The second week of April 2021 was the first time in quite a few weeks that a major severe weather outbreak was not expected to occur in the United States. Despite this, relatively active weather was still observed in several areas of the country. One such region was the Intermountain West, which was affected by a surface low-pressure system from Tuesday into Thursday. This cyclone initially formed in association with an upper-level positive potential vorticity (PV) anomaly. Potential vorticity is the product of absolute vorticity and static stability, and is measured in potential vorticity units (PVU). In general, PV is much higher in the stratosphere than in the troposphere, and the boundary between them is often defined as a 2 PVU surface. A region where higher PV values extend lower down in the atmosphere than surrounding areas (such as the one in the cross section below) is known as a positive PV anomaly. Through a somewhat complex cyclogenesis process, positive PV anomalies at the tropopause can cause a cyclonic circulation to form at the surface to the east, creating a surface low pressure center. This explains the formation of the surface low over Nevada on the 13th. The following image is a cross-section of this PV anomaly at 12Z 14 April: the fill pattern is PV (with a black dotted line marking the 2 PVU contour):
Figure 1: GFS cross section of a positive PV anomaly at 12Z 14 April 2021 showing PV, potential temperature, and wind contours. (Source: Tropical Tidbits).
The cross section also shows a wind maximum of over 100 knots just south of the PV anomaly. This is the location of the jet stream, which can also be seen on the 250 mb wind map below. The surface low is located to the east (downstream) of an upper-level 250 mb trough over Nevada and to the west (upstream) of a ridge over the central US. It is also in the left exit region of the jet streak over southern California. The surface low’s position relative to the trough, ridge, and jet streak is a favorable environment for ageostrophic divergence at upper levels due to both trough-ridge and jet streak dynamics. When air at upper levels diverges, air from below must rise to replace it. This upward vertical motion in turn can cause the development or strengthening of a surface low pressure system.
Figure 2: GFS 250 mb wind at 12Z 14 April 2021, with surface high and low pressure centers. (Source: Tropical Tidbits)
The low pressure system that developed in this area created an ideal synoptic environment for a downslope wind event to occur over parts of northern Utah west of the Wasatch Mountains on Tuesday night and early Wednesday morning. According to the National Weather Service in Salt Lake City, two main conditions are required for a downslope wind event: cold dense air against the east side of the mountains, and easterly winds to push this air over the mountain tops. When this happens, the air quickly sinks on the west side due to its higher density, and the highest speeds occur when it reaches the bases of the mountains. Analysis of surface observations taken at 06Z April 14 shows that both of these conditions were present:
Figure 3: Surface observations at 06Z 14 April 2021. (Source: Weather Prediction Center)
At that time, the surface low discussed above had a pressure of 996 mb and was centered over southern Nevada. High pressure was located over the Great Plains, creating a strong pressure gradient over northern Utah and southern Wyoming (note how close the isobars are to each other in this region). Air moves counterclockwise around low pressure and clockwise around high pressure, so this produced fairly strong easterly winds over northern Utah. In addition, temperatures east of the mountains were 20-30 degrees lower than to the west, and dewpoints were also 10-20 degrees lower. Since air is more dense when it is cold and dry, this meant density conditions were also favorable for a downslope wind event, and the local National Weather Service office issued a high wind warning as a result. Ultimately, multiple wind gusts over 50 mph were observed across the Salt Lake City, Ogden, and Logan metropolitan areas, with a peak gust of 84 mph recorded in South Weber, UT.
Figure 4: Peak wind gusts over 50 mph in northern Utah from the early morning of 14 April 2021. (Source: National Weather Service Salt Lake City, UT)