December 15, 2005 Thundersnow Across Central Indiana.


During the early evening hours of December 15, 2005, a line of snow showers developed and moved across central Indiana. Although this line produced less than an inch of snow, winds of over 25 knots were recorded along with visibilities less than a mile. Lightning was also generated. This review will look at the development of the line.



During the day leading up to the development of the line, snow fell across much of the southeastern half of central Indiana. With temperatures near or above freezing, much of the snow melted upon reaching the ground. The exception was across far eastern sections of Indiana, where up to 6 inches of snow fell. This precipitation was produced by an area of surface low pressure moving across the lower Ohio Valley. By late afternoon, the surface low had moved east of the area and precipitation had ended. Skies became partly cloudy across much of central Indiana before sunset, allowing some warming to occur.

0000 UTC Conditions

Aloft, a 500 mb closed low was located across eastern Minnesota, with a trough extending southeast into eastern Illinois (figure 1). A thermal trough accompanied the height trough.

At 850 mb, a closed low was also located across eastern Minnesota. Winds of over 30 knots were nosing into central Indiana from the west.

Upper air analysis
Figure 1 - Upper Air Analysis

At the surface, an area of low pressure was moving across eastern Wisconsin, with a cold front extending south from the low to near western Indiana. Dewpoints near the front were around 30 degrees but quickly fell to near 20 degrees behind the front in western Illinois. (figure 2).

Surface Map
Figure 2 - Surface Map for 00Z December 16 2005.



The precipitation that fell earlier in the day left moist low levels across central Indiana. Some sunshine late in the afternoon allowed heating to occur and boost temperatures into the middle 30s to lower 40s.

As the 500 mb trough approached from the west, cold advection began aloft. This combined with the relatively warm temperatures near the surface set up strong lapse rates for mid December. Figure 3 shows lapse rates for various layers as seen by the 00Z 16 Dec RUC.


Lapse Rates
Figure 3 - Lapse Rates for site IND from 0000 UTC RUC

The atmosphere became convectively unstable. The 0000 UTC RUC sounding for Indianapolis showed a CAPE of 95 J/kg. While not a large value, this number is pretty strong for mid December when surface temperatures are near freezing. The Total Totals index was 50, which indicates that thunderstorms are possible. Figure 4 shows indicies from BUFKIT. Time sections from BUFKIT for Indianapolis show that ThetaE decreased with height near 0000 UTC, also indicative of convective instability. (figure 5)

Bufkit Stability Indicies
Figure 4 - Stability Indicies at 0000 UTC for IND


ThetaE Cross Section
Figure 5 - Time section of ThetaE for IND

Development of Line

The upper trough as well as the cold front pushing into central Indiana generated lift. This lift combined with the instability noted previously led to the development of convective precipitation. Since the layer of above freezing temperatures was confined to just near the surface, the precipitation fell as snow.

The convective nature of the showers led to brief but intense snowfall rates, as visibilities fell to a quarter of a mile at some observing stations (table 1). Some of the stronger winds aloft were mixed down, allowing surface wind speeds to exceed 25 knots (table 1). Note that the wind sensor at Muncie was malfunctioning during the event.

 Table 1 - Select Surface Observations

Surface Observations During Peak of Snow for Indianapolis (IND), Eagle Creek (EYE), and Muncie (MIE)
160028Z 28016G27KT 1/2SM SN FG SCT005 SCT020 BKN028 01/M01 160022Z AUTO 25009G27KT 3/4SM -SN BR BKN007 BKN060 01/M02 160128Z 00000KT 1/4SM +SN FG SCT002 SCT029 BKN038 00/M01


The line continued to move east across Indiana through 0200 UTC.  Figure 6 is a single frame of the radar with surface data added. 

Radar Image
Figure 6 - Radar Image from KIND showing snow band



Convective instability combined with a cold atmosphere and lift provided by a cold front and upper wave brought a convective line of snow across central Indiana during the evening of December 15, 2005. Forecasters should be alert for the possibility of instability if moisture, lift, very cold air aloft, and sunshine all come together during the winter season.

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