Damaging waves hit the Marshall Islands on 19 January 2024, causing considerable damage, and the northern shores of American Samoa on 22 January 2024, leaving behind considerable debris, as shown below (photos courtesy NWS Pago Pago).

The waves could be tracked using Altimetric data as viewed at this NOAA/NESDIS website; data for the globe and also data around the Marshall Islands were saved from that website and shown below. A second website (that contains data over South Pacific around Samoa, which data are routinely taken from the NOAA/NESDIS website and stitched into a larger field) was used to save imagery around American Samoa as shown below. Plots at these websites show Significant Wave Height, that is, the mean of the highest 1/3rd of the waves. The global animation, below, centered on the Pacific, shows a region of higher waves moving southward through the western Pacific; stars show the location of the Marshall Islands (20 January) and American Samoa (22 January), at the approximate time of the waves’ arrival at those locations.

The animation below is zoomed in over the Marshall Islands, and covers 0^o to 20^oN Latitude, 180-150^oE Longitude. Note the southward progression of progressively larger waves through the animation. The purple box is the approximate location of Roi-Namur, the island where the damage discussed in the Army Times article above occurred.

The animation below shows American Samoa (highlighted in the Green Box). Wave heights increase from around 7 to 11 feet in advance of the large waves’ arrival on Tutuila and Manu’a. A High Surf warning was issued by the forecast office in Pago Pago.

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What caused these waves? Forecast model output (courtesy Eric Lau, NWS PRH), below, shows hurricane-force winds (in yellow, between 160^o and 170^oE longitude) generated by a potent extratropical cyclone to the east of Japan. The strong winds are present for about 24 hours, and the region of strongest winds translates with the predicted wind direction. The similarities between the wind direction and the motion of the speed maximum suggest that dynamic fetch may have been responsible for the extraordinary waves.

WPC OPC analyses, below, (also from Eric Lau) show the rapid development of the storm from 1003 mb on 1200 UTC.16 January 2024 to 964 mb on 1200 UTC/17 January 2024. The storm started to weaken starting at 1200 UTC/18 January 2024.

What did the satellite data look like during this time period? The airmass RGB animation, below, (created from geo2grid using Himawari-9 data supplied by JMA), shows the development of a strong, but not necessarily unusually strong, storm. Recall that the strongest winds were predicted to be between 160 and 170^oE, from 1200 UTC on the 17th to 1800 UTC on the 18th.

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A toggle between NOAA-20 VIIRS Day/Night Band and Infrared Window images valid at 0827 UTC (2:27 AM local time) on 22 January 2024 (above) showed an opening of the Kivalliq Polynya along the northwest coast of Hudson Bay — which was the result of a period of strong offshore winds (Arviat Airport CYEK observations), which tend to push thicker first year ice away from the Nunavut and Manitoba coast. With cold arctic air blowing across the polynya, a layer of thin ice quickly begins to grow where the thicker ice was displaced.

During the following daytime hours, a comparison of NOAA-20 VIIRS Visible, Near-Infrared and Infrared Window images valid at 1820 UTC (below) displayed the lower reflectance of the polynya ice in Visible imagery (which was also evident, via reflected moonlight, in the earlier Day/Night Band image) — along with horizontal convective rolls in the Near-Infrared image, highlighting bands of blowing snow moving east-southeastward across Hudson Bay.

In a toggle between NOAA-20 VIIRS Visible and ATMS MiRS Sea Ice Concentration images (below) the Sea Ice Concentration within the polynya was as low as 88% (darker red pixels).

Suomi-NPP VIIRS Ice Surface Temperature (IST) and Sea Ice Concentration (SIC) products (below) displayed gradients in those parameters within the polynya, with warmer IST and lower SIC values closer to the coast. The IST gradient was more notable, being about 10 K warmer near the coast.

A sequence of GOES-16 True Color RGB images (source) from 1540-2030 UTC on 21 January and 22 January (below) showed a widening of the Kivalliq Polynya during that 2-day period.

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The CSPP Geosphere site uses cloud-compatible technology with an on-site cluster and CSPP-Geo software (including geo2grid) to provide high-quality imagery from GOES-East and GOES-West. The site has recently been upgraded, with two level 2 products added (Cloud-top Height and Cloud-top Temperature). In addition, the landing (front) page of the site has been improved, as shown in the toggle above. The displayed image remains True Color (day time) and Night Microphysics (night time), but icons have been added on the left (as indicated) to change the Satellite (GOES-East and GOES-West are available); to change the Sector (Mode 6: Full Disk, CONUS/PACUS, 2 Mesoscale Sectors; Mode 4: Full Disk); to select the number of Frames (Default: 12 timesteps, no times skipped); to select a location on which to center; to select a latitude/longitude point on which to center; to toggle on/off auto-reloading, and to toggle on/off a mouse probe. On the right hand side, lower right, are icons to bring up the share menu, and also to enlarge the image to fit the screen. In the bottom left is link to a menu to select the products to be displayed, as shown below.

The Layers highlighted in Blue are what is currently displayed; these can be toggled off and on. Other layers that can be shown are in gray. Click a (grey) box to toggle on the band or product (or map/lat-lon grid); note that only three Bands/Level 2 fields can be displayed at once. When more than one field is displayed, they are overlain on top of each other, and can be toggled on and off for quick qualitative comparisons between fields. The Level 2 products — Cloud Top Height and Cloud Top Temperature — have information only where clouds are present, and are transparent otherwise.

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The screen capture below shows the result of two samplings at 1850 UTC on 22 January 2024 (Full Disk GOES-16 Data): for single band data, the probe is detecting the grey-scale in the 8-bit image, and uses knowledge of the enhancement to determine a brightness temperature or reflectance. For Level 2 products (Cloud Top Height and Cloud Top Temperature), the probe again detects the color shown, and displays the value range appropriate for the sensed color. In other words, the probe is not accessing the raw data used to create the image.

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NOAA-20 overflew the Samoan islands shortly after 1200 UTC on 22 January 2024, as shown above. The animation displays a series of 13 profiles and derived parameters to the east of American Samoa and Samoa. with locations as indicated by the circle. Most Unstable CAPE values derived from the profiles all are derived from the near-surface 986 mb — except for the profile location denoted in yellow, which has a MUCAPE level at 959 mb. That point also has the smallest value of MUCAPE: <200; Other values range from 2200 to as high as 4400. NUCAPS profiles are a useful source of thermodynamic information (independent of models) in regions where data are lacking.

GOES-18 also viewed this region, and the toggle below compares derived Total Precipitable Water and Lifted Index at 1200 UTC, the approximate time of the NOAA-20 overpass. The TPW shown below shows structures and values similar to the MIMIC TPW shown above. A careful observer will also see the convection below associated with the red NUCAPS sounding availability point above (many red points associated with the large convective complex near 17^o S, 166.5^o W are hidden in the figure above, but are viewable here). GOES Level 2 products are able to highlight relatively small-scale variability in moisture/stability (for example, the region of relatively stable air stretching southeast to northwest — tannish brown in the LI color enhancement — to east and north of the Samoan Islands. Use them in concert with NUCAPS fields (available in all sky conditions) to get a good feel of the moisture distribution in data-poor regions.

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Important NUCAPS note: NOAA-21 NUCAPS profiles are scheduled to flow via SBN to NWS forecast offices in February. At present, they will augment (not replace) NOAA-20 profiles, allowing forecasters to see short-term trends in NUCAPS fields.

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