Let’s examine the synthetic low cloud / fog product from the 4-km NSSL WRF-ARW model. This is from the 0000 UTC 17 October run valid between 0900-1600 UTC 17 October:
Low cloud / fog is depicted as blue in this color table, with mid- to high level clouds being black / dark grey. Early in the loop, note the region of blue (low cloud / fog) in northern Wisconsin. As the loop progresses, this region quickly becomes obscured by mid- to high level clouds that are moving eastward. It’s unclear when this low cloud / fog is forecast to dissipate in northern Wisconsin due to the higher levels clouds obscuring what is happening at lower levels.
When you come across a situation such as this, one option to get around this issue is to use the NSSL WRF-ARW experimental fog product. This is simply the model output of cloud liquid water at the lowest vertical level in the model. Here is the loop from 0900-1800 UTC, with cloud liquid water depicted as blue:
Fog can be inferred from the blue regions we observe in this loop. We avoid the issue of obscuration of higher level clouds, and we may also discriminate between low cloud and fog (as forecast by the model).
Another option for fog identification is the GOES-R Fog / Low Stratus Product from CIMSS. This product uses a blend of satellite imagery and 00 hour forecast model output (from the RAP, except it uses the GFS over oceans beyond the RAP domain) to assign a probability of fog. This differs from the previous experimental fog product above which is solely model output, run out to a longer forecast time. The following loop is the GOES-R FLS product for IFR fog:
The color bar is given as a percentage, so that greater percent probability of IFR fog exists in the orange to red colors. The visibility (bottom number, miles) and ceiling (top number, hundreds of feet AGL) observations are also overlaid so that locations of fog correspond to the regions of low visibilities. Note the regions of orange and red in northern Wisconsin indicating higher probabilities of fog which would cause IFR conditions, this closely corresponds to the observed fog from the observations.
For more detailed information on either the GOES-R fog / low stratus product, or the synthetic low cloud / fog product, see the training session from VISIT:
http://rammb.cira.colostate.edu/training/visit/training_sessions/