13-14 January 2005 warm episode and southeast Pennsylvania Flooding Event-DRAFT by Richard H. Grumm National Weather Service Office State College PA 16803

strong frontal passage on the 13th and or 1. INTRODUCTION the 14th of January. The surge of above normal moisture was also well forecast A strong cold front moved across by the MREF. The NCEP Short-Range Pennsylvania from around 0000 UTC Ensemble Forecast (SREF) system through 1500 UTC 14 January 2005. (SREFs) showed the frontal system in Ahead of this frontal system, the shorter range, but failed to capture unseasonably warm air moved into the the heavy rains observed in southeastern region. Temperatures soared into the 70s portions of the State. in western Pennsylvania and the 50s and 60s over most other areas. Along with The strong quasi north-south cold front this unseasonably warm air came tapped the warm moist air as it moved unseasonably high dew points. across the State. This triggered showers and areas of moderate to heavy rain. The frontal system was relatively well WSR-88D images showed narrow cold forecast by the National Centers for frontal rainband (NCFRB) and at times, Environmental Predictions (NCEP) a wide cold frontal rainband (WCRB) Medium-Range Ensemble Forecast over the State. Over the lower system (MREFs). As early as 7 days Susquehanna Valley, the NCFRB and prior to the event, the MREFs showed a WCRB produced a wide area of 1-2 inches of rainfall. With previous rains, this produced flooding over many counties in the lower- and middle- Susquehanna valley during the morning hours of 14 January 2004. Most of the reports of flooding came after the rainfall had ended.

This paper will serve to document the warm epispode of 13 January and the heavy rainfall of 14 January 2005. In addition to documenting the event, this paper will examine and show the utility Figure 1 Maximum temperatures over Pennsylvania observed on 13 January of using forecasts 2005. produced by the NCEP upon entering the boundary layer. For this approximation, the 850 hPa 0C isotherm was used.

The SREF doe implicitly produce rain, ice pellets, snow, and freezing rain data. At this time these data are not shown.

Some fields were displayed showing the mean of the EPS forecasts for that variable verse the 30-year climatology. In these instances, the departures are shown as standardized anomalies in standard deviations (SDs) from the 30-year normal. The reproductions software Figure 2 WSR-88D total estimated rainfall (in) and observed rainfall (in) used the same data and valid at 1300 UTC and 1200 UTC 14 January 2005 respectively. scripts used in real-time. All real-time graphics Ensemble Prediction System (EPS) in changes are based on improvements forecasting these types of events. made during the case study process.

2. METHODS The term “winter storm” is used loosely here and in fact this was more of a All data were available in real-time via winter precipitation event. There was no the PSU-NWS data feed. All EPS output significant surface cyclone associated images shown here were replicated using with this event. the operationally available data and the operationally deployed software. For ice 3. RESULTS storms, determining the temperature of the boundary layer is critical to Figure 2 shows the WSR-88D estimated determine if the boundary will be cold rainfall and the observed rainfall valid at enough to cause rain to freeze. For this 1300 and 1200 UTC 14 January 2005. purpose, EPS 2m temperature forecasts, The radar was in the warm season including spaghetti plots and probability convective mode and the overall plots were used. estimates were on the high side. One rain gage had an erroneous 7.10 inch reading. In addition to the temperature of the These data show that the heaviest boundary layer, it is critical to have an precipitation fell over eastern sections of estimate as to whether the precipitation central Pennsylvania with 1-2 inches of will be in the form of liquid or solid isolated thunderstorms were observed as the NCFRB crossed the State.

Figure 3 shows the NCFRB shortly after it reformed at 0449 UTC. Not the WCFBR to the west. The enhanced cores along the southern end of the line can be seen affecting the lower Susquehanna Valley between 0700 and 730 UTC in Figure 4. The WCFRB to the west would bring additional rain to this region later in the morning.

4. CONCLUSIONS Figure 3 KCCX 0.5 degree reflectivity valid at 0449 UTC 14 January 2005.

5. rainfall observed over much of the ACKNLOWEDGEMENTS lower-Susquehanna Valley. This area was where the NCFRB and WCFRB 6. REFERENCES provided rainfall to the region. A few

Figure 4 As in Figure 3 except valid at 0708 and 0724 UTC 14 January 2005.