10-minute Full Disk scan GOES-18 (GOES-West) Air Mass RGB images (above) showed a compact polar low as it migrated southeastward across the Bering Sea (not far south of the sea ice edge: surface analyses) on 27-28 January 2024. As the polar low moved through the Pribilof Islands on 28 January, St Paul Island (PASN) experienced a peak wind gust of 59 knots (68 mph) at 1216 UTC, and St. George (PAPB) had a peak wind gust of 47 knots (54 mph) at 1445 UTC. In addition, at PASN the visibility was briefly restricted to 1/4 mile with heavy snow as the polar low passed through the Pribilofs (plot of surface report data).

RCM-2 Synthetic Aperture Radar (SAR) surface wind speed imagery (source) at 1814 UTC on 27 January (below) depicted a compact semi-circular region of wind speeds around 50 knots (darkest shades of red) near or just west of the center of the polar low, as it was still located over the western Bering Sea (1810 UTC Air Mass RGB image | 1800 UTC surface analysis). Smaller pockets of orange-to-red along the trailing cold front were more likely a result of SAR signal backscatter — due to glaciation of convective clouds along the frontal boundary — rather than an indication of higher surface wind speeds.

During the relatively brief ~4 hour period of daylight hours on 27 January, GOES-18 Day Cloud Type RGB images (below) displayed a signature near the core of the polar low — and along its trailing cold front — suggestive of glaciating shallow convective clouds (darker shades of green). With the strong offshore flow of very cold arctic air from Southwest Alaska, widespread parallel cloud streets were evident across the open water of the Bering Sea (likely causing areas of ocean effect snow).

A sequence of 3 Suomi-NPP VIIRS Day/Night Band images (below) provided further evidence of shallow convective cloud banding near the low center and along the cold front.

Suomi-NPP VIIRS Day/Night Band images at 1223 UTC and 1403 UTC on 28 January (below) showed 2 nighttime views of the polar low as it was in the vicinity of the Pribilof Islands.

?Check out this stunning example of a #polar #low forming along the ice edge, behind the arctic front. These lows are classified based on how they form. But they tend to be small (mesoscale), but can be very strong. #akwx @uafgina pic.twitter.com/SsREA9oatQ

— NWS Fairbanks (@NWSFairbanks) January 27, 2024

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5-minute CONUS Sector GOES-16 (GOES-East) Ash RGB, Dust RGB, SO2 RGB and Split Window Difference images (above) displayed signatures of a series of volcanic cloud pulses produced by minor eruptions of Popocatépetl on 27 January 2024. This comparison highlights the fact that the Ash RGB might not always provide the best depiction of volcanic cloud structure or transport — particularly if the ash loading of the cloud is not high (as was often the case on this day) — so a comparison with other RGB types and/or Difference Products could prove to be helpful. Diurnal changes in the temperature of the underlying ground surface can also affect the contrast between volcanic clouds and the surface in RGB imagery.

Advisories from the Washington VAAC on 27 January placed the maximum ash heights in the 20000-23000 ft range.

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The Night Microphysics RGB animation below, taken from the CSPP Geosphere website (direct link to animation) shows strong convection over the central Gulf of Mexico along with cloud conditions in the surrounding states. Note, for example, the expanding region of low clouds over Texas. The annotated images below (0216, 0646 and 1046 UTC) highlight features.

At 0216, note the different colors associated with low clouds over Texas (where the stratus clouds are cooler) and over the northeast Gulf of Mexico/central Florida (where stratus clouds are warmer). The Blue component of the Night Microphysics RGB is the Band 13 (10.4 µm) brightness temperature; as the cloud top cools, the RGB will be less and less blue (and therefore more and more red and green, that is, yellow). There is a mid-level arced cloud feature to the south of the convection over the north-central Gulf that is likely an old outflow boundary. Strong convection south of Louisiana is denoted by red/speckled with yellow; the speckling occurs because the Band 7 (3.9 µm) imagery that is used in the ‘Night Fog Difference’ brightness temperature difference that is the green component of the night microphysics RGB has low precision at very cold temperatures as might be detected at the top of strong convection.

At 0646, the region of stratus clouds over Texas has expanded. (Do you think this is a region of fog? Why or why not? See below for the answer!) The old outflow boundary is still apparent, and could be a focusing point for new convection. A new outflow boundary has emerged from the convection over the north-central Gulf of Mexico and is propagating to the south. Convection that is developing on the western side of the convective complex is very yellow — one might infer the height/temperature/cloud type of the cloud tops there based on the RGB color.

At 1046, the stratus clouds over Texas continued to expand. Parts of the RGB over Texas show different colors, with different shades of yellow (compare the color near the arrow with areas to the northwest). That new Outflow Boundary — several hours old now — is still propagating southward, occasionally spawning convection. In addition, two more outflow boundaries are present as indicated. These colliding outflows might support convective initiation in the near future.

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Is there fog over Texas? It’s hard to tell just from the cloud top, which is the information given by the Night Fog Brightness Temperature Difference and Night Microphysics RGB. There is a Level 2 Product that can help: IFR Probability. GOES-R IFR Probability combines satellite detection of low clouds with Rapid Refresh estimates of low-level saturation to produce a probabilistic estimate of IFR conditions. (One could also look at webcams or surface observations that are plentiful over Texas). The toggle below shows Night Microphysics RGB in AWIPS, plus the IFR Probability field with and without observations. Fog is observed over much of Texas where the Night Microphysics RGB shows low clouds, and IFR Probabilities are high. Note conditions over Mississippi where high clouds block the satellite view of low stratus. Nevertheless, IFR Probability fields show large values because IFR probability fields include low-level saturation information from the Rapid Refresh model.

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What kind of rain rates are associated with the convection? Suomi-NPP overflew the Gulf around 0800 UTC, and the toggle below compares the GOES-16 clean window infrared (Band 13, 10.4 µm) imagery and the derived MiRS rain rate (available as AWIPS-ready files from the LDM feed that accesses data from the direct broadcast site at CIMSS). Peak values are 1″/hour.

How did the GOES-16 Infrared-derived Rain Rate (which is intercalibrated using available Microwave data) compare with the MiRS Rain Rate at that time? A cursor sample of GOES-16 Level 2 derived products associated with a cold convective overshooting top (below) indicated a Rain Rate of 1.47 in/hr.

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Target Sector (2.5-minute interval) JMA Himawari-9 AHI Red Visible (0.64 µm) and Clean Infrared Window (10.4 µm) images (above) showed Invest 90P as it intensified to become Tropical Cyclone Kirrily over the Coral Sea on 23-24 January 2024. Visible images showed multiple exposed low-level circulation centers, while Infrared images showed that deep convection was mainly developing west of the storm center.

Himawari-9 Infrared Window (11.2 µm) images from the CIMSS Tropical Cyclones site (below) showed that Tropical Cyclone Kirrily was moving through an environment of low deep-layer wind shear (in contrast to the high values of shear that were affecting Invest 90P on 20 January).

===== 25 January Update ====

2.5-minute Himawari-9 Visible and Infrared images (above) showed Cyclone Kirrily as it approached the coast of Queensland, Australia — making landfall near Townsville (YBTL) around 1200 UTC on 25 January 2024.

A plot of surface report data from Townsville (below) depicted a pressure minimum from 1120-1200 UTC (during landfall), with a maximum wind gust of 50 kts at 0942 UTC (prior to landfall).

Several hours prior to landfall, the Jason-3 satellite was detecting altimeter Significant Wave Heights (the average of the highest 1/3^rd of the wave distribution.) up to 20.34 ft off the coast of Queensland at 0422 UTC (below).

Around the time of landfall, ASCAT winds from Metop-B and Metop-C (below) were in the 40-50 knot range near the coast.

A toggle between daytime (pre-landfall) and nighttime (post-landfall) Suomi-NPP VIIRS Day/Night Band (0.7 µm) images is shown below.

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