At the start of the water vapor animation, near 0000 UTC, thick clouds cover southern California (and the sounding from San Diego shows saturated conditions); dry layers in the sounding appear by 1200 UTC. The 7.4 µm weighting function shows that information is detected by the satellite from lower down in the atmosphere; energy detected at 6.5 µm comes from higher in the atmosphere. This difference arises because of the better absorptive qualities of water vapor gas for 6.5 µm radiation vs. 7.4 µm radiation. By 1200 UTC, sufficient drying has occurred that the 7.4 µm Sounder Channel is detecting radiation that emanates from sea level. Note also at 1200 UTC that each individual moist layer influences the weighting function — but there is insufficient moisture at 1200 UTC in those moist layers that they are opaque to energy at either 6.5 µm or 7.4 µm.
— NWS Hanford (@NWSHanford) January 31, 2017
The tweet shown above was issued by the NWS forecast office in Hanford, California — using an image of the GOES-15 Low Instrument Flight Rules (LIFR) Probability, a component of the GOES-R Fog/low stratus suite of products — to illustrate where areas of dense Tule fog persisted into the morning hours on 31 January 2017.
AWIPS II images of the GOES-15 Marginal Visual Flight Rules (MVFR) product (below) showed the increase in areal coverage of Tule fog beginning at 0600 UTC (10 pm local time on 30 January); the fog eventually dissipated by 2030 UTC (12:30 pm local time) on 31 January. Note that Lemoore Naval Air Station (identifier KNLC) reported freezing fog at 14 UTC (their surface air temperature had dropped to 31º F that hour). In addition, some of the higher MVFR Probability values were seen farther to the north, along the Interstate 5 corridor between Stockton (KSCK) and Sacramento (KSAC) — numerous traffic accidents and school delays were attributed to the Tule fog on this day.
Legacy infrared Brightness Temperature Difference (BTD) products are limited in their ability to accurately detect fog/low stratus features if high-level cirrus clouds are present overhead. This is demonstrated in comparisons of GOES-15 MVFR Probability and BTD products from Aqua MODIS (above) and Suomi NPP VIIRS (below). Again, note the Interstate-5 corridor between Stockton and Sacramento, where the extent of the fog was not well-depicted on the BTD images (even using high spatial resolution polar-orbiter MODIS and VIIRS data). Daylight images of GOES-15 Visible (0.63 µm) data (below) showed the dissipation of the Tule fog during the 1600-2200 UTC (8 am – 2 pm local time) period. The brighter white snow pack in the higher elevations of the Sierra Nevada was also very evident in the upper right portion of the satellite scene. One ingredient contributing to this Tule fog event was moist soil, from precipitation (as much as 150-200% of normal at some locations in the Central Valley) that had been received during the previous 14-day period (below).
===== 12 January Update =====
As clouds cleared in the wake of the storm, a comparison of 375-meter resolution Suomi NPP VIIRS true-color and false-color Red/Green/Blue (RGB) images viewed using RealEarth (below) revealed the extent of the snow cover; snow appears as shades of cyan in the false-color image, in contrast to clouds which appear as shades of white. [Note: with 5 inches of snow remaining on the ground, a new record low temperature was set in Portland on 13 January]The fresh snowfall was also apparent in a 30-meter resolution Landsat-8 false-color RGB image (below) along the south face of Mount Hood (located about 98 miles or 158 km east of Portland). The ski slopes of Timberline Lodge and Mount Hood Meadows received 13-14 inches of new snow during this event; the snow base depth at Timberline was greater than the average amount for this time of year.
GOES-13 (GOES-East) Visible (0.63 µm) images (below) also displayed the dark smoke plume. The viewing angles from the 2 satellites were similar (~53 degrees from GOES-15 vs ~57 degrees from GOES-13), but the time sampling was slightly better from GOES-15 (due to the extra “SUB-CONUS” scan images at :11 and :41 minutes nearly every hour). Image frequency will be even better with the GOES-R series of satellites (beginning with GOES-16), with routine scans every 5 minutes; the visible image spatial resolution will also be improved (to 0.5 km, vs 1.0 km with the current GOES).MODIS Visible (0.645 µm), Shortwave Infrared (3.7 µm) and Infrared Window (11.0 µm) images from a 2036 UTC overpass of the Aqua satellite (below) showed the black smoke cloud in the Visible, but there was no evidence of a fire “hot spot” in the Shortwave Infrared (the media report indicated that the fire was extinguished about 2 hours after it started, which would have been around or just before the time of the MODIS images). On the Infrared Window image, the smoke plume actually did exhibit a slightly colder (darker blue color enhancement) signature, which is unusual since conventional fire and wildfire smoke is normally transparent to thermal radiation. A view of the 250-meter resolution Aqua MODIS true-color Red/Green/Blue (RGB) image from the MODIS Today site is shown below.