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By: Dr. Jeff Masters , 1:54 PM GMT on July 21, 2008
Tropical Storm Dolly has emerged over the Gulf of Mexico after a short passage over Mexico's Yucatan Peninsula, and is already beginning to organize. Dolly will potentially be a dangerous hurricane for Texas and northern Mexico. Satellite loops show a large and expanding circulation, with good upper-level outflow developing to the west and north. The surface circulation is not well formed yet. Dolly's intense thunderstorms lie mainly to the north of the center, and these thunderstorms brought heavy rains of 4-6" to the Cayman Islands yesterday. Dolly's heaviest rains passed through the Yucatan Channel last night and this morning. Rainfall amounts were less than four inches over Cancun, Cozumel, and Western Cuba. Winds were only 40-50 mph over this region, so there should have been little damage.
Figure 1. Current satellite image of Dolly.
The intensity forecast
The upper-level low that interfered with Dolly for so long has now moved away and weakened, and is no longer an issue. Wind shear over Dolly is essentially zero, and expected to remain below five knots over the next 2-3 days. Dolly will be over waters of 28-29°C. These SSTs are slightly below average for this time of year, but plenty warm enough to support Dolly intensifying into a hurricane before landfall. These warm waters extend to a moderate depth, with a Tropical Cyclone Heat Potential of about 40 kJ/cm**2. This is below the value of 80 usually associated with rapid intensification, but still high enough to allow Dolly to strengthen into a major hurricane, if it has enough time to do so. Our skill in making intensity forecasts is poor, but it currently appears that Dolly only has enough time to intensify into a Category 1 or 2 hurricane. I give a 10% chance that Dolly will go through a burst of rapid intensification shortly before landfall and achieve major hurricane status at landfall.
The track forecast
The surface center of Dolly is reforming a bit further to the north of the storm, near the heaviest thunderstorm activity. This will require a northward adjustment to the track forecast, increasing the danger to Texas. The models are pretty united in forecasting a track towards the Texas/Mexico border over the next two days, with a decrease in forward speed. Some significant uncertainty creeps into the forecast for Wednesday, when steering currents weaken and the models have differing solutions on the orientation and strength of the ridge of high pressure steering Dolly. The latest GFDL and HWRF models runs from 2 am EDT this morning have shifted significantly north, bringing Dolly ashore near Corpus Christi, Texas. The GFDL shows a very large borderline Category 1/2 hurricane, with tropical storm force winds extending from Galveston to Brownsville at landfall. Dolly will probably grow quite large and affect a 200-mile stretch of coast with tropical storm-force winds. Like her namesake, actress Dolly Parton, Dolly the tropical cyclone appears destined to become a media star.
Links to follow:
Cancun, Mexico radar
Texas marine forecasts and observations
Brownsville, TX weather
Tropical Storm Cristobal continues to chug along the U.S. East Coast after dropping welcome rains over the eastern portion of North Carolina yesterday. rainfall amounts of 1-2 inches were common, with a few isolated areas of four inches (Figure 2). Cristobal did little or no damage to the state, and the rains and wind of Cristobal have left North Carolina.
Cristobal will continue to the northeast today, and it currently appears that the rains of the storm will affect only the right side of its path, bringing little rain to Nantucket Island, southeastern Massachusetts, and coastal Maine. The storm could bring tropical storm force winds and heavy rains in excess of four inches to Nova Scotia. The combination of cooler waters, moderate wind shear, and dry air should prevent Cristobal from ever reaching hurricane strength. Cristobal will transition to an extratropical storm as it approaches Canada, and some of the models suggest that this system will rotate around the Bermuda High and head southward in the Mid-Atlantic Ocean late this week. There is a small chance that Cristobal would get born again as a tropical storm if it makes a complete circuit and passes over warmer water again.
Links to follow:
Nantucket Island, Massachusetts
Figure 2. Radar estimated rainfall over North Carolina from Tropical Storm Cristobal.
2008 Hurricane season notables
July 19, 2008, marked the first time on record that three July tropical storms were active on the same day in the Atlantic. Bertha, Cristobal, and Dolly were all tropical storms yesterday. Thanks to Margie Kieper for checking on this stat for me.
July 2008 already ranks 5th all-time for number of July named storms (three), and 4th all-time for the amount of Accumulated Cyclone Energy (ACE)--30 ACE units. Only 2005 (60.4 ACE units), 1916 (48.5 ACE units) and 1933 (34.0 ACE units) had more. All three of these years had at least five major hurricanes and were exceptionally active. Thanks go to Dr. Phil Klotzbach of Colorado State for compiling this stat.
Bertha: longest-lived July hurricane on record (and the longest-lived hurricane so early in the season). Bertha was a hurricane 7.75 days, which eclipses the previous record of 7 days held by Hurricane Emily of 2005.
Bertha: longest-lived tropical storm on record for July (and for so early in the season), 17.25 Named Storm Days (NSD). Bertha was the fourth longest lived named storm since 1950 to have formed in any month (trailing Ginger (1971) - 21.25 NSD, Carrie (1957) - 19.50 NSD, and Alberto (2000) - 19.25 NSD).
Bertha: farthest east forming tropical storm and hurricane for so early in the season.
Elsewhere in the tropics
There's no sign that this year's unusually early and active hurricane season is going to slow down. Three of our four reliable computer models continue to predict that a strong tropical wave about to emerge off the coast of Africa tonight will develop into a tropical depression later this week. This wave, located over Senegal (15N 15W), has a very strong rotation and a large circulation. Once it moves over the Atlantic, water temperatures that are up to 2°C above average for this time of year should allow this wave to develop.
I'll have an update later today.
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