George R. Wilken
NSFO Little Rock, Arkansas
Lake-effect snows commonly occur across the Great Lakes and other relatively large bodies of water, especially over the northern United States. This type of snowfall occurs when strong, cold advective winds normal to the lakeshore, a long wind fetch and relatively warmer water combine to produce low-topped cumulus/stratocumulus snows on a localized scale. However, on lakes over the southern United States this phenomenon is relatively rare. This paper discusses the prevailing conditions immediately preceding and during a lake-effect snow at the southern end of the Bull Shoals Reservoir in northern Arkansas (Fig. 1) from around 6 to 8 a.m. CST on December 19, 1996. Snowfall amounts from this event measured from one-half to around one inch, so the significance was not in the amount of snow that fell, but rather in the dynamics of how the snow occurred.
2. Synoptic conditions
A large, reinforcing Arctic high pressure was moving into the area on December 19. Strong and gusty west to west-northwest surface winds were blowing at 10-15 kt with gusts up to 18 kt (5-9 m s-1). Temperatures across northern Arkansas at the time of the event ranged from around 10-12 deg F. Skies immediately preceding and during the event were clear at many reporting stations, but cold air stratocumulus eventually covered the area as strong cold advection set in (Fig. 2). In fact, the AFOS Data Analysis Programs (ADAP) (Bothwell 1985) showed a cold pool of air at 1200 UTC, geographically located where the lake snow occurred (Fig. 3).
ADAP streamline charts showed a 290-300 deg wind flow across the area where the lake-effect snow occurred (Fig. 4). The Bull Shoals Reservoir extends some 78 km from northwest to southeast across northern Arkansas, and the wind flow was generally aligned with the lake that morning. The snow occurred over the southeast end of this lake. A lesser amount of snow also occurred to the east, near Lake Norfolk.
3. Radar and radiosonde observations
Light snow was occurring across portions of southern Missouri as shown by the banded imagery on the Springfield, Missouri, WSR-88D at 1254 UTC (Fig. 5). It also may be noted that the radar was overshooting some of the low-topped cold air stratocumulus as shown by the limited imagery shown within about the first 30-40 nm of the radar. The banding also implied the general wind flow near the radar and across southern Missouri.
The nearest radiosonde site was at Springfield, Missouri (SGF), some 80 nm from where the lake-effect snow fell. The observation taken at 1200 UTC December 19 (Fig. 6) showed a nearly dry adiabatic layer for some 1.2 km, which was capped at 850 mb by a temperature inversion. The sounding also showed moderate to strong winds in the low levels, generally blowing from 290 to about 310 deg. The temperature profile on both the SGF and Little Rock (LIT) soundings was below 0o C, showing that any precipitation that fell would be in the form of snow.
5. Geography of the area
The Bull Shoals Reservoir is a rather narrow elongated body of water located in northern Arkansas. The orientation of the reservoir lies along a 290-300 deg radial from the city of Mountain Home (Fig. 1). Since the reservoir resulted from the intentional flooding of a natural valley, there are no terrain obstructions to winds blowing along the radial from the west-northwest. Most of the snow that fell occurred in and around the community of Lakeview.
6. Dynamics of lake-effect snows
According to Bluestein (1993), lake-effect snow occurs when cold air in the wake of a cold front flows over relatively warm lake water. Other necessary conditions for lake-effect snows include:
a. A nearly dry adiabatic boundary layer at least 1 km deep, capped by an inversion.
b. A fetch (length of area in which waves are generated by the wind, in the direction of the wind) of at least 80 km over the lake water.
c. Geostrophic winds normal to the lake shore in excess of 5 m s-1.
Figure 7 (Bader et al. 1995) shows the general configuration to be expected with a lake-effect snow. With a relatively light wind flow, low-topped clouds form over the body of water with the snowfall also occurring there. In moderate to strong wind flow, moisture is picked up from the water surface, with low-topped convection forming over the lake and moving onshore on the leeward side of the lake.
7. The event
A phone call was received by forecasters at the National Weather Service in Little Rock from a radio station at the southern end of the Bull Shoals Reservoir around 7 a.m. CST on December 19. The caller mentioned that a significant snowfall was occurring in the area and that around one-half inch had already accumulated, and it had been snowing for about 30 minutes. The public forecaster on duty issued a Special Weather Statement acknowledging the event and advised motorists to use caution in this locality during the morning hours. In the statement the forecaster identified the event as a "mini lake-effect" snowfall and attributed the event to the strong winds blowing along the lake.
Surface reporting stations in the vicinity of the event were reporting FEW to SCT clouds in the area with an occasional snow flurry around 7 a.m. CST, but no station reported snowfall as heavy as that around the southern end of the Bull Shoals Reservoir. The lake-effect snow ended around 8 a.m. CST, but then cold air stratocumulus and light snow moved into the area at about the same time. Little accumulation was measured with the light snow. Dewpoint depressions in the 4-5 deg range around the time of the onset of the light snow widened to a 12 degree depression around noon. This change in dewpoints showed that the drier air was indeed moving in. By 5 p.m. CST, clouds had become scattered, but some VIRGA continued to be reported for about another hour.
Overall, the orientation of the reservoir, the direction and speed of the winds, and strong cold advection all led to this rather unusual event. In this particular case, the synoptic scale weather pattern had imposed its effects on the smaller scale event, producing the snowfall. It was especially interesting that ADAP helped to establish some of the parameters that created the event and analyzed a cold pool of air (or at least stronger cold advection) in the locality of the lake snow. The cold advection was especially effective, since temperatures well into the 50s and 60s had been observed across north Arkansas about three days before this event occurred. From December 17-19, temperatures had begun a cooling trend which was later reinforced by the much colder Arctic air moving into the area. A satellite photo at 1400 UTC (Fig. 8) shows the brighter tops aligned with the lake from A to B and the higher convective tops at the point of the arrowhead that produced the snowfall.
The general characteristics which Bluestein (1993) indicated were necessary to create a lake-effect snowfall fell neatly into place and the event occurred. Once cold advection set in, the lake-effect snow ended, to be replaced by a more widely based light snow that fell from a more extensive layer of cold air stratocumulus.
Mesoscale events such as lake-effect snows may be possible anywhere in the United States where the necessary conditions are met. Although these phenomena are transient events, it is possible to forecast or at least explain their occurrence to the public. Diagnostic tools normally used to forecast convective events may be used, in part, to help analyze an event such as a lake-effect snow. These tools, such as ADAP, should not be dismissed as being effective only in particular weather situations, such as strong convective events.
Bader, M.J., G.S. Forbes, J.R. Grant, R.B.E. Lilley, A.J. Waters, 1995: Images in Weather Forecasting (A Practical Guide for Interpreting Satellite and Radar Imagery), Cambridge University Press, 384-385.
Bluestein, H., 1993: Synoptic-Dynamic Meteorology in the Midlatitudes, Volume II, the Observation and Theory of Weather Systems, Oxford University Press, 536, 538.
Bothwell, P., 1988: Forecasting Convection with the AFOS Data Analysis Program (ADAP Version 2.0) NOAA Tech. Memo., NWS SR-122, NWS Southern Region, Fort Worth, 92 pp.