Dolly nears hurricane strength

Tropical Storm Dolly is building an eyewall and is near hurricane strength, as it approaches a Wednesday landfall in Texas or northern Mexico. Visible satellite loops show that heavy thunderstorm activity continues to increase near the core of the storm, with good upper-level outflow to the west and north, but restricted on the south side, where an upper level low pressure system is interfering. Maximum surface winds measured by the SFMR instrument on the current Hurricane Hunter aircraft inside Dolly were 74 mph (65 kt), measured at 8:39 am EDT. These are bottom end Category 1 hurricane winds, so Dolly is very close to becoming a hurricane. However, the winds at the flight level of 10,000 feet have been considerably less than 74 mph, so don't expect NHC to upgrade Dolly based on this measurement. Water vapor satellite loops still show some dry air on the south side of Dolly, but the storm has insulated itself from this dry air by building an eyewall that was 50% complete as of 4 am EDT (Figure 1). Dolly's outer spiral bands are approaching the Texas/Mexico coast, and the center of the storm is now visible on Brownsville, Texas long-range radar


Figure 1. Microwave satellite image of Dolly at 4:08 am EDT Tue July 22, 2008. Note the partial formation of an eyewall on the north and east sides. The eyewall was about 50% complete at this time. Image credit: Navy Research Lab, Monterey.

The intensity forecast
Wind shear over Dolly is less than five knots, and is expected to remain below ten knots over the next two days. An upper level high pressure system is moving into place over the storm, which should enhance Dolly's upper-level outflow and allow more rapid intensification. Dolly is over waters of 29°C. The waters cool to about 28°C as Dolly approaches the coast and passes over a cool ocean eddy. The warm waters Dolly is currently over extend to a moderate depth, with a Tropical Cyclone Heat Potential of about 40-60 kJ/cm**2. This is below the value of 80 usually associated with rapid intensification, but still high enough to allow Dolly to put on a burst of rapid intensification prior to landfall. Our skill in making intensity forecasts is poor, but it currently appears that Dolly only has enough time to intensify into a Category 1 hurricane. A Category 2 hurricane still a possibility, but this would require Dolly staying over water a longer period of time than forecast. Even if Dolly does slow down and stay over water until Thursday, the very slow motion of the storm would cause it to upwell cold water from the ocean depths to the surface, putting a damper on intensification. I put the chances of Dolly reaching major hurricane status (Category 3 or higher) at 2%.

The track forecast
A trough of low pressure approaching the Midwest U.S. tonight will weaken the steering currents driving Dolly west-northwest, and probably impart more of a northwesterly motion to the storm tonight and Wednesday. The exact influence this feature will have on Dolly is uncertain, with four models predicting a landfall in northeastern Mexico--the UKMET, ECMWF, NOGAPS, and GFS--and two models predicting a Texas landfall--the GFDL (near Brownsville) and the HWRF (near Corpus Christi). Dolly could come ashore anywhere within the cone of uncertainty, and one should not assume the storm will track down the "skinny black line" NHC has drawn through their official forecast. The timing of Dolly's landfall, as predicted by the computer models, will be anywhere from 10 am - 10 pm Wednesday. However, the GFS and ECMWF hint that Dolly may stall out right by the coast Wednesday, and some slow and erratic motion is possible tomorrow before the storm finally comes ashore.

Links to follow:
Brownsville, TX long range radar
Texas marine forecasts and observations
Brownsville, TX weather
Corpus Christi, TX weather


Figure 2. Visible satellite image from 1130 GMT (7:30 am EDT) Tue July 22, 2008, showing the new tropical low pressure system that moved off the coast of Africa. Note the stratocumulus clouds to the west of the low, indicating cooler waters that will slow down development of the system. Image credit: Navy Research Lab, Monterey.

Elsewhere in the tropics
Tropical Storm Cristobal continues to head out to sea, and is not expected to hit land. Nova Scotia and Newfoundland may feel the fringes of the storm as it races to the east today and transitions into an extratropical storm.

A large low pressure system (97L) moved off the coast of Africa last night, and is generating some heavy thunderstorm activity over the ocean waters near the Cape Verde Islands. This morning's 2:53 am EDT QuikSCAT pass shows a closed circulation near 17N 19W, with winds in the 10-20 mph range. Sea Surface Temperatures (SSTs) are about 27°C, which is one degree above the 26°C threshold needed for tropical storm formation. SSTs will cool to 25-26°C on Wednesday as the low moves westward, which will slow any development. Wind shear is low enough for development, 5-10 knots, and is forecast to remain low enough for development over the next few days. Once the low reaches the mid-Atlantic late this week, SSTs will warm once again above 26°C, and we may get yet another July tropical depression. However, systems that come off the coast of Africa this far north rarely affect the Caribbean or U.S., and Bermuda is probably the only location that needs to be concerned.

Quiz question: Since hurricane began getting names in 1950, only one major hurricane that hit the U.S. did not get its name retired. Which one was it? A hint: the track was similar to Dolly's expected track. I'll have the answer in my update on Dolly this afternoon.

Jeff Masters