The NWS Aberdeen forecast office issued a Snow Squall Warning on the morning of 09 April 2020 — and a sequence of GOES-16 (GOES-East) “Red” Visible (0.64 µm), “Clean” Infrared Window (10.35 µm) and Day Cloud Phase Distinction Red-Green-Blue (RGB) images (above) showed the southeastward movement of numerous convective cloud features responsible for the snow squalls. Cloud elements whose tops were glaciating exhibited shades of green in the RGB images; however, most cloud-top infrared brightness temperatures were generally rather warm (therefore lacking a color enhancement). No signatures of lightning were seen in GLM Flash Extent Density data with this shallow, low-topped convection. Of particular interest was the ~20 mile wide northwest-to-southeast oriented swath of fresh snowfall produced by these snow squalls, which passed through Aberdeen (KABR) and could be seen through gaps in the clouds on Visible (shades of white) and RGB images (shades of green); Aberdeen received 0.3 inch of snowfall.

Farther to the south and east, NWS Sioux Falls also issued a Snow Squall Warning — similar signatures of convective elements were seen in the GOES-16 imagery (below), including the glaciation of some of the cloud tops. Snowfall amounts were generally light (around 0.1 inch), but surface visibility was reduced to zero in some of the snow squalls.

In addition, NWS Duluth issued a Snow Squall Warning; the corresponding GOES-16 imagery is shown below. Snowfall reports included 1.4 inches at Duluth (accumulating within 30 minutes) and 2.5 inches at Butternut, Wisconsin.

It should be noted that these were the first Snow Squall Warnings issued by each of the 3 NWS forecast offices.

A snow squall warning continues until 9:45 AM CDT for US-12, US-281 near Aberdeen, SD. pic.twitter.com/Mth1pPxq5d

— NWS Aberdeen (@NWSAberdeen) April 9, 2020

A snow squall warning is in effect until 11:00 AM CDT for I-29, US-14 near Brookings, SD. pic.twitter.com/u0LuYBJ2WR

— NWS Sioux Falls (@NWSSiouxFalls) April 9, 2020

A snow squall warning is in effect until 8:30 AM CDT for I-35 near Cloquet, MN and I-35, I-535, US-2, US-53 near Duluth, MN–WI. pic.twitter.com/mz3R9UBjsy

— NWS Duluth (@NWSduluth) April 9, 2020

Important aspects of the boundary layer across the Upper Midwest were revealed by plots of rawinsonde data from Aberdeen, South Dakota and International Falls, Minnesota (below) — steep low-level lapse rates (surface – 3 km values of 7ºC per km) with ample moisture, and strong winds. The upper portion of the shallow convective layers were within the important -12 to -18ºC dendritic growth zone.

A time-lapse video of snow squalls that moved through Madison, Wisconsin can be seen here.

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GOES-17 (GOES-West) Mid-level Water Vapor (6.9 µm) images from the AOS site (above) showed the broad circulation around an anomalously-deep middle tropospheric cutoff low centered along the California coast on 08 April 2020. The system was producing record daily rainfall amounts, which in tandem with the clouds kept daily high temperatures at record low values at some locations across Southern California.

Overlapping 1-minute Mesoscale Domain Sectors provided GOES-17 “Red” Visible (0.64 µm) images at 30-second intervals (below) — which revealed the southeastward propagation of a compact circulation of clouds west of San Nicholas Island.

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CIMSS has long hosted the popular site ‘MODIS Today‘ that allows easy access to full-resolution imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board Terra (launched in 1999 and still producing imagery) and Aqua (launched in 2002 and still producing imagery). That site remains active and allows access to imagery back to 2007.

Ongoing work at CIMSS means a similar site for VIIRS imagery will soon be available, allowing access to full-resolution VIIRS imagery. For example, the Day-Night Band image above, during the Full Moon on 8 April 2020, is presented in one region of the USA — the Pacific Northwest just after 0900 UTC, on a clear night. The image below shows the midwestern United States at around 0700 UTC. (Reminder:  NOAA-20 Orbits can be viewed here).

Stay tuned!

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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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