The Big Chill of November 2013

Transcription

1 The Big Chill of November 2013 Value of anomalies for situational awareness By Richard H. Grumm National Weather Service State College, PA Trevor Alcott National Weather Service, Salt Lake City UT 1. Overview A fast moving trough deepened as it moved over eastern North America (Fig.1) November Cold air at 850 hpa (Fig. 2) suggested that the air mass was associated with pool of cold air in the -20 to -24C range as it moved across Minnesota, Wisconsin and eventually over New York. The 850 hpa temperature anomalies were on the order of -2 σ below normal with this pocket of cold air. The 850 hpa temperature anomalies were on the order of 3σ below normal over the Mid-Atlantic region (Fig.2d-e) where the 850 hpa temperatures were around -16C. At the surface, a strong anticyclone (Fig. 3) moved out of Canada on 23 November (Fig. 3a) reaching the southeastern United States on 25 November (Fig. 3f). The anticyclone had pressure anomalies on the order of -2 to -4σ as it moved across the Mid-West and a central pressure in excess of 1050 hpa with a closed 1048 hpa contour (Fig. 3b). In the strong gradient east the high pressure there were strong winds (Fig.4) which contributed to wind chill effects and lake effect snows downwind areas of the Great Lakes. The cold air produced several days where daytime high temperatures in many locations approached, tied, or set new record low daily high temperatures (Table 1). The low-high temperatures suggest that this was one of the coldest air masses to affect portions of the eastern United States in years. This paper will document cold episode of November The focus on the impacts is related to the traditional use of standardized anomalies and some of the limitations in using these data to identify extremely anomalous stations. The concept of using return periods and percentiles to compliment standardized anomalies is employed as a means to assess the potential of a HIWE. 2. Data and Methods The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global Forecast System (GFS) as first guess at the verifying pattern. The standardized anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter 1995). The standardized anomalies and the probability distribution functions are based on the re-

2 analysis climate (R-Climate). Though not shown here, they could be produce from internal model system climatologies (M-Climate). The traditional standardized anomalies were produced from the GFS 00-hour forecast using Climate Forecast System based means and standard deviations. The climatology spans a 30 year period. This 30-year period was used in the situational awareness (SA) tables to show the return period of the anomalies. The forecasts in the SA tables were derived from the 42 member North American Ensemble Forecast System (NAEFS). These data are displayed using the NAEFS mean fields relative to the climate. In addition to standardized anomalies, the return period and interval of the anomalies are produced relative to the 21-day centered values. The return period is in years such that a 30-year return period represents the extreme in the climatological data set. The percentile data represent where along the climatological probability distribution the current NAEFS forecast lies. Extremes in the tails would be in the 97 to 100 percentile (MAX) or in the 3 to 0 th percentile (MIN). In addition to finding the MAX and MIN relative to the time, the MAX and MIN are computed using the 4 climatological times of 00, 06, 12, and 18 UTC. This provides the ability to estimate the MAX (MIN) at the specified time or for all 4 times in the same 21-day centered climatological window. Temperature records were obtained from the National Climatic Data Center (NCDC). 3. Pressure Anomalies for record anticyclone The larger scale pattern was shown in Figures 1 to 4. These data showed the developing 500 hpa trough and the cold air at 850 hpa which moved over the eastern United States providing a period of unseasonably cold weather. Near the surface, the GFS and CFSR analyzed an intense surface anticyclone with mean sea-level pressure values between 1048 and 1051 hpa implying an intense surface anticyclone. The pressure anomalies near the anticyclone center were analyzed around +3s in the GFS (Fig. 3) and the CFSR (not shown). The mean sea-level pressure data for a point near Fargo, North Dakota was extracted from the CFSR 10 days either side 24 November. These data show that the hpa pressure near Fargo was the highest pressure recorded in the CFSR 21-day window since 1979 (Table 2). Despite being the record high pressure for the period, the pressure anomaly was only +3.14σ above normal. Though not shown, in this same window of time the record low pressure was hpa with a standardized anomaly of -3.21σ. A plot of the probability distribution function of the pressure and the cumulative distribution is shown in Figure 5. These data show that data is skewed to the left of the mean pressure (1018 hpa). The standardized anomalies show a similar distribution (Fig. 6). The raw pressure value as analyzed and the anomaly suggested that this was the strongest surface anticyclone to move through eastern North Dakota since 1979 in this 21-day centered time window.

3 The 850 hpa temperature at Fargo, ND was C which was -2.28σ below normal. This was the 26 th coldest 850 hpa temperature for the 21-day period but when sorted by anomaly value, it was the sixth most negative 850 hpa temperature anomaly near Fargo for the 21-day centered period (Table 3). The NAEF forecast of MSLP at 23/0600 and 23/1200 UTC over the northern plains showed the Date Broken Records Ties Sum Cumulative Sum Total Possible Locations 11/22/ West 11/23/ Southwest; TX, OK, KS 11/24/ Great Lakes and South 11/25/ South, Central, and Northeast Table 1. Preliminary data showing the record low high temperature records tied or broken and the number of stations available for comparison. Date Broken Records Ties Sum Cumulative Sum Total Possible Locations 11/22/ Northwest/Central US 11/23/ Northwest/ North Central 11/24/ Midwest 11/25/ Midwest and Mid-Atlantic Table 2 as in Table 1 except for record low temperatures. Return to text. large anticyclone over the region and the area +3 to +4σ pressure anomalies. As shown in Fig. 8, the pressure over 1048 hpa and anomalies over +3σ values corresponded to a 30-year return period in the NAEFS. These data imply that the NAEFS did credible job predicting a strong anticyclone with near record high pressure and pressure anomalies. The data in Figure 8 show that the NAEFS SLP anomalies in the +3.3 to +3.3 ranges had a return period of 30 years in the 30 year CFSR data. Additionally, the brighter reds indicated that the record was relative to all4 synoptic hours. The NAEFS clearly and correctly predicted a record anticyclone over the northern Plains. In addition to the pressure anomalies, the NAEFS was predicting cold temperatures and near record values for the time of year. The -2.4 to -2.8s temperature anomalies had a return period on the order of 30 years. The corresponding forecast of the temperatures (Fig. 9) showed anomalies in the -2 to -3 range which corresponded remarkably well with the data in Table Forecasts over eastern United States

4 .As the anticyclone moved eastward, the NAEFS did not predict as strong or as anomalous a system as it did over the northern Plains. However, the NAEFS did predicted some extreme temperature values (Fig. 10) with the return period of the temperatures at several time periods and levels (not shown) to be in the 30 year period. The data in Table 10 also show the hidden uncertainty information often contained in standardized anomalies when computed using an ensemble forecast system. As the forecast length decreased and the solutions converged the system predicted larger anomalies and more periods with record return periods. The 1200 UTC 22 November 2013 NEAES temperatures valid at 1200 UTC 24 November 2013 are shown in a plan view perspective. These data show that the largest return periods at lower levels were displaced into the Mid-Atlantic region and that at 500 hpa the air was extremely cold over New York, New England, and Quebec. Not surprisingly, on Sunday 24 November 2013 many locations in the northeast set or tied record low high temperatures for the date. The 850 hpa temperature data for Richmond, VA showed (Table 5) that the CFSR record a σ and -3.17σ 850 hpa temperature anomalies at 1200 and 1800 UTC on 24 November The only other time that -3.0 σ were record in this 21-day centered window was on 22 November 1987 and 24 November The cold over the Mid-Atlantic region as observed and forecasts was an extremely rare event (Fig. 13). The cumulative distribution (Fig. 13) of 850 hpa temperature anomalies for Richmond, VA is highly skewed with a long tail in the negative range. The frequency distribution shows several peaks to the right of mean (0.10). This implies periodic intrusions of extremely cold air into the region. The high number of events to the right of the mean show that the region is susceptible to intrusions of warm air but warm air with +1.8σ or greater anomalies is not very common.. The mean anomaly for the period is 0.10 and the 21-day centered mean 850 hpa temperature is 4.19C. The forecasts by the NAEFS and the analyzed values by the CFSR suggested that 24 November 2013 was an atypically cold day in Virginia. 5. Summary A strong trough and surface anticyclone brought an early season cold episode to much of the United States east of the Rocky Mountains. The deep trough and deep cold air were accompanied by a strong surface anticyclone. The strong anticyclone produced mean sea-level pressure values near 1050 hpa and based on the CFSR, implied many regions of the northern plains likely experienced both sea-level pressure values and pressure anomalies which occur about once every 30 years. The pressure data from the CFSR (Table 2) and the plots of the PDF and CDF imply that the anticyclone which tracked across eastern ND and MN was a record event for the 21-day window centered on 24 November. The NAEFS forecast data indicate that the NAEFS was able to predict a record event in close proximity the region where such a record event occurred. In addition to

5 the massive anticyclone and record surface pressure, the NAEFS also correctly predicting the intrusion of cold to near record cold air over the region. In the core of the cold air mass, many locations saw a once in 30 year event. The data for Richmond showed that Richmond and likely other parts of the Mid-Atlantic region had the lowest 850 hpa temperatures on record in the past 30 years. The C reading was the coldest record in this window and was associated with the most negative standardized anomaly. Though not shown, shorter range NAEFS 850 hpa temperatures forecast the -16C contour to reach the Richmond area. The more general forecasts of -12 to -14C at 48 to 72 hours in advance present successful forecasts of a cold episode. The data in Tables 1 & 2 show that the impact of the cold peaked on the 24 th and 25 th of November Part of this is related to temperature data collected at 1200 UTC at the end of the 24 hour. In the south and east the cold peaked between 1200 UTC 24 and 1200 UTC 25 November 2013 (Fig. 14). This case demonstrates the value of utilizing the entire PDF in the climate relative to the forecast to better quantify the potential for a record or near record synoptic weather event. 6. Acknowledgements The Pennsylvania State University for gridded data access and the Mid-Atlantic River forecasts and Trevor Alcott for return period data and images. Elyse Colbert for NCDC data extraction and summations. 7. References Associated Press, 2013a: More than 80 evacuated in Louisville floods and similar stories 6-7 October Associated Press, 2013b: South Dakota: Cattle Loss from Blizzard Estimated above 10,000 and similar stories 6 to 14 October Bodner, M. J., N. W. Junker, R. H. Grumm, and R. S. Schumacher, 2011: Comparison of atmospheric circulation patterns during the 2008 and 1993 historic Midwest floods. Natl. Wea. Dig., 35, [Abstract] Coniglio, M. C., D. J. Stensrud, and M. B. Richman, 2004: An observational study of derecho-producing convective systems. Wea. Forecasting, 19, Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS. Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H. Bredekamp, NASA, Washington, D.C.,

6 Graham, R.A, T. Alcott, N. Hosenfeld, and R. Grumm, 2013: Anticipating a rare event utilizing forecast anomalies and a situational awareness display: The Western Region US Storms of January 2010, BAMS, Graham, R A. and R. H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale Weather Events in the Western United States. Wea. Forecasting, 25, Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16, Grumm, Richard H., 2011a: The Central European and Russian Heat Event of July August Bull. Amer. Meteor. Soc., 92, Grumm, R.H. (2011b) New England Record Maker rain event of March NWA. Electronic Journal of Operational Meteorology, EJ4 12: Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp Junker, N. W., R. H. Grumm, R. Hart, L. F. Bosart, K. M. Bell, and F. J. Pereira, 2008: Use of standardized anomaly fields to anticipate extreme rainfall in the mountains of northern California. Wea. Forecasting,23, rainbands; J. Atmos. Sci., Vol. 41, p Stuart, N.A and R.H. Grumm 2006: Using Wind Anomalies to Forecast East Coast Winter Storms. Wea. and Forecasting,21,

7 DATE SLP SA 12Z23NOV Z23NOV Z23NOV Z21NOV Z24NOV Z20NOV Z14NOV Z14NOV Z23NOV Z29NOV Z29NOV Z02DEC Z20NOV Z27NOV Z02DEC Z03DEC Z26NOV Z29NOV Z14NOV Z21NOV Z22NOV Table 2. CFSR mean sea-level pressure centered on 24 November showing the date, highest pressure and associated standardized anomaly. Return to text. DATE Temp ( C) SA 00Z29NOV Z29NOV Z20NOV Z01DEC Z28NOV Z24NOV Z20NOV Z23NOV Z30NOV Z24NOV Z17NOV Z25NOV Z23NOV Z20NOV Z29NOV Z27NOV Z18NOV Z16NOV Z23NOV Z24NOV Z16NOV Table 3. As in Table 1 except for 850 hpa temperatures and temperature anomalies. Data sorted by anomalies in ascending order. Return to text.

8 Figure 1. GFS 00-hour forecasts of 500 hpa heights and standardized anomalies in 24 hour increments from a)_ 1200 UTC 20 November 2013 through f) 1200 UTC 25 November Heights every 60 m and anomalies in standard deviations as in the color bar. Return to text.

9 Figure 2. As in Figure 1 except for 850 hpa temperatures ( C) and temperature anomalies in 6-hour increments from a) 0000 UTC 23 November through f) 1200 UTC 25 November Isotherms every 2C. Return to text.

10 Figure 3. As in Figure 4 except for mean sea-level pressure (hpa) and pressure anomalies. Isobars every 4 hpa. Return to text.

11 Figure 4. As in Figure 3 except for 850 hpa winds and wind anomalies Winds in knots. Return to text.

12 Figure 5. Distribution of CFSR mean sea-level pressure for a point near Fargo, ND centered 10 days either side of 24 November spanning Upper panel shows the cumulative distribution and the lower panel s shows the frequency of pressures in 1 hpa bins. Return to text.

13 Figure 6. As in Figure 5 except for the standardized anomalies near Fargo, ND. Return to text.

14 Figure 7. NAEFS forecasts of MSLP and anomalies from 1200 UTC 20 November NAEFS showing 66 and 72 hour forecasts valid at 0600 and 1200 UTC 23 November Return to text.

15 Figure 8. The 1200 UTC 20 November NAEFS anomalies over the region shown in Figure 7 showing the largest, by absolute value, anomaly, by variable for the valid times listed on the left side of each image. The left table shows the standardized anomalies and the right table shows the return period of the anomalies. The bright red values show the largest return period in the 21-day period while the darker reds show the larges return period for the 21-day period at the synoptic 6-hour time. Return to text.

16 Figure 9. As in Figure 7 except for the NAEFS temperature anomalies valid at 1200 UTC 23 November showing the isotherm and standardized anomalies at 1000, 850, 700, 500, and 250 hpa. Return to text.

17 Figure 10. As in Figure 8 except showing the return period of anomalies over the northeastern United States from the NAEFS initialized at 0000 UTC 20, 21 and 22 November Return to text.

18 Figure 11. As in Figure 9 except for initialized at 1200 UTC 22 November valid at 1200 UTC 24 November 2013 showing temperatures and return period of the anomalies over the northeastern United States. Return to text.

19 Figure 12. As in Figure 11 except showing the traditional standardized anomalies. Return to text.

20 Date 850 Temp SA 12Z24NOV Z24NOV Z22NOV Z04DEC Z21NOV Z04DEC Z22NOV Z21NOV Z19NOV Z22NOV Z22NOV Z22NOV Z19NOV Z15NOV Z22NOV Z24NOV Z24NOV Z22NOV Z22NOV Z20NOV Table hpa temperatures and temperature anomalies for a point near Richmond, VA. Sorted by standardized anomalies lowest to highest showing 20 smallest anomalies. Return to text.

21 Figure 13. As in Figure 5 except for the distributions of 850 hpa temperatures anomalies at Richmond, VA centered on 24 November. Return to text.

22 Figure 14. Plots of the record lows and record low high temperatures set from 22 to 25 November Data from NCDC and used in Tables 1 & 2. Return to text.

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