Typhoon Atsani (which reached Category 5 intensity in the West Pacific Ocean on 19 August 2015) began to transition to a strong post-tropical storm on 25 August. JMA Himawari-8 water vapor (6.2 µm) images (above; click to play MP4 animation) showed the transformation as warm/dry air (yellow to red color enhancement) began to wrap into the western and southern portion of the storm. The animation is also available as a very large QuickTime movie (76 Mbytes) or an animated GIF (110 Mbytes).

Surface analyses from the Ocean Prediction Center (below; click to play animation) indicated that the post-tropical storm deepened to a minimum central pressure of 957 hPa at 12 UTC on 25 August, and was producing hurricane-force winds until 00 UTC on 26 August.

#Atsani transitions from typhoon into powerful HF post-tropical low impressive, short video: https://t.co/uMfMPOBh2M pic.twitter.com/InXpkA6B4Y

— NWS OPC (@NWSOPC) August 25, 2015

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1-minute interval GOES-14 SRSO-R visible images (above; click to play MP4 animation; also available as a 130 Mbyte animated GIF) showed the eye and surrounding cloud structure of Category 2 Hurricane Danny on 21 August 2015. The hazy signature of a dust-laden Saharan Air Layer (SAL) could be seen to the west, northwest, and north of the storm.

The compact circulation of Danny remained fairly “isolated” from the multiple pockets of SAL which stretched westward across much of the tropical Atlantic Ocean (above). The relatively clear dust-free air surrounding Danny was tropical moisture being wrapped northward into the circulation from the Intertropical Convergence Zone (ITCZ), as seen with the MIMIC Total Precipitable Water product (below).

A close-up view of Hurricane Danny (below; click image to play MP4 animation) showed some interesting detail in the convective bursts within the eyewall region, in spite of the very oblique satellite viewing angle. There is also a large (165 Mbyte) animated GIF available here.

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The first tropical storm of the season in the eastern Atlantic (Ana, Bill and Claudette all formed over the western Atlantic) has formed from a cluster of thunderstorms that emerged from Africa. The multi-day animation, above, from GOES-13, shows the steady progress of the disorganized system across the eastern Atlantic basin. Visible Imagery from GOES-13 and Meteosat-10, below, from the morning of 19 August, show the system near 40 W. The cyclonic curvature to the clouds is apparent; identification of the center in infrared imagery, below, is more difficult. Convection to the east of Danny has colder cloud top temperatures.

ASCAT winds from 0100 UTC on 19 August 2015 (above) show a compact circulation center with winds of 40 knots just north of 10 N and east of 40 W.

A persistent impediment to Tropical Cyclone initiation in the eastern Atlantic this year has been widespread Saharan Air Layer dust. The one-day animation, above (taken from this website), shows the SAL persists over the Atlantic; Danny has formed just to the south. Wind shear over Danny at present is weak (see below [source]), and slow strenghtening is expected as Danny approaches the Caribbean.

The Discussion from the National Hurricane Center at 0900 UTC on 19 August included this phrase: “The center is difficult to locate on infrared images,…” The Day Night Band from VIIRS on Suomi NPP can sometimes be used to locate low-level circulation centers of tropical systems. This relies on the presence of moonlight, however, and when Suomi NPP overflew Danny just before 0500 UTC on 19 August, the moon was below the horizon. Thus, the Day Night Band, below (Courtesy of William Straka) gave very little information.

More information on Danny is available at the CIMSS Tropical Weather Website. Consult the National Hurricane Center for the latest updates and official forecasts.

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Super Typhoon Atsani reached Category 5 intensity early in the day on 19 August 2015; the storm displayed a nearly annular eyewall signature during much of the day on Himawari-8 infrared (10.4 µm) images (above; click image to play MP4 movie file). A large (64 Mbyte) animated GIF is available here. Evidence of low-altitude mesovortices within the eye can be seen in the 2-km resolution infrared images.

A comparison of DMSP SSMIS microwave (85 GHz) and MSAT-2 infrared (10.8 µm) images around 20 UTC is shown below (courtesy of the CIMSS Tropical Cyclones site).

On the topic of eye mesovortices, a more detailed view of these features could be seen on daylight 0.64 µm visible images (below; click image to play MP4 animation). A large (28 Mbyte) animated GIF is available here.

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