The historic warm episode of February 2017

Transcription

1 The historic warm episode of February 2017 By Richard H. Grumm National Weather Service State College, PA 1. Overview A winter warm episode brought an extended period of record and near record warmth to much of the eastern United States in late February There were 6329 record high temperature records were tied or broken in February with 1534 records set in a 7 day period toward the end of the month (Table 1). During this period of record warmth comparatively very few low temperature records were set. In the eastern United States 24 & 25 February 1 (Tables 2 & 3) were record shattering days from Vermont to North Carolina. Allentown, PA hit 77, Burlington, VT 72F, Watertown, NY 64, Albany, NY 74, State College, 74. Areas farther east, such as Albany, NY, peaked on the 25 th (Table 3). These were just a few of dozens of record high temperatures for the month. The State College high of 74F was both the warmest recorded temperature for the month of February and for meteorological winter. The large scale pattern over the Unite States (Fig. 1) showed a large 500 hpa ridge with 1 to 2σ height anomalies over the central United States on February. The 500 hpa ridge gradually weakened and during the period of record warm in the east the 500 hpa heights were not impressive. This is in stark contrast to the record ridge which produced the warm episode of March 2012 (Grumm et al 2013). It is interesting to note that the 500 hpa ridge was sharper and had larger 500 hpa height anomalies over the Great Plains and Great Lakes (Figs. 1a & 1b) but weakened as the ridge moved eastward. Similar to the 500 hpa heights and anomalies, the 850 hpa temperatures were above normal during the warm episode but they were not remarkable (Fig. 2). The peak 850 hpa temperature anomalies were on the order of +2 to +3 σ above normal during the peak warmth on 24 February. The 850 hpa temperatures at 1800 UTC 25 February show the surge of warm air well into northern New England ahead of a very sharp north-to-south cold front. The more impressive feature was the above anomalous 2m temperatures observed over the eastern United States from 20 to 25 February (Fig. 2). These data show the widespread areas of +3 to +4σ 2m temperature anomalies at 1800 UTC 24 February 2017 and the surge of warm air into New England ahead of the cold front on 25 February This cold front triggered a significant severe weather event from New England to Virginia with over 222 reports of severe weather, including 5 tornadoes, and the first ever known February tornadoes in Massachusetts (SPC storm reports). 1 Records recorded as ending 12Z 25 February 2017

2 This paper presents the pattern associated with the record warm episode over the eastern United States with a focus on the two record warm days of 24 and 25 February. It is understood that the warmth and record warmth spread eastward overtime and that record high temperatures were set from Texas and Colorado into New England during the latter half of February In addition to a summary some of the records, forecasts from the NCEP GEFS are presented to show how well this event was forecast. To most people this was a pleasant break from winter but for fruit growers and ski resorts this may have been an economic disaster. News reports and social media showed flowering trees from Oklahoma to New Jersey. And many ski resorts closed prematurely due to the warm weather and associated snow melt. 2. Methods and Data The pattern was reconstructed using the CFSR- V2. These data were retrieved from the NCAR data repository and displayed using GrADS. The 2m temperatures were extracted from the 00- hour forecasts of the NCEP operational GFS. All climate records were retrieved using XMACIS and the National Centers for Environmental Information (NCEI) interactive website. Record highs at climate sites were retrieved from METAR observations and summarized in Tables 2 & 3. It should be noted that the NCEI data reports the high and lows for the 24 hour period ending which skews the appearance of high temperatures one day forward. Thus a high of 74F in State College is reported at 7 AM on 25 February. Information on ski resorts was obtained by web searches. In Pennsylvania only 4 of 15 ski resorts closed during the event which did not open up within 24 hours after the cold frontal passage. Resort Seven Springs Laurel Mountain Hidden Valley Whitetail Mountain Ski Roundtop Ski Sawmill Blue Mountain Jack Frost Big Boulder Camelback Spring Mountain Status Open Open Open Open Open Open Open Open Open Open Open Tussey Mountain Closed Ski Liberty Closed Blue Knob Closed Crystal Lake (Hughesville) Closed Table 2. A list of Ski resorts in Pennsylvania with available reports and status from February Several resorts closed on the 25 th be reopened on the 26 th or 27 th after making new snow behind the front. These data are summarized in Table 4. It should be noted that all ski resorts rely on snow making to remain open in the winter and that temperatures alone do not dictate whether or not a particular resort will or will not make snow. As a general rule most resorts can make snow when the sum of the dew point and wet bulb is less than 50F. These conditions returned to most of Pennsylvania late Saturday 25 February and Sunday 26 February. But each resorts business model reflected its decision as to whether or not to make snow.

3 The NCEP bias corrected GEFS (GEFBC) was used to illustrate how relatively well the pattern associated with the warm episode was. It should be note that the GEFS has a serious and operationally known cold bias for high temperatures over much of the eastern United States. During most of the warm episode, locally 2m GEFS temperatures were 5-10F too cold. The focus here is on the pattern and the GEFSBC. The NAEFBC was also used but showed a similar overall set of forecasts. The focus here will be on the GEFSBC forecasts for the record warmth on 24 February using 6 GEFFSBC forecasts periods initialized at 0000 UTC from 17 to 23 February Forecasts from 0600, 1200, and 1800 UTC cycles showed similar results. 3. Forecasts of the warm episode The GEFSBC 500 hpa forecasts are shown in Figure 4. Clearly, the shorter range GEFSBC forecasts were better able to predict the 500 hpa ridge with about +1σ above normal 500 hpa height anomalies. Looking at the mean and spread (not shown) implied relatively low uncertainty in the ridge with higher uncertainty with the short wave east of Florida. The corresponding 850 hpa temperatures (Fig. 5) are focused over the eastern United States. All of these forecasts predicted above normal 850 hpa temperatures though the longer range forecasts lacked the area covered by the +2σ temperature anomalies and failed to push the +8 to +10C air into the Mid-Atlantic region and southern New York (see Fig. 4e-f vs Fig 4a.). The GEFSBC 2m temperature forecasts are shown in Figure 6. These data show a wide area of +2σ and some areas of +3σ above normal 2m temperature forecasts. There is a similar trend in these data with 2m temperatures being too cold at longer ranges and a general cold bias relative to observations in all of these forecasts. The GFS 00-hour forecasts valid at 1800 UTC 24 February (Fig. 4e) were 22C in southwest Pennsylvania and the 18C contour covered most of the State. The GFS 00-hour forecasts too were too cold to apply as a ground truth as temperatures were in the 70 (21C) to 76F (24C) range over most of the State by 1800 UTC. The 3km NCEP HRRR 2m temperatures at 1800 UTC (Fig. 7) showed that temperatures were over 70F (20C) across most of the State of Pennsylvania while observations at this time ran several degree warmer. The 850 hpa forecasts valid 1800 UTC 25 February 2017 are shown in Figure 8. These data show the cold front moving into the eastern United States. Due to high spread in the members at longer ranges (not shown) the front does not look as sharp in longer range forecasts relative to shorter range forecasts. But all 6 forecasts show one last warm day in the east and a sharp northsouth cold front entering the region around by 1800 UTC. The value in these forecasts include the continued warmth near the coast, the cold air in the Midwest and the sharp front favoring potential severe weather.

4 4. A look at the records Table 1 summarized the records broken across the United States during discrete periods in February and over the past year. Clearly, February 2017 was a warm month dominated by an inordinate number of high temperature records being reached or exceeded. And there was a relative dearth of low temperature records (95) set in February 2017 relative to high temperature records (5857) and a similar trend was seen for the past year where the 8337 high temperature records set relative to 1230 low temperature records. Table 2 shows the records set using METAR sites. These data summarize the number of records set on Friday, 24 February. The records set a METAR sites on 25 February are displayed in Table 3. Clearly, 24 and 25 February were record warm days with tens of locations in the eastern United States setting new record high temperatures for the day and for the month. Including the record highs achieved during the overall period of warm temperatures, many sites set some other rather unique records. For example, in State College, PA station STCP1 set a record high of 74F on 25 February (observed on 24 th ) which higher than the old record of 73F. The station also recorded the first ever to back-to-back 70F plus days on 24 and 25 February 2. Other records set included the highest 7-day warmest mean maximum temperature (63.6: Table 4). Other sites in the northeast and Mid-Atlantic; such as Albany, NY; set similar records including the warmest multi-day stretch, record high for the month, and all-time metrological winter maximum high temperature Tables of successive warm days were produced for several sites in the eastern United States including Boston, Pittsburgh, and Philadelphia. These sites all had a long stretch of warm weather (Tables 4-8). It is interesting to note that Boston, MA, Pittsburgh, PA, State College, PA, and Philadelphia all had extended warm periods in 1930 and Other years that appear when examining mean high temperatures over 6 and 7 day periods include 1930, 1954, and 2017 at most locations. This suggests a similar pattern is required to create long term warm episodes in the cold season. The daily data and monthly running totals are shown in Figure 9. These images were plotted from the data shown in Table 1. Very few records were set during the first week of the month. Then, a warm episode developed between 7 and 13 February with brief break in the record warmth around February. The second surge of warm dominated the period of February before a late month cool down. During the peak period of prolonged warmth 170 to 504 records were broken every day. The twin warm episodes over the United States were focused over the eastern 2/3 of the country. The NCEI summary indicated record warmth from Texas to New York (Fig. 10) and this was the 2 nd warmest February on record, second only to the record warm February of Due to COOP site location the actual highs were on the days of 23 and 24 February.

5 5. Summary A record breaking warm episode affected much of the eastern United States from 18 to 25 February The peak warmth in the eastern United States and Mid-Atlantic region was focused on February On the 24 th and 25 th of many sites in the east set record high temperatures for the month of February and some sites achieved the all-time high for meteorological winter. Despite the record and the prolonged period of warmth, the pattern over much of the eastern United States was not that extreme. The 500 hpa height and 850 hpa tempature anomalies were ranges 1 to 2σ and 2 to 3σ above normal. Thus relative to the record warm episode of March 2012 the pattern was not overly remarkable. However, the 2m temperatures and 2m temperature anomalies were more impressive in the CFSR and the GFSBC forecasts. However, both were too cold relative to the observed conditions. The estimation of the GFS and CFSR 2m temperatures suggests they can capture the sense of an extreme event but miss the true impact and magnitude of the event. This event clearly illustrates some of the limitations of using gridded datasets, specifically 2m temperature datasets, to forecasts and analyzes actual shelter level temperatures. The GEFSBC had a serious cold bias and the 00-hour GFS analysis had a cold bias relative to the 3km HRRR which in turn ran colder than observations. Getting at local ground truth for surface temperatures requires observations. For example, in State College, the HRRR temperatures ranges 70 to 72F during the afternoon while observed temperatures in State College ranged about 74 to 78F. The official climate observing site (STCP1) hit 74F while many local Davis weather stations ranged as high as 78F 3. The gridded 2m temperatures data under represented the temperatures reached on 24 February. Despite the poor 2m temperatures, there was value in these forecasts. First, the models and the analysis indicated that the temperatures at 2m and 850 hpa were going to be above normal and above normal for several days. They also showed (Fig. 8) that a cold front would end the event on 25 February with warmth lingering near the coastal plain and much colder air entering the Ohio Valley by 1800 UTC 25 February. The frontal character also indicated a sharp north-south front which would favor the potential for showers and thunderstorms. And there was an extensive severe weather event on 25 February in the eastern United States which included 2 tornadoes in Pennsylvania and Massachusetts. The Massachusetts tornadoes were the first ever known tornadoes in the State. The economic consequences were directly felt by ski resorts. All suffered from a lack of business as temperatures soared to record levels people generally enjoyed alternate non-winter activities. 3 KPASTA18 northwest of town peaked at 74.9F at 2:19PM EST or 1919 UTC.

6 And the warm persistent warm temperatures caused rapid snow melt forcing some ski resorts to close early due to a lack of coverage (Table 3). Locally, in State College the record period of above normal high temperatures closed the local ski resort Tussey Mountain to shut down on 25 February The economic impacts on fruit growers will take longer to evolve. The March 2012 event caused fruit trees to bloom early and subsequent cold weather devastated fruit trees from Michigan to Pennsylvania. Some States lost 50 to 90% of the apple and cherry harvests. It will be several months before the full extent of this early warmth on fruit trees is fully realized. The often asked question is was this warm episode due to climate change. The short answer is we do not have the data to definitively state that it was. In State College, February 2017 will likely end up being the 5 th warmest February on record. It will hold the record for the 6-day highest mean maximum temperature, and it set the all-time February high temperature record and the highest all-time observed winter maximum temperature. These kinds of records were broken at many other stations. However, some of the record warm periods have occurred before with 1930 and 1954 showing up at many sites. Thus there is a strong meteorological component to this event. That said, over time climate change predicts we should have more warm episodes and fewer extreme cold episodes which is supported by the data in Table 1. This implies there could be a climate change component to this event. Based on NOAA/NCEI data February 2017 was second only to February 1954 for warmth across the United States. Globally, February 2017 was the second warmest on February on record in the dataset, second only to February 2016 (NOAA/NCEI). The three season winter of Dec-Jan-Feb was warm across the United States. 6. Acknowledgements The tables of record highs was provided by Jeff Waldstreicher of the NWS. Facts and data about the event from a National and Regional perspective were discussed and shared on the Albany MAP. 7. References Grumm, R., J. M. Arnott, and J. Halblaub, 2014: The epic eastern North American warm episode of March J. Operational Meteor., 2 (4), PDF (1619 KB) DOI: 3/7/2014

7 Figure 1. The CFSR based 500 hpa pattern over the United States for the six day period from a) 0000 UTC 20 February 2017 through f) 0000 UTC 25 February Data include 500 hpa heights (m) and 500 hpa height anomalies in standard deviations from normal. Return to text.

8 Figure 2. As in Figure 1 except for 850 hpa temperatures ( C) and 850 hpa temperature anomalies every 24 hours from a) 1800 UTC 21 to 1800 UTC 26 February Return to text.

9 Figure 3. As in Figure 2 except for 2m temperature focused over the northeastern USA in 24 hour increments from a) 1800 UTC 20 to f) 1800 UTC 25 February Return to text.

10 Figure 4. NCEP GEFSBC data showing the forecasts of the 500 hpa heights and 500 hpa height anomalies valid at 1800 UTC 24 February 2017 from GEFS forecasts initialized at 0000 UTC from a) 23 February through f) 16 February Height contours every 60m and anomalies as in the color bar. Return to text.

11 Figure 5. As in Figure 4 except for 850 hpa temperatures and temperature anomalies. Return to text.

12 Figure 6. As in Figure 5 except for GEFSBC forecasts of 2m temperatures and anomalies valid at 1800 UTC 24 February Return to text.

13 Figure 7. 3km HRRR 00-hour forecasts of 2m temperatures initialized at 0000 UTC 24 February Values in degrees F as per shading and contours every 10F. Return to text.

14 Figure 8. As in Figure 6 except for the 850 hpa temperatures and anomalies valid at 1800 UTC 25 February Note panel c has a 0600 UTC forecast. The 0000 UTC forecast for this time period was missing in our grib archive. Return to text.

15 7000 February 2017 Running total of maximum temperature records Ties Records Sums February 2017 Daily maximum temperature records Tie Records sum Figure 9. Plots of the daily records tied or broken and the sum of these quantities. The upper image shows the running totals and the lower image shows the daily records set or tied. Return to text.

16 Figure 10. NCEI plot of February 2017 mean temperature departures from normal. Data are in the color bar along the bottom of the image. Data courtesy of NOAA. Return to text.

17 Date Tie Records sum Ties Records Sums Table 1. List of record tied, broken, and total of both quantities by day of the month during February The monthly running totals are shown in the 3 rightmost columns. Return to text.

18 Table 2. Record highs at METAR sites on 24 February. Courtesy of Jeff Waldstreicher. Return to text.

19 Table 3. As in Table 2 except METAR site records set on 25 February Return to text.

20 Table 4. A List of the mean maximum high temperatures for State College, PA showing the rank and the mean high temperature over the 6-day period. Tables need to be re-accomplished at the end of Feb Return to text.

21 Table 5. As in Table 4 except for Boston, MA. Return to text. Table 6. As in Table 4 except for Philadelphia, PA. Return to text.

22 Table 7. As in Table 4 except for Pittsburgh, PA. Return to text.

23 Record Low Sea ice in NH