On 11 September 2017, the NWS Pueblo WSR-88D radar (KPUX) was taken down for emergency repairs. KPUX would not come back online until 24 September. While the Denver radar (KFTG) could be used by Pueblo forecasters during the outage, the loss of KPUX meant significantly reduced radar coverage for southern Colorado. Radar coverage is poor to begin with in the western part of the Pueblo CWA (mountains and mountain valleys) due to radar beam blockage. To help compensate, NWS Pueblo requested GOES-16 1-min mesoscale sectors when high impact weather threatened the region.
KFTG failed on 23 September, a day on which widespread heavy rainfall and strong-severe thunderstorms were expected across southern Colorado, and would not return to service until later that evening (0300 UTC). With both KPUX and KFTG out, NWS Pueblo forecasters were left using radars such as CYS and GLD, whose lowest beams scan roughly 20,000 ft AGL over Pueblo and Colorado Springs. NWS Pueblo requested a 1-min mesoscale sector, but was denied due to coverage over Puerto Rico (ongoing cleanup efforts from Maria with their radar down) and ongoing Hurricane Maria. Maria was out at sea with no TC Watches or Warning in effect, so it is unclear at the time of this writing why that sector was not moved over Colorado.
On 23 September, therefore, NWS Pueblo forecasters relied heavily on GOES-16 imagery (5-min), lightning data, and ground truth. One severe thunderstorm warning was issued for a storm in far eastern Colorado, where KGLD provided adequate information. The greatest forecast challenge for PUB existed over the southeast mountains and adjacent plains, where periods of heavy rainfall went undetected by radar. With shallow warm rain processes dominant, the precip was oriented below the lowest available radar beams. Two flood advisories were issued by PUB during the evening. The radar composite animation below demonstrates coverage over Colorado from 2300 UTC – 0400 UTC, during which both KPUX and KFTG were down (Fig. 1). The majority of the Pueblo forecast area was without coverage. The return of KFTG can be seen at 0300 UTC.
Reference figure 2 during discussion in the next two paragraphs. The first flood advisory was issued at 0027 UTC (24 Sep) for much of the Colorado Springs area. Abundant rain gauges, satellite imagery, lightning data, and spotter reports provided the information needed to issue the advisory. Prior to reports of heavy rain in Colorado Springs, heavy rain was reported in the more rural town of Pueblo West to the south. As the clouds associated with this rainfall streamed north, they cooled considerably, indicating to the forecaster that it was plausible Colorado Springs would receive rainfall at least to the degree of that being reported to the south. Nearby lightning strikes also indicated potential for localized heavy rainfall. Reports of heavy rainfall in Springs began to come in as the cooling clouds moved over the urban area, and along with the continued streaming of cooling clouds from the south, provided confidence to issue a flood advisory. Minor street and stream flooding was reported throughout the city along with one water rescue from Fountain Creek.
The second flood advisory was issued at 0149 UTC over the Junkins Burn Scar, located in the Wet Mountains southwest of Pueblo. Previous posts on this blog have referenced the Junkins burn scar, which is the result of an October 2016 Wildfire. Burn scars pose a significant flash flood risk given the lack of debris and less porous nature of the land surface. There aren’t any rain gauges on the burn scar and spotters are scarce in the area, forcing the forecaster to rely even more on GOES-16 IR data. Cold cloud tops detected in GOES-16 5-min IR imagery were analyzed heading towards the burn scar. The forecaster on shift compared these cloud top temperatures with those associated with the heavy rain that was confirmed to have occurred in the Pueblo West and Colorado Springs areas. Given the similarities (clouds over Junkins just slightly warmer than those over Colorado Springs, but cooler than those over Pueblo West), the local sheriff’s departments were notified of the potential flood threat and a flood advisory was issued. Thankfully, there were no reports of flooding over or near the burn scar, but it is unclear whether or not moderate-heavy rainfall occurred.
This event exemplifies the increased value of rapidly updating satellite imagery under unique circumstances such as radar outages.
-Bill Line, NWS
“The GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing. Users bear all responsibility for inspecting the data prior to use and for the manner in which the data are utilized.”