Target Sector (2.5-minute interval) JMA Himawari-9 AHI Red Visible (0.64 µm) and Clean Infrared Window (10.4 µm) images (above) showed Cyclone Errol as it was rapidly intensifying off the northern coast of Western Australia on 15-16 April 2025. About 2 hours after the end of the above animation, Errol peaked at 140 kts — Category 5 intensity — at 1200 UTC (SATCON). Errol exhibited a pinhole eye during the time period shown — a feature frequently observed with rapidly-intensifying tropical cyclones. Cloud-top infrared brightness temperatures in the eyewall region of Errol were in the -80s C (shades of violet to purple).

A NOAA-20 VIIRS Day/Night Band image valid at 0503 UTC (below) provided a higher-resolution view of the 10-mile diameter pinhole eye.

A Sentinel-1A Synthetic Aperture Radar (SAR) image (source) at 1610 UTC (below) sensed a maximum radial wind velocity of 124.8 knots in the northeast quadrant of Errol.

Factors favoring Errol’s rapid intensification included very warm water that the storm was traversing, and an environment of low deep-layer wind shear (below). Sea Surface Temperature and Wind Shear images were sourced from the CIMSS Tropical Cyclones site.

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Instrument issues at the weather station in Pago Pago (related to Hydrogen generation) means that balloon launches occur only once every other day, at 0000 UTC on odd days. The sounding for 15 April/0000 UTC (1 PM Samoa Standard time on 14 April) is shown below.

Two different JPSS satellites overflew the Samoan Islands just after 0000 UTC on 15 April 2025, and two profiles were created just west of Tutuila, as shown below.

The toggle below compares the two NUCAPS soundings shown in the image above to the 0000 UTC rawinsonde from Pago Pago. There is, overall, agreement, although (of course), the NUCAPS soundings are very much smoother.

At 1200 UTC, no rawinsonde was launched. But NUCAPS profiles were available at around 1200 and 1300 UTC as shown in the animations below.

If you compare the 0000 UTC NUCAPS profiles to those sensed around 1200-1300 UTC, what changes do you see, and how might those changes have been sensed by a rawinsonde if it had been launched. The later profiles all show dryer conditions at mid-latitudes (precipitable water values have dropped by about 0.4″). Temperatures above 700 mb are very similar in the two profiles, but the later profile is a bit cooler between the surface and 850 mb

The information change in the 12h between NUCAPS profiles (note also that there was a NUCAPS profile at around 0900 UTC!) can help you decide what a full-resolution sounding (made by a radiosonde) might have looked like.

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Thursday 17 April is the 125th anniversary of the beginning of American Sovereignty over American Samoa. Part of the celebration includes the Fautasi, a regatta that depends on weather. What tools are available to help forecast in the long-term and in the short-term? The animation above (from the CSPP Geosphere site) shows the Night Microphysics RGB during the early morning of 14 April, and periodic convection is occurring. The low clouds — white and pink against the blue background — are occasionally spawning deeper convection that is a deep red in the RGB. These convective towers are short-lived. This kind of information might be useful in the short-term for diagnosing where precipitation is occurring the night before the regatta.

MIMIC Total Precipitable Water fields, above, show the Samoan Islands along the southern edge of deeper moisture that is the South Pacific Convergence Zone between the islands and the Equator. Animations such as these can help a forecaster anticipate the approach (or retreat) of deep moisture that could support heavy rains.

The GOES-18 Clear-Air estimates of Total Precipitable Water, below, similarly show a gradient just south of the Samoan Islands, with values dropping from over 2″ over American Samoa (the purple enhancement) to about 1.5″ to the south (the yellow enhancement).

The green points above show where Legacy Profiles exist, and the animation below highlights the profile very close to Pago Pago in the center of Tutuila. The low-level stability in the sounding and mid-level dry air are consistent with the occasional convective development (and quick dissipation) observed in the Night Microphysics imagery above.

LightningCast probabilities (available at this site) can be used in the short term for Decision Support for outdoor activities. The animation below shows probability fields for the overnight hours, 1200-1500 UTC on 14 April 2025, matching the animations above. Probabilities with these showers to not become large, and GLM lightning observations are not present.

GREMLIN fields (1000-1500 UTC) that estimate radar returns are shown below (from here). Hit-or-miss showers are diagnosed.

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What can be used for winds and waves? Two PacIOOS buoys exist around American Samoa, and both are producing data. The plots below show that higher waves 7-7.5 feet, are at the Auunu buoy to the east. Waves are closer to 6 feet at King Poloa. At both locations, waves are approaching from the southeast, and the King Poloa site is somewhat in the lee of Tutuila. Altimetric wave heights are also available (link) — but at present the observations at that website are not up to date.

Scatterometry data gives sea-surface winds; strong winds near the Samoan Islands might build higher waves. Those data are available here, but as with the wave heights at that site, they are not up to date today. Scatterometry data are available at this osisaf website, however. ASCAT winds from MetopC (over the Samoan Islands) and from MetopB (to the east, i.e., upstream of the Samoan Islands) show values between 15 and 20 knots.

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Update, 15 April 2025: Today I noticed a feature (at the CSPP Geosphere site) in the Night Microphysics RGB that was moving over American Samoa, i.e., that hazy lighter blue, highlighted here. What does this feature in the RGB represent?

To answer that, I first loaded up the Band 7 (shortwave IR, 3.9 µm) infrared imagery, and one noteworthy feature is the lack of cloud development in/around that feature. This suggests the RGB image is detecting a region of dry air; it might show up in the RGB because the Split Window Difference — that highlights moisture differences in the atmosphere — is part of the Night Microphysics RGB.

The inset Split Window Difference field (source) has a darker smudge signifying dryer air in the same region as the lighter region in the RGB.

Of course, there are GOES-18 level 2 products that can help reveal the answer. The Total Precipitable Water field overlain on top of the Band 7 imagery, below, shows a distinct dry patch (TPW of less than 1.25″!) right where the feature in the RGB appears.

MIMIC Total Precipitable Water fields at about the same time (1400 UTC/15 April) show the dry patch as well.

For more information on the weather around Pago Pago, refer to the National Weather Service website and/or the Facebook page of the Pago Pago office.

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Himawari-9 clean window (Band 13, 10.4 µm) infrared imagery, below, (created using geo2grid software and HSD files from the SSEC Data Center) shows strong convection developing in the Philippine Sea between the Philippines to the southwest, the Ryuku Islands to the west, Japan to the north, and Guam far to the east. The structure of the the convection in the infrared imagery at the start of the animation strongly suggests development along an outflow boundary that persists through much of the animation.

The toggle below shows where the outflow might exist at 0800 UTC.

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This area of the western Pacific Ocean falls under the view of the Direct Broadcast antenna in the back yard of the National Weather Service on the island of Guam. What does the information downloaded from that antenna show? Three derived rain-rates from MetopC, NOAA-20 and NOAA-21 are shown below (imagery courtesy Douglas Schumacher, SSEC/CIMSS). The convection persisted for these five hours.

Day Night Band imagery from NOAA-21 shows that the strong convection was electrified. Lightning streaks are obvious in the Day Night band imagery.

Ground-based lightning observations, below (Courtesy Brandon Aydlett, Science/Operations Officer in Guam), also show extensive lightning with this convective system.

GCOM AMSR-2 imagery, below (also processed at the Direct Broadcast site on Guam, and ingested into the AWIPS machine, and also courtesy of Brandon Aydlett, WFO Guam), shows the strong microwave signal and the winds diagnosed to exceed 40 knots (and the region where wind diagnostics failed because of rain contamination). The wind speeds are also available at this NOAA/NESDIS site.

Himawari-9 Full Disk airmass RGB imagery (source) from late in the day on 10 April, below, shows that this convection was to the south of a developing cyclone moving into the northwest Pacific Ocean.

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Given all the lightning observed at 1700-1730 UTC, a natural question might be: what did LightningCast probabilities (computed with Himawari-9 data and CSPP Geo LightningCast software) show an hour earlier? Very large probabilities centered near 25^oN, 134^oE! A LightningCast animation (here) from 0000 to 2000 UTC on 10 April shows large value moving eastward through the domain below.

Thanks to Brandon Aydlett, WFO Guam, for kick-starting this blog post!

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