Squinting into blowing dust rivals trying to see through a lake-effect snow squall. However, unlike snow, this particular phenomenon is nearly invisible to radar and satellite alike. So the question is, how do we see and forecast for something so obvious to the naked eye and yet so invisible to our traditional instrumentation? This is a question that AWC forecasters at the FA desks face on various occasion and is what makes blowing dust SIGMETs so difficult to issue. One such occasion occurred this past Wednesday, April 8th. A strong upper level system was positioned over the Southwest U.S. and the associated upper level jet had surface winds in the afternoon gusting to 30+ mph, particularly across New Mexico (Figure 1).
Figure 1. 20140408 1800 UTC GOES-W visible imagery and surface obs. Winds were 20-30 mph, with 30 mph+ gusts.
The forecaster working the FA desk was expecting these winds to result in blowing dust and at the beginning of shift requested to use the VIIRS/MODIS dust enhancement (provided by CIRA) to try and better identify any areas of dust that would arise. With approval for a temporary transition of the product to operations, the tool was able to be utilized in this case. By 1800 UTC winds had begun to pick up and radar reflectivity from Albuquerque showed some weak echoes indicating perhaps some dust being lofted. However, surface observations for the next couple of hours remained clear.
At 1940 UTC the first useable MODIS pass arrived, shown below in Figure 2. While it showed some weaker indications of dust in the Four Corners area, the majority of New Mexico remained clear.
Figure 2. 20150408 1940 UTC MODIS dust enhancement over the Southwest. Note the pale yellows in the Four Corners area
Additionally, the forecaster took a look at GOES sounder dust enhancement imagery available on the web. Though a coarser resolution, the weak indications of dust in the Four Corners were noticeable in this imagery as well (Figure 3).
Figure 3. 20150408 1802 UTC GOES sounder dust imagery. Note the yellows in the Four Corners area.
Though there may have been some lofted dust, the few weak radar echoes and overall clear surface observations indicated that it was not widespread or thick enough to warrant the need for a SIGMET. The FA forecaster was collaborating with ZAB (Albuquerque CWSU) during the afternoon hours and passed on the link (http://rammb.cira.colostate.edu/ramsdis/online/goes-r_proving_ground.asp) to both versions of the dust enhancement imagery. As the satellite-derived imagery seemed to be supporting their original assessment and lacked any strong dust signals, a final decision was made not to issue the SIGMET.
Although this ended up as a null case, it still shows the value of the satellite-derived imagery as a decision support tool at the AWC. As previously mentioned, dust is very difficult to pick up on any traditional imagery. However, the VIIRS/MODIS dust enhancement is able to bring this invisible hazard into view and provides additional and valuable situational awareness for blowing dust SIGMETs.
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