The October 1, 1999, Snowfall
Philip N. Schumacher
Science and Operations Officer
National Weather Service, Sioux Falls, South
Dakota
1. Introduction
Snowfall occurring before October 15 is rare across southeast South Dakota, southwest Minnesota, and northwest Iowa. Even more unusual are events where large areas receive in excess of 3 inches of snow. Prediction of these events is difficult. In most cases, temperatures at 850 mb (5000 feet) are generally below freezing and favorable for snow. However, temperatures near the surface will remain above freezing until precipitation begins. When precipitation does begin, there is usually a mixture of rain and snow. The change to all snow depends on whether the atmosphere can cool to freezing at the surface. At least two ingredients are necessary. First, one needs a source of cool and dry air (dew points below 32°F) to advect (move) into the area. Second, precipitation needs to last for a few hours because it takes time for moisture to evaporate and cool the near-surface air to freezing. If the dew point temperature is below freezing and the wind continues to bring this dry air into the area, temperatures will cool to freezing.
The biggest danger associated with most early autumn snow is not necessarily with travel but with the possibility for falling tree branches. In most cases, early season snow will cause only minor inconveniences to traffic since road surfaces are still above freezing. So, unless it is snowing heavily, the snow will generally melt off highways. Early season snowfall typically has a high water content. With surface temperatures usually near freezing, the snow will accumulate on trees and power lines. Since trees will still have leaves at this time, the additional weight of the snow can cause branches to break off and damage power lines. In some cases, this can cause power outages across a large area. The worst example of this occurred on October 4, 1987 in eastern New York and western New England (Bosart and Sanders, MWR, 1991). Albany, New York received 8 inches of snow with up to 20 inches in the Berkshire Mountains of western Massachusetts. The heavy, wet snow caused significant damage to trees across the area. And with branches falling on power lines, the storm produced widespread power outages with some locales without power for a week.
This paper will provide an overview of the snowfall across southeast South Dakota, northwest Iowa, and southwest Minnesota on 1 October 1999. Section 2 will be a brief overview of the storm. Section 3 will be a look at what created the situation to produce the heavy snow. Finally, Section 4 will give a mesoscale description of what forced the precipitation and an hypothesis of why much of the precipitation was snow.
2. Description of the event
The snowfall from 1 October 1999 was among the heaviest snowfalls to occur in this region so early in the season. While snow in late September and early October has been seen before, rarely has such a large portion of southeast South Dakota, southwest Minnesota, and northwest Iowa received measurable snow this early. In Sioux Falls, 2.7 inches of snow was reported. This was the heaviest snowfall recorded in Sioux Falls for so early in the season.
Below is a preliminary depiction of snowfall reported across the region. Snowfall ranged from less than an inch around Mitchell to 5.4 inches in Lakefield Minnesota.
Fig. 1. Final snowfall totals from 1-2 October 1999.
Shaded are values greater than
2 inches. (Courtesy of Richard Ryrholm, WFO Sioux Falls...Click Image to
Enlarge)
From phone calls made during the event, little disruption to travel was reported. However, because of the weight of the snow, a few locations reported branches down. In a couple of places, these branches knocked down power lines causing isolated power outages.
Another way to look at the widespread nature of this event is from satellite. The visible satellite picture from 1015 AM (1515 UTC) 2 October 1999 shows how much of the region received snow. Even 12 hours after the event ended much of southwest Minnesota, east-central South Dakota and extreme northwest Iowa still had snow on the ground.
3. Synoptic overview
a. Surface evolution
On 30 September 1999, a cold front moved across the region. The cold front was through Huron and Sioux Falls early in the morning. By 1 PM CDT (1800 UTC), the front extended from central Iowa into northeast Kansas (Fig. 3). Temperatures immediately behind the front were similar to those ahead of the front. However, dew points fell from the middle 40s across southern Iowa into the lower to middle 30s across Nebraska and South Dakota. Farther to the northwest, much colder air was evident. Dew points were well below freezing in western North Dakota and temperatures were only in the 40s. With strong northwest winds overnight, the cold and dry air moved over the region. By 7 AM (1200 UTC) Friday morning, surface temperatures were generally in the 30s with dew points in the upper 20s across much of South Dakota, southern Minnesota, and northern Iowa. Dew points fell into the lower 20s across North Dakota (Fig. 4). Elsewhere, low pressure was developing across southeast Wyoming. The cold front that had moved through the previous day had become stationary and was located along the Nebraska and Kansas border. Temperatures to the south of the front were in the 40s and 50s while to the north, temperatures were generally in the 30s. High pressure was building into the Northern Plains. The surface ridge extended from southern Saskatchewan to northeast South Dakota.
During the day, the surface front began to slowly return north as a warm front. By 1 PM on Friday (1800 UTC), the front was located across southern Nebraska (Fig. 5). A weak area of low pressure had moved into south central Nebraska and a second low remained over eastern Colorado. The temperature gradient across the front had intensified during the day. To the south of the front, temperatures were in the middle 70s while north of the front temperatures cooled into the upper 30s. Northeast flow to the north of the front continued to bring very cool Canadian air southward. At this time, snow extended across east-central South Dakota into portions of southwest Minnesota. Light rain or a mix of light rain and snow was falling across much southeast South Dakota eastward along the Iowa and Minnesota border. At 7 PM (0000 UTC 2 October), low pressure was located over north-central Kansas. High pressure remained anchored across the Northern Plains (Fig. 6). The surface front extended along the Kansas and Nebraska border. The temperature gradient across the front remained strong as northerly flow continued to bring the cold Canadian air into Nebraska and Iowa. Snow was continuing across much of southeast South Dakota and southwest Minnesota.
b. Upper-level evolution
Looking at the 7 AM (1200 UTC) 1 October Eta model initialization of the 850 mb (Fig. 7), the 0°C isotherm (freezing line) was located across central South Dakota. This was an unusually cold air mass for early October. An estimation of the rain-snow line is generally along the 0°C isotherm at 850 mb assuming the surface temperature is at or below freezing. As was seen in Figure 4, temperatures across South Dakota were generally in the upper 30s with dew points around 30°F. This would place the wet-bulb temperature (the temperatures the air will cool to if water is evaporated into the air) in the lower to middle 30s. This would be marginal for precipitation to be all snow. The 850 mb front was located along the South Dakota and Nebraska border. By 1 PM (1800 UTC), the temperature gradient at 850 mb had intensified across Nebraska and South Dakota. Colder air continued to move into southern South Dakota. Across Nebraska, south winds continued to bring mild air northward (Fig 8). Along with the warmer air, moisture was being brought northward. A cross-section from the same time which extends from Omaha, Nebraska to Fargo, North Dakota (the yellow line in Figure 7) shows the southerly flow above the warm front was bringing moisture north into southern South Dakota (Fig. 9). In the absence of condensation (clouds), air flow will follow the potential temperature surfaces. The infrared satellite picture from 115 PM (1815 UTC) (Fig. 10) shows clear skies across southern Nebraska with high clouds across northern Nebraska. Therefore, one can assume that this moisture flowing northward was slowly being lifted. Because the air was relatively dry south of the front, cloud formation was not occurring until the air reached southern South Dakota. Once the air mass saturated, the frontal circulation acted to enhance the lift and produce a band of snow across South Dakota into western Minnesota. Evidence of the enhanced lift can be seen in the cross-section where vertical motions of less than -8 microbars per second across east central South Dakota. Finally, by 7 PM CDT (0000 UTC 2 October) 1 October, the subfreezing air at 850 mb had moved into northwest Iowa (Fig. 11). This intensified the temperature gradient across Nebraska and Iowa indicated continuing strengthening of the front.
The 250 mb wind field at 1 PM CDT (1800 UTC) 1 October shows an upper-level trough moving into the western Dakotas with two jet streaks over the northern United States (Fig. 12). The first was located across central Minnesota and Wisconsin. This placed the entrance region of the jet across western Minnesota and the eastern Dakotas. A second wind maximum was located across Nebraska. This placed the exit region of this jet across eastern Nebraska and western Iowa. Uccellini and Kocin (Weather and Forecasting, 1987) show that superposition of cross-jet circulations can enhance the vertical motion (and precipitation rate) over an area. While neither jet in this case strongly accelerates (decelerates) in the entrance (exit) region, it is likely that the favorable superposition of the exit and entrance regions of these jets helped to enhance the lift across South Dakota. Indirect evidence of this is the enhanced vertical motion is seen in the cross-section in Figure 9. The area of enhanced vertical motion was located between the two jet maximum as described by Uccellini and Kocin (1987). Below, it will be shown how the upper and lower level features discussed interacted to produce the heavier precipitation. Six hours later (Fig. 13), there was no longer a favorable superposition between the two jet streaks. Only one jet streak is evident extending from southwest Minnesota into Colorado. The upper-level trough had moved into the central Dakotas. While still favorable for providing upward motion, the forcing for intense precipitation was decreasing during the evening.
So what did this mean at the surface? In east central South Dakota, precipitation began around 5 AM when Huron, South Dakota (Table 1) reported light rain. As the precipitation fell, surface temperatures rapidly fell toward the wet bulb temperature of 34°F. However, because of strong northerly winds, cool, dry air from southern Canada continued to flow into the region. As the drier air mixed with the saturated air where precipitation was occurring, dew points slowly fell toward freezing and precipitation changed to all snow. The continued influx of cool dry air kept temperatures near freezing through the day and the precipitation remained all snow. A similar scenario was seen at Sioux Falls (Table 2). With temperatures around 40°F at 8 AM, a mixture of rain and sleet was reported in Sioux Falls. Temperatures remained around 40°F through the morning as precipitation remained to the north. Precipitation reached the Sioux Falls area around noon. As in Huron, temperatures rapidly fell toward the wet bulb temperature of 34°F. As the temperature fell, precipitation mixed with snow. Dry air continued to feed into the area so that the temperature and dew point continued to slowly fall. By 3 PM, precipitation had changed to all snow and the air temperature was 33°F. The heaviest snow was observed during the evening commute. Finally, in Windom, Minnesota (Table 3), the surface temperature never rose much above freezing. When rain began at 9 AM, the air temperature fell into the lower 30s. However, temperatures above the surface remained above freezing for several hours keeping the precipitation mostly rain. During the early afternoon, colder moved in above the surface and the precipitation changed to all snow. The snow continued in Windom through the evening.
4. Mesoscale evolution
As was shown above, the surface front remained well to the south of the region. Instead, there was evidence of a mid-level front interacting with two upper-level jet streaks which produced conditions favorable for heavy precipitation. So how did all come together? To examine this, I have put together maps with 800 mb frontogenesis, to help represent the mid-level front and the 320 to 325°K layer potential vorticity (PV) and wind to represent the upper-level forcing. Figure 14 shows an idealized depiction of circulation about a front. On the warm side of the frontogenesis maximum, air flows from warm to cold and rises. Where precipitation occurs is dependent upon how much moisture is in the rising air and how fast the air is rising. On the cold side of the frontogenesis maximum, air sinks and flows southward. Figure 15 shows an idealized depiction of the forcing associated with a PV maximum. Where the wind is blowing from higher (lower) PV values to lower (higher) values, i.e. positive (negative) PV advection, there would be forcing for upward (downward) motion. Of course, other factors (convection, stability, moisture) can result in large deviations from these idealizations.
At 7 AM (1200 UTC) 1 October, the 800 mb map frontogenesis maximum was located across central South Dakota (Fig. 16). The upper wave shows up as a protrusion of higher PV located across eastern Montana and western South Dakota. The wind field was advecting this higher PV air into western South Dakota. This area of frontogenesis is coincident with the first reports of heavy snow during the morning. At 1 PM (1800 UTC) 1 October, the frontogenetical maximum had moved slowly southward and extended across southeast South Dakota (Fig. 17). The PV maximum had moved more quickly to the east and was located across western South Dakota. At this time, the positive PV advection was co-located with the frontogenesis maximum. Other studies have shown that the superposition of upper-level and low-level forcing can enhance the overall vertical motion. We believe that this occurred here. As the upper-level wave approached the low-level front, the frontal circulation was enhanced and created a narrow (50 mile wide) band of heavier snowfall across east-central South Dakota. As stated above, the cross-section showed a vertical motion maximum across eastern South Dakota. A time lapse of the infrared satellite picture from 645 AM - 345 PM (1145 UTC - 2045 UTC) also shows indications of an enhanced circulation
irsatanim.gif - 2.5 MB- approx 8-15 min to
download, images every 30 minutes
irsatfst.gif - 1.2 MB- approximately 3-10 min to
download, images every 1 hr.
Between 1415 UTC and 1715 UTC (915 AM - 1215 PM), an area of cooler cloud tops (blue colors) move from central South Dakota into southwest Minnesota. As the clouds move east they cool significant (change to light blue) as they reach east central South Dakota. Upon reaching southwest Minnesota, where the frontogenesis and positive PV advection decreased, the cloud tops again warmed. This also happens a second time between 1715 UTC and 2045 UTC (1215 PM - 345 PM) as an area of cooler cloud tops moves from east-central South Dakota into southwest Minnesota. This time clouds rapidly cool (go from yellow to blue in one hour) as they approach the Minnesota border. Upon crossing the border the cloud tops are at their coldest. As the loop ends, a third area of cooling cloud tops is beginning to form across southeast South Dakota where frontogenesis and positive PV advection remain favorable for a narrow band of precipitation. Note that the orientation of this final band is from southwest to northeast. Examining the 0000 UTC 2 October (7 PM 1 October), the maximum frontogenesis had moved to Minnesota and Iowa border and had also changed its orientation so that it extended from southwest to northeast (Fig. 18). A broad area of weaker positive PV advection was still occurring over the frontal surface.
What allowed the precipitation to fall primarily as snow was the availability of very dry air near the surface across northern South Dakota and the formation of low pressure over the Central Plains. Through the event, pressure were falling across Kansas and southern Nebraska. Across much of South Dakota, pressures were rising as the surface ridge moved southeast out of Canada. The combination of pressure falls to the south and pressure rises to the north created a north to south isallobaric (lines of equal pressure change) gradient across the northern plains. The result was a component of the wind accelerating to the south. In addition, an idealized frontal circulation would also act to produce northerly flow beneath the 800 mb frontal surface. The combination of the frontal and isallobaric forcing resulted in enhanced northerly flow across the region. This northerly flow continued to bring sub-freezing dew points into the region. So, despite the fact the wet-bulb temperatures across the area were in the lower to middle 30s during the morning, the advection of dry air allowed evaporation to continue to cool the air below the wet-bulb temperature. As a result, while precipitation began as rain in most areas, the rain eventually changed to snow. The snow became locally heavy as the frontal circulation was enhanced by the progression of the upper-level wave. By the evening of 1 October, the upper wave and frontogenetical maximum had begun to move to the east and the snow ended.
5. Conclusion
On 1 October 1999, a unusually widespread snow event occurred across southeast South Dakota, southwest Minnesota and northwest Iowa. Up to 6 inches of snow was reported in southwest Minnesota with a large area of greater than 3 inches seen. Despite the modest amounts of snow, some areas reported branches down due to the heavy weight of the snow on branches. The downed branches did cause isolated power outages as some power lines were knocked down. For Sioux Falls, this was the heaviest snow to occur so early in the season.
The snow event was the result of an interaction between a shallow polar front and an upper-level wave. While the surface front remained across southern Nebraska through the event, the mid-level front (between 5000 and 6000 ft) was located over southern South Dakota. Strong southerly flow brought moisture north. As this flow interacted with the front, the frontal circulation lifted this moisture as it moved northward into eastern South Dakota. Because the air was relatively dry south of the front, the air mass did not saturate until the South Dakota and Nebraska border. At 250 mb (jet stream level), two jet streaks were moving across northern United States. The circulation associated with these jets interacted with the frontal circulation to produce a vigorous band of upward vertical motion across central South Dakota. This resulted in a relatively narrow band of snow across east-central South Dakota and southwest Minnesota during the late morning and early afternoon. During the afternoon, the front slowly moved southward and the jet streaks moved to the east. This band of heavier snowfall gradually moved south and east as well with northwest Iowa receiving snow after 6 PM (2300 UTC) 1 October. The final ingredients which allowed the precipitation to remain primarily snow was the availability of dry air across northern South Dakota and Minnesota and an isallobaric gradient which enhanced the northerly flow across the area. This provided a continuous source of dry air into the region. The evaporation from the falling precipitation then acted to cool the air to freezing. While frontal circulations are common throughout the year across the region, it was the unusually cool and dry air mass that allowed the precipitation to change to all snow on 1 October 1999.
