AN EXAMINATION OF THE AREAL EXTENT OF HIGH WINDS ASSOCIATED WITH MOUNTAIN WAVES ALONG THE WESTERN FOOTHILLS OF THE SOUTHERN APPALACHIAN MOUNTAINS

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1 A XAA F H AA X F HH D AAD H UA A A H FH F H UH AAAHA UA David. affin AA/ational eather ervice eather Forecast ffice orristown, ennessee Abstract High winds associated with mountain waves have been found to occur frequently along the western foothills of the southern Appalachian ountains from ovember through arch. hen these high wind events occurred, reports of significant wind damage were normally received from two main areas in east ennessee: the reat moky ountains ational ark and the amp reek community of southeast reene ounty. t was not known whether the ational ark and the amp reek areas were just more reliable at reporting wind damage or actually had stronger winds compared to other areas along the western foothills. hus, a study was conducted to examine the available observations along the western foothills of the southern Appalachian ountains to determine the areal extent of these high winds during mountain-wave events. Due to the lack of Automated urface bservation ystem (A) and Automated eather bserving ystem (A) stations in the western foothills, 8 eatherug and 3 emote Automated eather tations (A) stations were examined on the 52 dates between 999 and 2007 when mountain-wave-induced warning-level winds were reported at ove ountain in the ational ark. Although the ove ountain site was at a higher elevation than most of the other sites in the western foothills, it was concluded that the higher wind speeds at ove ountain were mainly due to its location in the foothills of the highest ridges on the western side of the southern Appalachian ountains. his conclusion was made because the wind speeds at ove ountain were found to be even stronger than those observed at the highest elevation site at lingmans Dome. t was found that only the amp reek site also reported at least advisory-level winds on 87% of the dates that ove ountain reported warning-level winds. n addition, it was found that only the amp reek, oker reek, hady alley, and rwin sites reported warning-level winds on a few of the ove ountain dates. he amp reek and oker reek sites reported warning-level winds on roughly a third of the dates that they were available, while the hady alley site reported warninglevel winds on roughly a tenth of the dates that it was available. omposite maps of surface isobars, surface isotherms, and 850-mb isotachs from the warning-level wind dates revealed that the warninglevel winds at amp reek, oker reek, and hady alley resulted from a stronger low pressure system with stronger 850-mb winds compared to the low that produced warning-level winds at ove ountain. Also, weaker cold-air damming on the eastern side of the southern Appalachian ountains appeared to allow warning-level winds at the oker reek and hady alley sites, with the weakest cold-air damming observed during the warning-level wind dates at hady alley. orresponding Author: David. affin AA/ational eather ervice 5974 ommerce lvd. orristown, ennessee 3784 mail: David.affin@noaa.gov

2 affin. ntroduction High winds associated with mountain waves have been found to frequently occur from ovember through arch within a narrow area along the western foothills of the southern Appalachian ountains (affin 2009). hen these mountain wave events occur along the western foothills, reports of wind damage are normally received from two main areas in east ennessee: the reat moky ountains ational ark (hereafter referred to as ational ark) and the amp reek community of southeast reene ounty (Fig. ). ome of the stronger wind events have produced reports of numerous large trees down in the ational ark, which closed almost all the roads in the park, while several homes were damaged and a few barns destroyed in the amp reek area. However, it was not known whether the ational ark and the amp reek areas were just more reliable at reporting wind damage or actually had stronger winds compared to other areas along the western foothills of the southern Appalachian ountains. t was previously noted in affin (2009) that the location of the highest ridges in the southern Appalachian ountain range within the ational ark likely caused higher mountain-wave-induced winds in the foothills. Also, it was theorized that the frequent reports of wind damage at amp reek were possibly the result of its location on the other side of the mountains from the French road alley where southeast winds may have accelerated up through the valley. he purposes of this study are to () examine the available observations in the western foothills of the southern Appalachian ountains to determine the areal extent of the high winds during mountainwave events and (2) examine the synoptic conditions that produced warning-level winds at the different sites. winds and peak one-second wind gusts each hour. ince the ove ountain observation in the ational ark normally reports the highest winds during mountain-wave events, affin (2009) conducted a mountain-wave-induced high wind climatology of the ove ountain site. High wind warnings are issued by the ational eather ervice when sustained winds of at least 40 mph occur for an hour or longer, or wind gusts of at least 58 mph occur. hus, the affin (2009) climatology found those dates when southeasterly (direction between 90 and 80 ) sustained winds of at least 40 mph occurred at ove ountain along with a stable boundary layer and a veering wind profile observed on the Atlanta sounding. his methodology likely gave a good estimate of how many warning-level wind events at ove ountain resulted from mountain waves, since it covered many of the necessary ingredients for mountain wave formation including a strong south to 2. Data and ethodology he ational ark ervice Air esources Division maintains a network of anemometers within the ational ark (Fig. 2), including sites at ove ountain (elevation of 4,50 ft ), ades ove (,850 ft ), ook ock (2,700 ft ), and lingmans Dome (6,670 ft ). hese anemometers are.. oung model ind onitors, which record hourly-averaged sustained Fig.. ocation of southern Appalachian region (shaded area) in relation to the United tates (top) with relief map of the southern Appalachian region (bottom) [bold lines denote state boundaries with state names in bold capital letters; regular lines denote county boundaries with county names in small text; medium text denotes wind damage locations; large text denotes geographic features]. 48 ational eather Digest

3 Areal xtent of ountain-ave inds ear the outhern Appalachians Fig. 2. ontoured elevation map of the reat moky ountains ational ark and surrounding areas (contours in feet; bold line denotes park boundary) with the locations of the observations sites in or near the park. southeast wind perpendicular to the mountain ridges, an upstream stable boundary layer extending above the mountain tops, and a veering wind profile that eventually produced a critical layer where the wind flow became parallel to the mountain ridges. ecause A and A sites are non-existent in the western foothills of the southern Appalachian ountains, 8 eatherug * sites near the western foothills of the southern Appalachian ountains were examined for high winds (Fig. 3; able ). All of the eatherug sites in this study were located on the roofs of schools or buildings at around 0 ft A, with the anemometers located on a 0-ft mast making the total height around 20 ft A. eatherug anemometers are fairly similar to those at A and A sites in that they report two-minute-averaged sustained winds with one-second wind gusts. n addition to the eatherug sites, three A sites maintained by the U.. Forest ervice located near the western foothills of the southern Appalachian ountains were examined: oker reek (K), ndian rave (), and Unicoi (). A anemometers are mounted on a 20 ft tower with ten-minute-averaged sustained winds and five-second-averaged wind gusts. hus, A winds are expected to be lower than eatherug winds. Although many of the eatherug Fig. 3. elief map of the southern Appalachian region (color legend at top left denotes elevations [ft x,000 ]) with the locations of observation sites in the foothills of east ennessee. *: ention or display of a trademark, proprietary product, or firm in text or figures does not constitute an endorsement by the ational eather ervice, AA, or the Department of ommerce, and does not imply approval to the exclusion of other suitable products or firms. olume 35 umber ~ August 20 49

4 affin eatherug tations ristol () Ducktown (DUK) lizabethton (Z) nglewood () rwin () towah (A) atlinburg () amp reek () Holston ountain (H) Johnson ity (JH) Johnson ity (J) aryville (A) ountain ity () ld Fort (DF) igeon Forge (F) arrottsville () oan ountain () hady alley () levation () 634 feet 729 feet 627 feet 853 feet 857 feet 889 feet 470 feet 427 feet 423 feet 650 feet 847 feet 027 feet 2520 feet 906 feet 00 feet 27 feet 2769 feet 2822 feet A tations oker reek (K) ndian rave () Unicoi () ational ark tation ove ountain levation () 700 feet 2700 feet 2400 feet levation () 450 feet able. bservation sites along the western foothills of the southern Appalachian ountains. and A sites were not always available on the dates that high winds were reported at ove ountain, the available sites were examined to determine if sustained winds were at least 40 mph or wind gusts were above 58 mph. n addition to searching for warning-level winds, the occurrence of advisory-level winds, which are defined in this study as sustained winds of at least 25 mph, was noted as well. he other three sites in the ational ark (lingmans Dome, ook ock, and ades ove) were not evaluated in this study since wind gust data were not available at these three sites. n addition, the lingmans Dome observation was only available on a seasonal basis from ay through ctober. After the eatherug and A stations were evaluated for high winds on the same dates that produced mountain-wave-induced high winds at ove ountain, composite maps of mean sea level pressure, surface isotherms, and 850-mb isotachs were constructed from the dates that warning-level winds were observed at the different sites. hese composite maps were constructed from the ational enters for nvironmental rediction- ational enter for Atmospheric esearch reanalysis dataset (Kalnay et al. 996) on the ational ceanic and Atmospheric Administration (AA) ooperative nstitute for esearch in nvironmental ciences limate Diagnostics enter s eb site ( psd/data/composites/hour/). 3. esults f the 52 dates when mountain-wave-induced warning-level winds were reported at ove ountain between 999 and 2007, there were only five sites (rwin, amp reek, Holston ountain, hady alley, and oker reek) that reported advisory-level winds or stronger during at least a third of the time that they were available (able 2). t was found that only the amp reek site reported at least advisory-level winds on 87% of the dates that warning-level winds were observed at ove ountain. n addition, it was found that only the amp reek, oker reek, hady alley, and rwin sites reported warninglevel winds on a few of the ove ountain dates. he amp reek and oker reek sites reported warning-level winds on roughly a third of the dates that they were available, while the hady alley site reported warning-level winds on roughly a tenth of the dates that it was available. hile the Holston ountain site reported advisory-level winds on 60% of the dates that it was available, this finding was likely the result of its higher elevation compared to the other sites. he ove ountain site was also around the same elevation as Holston ountain, but the higher wind speeds at ove ountain were likely due to its location in the foothills of the highest ridges on the western side of the southern Appalachian ountains. A comparison of ove ountain to lingmans Dome (which was 2,59 ft higher in elevation) on the available dates (5 ay 2003, 50 ational eather Digest

5 Areal xtent of ountain-ave inds ear the outhern Appalachians 22 eptember 2003, 23 eptember 2006, 7 ctober 2006, and 27 ctober 2006) revealed that lingmans Dome reported weaker sustained winds on all five of these dates. n fact, the sustained winds at lingmans Dome during four of the events were generally 5 to 20 mph less than ove ountain, except on 7 ctober 2006 when the sustained winds were only 2 to 5 mph less. omposite maps of mean sea level pressure, surface isotherms, and 850-mb isotachs from the warninglevel wind dates revealed that warning-level winds at amp reek were the result of a slightly stronger low pressure system with slightly stronger 850-mb winds compared to the low that produced warning-level winds at ove ountain (Fig. 4). Also, a low pressure system with 850-mb winds of similar intensity to the amp reek composite was observed in both the oker reek and hady alley composites (Fig. 5). A cold-air damming scenario (ell and osart 988) was observed on the eastern side of the southern Appalachian ountains in all four composites of surface isotherms. hile the coldair damming in the ove ountain and amp reek composites was of a similar intensity, weaker cold-air damming was observed in the oker reek and hady alley composites with the weakest cold-air damming in the hady alley composite. of all the available data. t s possible that the amp reek site may have experienced even higher winds than were reported, since a visit to this site found that there was some potential blockage from a higher gymnasium roof to the southeast of the anemometer. he fact that ove ountain had the highest wind speeds of any reporting site along the western foothills was likely the result of ontinued page onclusions From the available eatherug and A sites in the western foothills of the southern Appalachian ountains, it was found that the ove ountain and amp reek observation sites reported the highest winds Fig. 4. omposite maps of mean sea level pressure (mb; top), surface isotherms ( ; middle), and 850-mb isotachs (m s - [ m s - =.944 kt]; bottom) from warning-level wind events at ove ountain (left) and amp reek (right). olume 35 umber ~ August 20 5

6 affin Dates of ove ountain arning- evel inds D U K Z A H /2/99 x x x x x x x x /22/99 x x x x x x x x /3/99 x x x x x x x 3/6/99 x x x x x x x 3/9/99 x x x x x x x 2/0/99 x x x x x x /4/00 x x A x A 3/20/00 x x x 2/7/00 x /30/0 x A x A x x 2/25/0 x A x x x A 3/2/0 x x x x A x x x /25/0 x x x x x x A x x A 2/4/0 x x x A x x A x x x 2/7/0 x x x x x x x x x 2/23/0 x x x x x x x A 2//02 x x x x A x A x x x x x 2/20/02 x x x A x A A x x x x x A 4/8/02 A x x x A x x x x x x A /5/02 x x x x x x x x x x x x x x x /0/02 x x x x x A x x x x x x A x x x 2/9/02 x x A x x x x x x x x A A x x 2/24/02 x x x x x x x x x x x x x x x 3/9/03 x x x x x A x x x x x A A x x 5/5/03 x x x x x x x x x x x x x 9/22/03 x x x x A x x x x x x x x /8/03 x x x x x x x A x x /24/03 x x x x A x x x x A A x x 2/30/03 x x x x A x x x A A x x /25/04 x x x x x x x x x x 2/6/04 x A A x x x x x A J H J A D F F K able 2. he availability of observation sites along the western foothills on the warning-level wind dates at ove ountain. An x indicates availability but no high winds observed, A indicates advisory-level winds observed, indicates warning-level winds observed. he definitions of the five-letter station identifiers are given in able. able 2 continues on the next page. 52 ational eather Digest

7 Areal xtent of ountain-ave inds ear the outhern Appalachians Dates of ove ountain arning- evel inds D U K Z A H 3/2/04 x x x x A x x x x x x x 2//04 x x x x x A x x x x A x x 2/23/04 x A x A x A x A A x x /3/05 x x x x x x x x A x x 2/4/05 x x x x x x x x x x x x x /28/05 x x x x x A A x x x x A A x x 2/9/05 x x x x x A A x x x x A x x x /7/06 x x x x x A A x x x x A x x /29/06 A x x A x A x x x x x x x x 2/2/06 x x x A x A A x x x x A x x 3/0/06 x x x A x A x x x x x 9/23/06 x x x x x x x x x x x x x x x 0/7/06 x x x x x A x x x x A x x 0/27/06 x x x x x A x x x x A x x x /6/06 x x x x x x x x x A x x 2/3/06 x x x x x x x x x x x x x x /7/07 x x A x x A x x x x x x x 2/25/07 x x x A x x x x x x x 3//07 x A x A A x x x x A x x 4//07 x x x A A x x x x x A A x x 2/23/07 x x x x x x x A x A x x A Advisory # % of total arning # % of total J H J A D F F K able 2. ontinued from facing page. olume 35 umber ~ August 20 53

8 affin Fig. 5. omposite maps of mean sea level pressure (mb; top), surface isotherms ( ; middle), and 850-mb isotachs (m s - [ m s - =.944 kt]; bottom) from warning-level wind events at oker reek (left) and hady alley (right). 54 ational eather Digest

9 Areal xtent of ountain-ave inds ear the outhern Appalachians its location near the highest ridges of the western side of the southern Appalachian ountains. his conclusion was made because the wind speeds at ove ountain were found to be even stronger than those observed at lingmans Dome (2,59 ft higher than ove ountain) during the five available events. ecause mountain waves normally produce high winds in a narrow area along the foothills, it s still possible that some of the sites examined in this study were not at the proper location to directly measure mountain-wave-induced high winds in their immediate vicinities. hus, high winds due to mountain waves may be more extensive and of stronger intensities along the western foothills of the southern Appalachian ountains than can be determined in this study. However, this study should be considered a first step in confirming that the ational ark and amp reek areas are not just reliable damage-reporting sites, but also experience some of the strongest winds during mountain-wave events along the western foothills of the southern Appalachian ountains. After examining composites of surface isobars, surface isotherms, and 850-mb isotachs, it was found that the warning-level winds at amp reek, oker reek and hady alley resulted from a stronger low pressure system with stronger 850-mb winds compared to the low that produced warning-level winds at ove ountain. old-air damming was observed on the eastern side of the southern Appalachian ountains in all four composites of surface isotherms, but weaker cold-air damming was observed in the oker reek and hady alley composites. his weaker cold-air damming likely allowed warninglevel winds to develop at these sites due to less blockage of the low-level southeasterly winds. he weakest coldair damming was observed in the hady alley composite, which was likely necessary to allow strong low-level southeasterly winds to penetrate further north into northeast ennessee. t s possible that warning-level winds were still able to occur at ove ountain and amp reek despite the stronger cold-air damming, since Acknowledgments he author would like to thank im Hyatt, erry Hambrick, and ill allahan of A onvergence echnologies for the eatherug data. Also, thanks to Jim enfro of the reat moky ountains ational ark and Jessica ard of Air esource pecialists, nc. for their help in retrieving the ove ountain wind data. hanks to Dr. am g (etropolitan tate ollege of Denver) and Dr. Alan zarnetzki (University of orthern owa) for their helpful reviews of this manuscript. mountain ridges with higher elevations were located near these two sites. hus, strong southeasterly winds at a slightly higher elevation over-riding the cold air at the surface could still produce mountain waves at these two sites, while not producing mountain waves at oker reek or hady alley where the mountain ridges are not as high, which would require strong southeasterly winds at a lower elevation. ince the amp reek composites cannot fully resolve the question of whether the observed cold-air damming blocked southeasterly winds in the French road alley, future research with high-resolution computer models should be conducted to determine if the frequent reports of wind damage at amp reek were possibly the result of its location on the other side of the mountains from the French road alley where southeast winds may have accelerated up through the valley. Author David affin has been a senior forecaster with the ational eather ervice since January 2002, and has been stationed at orristown, ennessee since August 998. He began his career at F emphis, ennessee in arch 994. His research interests include gravity/mountain waves, tornadoes, wake lows, and local climatological studies. He has a.a. degree (990) in economics from the University of ennessee-knoxville and an.. degree (993) in meteorology from exas A& University. eferences ell,. D., and. F. osart, 988: Appalachian old-air Damming. on. ea. ev., 6, affin, D.., 2009: n High inds and Foehn arming associated with ountain ave vents in the estern Foothills of the outhern Appalachian ountains. ea. Forecasting, 24, Kalnay,.,. Kanamitsu,. Kistler,. ollins, D. Deaven,. andin,. redell,. aha,. hite, J. oollen,. Zhu,. helliah,. bisuzaki,. Higgins, J. Janowiak, K.. o,. opelewski, J. ang, A. eetmaa,. eynolds,. Jenne, and D. Joseph, 996: he / A eanalysis 40-ear roject. ull. Amer. eteor. oc., 77, olume 35 umber ~ August 20 55