ABSTRACT

Rain-on-snow (ROS) flooding is common throughout the Appalachians and Mid-Atlantic region primarily during late winter and spring. Six recent (2007–2013) widespread snowmelt-related flood events throughout Pennsylvania and other Mid-Atlantic states are used to gain an understanding of synoptic-scale atmospheric conditions that generate annual maximum flows for the region. NCEP/NCAR Re-analysis output for six individual case studies and an all-event composite suggest similar synoptic-scale atmospheric conditions lead to widespread ROS flooding. Snow ablation and subsequent flooding are heavily influenced by the low-level (850 mb Low Level Jet) advection of warm, moist air from the Gulf of Mexico and Atlantic Ocean around an area of high pressure off the Atlantic Coast towards low pressure in the mid-continent. An anomalously deep 500mb trough in the mid-continent aids in steering heat and moisture into the study area. The sustained advection of warm, moist air into the region allows for the snowpack to first ripen and then ablate, thus producing widespread ROS flooding throughout the study area. Although regional snowpack ablation combined with liquid precipitation drive ROS flooding, the impact of antecedent soil moisture or frozen ground should not be overlooked. New information presented herein provides flood forecasters identifiable synoptic-scale features that may precede ROS flooding.

摘要

雨雪 (ROS) 洪水在整个阿巴拉契亚山脉和大西洋中部地区很常见，主要发生在冬末和春季。最近（2007-2013 年）遍及宾夕法尼亚州和其他大西洋中部各州的六次与融雪相关的广泛洪水事件被用来了解天气尺度大气条件，这些条件为该地区产生年度最大流量。NCEP/NCAR 对六个单独案例研究和全事件复合的再分析结果表明，类似的天气尺度大气条件导致了广泛的 ROS 泛滥。积雪消融和随后的洪水严重受到来自墨西哥湾和大西洋的温暖潮湿空气的低层（850 mb 低层喷流）平流的影响，该平流围绕大西洋沿岸的高压区向中部的低压-大陆。大陆中部一个异常深的 500 mb 槽有助于将热量和水分引导到研究区域。温暖、潮湿的空气持续平流进入该地区，使积雪首先成熟然后消融，从而在整个研究区域产生广泛的 ROS 泛滥。尽管区域积雪消融与液体降水相结合导致 ROS 泛滥，但不应忽视前期土壤水分或冻土的影响。此处提供的新信息为洪水预报员提供了可识别的天气尺度特征，这些特征可能先于 ROS 洪水。尽管区域积雪消融与液体降水相结合导致 ROS 泛滥，但不应忽视前期土壤水分或冻土的影响。此处提供的新信息为洪水预报员提供了可识别的天气尺度特征，这些特征可能先于 ROS 洪水。尽管区域积雪消融与液体降水相结合导致 ROS 泛滥，但不应忽视前期土壤水分或冻土的影响。此处提供的新信息为洪水预报员提供了可识别的天气尺度特征，这些特征可能先于 ROS 洪水。