GOES-16 True Color RGB images from the CSPP GeoSphere site (above) showed multiple mesoscale vortices embedded within the exposed Low-Level Circulation Center (LLCC) of a Tropical Depression off the southeast coast of Brazil on 18 February 2024. This disturbance continued to organize and gradually intensify during the day and into the evening hours — with part of the LLCC being drawn beneath deep convection in the SW quadrant of the system — becoming Tropical Storm Akará at 0000 UTC on 19 February (surface analysis | warning text). Tropical Storms in the South Atlantic are relatively rare — the last was Tropical Storm Iba in 2019.

A sequence of Meteosat-10 Water Vapor images with an overlay of deep-layer wind shear from the CIMSS Tropical Cyclones site (below) indicated that Akará was located within a corridor of relatively low shear — a factor that favored intensification. Sea Surface Temperature values in the area where Akará first developed (near 25ºS latitude, 40ºW longitude) were around 27ºC.

A 6-day animation of the MIMIC Total Precipitable Water product — from 13-18 February (below) — showed that Akará first began to develop along the trailing edge of a stalled cold front that had been moving northward (surface analyses). Beginning on 15 February, a broad plume of moisture from the tropics began to move south along the coast of Brazil — which then fed into the circulation of the developing tropical disturbance (which formed as a subtropical depression on 16 February), helping it to intensify.

A closer view of the MIMIC Total Precipitable Water product (below) included plots of surface and ship reports from 13-18 February. Maximum TPW values within the circulation of Akará on 18 February were around 3.5 inches (brighter shades of white). The relatively compact system was far enough offshore to not have any adverse impacts (such as strong winds) that showed up in any of the surface/ship reports.

Surface wind information from Metop-B/C ASCAT and GCOM-W1 AMSR2 (below) showed the flow within portions of the developing tropical disturbance during the 17-18 February period (source). The circulation of Akará was well-sampled by Metop-B ASCAT at 0003 UTC on 19 February, just after it reached Tropical Storm intensity.

Significant Wave Height values derived by Sentinel-3A increased from 11.27 ft at 1216 UTC on 17 February to 14.98 ft at 0038 UTC on 18 February (below) — along the southern periphery of what was still a subtropical depression.

===== 19 February Update =====

GOES-16 True Color RGB images (above) showed that there was a notable lack of sustained deep convection near the exposed LLCC on 19 February — this was likely due to an increase in shear in the vicinity of Akará (below). The tropical storm had also moved far enough south to be located over colder water, where Sea Surface Temperature values were only around 25ºC.

===== 20 February Update =====

For the second consecutive day, sustained deep convection failed to develop near the exposed LLCC of Akará (above). The MIMIC TPW product (below) indicated that a ribbon of dry air had begun to wrap into the circulation of the tropical storm, beginning on 19 February.

Akará was then downgraded to a Tropical Depression as of 0000 UTC on 21 February (below).

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Altimetric measurements of the sea surface (from this site), above, show a region of very tall waves early on 14 February, with Significant Wave Heights exceeding 30 feet. Those waves are also show below in an analysis from OPC (link); they are southwest of a hurricane-force low (surface analysis; here is a toggle between the Significant Wave heights and the surface analysis).

Forecast Waves, below, show the long wave period associated with the large waves starting to affect the Hawai’ian Islands by 1200 UTC 15 February. The north-facing shores of the Hawai-ian islands are under a High Surf Warning.

What does the satellite imagery show for the storm that supported such strong waves? Airmass RGB imagery from Himawari-9, below, (from 0000 UTC 9 February through 2300 UTC 12 February) show the development of the system and a long fetch of strong winds (by 2300 UTC 12 February). The strong storm exits the eastern edge of the domain by the end of the animation

GOES-West imagery (from the CSPP Geosphere site), hourly below from 1500 UTC on 12 February through 1900 UTC on 14 February shows the strong storm moving eastward across the Pacific, with strong nortwest winds inferred behind a propagating cold front.

For more information on this wave event, refer to the forecast office of the National Weather Service in Honolulu.

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GOES-16 Mid-level Water Vapor (6.9 µm) images with an overlay of GLM Flash Extent Density (above) showed that there were isolated brief periods of lightning activity over parts of West Virginia / Maryland / Pennsylvania during the nighttime hours, followed by more activity off the south coast of Massachusetts during the daytime hours on 13 February 2024. Although this lightning was occurring near areas receiving moderate to heavy snowfall, there were no METAR sites that explicitly reported thundersnow.

As clouds slowly began to clear, GOES-16 Day Cloud Type RGB images (below) began to reveal areas with appreciable snow cover (darker shades of green).

On the following day, with minimal cloud cover RGB imagery showed the areal extent of the swath of fresh snow cover that extended from West Virginia to Massachusetts (below). Notable snowfall accumulations included 15+ inches in Pennsylvania, New Jersey and Connecticut (storm summary).

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A sequence of Suomi-NPP VIIRS Infrared Window (11.45 µm) images (above) displayed the development of a coma-shaped cloud structure associated with a low pressure system near the North Pole (northwest of Greenland) on 13 February 2024. The single METAR surface report plotted is Svalbard, Norway (the reports for CWLT — Alert, Nunavut, Canada — were not available).

Analyses from the Canadian Meteorological Centre (below) showed the evolution of the surface low.

Given the relatively frequent overpasses of polar-orbiting satellites over the high latitudes, cloud-tracked Atmospheric Motion Vectors (AMVs) can be calculated using Infrared data from VIIRS — examples that combine AMVs from Suomi-NPP and NOAA-20 (source) are shown below.

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