At 21Z yesterday, the NHC upgraded TD4 to a Tropical Storm, the forth one of the season, BUT the first "purely" tropical storm... meaning that it formed in the deep tropics, void of baroclinic influences, unlike the previous three storms that never crossed below 25N. The convective pattern has been very encouraging, and is the reason for the upgrade. The CDO is still expanding and quite cold (still around -80C peak), and there are now obvious bands forming. All features appear to be symmetric about the center, meaning vertical shear is very low. SSTs there are approximately 28C. At 15Z today, Dolly was located at 10.4N 37.2W and tracking W at 14kts. Maximum sustained winds have reached 55kts and the central pressure has fallen to 994mb. She is VERY close to reaching hurricane status, probably later today. She would be the first hurricane of the season. (Recall that Erin was the first hurricane of last year, on 8 Sep.) Microwave imagery is one of the best tools to use at this stage of a tropical cyclone's life, because the core is hidden from visible and infrared wavelengths by the CDO. Micowave, with a much longer wavelength, can "see" through the clouds to reveal the inner precipitation structure. HOWEVER, the most recent pass of such an instrument just barely hit the storm, not really showing enough of it to be useful, and the last good pass was too long ago to be useful. So, we wait (and wish there were more microwave instruments on our growing array of satellites).
Please visit my tropical Atlantic headquarters.
P.S. I may send out an update later today if she changes much, and then none until Tuesday morning, as I'll be out of town for the long weekend. Dolly will not be near any land by Tuesday; perhaps 1000km away from the closest land, which will be the Leeward Islands.