10-minute Full Disk scan GOES-18 (GOES-West) daytime True Color RGB and Nighttime Microphysics RGB images — created using Geo2Grid (above) — showed the offshore transport of glacial silt from the Copper River Delta in Southcentral Alaska from 11-23 December 2025. Even though cloud cover occasionally obscured the signature of this plume of airborne glacial silt, its southward transport over the northern Gulf of Alaska was occurring during the entire 13-day period.

Focusing on one particular day (18 December), a GOES-18 Near-Infrared image at 2100 UTC (below) included plots of Metop-C ASCAT surface scatterometer winds — which showed wind speeds of 35-37 kts immediately offshore of the Copper River Delta. Farther inland, note the peak wind gust of 53 kts at Valdez (PAVW) as gap winds were being channeled down Valdez Glacier.

Metop-B Ultra High Resolution (UHR) ASCAT winds (below) displayed the narrow plume of higher wind speeds (shades of red) that arced southward from the Copper River Delta.

RCM-2 Synthetic Aperture Radar (SAR) winds at 1558 UTC (below) also highlighted the plume of stronger winds (shades of yellow to red) emerging from the Copper River Delta.

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Surface analyses during the 11-23 December period (above) depicted the strong coastal pressure gradient (closely-spaced isobars) between a cold inland area of high pressure and migratory areas of low pressure over the northern Gulf of Alaska. This strong pressure gradient forced gap winds to accelerate down the Copper River Valley (topography) and emerge from the coast.

The effect of the persistent inland area of high pressure was seen in a map of surface air temperature departure (below) — which ranged from -15 to -21ºF along the coast near the Copper River Delta (medium shade of blue) to -27ºF and colder (magenta) near the Alaska/Yukon border. On 22 December, minimum temperatures were quite cold across Alaska and Yukon (and included -62ºF at Chicken).

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One of the most common sayings that a young meteorologist learns early in their education is that “air acts as a fluid.” Gap winds are an excellent illustration of that idea. As a fluid moves through a narrow passage, it has to speed up in order to ensure a constant mass flow rate, just like putting a finger on a garden hose to make a jet of water shoot faster. As air flows through gaps in mountain ranges, the ssame thing happens: the speed of the air increases so that the total mass of air keeps moving at the same rate as it was before it reached the gap.

Southeast Alaska is an ideal location for gap winds. The high, cold territory of British Columbia and the Yukon create large airmasses that flow downhill toward the Pacific Ocean. As these air masses reach southeast Alaska, they are filtered between the rugged islands of the Alexander Archipelago (the Pacific islands that comprise much of southeast Alaska’s territory). Thw air has to accelerate as it is being pushed from behind, and thus rapid winds form through and downstream of the gaps.

This can be seen in satellite products from 22 December 2025. Cross Sound is the body of water between mainland North America and Chichagof Island, the nation’s fifth largest island. The sound is at sea level, of course, but is flanked on either side by ridges that exceed 2000 feet (600 m) with only a 13 mile (20 km) gap between them. This creates a significant opportunity for gap winds. The following image shows the GOES-18 Channel 13 (infrared window) view overlaid with ASCAT winds.

Note that the winds downstream of the Cross Sound gap are 35-40 kts strong. With speeds that fast, these are officially gale force winds. Also note the cloud streets forming parallel to the winds. These are likely caused by the instability created when the cold continental air flows over the relatively warm sea. The “warm” maritime air rises to create convective clouds that are then organized by the winds into bands parallel with the flow.

As noted above, these gap winds form because the cold, dry, dense air masses of northern North America flow downhill to the sea. How cold is it? Let’s zoom out.

Here, we see the loop of GOES-18 Channel 13 imagery. The default color table in use here is designed for the continental United States. Those blues and greens represent temperatures around -40 F (-40 C) which are associated with moderately deep convection in the Lower 48. However, in the Yukon in December, those are surface temperatures. We can easily see that the Yukon is largely clear just by animating the satellite imagery. Those very cold structures are not moving, which means they’re part of the surface. Thus, we can tell that a very clear night produced bitterly cold air at the surface, a ripe setup for gap flow once that air flowed downhill to the Pacific. This animation also shows that the wind observations are only available for certain frames; this is because ASCAT is mounted aboard polar orbiting satellites and thus isn’t constantly available.

The ASCAT winds aren’t the only satellite-based wind instrument to measure the gap winds. Synthetic Aperture Radar (SAR) is another useful tool. These observations are available here and provide a nigher spatial resolution view of the surface winds over water at the expense of narrower swaths (and thus less frequent coverage). The finer spatial resolution enables us to see not only the gale force winds flowing out of the Cross Sound gap (at the top of the image) but highly localized regions of fast winds emanating from much smaller gaps as well.

A postscript: What does Chicaghof Island, Alaska, have in common with Chicago, Illinois, besides their very similar names? They’re both famous for bears! Chicaghof Island has the highest concentration of bears per unit area of any place on earth, while American football fans are quite familiar with Chicago’s Bears.

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A 29-day sequence of 10-minute Full Disk scan GOES-18 (GOES-West) daytime True Color RGB and Nighttime Microphysics RGB images — created using Geo2Grid (above) showed the evolution of a long-duration Tule fog event across the Central Valley of California, from 21 November to 19 December 2025.

The fog was more sparse in its areal coverage during the initial 3 days of the period — but became more persistent and widespread beginning on 24 November. The trend of increasing dense middle to upper-level cloud cover on 19 December (associated with the arrival of an atmospheric river) began to obscure the fog across the northern 2/3 of the Central Valley.

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As strong westerly winds continued across Colorado on 19 December 2025, a mountain wave became established over the Front Range. Wind shear associated with this mountain wave resulted in numerous Pilot Reports (PIREPs) of Moderate to Severe turbulence (above).

One notable PIREP mentioned 5 seconds of Severe Turbulence at an altitude of 13800 ft, just NW of Denver (below).

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