Tropical Cyclone Harold made landfall on Vanuatu’s largest island of Espiritu Santo as a Category 4 storm (ADT | SATCON) on 05 April 2020. JMA Himawari-8 Infrared Window (10.4 µm) images (above) showed the emergence of a well defined eye — which exhibited some trochoidal motion. Cloud-top infrared brightness temperatures were -90ºC and colder (yellow pixels embedded within dark purple) before the eye became evident. The structure of the eye was seen to deteriorate somewhat after Harold made landfall.

Himawari-8 “Red” Visible (0.64 µm) images (below) revealed a similar deterioration of the eye after landfall.

VIIRS Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP as viewed using RealEarth (below) provided more detailed view of Harold several hours before landfall.

A toggle between NOAA-20 VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images at 1359 UTC (below) helped to highlight the cloud-top gravity waves between the eye of Harold and the west coast of Espiritu Santo.

A DMSP-18 SSMIS Microwave (85 GHz) image from the CIMSS Tropical Cyclones site (below) showed Harold at 1719 UTC.

===== 06 April Update =====

Himawari-8 Infrared images (above) showed Harold as it intensified to a Category 5 storm while moving across the islands of Vanuatu on 06 April.

 

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A thunderstorm over Jackson County, Oregon late in the day on 5 April 2020 produced abundant hail that accumulated to depths in excess of an inch over a wide swath of the county.  Visible animation from 1500 UTC on 5 April through 0300 UTC on 6 April show the region experiencing plentiful sun during the day, helping to destabilize the lower troposphere.  Much of the hail fell in the hour between 0030 and 0130 UTC on 6 April.  That time is captured in the visible loop above, and in the rocking animation from 00 to 02 UTC on 6 April 2020 (and back) below.  Similar animations (from 1500 UTC 5 April to 0300 UTC 6 April) are also available for GOES-17 Band 13 (10.3 µm) and the Day Cloud Phase RGB.

The storms formed in a trough at 500 mb that was associated with abnormally cold temperature (image below courtesy Mike Stavish, WFO MFR). This feature is also discussed in this blog post.

The 0000 UTC Upper-Air sounding at Medford, below, (also courtesy Mike Stavish) shows modest Convective Available Potential Energy (CAPE) amounts and a low freezing level (less than a mile above ground) suggest the possibility of hail.

Indeed, the storm in question was a prolific hail producers, as shown in photographs below (photos courtesy Dan Weygand, WFO Medford)

 

NOAA/CIMSS ProbSevere readouts in a timeseries, below (courtesy John Cintineo, UW-Madison CIMSS), shows very low values of parameters typically associated with severe weather.  ProbHail peaks at only 2%. The animation above shows very small values (Click here for a ProbSevere image at 0130 UTC with a Probe that shows values).   Such small values during this event amplify the message that local knowledge of conditions that are favorable for severe weather (and hail events like this that coat roads can lead to traffic crashes and fatalities) are important.

Rocking animations that cover the time of hail fall are shown below.  Because the storm formed/decayed as the sun was setting, use of visible imagery (at top) or Day Cloud Phase Distinction (that uses the visible 0.64 µm in the ‘green’ and the snow/ice near-infrared 1.61 µm in the ‘blue’) was a challenge.  Clean window imagery of course maintained a consistent signal through sunset.

Note the Day Cloud Phase Distinction does capture the glaciation of the cloud.  Sometimes this can be used to judge when lightning might become a hazard.  Lightning was a rare event on this day.

Rocking Animation of GOES-17 Band 13 Clean Window Infrared imagery (10.3 µm) from 0000 UTC to 0200 UTC (and back) on 06 April 2020 (Click to enlarge)

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NOAA-20 overflew the West Coast, and NUCAPS soundings were produced, at 2047 UTC on 5 April.  (Click here to see NOAA-20 orbits on that day, from this site).  Gridded NUCAPS fields (from this site; update: from this site) of 500-mb Temperatures, Ozone Anomalies, and 850-500-mb Lapse Rates, below (imagery courtesy Emily Berndt and Frank LaFontaine, NASA Sport), show the cold air in the cutoff and the unstable environment that supported the convection.

(Thanks to Mike Stavish, Science and Operations Officer — SOO — in Medford for alerting us to this case)

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GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) showed a variety of precipitation types as a strong cold front moved slowly eastward across the Upper Midwest during the 02 April – 03 April 2020 period — including thundersnow snow at Aberdeen, South Dakota and Oakes, North Dakota (at 18 UTC on 02 April) and freezing rain at a few sites in northern Iowa, the eastern Dakotas and western Minnesota.

On the following day, in a comparison of GOES-16 “Red” Visible (0.64 µm) and Day Snow Fog Red-Green-Blue (RGB) images (below), the RGB images revealed darker red swaths across southeastern South Dakota, northwestern Iowa and western Minnesota (at the 1.61 µm wavelength, ice absorbs radiation more strongly than snow — which contributed to the darker appearance of those swaths). Note that these ice accrual swaths exhibited no signature in the Visible imagery.

Props to Carl Jones (NWS Grand Forks) for pointing this feature out.

Dark red swath extending through Minnesota is #ice accretion from April 2-3 winter storm. Strong infrared absorption in the 1.61 component of the Day Snow-Fog RGB makes it darker compared to snow. Clearly shows where ice accretion occurred whereas visible cannot. #ndwx #mnwx pic.twitter.com/L1rs7EskLa

— Carl Jones (@northflwx) April 4, 2020

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A sequence of GOES-16 (GOES-East) “Red” Visible (0.64 µm), Mid-level Water Vapor (6.9 µm) and Air Mass Red-Green-Blue (RGB) mages (above) showed an occluding Hurricane Force low pressure system (surface analyses) off the US East Coast on 01 April 2020. In the Air Mass RGB images, darker red areas just south of the storm center indicated the presence of higher amounts of total column ozone, brought about by a lowering tropopause — RAP40 model fields of the PV1.5 pressure (representing the height of the “dynamic tropopause”) suggested that the tropopause had descended below the 500 hPa pressure level later in the day.

The hurricane-force winds at the surface were creating seas as high as 33 feet. The milky/hazy signature of a highly-agitated sea surface + sea spray — immediately south of the convection around the core of the storm — was evident in GOES-16 True Color RGB images, created using Geo2Grid (below).

GOES-16 Visible images with an overlay of GLM Flash Extent Density (below) revealed that lightning activity gradually decreased within convection surrounding the core of the low during the day.

As the storm was becoming organized near the Southeast US coast during the preceding overnight hours, a toggle between Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images at 0627 UTC (below) showed widespread mesospheric airglow waves (reference) in the Day/Night Band, propagating across the western Atlantic Ocean — these waves were likely generated by the approach of an upper-tropospheric jet streak. Note that the wave features were not apparent in the corresponding VIIRS Infrared image.

Addition information about this event is available on the Satellite Liaison Blog.

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