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