Blog Post 2 (author: Thomas Silas)

The second full week of April 2021 was the first week in quite some time that no major severe weather outbreak was forecast for the southeastern United States. That said, though, interesting weather did occur in the US, including some significant precipitation over the Intermountain West region. This can be seen on a radar reflectivity image from the afternoon of Wednesday, April 14:


Figure 1: NEXRAD radar reflectivity over Utah and southwest Wyoming, 1945Z 14 April 2021 (source: College of DuPage)


Two distinct areas of precipitation can be seen affecting northern Utah: a stratiform region stretching from eastern Utah to southeast Idaho, and a convective line over western Utah. While radar reflectivity alone does not directly distinguish between rain and snow, snow tends to appear much smoother on maps of this type, while rain echoes are better defined with sharper edges. In addition, reflectivity values tend to be higher for rain than for snow. Based on these characteristics, some of the precipitation in the first stratiform region appears to be in the form of snow, especially in the northwest Utah/southern Idaho border areas. However, most locations only experienced rain, with snow limited to areas of higher terrain. This is likely due to the tendency for the radar beam to increase in elevation with distance from the radar. In addition, many radar sites in the western US are placed on top of hills or mountains in order to see over nearby terrain, enhancing this elevation effect. Precipitation is probably falling through the radar beam as snow, but melting before it reaches the surface. This is confirmed by a map of surface observations from 2200 UTC (1600 MDT) that afternoon, which shows temperatures well above freezing (40s-50s F) in the valley regions.


Figure 2: Surface observations, 22Z 14 April 2021. (source: UW-Madison AOS Department)


Significant snow did fall in the mountains, with some locations seeing well over a foot of accumulation. However, perhaps more interesting was the convective line that formed behind the earlier round of stratiform precipitation. At 2030 UTC, this line extended from the southern end of the Great Salt Lake southward through Salt Lake and Utah Counties as seen on the radar image below. This may not technically qualify as a squall line or mesoscale convective system (MCS) since the area covered was not large and the system was relatively weak. However, a shelf cloud and gusty winds over 40 mph were observed with much of the line, and a few stronger cells can be seen within it – especially the one over northwestern Utah County. That particular cell produced half-inch diameter hail in addition to frequent thunder and lightning.


Figure 3: NEXRAD base reflectivity from KMTX radar site, approximately 2030Z 14 April 2021. (source: College of DuPage)


Interestingly, surface temperatures were only in the mid 40s at the time. Although thunderstorms are unusual with temperatures this low, they are not unheard of if other atmospheric conditions are right. This can be seen from the 0000 UTC sounding from Salt Lake City later that evening: the surface temperature was only 44F, but mid-level temperatures were cold enough to result in some weak instability – around 250 J/kg of CAPE (Convective Available Potential Energy). While this is not a lot and is too low for any significant threat of severe weather, it was clearly enough to allow for the scattered thunderstorms observed Wednesday afternoon across northern Utah.


Figure 4: 00Z 15 April 2021 sounding from Salt Lake City. (Source: Storm Prediction Center)