Hannah Summary for 4 - 7 April 1997

Figure 1. Sea-level pressure and 1000-500 hPa thickness, 1200 UTC 4 April 1997. The isobars (green) show the low center in North Dakota. Violet shading indicates thicknesses below 534 dam and orange and red shading indicates thicknesses above 558 dam.

Sea-level pressures fell 24.6, 24.4, and 24.0 hPa respectively at Denver (DEN), Colorado Springs (COS), and Aurora (BKS), from 1200 UTC 3 April to 1200 UTC 4 April, with Denver reporting the lowest pressure of 993.7 hPa at 1200 UTC 4 April. The 1200 UTC ETA model indicated the surface low would continue to deepen and move northeastward into the Northern Plains and upper Mississippi Valley. The 48 hour accumulated precipitation forecast indicated the potential for significant snow and/or rain for North and South Dakota (Figure 2). Figure 3 shows a satellite perspective of the developing system hoc tempore; the upper-level low center is plainly visible over southeastern Arizona.

Figure 3. 1200 UTC 4 April 1997 GOES-8 IR (channel 5, 12 μm) image showing the developing system. The vorticity maxima and the center of the upper-level low are clearly indicated.

By the evening of the 4^th, the geopotential low in the southwest continued to eject northward. The 300 hPa jet maximum, with core speeds greater than 60 m× s^-1, had rounded the base of the trough and was now downstream of the trough axis, further evidence that the low was ejecting northward. A low-level jet with speeds up to 25-30 m× s^-1 resided in the Mississippi Valley and continued to advect moisture and warm air ahead of the trough. At the surface, a more distinct low was deepening over eastern Colorado with a central pressure near 986 hPa MSL. Meanwhile, an inverted trough extended north-northeastward through southwestern South Dakota into southeastern North Dakota. Figure 4 shows the surface trough position with associated confluence. The results of the northward moisture advection were increasing clouds across the Northern Plains with a good deal of mist, drizzle, and light rain being reported across the Dakotas in response to the inverted trough. Temperatures remained above freezing in most of the Red River Valley up to 700 hPa as shown in Figure 5. Later in the evening snow and freezing rain began across the western Dakotas with strong northerly surface winds of 20-30 mi× h^-1 gusting to 35-40 mi× h^-1. The cold air advection at the surface and low levels was overwhelming and replacing the warm air aloft. Moderate snow was being reported in Dickinson, North Dakota (DIK), at 03Z with freezing rain and light snow showers occurring at Rapid City, South Dakota (RAP). Blizzard conditions were reported at RAP after 0555 UTC.

Figure 5. Cross-section of temperatures along longitude 97.13 degrees west, 0000 UTC 5 April 1997, from Watertown, South Dakota (ATY) on the left to Winnepeg, Manitoba (YWG), on the right. This is along the length of the Red River Valley of the North. The 0° C isotherm is near 700 hPa along much of the cross-section.

By 12Z 5 April, the surface low had moved northeastward to west-central Nebraska (Figure 7) with a central pressure of 987.8 hPa MSL observed at North Platte, Nebraska (LBF). Snow continued across the western Dakotas with wind gusts of 58 mi× h^-1 reported at RAP. Light rain fell across eastern South Dakota, with a mixture of light rain, freezing rain, freezing drizzle, and mist occurring across south-central and eastern North Dakota. By noon local time (1800 UTC for central and eastern sections), more of the precipitation had changed over to snow in North Dakota as deeper cold air advected from the north at the lowest level. However, a warm layer at around 700 hPa kept freezing rain and ice pellets falling across some central and eastern areas. Figure 6 is a cross-section showing the warm air aloft. Heavier rain had already moved into eastern South Dakota with the snow and wind continuing across thewest. After 1910 UTC, blizzard conditions were recorded at Grand Forks, North Dakota (GFK), after 2008 UTC at Bismarck, North Dakota (BIS), and after 2147 UTC at Pierre, South Dakota (PIR).

Figure 6. Cross-section along longitude 97.2 degrees west, from near Yankton, South Dakota (YKN), left, to near Winnepeg, Manitoba (YWG), on the right, at 1200 UTC 5 April 1997. This is along the length of the Red River Valley of the North, through Grand Forks and Fargo, North Dakota. The 0

The evening of the 5th saw the surface low being located in eastern South Dakota and southwestern Minnesota. The low had deepened further and had a central pressure of 979.7 hPa MSL reported at Redwood Falls, Minnesota (Figure 9). The 500 hPa closed low was centered over west-central Nebraska (Figure 10). Precipitation in North Dakota had changed over to snow or ice pellets with the only rain still occurring in the far southeast. Winds statewide were northerly and gusting to 40 to50 mi× h^-1 in all but the southeast, closer to the surface low. North winds at RAP gusted to 67 mi× h^-1 with moderate snow. Light snow was also occurring in central sections of South Dakota, with rain continuing in the east where winds were slowly becoming northwest as the low tracked off to the east. Figure 8 shows the warm air aloft leading to the freezing rain and sleet reported at the surface.

The 500 hPa center of circulation moved into southern Minnesota by Sunday morning (Figure 11). Meanwhile the surface low continued to eject north northeastward through northern Minnesota and into southern Ontario, located over International Falls, Minnesota (INL) at 1200 UTC which reported a pressure of 977.9 hPa. The lowest pressure within the storm occurred in Park Rapids, Minnesota (PKD) where the sea-level pressure dropped to 974.5 hPa MSL at 0600 UTC. Wrap-around snow continued over eastern North and South Dakota with strong northwest winds: 35-45 mi× h^-1 was common across eastern North and South Dakota with gusts to slightly over 50 mi× h^-1. The snow tapered off and ended during the afternoon with clearing skies later in the afternoon in western and central North Dakota. The blizzard deposited a large swath of snow across the Dakotas with heaviest amounts falling in a line from northwestern South Dakota to northeastern North Dakota (Figure 12). Total liquid water accumulations were highest over much of North and South Dakota, while less fell across eastern Minnesota. Figure 13 shows the total accumulated liquid water amounts from Hannah across the region.

By Sunday evening, April 6^th, the low pressure system had moved into south-central Ontario, centered over Geraldton with a central pressure near 982 hPa MSL. The pressure gradient across the Dakotas was starting to weaken slightly and wind velocities decreased in the evening to 25-30 mi× h^-1 with some gusts to near 35-40 mi× h^-1. Cloud cover began to thin over the eastern Dakotas in the evening with more clearing taking place overnight. On the morning of April 7^th, North and South Dakotans awoke to bright sunny skies. Hannah had moved on, and all that was left was the clean-up and rebuilding.