Himawari-9 imagery for the hour surrounding an RCM-1 pass over the center of Severe Tropical Cyclone Herman, below, shows strong convection (with widespread brightness temperatures colder than -85^oC — light purple in this enhancement — with isolated pixel values cooler than -90^oC — in purple). Peak Synthetic Aperture Radar (SAR) winds from the Radarsat Constellation Mission-1 satellite exceeded 80 m/s! Click here to view the storm track, and text describing the storm, both from this Australian Bureau of Meteorology website. (Here’s the 1-hour animation without the SAR winds).

A zoomed-in picture of the SAR wind analysis is shown below. The data can also be viewed at this SAR Tropical Cyclone site specific to Herman (Direct link to image). Strong winds encircle the entire eye.

The image below, from the Australian Bureau of Meteorology ‘MetEye‘ website, shows Herman’s track relative to Australia.

Herman is moving through an atmosphere of low shear, as shown below (image from this site), and is over waters with sea-surface temperatures of 27-28^oC (not shown).

The MIMIC-TC animation below (from this site) shows the closing off of the eyewall that accompanied Herman’s rapid intensification

More information on Herman is available at the Joint Typhoon Warning Center. Also, note that the Target Sector from JMA Is over Herman (link).

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2.5-minute Target Sector Himawari-9 Infrared Window (10.4 µm) images (above) showed Herman as it rapidly intensified to a Category 3 storm, eventually displaying a pinhole eye. Note that the eye exhibited some trochoidal motion (wobble) as the storm moved south-southeastward — a behavior often seen with intense tropical cyclones.

===== 31 March Update =====

Herman continued its remarkable rapid intensification on 31 March, with 2.5-minute Target Sector Himawari-9 Infrared images showing an intermittent open eye persisting past 15 UTC (above).

A Himawari-9 Infrared – Water Vapor Difference product from the CIMSS Tropical Cyclones site (below) displayed large values (shades of red to violet) — indicating that cloud tops within the eyewall were overshooting the tropopause (reference).

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GOES-16 (GOES-East) Shortwave Infrared (3.9 µm), Night Fog BTD (10.3 µm – 3.9 µm) and Nighttime Microphysics RGB images (above) displayed the thermal signature — via a single warm pixel — of a fire that resulted from a BNSF train derailment (media report) in Raymond, Minnesota at 0602 UTC (1:02 AM CDT) on 30 March 2023.  Although the fire itself was relatively small (only 3 train cars were burning) and patchy clouds were drifting overhead, the subpixel sensitivity of thermal anomalies is very good with the 3.9 µm spectral band (and any products which include 3.9 µm data).

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Tropical Cyclone Herman has upgraded to Tropical Storm status and currently sits in the Eastern Indian Ocean, approximately 1600 km from the coast of Western Australia. It has been measured to produce gusts up to 55 mph. As of yet, the storm poses no threat to human populations and is not forecast to make landfall. However, its outer rainbands could bring heavy precipitation to Indonesian islands Sumatra and Java.

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1-minute Mesoscale Domain Sector GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) include an overlay of GLM Flash Extent Density — and showed the distinct circulation of a middle-tropospheric closed low (400 hPa analysis) as it moved northeastward across Illinois and Indiana into southern Lower Michigan on 25 March 2023. Isolated clusters of brief lightning activity were seen across the region, but there were no reports of thundersnow. A number of locations in Iowa, Illinois and Wisconsin received heavy snowfall, with several new daily records being set.

The corresponding 1-minue GOES-16 “Clean” Infrared Window (10.3 µm) images below helped to highlight areas of mesoscale banding where precipitation rates were enhanced (some sites reported snowfall rates of 1 to 2 inches per hour).

Later in the day, as clouds cleared along the back edge of the storm system, much of the southwest-to-northeast swath of fresh snow cover was revealed in GOES-16 Day Snow-Fog RGB images (darker shades of red) and Day Cloud Phase Distinction RGB images (brighter shades of green).

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Microwave estimates of snowfall rate (SFR) are available at this website, and imagery during the day on the 25th are shown below. The heavy band of snow is apparent early in the morning of 25 March; by afternoon, it has shifted eastward to hug the Lake Michigan shoreline in Wisconsin.

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