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Tropical Storm Ernesto forms

By: Dr. Jeff Masters, 10:02 PM GMT on August 02, 2012

Observations from an Air Force hurricane hunter aircraft indicate that Tropical Storm Ernesto has arrived. The season's fifth tropical storm has surface winds of 50 mph on the north side near 14°N, but has an ill-defined surface circulation with almost no winds out of the west. Ernesto is not a pretty sight on visible satellite loops, with patchy heavy thunderstorm activity and little spiral banding. Arc-shaped low clouds can be seen racing away from Ernesto on its north side, an indication that the storm is ingesting dry air that is causing strong thunderstorm downdrafts. This sort of phenomena is seen in storms that are struggling to hold together in the face of wind shear and dry air. Wind shear over Ernesto is at the moderate level, 10 - 20 knots. Water vapor satellite loops show that Ernesto is at the southern edge of a large area of dry air. Wind shear due to strong upper level winds from the west is driving this dry air into the core of the storm, disrupting it. A series of Air Force hurricane hunter aircraft are scheduled to visit Ernesto every six hours to keep tabs on it.


Figure 1. Afternoon satellite image of Ernesto. Note the arc-shaped low clouds on the north side of Ernesto, which mark the boundaries of where thunderstorm outflow due to ingestion of dry air is occurring.

Forecast for Ernesto
Since the Hurricane Hunters found Ernesto's strongest winds to be near 14°N, I expect that Martinique, Dominica, St. Lucia, and Barbados will see the strongest winds on Friday during Ernesto's passage through the islands. Wind shear is expected to remain moderate, 10 - 20 knots, through Friday, then fall to the low range through Tuesday, according to the 2 pm EDT run of the SHIPS model. Given Ernesto's poor organization, I give a 20% chance that the storm will degenerate into a tropical wave on Friday. If this happens, the storm will still be capable of bringing winds of 50 mph to the Windward Islands, though--and will still be capable of regenerating into a tropical storm in the Western Caribbean. Once Ernesto clears the Lesser Antilles, the reliable computer models predict a west to west-northwest motion through the Caribbean, with the storm's heavy rains staying south of Puerto Rico and the Dominican Republic. The more westerly path predicted by the usually reliable ECMWF model, which brings Ernesto to a landfall in Honduras on Monday night, is being discounted by NHC, since they are assuming Ernesto will stay stronger than the ECMWF model is forecasting. Once Ernesto enters the Central Caribbean on Sunday, it is possible that the storm's outer spiral bands will cause flooding problems in Southwest Haiti, Jamaica, and the Cayman Islands. Of the major models NHC uses operationally--the ECMWF, GFS, NOGAPS, UKMET, GFDL, and HWRF--only the HWRF clearly shows Ernesto reaching hurricane strength over the next five days. So, the official NHC forecast of a hurricane near Jamaica on Monday is an aggressive one. By Monday, a trough of low pressure passing to the storm's north may be capable of turning Ernesto more to the northwest, resulting in the storm entering the Gulf of Mexico by the middle of next week. Stay tuned.


Figure 2. Radar image taken just before landfall as Typhoon Damrey approached the coast of China. Image credit: weather.com.cn.

Typhoon Damrey hits China
Typhoon Damrey hit China near the Chenjiagang Township of Xiangshui County around 9:30 p.m. local time Thursday as a strong Category 1 storm with winds near 85 mph. It is too early to know what sort of damage the storm may have caused. The Western Pacific is entering a very active period, and is likely to get at least two more new named storms in the next week.

Jeff Masters

Hurricane

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

Reader Comments

2001. kwgirl
Quoting jeffs713:

Over land, yes. Over water, nighttime is best.

Storms intensify during the day over land due to the difference between surface temp and air temp. (land heats faster than water). So over land, the ground temp is hotter than the air, which means warm air rises faster, causing storms.

Over water during the day, the water doesn't warm that much or quickly, so the warmer air temps cause the air to actually stabilize (due to the decreased difference between air and surface temp). At night, the water is warmer than the air, causing increased lift and evaporation - feeding storms.
Hence causing RI! Good morning all.
I released my 70th tropical update of this season this morning...covering Ernesto and what is now 90L...and more...

I've been told that these in-depth updates help folks understand things about the tropics they didn't before...that's why I keep doing them. Let me know what you think too...or feel free to leave comments.