Himawari-8 “Target” infrared imagery at 10.41 µm (above) and 7.35 µm (below) (courtesy JMA, the Japanese Meteorological Agency) show Typhoon Vamco as it crossed the South China Sea on 13 November, approaching Vietnam. Strong convection develops frequently in the region surrounding the not-quite-circular eye (click here for an mp4 animation), and dry air is far removed from the center, based on the low-level water vapor imagery below (click here for an mp4 animation), although it is wrapping around the southern half of the storm by the end of the animation.  (Click here for more information on Vamco from JMA).

MIMIC Total Precipitable Water fields (from this site), below, show that Vamco is preceded by relatively dry air that appears to be wrapping closer and closer to the storm (The storm is however followed by abundant moisture).  Dry air and relatively cool sea-surface temperatures (from this site) may be the reason that weakening is forecast before landfall.  Shear values remain low but are forecast to become less favorable.

For more information on Vamco, refer the SSEC Tropical Page, the JTWC or to JMA.

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GOES-16 (GOES-East) “Red” Visible (0.64 µm), Near-Infrared “Snow/Ice” (1.61 µm) and Day Snow-Fog Red-Green-Blue (RGB) images (above) showed a broad swath of fresh snow cover across parts of Nebraska, South Dakota, Iowa, Minnesota, Wisconsin and Upper Michigan on 11 November 2020. The band of heavy snow developed northwest of an intensifying midlatitude cyclone on the previous day (surface analyses) — storm total snowfall amounts included 11 inches in Nebraska, 11 inches in Minnesota, 8 inches in South Dakota, 7.5 inches in Iowa and 6.8 inches in Wisconsin.

In a comparison of images at 1501 UTC (above), note the darker shades of gray/black on the 1.61 µm image and the darker shades of red on the RGB image along the southeastern edge of the heavy snow band — this highlighted areas where a significant accrual of ice resulted from freezing rain/drizzle. As seen in a plot of ABI Spectral Response Functions (below), snow and ice are efficient absorbers of radiation (and therefore exhibit a low relectance) at the 1.61 µm wavelength, making them appear darker on the Snow/Ice and RGB images — and since ice absorbs more strongly than snow, it appears as the darkest shades of gray/black (1.61 µm) or red (RGB).

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A toggle between Suomi NPP VIIRS Infrared Window (11.45 µm) and Day/Night Band (0.7 µm) images at 0734 UTC (above) showed Tropical Storm Eta over the Gulf of Mexico (northwest of Cuba) on 10 November 2020. A large convective burst was seen southeast of the storm center, with concentric cloud-top gravity waves propagating radially outward from its lightning-illuminated core (intense lightning activity was causing the cluster of bright pixels on the Day/Night Band image).

1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Clean” Infrared Window (10.35 µm), GLM Flash Extent Density and “Red” Visible (0.64 µm) images (below) showed Tropical Storm Eta from sunrise to sunset, as it continued a slow northward movement — pulsing overshooting tops occasionally exhibited infrared brightness temperatures of -90ºC and colder (yellow pixels embedded within darker shades of purple), and lightning activity persisted for much of the day.

===== 11 November Update =====

Eta once again reached Hurricane intensity at 1235 UTC on 11 November, as it approached the west coast of Florida. 1-minute GOES-16 Visible images (above) showed the partially exposed low-level circulation of Eta; however, it then weakened back to a Tropical Storm several hours later, at 1800 UTC.

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GOES-16 (GOES-East) Upper-level Water Vapor (6.2 µm) and Air Mass Red-Green-Blue (RGB) images (above) displayed a series of shear vortices migrating southwestward over the western US on 09 November 2020. The “dynamic tropopause” — taken to be the pressure of the PV1.5 surface — descended to the 500-600 hPa level within the largest and most well-defined vortex that was moving over Montana and Idaho. These features displayed hues of red to orange in the Air Mass RGB images (for example, at 2101 UTC), indicative of the dry and ozone-rich stratospheric air within the vortices. Aircraft reports of turbulence are sometimes seen in the general vicinity of these shear vortices, as the local tropopause is deformed (such as on 07 June 2017); in this case, there were only two instances of turbulence reported (at 1545 UTC and 0200 UTC).

The GOES-16 Water Vapor image at 2101 UTC (above) showed the northwest-to-southeast oriented cross section line I-I’ — and RAP40 model fields along that line (below) revealed the descent of stratospheric air (characterized by low values specific humidity along with high values of potential vorticity) within the shear vortex when it was located near the Idaho/Montana border.

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