Strong gap winds accelerating out of the Copper River Valley along the southern coast of Alaska were lofting fine particles of glacial silt/sand and transporting those aerosols southward across the Gulf of Alaska on 20 October 2020. 1-minute Mesoscale Domain Sector GOES-17 (GOES-West) “Red” Visible (0.64 µm) and Dust Red-Green-Blue (RGB) images (above) displayed the plume of airborne dust during the period of 1700-2300 UTC.

A good visualization of the dust plume was provided by GOES-17 True Color RGB images created using Geo2Grid (below).

A comparison of Suomi NPP VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images at 2040 UTC is shown below — note that the dust plume appeared warmer (darker shades of gray) on the Shortwave Infrared image, since the small dust particles were efficient reflectors of incoming solar radiation.

The dust plume also exhibited a good signature in the VIIRS Aerosol Optical Thickness (AOT) product from the eIDEA-AK site (below).

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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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