Tropical system Marco (formerly a category 1 Hurricane) was weakened by shear overnight and early morning on 24 August (as forecast).  The half-hourly animation above, of GOES-16 Band 10, i.e., infrared “low-level” water vapor (7.3 µm), overlain with derived motion winds at upper (red) and lower (blue) levels (Click here to see only the water vapor animation;  here is the shear analysis from the SSEC Tropical website; Note in the animation how the number of low-level vectors increases greatly at the end of the animation as visible imagery becomes available at sunrise.  Also:  the shear in the eastern part of the domain, over the Florida Straits, suggests a favorable environment for Tropical Cyclone Laura, approaching western Cuba from the east) shows the effect of shearing.

Low level easterlies and upper-level southwesterlies mean that the vertical structure of the storm was interrupted:  the low-level circulation decoupled from the upper-level.   That is, the low-level circulation moved to the west as the mid- and upper-level parts of the storm moved north and east.  Result:  By sunrise, visible imagery (0.64 µm) showed the low-level swirl of the elongating near-surface circulation southeast of the mouth of the Mississippi River, and south and west of the main convection over and offshore of the northwest Florida panhandle.

For more information on Tropical Storm Marco, refer to the pages of the National Hurricane Center.

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1-minute Mesoscale Domain Sector GOES-17 (GOES-West) “Red” Visible (0.64 µm), Shortwave Infrared (3.9 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed the formation of a pyrocumulonimbus (pyroCb) cloud that was spawned by the Castle Fire in California on 23 August 2020.

The coldest pyroCb cloud-top infrared brightness temperatures were around -50ºC –according to 00 UTC rawinsonde data from Las Vegas, Nevada (below) indicated that this temperature corresponded to altitudes around 12 km.

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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 — with and without an overlay of GLM Flash Extent Density (above) showed Marco as it intensified from a Tropical Storm to a Category 1 hurricane at 1630 UTC on 23 August 2020. A pronounced semi-circular convective burst was seen to develop near the storm center shortly before 19 UTC.

A toggle between time-matched Infrared images from Suomi NPP and GOES-16 (below) indicated that the coldest cloud-top infrared brightness temperature on the Suomi NP VIIRS image was -86.8ºC, compared to -81.5ºC from the GOES-16 ABI instrument (the same color enhancement has been applied to both images). The northward parallax displacement associated with GOES-16 imagery over the southern Gulf of Mexico was also apparent.

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Tropical Storm Marco over the northwest Caribbean Sea, shown above in a toggle of GOES-16 ABI Clean window imagery (10.3 µm) and a NUCAPS diagnosis of 850-500mb lapse rates, is over very warm waters and in a region of favorably low diagnosed vertical wind shear. (Wind shear is from this website, (direct link to shear); the product is described here)  An inhibiting feature in strengthening, as noted in the National Hurricane Center discussion, is stable air.

NOAA-20 overflew Marco shortly before 0700 UTC on 22 August, and NUCAPS soundings derived from CrIS and ATMS data on NOAA-20 showed the stable mid-tropospheric air surrounding the storm. The 850-500 mb lapse rates were around 5.5 C/km. (Gridded NUCAPS data can also be viewed here) Individual soundings over western Cuba and just off the northwest tip of the Yucatan peninsula also show the relatively stable air. (Click here to see the Lapse Rate Analysis overlain with NUCAPS Sounding Availability points from the AWIPS display)

Marco’s future path is forecast to move over very warm waters that are shown in the ACSPO analysis of SSTs, below, from Suomi-NPP VIIRS data.  Consult the National Hurricane Center for the latest on this storm that will likely affect the Texas/Louisiana Gulf Coasts.

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