For the second day in a row, residents of the southern United States have awoken to dense fog advisories. A view of the continental United States surface map shows that overall, there is very weak synoptic forcing across the south, which results in calm winds and little mixing or advection. This allows radiative cooling to dominate other processes in the overnight hours, and eventually air temperatures cool until they reach the dew point temperature at which point condensation occurs and fog forms. With little wind to disturb things, the fog persists overnight. During the day, the sun rises and heats the ground, which causes near-surface air temperatures to rise. However, the absolute moisture content of the air is relatively unaffected, so the dew points remain unchanged. The warmer air temperatures mean that the air is no longer saturated, and so the fog dissipates.

Band 13 from the GOES-16 ABI measures the atmospheric window brightness temperature and is often the first stop for observing conditions at night when no visible channels are available. Since fog has effectively the same temperature as the land, it can be challenging to discern where the fog ends and clear air begins using just that channel. This is particularly evident in the pre-dawn hours of 28 January 2025. The Band 13 loop over Alabama and Mississippi from 10:00 – 12:00 UTC (4:00 AM to 6:00 AM Central Time) shows little of note. The clouds moving in from the west indicate some elevated instability due to their convective shape, but otherwise conditions look clear and calm.

However, the Nighttime Microphysics RGB tells a very different story. Here, three different parameters are combined into one image to help users identify various features. In this case, the 12.4–10.4 micron brightness temperature difference (sensitive to optical depth) is assigned to the red channel. The 10.4–3.9 micron brightness temperature difference (sensitive to cloud particle size and cloud phase) is assigned to green, and the 10.4 micron brightness temperature (sensitive to temperature) is assigned to blue. Fog, which is a warm liquid cloud, is going to be a light blue or mint green in this combination. Looping this RGB for the same time as above shows the undeniable presence of fog over the region. Of particular note is the long, narrow Tennessee River valley stretching diagonally from northeastern Alabama into eastern Tennessee. This feature is practically invisible in the traditional IR window, yet the fog that formed in this valley is easily seen in the RGB.

It is important to note that this RGB depends on the 3.9 micron brightness temperature channel. This channel straddles the border between terrestrial emission and solar reflectivity. At night, it represents the infrared emission of the surface, but during the day, it is dominated by reflected radiance from the sun. Because of this, the interpretation of the RGB cannot be used during the day as the relationship between the 3.9 ?m brightness temperature and cloud microphysics is only valid at night.

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Night Microphysics RGB imagery from the CSPP Geosphere site (link) show the blossoming of high clouds (deep red in the RGB) north of the Samoan Islands, clouds that subsequently sag southward. Careful inspection of the animation also shows clouds that are more pink than deep red; these are likely mid-level clouds that might be developing in the vertical. Water vapor imagery (below, from this site) also show the development of cold cloud tops north of the Samoan Islands, part of a region of colder cloud tops that extends to the east of American Samoa (circled in this annotated water vapor image from 1320 UTC).

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As you’re pondering the satellite imagery, perhaps you recall the numerical models from the previous days. The animation below (taken from the tropicaltidbits website) shows precipitation moving towards the Samoan Islands (in the upper right quadrant of the domain) from the east and northeast. This animation shows a nine sequential forecasts all valid at 0600 UTC on 28 January 2025. The forecasts were consistent in bringing rain towards the Samoan Islands (near 12^oS, 170^oW). And the satellite imagery above shows convection dropping southward.

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A timely Metop-C overpass provided ASCAT sea-surface winds at 0930 UTC over the Samoan Islands. Strong winds to the north of Samoa, and lighter winds near Samoa, suggest surface convergence that might be helping to force the convection. (Image from this website).

LightningCast Probabilities (available here) show the likelihood of a GLM observations within the next 60 minutes. The animation below shows LightningCast increasing to the north of the Samoan Islands overnight on early 28 January; those probabilities move southward as convection continues, and eventually lightning (in the form of GLM Observations) occurs over Independent and American Samoa. The lightning is north of American Samoa at 1040 UTC, and overspreads the islands from 1140-1410 UTC, when the animation stops. (Click here for an animation with a faster timestep). The increasing LightningCast probabilities suggest strong convection .

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Pago Pago, American Samoa reported light rain at 1600 UTC on 28 January. When you try to determine if rain is imminent, use as many satellite products as you can!

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5-minute CONUS Sector GOES-16 (GOES-East) Visible images (above) showed ice across much of Lake Erie on 27th January 2025. The far western and far eastern parts of the lake were covered in fast ice — while the motion of drift ice within the central portion of the lake was being influenced by strong SW winds gusting to 30-40 kts (near the east end of Lake Erie, there was a peak wind gust of 51 kts at Niagara Falls KIAG at 1921 UTC). Lake Erie average ice cover had quickly risen above the historical average during the preceding week, as cold temperatures prevailed across much of the eastern US.

GOES-16 True Color RGB images from the CSPP GeoSphere site (below) provided better contrast between the lake ice and areas of open water.

A closer look at GOES-16 True Color RGB images centered over the fast ice covering the far western part of the lake (below) revealed that some of that ice began to become detached from the shoreline later in the day, due to wind stress.

A closer look at GOES-16 True Color RGB images centered over the fast ice covering the far eastern part of the lake (below) displayed a large section of the western ice edge that was abruptly pushed eastward by strong winds late in the day. As that rapid deformation of the fast ice occurred, the pressure appeared to force a small segment of ice northward up the Niagara River.

RCM-1 and RCM-2 SAR Normalized Radar Cross Section imagery (source) at 1137 UTC and 2331 UTC (below) provided a detailed view of the intricate ice structure across the western portion of Lake Erie — in addition to linear channels created by icebreaker ships.

A toggle between Sentinel-2 Optimized Natural Color RGB and Normalized Difference Water Index images (below) demonstrated that the NDWI product was more useful for highlighting ice leads as well was the linear channels created by icebreaker ships.

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GOES-18 Upper Level water vapor infrared imagery, above, (source) shows a stream of upper-level moisture moving over southeastern Alaska on the east side of a cyclonic system over western Alaska on 27 January 2025. This large storm moved warm and moist air northward into Alaska; several stations tied record warm temperatures on 26 January (albeit weak records, compared to the days before the 26th), including Fairbanks and Anchorage and Kodiak Island. GOES-18 Airmass RGB images, below (from here), show the characteristic red/orange enhancement over central Alaska and the northern Gulf of Alaska expected when a strong potential vorticity anomaly is present. The green enhancement over southeast Alaska is more typical of an airmass with at least some tropical origins. (You can find a Quick Guide for the airmass RGB here, or here).

MIMIC Total Precipitable Water for the 24 hours ending 1400 UTC on 27 January 2025, below, shows a concentrated ribbon of northward-moving enhanced moisture (one might call this an atmospheric river) moving eastward, from south-central Alaska at the start of the animation to southeast Alaska at the animation’s end. The south coast of Alaska had generous rains on the 26th as this moisture source moved through.

The atmosphere river resulted in Total Precipitable Water amounts that were well above normal. The NOAA/NESDIS/OSPO Percent Of Normal field for 1200 UTC on 27 January 2025, below, from this source, showed values near 200% of normal.

The 1200 UTC/26 January 2025 sounding from Anchorage (from the handy Wyoming Sounding site) showed a TPW at 17.31 mm, 0.68 inches. (Here’s the blended TPW percent of normal for 1200 UTC on 26 January — note the exception moisture to the west of where it occurs above 24 hours later). The plot below is from the SPC Sounding Climatology page, and shows the observed maximum/mean/minimum sounder TPWs at Anchorage. The values with this system although not record breaking are certainly on the high side of the distribution for January.

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This website shows microwave estimates of snowfall rate from the many polar-orbiting satellites that give excellent coverage over Alaska. Data from 0444 through 1342 UTC on 27 January 2025, below, shows heaviest snows along the coast of southeastern Alaska, and considerable snow along the Brooks Range in northern Alaska as well.

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