With GOES-17 now at the 137.2W GOES-West position (not yet operational, though), Alaska has significantly improved satellite coverage. Imagery from GOES-17 is available every 15-min over Alaska from the full disk sector, and every 1-min (or 30 seconds) in a mesoscale sector upon request (previous generation GOES provided 15-min coverage over Alaska with a 30-min gap every 3 hours). However, spatial resolution is degraded so far from the satellite subpoint over Alaska. A 1 km pixel will have an approximate pixel area of around 4 km over southern Alaska and the Aleutian Islands, degrading to 8+ km over northern Alaska. Figures 1, 2, and 3 show 15-min full disk VIS, IR and WV imagery during the day on 19 Nov. An occluded low pressure system is diagnosed along the southern Alaska coast.
Figure 1: 19 Nov 2018 GOES-17 0.64 um VIS. Full res
Figure 2: 19 Nov 2018 GOES-17 10.3 um IRW. Full res
Figure 3: 19 Nov 2018 GOES-17 6.2 um WV. Full res
Since GOES-17 arrived at GOES-West and dataflow resumed, NWS offices in Alaska have taken advantage of the 1-minute mesoscale sectors. On the evening of 16 Nov, WFO Juneau requested and was granted a sector through the next evening for “Winter Storm Warning in area with no RADAR coverage.” During the morning of 17 Nov, WFO Anchorage requested a sector to run through the evening for “Winter Wx Advisory, no radar coverage, models under performing.” These sectors were positioned next to each other, providing widespread 1-min imagery over Alaska. The mesoscale sector coverage from the AWIPS GOES-West perspective is shown in Figure 4. For a more direct view of Alaska, AWIPS users can take advantage of the Alaska perspective, shown in Figure 5. The meridional stretching of the sectors (and embedded pixels) is apparent in the Alaska perspective. The southern extent of the sectors are cut off in the Alaska perspective in this example. Alaska offices have requested mesoscale sectors numerous additional times since the 17th.
Figure 4: 17 Nov 2018 GOES-17 1-min meso-1 and meso-2 sector 0.64 um VIS from GOES-West perspective. Full res
Figure 5: 17 Nov 2018 GOES-17 1-min meso-1 and meso-2 sector 0.64 um VIS from Alaska perspective. Full res
Most derived products are not yet available in AWIPS from GOES-17, but all imagery channels, channel differences, and RGB’s are. The Day Cloud Phase Distinction (DCPD) RGB, which provides great contrast of snow (green) vs low clouds (light blue) vs high clouds (red), benefits from some slight modifications to the recipe. These changes are necessary due to lower reflectance and cooler temperatures over Alaska compared to over CONUS, especially this time of year. Figure 6 shows the default DCPD RGB over Alaska on 17 Nov, while figure 7 shows the same RGB and time period with some adjustments to the RGB component ranges. The max value for the vis/nir components was reduced, and the min value for the IR component was reduced. Similar tweaks can be made to other RGBs in order to make them more usable in cold airmasses.
Figure 6: 17 Nov 2018 GOES-17 1-min Day Cloud Phase Distinction RGB over Alaska. Full res
Figure 6: 17 Nov 2018 GOES-17 1-min *modified for AK* Day Cloud Phase Distinction RGB over Alaska. Full res
As of the writing of this post, GOES-17 imagery is preliminary and non-operational.
Bill Line, NWS