Much of the mid-Mississippi River valley was placed under a Moderate Risk of severe weather by the Storm Prediction Center on 31 March 2023. Day Night Band imagery from NOAA-20 (mislabeled as Suomi NPP), above, shows two bore-like features with very different orientations (there is also a notable lightning feature over eastern South Dakota!). The feature over Iowa is oriented southeast to northwest, the one over Minnesota — at a lower level (note the shadow cast by the higher clouds over Iowa on the lower clouds over Minnesota) — is oriented southwest to northeast. Winds are typically perpendicular to such cloud bands, so this one still image suggests very strong shear (consistent with the SPC forecast). The 0000 UTC SkewT from Minneapolis/Chanhassen, below, from the Wyoming Sounding site (here’s the AWIPS NSharp image), shows very strong low-level shear and veering winds. Winds below 850 are nearly perpendicular to the cloud bands in southern Minnesota; winds above 850 are nearly perpendicular to the cloud bands in north-central Iowa.

Surface observations overlain on top of the Day Night Band imagery, below, show a 180-degree windshift across the cloud band feature in Minnesota. The influence of the Iowa Bore does not appear to extend to the surface, perhaps because of the strong inversion present.

Shear Analysis (Surface to 3 km) from the Storm Prediction Center, below, for 0800 UTC, does show a change in shear from the environment near the northern bore feature over southern Minnesota to the environment surrounding the southern bore feature over northern Iowa.

The Day Night band image above is also available (for 5 days) at the CIMSS Direct Broadcast ftp site (here) and at the CIMSS VIIRS Image Viewer (here).

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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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