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.

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.

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

[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).

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.

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).

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.

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).

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

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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.

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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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.

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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.

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.

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).

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.

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

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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).

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.

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.

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.

Some official news from us and our friends @NWSGrandForks:
Following the numerous tornadoes that occurred across central ND yesterday, Sunday the 14th, the record for the total number of tornadoes occurring in the state of North Dakota in a calendar year has been broken. #NDwx pic.twitter.com/u11opgbxmf

— NWS Bismarck (@NWSBismarck) September 15, 2025

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