A sequence of Suomi-NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images (above) showed the cyclonic swirl of clouds associated with a low pressure system that was moving eastward across the Beaufort Sea on 25 October 2024. The relatively tight pressure gradient south of this low (surface analysis) was forcing fairly strong W-SW winds across the North Slope and Arctic coast of Alaska (as well as the adjacent Canadian provinces of Yukon and the Northwest Territories).

A Suomi-NPP VIIRS Day/Night Band image with an overlay of Metop-B ASCAT winds (below) depicted the flow across the offshore waters north of Alaska — where Heavy Freezing Spray Warnings were issued for the ice-free waters. Numerous ASCAT wind speeds were in the 30-35 knot range.

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The Joint Typhoon Warning Center is currently monitoring two areas of interest of near Guam for potential tropical cyclone formation. The first, 99W, is located due east of Guam, while the second invest, 98W, is further southeast. During the local overnight hours today between 14:54 and 16:11 UTC, the VIIRS instruments onboard S-NPP, NOAA-20, and NOAA-21 captured the following sequence of day-night band imagery:

Overall, it is clear that the situation is a bit messy. In the DNB imagery, it is easy to pick out a few features of note, including a broad region of convection on the northern side of invest 99W, a narrow stripe of convection extending to the south all the way to invest 98W, and some hints of the low level circulation of 98W. This final feature highlights a case where DNB imagery can be very helpful in conjunction with geostationary IR imagery during the overnight hours.

Of course, some other polar satellite instruments can help diagnose the state of these circulations a bit further. For example, the 183 GHz band from ATMS is sensitive to water vapor, and that band’s imagery (processed via CSPP MiRS) from the NOAA-20 pass at 15:20 UTC is below:

Over Guam and points west, there is a rather pronounced “tongue” of drier air. With invest 99W moving across the edge of this gradient, it will likely struggle to gain symmetry to its structure on its west side. Additionally, a key forecast challenge for invest 98W mentioned in the latest Area Forecast Discussion from NWS Guam is uncertainly regarding how this drier air mass will be influenced by the passage of 99W.

And finally, the AMRS2 instrument aboard GCOM-W1 offers a look inside the clouds of invest 99W at 89 GHz just a bit later at 16:05 UTC. This data confirms that no clear organization of the precipitation bands has taken place at this point.

All imagery in this post was collected by the SSEC direct broadcast antenna system located at NWS Guam. Those with interests in the western Pacific should stay tuned to local authorities for the latest updates. Thank you to Scott Lindstrom for guiding me through the process of writing my first post on the CIMSS Satellite Blog!

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1-minute Mesoscale Domain Sector GOES-18 (GOES-West) Visible and Infrared imagery of Hurricane Kristy in the East Pacific Ocean on 23 October (above) highlighted a well-defined eye, with GLM-indicated lightning activity increasing within the inner eyewall. Cloud-top infrared brightness temperatures in the -80s C (shades of violet to purple) were apparent at times. Kristy reached Category 4 intensity as of the 2100 UTC advisory from NHC.

A closer view of GOES-18 Visible images (below) revealed low-altitude mesovortices within the eye.

Hurricane Kristy was moving through an environment characterized by low values of deep-layer wind shear, as seen in an analysis from the CIMSS Tropical Cyclones site (below) — a factor which favored its rapid intensification.

===== 24 October Update =====
On 24 October, 1-minute GOES-18 Visible and Infrared imagery (above) again displayed a well-defined eye, with GLM-indicated lightning activity increasing within the inner eyewall — and cloud-top infrared brightness temperatures in the -80s C (shades of violet to purple) were apparent at times. Kristy continued to further intensify, becoming a Category 5 hurricane as of the 2100 UTC advisory from NHC.

In a closer view of GOES-18 Visible images (below), distinct low-altitude mesovortices were once again seen within the eye.

Earlier in the morning, when Kristy was still a Category 4 hurricane, a Synthetic Aperture Radar (SAR) image at 1401 UTC (below) indicated that a derived maximum wind speed of 130 knots was present in the NE quadrant of the eyewall (source).

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MIMIC TPW fields for the first half of 22 October (at which point the data feed from NOAA/NESDIS was interrupted) shows the Samoan Islands within an isolated region of relatively dry air with moisture moving in from the east. GFS fields showing the Galvez Davison Index (GDI) that is sometimes used to anticipate rain in the tropics, below (from this site), shows a predicted increase in index values (suggesting an increase in rain probabilities; GDI has been discussed on this Blog previously here and here).

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Given this background, what might you examine to determine if rain is imminent? Visible imagery from GOES-18 might be one example, as shown in the animation below from the CSPP-Geosphere site. Convective clouds approach the Samoan Islands from the east, coming very close to American Samoa but not overspreading the islands. Convection does overspread Upolu by the end of the animation.

GREMLIN is a product that predicts what radar would show given a distribution of GOES data (from bands 7, 9, and 13). The animation below shows predicted radar echoes getting very close to Tutuila (the island including Pago Pago), and overspreading Upolu, the island of Samoa that includes its Capital city Apia.

A meteorogram for Pago Pago, American Samoa (station NSTU) from 0300 UTC 22 October through 1500 UTC 23 October, below, shows that light rain was observed at Pago Pago airport at 22 UTC on 22 October. Heavier rain overspread the region on 23 October. A similar meteorogram for Apia, Samoa (station NSFA), follows, showing a similar tale, but does it show the rain that might have been expected (at Apia especially) given the imagery above?

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Use large-scale products such as MIMIC or model fields of GDI to get a general feel for how precipitation might be approaching and/or evolving, then examine satellite imagery, either individual bands or derived products, as guidance for your precipitation forecast in the short-term.

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(Added, 24 October)

You will note above that the meteorogram for Pago Pago shows rain starting at 1200 UTC on the 23rd. What did the GREMLIN fields look like at that time? The animation below spans 1100-1850 UTC on the 23rd. GREMLIN gradually increases its estimations of radar echoes during this time, including the times in the meteorogram when rain was observed at NSTU.

The National Weather Service in Pago Pago used GREMLIN imagery (from 1850 UTC) in the Facebook post, below, to alert their followers to the extent of the rain.

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GREMLIN fields are now available at the CIRA SLIDER.

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