Thanksgiving Snow and Arctic Front 25 November 2005 By Richard H. Grumm National Weather Service State College, PA 16801

Transcription

1 Thanksgiving Snow and Arctic Front 25 November 2005 By Richard H. Grumm National Weather Service State College, PA INTRODUCTION An approaching arctic front brought light snow to most of western and central Pennsylvania on Thanksgiving eve and early on Thanksgiving data. Snowfall with the approaching system was generally on the order of 25 to 75 mm (1-3 inches). The arctic front, with strong gusty winds, on the order of 35-45KTS came through State College around 1900 UTC. Temperatures fell rapidly behind the front and the melting snow quickly froze. This led to numerous traffic accidents throughout central Pennsylvania during the afternoon hours. Behind the front, temperatures fell well below normal with many locations approaching near record low values on the morning of the 25 th. Lake effect snow (LES) impacted Ohio, New York, and northwestern Pennsylvania. A few locations in the State College County warning area (WFOCTP), in Warren County received 6-12 inches of snow. The higher snow fall amounts were near and along the New York border. This paper will document the thanksgiving snow and arctic front. It will also document some short-term ensemble forecast tools which were useful in forecasting this event. 2. METHOD Observational data was retrieved off the WFOCTP Advanced Weather Interactive and Processing System (AWIPS). Using case playback mode, a mix of radar, satellite, and surface observations were used to document the event. From a forecast perspective, data will be limited to forecasts from the National Centers for Environmental Predictions (NCEP) short-term ensemble forecast (SREF) system (SREF). These data are displayed using GrADS. A mix of ensemble output is produced and some of these data include the departure of the ensemble mean fields from the 30-year climatological values in standard deviations (SDs) from normal. 3. OVERVIEW Based on surface and radar observations, light snow moved across southwestern Pennsylvania during the late afternoon and evening hours of 23 November The snow spread to the northeast as a quasi northwest to southeast oriented series of snow bands. The snow shield, with embedded stronger bands is shown at 24/0000 and 24/0200 UTC in Figure 1. Steady light snow reached to State College around 24/0200 and the snow continued to move northeastward. A more organized are of snow moved over the region between 24/0400 and 24/0600 UTC with a large area of 30dBZ returns. The stronger bands can be seen in the radar imagery displayed in Figure 2. A sharp back edge to the precipitation shied was visible on radar

2 feature moved rapidly to the east. Observations showed very light snow or flurries west of this feature. This feature passed through KJST around 0615 and State College around After 24/0700 UTC the area to the west began to fill in with weak returns. Observations showed areas of drizzle, freezing drizzle, and light snow in the warmer moist air. Most of the accumulating snow was observed with this feature. By 0800 UTC this steady and accumulating snow had moved off to the east leaving a good 1-3 inch snowfall over the region. Temperatures slowly rose to near or a few degrees above freezing during the morning and early afternoon hours ahead of the approaching artic front. Figure 3 shows the visual satellite image from 25/1600 UTC. Most of the region was cloud free allowing the snowfall across Ohio, West Virginia, and Pennsylvania. Some LES bands were present over northwestern Pennsylvania as well some distinct LES bands over Lakes Superior, Huron, Erie and Ontario. Figure 1 KCCX composite reflectivity and surface observations valid at a) 0000 and b) 0200 UTC 24 November to the west (Fig. 2 lower panel) and this Figure 4 shows the surface observations at 1700 and 2000 UTC. The 1700 data is underplayed with GOES visual imagery. These data show the strong gusty northwest winds along and behind the arctic front. The rapidly falling

3 leading to numerous traffic accidents. At University Park airport, the temperature fell from 36 to 25F between 1700 and 2000 UTC. 4. SREF FORECASTS Figure 5 shows the 23/0900 UTC SREF forecasts of MSLP and 850 hpa temperatures valid at 24/1500 UTC. These data show the deep surface cyclone (-3.5 to -4 SDs below normal) passing over Lake Ontario, and the cold air at 850 hpa over the Great Lakes. The leading edge of this arctic air mass was crossing Pennsylvania at this time. Figure 2 As in Figure 1 except composite reflectivity valid at 0500 and 0.5 degree data valid at 0636 UTC. temperatures produced icy conditions Figure 6 shows plume diagrams of accumulated precipitation, colored by type and 2m temperature forecasts for State College, PA also from the SREF at 23/0900 UTC. The precipitation forecasts shows light precipitation beginning around 24/00000 UTC with most members showing the time of maximum precipitation between 0300 and 1200 UTC. All SREF members forecast snow. The mean was around 0.27 inches with a first guess of 2.7 inches of snow and the range was (10:1 ratio) about 1.0 to 4 inches of snow.

4 Figure 3 GOES visual imagery valid at 1615 UTC 25 November Except for some cirrus and narrow lake effect snow bands, most of the snow coverage from Ohio and West Virginia is visible in this image. The lower panel of Figure 6 shows little temperature change overnight with rising temperatures, due to the warm advection associated with the snow. The normal time for the overnight low is close to the time of the high for the day. The SREFs quite consistently forecast the precipitous fall of temperatures associated with the passage of the arctic front around 24/1500 UTC. Figure 7 shows the SREF wind forecasts valid at 24/1500 UTC. North of the cyclone, anomalous U-winds were forecast. Southwest of the surface cyclone anomalous west-northwest winds were forecast. These strong winds translated to near the surface along and behind the front as indicated in Figure 4. Figure 8 shows the SREF 850 hpa forecasts valid on Friday 25 November These data show the unseasonably cold air mass the settled over the region

5 for the day. This led to overnight lows Thursday into Friday that were close to record low values and high temperatures 25 November This provided a White Thanksgiving for many locations of the state. An arctic air mass swept Figure 5 SREF forecasts of MSLP (hpa) and 850 hpa temperatures (C) from forecasts initialized at 0900 UTC 23 November Upper panels show spaghetti and spread and lower panels show ensemble mean and the departures from normal in standard deviations from normal. that were about 20F below normal for the date. This cold air mass, with 850 hpa temperatures near -14C produced the instability over the Great Lake contributing to the LES bands shown in Figure CONCLUSIONS A fast moving clipper storm brought 1 to 3 inches of snow across much of western and central Pennsylvania during the evening and early overnight hours of 24- across the State during the afternoon and early evening hours of the 25 th. Strong gusty winds buffeted the region along and several hours behind the cold front. The NCEP SREF, using the forecasts initialized at November 2005, did an excellent job on forecasting the potential snow, the strong cold front, the strong winds associated with the cold front, the unseasonably cold temperatures behind the front, and the potential for Lake Effect snows.

6 Figure 6. SREF plumes from forecasts initialized at 0900 UTC 23 November Upper plume shows accumulated precipitation color coded by precipitation type and instantaneous 3-hour precipitation (gray) from each member. Lower panel shows 2m temperatures by member. The thick black line is the ensemble mean value. Return to text. Furthermore, the SREF plumes, as demonstrated by the plume for State College (other locations were examined but not shown) demonstrated the shortterm forecast utility of point data forecasts from the SREFs. The potential value of plumes in short-term weather forecasts for timing and high probability time of precipitation along with frontal passages appears to hold great promise.

7 Figure 4. Surface observations centered over Pennsylvania at 1700 and 2000 UTC 24 November GOES visual picture is underplayed with the 1700 UTC data. Return to text.

8 Figure 7 SREF forecasts initialized at 23/0900 valid at 24/1500 UTC showing ensemble mean winds and departures of the U (upper) and V (lower) winds from normal in standard deviations from normal. Return to text. Figure 8 As in Figure 7 except SREF 850 hpa temperature forecasts initialized at 23/0900 valid at 25/1200 UTC. Return

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