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Tropical Storm Danny Gathers Strength in Atlantic

By: Bob Henson 11:33 PM GMT on August 18, 2015

After more than a month between named Atlantic storms--a somewhat unusual occurrence between late July and late August over the last few years--Tropical Storm Danny was christened at 5:00 pm EDT Tuesday. Located at 10.9°N, 37.5°W, or about 1600 miles east of the Windward Islands, Danny was moving west at 12 mph. Sustained winds were at minimal tropical storm strength: 40 mph. Danny has a small core of heavy showers and thunderstorms (convection), surrounded by a fairly large envelope of clouds and scattered storms. Visible and infrared satellite loops show convection rapidly strengthening over the last several hours near Danny’s center and in surrounding bands.

Figure 1. Visible image of Danny collected by the GOES-East floater satellite at 1945 GMT Tuesday (3:45 pm EDT), just before it was upgraded to tropical-storm status. Image credit: NOAA/NESDIS.

Figure 2. NHC’s outlook for Tropical Storm Danny as of 5:00 pm EDT Tuesday.

There was little change in the prognosis for Danny from the NHC’s 11:00 am EDT advisory to the 5:00 pm edition (see Figure 2 above). Wind shear around Danny should remain below 10 - 15 mph for most of the next five days, and sea-surface temperatures along Danny’s path will average near 28°C (82°F) for that entire period. Both of these factors point toward a good chance that Danny will strengthen over the next several days. The NHC projects that Danny will become the year’s first Atlantic hurricane by Thursday morning and grow to Category 2 strength (sustained winds of 100 mph) by Saturday. This long-range forecast is consistent with the statistical models that show more skill than dynamical models at intensity prediction beyond 3 days. Of the two dynamical models most trusted for intensity forecasting, the recently upgraded HWRF has consistently called for Danny to develop into at least a strong Category 1 hurricane. The GFDL failed to develop Danny until today’s runs, but its 1200 GMT Tuesday run brings Danny well above the hurricane-strength threshold, much more in line with the HWRF. If Danny continues to develop at a healthy clip, a period of more rapid intensification later this week cannot be ruled out. Such phases remain very difficult to predict.

Most of the dynamical track models now move Danny toward the west-northwest at a fairly modest pace until this weekend, when a building ridge to the north should help push it at a faster rate. By that point, Danny would draw on oceanic heat content that gradually increases along its path. However, a large area of dry air and Saharan dust north of Danny may inhibit its development at times. With a solid convective core, Danny might be able to fend off interference from this dry, dusty air until it encounters pockets of stronger wind shear, a possibility that long-range models are suggesting for this weekend into early next week. Thus, there is no guarantee that Danny would maintain whatever strength it attains in the deep tropics, and it is still far too early to predict with any confidence how much of a threat Danny might pose to the United States if it survives the long trek. A small change in trajectory now would have big implications for the track many days from now.

Figure 3. Intensity forecasts for Tropical Storm Danny as of 1800 GMT Tuesday, August 18. Models shown are GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; ICON and IVCN = blends of statistical and dynamical model guidance used at the National Hurricane Center.

Figure 4. Infrared satellite image of Typhoon Atsani from NASA’s Visible Infrared Imaging Radiometer Suite (VIIRS) at 1509 GMT Tuesday (9:09 am EDT). Image credit: NOAA/NASA and RAMMB/CIRA, courtesy Dan Lindsey.

In the Pacific: Goni, Atsani continue to rage
Two massive tropical cyclones in the Northwest Pacific--Typhoon Goni and Typhoon Atsani--continue to churn over open water, with no significant changes in strength over the last few hours. See this morning’s post for more on Goni and Atsani.

We’ll have a full update on Danny, Goni, and Atsani by midday Wednesday.

Bob Henson


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.