A Case Study of the 13-14 February 2007 Extended Sleet Event

1. A Case Study of the 13-14 February 2007 Extended Sleet Event Potter, Matthew S.; Selmer, Patrick A.; Clark, Richard D. Department of Earth Sciences Millersville University of Pennsylvania Abstract Methods Conclusions A major winter storm affected much of the Mid-Atlantic and Northeastern United States on 13-14 February 2007. For an extended and nearly uninterrupted period of 15 hours between approximately 23 UTC 13 FEB and 16 UTC 14 FEB, precipitation in the form of sleet (ice pellets) fell over much of south central Pennsylvania, northern Maryland, and parts of New Jersey. This research investigated the factors contributing to this unusually long precipitation event. The influence of a persistent easterly low-level jet, which helped to maintain a consistent pool of cold air in the near-surface layer, was an important factor in maintaining precipitation in the form of sleet. Temperature and humidity profiles from the NAM, RUC, and GFS were analyzed to determine the mechanism by which this extended sleet was sustained. Bourgouin’s method of determining precipitation types was implemented in the model soundings to show that sleet was favorable during the times of interest. Using Bourgouin’s Method of determining precipitation type, various modeled sounding data was used to determine if sleet was the most favorable type of precipitation to be occurring during the storm at various point (station) locations and times (with the exception of the GFS, which had a time step of three hours, the models used time steps of one hour). To validate this information that was gleaned from the modeled data, surface observations, METAR and CoCoRaHS reports, were used. Radar data was used to indicate how long and when it was precipitating at the various station locations. A computer program was written and used to graphically show and calculate some useful characteristics of the wind in the vertical at each of the different point locations. BUFKIT was used to look at various meteorological parameters in the modeled sounding data such as relative humidity (subsaturated regions in the warm layer), wind direction, temperature, and others. Upper air and surface analysis maps were used to look at the synoptic features and progression of the storm. The locations of highest total amounts of sleet, as suggested by the model data, are KMDT (Harrisburg, PA), KHGR (Hagerstown, MD), KMRB (Martinsburg, WV), KRDG (Reading, PA), KABE (Allentown, PA), and W54 (Westminster, MD). These locations matched well with historic known areas to receive the most amount of sleet. It was difficult to validate the times at which sleet fell because the spatial and temporal coverage was not as good as we hoped that it would be for the CoCoRaHS and METAR reports. The relative humidity profiles showed no subsaturation in the warm layer; subsaturation would aid in the maintenance of sleet as described by Hanesiak and Stewart [2]. This could have been attributed to the lack of spatial resolution of the data in the BUFKIT program. Warm air advecting in from the mid-levels provided a consistent warm layer (warmer than 0°C) for precipitation to fall as liquid, before refreezing in the boundary layer. The most important factor in this extendedsleet event was a strong, low-level easterly jet. This jet, which was ageostrophic in nature (see figure below and to the right) was important in the maintenance of a cold, moist boundary layer conducive for precipitation in the form of sleet. As the reconstituted low pressure deepened and moved up the coast of New Jersey the low-level jet broke down and winds at all levels shifted to northerly, ending the extended period of sleet and changing the remaining precipitation to snow. This changeover of precipitation type (according to Bourgouin’s Method of determining precipitation type [1]) corresponding to the change in winds is generally observed in the model data and is consistent with the actual timing of the low off the coast of New Jersey. Results A map showing the point locations of modeled data used in the investigation (left side) and example of one particular model (RUC) sounding at a particular time (6 Z Feb 14th) for Reading, PA during the event (right side) NEXRAD Radar composite image from 8 Z 14 February 2007 [3]. Overview 925mb Winds 700mb Winds A strong anticyclone (1037 mb) was centered over eastern Ontario providing low-level cold air in the Northeast United States on February 13. During the day a low pressure center (1000 mb) was approaching the Appalachian Mountains. Starting out of snow, warm southerly flow at the mid-levels changed precipitation to sleet and freezing rain in the southern part of our focus area by evening. During the nighttime hours, all parts would change to sleet and freezing rain. By 09z February 14, all of the focus area was experiencing heavy sleet/freezing rain, and southeastern sections were changing over to rain. At this stage of the event the Appalachian low had since reconstituted itself over the Carolinas and moved over the Delmarva with a pressure of 993 mb. Heavy sleet would continue into the morning hours of the 14th before finally ending as a quick period of snow as the wind direction became north-northwesterly at all levels. In the end, a widespread area of 2-4 inches of sleet fell in northern Maryland, south-central Pennsylvania, and parts of New Jersey . Photograph of accumulated sleet and snow near Trenton, NJ (KTTN on map above and to the left) 0 Z 14th FEB 925 mbgeopotential height and temperature analysis overlaid with arrows indicating the primary wind direction at 925 mb and 700mb over our focus area References Average resultant (vector wind) wind magnitude as a function of pressure for model levels in the 0 Z 14 February NAM model run for KMDT (Harrisburg, PA) whose direction (according to the meteorological coordinate system) was between 0º and 100º. The labels on the top right-hand corner of the graph are: the maximum resultant wind magnitude included in the calculation of the average resultant wind calculations and the pressure level at which it occurred, and the time range (inclusive) where wind directions were between 0º and 100º. This is the whole graph; above where the line stops there were no wind vectors with directions between 0º and 100º. The low-level maximum seen in this graph was pretty indicative of all other models at all other stations. [1] Bourgouin, Pierre. "A Method to Determine Precipitation Types." Weather and Forecasting 15 (2000): 583-92. [2] Stewart, Ronald E., and John M. Hansiak. "The Mesoscale and Microscale Structure of a Severe Ice Pellet Storm." Weather Monthly Review 123 (1995): 3144-162. [3] Martin, Raymond Jr. C. "Snow and ice storm, February 13-14, 2007 - Satellite Imagery." New Jersey Expressways and Tollways. Web. 07 Jan. 2010. <http://www.njfreeways.com/weather/2007/14-Feb-07-SatelliteImagery. html>. Number of total hours favorable for sleet for the 0 Z 14 February 2007 run of three different forecast models according to Bourgouin’s Method for determining precipitation type [1]. The Stations are oriented geographically from Northwest to Southeast.