The abrupt end of the Persistent Subtropical Ridge of the autumn of 2016

The abrupt end of the Persistent Subtropical Ridge of the autumn of 2016 1. Introduction by Richard H. Grumm National Weather Service State College, PA 16803 Contributions by Elyse Hagner National Weather Service Morristown,TN A persistent subtropical ridge dominated the pattern over southern United States and the Gulf of Mexico during the autumn of 2016. This feature led to 3 months of above normal 500 hpa heights over much of the eastern United States (Fig. 1a) and above normal 850 hpa temperatures over a similar region (Fig. 1b). In the mean, the surface pressure was close to normal with a weak ridge focused over the southeastern United States (Fig. 1c). This feature kept deep moisture and high precipitable water (PW) air out of the southern United States (Fig. 1d). There was some return flow moisture over the western plains and into central Canada where the PW values were above normal during the 3-month period. The impact of the ridge from 1 September 2016 through 28 November 2016 (Fig. 2a) was a dearth of precipitation in the southeastern United States. The data is shown minus the last 2 days of November 2016 to highlight the dry conditions in the southeastern United States prior to the abrupt change. The data for the full 3-month period (Fig. 2b) reveals a significant change which produced heavy rainfall over a significant portion of the dry region in the southeastern United States. This rainfall event at the end of the month had the unfortunate impact of producing strong winds which spread fires in Tennessee destroying significant portion of Gatlinburg, TN. The severe weather and heavy rains closer to frontal passage put an end to the fires and dramatically changed the rainfall over most of the region. The strong frontal system of 28-30 November 2016 was notable for the strong winds ahead of the frontal system, the multi-day severe weather event it triggered, and the heavy rainfall which put a significant dent in the rainfall deficit over the region. This paper will document the odd convergence of conditions which produced the southeastern drought, the devastating fires, and the heavy rainfall. 2. Methods and data The climate forecast system re-analysis (CFSR) data was used to reconstruct the pattern and the standardized anomalies associated with the event. The CFSR was used to create composites for the season and for the months. The compositing required computing the mean and the mean of the departures in 6-hour increments covering the range. The primary ranges used included 0000 UTC 1 September through 1800 UTC 30 November, times for individual months, and ranges ending up to the time of the heavy rainfall and severe weather. Individual times were also produced during the key times of the rapidly spreading fires and the severe weather. 1

The Stage-IV rainfall data (Seo 1998) was used to estimate the rainfall over several 6, 12 and 24, monthly, and case related discrete times to show the impact of the pattern changing rainfall event. The rainfall pattern also revealed some interesting information about the southeastern drought and how both meteorologically and climatologically significant the precipitation event of 28-30 November 2016 was for the southeastern United States. Forecasts from the NCEP GEFS and NCAR 3km are ensemble are used to illustrate how well the change in the pattern occurred. The strong winds ahead of the front had a devastating impact on fires in the southern Appalachian Mountains and the rainfall but a significant dent in the rainfall deficit. The GEFS and NCAR EFS both performed well for this significant high impact weather event. 3. Results a. The Large scale pattern The large scale pattern over North America during the autumn of 2016 (Fig. 1) showed the persistent ridge over southern North America and the Gulf of Mexico. This feature remained quite strong and persistent during the month of November 2016 (Fig. 3). The key features and anomalies persisted over the month despite the dramatic change in the pattern (Fig. 4) which brought lower heights and colder temperatures into the south-central United States. It also produced a plume of deep moisture across the southeastern United States. This system clearly was a game changing weather event over much of the eastern United States. b. Regional Pattern The regional pattern focus is on the frontal system and the strong winds ahead of the front. The 850 hpa winds in 6-hour increments from 1200 UTC 28 to 1800 UTC 29 November (Fig. 5) show the strong low-level jet ahead of the front. Winds in excess of 40 ms -1 were observed over Tennessee at 0000 UTC 29 November 2016 (Fig. 5c). The v-wind anomalies at the same time (Fig. 6) indicated 3 to 4σ above normal southerly winds in the region. The observations in Knoxville, TN (Appendix I) showed strong winds and frequent wind gusts of 30 to 43kts during the afternoon hours of 28 November 2016. Ahead and along the frontal boundary (Fig. 7) there was a surge of high PW air. The PW anomalies peaked at around +2 to +3σ above normal just ahead of the frontal system (Fig. 8). The 850 hpa temperature change was not remarkable ahead of the first front and was more like a dry surge than a strong cold front. Examining the 850 hpa temperatures in 12 hour increments reveals that a second stronger frontal boundary pushed through the region on 1 December 2016 (Fig. 9). It will be shown that this second system produced a widespread severe weather event and produced heavy rainfall over the region. c. Rainfall and severe weather The rainfall displayed in 12 hour increments from 1200 UTC 29 to 1200 UTC 01 December 2016 are shown in Figure 9. The period of heaviest rainfall was the period of 1200 UTC 30 November through 0000 UTC 1 December 2016 (Fig. 9e). These data show the rainfall entering the central Tennessee for the period ending at 0000 UTC 29 November 2016. 2

The total rainfall for this period (Fig. 10) showed areas of over 128 mm of rainfall in eastern Tennessee with a broad region of over 32 mm covering all but western Tennessee. The surge of the two fronts brought a heavy rainfall event to the region putting a significant dent in the rainfall deficit. This rainfall deficit at 3 observations locations was in excess of 10 inches (Table 1) leading up the event. In addition to the winds ahead of the front and the rainfall, the strong frontal system produced severe weather (Fig. 11). The 3-day event produced mainly severe convective winds on 28 November, a significant tornado event was observed on 29 November with at least 56 reported tornadoes including 3 deadly tornadoes in Alabama 1 and one deadly tornado in Tennessee. The third day of the event included a mix of severe thunderstorms and tornadoes from eastern Alabama into North Carolina (not shown) with 9 tornadoes and 44 severe wind reports. d. Forecasts The GEFS forecast the passage of a strong low-level southerly jet over Tennessee on between 1800 UTC 28 to 0600 UTC 29 November 2016 (Fig. 12-14). The 850 hpa winds and v-wind anomalies are shown for the periods of 0000 UTC and 0600 UTC 29 November when the peak winds were forecast to move across eastern Tennessee. Clearly, shorter range forecasts had stronger winds and larger v-wind anomalies, on the order of 4 to 5σ above normal. The 925 hpa winds peaked around 0000 UTC (Fig. 14) and farther to the west than the 850 hpa winds. The GEFS had clear signals for a strong low-level jet and the potential for strong and gusty winds with several days of lead time. The GEFS QPFs (Fig. 15-16) showed the potential for 25 to 50 mm rainfall event over the 48 hour period from 1200 UTC 29 November through 1200 UTC 01 December 2016. These numbers are low relative to the observed rainfall over this period. They do however reflect a significant rainfall event in what was a drought stricken area. Despite the low QPF this was still a significant QPF event with the potential to produce a fire squelching rainfall. The potential for strong winds at or near the surface was examined using the NCAR 3km ensemble forecast system. The probability of hourly maximum wind speed of greater than 20 ms-1 is shown from the 0000 UTC 28 November forecast cycle. These data suggest the increased threat of 20 ms-1 winds over eastern Tennessee and the southern Appalachians by 1600 UTC 28 November, peaking near 0600 UTC 29 November before diminishing by 1300 UTC 29 November 2016 (Fig. 15). e. Impacts The synoptic period of high winds occurred on 28 November 2016. The most representative METAR site was Knoxville (KTYS) which had high winds during the afternoon and evening hours of 28 November. The winds peaked at 43kts and there were several hours with wind gusts over 30kts. Other of the METAR sites (not included here) also had high winds on 30 November; however convection was coincidence with the high winds on 30 November 2016. 1 Based on the Storm Prediction Center text products associated with the storm reports. 3

The impact of the high winds on the 28 th of November was to rapidly spread the fires in eastern Tennessee. The rapidly spreading Chimney Tops 2 fire produced massive destruction in and around Gatlinburg, TN (CNN 2016). There were numerous other fires. The fires led to at least 14 fatalities, destroyed over 44,000 acres of woodlands, and destroyed entire neighborhoods in and around Gatlinburg and other lesser known towns. The fires were the result of dry conditions and people setting fires which either escaped them or were deliberated set. The dry conditions, the fires, and then the high wind event resulted in regional disaster. As of 9 December 2016 the Chimney Tops 2 fire was about 90% contained and had not been completely extinguished by the rain. The Chimney Tops 2 fire was likely resulted in 14 deaths and the destruction of over 700 homes and other structures. The drought had significant agricultural impact. The dry conditions limited hay production causing a regional hay shortage and has reduced the availability of water for animals and other activities. The heavy rain on 29 and 30 November led to flooding in portions of Alabama. The rains also aided in controlling and extinguishing most of the fires. The severe storms on 29-30 November produced 10 fatalities and multiple injuries. Most of the strong tornado activity was in Alabama and portions of Tennessee. The pattern changed dramatically in the southeastern United States. This change produced a meteorologically and climatologically significant high wind event, rainfall event, and severe weather event. The winds caused rapid spreading fires which likely contributed to the impact of this event.. 4. Conclusions The autumn of 2016 was very warm and dry over the southeastern United States (Fig. 1) due to the persistent ridge which extended from the Gulf of Mexico into central Canada. This feature dominated until about 28 November 2016 (Fig. 3) when there was a relatively abrupt pattern change (Fig. 4). The strong frontal system with this abrupt change produced strong winds in the warm sector which rapidly spread fires in eastern Tennessee 28-30 November 2016. The strong front triggered severe weather and tornadoes, and a heavy rainfall. The strong frontal system which caused the high winds, heavy rainfall and the severe weather event was relatively well predicted by the NCEP GEFS. With at least 4 days of lead-time the GEFS showed both the potential for strong winds over the southern Appalachians and a multiday rain event which was forecast to produce 25 to 50 mm or rainfall. Observations imply the GEFS underestimated the total rainfall. But the GEFS had a useful signal for both events. Though not shown, the GEFS also provided clues with high PW air, high CAPE, and shear for a severe weather event. 4

Parameter/Threshold Tri-Cities/Bristol-Johnson City Area (KTRI) Knoxville Area (KTYS) Chattanooga Area (KCHA) Sept observed 2.57 1.42 1.63 Sept normal 2.99 3.24 4.04 Oct observed 0.74 0.18 0.08 Oct normal 2.10 2.51 3.28 Nov month to date observed as of midnight 11/27 0.19 0.27 0.06 Nov normal month to date as of midnight 11/27 2.74 3.53 4.42 Sept-Nov observed as of midnight 11/27 3.50 1.87 1.77 Normal Sept-Nov as of midnight 11/27 7.83 9.28 11.74 Oct-Nov observed as of midnight 11/27 0.93 0.45 0.14 Normal Oct-Nov as of midnight 11/27 4.84 6.04 7.70 Year to date observed as of midnight 11/27 27.66 32.69 25.11 Normal year to date as of midnight 11/27 37.28 42.88 47.00 Current (as of midnight 12/4) year to date 31.11 39.52 31.95 Normal (as of midnight 12/4) year to date 38.10 43.98 48.29 Table 1. List of rainfall amounts (inches) at 3 locations in eastern Tennessee. These data show the significant rainfall deficit at each location. Each location was running a deficit of 10 or more inches through 27 November 2016. The data also show that as of 4 December each location had made up at least 4 inches of the rainfall deficit. Shorter range forecasts from the NCAR 3km ensemble showed that this ensemble forecast system provided incredibly detailed, timely, and accurate information related to the high wind event. These high wind forecasts were in the correct time window and focused over the region where the fires were present. This historic high wind, fire, rainfall, and severe weather event over the southern Appalachians was the result of an odd convergence of circumstances. These included the persistent ridge from September into late November. This feature reduced the rainfall and produced drought conditions. A strong trough associated with a pattern change of North America brought strong winds into the dry region. The winds accelerated fire growth. The frontal system eventually brought moderate to heavy rainfall to the region. The rainfall put a significant dent in the rainfall deficits and extinguished many of the fires 2. And for good measure the stronger of two fronts produced a significant severe weather event which included deadly tornadoes. 5. References Galarneau, T. J., Jr., L. F. Bosart, and A. R. Aiyyer, 2008: Closed anticyclones of the subtropics and midlatitudes: A 54-yr climatology (1950 2003) and three case studies. Synoptic Dynamic Meteorology and Weather Analysis and Forecasting: A Tribute to Fred Sanders, Meteor. Monogr., No. 55, Amer. Meteor. Soc., 349 392. 2 As of 9 December the Chimney tops 2 Fire was 91% controlled. 5

Figure 1. The composite pattern and the anomalies in the pattern over the 3 month period from 1 September 2016 to 30 November 2016. Data include the a) 500 hpa heights, b) 850 hpa temperatures, c) mean sea-level pressure, and d) precipitable water. Return to text. 6

7 Figure 2. Upper panel shows the total QPE from the 6-hourly Stage-IV data from 0000 UTC 1 September 2016 through 1200 UTC 28 November 2016. The lower panel ends at 1800 UTC 30 November. Return to text.

Figure 3. As in Figure 1 except for the period of 0000 UTC 1 November through 0000 UTC 1 December 2016. Return to text. 8

Figure 4. As in Figure 3 except the mean pattern for the period of 1200 UTC 29 November through 1200 UTC 1 December 2016. Return to text. 9

Figure 5. The CFSR 850 hpa winds for the 6-hour periods from a) 1200 UTC to f) 1800 UTC 29 November 2016. Return to text. 10

Figure 6. As in Figure 5 except for 850 hpa winds and v-wind anomalies. 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 7 except for precipitable water and precipitable water anomalies. Return to text. 13

Figure 9. As in Figure 7 except for 850 hpa temperatures every 12 hours from a) 0000 UTC 29 November through f) 1200 UTC 01 December 2016. Return. 14

Figure 10. As in Figure 9 but the total QPE for the period from 1200 UTC 28 November through 1200 UTC 01 November 2016. Values as in the color bar. Contours in powers of 2. Return to text. 15

Figure 11. Storm Prediction Center storm reports by time for the 28 th and 29 th of November 2016. Return to text. 16

Figure 12. GEFS forecasts of 850 hpa winds (ms-1) and v-wind anomalies valid at 0000 UTC 29 November 2016. Forecasts initialized at a) 1200 UTC 25 November, b) 0000 26 November, c) 1200 UTC 26 November, d) 0000 UTC 27 November, e) 1200 UTC 27 November, and f) 1200 UTC 28 November 2016. Return to text. 17

Figure 13. As in Figure 12 except valid at 0600 UTC 29 November 2016. Return to text. 18

Figure 14. As in Figure 12 except valid at 925 hpa winds and v-wind anomalies. Return to text. 19

Figure 15. As in Figure 12 except for the GEFS potential for 50 mm or more QPF for the 48 hour period ending at 1200 UTC 1 December 2016. Return to text. 20

Figure 16. As in Figure 15 except for the ensemble mean precipitation and each member s 50 mm contour if forecast. The thick black contour is the mean 50 mm contour from the ensemble. Return to text. 21

Figure 17. NCAR 3km ensemble initialized at 0000 UTC 28 November 2016 showing the probability of the hourly maximum wind speed exceeding 20 ms-1 within 25 miles of a grid point. The times show the first threat time at 1600 UTC 29 November, the height of the event at 0000 and 0600 UTC and the end of the forecast event at 1200 UTC. Images courtesy of NCAR. Return to text. 22

Appendix I: Knoxville Observations during the synoptic high wind period: PK WND dddff(f)/(hh)mm 2016-11-29 02:48 METAR KTYS 290748Z 23008KT 4SM RA BR SCT008 OVC043 14/12 A2973 RMK AO2 PRESRR P0024 $ = 2016-11-29 02:29 METAR KTYS 290729Z 27012G18KT 3SM +RA BR OVC007 14/13 A2971 RMK AO2 P0019 T01390128 $ = 2016-11-29 02:10 METAR KTYS 290710Z 28010KT 2 1/2SM +RA BR BKN008 BKN014 OVC046 14/13 A2971 RMK AO2 P0008 T01440128 $ = 2016-11-29 02:07 METAR KTYS 290707Z 28014KT 3SM +RA BR SCT008 BKN014 OVC046 14/13 A2971 RMK AO2 P0005 T01440128 $ = 2016-11-29 01:53 METAR KTYS 290653Z 29015KT 10SM -RA FEW031 OVC047 16/13 A2969 RMK AO2 PRESRR SLP048 P0004 T01560128 $ = 2016-11-29 00:53 METAR KTYS 290553Z 20010G22KT 7SM -RA SCT049 OVC060 17/12 A2965 RMK AO2 PK WND 19030/0524 RAB25 SLP032 P0003 60009 T01670117 10200 20161 55004 $ = 2016-11-28 23:53 METAR KTYS 290453Z 19019G33KT 10SM FEW055 SCT075 SCT110 OVC250 18/10 A2960 RMK AO2 PK WND 19033/0451 RAE26 SLP020 P0000 T01780100 402000033 $ = 2016-11-28 22:53 METAR KTYS 290353Z 17018G29KT 10SM -RA SCT055 BKN090 BKN110 OVC250 16/11 A2965 RMK AO2 PK WND 18038/0254 SLP036 P0006 T01610111 $ = 2016-11-28 21:53 METAR KTYS 290253Z 19030G42KT 8SM -RA BKN050 BKN070 OVC090 18/11 A2966 RMK AO2 PK WND 18043/0220 RAB36 SLP039 P0000 60000 T01780106 53003 = 2016-11-28 20:53 METAR KTYS 290153Z 18021G38KT 10SM BKN065 OVC150 19/08 A2962 RMK AO2 PK WND 16042/0124 SLP026 T01940078 = 2016-11-28 19:53 METAR KTYS 290053Z 18025G32KT 10SM FEW065 BKN110 OVC250 19/07 A2964 RMK AO2 PK WND 17032/0053 SLP033 T01940072 = 2016-11-28 18:53 METAR KTYS 282353Z 18016G31KT 10SM FEW060 SCT120 OVC250 19/07 A2965 RMK AO2 PK WND 20031/2345 SLP038 T01890067 10200 20167 56039 = 2016-11-28 17:53 METAR KTYS 282253Z 15006KT 10SM FEW060 BKN120 OVC250 17/06 A2968 RMK AO2 PK WND 14026/2159 SLP047 T01720056 = 2016-11-28 16:53 METAR KTYS 282153Z 16020G33KT 10SM FEW060 BKN150 OVC250 18/06 A2972 RMK AO2 PK WND 18033/2144 PRESFR SLP060 T01780056 = 2016-11-28 15:53 METAR KTYS 282053Z 17018G27KT 10SM FEW060 BKN150 BKN250 18/05 A2977 RMK AO2 PK WND 18036/2011 SLP078 T01780050 56031 = 2016-11-28 14:53 METAR KTYS 281953Z 17021G28KT 10SM FEW060 SCT150 BKN250 19/06 A2979 RMK A02 PK WND 16038/1943 SLP084 T01890056 = 2016-11-28 13:53 METAR KTYS 281853Z 18020G34KT 10SM FEW050 SCT170 BKN250 18/06 A2983 RMK AO2 PK WND 18036/1841 SLP097 T01780061 = 2016-11-28 12:53 METAR KTYS 281753Z 20010KT 160V220 10SM FEW049 SCT170 OVC250 17/04 A2986 RMK AO2 SLP110 T01720039 10172 20039 58040 = 2016-11-28 11:53 METAR KTYS 281653Z 02005KT 8SM FEW049 BKN140 BKN200 11/01 A2993 RMK AO2 SLP134 T01110006 = Convective High Winds 30 November: 2016-11-30 05:53 METAR KTYS 301053Z 21016G28KT 3SM VCTS +RA BR SCT040CB BKN070 OVC100 16/14 A2976 RMK AO2 PK WND 27045/1026 WSHFT 1017 LTG DSNT ALQDS RAB0954E01B21 TSB23E52 SLP069 FRQ LTGIC VC N-E-S TS VC N-E-S MOV NE P0032 T01610139 $ = 2016-11-30 05:38 METAR KTYS 301038Z 22024G30KT 3SM +TSRA BR BKN037CB BKN050 OVC070 17/15 A2976 = 2016-11-30 05:31 METAR KTYS 301031Z 24021G45KT 1 1/2SM +TSRA BR SCT045CB BKN060 OVC100 16/14 A2976 RMK AO2 PK WND 27045/1026 WSHFT 1017 LTG DSNT ALQDS RAB0954E01B21 TSB23 FRQ LTGIC OHD TS OHD MOV NE P0000 T01610144 $ = 2016-11-30 05:27 METAR KTYS 301027Z 26034G45KT 1/2SM +TSRA SCT045CB BKN060 OVC100 19/16 A2978 RMK AO2 PK WND 27045/1026 LTG DSNT ALQDS RAB0954E01B21 TSB23 PRESRR FRQ LTGIC OHD TS OHD MOV NE P0000 T01940156 RVRNO $ = 2016-11-30 05:23 METAR KTYS 301023Z 25022G26KT 10SM -TSRA SCT045CB BKN060 OVC100 21/16 A2975 RMK AO2 PK WND 25026/1023 LTG DSNT ALQDS RAB0954E01B21 TSB23 PRESRR FRQ LTGIC OHD TS OHD MOV NE P0000 T02060161 $ = 23

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The National Park Service has wind sensors within the bounds of GSMNP. Samples of the wind sensors in the fire regions. Values in ms-1. 25