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Category 5 Super Typhoon Ragasa moves WNW across the Philippine Sea and into the Luzon Strait

2.5-minute Target Sector JMA Himawari-9 Infrared images (above) showed Ragasa as it intensified from a Category 4 Typhoon at 0000 UTC on 21 September 2025 to a Category 5 (140-knot) Super Typhoon at 0600 UTC and then further to a 145-knot Super Typhoon by 1200 UTC (SATCON). There was a... Read More

2.5-minute Himawari-9 Infrared images, from 0002-2042 UTC on 21 September [click to play animated GIF | MP4]

2.5-minute Target Sector JMA Himawari-9 Infrared images (above) showed Ragasa as it intensified from a Category 4 Typhoon at 0000 UTC on 21 September 2025 to a Category 5 (140-knot) Super Typhoon at 0600 UTC and then further to a 145-knot Super Typhoon by 1200 UTC (SATCON). There was a ~2 hour gap in Target Sector images, from 1100-1250 UTC; during that period, 10-minute Full Disk imagery was used to fill the gap.

The primary environmental factor that favored this intensification was low values of deep-layer wind shear (below) — in addition, Ragasa traversed a lobe of slightly warmer waters (Sea Surface Temperature | Ocean Heat Content) as it reached Category 5 intensity.

Himawari-9 Infrared images, with contours and streamlines of deep-layer wind shear at 1800 UTC on 21 September

Ragasa was undergoing an eyewall replacement cycle (ERC) on 21 September, which was evident in the MIMIC-TC product (below) — the smaller-diameter inner eyewall was eventually replaced by a larger-diameter outer eyewall. However, the ERC did not adversely affect the intensity of the Super Typhoon during the course of the day.

MIMIC-TC product on 21 September

3 snapshots of microwave imagery during the day (from DMSP/SSMIS and NOAA-21/ATMS) are shown below.

DMSP-16 SSMIS image at 1003 UTC on 21 September

NOAA-21 ATMS image at 1731 UTC on 21 September

DMSP-17 SSMIS image at 2152 UTC on 21 September

[the wind shear, sea surface temperature, ocean heat content and microwave products shown above were sourced from the CIMSS Tropical Cyclones site]

An RCM-3 Synthetic Aperture Radar (SAR) Wind Speed image (below) also highlighted a narrow “moat” of lower wind speed that was surrounding the inner eyewall at 2131 UTC (a “brightness temperature moat” feature was also seen in the 2152 UTC DMSP-17 SSMIS microwave image).

RCM-3 Synthetic Aperture Radar image at 2131 UTC on 21 September [click to enlarge]

A sunrise-to-sunset animation of 2.5-minute Himawari-9 Visible images (below) revealed the presence of low-level mesovortices within the large eye. Super Typhoon Ragasa made its first landfall on the tiny island of Babuyan in the Luzon Strait after 0300 UTC on 22 September.

2.5-minute Himawari-9 Visible images, from 2152 UTC on 21 September to 0832 UTC on 22 September [click to play animated GIF | MP4]

A Suomi-NPP VIIRS Day/Night Band image at 0511 UTC on 22 September — when Ragasa was still at Category 5 intensity (below) displayed the 40-mile diameter eye as its western edge was near Synoptic Station 981330 (Calayan Island).

Suomi-NPP VIIRS Day/Night Band image valid at 0511 UTC on 22 September [click to enlarge]

Sentinel-3B Altimetry data, showing a Significant Wave Height value of 27.75 ft east of Luzon at 0129 UTC on 21 September

The strong winds associated with Super Typhoon Ragasa caused high waves (above), which resulted in an upwelling of cooler sub-surface water in the northwest Philippine Sea — late in the day on 22 September (when Ragasa was located far to west, in the South China Sea), note the pocket of 27-28ºC Sea Surface Temperatures (yellow to light orange) along and north of the track of Ragasa at the point where it first reached Category 5 intensity from 0600-1200 UTC on 21 September (below). Sea Surface Temperatures over that area of upwelling were around 30ºC just prior to the passage of Ragasa.

Sea Surface Temperature analysis at 2233 UTC on 22 September, with a plot of Ragasa’s track from 18-23 September

As Ragasa was beginning to move west-northwestward away from northern Luzon on 22 September, a Significant Wave Height of 58.26 ft was sensed at 1352 UTC (below).

Sentinel-3B Altimetry data, showing a Significant Wave Height of 58.26 ft north of Luzon at 1352 UTC on 22 September

Additional information pertaining to Ragasa is available in this blog post.

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Aircraft turbulence associated with transverse cirrus bands

Transverse cirrus bands — narrow cirrus cloud filaments oriented perpendicular to the mean wind flow at the altitude of those cloud features — have long been recognized as indicators of potential aircraft turbulence. Such was the case on 20 September 2025, when a Mesoscale Convective System over Kansas/Oklahoma began to exhibit transverse... Read More

5-minute GOES-19 Upper-level Water Vapor (6.2 µm) images, with Pilot Reports of turbulence plotted in red, from 0946-1501 on 20 September [click to play MP4 animation]

Transverse cirrus bands — narrow cirrus cloud filaments oriented perpendicular to the mean wind flow at the altitude of those cloud features — have long been recognized as indicators of potential aircraft turbulence. Such was the case on 20 September 2025, when a Mesoscale Convective System over Kansas/Oklahoma began to exhibit transverse banding along its northern to eastern periphery as it moved eastward toward the mid-Mississippi Valley (above). Several high-altitude pilot reports of light to moderate turbulence were seen in the area of transverse banding.

A stepped comparison of GOES-19 Infrared Window (10.3 µm), Upper-level Water Vapor (6.2 µm), Near-Infrared Cirrus (1.37 µm) and Red Visible (0.64 µm) images at 1301 UTC (below) showed that the transverse cirrus bands were best visualized using the Near-Infrared Cirrus and Water Vapor (and to a somewhat lesser extent, the Infrared) imagery — the presence low- to mid-level clouds tended to mask the appearance of some of the thin high-altitude cirrus bands.

GOES-19 Infrared Window (10.3 µm), Upper-level Water Vapor (6.2 µm), Near-Infrared Cirrus (1.37 µm) and Red Visible (0.64 µm) images at 1301 UTC on 20 September, with Pilot Reports of turbulence plotted in red [click to enlarge]

Transverse cirrus bands are also occasionally observed near the axis of strong jet streaks and around the periphery of tropical cyclones.

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Resuspended volcanic ash from the 1980 eruptions of Mount St. Helens

5-minute PACUS Sector GOES-18 (GOES-West) Visible images (above) showed the hazy signature of resuspended volcanic ash (remaining from the 1980 eruptions of Mount St. Helens) that was being lofted from the surface by strong east-southeast winds — which were gusting as high as 38-42 mph at nearby RAWS sites — and being... Read More

5-minute GOES-18 Visible images with plots of 30-minute RAWS surface observations (yellow) and intermittent Pilot Reports of volcanic ash (red), from 1421-1921 UTC on 16 September [click to play MP4 animation]

5-minute PACUS Sector GOES-18 (GOES-West) Visible images (above) showed the hazy signature of resuspended volcanic ash (remaining from the 1980 eruptions of Mount St. Helens) that was being lofted from the surface by strong east-southeast winds — which were gusting as high as 38-42 mph at nearby RAWS sites — and being transported west-northwestward on 16 September 2025. Pilot reports indicated that the resuspended ash was reaching altitudes of 10000 ft at 1610 UTC and 7000-8000 ft at 1753 UTC.

In the corresponding GOES-18 Split Cloud Top Phase brightness temperature difference images (below), the resuspended ash exhibited darker shades of blue.

5-minute GOES-18 Split Cloud Top Phase brightness temperature difference images with 30-minute plots of RAWS surface observations (yellow) and intermittent Pilot Reports of volcanic ash (red), from 1421-1921 UTC on 16 September [click to play MP4 animation]

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5-minute GOES-18 True Color RGB images, from 1421-2101 UTC on 16 September [click to play MP4 animation]

True Color RGB images (source) from GOES-18 (above) provided good contrast between the hazy resuspended ash and the underlying vegetated landscape — and True Color RGB images from GOES-19 (GOES-East) (below) leveraged a more favorable forward scattering geometry later in the day to reveal that the transport of resuspended ash continued for a longer time period than was apparent in the GOES-18 imagery. The 2 sets of GOES True Color RGB images (both centered at Mount St. Helens) are displayed in the native projection of each satellite.

5-minute GOES-19 True Color RGB images, from 1541 UTC on 16 September to 0031 UTC on 17 September [click to play MP4 animation]

A side-by-side comparison of True Color RGB images from GOES-18 and GOES-19 — remapped to a common projection using Geo2Grid (below) — similarly showed that the hazy signature of resuspended ash was more apparent later into the day in GOES-19 imagery, due to enhanced forward scattering associated with the GOES-East viewing angle.

True Color RGB images from GOES-18 (left) and GOES-19 (right), from 1411 UTC on 16 September to 0001 UTC on 17 September [click to play animated GIF | MP4]

In a plot of rawinsonde data from Salem, Oregon KSLE (below) note the transition from east-southeast winds below the 790 hPa (2.1 km) level to southerly winds between that and the 635 hPa (4 km) level — this change in wind direction influenced the transport of resuspended ash, carrying it more northward as the ash reached higher altitudes (which was apparent in the GOES True Color RGB images).

Plot of rawinsonde data from Salem, Oregon at 1200 UTC on 16 September [click to enlarge]

South-facing USGS Johnston Ridge Observatory webcam images (below) offered a ground-level view of the resuspended ash that was being lofted by strong winds on 16 September.

USGS Johnston Ridge Observatory webcam images of resuspended ash on 16 September [click to play animated GIF | MP4]

Resuspended ash from Mount St. Helens has also occurred with strong westerly winds.

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Outbreak of tornadoes across North Dakota

1-minute Mesoscale Domain Sector GOES-19 (GOES-East) Visible and Infrared images (above) included time-matched (+/- 3 minutes) plots of SPC Storm Reports — which showed supercell thunderstorms that produced a south-to-north oriented swath of at least 12 tornadoes across central North Dakota on 14 September 2025. With this outbreak, a new record was set for the number of... Read More

1-minute GOES-19 “Red” Visible (0.64 µm, left) and “Clean” Infrared Window (10.3 µm, right) images with time-matched (+/- 3 minutes) SPC Storm Reports plotted in red/blue, from 1730 UTC on 14 September to 0022 UTC on 15 September [click to play animated GIF | MP4]

1-minute Mesoscale Domain Sector GOES-19 (GOES-East) Visible and Infrared images (above) included time-matched (+/- 3 minutes) plots of SPC Storm Reports — which showed supercell thunderstorms that produced a south-to-north oriented swath of at least 12 tornadoes across central North Dakota on 14 September 2025. With this outbreak, a new record was set for the number of tornadoes in North Dakota during a calendar year.

A plot of rawinsonde data from Bismarck, North Dakota at 1800 UTC on 14 September (below) indicated that the Most Unstable (MU) air parcel’s Equilibrium Level (EL) was at an altitude near 12 km, where the air temperature was around -55C (which closely corresponded to the coldest cloud-top infrared brightness temperatures seen in GOES-19 Infrared imagery).

Plot of rawinsonde data from Bismarck, North Dakota at 1800 UTC on 14 September [click to enlarge]

GOES-19 Visible and Infrared images at 2231 UTC (below) included plots of three SPC tornado reports (T) that were received very near that time — plotted at both their observed surface locations, and at their “parallax-corrected” location (assuming a mean cloud-top height of 12 km). It can be seen that that the parallax-corrected locations were moved NW, closer to the parent supercell thunderstorms.

GOES-19 Visible (left) and Infrared (right) images at 2231 UTC on 14 September, with SPC Storm Reports of three tornadoes plotted at their surface locations as well as at their “parallax-corrected” cloud-top locations [click to enlarge]

Another important ingredient helping to produce this tornado outbreak was the presence of climatologically high values of Precipitable Water (below) — the value of 1.39 inch derived from 1800 UTC Bismarck rawinsonde data was above the 90th percentile for 14 September, and not far below the daily maximum value of 1.68 inch.

Plot of annual Precipitable Water climatology for all Bismarck soundings — with the values for 14 September highlighted [click to enlarge]

Hourly SPC Mesoscale Analyses of Precipitable Water (below) depicted the broad corridor of high moisture that was being transported northward across the Dakotas on 14 September.

Hourly SPC Mesoscale Analysis of Precipitable Water across the north-central US, from 1500 UTC on 14 September to 0000 UTC on 15 September [click to play MP4 animation]

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