The animation above shows the Nighttime Microphysics RGB (saved from the CSPP Geosphere site) over the islands of American Samoa. (An animated gif is available here). The annotated animation below highlights several features that are typical of convective development detection at night. Low clouds become apparent shortly before 1450 UTC (when they are circled); subsequent color changes in those low clouds (in this case, a reddening that then detaches from the cloud) are most likely orphaned anvils (highlighted with arrows in the 1500-1530 UTC imagery — but the orphaned anvil can be tracked at later times, as it moves farther to the south) whose appearance frequently precedes successful convection; further reddening of the low clouds, as highlighted by arrows from 1610 to 1630 UTC, shows a transition of the low clouds (made up of water droplets) to higher clouds that are glaciating. At the end of the animation, note that active convection is near the Manu’a Islands.

Careful inspection of the Night Microphysics RGB can help you anticipate where convection might be developing at night.

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On 14 November 2022, American Samoa (indeed, all the Samoan Islands) were withing deep moisture associated with the South Pacific Convergence Zone, a common occurrence. The animation of MIMIC Total Precipitable Water, above, shows the evolution of the moisture hourly from 0000 to 1200 UTC on 14 November. (MIMIC fields in various domains are available in real time at this website; archived imagery is here). Note that 1200 UTC is 1 AM in American Samoa.

A forecaster in the morning might look at GDI (Gálvez-Davison Index, also discussed on this blog here) that is created from the GFS, as available at this NCEP link. (one can also search on ‘GDI NCEP’). The forecast fields from the 1200 UTC/14 November run are shown below (courtesy Jose Galvez and Bonnie Acosta, NOAA). The GDI shows an increase in values over Samoa first, then those large values overspread Tutuila (the largest of the islands of American Samoa) by 0000 UTC; by 0600 UTC, the back edge of the larger values is approaching Tutuila from the west. (For reference, Click here to see a map of the Samoan Islands; from west to east the main islands are Savai’i and Upola (the country of Samoa), then Tutuila, Ofu, Olosega and Ta’u, the four main islands of American Samoa).

Bearing the GDI fields above in mind, consider the evolution of Band 13 imagery shown below from GOES-18. Deep convection is persistent to the south of the Samoan Islands in a band that is roughly aligned with the largest forecast GDI values at 1200 and 1800 UTC. A region of stronger convection with an axis south-southwest to north-northeast develops across Savai’i and Upola. This development in the infrared imagery is very much in line with the forecast evolution of GDI fields that shows increasing values of the GDI in a line stretching southwest to northeast of Samoa.

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The evolution of the Band 13 imagery from 2110 UTC to 0610 UTC is also consistent with the GDI predictions — a back edge to the convection is approaching Tutuila by 0610 UTC. This edge is apparent in the infrared imagery, but also in the MIMIC Total Precipitable water (here is the 0600 UTC image).

The Weather Office in Pago Pago issued a Flood Watch at around 4 PM Samoa Time on 14 November (that is, around 0300 UTC on 15 November). Trends in the satellite imagery, and in total Precipitable Water fields, and in the GDI forecast were all used to help decide on whether extra staffing would be needed after sunset to deal with flooding-related work. (Extra staff were not called in).

GDI forecasts and satellite-based trends in infrared imagery and microwave estimates of total precipitable water can help with staffing decisions at forecast offices.

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A toggle between NOAA-20 VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images valid at 0805 UTC or 2:05 AM CST on 12 November 2022 (above) showed a broad plume of lake effect clouds streaming south-southwestward from Lake Nipigon, Ontario (north of Katatota Island, station CWKK) toward Lake Superior. A strong northeasterly flow of cold air across the still-unfrozen Lake Nipigon was helping to create this plume of lake effect clouds.

During the subsequent daytime hours, GOES-16 True Color RGB images from the CSPP GeoSphere site (below) displayed the lake effect snow (LES) bands that were streaming southwestward from Lake Nipigon and across the western portion of Lake Superior. The dominant LES band produced snowfall amounts as high as 10 inches in western Upper Michigan and 13.8 inches in northwestern Wisconsin.

GOES-16 Day Cloud Phase Distinction RGB images (below) suggested that there was some glaciation of the dominant LES band, as indicated by the shades of yellow to green.

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A comparison of GOES-16 Day Cloud Phase Distinction RGB, Cloud Top Pressure, Cloud Top Height, Cloud Top Phase and Cloud Optical Depth at 1756 UTC (above) highlighted the southern portion of the dominant LES band that produced the heaviest snowfall reports in Upper Michigan and northwestern Wisconsin.

Cursor-sampled values of GOES-16 Day Cloud Phase Distinction RGB components, Cloud Top Pressure, Cloud Top Height and Cloud Top Phase (below) showed typical values of those parameters along the dominant LES band at 1756 UTC.

Cursor-sampled values of Level 2 Derived Products (such as Cloud Top Phase and Cloud Top Height) can also be obtained directly from RGB imagery — in this case, Day Cloud Phase Distinction (below) — using an AWIPS “Local Menu Items” option, as discussed in this Satellite Book Club presentation.

Animations of GOES-16 Cloud Top Pressure, Cloud Top Height, Cloud Top Phase and Cloud Optical Depth derived products during the period from 0001-1956 UTC on 12 November are shown below. Note that Cloud Optical Depth values are only displayed across the southern part of the satellite scene, since the operational version of this product is only created for pixels having a local solar zenith angle of 65 degrees or less (and the November sun angle was low across that region).

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RADARSAT-2 overflew Guam early on 10 November 2022, and the toggle above shows both Himawari-8 imagery at that time with the SAR wind estimates overlain. This was a time of light winds and minimal shower activity. It’s interesting that the wake of weak winds downwind of the islands of Guam (and Rota) are apparent in this regime of weak easterly winds. (Metop-B ASCAT winds from late on 9 November are here, and late on 10 November are here; both images are from the NOAA/NESDIS ‘manati’ website).

A zoomed-in version of the image above is shown below. The windspeed downwind of Guam shows downstream variations in speed, oriented north-south. Those speed changes are very small, from 11-12 knots (cyan enhancement) to 8 or 9 knots (dark blue) enhancements. The coldest cloud tops in the Himawari-8 imagery — to the west of Guam — are associated with strong/weak wind dipoles. The region of stronger winds (around 18 knots, green in the enhancement used) to the west-northwest of Guam are associated with slightly cooler cloud tops, but this line of tropical convection has brightness temperatures in the 15-18^oC range: not very tall at all!

Imagery is also available at the NOAA/NESDIS STAR SAR Winds calendar here; the wind speed and the Normalized Radar Cross Section information are both available, and they are shown in a toggle below.

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