Fog development challenges along a boundary in South Texas

A frequently occurring forecast challenge in South Texas, is dense fog development along a boundary.  A prolonged period of southeasterly winds will elevate dewpoints in this region due to the proximity of the Gulf of Mexico.  The winds will tend to relax with the approach of most cold fronts, leading to a period of a few hours with light winds.  In some cases prefrontal troughs will bring a temporary wind shift which will return to onshore flow due to a weak seabreeze.  It other situations, similar to March 28th, the wind will decrease to very light prior to the wind shift.  If the associated precipitation is insignificant and cloud cover is only scattered (typical with many weaker boundaries) during the evening or overnight hours, conditions can become ideal for radiational fog.  In addition, if sea surface temperatures are in the 50s and 60s (common in the cool season) there may also be development of sea fog which combined with the land-based fog can produce high impact events.

On March 28th, the GOES combined fog depth product depicted the development of fog along a boundary.  Following the passage of the boundary the dry air advection and increasing wind will dissipate the fog resulting in rapid clearing. The weaker fronts can be more problematic, as was the case on April 3rd, 2010 when very light winds and mostly clear skies led to widespread dense fog across the entire Coastal Bend.  The dry air advection was weaker in this case and resulted in deeper fog which became more widespread.

GOES fog depth product from 0331 to 0845 UTC 28 March 2010.

One forecast challenge of these events include identifying the length of window for ideal fog conditions which requires accurate timing of the frontal passage.  In some cases, model forecast soundings depict favorable fog conditions but real-time monitoring of satellite data and mesonet plots can be the best tools for short-term public, marine or aviation forecasts.  Satellite trends and be very useful in depicting the expansion and advection of ground fog (light wind scenario) or low stratus development (more often when a low level jet is present).

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