FLS, ACHA, and SRSOR over San Francisco Center

As all the activity out east has been ongoing since early this morning, there hasn't been much chance to use the CI and CTC algorithms (although we are watching some scattered convective fire north of Atlanta... but another post on that later). Because of this and the SRSOR being out on the West Coast we've been keeping an eye on San Francisco Center and the fog.

SFO has been in a ground delay since early this morning as the marine stratus pushed onshore and thickened. The simulated fog difference from the NSSL-WRF (Figure 1) forecast these low ceilings to remain until 1700 UTC, quickly dissipating from the Bay area after that while keeping the marine layer just offshore of the airport.

Figure 1. 20130822 1300 - 2300 simulated fog difference forecast

 Another simulated fog tool from the NSSL-WRF is shown below in Figure 2.

Figure 2. 20130822 1300 -2200 WRF fog forecast

This loop shows non-zero liquid water near the surface. Note the blue over SFO, indicating that the presence of liquid water near the airport. This provides further confidence of the thick marine layer around the Bay area. As with the the simulated fog difference, this fog forecast is suggesting the thinning and dissipation of the fog by 1700 UTC.

The GOES-R Fog and Low Stratus from this morning (shown in Figure 3), verified the simulated fog difference forecast, showing high IFR probabilities through about 1700 UTC before probabilities began to decrease.

Figure 3. 20130822 1200 - 1715 UTC IFR probabilities from the Fog and Low Stratus overlaid with observed ceilings

During the planning telecons, SFO Center was concerned with the thickness of the fog and whether or not they would be able to maintain their current rates. Given the small peninsula in the Bay area on which SFO is situated, their runways are configured parallel to each other with little distance between them. As planes approach each one, they briefly fly head on before making the turn towards the runway. When the Bay is socked in this maneuver becomes dangerous as not only can the planes not see each other, but the tower also cannot see the planes. As such, operators have to reduce flow rates.

So, in this case the thickness of the fog layer is very important. Shown below is the fog thickness product of the Fog and Low suite (Figure 4).

Figure 4. 20130822 1530 - 1715 UTC fog thickness (in meters) overlaid with observed ceilings

Note the yellow and green colors just west of the Bay towards the beginning of the loop indicating thicknesses of ~320 meters (~1100 feet). This indicates a fairly thick fog layer, however it is also important to know where the base and top of this layer are. As such, take a look at the cloud top height algorithm in Figure 5.

Figure 5. 20130822 1445 - 1717 Cloud Top Heights (meters)

Cloud heights over the Bay area at the beginning of the loop showed at roughly 420 meters (~1400 feet). With the fog thickness indicated at ~160 meters (525 feet) over SFO and the cloud tops at 1400 feet, it could be inferred that the base of the fog is roughly 1000 feet off the ground... definitely an interesting tool to use in providing situational awareness for fog events.

On a side note, check out the cloud tops at ~140 meters (roughly 500 feet) east of SFO in central CA. This is actually smoke from a rather large fire burning in the area. Further proof of this is seen in the GOES-14 1-min SRSOR data in Figure 6.

Figure 6. 20130822 1648 - 1747 UTC GOES-14 SRSOR 1-minute imagery over California

Aside from the smoke, also notice the very slow dissipation of the fog out of the Bay area. It appears as though the WRF simulated forecasts were both quite accurate here as the fog generally lifted by 1745 UTC. Notice though that the marine layer remains just off shore, also forecasted by the simulated fog difference. SFO has been able to up their rates somewhat, but still remains in a delay program due to that lingering marine layer just off the coast.