VISIT: Meteorological Interpretation Blog

Nighttime Visible Imagery over the Atlantic

Over the past two weeks, a few hurricanes (Gabrielle, Humberto, and Imelda) traversed over the Atlantic Ocean, and either skirted along or directly impacted the United Kingdom (U.K.) territory of Bermuda. Nighttime visible imagery provided from the VIIRS Near-Constant Contrast (NCC) product captured the synoptic scaled systems across the Atlantic.

Note, for the best optimal viewing of VIIRS NCC, it is recommended to be used during the full moon phase of the lunar cycle (i.e., from ~2 days after the first quarter to ~2 days after the last quarter), since the illumination of features at night is a function of moon phase and elevation above the horizon. However, VIIRS NCC may also be utilized during the new moon phase of the lunar cycle (i.e., moonless nights) to observe atmospheric and surface features, which are predominately illuminated by nightglow. During this period, emitted lights (e.g., city lights) will appear significantly brighter than their surroundings, while atmospheric and surface features (e.g., clouds, snow and ice cover) will appear dim and fuzzy.

VIIRS NCC observes Category 4 – Hurricane Gabrielle at 0549Z, 23 September 2025

At this time stamp, Gabrielle was a Major Hurricane, but stayed east of Bermuda and eventually became a post-tropical cyclone, impacting the Portuguese Azores later that week. While the image is taken during a moonless night, the eye of the hurricane can still be discerned in the imagery. Emitted city lights from Bermuda are seen west of the hurricane as well.

VIIRS NCC observes Category 5 – Hurricane Humberto at 0559Z, 28 September 2025

Humberto experienced peak intensities of 160+ mph winds, where the large cyclone churned through the Atlantic over the course of a week and migrated anticyclonically (clockwise) around Bermuda before dissipating to the north/northeast of the territory. The image below shows the magnitude of Humberto while it was classified as a Category 5 hurricane. The cyclone was located northeast of Puerto Rico and the U.S. and British Virgin Islands. Notice the scattered emitted city lights across the islands.

VIIRS NCC observes Category 1 – Hurricane Imelda at 0632Z, 1 October 2025

Hurricane Imelda and its eye, was located northeast of the Bahamas, and directly east of Florida. Later that day, Imelda intensified into a Category 2 hurricane, before making landfall in Bermuda. Imelda brought heavy precipitation and flooding along with high winds to the remote island. West of Imelda, saturated emitted city lights can be observed along the southeastern U.S., spanning from Florida to the Carolinas.

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Typhoon Ragasa

Over the last 48-hours, Typhoon Ragasa barreled through the South China Sea and its forecasted to make landfall in the Guangdong Province of China by 24 September 2025. Chinese megacities like Hong Kong and Macao reside along the coastline of the Guangdong Province, and have been put on high alert of the incoming storm. Typhoon Ragasa has already battered the northern Philippines producing significant rainfall, flooding, and high winds that led to widespread damage to infrastructure and also triggered landslides.

Himawari-9, a geostationary satellite operated by the Japan Meteorological Agency, provides observations of the typhoon at a high temporal refresh rate of every 10 minutes (i.e., Full Disk sector). The Himawari-9 AHI GeoColor product animation captures the cyclone offshore, south of Hong Kong, and moving westward towards the Chinese coastline from ~16-19Z, 23 September 2025. During the nighttime, GeoColor not only monitored the massive typhoon and observed its well defined eye, but the product also consists of a static, background city lights layer that helps viewers identify the populated areas that may be impacted by the storm.

Himawari-9 AHI GeoColor Product from 1600-1850Z, 23 September 2025

The Advected Layer Precipitable Water (ALPW) product, derived from multiple polar-orbiting satellites, captured the horizontal moisture transport of Ragasa over a ~30 hour period, as the typhoon trekked westward over the South China Sea. Refer to the ALPW animation below. The 4-panel product depicts precipitable water values within four atmospheric layers: from the surface to 850-mb (top-left), 850-700mb (top-right), 700-500mb (bottom-left), and 500-300mb (bottom-right). Note, the center of the typhoon can be spotted in the surface to 850mb precipitable water layer. The high moisture transport lead to torrential rainfall and flooding over southern China as Ragasa made landfall.

ALPW Product observations from 5Z, 22 September 2025 to 13Z, 23 September 2025

At ~9Z, 24 September 2025, Typhoon Ragasa made landfall near Hailing Island, China. VIIRS imagery from JPSS polar-orbiting satellites observed the typhoon as it moved towards the Chinese coastline prior to landfall. The infrared imagery is at a 375-m spatial resolution and shows the evolution of the typhoon, the typhoon’s eye and the convective bands.

VIIRS 11.45 um (I-5 Band) from ~5Z, 23 September 2025 to ~6Z, 24 September 2025

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Rio Blanco County, Colorado – Wildfires

On 2 August 2025, two wildfires initiated in Rio Blanco County, located in western Colorado. As of 11 August 2025, the Lee Fire burnt over 116,000 acres with 7% containment. Within the past three days, the fire has rapidly moved to the south towards Garfield County that includes the town of Rifle, Colorado. At the time of this blog entry, the Lee Fire was the 5th largest fire in Colorado state history and is still growing. The second fire, named the Elk Fire, burned over ~14,000 acres and is 30% contained while located east of the Lee Fire. Refer to the Google Map image that shows the approximate fire perimeter extent of both fires on 11 August 2025.

GOES-19 ABI 3.9 um imagery observes the emitted hotspots from both fires (i.e., white and red pixels) throughout the afternoon on 2 August 2025, where clouds (i.e., grey and black pixels that exhibit colder brightness temperatures) also move over the scene.

GOES-19 ABI 3.9um, 5-min temporal resolution, from ~16-02Z, 2 August 2025

After a few days of fire growth, on 5 August 2025, VIIRS observed the fire hotspots, intensities, and smoke from both wildfires. Refer to the VIIRS 4-panel below. Note, the elongated smoke plume, shown distinctly in the VIIRS Visible (0.64 um) and Day Fire RGB imagery that extends ~240 miles eastward towards the Front Range of Colorado. Smoke was also observed along the northern Interstate-25 corridor later that day.

VIIRS 4-Panel –> Top Left: Visible, Top Right: VIIRS Day Fire RGB, Bottom-Left: VIIRS 3.7 um, and Bottom-Right: VIIRS Fire Temperature RGB at 2040Z, on 5 August 2025

A 10-day VIIRS 3.7 um animation shows the evolution of the fires over western Colorado. Notice both fires have an initial rapid fire spread to the east, then the Lee Fire perimeter advances to the south after 8 August 2025. The shortwave infrared imagery is at 375-m, where black pixels indicate the fire hotspots. Notice the white pixels within the fire perimeter as well; this indicates that the VIIRS sensor is saturating, since it’s observing very intense fire pixels. VIIRS exhibits a low saturation temperature of 95C (or 368K), where any fire pixels that are observed above that temperature threshold will produce unrealistic cold pixel anomalies.

VIIRS 3.7um (I4) – Day and Night observations from 2-11 August 2025

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Dragon Bravo Fire, Arizona

At the time of this blog entry, the Dragon Bravo Fire burned 130,000 plus acres along the Grand Canyon National Park’s North Rim and is only ~13% contained. The fire initiated on 4 July 2025 and several structures along the North Rim, including the historic Grand Canyon Lodge, have already been destroyed. The cause of the fire was due to lightning. Prolonged hot, dry and windy conditions aided in the fire spread.

The high temporal resolution GOES ABI captured the rapid fire spread throughout the course of a week and half, from 24 July – 4 August 2025. Notice the active hotspots (i.e., white and red pixels) and the increased fire spread to the north.

GOES-18 ABI 3.9 um hourly observations from 0000Z, 24 July 2025 to 0000Z, 4 August 2025

VIIRS 3.7 um (I4) imagery zooms in on the finer details of the hotspots and the advancing fire perimeter during a similar time period. VIIRS observes these features at a high spatial resolution of 375-m compared to the coarser GOES 2-km resolution. The shortwave infrared imagery from VIIRS and GOES can be used in complement with one another during the day and night to observe fire hotspots, along with other thermal anomalies.

VIIRS 3.7 um (I4) observations from ~9Z, 24 July 2025 to ~9Z, 3 August 2025

Another way to view the Dragon Bravo Fire is from the VIIRS Day Fire RGB that utilizes three individual spectral bands (I4 – 3.7 um, I2 – 0.86 um, and I1 – 0.64 um) to detect fires, monitor vegetation changes and observe smoke. The RGB has the same 375-m spatial resolution and captures the active fires, burn scars and smoke produced from the Dragon Bravo Fire. Note, one of the limitations of the RGB is that the dataset can only be used during the daytime and is not available at night.

VIIRS Day Fire RGB observations from ~20Z, 24 July 2025, to 21Z, 3 August 2025

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A Tropical Upper Tropospheric Trough (TUTT) in Florida

The Tropical Upper Tropospheric Trough – TUTT (Sadler, 1976) is a trough or cyclonic circulation in the mid-upper troposphere located in tropical locations. Sadler defined the TUTT as an upper trough that lies between upper ridges in the subtropics and ridges in tropical locations. The TUTT is a complex system that often enhances convection along its periphery and limits it near its center. However, strong TUTTs can also enhance convection near their centers, given destabilization stimulated by the cold air present in their cores. In rare occasions, a strong TUTT that develops weak upper winds and strong convection near its center can yield to a cyclonic circulation at the surface that eventually triggers tropical cyclogenesis.

GOES-19 6.9um channel satellite imagery (link to quick guide) shows a TUTT located over the Bahamas and southeast Florida on the morning of 23 July 2025. The satellite presentation of the system shows a classical TUTT, characterized by strong convection along its periphery and a minima in convection near its cyclonic core. Animating water vapor channels such as the 6.9um one is an effective method to observe systems located in the mid-and upper troposphere, as these channels capture the motion of water vapor in these layers. The following animation shows the cyclonic rotation associated with the TUTT, which centers near Andros Island in the northwest Bahamas. It also shows convective tops in the periphery of the TUTT in shades of light gray and blue (brightness temperatures near and below -30°C), which rotate cyclonically around the center. The orange coloration (warmer temperatures) indicates drier air, which was entering the system from the northeast. When dry air in the mid troposphere is present very close to convection, it can enhance its strength by stimulating evaporation. This yields to cooling and, in turn, enhances vertical motion in the storms. The common presence of dry air near the center of a TUTT is one reason why they are often associated with strong convection that contains lightning, large rainfall rates and sometimes gusty winds.

The TUTT can also be seen on other satellite products. The following animation shows CIRA’s GeoColor product. In order to evaluate the location of thunderstorms, CIRA’s GLM Optical Energy product (link to quick guide) has been overlaid. The GeoColor is able to measure the surface and low-level clouds when higher clouds are not obscuring the view. This capability illustrates that the cyclonic rotation evident in the 6.9um channel is not occurring at the surface. In fact, the movement of low-level clouds is different. Lightning data from the GLM also confirms the presence of thunderstorms. The stronger storms are occurring over and off the coast of southeast Florida, in the periphery of the TUTT, given the interaction of three processes that are interacting to highlight ascending motions: (1) horizontal moisture gradients in the mid troposphere, (2) upper jet dynamics and (3) TUTT propagation. Role of moisture gradients: In the northwest portion of the TUTT, the dry air intrusion from the northeast of the TUTT (described in the previous paragraph) meets the very moist air mass located near southeast Florida, highlighting evaporative cooling processes and vertical motions. Role of upper jet dynamics: stronger upper winds over Florida, evidenced by the fast motion of the higher clouds, produces horizontal wind gradients that highlight ascent. Role of TUTT propagation: rapidly decreasing geopotentials in the mid-troposphere, as the TUTT approaches Florida from the southeast, enhance ascent. A fourth factor (not shown in these animations) is the presence of high values of available moisture over Florida. For this, precipitable water products are recommended. The combination of all of these processes yields to strong thunderstorms with frequent and dense lightning.

Not related to the TUTT but also of interest, CIRA’s GeoColor product shows that a Saharan Air Layer Intrusion is propagating northwestward in eastern portions of the domain. How can we tell? By the presence of a milky or whitish color shade in eastern portions of the image, which contrasts with the dark blue color of the ocean elsewhere in the figure. This contrast is more defined near sunrise. The reason is that the radiation beams returning to the satellite from the earth-ocean-atmosphere system have to travel in a low angle to reach the satellite. This forces them to encounter larger amounts of dust before reaching the satellite. The scattering of solar radiation caused by the dust produces the bright coloration that appears in CIRA’s GeoColor product.

An additional impact of the TUTT is the modulation of vertical wind shear, due to the vertical differences in wind directions and speeds in different quadrants of the TUTT. This has an impact in convection. The Day Cloud Phase Distinction RGB (link to quick guide) product is very useful to understand cloud types, and animating it shows how clouds at different levels move. It can provide a qualitative assessment of wind shear. It also illustrates convection types. Shallow convection in the tropics consists of water clouds, which appear blue and green. Yellows and oranges show ice clouds, which relate to deep convection and cirrus. The following animation shows this product from 12:50 through 18:00 UTC, capturing the diurnal cycle of solar heating. The impacts of the TUTT and how it relates to the diurnal cycle and to the Saharan Air Layer are described in the animation.

Reference:

James C. Sadler, “A Role of the Tropical Upper Tropospheric Trough in Early Season Typhoon Development,” Monthly Weather Review, October 1976, Volume 104, pp. 1266–1278.

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