1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) and “Clean” Infrared Window (10.3 µm) images (below) showed the formation of Tropical Storm Iba off the east coast of Brazil at 16 UTC on 24 March 2019 (surface analyses). Plots of GLM Groups revealed some intermittent lightning activity. Tropical cyclones in the South Atlantic basin are rare — the last was in 2010, and only one example (Catarina in March 2004) is known to have reached hurricane intensity.

A toggle between NOAA-20 VIIRS True Color Red-Green-Blue (RGB) and Infrared Window (11.45 µm) from RealEarth (below) showed Iba at 1610 UTC.

GOES-16 Infrared images with an overlay of deep-layer wind shear valid at 18 UTC from the CIMSS Tropical Cyclones site (below) revealed a very tight gradient of shear over Iba. However, the shear gradient began to relax somewhat by 21 UTC.

In a sequence of GOES-16 “Clean” Infrared Window (10.3 µm) and Infrared-Water Vapor (10.3-6.9µm) brightness temperature difference (BTD) images (below), the clusters of deep convection propagating southward — east of Iba’s center of circulation, denoted by “I” — exhibited large negative BTD values (darker shades of red) suggestive of significant cloud-top penetration into the lower stratosphere (reference).

GOES-16 Visible images with an overlay of 1138 UTC ASCAT surface scatterometer winds from the Metop-A satellite (below) showed speeds in the 40-49 knot range (yellow barbs).

The MIMIC Total Precipitable Water product (below) showed that Iba was embedded within a plume of moisture that extended southeastward off the coast of Brazil.

Sea Surface Temperature values (below) were around 30ºC in the waters where Iba intensified.

===== 25 March Update =====

A comparison of GOES-16 Visible and Infrared images (above) showed that increasing deep-layer wind shear had exposed the low-level circulation center of Iba. However, GLM Groups plotted on the Visible images revealed an increasing amount of lightning activity associated with a convective burst that began to wrap around the southern edge of the storm center after 15 UTC — and a brief cloud-top infrared brightness temperature of -90ºC (yellow pixel embedded with darker purple shades) was seen at 1635 UTC.

A timely overpass of the Suomi NPP satellite at 1639 UTC provided 375-meter resolution VIIRS True Color RGB and Infrared Window (11.45 µm) images (below), which showed a large overshooting top that exhibited infrared brightness temperatures of -90ºC and colder (yellow), with a warmer ring of compensating subsidence immediately surrounding it. The coldest pixel had a brightness temperature of -103.7ºC — which is almost 1ºC colder than the -102.96ºC value noted over Australia in 2008.

The explosive growth of that convective burst was very apparent in a toggle between VIIRS Infrared images from NOAA-20 at 1549 UTC and Suomi NPP at 1639 UTC (below, courtesy of William Straka, CIMSS). Note that the images use a slightly different variant of the color enhancement. A comparison of VIIRS True Color and Infrared images from NOAA-20 and Suomi NPP viewed using RealEarth is available here.