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Foehn gap and orographic crest cloud downwind of the Sierra Nevada

By Scott Bachmeier •

GOES-18 Mid-level Water Vapor (6.9 µm) images, with pilot report (PIREP) plots of turbulence [click to play animated GIF | MP4]

GOES-18 (GOES-West) Mid-level Water Vapor (6.9 µm) images (above) revealed a well-defined Foehn gap (narrow ribbon of darker blue enhancement) near the spine of the Sierra Nevada range in California, along with a broad “orographic crest cloud” (darker green enhancement) that extended eastward across southeast California into southern Nevada on 06 January 2024.

The corresponding GOES-18 “Clean” Infrared Window (10.3 µm) images are shown below.

GOES-18 “Clean” Infrared Window (10.3 µm) images, with pilot report (PIREP) plots of turbulence [click to play animated GIF | MP4]

A toggle between GOES-18 Water Vapor and Infrared images at 2201 UTC + Topography (below) helped to illustrate that the location of the Foehn gap was immediately downwind (east) of the spine of the Sierra Nevada.

GOES-18 Water Vapor and Infrared images at 2201 UTC + Topography [click to enlarge]

There were Pilot Reports of Mountain Waves in the vicinity of the Foehn gap, as well as a report of Moderate to Severe Turbulence within the crest cloud feature (below).

GOES-18 Water Vapor image at 2106 UTC, with a Pilot Report of a Mountain Wave at 2105 UTC [click to enlarge]


GOES-18 Water Vapor image at 2131 UTC, with a Pilot Report of a Mountain Wave at 2131 UTC [click to enlarge]


GOES-18 Water Vapor image at 2206 UTC, with a Pilot Report of Moderate to Severe Turbulence at 2208 UTC [click to enlarge]

In a comparison of GOES-18 10.3 µm infrared brightness temperature, CLAVR-x Cloud Top Temperature derived product and CLAVR-x Cloud Top Height derived product within the crest cloud feature at 0021 UTC (below), the Cloud Top Temperature product was nearly 5C colder than the single-channel infrared brightness temperature, and the Cloud Top Height was around 45000 ft.

Cursor sample of GOES-18 10.3 µm brightness temperature (white), CLAVR-x Cloud Top Temperature derived product (cyan) and CLAVR-x Cloud Top Height derived product (green) at 0021 UTC [click to enlarge]

Categories: Aviation, CLAVR-x, GOES-18