National Weather Service explains "doughnut hole" in Colorado snow coverage

On Sunday, the National Weather Service (NWS) released a satellite image showing the "rare sight" of snow on the ground across most of Colorado, following a snowy last few weeks.

The only "hole" on the map where there was no snow was located in and around Pueblo County. According to the service, it is not surprising that this area, known as the  "Pueblo Precipitation Doughnut Hole" (PPDH), has been missed by the last few snow storms. 

In a series of tweets on Monday, NWS shared some science behind why the PPDH tends to stay dry while other, even neighboring, areas get hit hard by snow. 

"First, let's take a look at some basic meteorology. Stated simply... precipitation likes rising air, and inversely of that, precipitation doesn't like sinking air," the service said.

According to the thread, rising air occurs when strong low pressure systems force air to rise along a boundary or up terrain (upslope wind). Meanwhile, strong high pressure systems that force air downslope cause sinking air. 

As illustrated in the image below, the downslope terrain across the plains contributes to the PPDH phenomenon. 

"Most of the time, winds will be northerly, southerly, or westerly for southern Colorado. These winds will upslope along the Palmer Divide and Raton Mesa areas (precipitation) and downslope into the Arkansas River Valley (no precipitation)," the service said. 

In order for air to rise in the Arkansas River Valley area, where PPDH is located, easterly winds are needed to upslope along the rising terrain to the west. 

"Now with all of that said there are exceptions to this... Such as when a strong and dynamic storm system passes over, it can overcome the mitigating downslope winds and either push precipitation off the terrain over the PPDH or just develop it over the PPDH," NWS reports.

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