GOES-18 Airmass RGB imagery, above, from 1500-1820 UTC on 28 October, show light snow over much of central Alaska as a storm system moves into the state. Temperatures at 500 mb, below, derived from NUCAPS profiles (as shown in the toggle), show a tongue of warmer air positioned with the cloud feature (shown as white) in the RGB above. Temperatures in that warm(ish) tongue of air (bluish in the enhancement) are around -25 to -27^oC versus the Bering Sea (magenta in the enhancement) where they are around -35^oC. Temperatures at 850 mb over the Bering Sea (shown here with a color enhancement that ranges from -25^oC to +10^oC) are around -6^oC.

Snowfall Rates, estimated from ATMS on Suomi-NPP (from this site) show snow moving slowly north between 1324 and 1504 UTC over western Alaska.

Did your eye jump on the heavier Snowfall rates over southern Alaska? The region around Anchorage is in a Winter Storm Warning; 6-12″ is expected, more than the lighter values expected across the central part of the state.

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Infrared imagery over the Hawai’ian Islands on 25/26 October, below, shows convective development over Maui and Oahu. This was also a region of strong vertical wind shear, as shown below, as upper level cloud features were moving southeastward, low level clouds were moving to the west).

GREMLIN is a machine-learning tool that uses Bands 7, 9 and 13 fields from GOES-18 to emulate what NEXRAD radar echoes might look like. The animation below shows GOES-18 infrared data, and GREMLIN radar estimates. GREMLIN shows radar returns over both Oahu and Maui.

Strong convection with radar returns occur over both Maui and Oahu after 1500 UTC. The National Weather Service in Honolulu issued Flash Flood Warnings at 1638 UTC for Maui and at 1640 UTC for Oahu. The side-by-side animation, from 1501-1701 UTC shows the heavy rain indicated by GREMLIN as the Flash Flood Warning was issued.

The Hilo sounding from 1200 UTC on 26 October, below, (from here) shows a moist atmosphere (55 mm of Total Precipitable Water) and mid-level instability. MIMIC Total Precipitable Water (TPW) at that time (here) shows the horizontal distribution of the moisture.

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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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