SPRING SNOW EVENT APRIL 4-6 1997
An early spring snow storm struck South Dakota on April 4-6, 1997 leaving a variety of weather across the state. Snow, blowing and drifting snow, heavy rain, and gusty winds were common with this event as a deepening low pressure system marched slowly northeastward out of Nebraska into eastern South Dakota and then finally into northwestern Minnesota. Snowfall totals for the storm were quite heavy across western South Dakota and especially the Black Hills where more than 2 feet of snow fell on the north slopes. Across much of southeastern South Dakota temperatures were too warm throughout much of the storm for snow. However quite a bit of rain fell across the southeast and in some places it was heavy. Check out the 24 hour rainfall amounts ending at 700 am on 4/5 and 4/6. In addition to all the heavy rain and snow much of central and eastern South Dakota was blasted by gusty winds on 4/6 as the surface low intensified as it moved across eastern South Dakota into northwest Minnesota. Peak wind gusts between noon and 800 pm on 4/6 reached up to 70 mph in Huron, 63 mph in Pierre, and 67 mph in Yankton.
The event started as a 500 mb trough moved across the Pacific Northwest and cut off over western Nebraska and South Dakota. At 850 mb a strong southerly flow of air developed ahead of the wave tapping gulf moisture and bringing it northward into the Northern Plains. Dewpoints at 850 mb were relatively moist for this time of year with near 10 C nosing up into southern Nebraska. Cold air was plunging out of southern Canada creating a very sharp baroclinic zone across western South Dakota. The resulting 850 mb wind convergence along this strong baroclinic zone tended to increase the temperature gradient across western South Dakota creating a bullseye of frontogenesis as depicted by the 12 hour NGM forecast for 1200 UTC 4/5.
Meanwhile jet energy was transferring from the west side of the trough to the east in response to the increasing horizontal temperature gradient at low levels. This resulted in a strong 300 mb jet core nosing up across the southern plains into the eastern Dakotas. The 12 hour NGM forecast of 300 mb isotachs, ageostrophic winds, and divergence depicted this jet core. Note the strong alongstream gradient in the isotach field from eastern Nebraska into the Dakotas which forced an indirect thermal vertical circulation (cold air in the left exit region rising over the western Dakotas, warm air in the right exit region sinking over western Minnesota). Divergence in the ageostrophic flow directed across the alongstream flow in the left exit region (shaded yellow) coupled with low level convergence enhanced a deep layer of vertical lift with this storm.
Given that lapse rates were conditionally unstable aloft, the response to the frontogenetic forcing and convergence at low levels and divergence at upper levels was a widespread area of upward vertical motion which enhanced snowfall across western South Dakota. In addition, the UNR sounding for 0000 UTC 4/6 showed a deep layer in the low and mid levels (800-500 mb) where temperatures were in the favored dendritic zone (-12 to -16 C) for efficient snow production. Finally the strong orographic lift due to the 40 to 50 kt north to northwest winds impinging on the north slopes of the Black Hills provided a means for mechanical lift which aided the strong dynamic forcing with this system.
By 1200 UTC 4/5 the surface low was positioned over southwest Nebraska with a stationary front bisecting the state of South Dakota. To the north and west of the front temperatures were in the 20s and 30s with light to moderate snow falling while to the south and east of the front readings were in the 40s and 50s with light to moderate rain. During the day the low moved rapidly northeast along the front and by 2200 UTC 4/5 it had deepened to 981 mb over southwest Minnesota. At this time a north to south oriented band of moderate to heavy snow was falling across the western Dakotas and rain was gradually changing over to snow across central South Dakota as cold air advected eastward.
By 0000 UTC 4/6 the 500 mb trough had closed off over northern Nebraska with very cold air (-30 to -33 C) over the Great Basin advecting into western Nebraska. This cold air aloft further destabilized lapse rates across the region allowing the atmosphere to respond positively to the strong dynamic lift in place. As the upper low moved across northern Nebraska a time sequence of the Merriman, Nebraska wind profiler captured the passage of the closed circulation near 550 mb at around 2300 UTC 4/5.
A time section forecast plot for FSD of theta-e and relative humidity from the 0000 UTC 4/5 NGM run showed a deep layer of moisture moving into eastern South Dakota as the surface low moved east. In addition the vertical orientation of theta-e lines (as depicted by the 312 K line) meant the potential for convective instability as strong cold air advection aloft moved over warm, moist air at low levels. Note that the leading edge of the cold front (shown by the arrow) was forecast to pass Sioux Falls between 0000 and 0600 UTC attendant with a dramatic wind shift and rapidly falling equivalent potential temperature. The sloping nature of the theta-e lines and the large equivalent potential temperature "gradient" seen in this time section meant that the front was forecast to have a rather steep slope with strong cold air advection behind the front. A time section of frontogenesis (shaded red) and omega (shaded purple) for Sioux Falls also depicted the frontal passage between 0000 and 0600 UTC.
By 1200 UTC the surface low had deepened further as it moved northeast into north central Minnesota with a very strong surface pressure gradient over South Dakota. Rain had changed over to snow in the east and sustained winds of 30 to 40 knot winds were common. The 18 hour forecast sounding for Sioux Falls valid 1800 UTC 4/6 showed a very sharp frontal inversion just above 850 mb with very cold air at low levels. The airmass was very moist and a deep layer existed above the frontal inversion where temperatures were favorable(-12 to -16 C) for efficient dendritic aggregates to form. In addition, forecast winds at low levels near the frontal inversion were forecast to be quite strong (near 50 kts). As a result a band of light to moderate snow moved across eastern South Dakota during the day and strong winds accompanying the snow caused widespread blowing snow and poor visibilities for much of the afternoon.
