This blog entry will look at an example of the synthetic fog product (from the 4-km NSSL WRF-ARW model) for an event that took place during the overnight hours of November 19 to 20, 2011.
Here is the synthetic fog product during the overnight hours:
There is southerly flow advecting moisture from the Gulf of Mexico northward. The increasing moisture, in combination with nighttime cooling leads to the development of a large area of stratus clouds across the Plains from Texas to Nebraska and eastward across the southeast states. This enhancement will depict regions of low-level clouds in the blue colors. Note that the imagery from 0000-08000 UTC is based on the 0000 UTC November 18 model run, and the imagery between 0900-1200 UTC is based on the 0000 UTC November 19 model run. There is a discontinuity between 0800 and 0900 UTC for this reason, but the trend in the low-level clouds as discussed earlier is still valid.
Let’s assess how well the model forecast was. We’ll look at the GOES Low Cloud / Fog product (also known as the shortwave albedo product):
In this color table, low-level clouds / fog are depicted by the lighter / white colors. High-level clouds are darker / black and color enhanced above a temperature threshold. The model did a pretty good job with the development of low-level stratus across the Plains and into the southeast. Fog can be assessed by looking at surface observations (not shown).
The key to remember here is that this product aids in visualization. Model fields of relative humidity could have yielded the same conclusions, however by looking at a forecast model field of something you’re already familiar with for diagnosing stratus / fog (i.e., the GOES fog product), it offers a different perspective on assessing the possibility of low-level stratus / fog during the forecast period of interest.