Warm Episode of December 2015:

Transcription

1 Warm Episode of December 2015: Record Warmth over Mid-Atlantic Region By Richard H. Grumm National Weather Service State College, PA 1. Overview A strong 500 hpa ridge over the eastern United States (Fig.1) brought a period of above normal warmth to much of the eastern United States from 11 to 15 December 2015 (Table 1). The warm air affected much of the central and western United States on the 10 th and moved eastward with time. From December the high temperature in State College, PA exceeded 60F and the high temperature exceeded 50F from December Across Pennsylvania and much of US records broken Broken Tied Total Dec 10 th Dec. 11th Dec. 12th Dec. 13th Dec. 14th Dec. 15th Dec 16 th day total Table 1. List of high temperature records broke, tied, and all records tied or broken for the 5 day period from 11 to 15 December NCDC site for records. the eastern United States many daily maximum-high temperature (Table 1) and record highminimum temperature (Table 2) records were tied or broken during this period. In Pennsylvania where there are 60 stations which can be compared, 78% percent of the reporting stations set or tied the record maximumlow temperature. During this period the number of daily high temperature records set or tied (63 to 65 records were possible per day) several days saw in excess of 60% of the high temperature records set or tied. The evolution of the 500 hpa pattern (Fig. 2) and the 850 hpa pattern (Fig. 3) and temperature anomalies show the US Max-Min Broken Tied Total warm air move out Dec. 11th of the eastern Plains Dec. 12th peaking in the Dec. 13th eastern United States Dec. 14th on 14 December Dec. 15th before falling on the Dec th. Due to clouds 17-Dec and rain, fewer Table 2. As in Table 1 except for records highest minimum temperatures records were set on the 16 th and the warm episode ended over most of the Mid-Atlantic region.

2 PA maximumlowest temperature Broken Tied Total Dec. 11th Dec. 12th Dec. 13th Dec. 14th Dec. 15th Dec Dec Table 3. Listing of highest minimum temperature records broke or tied in Pennsylvania by day. Based on the NCDC site there are 60 potential stations available on most days to compare to the current observations. Return to text. Thus on the 15 th 78% of the reporting stations set or tied the record high minimum temperature. PA Maximum Temperature Broken Tied Total Dec. 11th Dec. 12th Dec. 13th Dec. 14th Dec. 15th Dec Dec Table 3. Listing of highest maximum temperature records broke or tied in Pennsylvania by day. Based on the NCDC site there are 65 potential stations available on most days to compare to the current observations. Thus on the 13 th 67% and on the 14 th 75% of the reporting stations set or tied the record high maximum temperature. The precipitable water (PW: Fig. 4) went above normal on the 12 th and stayed above normal though about 0600 UTC on the 15 th (not shown). The surge of high PW air on the 14 th ahead of the front produced the high number of maximum-low temperature records on the 15 th. The impact of high humidity on the number of maximum low-temperatures was notable. The period of the PW is shown as 0000 UTC rather than 1200 UTC as on the 15 th at 1200 UTC drier air had already entered much of the eastern United States behind the cold front implied in Figure 5e. The strong pressure gradient in Figures 5d & 5e combined with the high PW values (Fig. 4d & 4d) likely contributed to the large number of overnight highminimum temperature records set or tied. Due to the period of warm and relatively humid weather record highs and record low highs contributed to period of significantly above normal weather. By 17 December 2015 all the climate sites in Pennsylvania were running 1 to 8F above the previous record warm December. This warm episode set the month up to be one the warmest Decembers on record in Pennsylvania since recording keeping began at most stations. Site YEAR NOV DEC Annual

3 Harrisburg Site YEAR NOV DEC Annual Williamsport Site YEAR NOV DEC Annual State College Site YEAR NOV DEC Annual Bradford Table 5. Data for the top 3 warmest Decembers as implied by the mean monthly temperature for four select sites in central Pennsylvania. The Harrisburg site is Middletown-Harrisburg Area on xmacis2. The data are sorted by warmest mean temperature during the month of December. Thus the November data is just a reference and is not sorted. The data for 2015 is through 20 December Return to text. The mean temperatures for December through 20 December 2015 sorted by the warmest mean December temperature is shown in Table 5. As of 20 December the mean temperature at the 4 sites shown was warmer than any previous December with Williamsport running nearly 5F above the previous record December of The data were truncated showing on the 3 warmest Decembers on record for each site and the data included the November data for comparison only. It is interesting to note that at most sites many of the top 10 warmest Decembers have occurred in the last years with the notable exception of December 1923 which showed up in the Harrisburg and State College records. 2. Forecasts Forecasts of this event are shown from a pattern perspective, which was overall quite well predicted with relatively long lead-times. And though not shown the Climate Forecast System (CFS) predicted warm temperatures across eastern North America for most of December and thus the Climate Prediction Center forecast a high probability of much above normal weather over much of the eastern United States in mid-december. The focus will be 6 GEFS pattern forecasts before the event all valid at 1200 UTC 14 December near the peak of the episode. The data shown include the forecasts for the North American 500

4 hpa pattern (Fig. 6), the 850 hpa temperatures over the eastern United States (Fig. 7), and the precipitable water forecasts over the eastern United States (Fig. 8). The ridge and surge of warm moist air was relatively well predicted by the GEFS with at least 6 days of lead-time. Plume diagrams from the 0000 UTC GEFS on 8 & 9 December (Fig. 9) show the warm temperature forecasts for State College. The GEFS clearly indicated much above normal 2m temperatures and a prolonged period of warm temperatures. The normal December maximum is 38 and the minimum is 25. Note the low temperatures were forecast to remain over 20F above normal most of the period. Thought the longer range GEFS 2m temperatures were too cold for daily maximum they were still well above normal for December. Though not shown, shorter range GEFS 2m temperature forecasts were in the 60 to 65F range. 3. Summary A period of above normal temperatures affected most of the eastern United States to include Pennsylvania from December There were successive days were many record high temperatures and record high-minimum temperatures were set. The period of extreme warmth was so extreme that by 17 December all COOP sites examined in Pennsylvania were running several degrees warmer than any other previous December on record. The warmth and forecast warm episode around Christmas should help ensure that December 2015 will be one and most likely, the warmest December on record. The pattern which produced this warm episode included a strong 500 hpa ridge (Fig. 1 & 2) which allowed unseasonably warm air to dominate much of eastern North America. During this period the Arctic Oscillation (AO) was highly positive implying a strong polar vortex locked up in higher latitudes. The ridge and the warm temperatures were well predicted by both the NCEP GEFS (shown Fig. 6-9) and the Climate Forecast System (examined but not shown). Both of these systems are forecasting a second warm episode in the eastern United States. The record warmth of December 2015 will likely lead to one of the warmest Decembers on record over much of the eastern United States. It will likely be attributed to El Niño though the signal over North America was not a classic El Niño signature. In a medium range forecast sense this event was relatively predictable. 4. Acknowledgements 5. References

5 Figure 1. The composite mean pattern and anomalies from the CFSV2 analysis for the period of December 2015 showing the mean a) 500 hpa heights and anomalies, b) 850 hpa temperatures and anomalies, c) mean sea-level pressure and anomalies, and d) precipitable water and anomalies. Return to text.

6 Figure 2. As in Figure 1 except for the 500 hpa pattern and anomalies in 24 hour increments from a) 1200 UTC 11 December through f) 1200 UTC 16 December Return to text.

7 Figure 3. As in Figure 2 except for 850 hpa temperatures and anomalies over the eastern United States. Return to text.

8 ! Figure 4. As in Figure 3 except for precipitable water and precipitable water anomalies. Data run 0000 UTC December to show warm suge overnight on the 15 th. Return to text.

9 Figure 5. As in Figure 3 except for mean sea-level pressure and pressure anomalies. Return to text.

10 Figure 6. Forecast from 6 GEFS runs showing the 500 hpa heights and anomalies valid at 1200 UTC 14 December 2015 over North America from 1200 UTC GEFS runs initialized at in 24 hour increments from a) 8 December through f) 13 December Return to text.

11 Figure 7. As in Figure 6 except for 850 hpa temperatures and temperature anomalies. Return to text.

12 Figure 8. As in Figure 6 except for precipitable water and precipitable water anomalies. Return to text.

13 Figure 9. GEFS plume diagrams from the 0000 UTC 9 and 9 December 2015 GEFS valid at State College. The data include top to bottom in each panel, the 2m temperatures (F), the 850 temperatures (C), and the 700 hpa temperatures (C). Thick black is the ensemble mean and yellow is the median. Return to text.