During the cooler season WFO Corpus Christi often has forecast challenges for predicting the development and coverage of low stratus clouds and raditional fog. In some situations, a low level jet will develop as radiational effects cool the interior land and rather stable mid level lapse rates exist. Significant low level moisture, including high dew point temperatures, can make for prime conditions that support the development of surface-based fog or subtle vertical motions generated from the air being forced over a shallow surface-based stable layer present at night over the land. This can develop a stratus cloud deck within the low level jet. This can also be enhanced over our forecast area due to speed convergence along the coast or further inland when stronger winds that occur over the better mixed and warmer Gulf of Mexico pile up when they meet relatively lighter winds in the low levels across interior sections. The timing and areal coverage of the stratus can be particularly challenging since it typically results in MVFR ceilings and at times IFR conditions for TAF sites. While there are areas that are known for raditional fog, such as KALI (Alice), the fog can occur in any location, especially during the winter months. In addition, since the Gulf of Mexico remains mild through the winter there is occasionally a significant threat for sea fog when relatively mild air returns over a cooler air mass brought by recent cold fronts. We typically look for transitional periods when air mass dewpoints are near or lower than dewpoints over the ocean and this is capped by a warmer stable layer above the shallow boundary layer. The worse conditions can occur when a weak frontal boundary results in very light winds, yet sufficient clearing occurs and we get raditional and advection fog (land and sea fog) that quickly spreads across the forecast area. This occurred a couple times during the winter of 2008-09 and greatly affected the coastal bend of South Texas.
One of the primary tools that forecasters at WFO Corpus Christi use for real-time monitoring of fog or low cloud development is the GOES low cloud base and fog depth products that are provided by SPoRT and available in AWIPS. This imagery is most useful during mostly clear evenings when fog or low clouds are anticipated but the location and coverage is uncertain. Forecasters can run a loop of these satellite products and identify the first development of fog or low clouds. The example provided here
was on October 8 during the early morning hours when a low level jet was present over the forecast area and abundant low level moisture existed (dewpoints in the 70s and precipitable water values around 1.80 inches). An MVFR broken ceiling affected much of the western portion of our forecast area as evident by the surface METAR observations overlaid with the GOES fog products.
Many examples were provided for the Fog Intensive Study Period in January 2009 which demonstrated considerably lead time (10 to 120 minutes) for incorporating changes to routine or amended TAFs. The GOES products were also used to issue Dense Fog Advisories during the evening shifts prior to the onset of a couple significant events. The WFO relies on using these products combined with other tools to better anticipate fog and low cloud development as we enter the heart of the fog season in December and January.