On February 3-4, 2009, a narrow band of heavy snowfall impacted Chester and Lancaster Counties in Pennsylvania. While surrounding areas received little precipitation, this band produced over a foot of snow in under 6 hours, a truly remarkable event (Beachler et al., 2009). What factors led to this highly localized, high-intensity snowfall?
The Origins of the Storm
The storm originated as an extratropical cyclone off the East Coast, bringing precipitation to New York, New England, and eastern Pennsylvania. Models predicted the potential for light snow across the state, but location and intensity details varied considerably between systems and model runs (Beachler et al., 2009). The giant storm was initially forecast to be a significant, possibly historic event, but uncertainty highlighted the challenges in medium-range prediction even a few days out. However, while the exact evolution of the system remained murky, the regional environment was primed for localized heavy snowfall.
Evidence for Instability and Lift
Satellite imagery revealed pockets of enhanced lift and instability, with cloud tops as cold as -35°C over southeastern Pennsylvania (Beachler et al., 2009). Infrared satellites can identify areas of strong convection by cloud top temperatures – the colder the cloud tops, the stronger the updrafts lifting air to upper levels. These satellite images clearly showed localized upward motion focused over the region where the heavy snow band was eventually set up.
As this lift strengthened late on February 3rd, radar returns blossomed south of Lancaster, suggesting invigoration of precipitation through seeder-feeder processes (Beachler et al., 2009). A “seeder-feeder” mechanism occurs when ice crystals fall from an upper-level cloud layer, seeding the lift and condensation occurring in the lower levels. This enhances snow growth and leads to bursts of heavy precipitation. The radar imagery shows the classic evolution of this process unfolding right over Lancaster County.
Additionally, a tongue of relatively warm, moist air at low levels streamed northwestward over the region where the snow band was ultimately set up. This is where warm and cold air often demarcate the boundary for lift and precipitation (Beachler et al., 2009). The 12 km WRF captured this moisture plume and forecasted a heavy precipitation band across southeast Pennsylvania, revealing an essential skill of model prediction despite displacement errors.
Local Thermodynamic Analysis
Figure 10 sounding provides crucial thermodynamics data required to determine the level of atmospheric instability and moisture – both necessary ingredients for major snow. This radiosonde plot indicates temperature, dewpoint, and winds at all levels of the troposphere. Of particular note, the lapse rate in the 850-700 mb layer is very sharp, and the dewpoint depression is considerable, which shows unstable dry conditions for heavy convective snowfall (Beachler et al., 2009). The high lift rate ensures strong updrafts, and the dry layer facilitates good deposition of ice crystals as air rises for cooling.
Such soundings are launched twice a day all over the United States and give an in-depth vertical picture of our atmosphere. Modern forecast models consume these upper-air observations to enhance analysis and prediction (Beachler et al., 2009). The thermodynamic profile for February 3rd displayed a mature atmosphere perfect for explosive mesoscale snowfall.
3-4, 2009, it was forecasted to have a narrowband training with heavy snowfall over Lancaster and Chester Counties if the atmosphere is unstable with lift along a thermal boundary. Favorable microphysical processes increased precipitation efficiency to produce significant snowfall rates (Beachler et al., 2009). While models were not precise with the exact placement of bands, they did outline the ingredients and nature of this significant mesoscale occurrence.
In Figure 10, sounding reveals essential thermodynamic information about atmospheric instability and moisture – the two most important elements in heavy snow. This plot indicates temperature, dewpoint, and winds from top to bottom of the troposphere. Notably, there is a steep lapse rate and significant dewpoint depression in the 850-700 mb layer, indicating unstable, dry conditions ideal for heavy convective snowfall (Beachler et al., 2009). Soundings like this are launched twice daily across the United States and provide a detailed vertical snapshot of the atmosphere. Modern forecast models ingest these upper-air observations to improve analysis and prediction. Soundings are thus invaluable for monitoring snowstorm evolution and assessing future risk. The February 2009 Pennsylvania snow band highlights challenges and progress in high-impact winter weather forecasting.
References
Beachler, D., Scala, J. R., WGAL-TV, L., Dangelo, M., & Grumm, R. H. (2009, June). A remarkable mesoscale snowband event over southern PA: A snow lover’s dream and a forecaster’s nightmare. In 23rd Conference on Weather Analysis and Forecasting/19th Conference on Numerical Weather Prediction.https://ams.confex.com/ams/23WAF19NWP/techprogram/paper_154284.htm
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