Very interesting Fog and Low Stratus case over Denver Center

As today has been a rather quiet day in regards to convection we took the opportunity to explore the Fog and Low Stratus (FLS) over Denver Center. The 8:15 am traffic planning call from the Command Center had Denver as one of their concerns because of an area of fog that developed around 1000Z (Figure 1).

Figure 1. 20130814 0832 - 1402 Z visible imagery over the Western U.S. Note the fog developing over Denver Center as well as the smoke from the fires in Southern Idaho

The FLS indicated this area as well... and as Denver is right on the edge of both the GOES-E and GOES-W domains, we compared the FLS displays from both satellites (GOES-E in Figure 2 and GOES-W in Figure 3).

Figure 2. 20130814 0832 - 1402 Z GOES-E Fog and Low Stratus IFR probability. Note the abrupt decrease in probabilities over Denver at 1315 Z

Figure 3. 20130814 0832 - 1402 Z GOES-W Fog and Low Stratus IFR probability. Note the stray light correction at 0845 Z as well as the abrupt decrease in probabilities over Denver at 1315 Z

Several things to note... first, the the 0845 Z GOES-W IFR probability shows an abrupt increase in probability over western Colorado. Given the steeper viewing angle of the western satellite this is a stray light correction.

Additionally, and more significantly, note the probabilities from both satellites from 1302 to 1315 Z. The IFR probabilities directly over Denver Center abruptly decrease. However, the visible for this same time period showed no change in the fog coverage. After further investigation we discovered a trail of smoke originating from the ongoing fires in southern Idaho, particularly noticeable in the GOES-W imagery. This smoke blew through Colorado and over Denver Center. The thought is that given the steep angle at which the both satellites view Colorado and the associated light during the sunrise (adding the visible optical properties during the daytime), the smoke interfered with the ability to detect the fog. So while the night time performance was great, the performance as the sun rose was impeded by the smoke.

Given this issue with the FLS we then searched for some other means to monitor this fog. Shown below in Figure 4 is yesterday's (20130813) 00Z simulated fog difference forecast for 0800 - 1200 Z this morning. Note the area of forecasted low-level liquid clouds over Denver Center in the general vicinity of where the fog developed.

20130814 0800 - 1200 Z Simulated Fog Difference from the 20130813 00Z run of the NSSL-WRF

Now take a look 0800 - 1200 Z this morning from today's (20130814) 00Z NSSL-WRF run. Interestingly, yesterday's NSSL-WRF forecast picked up on the fog development over Denver while today's forecast missed it completely.

20130814 0900 - 1200 Z Simulated Fog Difference from the 20130814 00Z run of the NSSL-WRF

It was a very interesting case with which to explore the behavior of these various products. Denver Center did end up with some delays earlier this morning do the rather rapid development of the fog. In fact, at one point visibilities were actually down to 1/2 mile with 1/4 mile RVR (runway visible range... or how far one can see down the runway).