09 December 2005 snow event by Richard H. Grumm National Weather Service Office State College, PA 16803

Transcription

1 09 December 2005 snow event by Richard H. Grumm National Weather Service Office State College, PA INTRODUCTION A winter storm produced heavy snow over a large portion of Pennsylvania on 8-9 December The winter storm which produced this snowfall produced heavy snow from Illinois to the East Coast. The observed snowfall from Ohio to New England is shown in Figure 1. As the secondary storm developed along the coast, it produced blizzard conditions with heavy snow and strong winds over southern New England. This 4 hour blizzard struck coastal regions of Massachusetts on Friday the 9 th of December. Snow on the runways at Chicago s Midway airport may have contributed to the crash of Southwestern Airlines Flight The plane skidded off the runway and into a nearby street killing one person on the ground. No serious injuries were incurred by those on the plane. The National Transportation Safety Board was still investigating the causes of the accident as this was written. However, this case may serve to illustrate the impact that winter weather can and often does have on transportation in the United States. This event was relatively well forecast by the National Centers for Environmental Predictions (NCEP) deterministic models and ensemble prediction systems (EPS). This is in stark contrast to the event of 5-6 December 2005 where the short-term ensemble forecast (SREF) system forecast snow in southeastern Pennsylvania and New England and points south and east. Most of the inland areas saw little or no snow. In that event, the NAM members were too wet and brought the precipitation shield too far north and west of the observed location. Figure 1 Regional snowfall plot (inches) from spotter reports that made NWS public information statements. Snowfall is for the period ending 1158 PM Friday 09 December This paper will document the event of 9 December 2005 as it affected Pennsylvania. It was a success story for the NCEP SREF and medium range ensemble forecast system (SREF) both EPS s performed

2 Figure 2 MREF forecasts initialized at 0000 UTC 6 December 2005 valid 1200 UTC 9 December Upper panel shows spaghetti plots of select contours and spread. Lower panels shows the ensemble mean (hpa) and the standardized anomalies. admirably in this event. Products from the SREF will be emphasized and how well they performed will also be emphasized. However, the fact that the NCEP EPS s sometime do not perform well should not be forgotten as we examine this case. 2. METHODS Most data shown here was extracted from NCEP after the event. Due to the move of the National Weather Service (NWS) Office in State College, much of the original data was lost or never retrieved. NCEP provided a backup set of data for the case study. Figure 3. MREF forecasts initialized at 0000 UTC 6 December 2005 valid for the 24-hour period ending at 0000 UTC 10 December Upper panels show the probability of 0.60 inches of QPF and the lower panels shows each member 0.60 contour and the ensemble mean precipitation. Observational data used include cooperative observer and snow spotter reports of snowfall retrieved in near realtime. Surface observations, satellite, and radar data were extracted using AWIPS data. These data were burned on to DVD s for replay on the Weather Event Simulator after the fact. Much of the model data is shown with the field or the EPS field s departure from normal in standard deviations from normal. The NCEP global re-analysis data set was used to compute the means and standard deviations. 3. FORECASTS

3 i. 6 December 2005 Pennsylvania, let alone Illinois. This was Figure 4 As in Figure 2 except for SREF forecasts initialized at 06/0900 UTC. Forecast from the NCEP MREF initialized at 06/0000 UTC 6 December 2005 are shown in Figures 2 and 3. The key feature was a massive anticyclone with above normal mean sea level (MSLP) over the eastern United States. The anticyclone was forecast to split and a surface cyclone was forecast to develop along the East Coast (Figure 2) and track up along the coast. Though not shown, MREF precipitation types and 850 hpa temperatures implied snow in most locations outside the immediate coastal plain. The MREF precipitation is shown in Figure 3. These data suggested no significant precipitation in western Figure 5 As in Figure 4 except SREF forecasts initialized at 06/0900 UTC showing 24-hour precipitation for the period ending at 09/2100 UTC 9 December forecast to be a coastal storm. Though there was some confidence in snowfall in Pennsylvania in New York, the scenario that would play out would be markedly different than shown by this forecast. By 06/0900 UTC, this storm fell into the forecast range of the NCEP SREF. Similar to the MREF, the SREF forecast an east coast winter storm (ECWS) to sneak up between the splitting anticyclones (Figure 4). The precipitation forecasts (Figure 5) showed a similar area to be affected by this storm as the MREF from 9 hours earlier. These forecasts implied little threat to Chicago.

4 The plume diagram from the SREF showing precipitation by type at State College, Pennsylvania showed the potential for either a heavy or light snow fall event. There was clustering to large precipitation and low precipitation amounts. Most members forecast snow. The mean of 0.54 would appear prescient on 9 December when around 0.67 liquid equivalent and 7-8 inches of snow fell in and around State College. For east and central Pennsylvania was the SREF correctt for the wrong reason? ii. 7 December 2005 The first hint that the coastal storm would not be the primary low appeared in the NCEP MREF s initialized at 1800 UTC (not shown). Forecasts initialized on the 7 th began to show the changing evolution of this system. The 07/0900 UTC SREF forecast showed a significant change in the forecasts. A significant inverted trough and potential deformation zone appeared in the EPS mean MSLP forecasts (Figure 7). A low was forecast to move over the Ohio Valley and weaken early on the 9 th as a secondary low developed along the Carolina coastal plain (not shown). The resulting QPFs were farther west than previous forecasts (Figure 8). However, they did not produce significant precipitation (snow) over the Chicago region. Chicago was still forecast to be just north and west of the Figure 6 SREF plume of precipitation by time from forecasts initialized at December 2005 for a point near State College, Pennsylvania. Accumulated precipitation in color coded by precipitation type. Gray lines show each members instantaneous 3-hour precipitation. more westward precipitation shield. The forecast showed the high probability of 0.5 inches or more QPF moving up over Pennsylvania and across southern New York State. The 850 hpa temperature forecasts are shown in Figure 9. Most locations were below 0C and would remain so supporting snow. SREF precipitation types limited mixed and rain to southern areas and along the coastal regions (not shown). Though not shown, strong warm advection was forecast on the west side of the retreating anticyclone. This feature would prove to be poorly forecast. The threat of snow farther west continued with subsequent model runs. The 2100 UTC SREFS continued this trend with a cyclone moving into the Ohio Valley. Though not shown the secondary low was still forecast to

5 Figure 7. As in Figure 4 except for SREF forecasts initialized at 07/0900 UTC valid at 0000 UTC 9 December Figure 8 As in Figure 5 except SREF forecasts initialized at 07/0900 UTC showing 24-hour precipitation for the period ending at 09/1500 UTC 9 December develop along the coast. The precipitation shield continued to shift farther westward reaching to Chicago. iii. 8 December 2005 Figures 10 and 11 show the MSLP and precipitation during the time of maximum extent of the storm. The MSLP forecasts show a broad trough in the ensemble mean. Several members produced a closed low over the Midwest. The precipitation shield showed that 0.50 inch contour was forecast to extend back over Illinois. The focus of the heavy precipitation for the 24 hour period ending at 1500 UTC 9 December was over the Ohio Valley, in the deformation zone between the two Figure 9. SREF forecasts of 850 hpa temperatures initialized at 0900 UTC 7 December 2005 valid at 0300 UTC 7 December 2005.

6 Figure 10. As in Figure 7 except for SREF forecasts initialized at 08/0900 UTC valid at 0000 UTC 9 December anticyclones, and along the coast with the forecast development of the secondary surface cyclone. The plume diagram for Chicago is shown in Figure 12. These data show that all SREF members forecast snow and that 2 members forecast in excess of 0.60 inches of QPF at Chicago. A third member forecast just under 0.50 inches of total QPF. The accumulation contours show the rapid onset and accumulation of the precipitation. The gray instantaneous precipitation shows that most members forecast the high probability of QPF about 6-hours either side of 0000 UTC 9 December This proved to be an excellent forecast. The SREF precipitation type and accumulation plume for State College is shown in Figure 13. Similar to the Chicago plume, these data show the rapid onset of precipitation with most Figure 11. As in Figure 8 except SREF forecasts initialized at 08/0900 UTC showing 24-hour precipitation for the period ending at 09/1800 UTC 9 b 200 members forecasting 0.40 inches or more QPF between 0000 and 0900 UTC on the 9 th. There was incredible agreement with the time of maximum 3- hourly accumulated precipitation, between 0600 and 0900 UTC. A few members brought the precipitation in earlier, around 0000 UTC and a few lingered it after 1200 UTC. The snow arrived shortly after 0300 UTC and was over before 1200 UTC. The ensemble mean appeared to work well in this instance with 7-8 inches observed locally. This plume showed great skill in the short term application of plumes and ensembles in the forecast process. 4. CONCLUSIONS. A widespread snow event affected the eastern United States from Illinois to Maine. This event was relatively well

7 Figure 12 SREF plume from forecast initialized at 0900 UTC 9 December 2005 for a point near Chicago, Illinois. forecast by the NCEP SREF and MREF systems, in stark contrast to the failure of the SREF to correctly forecast the winter storm of 5-6 December MREF forecasts from the 5 th and 6 th of December implied a coastal storm. The MREF slowly evolved toward the more complex Miller Type B system which was ultimately observe red. Thus the precipitation shield advancing up the coast, as forecast on the 6 th (Figure 3) change to a precipitation shield coming up from the southwest with significant precipitation west of the Appalachian mountains (Figures 8 and 11). As the time of the event drew nearer, the SREFs converged on the solution. Shortterm forecasts from the SREFS proved quite valuable. The correctly forecast snow, the timing of the snow, and the period of rapid accumulations. It is somewhat discouraging at how poorly the western edge of the precipitation shield was initially forecast by both NCEP EPS s. The initial implications point to initial conditions, but this needs to be examined closer. The impact was clear, as the event drew near the EPS system skill increased markedly. At forecast lengths of hours the EPS data proved most useful. The short-term plumes revealed how point data from EPS s can be used for improved short term and aviation weather forecast applications. 5. ACKNOWLEDGEMENTS I would like to thank Jun Du of NCEP for providing the data for this case. Due to the move of the National Weather Service office in State College, computers failed and were disconnected. This led to sporadic data flow and

8 Figure 13 As in Figure 12 except for State College Pennsylvania. missing data on the 7, 8, and 9 th of December.

9