Tropical Depression Eighteen Discussion Number 2
Statement as of 5:00 am EDT on September 18, 2005 the reconaissance aircraft that was en Route to Tropical Storm Philippe was diverted to the depression...and we very much appreciate the last minute flexibility of the aircrew. When they first got to the depression...they didn't find much...including only 5 kt of west wind and a center southwest of the deep convection. However...on their second pass through they found that the center had migrated or redeveloped closer to the convection... which is now taking on a slightly more banded appearance on the first few images after the satellite eclipse. These observations suggest that the depression is becoming better organized. The peak flight-level winds were 36 kt...which supports an initial intensity estimate of 25 kt. The aircraft data require a relocation of the center to the south and west of the previous track...and this results in a slight southward shift of the official forecast. However...the basic track reasoning remains the same. The depression is expected to move west-northwestward for the next couple of days toward a weakness in the subtropical ridge just east of Florida. After that...high pressure over Texas and the western Gulf should turn the depression westward. The official forecast is in best agreement with the GFS and GFDL guidance. The UKMET is farther south...taking the cyclone across central and western Cuba. The Canadian model is also south of its previous track...taking the center just south of Key West. Upper-level low pressure is centered over central Cuba...and this low is producing some southerly shear over the depression. This should limit intensification in the short term...however...this low is forecast by all the global models to weaken and be replaced by a narrow ridge of high pressure over the next two to three days. This upper pattern...coupled with very warm sea surface temperatures along the path of the cyclone...would favor more rapid development as long as the cyclone avoids the land mass of Cuba. The GFDL is not nearly as agressive as it was earlier today...not making the system a hurricane until it is well into the Gulf of Mexico. The official forecast is a little slower than the previous advisory in bringing the system to hurricane strength and follows a blend of the SHIPS and GFDL guidance. Forecaster Franklin forecast positions and Max winds initial 18/0900z 21.7n 71.2w 25 kt 12hr VT 18/1800z 22.2n 73.0w 30 kt 24hr VT 19/0600z 22.8n 75.3w 35 kt 36hr VT 19/1800z 23.4n 77.5w 45 kt 48hr VT 20/0600z 23.8n 80.0w 55 kt 72hr VT 21/0600z 24.0n 84.5w 65 kt 96hr VT 22/0600z 24.0n 89.5w 75 kt 120hr VT 23/0600z 24.0n 93.5w 80 kt