WESTERN REGION TECHNICAL ATTACHMENT
NO. 00-06
MARCH 28, 2000

AN EXAMPLE OF USING THE "SATELLITE FOG
PRODUCT" IN PREDICTING DENSE FOG
OVER SOUTH-CENTRAL AND SOUTHEAST MONTANA
AND NORTH-CENTRAL WYOMING ON NOVEMBER 28, 1999

Richard Canepa and Mark H. Strobin, NWSO Billings, Montana

[Note: Satellite imagery will appear only on the web version of the Technical Attachment.]

Introduction

During the late afternoon and evening of November 28, 1999, dense fog moved from southeast Montana into the Billings area. The fog was dense enough to close Billings Logan International Airport. Dense fog, defined as visibilities of 1/4 mile or less, is uncommon across NWSO Billings County Warning Area (CWA). NWSO Billings CWA incorporates south-central Montana, southeast Montana, and Sheridan County Wyoming. Based on National Climatic Data Center normals, dense fog during the month of November across NWSO Billings county warning area occurs on average one day during the month.

This Technical Attachment will show the utilization of satellite imagery, in particular the fog product, and the synoptic situation which allowed forecasters to accurately predict the fog. Dense Fog Advisories were issued well ahead of the occurrence of dense fog in the Billings area.

The Synoptic Situation

A surface low developed along the east slopes of the northern Rockies on November 26. The low migrated to eastern Wyoming late on the 26th. Temperatures were mild enough to support rain at Billings and Sheridan, mainly rain in Miles City, but cold enough to support snow in the extreme southeastern corner of Montana. Total precipitation at all three cities was 0.15" or less on the 26th (see table below). Skies cleared late on the 26th with no precipitation on the 27th or 28th.

A strong 1040 mb surface high was located over southern Saskatchewan and Manitoba on the evening of November 28. The surface weather analyses at 00Z and 03Z on November 29 showed slight pressure increases, with 3-hour pressure changes of less than 1.0 mb. Surface pressure gradients over south-central and southeast Montana were rather weak with a tendency for an upslope east to northeast wind. The upper-air data at Glasgow, MT for 00Z and 12Z on November 29 clearly indicated a very moist and stable layer below approximately 875 mb with an easterly component to the wind on both soundings (Figs. 1 & 2).

Importance of Satellite Fog Product Imagery

In viewing low clouds and fog, visible imagery (except with snow cover) is the most ideal method. Naturally, visible imagery is only available during daylight hours. Infrared imagery is a poor choice for night viewing because low clouds and fog are hard to detect. This is because low clouds and fog have a similar radiating temperature as the underlying land.

The best satellite imagery to use for identifying fog and stratus clouds during the night is commonly called the "fog product." The identification of fog and stratus at night is an application of the GOES Imagery data (CIRA - Cooperative Institute for Research in the Atmosphere tutorials, 1999). The GOES Imagery data utilizes bi-spectral satellite imagery (11u - 3.9u on AWIPS, i.e. fog product) since low clouds and fog have different emissive properties in the two wavelengths. The imagery is reduced by subtracting the 3.9u brightness temperatures from those at 10.7u . This imagery highlights low clouds and fog in white gray shades (a positive difference), clear conditions with little temperature variability shows up as a mid-gray shade, and high clouds in a dark shade (a negative difference). High clouds appear dark in this imagery since much of the sensed energy comes from the ground, and the 3.9u channel response to warm sub-pixel temperatures is greater than that at 10.7u . This is true even though the emissivity of an ice cloud is about the same as at the 3.9u and 10.7u . Using the AWIPS Image Properties the forecaster can colorize the "fog product" making it easier to identify the low clouds and fog. Additionally, putting these images in animation, fog and low clouds would be easily observed since the higher ice clouds would show more rapid movement.

There are limitations in viewing the fog product imagery at night. The "fog product" is also a "stratus product" since the satellite only observes the top of clouds, not the cloud bases. The forecaster must utilize additional data such as surface observations to differentiate between stratus and fog. At times, it will be difficult to distinguish between high clouds and land. This problem is alleviated by animating the product. During the daytime, the fog product uses the 1 km spatial resolution GOES visible data which would give the product a highly defined appearance. During the night, the product utilizes bi-spectral satellite imagery which has a resolution of 4 km. Therefore, fog product imagery at night will have less sharpness and definition.
The Event

On November 28, the end of the Thanksgiving holiday weekend, dense fog persisted all day at Miles City, MT and to some degree at Sheridan, WY. The lower atmosphere mixed out enough during the day farther west with improved visibilities at Billings, MT. It is worth noting that prior to November 28, airport holiday traffic at Billings Logan International Airport had already been impacted with many flights delayed or canceled due to dense fog. With little mixing during the day over south-central Montana on the 28th, the lower atmosphere remained very moist and stable into the evening. Dense fog over southeast Montana began to expand and, with a persistent upslope wind component, advected westward in the Yellowstone and Tongue River valleys on the evening of the 28th (Fig. 3). Dense fog reached the Billings, MT airport (located approximately 400 feet higher than the city center) and lowered the visibility to less than 1/4 mile at 8:38 pm MST. Visibility remained very low overnight (Fig. 4). While the airport observed a very light northeast to northwest wind, there was a 10 to 15 knot easterly wind during the evening at Livingston, MT. The easterly upslope wind was directly supported by the slight pressure rises over southeast Montana during the evening of the 28th. By the morning of the 29th, pressure began to decrease along the east slopes (lee-side troughing) and a downslope, drier south to southwest wind developed. This completely eroded the fog before noon at Billings, MT, while visibilities gradually improved at Sheridan, WY and Miles City, MT.

Conclusion

Fog producing processes begin with a very moist and stable lower atmosphere with light mixing. In this case, rather light precipitation amounts over the area (0.15 inches or less) was sufficient to moisten the lower atmosphere. Other factors include slight pressure rises over south central and southeast Montana which resulted in an easterly upslope wind as far west as Livingston, MT, and a clear sky for radiational cooling processes.

Dense fog always results in some degree of impact on the livelihood of people, affecting travel both in the air and on the ground, especially if it occurs over a holiday when traffic volume dramatically increases. Fog is rare along the east slopes of the Rockies in south- central and southeast Montana, and north-central Wyoming. Moreover, it is very rare for dense fog to occur over such a widespread area for a long duration as reviewed in this example. When forecasters anticipate conditions favorable for fog development, in combination with other weather data, the satellite "fog product" becomes one of the most essential tools to use.