GOES-16 (GOES-East) Day Cloud Phase Distinction Red-Green-Blue (RGB) images with a overlay of GLM Flash Extent Density (above) displayed clusters of convection — some with brief bursts of lightning activity — which were moving eastward and enhancing snowfall rates from Watertown, South Dakota (station identifier KATY) into far western Minnesota on 20 October 2020. The resulting snowfall amounts included 6.0 inches near Watertown and 8.0 inches near Ortonville, Minnesota.

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1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed Tropical Depression 27 as further intensified to become Tropical Storm Epsilon at 15 UTC on 19 October 2020. While the low-level circulation (LLC) generally remained exposed during that time, deep convection was increasing around the LLC (including a small convective burst near the storm center forming around 1630 UTC).

A GOES-16 Visible image with plots of available NOAA-20 NUCAPS profiles (above) showed one valid infrared (CrIS) + microwave (ATMS) sounding (green dot) just southeast of the center of Epsilon at 1640 UTC — that sounding profile (below) revealed a moist (PW = 1.95″) and unstable (MU CAPE = 1066 J/kg, and LI = -3) atmosphere just after the time of development of the convective burst near Epsilon’s center.

GOES-16 Visible images with overlays of deep-layer wind shear and GLM Flashes from the CIMSS Tropical Cyclones site (below) indicated that Epsilon was in an environment of moderate shear, with limited lighting activity near the storm center.

A toggle between the MIMIC Total Precipitable Water and Saharan Air Layer products (below) showed that Epsilon was embedded within a pocket of abundant moisture, with dry air situated to the north and northwest.

Tropical Storm Epsilon was located over water having Sea Surface Temperature values around 28ºC and a modest Ocean Heat Content (below).

#Epsilon has formed in central subtropical Atlantic – the earliest 26th Atlantic named storm formation on record. Prior record was November 22, 2005 (Delta). Additional storm in October 2005 added after the season, which is why Epsilon breaking record set by Delta. #hurricane pic.twitter.com/NeyB1l6yrD

— Philip Klotzbach (@philklotzbach) October 19, 2020

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Low-level water vapor imagery, above, from early morning on 17 October 2020, shows the characteristics of strong low-level winds in the lee of the Colorado Rockies, namely a warm trench and herringbone-like structures that suggest turbulent flow. This region is near the Cameron Peak fire, a long-lived conflagration to the west of Fort Collins (previous blog posts on this event are here, here and here).

Shortwave infrared imagery, below, captures the regions of hottest fire activity, both with Cameron Peak and with the newer East Troublesome fire to its southwest. Clouds moving down from the north impede the satellite view of the fires at the end of the animation.

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NOAA-20 overflies Colorado twice daily; do the NUCAPS soundings produced from radiances observed by the CrIS (Cross-track Infrared Sounder) and ATMS (Advanced Technology Microwave Sounder) instruments on board NOAA-20 detect atmospheric structures (i.e., inversions) that trap energy and accompany downslope wind events?  On 16 October, in the afternoon, NOAA-20 NUCAPS soundings, below, did not show inversions.

Similarly, NUCAPS profiles from the morning pass on 17 October, however, around 0900 UTC,  did not show mid-tropospheric inversions over eastern Colorado, over the High Plains.

Some NUCAPS profiles upstream of the Front Range of the Rockies, however, showed ample evidence of inversions, especially in a region over central Colorado.

The 7.3 µm infrared image (Band 10, low-level water vapor), below, has turbulent structure near the regions where inversions were detected by NUCAPS.

A wind speed plot from the NCAR Mesa Lab in Boulder, below (source), shows the periodic strong and gusty winds on 17 October.

Use NUCAPS profiles to gauge the strength of the inversion that is associated with downslope events.

Thanks to Paul Schlatter, SOO at WFO BOU, for the idea for this blog post!

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GOES-16 (GOES-East) “Red” Visible (0.64 µm) and Day Cloud Phase Distinction Red-Green-Blue (RGB) images (above) showed multiple long, narrow northwest-to-southeast oriented swaths of snow cover extending across much of North Dakota into western Minnesota early in the day on 16 October 2020. The snow swaths — which appeared as brighter shades of green in the RGB images — slowly melted during the late morning and early afternoon hours.

A toggle between NOAA-20 VIIRS Sea Surface Temperature (SST) product and Infrared Window (11.45 µm) images at 0843 UTC (below) displayed a northwest-to-southeast cloud band that extended from Lake Sakakawea (which exhibited SST values in the low/mid 50s F) to the Bismarck (KBIS) / Mandan area. Note that Mandan (located just west of KBIS) was reporting “precipitation of unknown type” with an air temperature of 32ºF — indicating that this feature was a lake effect cloud band which was producing light precipitation (predominantly snow).

Yes there was. In fact we had some flakes from that band here at the office in Bismarck too. pic.twitter.com/h8h5Aaljlv

— NWS Bismarck (@NWSBismarck) October 18, 2020

=====17 October Update =====

On the following day, GOES-16 Day Cloud Phase Distinction RGB images (above) showed a new, broader swath of snow cover from southern Saskatchewan and northeastern Montana into North Dakota that was produced by a clipper-type disturbance (surface analyses). For this event, snowfall amounts were as high as 3.5 inches in northeastern Montana and 2.1 inches in North Dakota (NOHRSC) — so the rate of snow melt was slower than what was seen on the previous day.

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