Due to a lack of radar coverage over American Samoa, WSO Pago Pago requested 1-minute Mesoscale Domain Sector coverage over the islands during a prolonged period of heavy rainfall risk. GOES-18 (GOES-West) Clean Infrared Window (10.3 µm) images (above) displayed areas of deep convection that moved near and over the American Samoa island of Tutuila (where Pago Pago NSTU is located) on 9th January 2025 — which produced periods of moderate to heavy rainfall (leading to flash flooding and landslides, prompting the issuance of Flash Flood Warnings; Pago Pago received 3.19 inches of rainfall that day), strong winds (gusting 45 mph at Pago Pago, which led to flight cancellations) and power outages across parts of the islands. The coldest cloud-top infrared brightness temperatures were around -80ºC (shades of white embedded within dark black regions).

The GOES-18 Infrared image at 2100 UTC (below) included cursor samples of the Cloud Top Temperature, Cloud Top Height and Rain Rate Level 2 derived products for an area of convection that was approaching the island of Tutuila.

These thunderstorms and strong winds developed as Tropical Disturbance TD04F was located southwest of American Samoa (Fiji Meteorological Service surface analyses: 0300 UTC  | 1230 UTC | 2100 UTC), in addition to the South Pacific Convergence Zone being oriented NW to SE across the Samoan Islands.

A toggle between plots of rawinsonde data from Pago Pago (below) depicted an atmosphere with deep moisture throughout the troposphere (Total Precipitable Water values of 2.45-2.49 in) and modest instability (CAPE values of 1242-1642 J/kg, and Lifted Index values of -3ºC).

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Screen captures of VIIRS Day night imagery from NASA Worldview, above, show the distinct signature of light emitted from fires on 8 January. Three separate fires are apparent. The toggle below compares the views early on 7 and 8 January.

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Day Night Band Imagery from early on 9 January 2025, shown below in a toggle with VIIRS I04 (3.74 µm) shortwave infrared imagery shows two main fire areas, the Eaton and Palisades fires, have strong visible light signatures and strong shortwave infrared signatures as well. On 9 January, the faint outline of a smoke plume is apparent over the Pacific Ocean to the south of Malibu and west of Manhattan Beach.

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Strong Santa Ana winds and dry conditions caused multiple fires in Los Angeles area on 7-8 January 2025 (here is a blog post on the Palisades fire, for example). What information on these fires is provided by the Next-Generation Fire System (NGFS) website? The Alerts Dashboard at that site shows the latest Thermal Anomalies; when a previously un-detected fire develops, it is highlighted on that website. Consider the screenshot below, from ca. 0214 UTC on 7 January. Two separate fires are indicated in Los Angeles county; the county name is highlighted in magenta because Los Angeles County is ‘Rank 1’ as shown at the bottom of the page: SPC has highlighted the county as one defined by Extreme Fire Weather criteria, and/or there is a Red Flag warning. In other words, the likelihood of rapid fire growth is present. Other counties listed show less hazardous ranks. For Los Angeles County at 0214 UTC, the oldest detection (of the Palisades fire), from 7h 48 minutes before, occurred with GOES-18 Mesoscale Sector 2; GOES-18 CONUS data detected this fire 5 minutes after that, and GOES-16 CONUS’s detection lagged the GOES-18 Mesoscale sector detection by 16 minutes! The lesson from this: Use a mesoscale sector when possible for fire detection.

At 1439 UTC, the Alerts Dashboard for Los Angeles County shows much more activity, with at least three new thermal anomalies shown. For all the fires, the earliest detection was achieved by the GOES-18 Mesoscale domain (1-minute imagery) sector.

Clicking on the ‘Satellite Imagery’ box for the detection that’s 4 lines down in the image below — GOES-18 Mesoscale Sector 2, from 8h and 26 m before the time of the ‘Last Updated’ time (1439 UTC) yields the display below, NGFS Microphysics imagery centered on the new detection (to become the ‘Hurst’ Fire) between I-210 and I-5 south of Santa Clarita. The very obvious ongoing Palisades fire — near the coast — and the Eaton fire — near Altadena/Pasadena — are present. The focus for the figure below, however, is the newly-developing fire in the center of the image that is circled in the toggle. A user would already know of the two other fires ongoing.

On the right hand side of this display are right/left arrows that let a user step forward and backward with time for up to two hours. The slow toggle below shows the Night Microphysics for this fire at 0610 (no detection), 0611 (first detection), 0615 and 0811 UTC on 8 January. The fire grew rapidly.

Note the link on the side: ‘Open in Real Earth’. When that’s clicked, a new tab appears in your browser with the RealEarth display as shown below. The functionality of RealEarth allows a user to change the start/end times of the images loaded. RealEarth adds flexibility to the times displayed.

If you mouse over a pixel in RealEarth, the data can be probed, and the results at 1031 UTC are shown below for this fire. The land surface and potential fuels for the detected fire are shown in piechart form (this probe feature is also available at the NGFS website).

The RealEarth website makes it relatively simple to zoom out and see how things are changing over a long period of time. The toggle below, showing 0710 and 1710 UTC imagery, shows how the three fires have changed in those 10 hours. All three fires grew in size during these ten hours.

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This is a life-threatening fire event over Los Angeles county. Residents should heed warnings from local officials. For more information, view the NWS Los Angeles website. In particular, very strong winds will lead to rapid fire spread, and inhibit fire-fighting efforts.

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Antonio B. Won Pat International Airport on Guam experienced a record wet 7 January this year when 3.32″ of rain fell, mostly between 1700 UTC/6 January and 0500 UTC/7 January. Himawari-9 Band 13 imagery, above, for the 3 days ending at 0000 UTC 08 January 2025, show a region of thunderstorms approaching the Marianas. A slower animation covering the times of the heavy rainfall is below; the heavy rain was fairly isolated; much of the Marianas remained dry.

A closer look using 10-minute Himawari-9 Infrared images centered on Guam International Airport PGUM (below) showed the series of convective storms that produced the bulk of the record-setting rainfall (plot of surface observations). Cloud-top infrared brightness temperatures briefly reached -80ºC (violet pixels) with one of the convective elements.

It is noteworthy that the Total Precipitable Water value of 63.33 mm (2.49 in) derived from 7 January / 0000 UTC PGUM rawinsonde data exceeded the previous record TPW value of 2.34 in for that date/time (based on SPC climatology).

10-minute Himawari-9 Visible and Infrared images (below) displayed the rapid development of a convective cell which produced a brief period of heavy rain at PGUM as it traversed the island.

The large-scale conditions that allowed the heavy rain were well-forecast. GFS forecasts of the Galvez-Davison Index (GDI) (source) from the forecast started at 1800 UTC on 4 January 2025 are shown below, and they show a narrow tongue of higher values moving over the Marianas that corresponded with the time of the heavy rains.

MIMIC Total Precipitable Water (TPW) fields, below, from 0000 UTC 6 January to 0000 UTC on 8 January 2025, show abundant moisture moving over Guam (at 144^oE Longitude, 13.4^oN Latitude)

The Guam forecast office of the National Weather Service (WFO GUM) receives polar orbiting data from a Direct Broadcast antenna onsite (signals are processed by CSPP software). True-color VIIRS imagery from ca. 0300 UTC on 7 January is shown below, and a line of convection bisecting Guam is apparent.

CSPP also processes microwave imagery, and the computed MIRS estimates of TPW are shown below. The MIRS diagnostics also show the enhanced amount of TPW over the southern Marianas as the rain fell. TPW values dropped quickly by mid-day (UTC) on the 7th (at the end of the animation below).

My thanks to Landon Aydlett, WCM on Guam for alerting me to this wet event, and to Douglas Schumacher, CIMSS, for the Direct Broadcast imagery.

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