Gusty winds, low relative humidity, and dry fuels resulted in high fire danger across parts of Colorado in mid-November, 2021. These conditions allowed for the growth of the Kruger Rock Fire near Estes Park, CO on 16 Nov 2021.

GOES-East shortwave IR imagery, however, did not detect the wildfire hot spot during the morning and afternoon, despite the fire quickly growing to over 75 acres. However, when one viewed the GOES-West shortwave IR imagery, the hot spot was easily diagnosed from initiation through the afternoon. An animation comparing SWIR imagery from GOES-East and GOES-West is shown in Figure 1.

The explanation for this appears to be a couple of factors. The most significant factor was a persistent and optically thick mountain wave cloud, oriented above a point just east of the fire, blocking the heat signal to GOES-East, but not to GOES-West. Note that at 105W, this location is in the middle (longitude) of the two GOES satellite (137.2W and 75.2W), meaning pixel sizes are similar. Another 2-panel GOES-East/West comparison is shown in Figure 2, but this time comparing Natural Fire Color RGB Imagery, which shows the hot spot and cloud cover in the same image. GOES-West has an unobstructed view of the fire, while a cloud appears to mask the view from GOES-East. A similar case of a mountain wave cloud blocking a wildfire hot spot signature occured last eyar with the Cameron Peak Fire.

Another factor that may have weakened the SWIR hot spot signal from GOES-East compared to GOES-West was that the wildfire was burning on a west facing slope. Therefore, while the GOES-West satellite had a direct and unobstructed view of the heat signature, the GOES-East satellite view of the heat signature may have been partially degraded due to terrain blocking. Figure 3 shows the satellite comparison again, but a little later in the afternoon as the primary wave cloud shifted to the east. The hot spot signature becomes apparent in the GOES-East imagery, but not to the degree that it was earlier from GOES-West. Note, during the same time period, additional clouds developing from the west begin to obstruct the view of the hot spot from GOES-West. Not a clear comparison, unfortunately, so the influence of terrain is not straightforward with this case.

VIIRS passes during the early afternoon provided yet another view that captured the wildfire hot spot signal during the early afternoon. The VIIRS SWIR imagery is available at 375 m resolution from the I bands (compared to 2 km at nadir from ABI), and is a component of the Natural Fire Color RGB (Fig 4). Parallax causing shifting of clouds also plays a role here, from scan to scan, depending on where the viewing target lies within the VIIRS swath. Terrain correction allows the hot spot to remain at the same location from scan to scan. The ~1850 UTC scan captured the fire heat signature beneath the cloud, albeit under what appeared to be a relatively transparent one. Subsequent scans detected the hot spot from the clear sky.

The example underscores the importance of forecasters viewing imagery from both GOES-East and GOES-West when possible in such situations. Timely VIIRS imagery can also be useful in determining the exact location of the hot spot consistently from scan to scan and in more detail, and when viewing among clouds and on terrain.

Bill Line, NESDIS and CIRA

GOES Imagery has long been a valuable tool for fog detection and tracking, both during the day and night. With GOES ABI, forecasters are able analyse low clouds and fog in even better detail, spatial and temporally. Further, the additional spectral bands allow for more insight into the features through the generation of multispectral products such as RGBs.

NWS Bismark, ND forecasters leveraged GOES-East ABI Imagery in tandem with area webcams and surface obs to analyze fog evolution prior to and after sunrise on 09 Nov 2021, informing their forecast decision-making. While GOES imagery provides a great look at the spatial extent of low clouds and fog and their evolution with time, the webcams and surface obs allow the user to determine actual surface conditions associated with the satellite cloud feature. At 1213 UTC, the forecaster issued an update to the Area Forecast Discussion that read: “GOES Nighttime Microphysics shows extent of fog in southwest North Dakota, possibly nudging into the south central this morning. Based on cameras and coverage depicted on satellite imagery, coverage is somewhat patchy, but has lowered visibility at Hettinger down to 1/4 mile. An SPS was issued to address fog this morning for this area. Fog will gradually dissipate mid-morning as boundary layer mixing increases after sunrise.”

The Nighttime Microphysics RGB imagery that goes with the above discussion captures the low clouds and fog as a light orange/light brown/yellow color, compared to the pink clear-sky land background, blue/magenta bodies of water, and red (thick) to black (thin) high clouds (Figure 1). The imagery shows the development and northward surge of low clouds and fog from beneath exiting upper level cloud cover over a 5-hour period ending just prior to sunrise. The webcams (and METARs) confirmed conditions beneath the suspected low clouds, per the discussion.

Forecasters continued to analyze satellite imagery and webcams after sunrise to inform their decision-making, as the low clouds and fog hung around. At 1505 UTC, Forecasters wrote: “Latest satellite and webcams indicate fog over the southwest and south central is beginning to dissipate. We did extend the areas of fog farther east and extend the SPS for fog through 16 UTC. Otherwise no changes to the current forecast.” During the daytime, multispectral products such as the Day Cloud Phase Distinction (Fig 2) and Day Snow Fog (Fig 3) RGBs may be utilized, in addition to visible imagery (Fig 4). In the three hour period following sunrise, these animations show the fog and low clouds dissipating through the morning hours, with surface conditions confirmed by webcams and METARs.

Bill Line, NESDIS and CIRA