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Dolly slowly organizing, having trouble with dry air

By: Dr. Jeff Masters, 11:48 PM GMT on July 21, 2008

Tropical Storm Dolly is slowly organizing over the Gulf of Mexico as it approaches a Wednesday landfall in Texas or northern Mexico. Visible satellite loops show that heavy thunderstorm activity is increasing near the core of the storm, with good upper-level outflow to the west and north. The surface circulation is now well formed, and Dolly will soon be forming an eyewall as the inner core continues to organize and consolidate. Until this process gets further along, the winds and pressure will remain fairly steady. Maximum surface winds measured by the SFMR instrument on the current Hurricane Hunter aircraft inside Dolly were 50 mph (45 kt), measured at 6:25 pm EDT. There are indications on water vapor satellite loops that a pocket of dry air on the south side of Dolly is interfering with the organization of the storm. This dry air was sucked in from the Yucatan Peninsula, and created strong downdrafts in the thunderstorms on Dolly's south side that sapped moisture and energy from the storm (Figure 1). If you watch the visible loop carefully, you can see evidence of these downdrafts in the form of surface lines of cumulus clouds propagating away from Dolly's center on the south side of the storm.

Figure 1. Visible satellite image of Dolly at 6:41 pm EDT. Note the band of cumulus clouds that mark the boundary of outward flowing air along the surface, generated when dry air from the Yucatan Peninsula got ingested into thunderstorms on the south side of Dolly, creating strong downdrafts that spread out in all directions upon reaching the surface.

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 two days. Dolly is having trouble with dry air sucked in from the Yucatan, but this will grow less of an issue by Tuesday as the storm draws away from the dry air source. 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 hurricane. A Category 2 hurricane now looks unlikely, and I put the chances of Dolly reaching major hurricane status (Category 3 or higher) at 2%.

The track forecast
The models are pretty united in forecasting a track towards the Texas/Mexico border over the next day, with a sharp decrease in forward speed Tuesday. Significant uncertainty creeps into the forecast when this deceleration occurs and the steering currents weaken. The models have differing solutions on the orientation and strength of the ridge of high pressure steering Dolly, and the storm could come ashore anywhere within the cone of uncertainty--from northern Mexico to Corpus Christi, Texas. Tropical storm force winds will impact a 200-mile long stretch of coast.

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

I'll have an update Tuesday morning by 9 am EDT.

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.