Tropical Storm Henri intensified to become a hurricane at 1500 UTC on 21 August 2021  — and 1–minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) images and “Clean” Infrared Window (10.35 µm) images (above) showed Henri during the 1400-2300 UTC period. Visible images indicated that a small-diameter inner core began to form later in the day.

Henri was moving through an environment of light to moderate deep-layer wind shear, as seen in an animation of GOES-16 Infrared images from the CIMSS Tropical Cyclones site (below),

During the preceding nighttime hours, a toggle between Suomi NPP VIIRS Infrared and Day/Night Band images at 0710 UTC (below) showed a large convective burst east of the surface center.

 

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Training thunderstorms early Saturday caused historic rains and devastating (and deadly) flash floods early on Saturday morning 21 August 2021. (Click here for a listing of hourly precipitation totals at McEwen, TN; this excellent tweet from Tomer Burg shows the radar in the region). Were there satellite products that described the environment, rich in moisture, that allowed the flooding rains to occur? The animation above shows MIMIC Total Precipitable water at hourly timesteps from 1000 UTC on 20 August 2021 to 1200 UTC on 21 August 2021. There is a pronounce gradient in moisture established over middle Tennessee by 0400 UTC on 21 August, and the amount of moisture is very large!

The rain that fell is analyzed below (data source: MRMS). The heavy rain caused drastic (and record-setting) stream flows and water heights on the Big Piney River. The flooding also closed I-40 temporarily.

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The animation below shows the GOES-16 Low-level water vapor imagery. A pronounced boundary is apparent near the region where the flooding rains developed. (Band 8 imagery — upper-level water vapor infrared (6.19 µm) imagery — shows a gradient in the same region as well)

GOES-16 Band 13 Clean Window infrared (10.3 µm) imagery, below, overlain on top of the Level 2 Total Precipitable Water (TPW) product document the development of the training thunderstorms along the TPW gradient. The motion of the storms is along the gradient. Numerous blog entries at the Hazardous Weather Testbed discuss how convective development is favorable along gradients, and in this case the developing thunderstorms maintain access to the moisture.

Derived Motion Winds can be used to estimate the winds surrounding the developing convection, and the animation below, from 0000 to 0956 UTC, show primarily northwesterly flow, so if the storms are moving with the ambient flow — and there’s no guarantee that that’s happening — you can infer their motion. Derived motion winds in the vicinity of the developing storms are primarily northwest and north-northwest. That might help a forecaster anticipate training with convection.

How do you know when anticipated thunderstorms within a primed region are ready to erupt — especially at night when visible imagery are unavailable (except for Day Night Band imagery; with Suomi-NPP (flying over at 0641 UTC and NOAA-20, flying over at 0720 UTC you can get a 50-minute animation in the middle of the night, shown here, with data from VIIRS Today). The animation below, of the GOES-16 Night Time Microphysics RGB, follows an example discussed here by Carl Jones, WFO FGF. The appearance of red values in the RGB over west-central TN around 0501 UTC heralds the development of deeper convection. This should also be the time when GLM lightning observations develop. See the second animation below.

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It turns out the relationship between color change in the RGB and lightning initiation isn’t quite so clear-cut. The stepping animation below shows the Night Time Microphysics and the GOES-16 Cloud Phase product. It is a challenge to relate a cloud phase to a particular color. GLM observations are occurring in regions where ice clouds are present. However, there are also regions where ice clouds are diagnosed and lightning is not happening.

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The Weather Prediction Center (WPC) had two Mesoscale Discussions on this event, here and here.

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This event was discussed at an FDTD Webinar (link) led by Matt Reagan and Brendan Schaper from the Nashville forecast office. (Storyboard Link)

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GOES-16 Visible Imagery, above, shows Tropical Storm Henri east of Florida and to the north of the Bahamas. Cirrus outflow is well-developed to the east and south, but the storm is sheared; the surface circulation is near the northwestern edge of the ongoing deep convection. Persistent northerly shear, as shown below in a figure taken from the CIMSS Tropical Website, shows Henri moving from a region of relatively high shear towards a region of relatively smaller shear.

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GCOM-W1 overflew Henri at 0616 UTC on 20 August. Microwave imagery, below, (retrieved from the AOML Direct Broadcast site) confirm the sheared nature of the storm at that time. Both frequencies show that deep convection is displaced to the south of the storm center.

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Suomi NPP overflew the storm at around 0600 UTC, and the toggle below shows the Day Night band visible (0.7 µm) imagery and the I05 infrared (11.5 µm) imagery; included in the toggle are Advanced Clear Sky Processor for Oceans (ACSPO) SST values in clear regions. Atlantic Ocean waters have surface temperatures in the 84-85º F range. Cloud top temperatures are as cold as -86º F for this overpass. (Suomi NPP VIIRS data were processed at the CIMSS Direct Broadcast site and injected into AWIPS).

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The animation of the Airmass RGB, below, shows a prominent feature over the mid-Atlantic states of the United States that will have a big role in Henri’s future path: a mid-tropospheric potential vorticity maximum highlighted as red/orange in the Airmass RGB. The toggle at bottom compares the 1211 UTC Airmass RGB with contours of pressure (mb) on the 1.5 PVU surface.

For the latest on Henri, refer to the National Hurricane Center Website. Residents of southern New England and Long Island and New Jersey in particular should pay close attention to this storm.

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Grace (then a tropical storm) is shown in AWIPS above, in between Jamaica and the Caymans, in Day Night Band Visible imagery, along with VIIRS I05 Infrared imagery, from 0626 UTC, using data from the Direct Broadcast site at CIMSS. The coldest cloud tops with the most vigorous convection, shown as white/blue in the color enhancement, are near -89º C. The visible imagery has sufficient lunar illumination (more than on Monday 16 August with Fred!) that the tallest cloud tops are casting shadows. The few cloud-free pixels that are present allow ACSPO sea-surface temperatures to be computed: 30º C or 86º F.

NOAA-20 and Suomi-NPP give infrequent views of the storm. GOES-16 is the better choice for storm monitoring. The animation below shows the Convection RGB over the storm. The Convection RGB is useful because it can highlight regions of vigorous convection (in orange/yellow). The RGB highlights convective banding near the center of the storm and also on the periphery of the storm.

GOES-16 Water Vapor imagery, below, shows the environment in which the storm is forming. The upper-level water vapor imagery, 6.19 , below, begins near the time of the Suomi-NPP overpass (imagery shown above)and continues until about 1600 UTC. It shows a rapid organization to the storm — certainly the satellite presentation changes over the 10 hours of this animation! There does seem to be dry air to the north of Grace, just north of Cuba. How that affects the storm in the future is to be determined.

Total Precipitable Water fields, below, for the 24 hours endings at 1500 UTC on 18 August, also show some dry air near the storm. However, Grace has access to plenty of moisture in the near term. Tropical storm Henri, Pacific Hurricane Linda, and the remains of Fred are also apparent in the imagery.

Grace is in an environment of low shear, and is over very warm water, as shown in this image, a 1500 UTC 200-850 mb shear and SST analysis taken from the CIMSS Tropical Website.

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JAXA‘s GCOM-W1 satellite, carrying the AMSR-2 instrument, overflew Grace near 1900 UTC on 18 August, and imagery (taken from the AOML Direct Broadcast website) from 36.5 and 89.0 GHz is shown above (note that 89.0 GHz imagery is used in MIMIC-TC imagery available at the CIMSS Tropical Website). Both microwave representations show a distinct eye in the storm, most especially at lower levels.

True-Color imagery from CSPP Geosphere, below, shows convection persistently developing around the storm center as it moves through the northwest Caribbean towards the Yucatán Peninsula.

1-minute Mesoscale Domain Sector #GOES16/#GOESeast Infrared images of #Grace, from the time it reached hurricane intensity (15 UTC on 18 August) to it making landfall along Mexico's Yucatan Peninsula (0945 UTC on 19 August): https://t.co/Qa6m8flCs5 pic.twitter.com/VfgnjnEuv0

— Scott Bachmeier (@CIMSS_Satellite) August 20, 2021

More information on Grace is available at the National Hurricane Center.

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