph winds, 1 - 3 inches of rain, and a 1 - 3 foot storm surge.
Damage estimates from Ophelia's long unwelcome stay over North Carolina are said to be less than $800 million--a remarkably high figure for what was only a tropical storm for all but a two square miles area of the state. Perhaps the category system for ranking hurricanes also needs to take into account how slow a storm moves. What we really need is a separate "storm damage potential" category for hurricanes, as Steve Gregory has championed on his blog.
For those of you who missed by blog last night, there was a scientific first accomplished in Ophelia last night--the first ever remotely-piloted aircraft penetration of a tropical cyclone. A aerosonde flew through Ophelia at 2,500 foot altitude, and measured winds of 74 knots. The project is described in detail on the NOAA Hurricane Research Division's web site. The objective is to use the pilotless aircraft in regions where it is too dangerous for humans to fly:
Simply stated, continuous observation of thermodynamic (temperature and moisture) and kinematic (wind) structure of the near-surface hurricane environment has never been documented in a hurricane. This environment, where the atmosphere meets the sea, is critically important since it is where the ocean's warm water energy is directly transferred to the atmosphere just above it. The tropical cyclone surface layer is also important because it is where we find the strongest winds in a hurricane and coincidentally, the level at which most of us live (i.e. at/near the surface). As such, observing and ultimately better understanding this region of the storm is crucial if we hope to improve our ability to make accurate forecasts of TC intensity change. Enhancing this predictive capability would not only save our economy billions of dollars but more importantly it would save countless lives.
Well done, Aerosonde Corporation and NOAA!
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