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Shary forms; potentially dangerous 91L approaching Lesser Antilles

By: Dr. Jeff Masters, 3:23 PM GMT on October 29, 2010

Tropical Storm Shary is here, the eighteenth named storm of this remarkably active 2010 Atlantic hurricane season. Shary's formation makes 2010 tied for 5th place with 1969 for most number of named storms in an Atlantic hurricane season. Only 2005 (28 named storms), 1933 (21 named storms), 1995 (19 named storms), and 1887 (19 named storms) had more named storms than 2010 has had. Atlantic hurricane records go back to 1851, though there were likely many missed named storms prior to the beginning of satellite coverage in the mid-1960s. Shary is going to be a weak and very short-lived storm, and Bermuda is the only land area that needs be concerned with the storm. A Tropical Storm Warning is posted for the island, and rain bands from the storm can be seen on Bermuda radar.

Potentially dangerous 91L approaching South America and Lesser Antilles
A very impressive tropical wave (Invest 91L), about 350 miles east-southeast of the southernmost Lesser Antilles Islands, is headed west-northwest towards the islands at 15 - 20 mph. In discussions I had with hurricane experts at NHC and NOAA's Hurricane Research Division yesterday, it was widely agreed that this system was unusually large and well-organized for this time of year--something one would expect to see in early September, but not late October. The historical Atlantic hurricane data base shows no cases where a tropical depression has formed so far south and east so late in the year. "Ominous" and "unprecedented" were a few of the adjectives I heard used to describe 91L, and this system has the potential to be a dangerous storm for the islands of the eastern and central Caribbean.

Figure 1. Morning satellite image of 91L.

91L is centered very far to the south, near 10°N latitude, and this close proximity to the Equator has slowed development. Also slowing development has been the system's very large size--it takes time to spin up such a large circulation. Aiding development has been low wind shear of 5 - 10 knots, warm ocean temperatures of 29°C, and a very moist atmosphere. A pass by the ASCAT satellite last night revealed a nearly closed circulation, and top winds of about 30 mph. Visible satellite loops do not show a clear surface circulation yet, though the storm has plenty of heavy thunderstorm activity that is increasing in organization, with several impressive low-level curved rain bands.

Forecast for 91L
91L will continue moving west-northwest at a decreasing forward speed through Monday, bringing very heavy rain tonight and Saturday to the northern coast of South America and most of the Lesser Antilles. The center of the storm will track very close the coast of South America this weekend, and it is likely that this will slow or halt development over the weekend. By Monday, the center of 91L may pull far enough away from South America that more substantial development can occur. However, steering currents are expected to substantially weaken in the eastern Caribbean beginning on Monday, as a strong trough of low pressure develops over the Eastern U.S., weakening the ridge of high pressure steering 91L. The trough may be strong enough to pull 91L to the north, resulting in a potential threat to Haiti, the Dominican Republic, Puerto Rico, and the Lesser Antilles Islands by Wednesday of next week. We do have several models--the HWRF and GFS--that develop 91L into a hurricane by Wednesday. Shear is predicted to remain low, 5 - 10 knots, for most of the next five days, and water temperatures are at near record highs, 29 - 29.5°C. There is the potential for 91L to reach hurricane status if passage over South America this weekend does not disrupt the storm sufficiently. An Air Force hurricane hunter aircraft is scheduled to investigate 91L at 2pm this afternoon to see if a tropical depression has formed.

Figure 2. Hurricane Specialist Dan Brown coordinates with the Bermuda Weather Service, alerting them to the possibility that 92L might be upgraded to Tropical Storm Shary, necessitating issuance of a Tropical Storm Watch for the island that evening.

A Thursday evening shift at NHC
I spent another shift yesterday evening at the National Hurricane Center, where I've spent the week as a participant in their visiting scientist program. Each week during hurricane season, NHC invites a hurricane researcher or forecaster in academia, government, or private industry to spend a week shadowing the NHC forecasters as they prepare their forecast products. The evening shift is chosen, since it is less of a zoo, and the presence of the visiting scientist will present less of a distraction to the forecasters.

Once again, I spent the first portion of the shift working with Tropical Analysis and Forecasting Branch (TAFB) forecaster Wally Barnes, who made the intensity and position estimates of 90L, 91L, and 92L based on infrared satellite imagery. This task was accomplished using the Dvorak technique, a system of classifying cloud patterns of tropical cyclones based on how cold the cloud tops are, how much spiral banding is present, and other factors. We classified 92L (later to be Shary) as an ST2.5--a subtropical storm with 40 mph winds. The more dangerous tropical wave Invest 91L approaching the coast of South America got a far weaker classification, since the tops of its thunderstorms were not very cold, and the bands of clouds were fairly fragmented.

We presented our data to Senior Hurricane Specialist Dan Brown, who would be responsible for the decision whether or not to upgrade 92L to Shary. He was impressed with the ST2.5 classification we'd come up with for 92L, but wanted more evidence that the storm was as strong as this satellite estimate indicated. At 9:10pm, we had our evidence. The latest wind observations from NOAA buoy 41049 showed 33.4 knots (38 mph) as 92L passed by. This wind speed is just at the boundary of tropical storm force winds--39 mph. However, since the buoy's anemometer is at an elevation of 5 meters, an adjustment upwards to the wind speed is necessary to correct the winds to the standard measurement height of ten meters, due to frictional slowing of the wind near the surface. Thus, the buoy winds were more like 40 mph, above tropical storm force, and this was Tropical Storm Shary--if a closed circulation existed. Dan told us he was going to start writing an advisory package, in case additional data came in indicating 92L had a closed surface circulation. He called the Bermuda Weather Service to alert them that he was considering naming this system Shary, and that a tropical storm watch or warning might be required for the island that evening. Dan also called the head of the hurricane forecasting branch of NHC, James Franklin, to alert him of the impending new storm.

Figure 3. "This is the part where the world finds out about Shary," Hurricane Specialist Dan Brown told me as he filled out this form on his computer. About an hour before the first package of official advisories on a new tropical depression or tropical storm are sent out, NHC renumbers an Invest with the "AL" prefix and a number indicating how many tropical storms or depressions have occurred so far this year. In this case, 92L got renumbered AL20, since there have been 18 named storms and 2 tropical depressions that did not reach tropical storm strength. The newly numbered storm appears on the Navy Research Lab web site about an hour before the first advisory is sent out to the world. It is rare for NHC to change their mind and not issue advisories after renumbering occurs.

At 9:20pm, we had our proof of a closed circulation. A ship heading towards the center of Shary from the south measured west winds of 15 knots just south of the center, strongly suggesting that Shary had a closed circulation and was a legitimate tropical storm. Dan accelerated his work on the 11pm advisory package--there was a lot of work to do between now (9:30) and 10:30, when he wanted to get the advisories out. The other hurricane specialist on duty, Robbie Berg, helped out, and the two of them worked hard over the next hour to plot positions, scan the latest observations and model data, and type up advisories. Sandwiched between these efforts were several phone calls--a coordination call with other branches of NOAA and the Navy, another call to the Bermuda Weather Service, plus a conversation with Trinidad's weather service, which was concerned about the tropical wave (Invest 91L) approaching their island. Finally, at 10:30pm, the advisory package was complete, and Dan hit the "Send out to the Whole World?" button on his screen, making Tropical Storm Shary the eighteenth named storm of the 2010 Atlantic hurricane season.

Figure 4. Hurricane Specialist Robbie Berg updates NHC's big hurricane tracking board with its newest addition, Tropical Storm Shary.

It's worth noting that we would not have known Shary was a tropical storm without data from the buoy the storm passed over. This buoy was one of the new buoys financed by a special supplemental funding bill approved by Congress several years ago, in an effort to improve hurricane forecasts. Money well spent in this case!

Next update
I'll have an update later today or Saturday morning on the latest from the tropics.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.