Local Review: December 20-21 2005 stratus-build down fog event
In this event, the models had a difficult time with clearing out the low level moisture too quickly, which made forecasting this event quite a challenge. Afternoon crossover values ranged from the lower to mid 20s, but the models were projecting these values to lower during the course of the night. There was some snow on the ground and low clouds across the forecast area during the afternoon.
The lowest ceilings were found over central and south central Kansas while southeast Kansas had ceilings mainly in the MVFR category. During the late afternoon and early evening, there was some clearing across the western sections of the forecast area, but the lack of isentropic downglide displayed on the 270k theta surface really slowed down the eastward progression of the clearing line.
The NAM/RUC were both showing the clearing line arriving at Wichita and Cottonwood Falls by midnight according to the boundary layer RH and soundings. This indeed was not the case. Around 8 pm, the forecasters made some phone calls to the western fringes of the clearing line around Lincoln, Ellsworth and Rice counties. At that time, dense fog was forming in those areas, and the crossover values were being met. The satellite fog imagery displayed this fog layer fairly well. This was also where the cloud layer was the thinnest which allowed for ideal fog top cooling with good decoupling across this region.
As the night wore on, the clearing line made very slow progress to the east. It finally made it to Salina and Hutchinson before 12z, but not before the widespread dense fog developed and persisted for several hours. The dense fog mostly formed where the ceilings started out the lowest in Central Kansas. During the evening, the ceilings in this region were mainly below 500ft and continued to lower until the dense fog settled in. South central Kansas started out with ceilings around 800ft and then the dense fog developed between 06 and 08z when things started to decouple. Southeast Kansas never did receive dense fog, possibly due to thicker cloud decks which did not allow for good fog top cooling, and lack of snow cover. However, the ceilings did lower about a 1,000 ft during the night.
To sum it all up, we all need to cautious on clearing out trends suggested by the models, especially when there is a lack of isentropic downglide. This can play a crucial role where dense fog could develop, especially if the ceilings are below 1,000ft and there is good decoupling. Sometimes you do not even need a cold ground to develop dense fog. Fog top cooling may be enough to cause the stratus build down effect to take place. I saw this happen in SGF on January 3, 2006 when the ground was really warm. Be especially watchful for those areas around the back edge of the stratus, where horizontal moisture flux may cause the stratus to backbuild. This appeared to happen in a case on January 23, 2006.