Atmósfera 22(3), 315-330 (2009)

A comparison of two cases of low-latitude thundersnow

G. DOLIF NETO Centro de Previsao del Tempo e Estudos Climáticos- Instituto Nacional de Pesquisas Espaciais, São Paulo,

P. S. MARKET, A. E. BECKER, B. PETTEGREW, C. MELICK, C. SCHULTZ, P. I. BUCKLEY, J. V. CLARK, A. R. LUPO Department of Soil, Environmental and Atmospheric Sciences, University of Missouri-Columbia, MO, USA Corresponding author: P. S. Market; e-mail: [email protected]

R. HOLLE, N. DEMETRIADES Vaisala Incorporated, Tucson, AZ, USA

C. E. BARBIERI Department of Parks, Recreation, and Tourism, University of Missouri-Columbia, MO, USA

Received April 16, 2008; accepted May 11, 2009

RESUMEN Se examinan dos eventos de nevada con relámpagos en latitudes bajas a fin de identificar sus características. Ambos eventos se originaron a escala sinóptica y tuvieron impactos de pequeña escala (por ejemplo, ascenso orográfico e inestabilidad elevada). El primer evento ocurrió en el hemisferio sur y se desarrolló a finales del invierno influenciado por la orografía subyacente. Este evento presentó abundancia de relámpagos (94 destellos nube-suelo en la región); sin embargo, la acumulación de nieve fue insignificante. Este evento fue dominado por relámpagos con polaridad negativa, con una amplitud máxima promedio de -43.2 kA. El segundo evento fue un caso de convección elevada en el hemisferio norte con frontogénesis debajo de una capa extensa de inestabilidad potencial. Este evento también presentó abundantes relámpagos (706 destellos nube-suelo en la región), así como considerable acumulación de nieve sobre un área extensa. Este evento también fue dominado por relámpagos con polaridad negativa, con una amplitud máxima promedio de -23.7 kA. Ambos eventos ameritan mayor investigación dado que estudios sobre tormentas similares no son abordados frecuentemente en la literatura. Más aún, teniendo en consideración que las regiones cálidas sub-tropicales usualmente no están preparadas para enfrentar ni siquiera los efectos de pequeñas acumula- ciones de nieve o hielo. En el futuro, los pronosticadores pueden anticipar mejor estos eventos anómalos observando las siguientes características generales: 1) sistemas climáticos sinópticos significativos y bien definidos en bajas latitudes; 2) una zona baroclínica fuerte con estructura de chorro superpuesta bien definida (≥60 ms-1); 3) una extensión considerable y profunda de aire frío más próximo al ecuador de lo típico; y 4) una capa con humedad neutral a inestabilidad condicional sobre la zona frontal.

ABSTRACT Two cases of low-latitude with are studied to determine their characteristics. Both cases had synoptic-scale origins, but also featured smaller-scale influences (e. g. orographic lift and elevated instability). 316 D. Dolif Neto et al.

The first event occurred in the Southern Hemisphere and was a late case that developed under the influence of underlying orography. Lightning was plentiful in that event (94 -to-ground flashes in the region), but snow accumulations were not significant. Lightning flashes of negative polarity dominated this case, with a mean peak amplitude of -43.2 kA. The second event was a case of eleva- ted convection, with frontogenesis beneath an extended layer of potential instability. Appreciable lightning occurred with this event as well (706 cloud-to-ground flashes in the region), and snow accumulations were significant over a broad area. Lightning flashes of negative polarity dominated this case also, with a mean peak amplitude of -23.7 kA. Each of these events is worthy of further scrutiny, as studies of such do not appear often in the literature. Indeed, such warm, subtropical regions are often unprepared for the effects of just a little snow or ice accumulation. Future forecasters can anticipate better such anomalous events by looking for these broad features: 1) significant and well-defined synoptic-scale systems at low latitudes, 2) a strong baroclinic zone with a well-defined (≥60 ms-1) jet structure aloft, 3) cold air of appreciable depth and areal extent drawn much closer to the equator than is typical, and 4) a moist neutral to conditionally unstable layer above the frontal zone.

Keywords: Thundersnow, low-latitude, winter convection.

1. Introduction Thundersnow can lead to deep snow accumulations and dangerous travel conditions in areas where it occurs (Crowe et al., 2006). Very often a middle-latitude occurrence (e.g. Market et al., 2002), these thundersnow storms can deposit anomalous snow totals, usually on human populations who are well-prepared to cope with their impacts (impassable roads, diminished emergency services, etc.) and return daily life to normal fairly quickly (chemical treatment of roads, physical snow removal, etc.). In short, thundersnow tends not to be a tropical or subtropical phenomenon. Yet, low latitude events do occur (e.g. Guinan 1988) -deep snow accumulations make it difficult for all, but especially problematic for those who live in areas totally unprepared for such severe winter weather. Until recently, snowstorms with lightning had received little attention in the literature; such events in lower latitudes have received even less. The current study looks at two low-latitude snow events with lightning. One event occurred in the Southern Hemisphere in the Brazilian state of . This event featured lightning and thunder during snowfall as a result of convection originating in the boundary-layer. This convection also occurred in association with a cold frontal passage and forced ascent over significant terrain; a moist neutral layer in the mid-levels was present as well. The second event considered occurred in the Northern Hemisphere over the state of Texas in the United States. This event featured lightning and thunder with elevated convection, the result of instability released above the frontal inversion well north of a surface cold front. For these case studies, we employ a quasi-geostrophic framework for the synoptic analysis using observed data, along with output from the United States National Weather Service’s Global Forecasting System (GFS). This approach is employed to promote a more uniform, larger-scale comparison of meteorological fields between the events. On the mesoscale, comparisons are made using initial fields from the GFS as well as observed data from radiosonde flights, lightning networks, and satellite observations. Having introduced the topic, this brief paper will present both cases concurrently and in a similar fashion. In section 2, a synoptic overview precedes mesoscale analyses of the Southern Hemisphere case; the same approach is used in section 3 for the Northern Hemisphere case. Concluding remarks on these two unique cases will be offered in section 4. Low-latitude thundersnow 317

2. Case 1- Southern Hemisphere 2.1 Synopsis On the morning of 12 September 2005, a brief thundersnow event occurred near São Joaquim, Santa Catarina, Brazil, (latitude: 28.29°S and longitude: 49.93°W). Lasting only a few minutes, the snow and thunder occurred between 1043 and 1115 UTC 12 September 2005 in São Joaquim (Cruzeiro), at an altitude of 1510 m and in the Morro da Igreja (latitude 28.12°S and longitude 49.49°W), at an altitude of 1828 m, with the surface weather station air temperature oscillating between -0.6 and +1.2°C. No measurable snowfall was reported at either station. Santa Catarina is located between latitudes 25 and 30°S, and with much of the state elevated above mean sea level by 500 m or more, snow is not unheard of there. Nevertheless, the unusual nature of this event demands further scrutiny. The synoptic analysis begins aloft, where at the outset the parent was an aging, occluded and already quite deep (952 hPa), with a surface center far southeast of the thundersnow location (Fig. 1). The thundersnow event was situated beneath the equatorward, entrance region of the 300-hPa jet streak (Fig. 2a) embedded in quasi-linear flow. Wind speeds in excess of 60 ms-1 were found to begin north of 30°S, highlighting the subtropical origins of this jet streak. This arrangement suggests divergence aloft, and the encouragement of vertical motions over the Santa Catarina region. While across-stream ageostrophic winds and upper air divergence values were diagnosed in the appropriate quadrant at several levels (up to +6 × 10-5 s-1 at 300-hPa), they were maximized over the state of São Paulo, some 200-250 km to the north of the event site. The 300- hPa divergence values approached zero over Santa Catarina at 0000 UTC, but vertical motions were still robust in the lower , as the reader will see shortly. At 500 hPa, a closed low was found near 56°S 33°W (Fig. 2b). To the north and west, elongated trough and vorticity axes were diagnosed well south of Santa Catarina, over southern Uruguay. The 500-hPa level gives a clearer picture of the age of the parent cyclone in which this event occurred. In addition to the low heights (<4980 gpm) found with the cyclone center at this level, three distinct vorticity lobes were found along and behind the leading trough. Yet, the trough axis at more northern latitudes (and thus, over the continent) was located north and downstream of the one at 300 hPa, so some baroclinic organization was still in evidence at this time. In many ways, this system resembles the cold air surges described by Lupo et al. (2001). The vertical slope of the trough axis was in evidence at 700 hPa also (Fig. 2c), along with significant cold advection associated with the cold front at that level. However, note that this analysis was valid nearly twelve hours before the time of thundersnow in Santa Catarina, with the leading edge of the frontal zone at this level well south of the state at 0000 UTC. The vertical slope of the trough downward toward the warmer air persisted at 850 hPa (Fig. 2d), but frontal structure is more difficult to diagnose. While cold advection was clearly evident off the south coast of Brazil, the signature was weak at best for onshore locations to the south of Santa Catarina. Although the location where the thundersnow occurred was on the trailing edge of the surface cold frontal zone, a significant low-level thermal gradient accompanied this boundary, as evidenced by the surface potential temperature gradient across the Santa Catarina region at 1000 UTC (Fig. 3). 318 D. Dolif Neto et al. 73 77 84 3 132 3 18 64 77 61 5580 195 6 1003 1006 55 270 1024 5460 H 67 2 167 1026 26 79 5700 230 2 23 150 66 27 1024 82 80 66 203 66 82 5220 24 2 172 77 24 82 994 226 992 72 259 1000 5580 5460 69 5340 1008 73 54 82 75 50 50 5220 79 300 68 3 217 15 52 H 37 52 55 1016 59 55 45 39 73 34 52 5580 36 30 30 23 H H H 75 81 1016 5340 1038 34 52 1038 39 45 81 72 37 17 36 77 52 73 61 50 16 55 23 50 59 H 1021 75 77 H 1039 5460 55 59 66 1032 52 52 55 61 61 70 351 3 14 75 5580 61 092 1 5700 28 81 Fig. 1. South American surface analysis valid at 1200 UTC 12 September 2005. Station plots are supplemented with output from the Gridded Gridded the from output with supplemented are plots Station 2005. September 12 UTC 1200 at valid analysis surface American South 1. Fig. Forecast System (GFS) initial fields, showing sea level pressure (solid, every 4 hPa) and 1000-500-hPa geopotential thickness (dashed, shown section cross the represents line dashed bold, the and 4, Figure in soundings the of locations the mark symbols Star gpm). 60 every Area of interest exists between star symbols. in Figure 5. Low-latitude thundersnow 319

a) b)

5880

-10 -10

5880

-20 -20

9600 5820 -30 -30 9480 9480 44 5760 5640 5700 80 9240 9360 5580 9240 9360 9120 40 18 5520 14 9000 10 5460 70 36 10 -40 -40 5400 14 8880 32 10 5340 10 60 8760 5280 28 14 5220 8640 4 50 5160 -50 8520 24 -50 18 5100 10 20 18 5040 40 14 16 10 4 10 10 4980 -70 -60-50 -40 -30 -70 -60 -50 14 18 -40 -30 050912/0000v000 300 MB HEIGHTS, ISOTACHS (m/s) 050912/0000v000 500 MB HEIGHTS, ABS VORTICITY (1/S*10-5)

c) d) 1530 5 5

-10 -10 10 1500 15 20 3180 10 1530 5 10 10 -20 -20 1560 3120 1530 3150 1560 15 5 10 01590 3120 10 1590 -30 -30 1500 3090 5 3060 9360 9360 9240 5 9240 3060 9120 10 91201530 3030 3000 1470 2970 3090 9000 1500 1440 2940 5 1410 -40 3120 2910 1640-40 1380 3150 88802890 2850 1350 2820 1320 87602790 1290 2760 1260 2730 1230 2700 -50 -50 1200 2670 1170 2640 1140 2610 1110 20 2580 1180

5 10 10 930 960 24602490 990 -70 -60-50 -40 -302520 1470 -70 -60-50 -40 10 -301020 050912/0000V000 700 MB HEIGHTS, TEMP (C) 050912/0000V000 850 MB HEIGHTS, TEMP (C)

Fig. 2. Objective upper air analyses from the GFS initial fields for South America valid at 0000 UTC 12 September 2005: a) 300-hPa geopotential heights (solid, every 120 gpm) and isotachs (dashed, every 10 m s-1; shaded, every 10 m s-1 above 40 m s-1); b) 500-hPa geopotential heights (solid, every 60 gpm) and absolute vorticity of the geostrophic wind (dashed every 4 × 10-5 s-1; shaded, every 4 × 10-5 s-1 above 16 × 10-5 s-1); c) 700-hPa geopotential heights (solid, every 30 gpm) and temperature (dashed, every 5°C); d) 850-hPa geopotential heights (solid, every 30 gpm) and temperature (dashed, every 5°C). 320 D. Dolif Neto et al.

299 209 297 312 297 297 293 302 293 295 294

299 297 295 288 298 294 295 296 289 287 312 290 288 297 288 288 312 315 308 288 285 286 298 267 289 290 290 286 296 287 296 297

285 283 284 291 286 281 280 281 280 281 274 281 273 278 270 278 279 271 275 281 269 276 279

274 281 271 270

274 269

275 MON Sep 12 2005 1000 METARS Obs

Fig. 3. A plot of surface potential temperature for various stations in South America valid at 1000 UTC 12 September 2005. Dark box approximates the region for which lightning strokes are shown in Figure 7.

2.2 Mesoscale analysis Two radiosonde ascents occurred nearby, just after the time of the thundersnow event. The first flight from Porto Alegre-Salgado Filho, Brazil (SBPA; latitude: 30.00°S and longitude: 51.18°W), was south of the surface cold front location, thus representing the cold post-frontal air (Fig. 4a). The surface layer at SBPA was nearly dry adiabatic over the lowest ~100 hPa while winds veered from southerly to westerly through the lowest ~250 hPa, marking the top of the frontal inversion layer. The second radiosonde flight (Fig. 4b) occurred in the warm air north of the cold frontal zone, from Curitiba-Afonso Peña, Brazil (SBCT; latitude: 25.52°S and longitude: 49.17°W). Although an inversion was present and based some ~50 hPa from the surface (likely the remnants of a radiation inversion), winds backed from easterly to northwesterly, suggesting a warm advection signature ahead of the cold frontal zone. A deep, nearly moist adiabatic layer occurred with this sounding, but none of the stability metrics were particularly robust; even the convective available potential energy (CAPE) for a parcel lifted from the most unstable level yielded a value of zero. Thus, the cold frontal zone and thundersnow event occurred near the time of but between these radiosonde observations, neither of which revealed significant mid-level instability. Yet, the sounding for SBPA (Fig. 4a) was conditionally unstable over the lowest ~200 hPa. Along with onshore flow (Fig. 1) and orographic lift, this sounding (Fig. 4a) suggested conditions suitable for post-frontal, low-level convection. Meanwhile the SBCT sounding (Fig. 4b), farther north and ahead of the surface cold front, revealed a deep layer that is convectively neutral. Low-latitude thundersnow 321

a) RAOB - 20050912_1200_BRAZIL_SBPAUPA_WMO.TAT -- SBPA - PORTO ALEGRE/SALGAD, BZNM at FT12Z (x1000) 12 Jul 2007 mb 100 TROP Lvl : 12470 m AGL FRZG Lvl : 1256 m AGL 16 cclEL Hgt : 1516 m AGL lfcEL Hgt : m AGL 50 LFC Hgt : -- m AGL 15 CCL Hgt : 980 m AGL LCL Hgt : 4463 m AGL Water : 1.57 cm 150 : -- cm 14 T2Gust : -- kt 45 WindEx : 34 kt SWEAT : 76.0 13 CAP : -- Boyden : 86.2 40 200 S {TT} : -9.8 12 KO : 24.2 LJ : 5.9 TT : 9.2 Kl : -15.9 11 250 Tc : 11.6 °C 35 Storm : 268/41 0-6km s-rH : 52 0-3km 10 s-rH : -14 0-2km 300 s-rH : 11 0-1km 30 CAPE+ only : 0 J/kg 9 CIN total : 0 J/km DCAPE 6km : 0 J/kg VGP 0-4km : -- 8 400 EHI 0-2 km : -- 25 MVV : 0 m/s BRN : -- 7

LFC Lift / LPL 680 mb 6 20 500 FSI : 31.8 Threat : 10.0 5 Point : -2.5°C 600 15 4

700 3 10

800 2 850 5 900 1 925 Stn Elev: 3 m 1000 00 QNH = 1031.1 mb 1050 DA: -304 m, ISA -30 -20 -10 0 10 20 30 °C MSL KNOTS

RAOB - 20050912_1200_BRAZIL_SBPAUPA_WMO.TAT -- 83840 - SBCT - ACURITIBA/ALFONSO PEN, BZ at 12Z 12 Jul 2007 b) NM FT (x1000) mb 100 TROP Lvl : -- m AGL FRZG Lvl : 3189 m AGL 16 cclEL Hgt : 501 m AGL lfcEL Hgt : m AGL LFC Hgt : -- m AGL 50 15 CCL Hgt : 307 m AGL LCL Hgt : 2455 m AGL Water : 3.21 cm 150 Hail : -- cm 14 T2Gust : -- kt 45 WindEx : -- kt SWEAT : 150.7 13 CAP : -- Boyden : 90.7 S {TT} : 26.4 40 200 12 KO : 15.2 LJ : 2.2 TT : 33.9 Kl : 23.6 11 250 Tc : 11.8 °C 35 Storm : 261/25 0-6km s-rH : 259 0-3km 10 s-rH : 99 0-2km 300 s-rH : 0 0-1km 30 9 CAPE+ only : 0 J/kg CIN total : 0 J/km DCAPE 6km : 0 J/kg 8 VGP 0-4km : -- 400 EHI 0-2 km : -- 25 MVV : 0 m/s 7 BRN : -- LFC Lift / LPL 680 mb 6 20 500 FOG FSI : 17.2 Threat : 14.4 Point : 6.9°C 5 600 15 4

700 3 10

800 2 850 5 900 1 925 1000 1050 00 -30 -20 -10 0 10 20 30 °C MSL KNOTS

Fig. 4. Skew-T log-p plots of radiosonde data valid at 1200 UTC 12 September 2005 from a) Porto Alegre- Salgado Filho, Brazil (SBPA), which is represented by the southern star in Figure 1, south of the surface frontal zone, and b) Curitiba-Afonso Peña, Brazil (SBCT), which is indicated by the northern star in Figure 1. 322 D. Dolif Neto et al.

A cross section of the frontal zone (Fig. 5) revealed a well-developed baroclinic structure, with isentropes sloping down toward the warmer air, and veering winds in the lower troposphere from the warm side to the cold side of the front (cold southerlies south of the front and warm northerlies to the north of the frontal zone). The frontal zone intersected the surface of the earth (in a region where the terrain extends as high as ~900 hPa) near the center of this analysis (Fig. 5). A jet structure was present at 300 hPa with speeds in excess of 100 kt, aloft and poleward of the surface frontal location. In addition, a direct thermal circulation was suggested in the thinned (grid spacing of 2.5° latitude × 2.5° longitude) GFS fields, with ascent (<-3 μb s-1) embedded in a nearly-saturated region of the mid-troposphere just poleward of the surface frontal zone. Farther south, a region of descent (>3 μb s-1) was found just above the frontal zone and beneath the upper level jet streak. Finally, note that the best moisture and ascent not only occurred near the location of the surface frontal zone, but above a region of significant terrain and with a shallow layer of elevated potential instability.

300

336

400

324

70 70 320 500

3 316 328 312

308 304 332 90 300 700 296

292 328 324 80

850 288 70 925

1000 MON SEP 12 2005 1200V000 GFS thinned Pressure Vertical Velocity South MON Sep 12 2005 1200V000 GFS thinned Wind Vectors North MON Sep 12 2005 1200V000 GFS thinned Equivalent Potential Te mperature Mon Sep 12 2005 1200V000 GFS thinned Relative Humidity

Fig. 5. Cross-section from the GFS initial fields for South America valid at 1200 UTC 12 September 2005 along the line shown in Figure 1. Winds are plotted in station model format (in knots), with relative humidity (shaded every 10% above 70%), equivalent potential temperature (solid, every 2 K), and pressure vertical velocity (dashed, every 1 μb s-1).

Satellite imagery (Fig. 6) showed the coldest over-land temperatures were found over the western reaches of Santa Catarina, with onshore cloud top temperatures generally warmer than -40°C. Nevertheless, the analysis of cloud-to-ground lightning flashes (Fig. 7) revealed a steady northward progression of the flash patterns that attended the motion of the frontal zone, with numerous flashes detected over Santa Catarina and Paraná during the period in question. Lightning flash frequency was never very high with this system, and was also on the decline between 0800 and 1100 UTC over the state of Santa Catarina. There were 47 cloud-to-ground lightning flashes over the entire state between 0800 and 0900 UTC, 27 between 0900 and 1000 UTC, but only 20 flashes between 1000 and 1100 UTC (the time period of interest) for the same area. Interestingly, no flashes occurred in the area after 1100 UTC, so we speculate that reports of thunder by human Low-latitude thundersnow 323

observers as late as 1115 UTC may well have been borne of in-cloud lightning flashes. Of the 94 cloud-to-ground flashes, 79 (84%) were of negative polarity and 15 (16%) were of positive polarity. Negative flashes had a mean peak amplitude of -43.2 kA (±33.6 kA); the median peak amplitude was -29 kA. Positive flashes had a mean peak amplitude of +38.1 kA (±17.5 kA); the median peak amplitude was +37 kA. The relatively widespread nature of these lightning flashes suggests that thundersnow may have been experienced more widely than was reported. However, that conclusion must also be tempered by the notion that the stations reporting thundersnow were at a relatively high altitude.

PR Cuchilla

Florianopolis M. Igreja Morianopolis RS S. Joaquim Porto Alegre

-30 -40 -50 -60 -70

(graus Celsius) 1030z 20050912

Fig. 6. GOES-12 infrared satellite image of South America from 1030 UTC 12 September 2005. The temperature scale °C is show at bottom right.

25S

25.5S

26S

26.5S

27S Florianopolis 27.5S

28S Sao Joaquim (Cruzeiro) Morro da Igreja 28.5S

29S

29.5S

30S 55W 54.5W 54W 53.5W 53W 52.5W 52W 51.5W 51W 50.5W 50W 49.5W 49W 48.5W 48W

8910 11 GMT Fig. 7. Lightning strokes from the Brazilian Lightning Detection Network (BrasilDat) for the 3-hour period 0800 to 1100 UTC 12 September 2005. Color coding breaks flashes into 1-hour subperiods. 324 D. Dolif Neto et al.

3. Case 2 – Northern Hemisphere 3.1 Synopsis Thundersnow occurred along the Gulf of México coast of the state of Texas in the United States late in the day of 24 December 2004. Situated between 24 and 29°N latitude, snow totals of 31.8 cm and 11.2 cm were measured at Victoria, Texas, and Corpus Christi, Texas, respectively. Even Brownsville, Texas, on the US-México border measured 3.8 cm of snow. The thundersnow event that developed late on 24 December 2004 across southern Texas occurred in the wake of a previous cold frontal passage (Fig. 8); this surface frontal zone was supported by a significant long wave pattern in the mid- to upper-troposphere and a surge of cold air in the lower troposphere. By 0000 UTC 25 December 2004, a closed low had appeared over the Big Bend region of Texas at the 300-hPa level, with the Texas coast found beneath the axis of the subtropical jet (Fig. 9a). Indeed, the 300-hPa low and associated short wave trough lagged behind the larger longwave system, whose center at this time was far to the north over the Hudson Bay area. The development of the closed low was partially a response to the deep column of cold air that had become concentrated beneath the upper low, which will be discussed at greater length shortly. At 500-hPa (Fig. 9b), a distinct circulation was present in the vorticity field over the northern third of México, although a closed low was not present at this level. Cyclonic vorticity advection by the geostrophic wind was indicated downstream of the trough axis, although the signature was weak over coastal Texas. Of interest, too is the band of shear vorticity across the southern and eastern United States that served as a testament to the history of this system and the greater phase speed of the northern wave which, until recently, had been a part of the much larger long wave pattern.

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7 23 204 44 242 1024 24 31 1 19 32 1829 L 7 -8 -3 3 +6 +6 +7 49 233 -10 -2 7 258 -21 27 37 205 -6 0 15 215 47 174 -19 -5 068 21 0853 112 0 -2 1 242 10 44002520 11003 +15 41 275 19 295 14 15 +11 +5 1 -15 4 23 +10 46652 46015 11704 -18 -7 236 12 246 ** 5 0 9 1310034400438 10906 -16 45 38 26 17 0 6 0 16 244 +12 0 24 227 30 216 14 10 296H10 6 -4 +5 +4 15 000905 42 272 26 -16 -3 38 124 1005 SGNW3 ** +6 -20 221 7 LSCM14 16 15 52 244 42 2748 1 18 -1 40 109 28 9 10 -15 12 207 70 200 +13 096 9 0 -2 25 -13 -7 23 +9 15 -7 10 0 22 31 279 02525 -1 21 0 1 224 29 22 10903 PFRX62 20301 49 -3 24 0 ** -21 -7 1028 092 218 68 56 258 58 238 -11 21 22230 178 28 10 21 090905 -3 +3 38 132 14 14 -9 +1 22 240 000902 -12 -2 19 1034 +5 3 ** 198 -17 8 271 12 63 18 -14 -13 +3 -3 19 46059 11405 WRYW46 40 307 12 -6 43 180 -14 24 25 -1 0 12 70 211 3 15 12 56 237 37 300 21 266 23 -6 0 -16 19 -1 25 18 48 26544 284 +3 23 249 30 34401436 10804 PFASG48200402 57 260 35 37 21 -3 -7 -30 28060 56 246 -9 -16 3 0 31 168 35 135 34 123 138 12 251 -1 279 050803 L 22 14 +11 +8 -8 -9 1 214 -17 -10 15 12 21 354 20000 -20 -13 40 18 15H1032 20 304 H-8 19 70 125 -18 39 +4 14480 20301 46013 11604 -3 2127 20 15 10 -5 -8 17 02080 257 34 304 +7 -2 13 278 21 251 23 15 030802 13 001604 -13 22 315 18 0 -3 34 219 WPV063 +1 21 21 199 262 2 +11 24 37 15 -17 15 233 -20 2 12 14 35 193 9 260 56 56 44 259 3 26 296 31 170 26 184 -21 -20 19 +7 -30 -8 -10 16 223 28 207 -4 30 323 -3 +7 -9 5 200 58 250 62 211 -1 +9 27 -1 -15 33 CLKN745 6--20301 0 46042 160350 257 14 30 -7 7 +3 36 213 6 6 17 21 -8 +13 1012 ELWR551 20100 13 281603 -1515 16 -10 12 5 280 271 236 17 6 27 24 191 216 1029 -14 -5 19 30 178 14 341206 57 25039 32 197 -19 20 20 20 272 37 +12 71 140 -3 50 237 45 -5 27 190 -28 231 +2 13 211 15 257 -7 -12 11 -26 +5 FPSN7 +5 50 -15 34 264 7 17 301 19 0 5 14 39 213 25039 11 12 -13 +4 15 0 50 19 +10 4100261 10804 61 27 286 -1 24 220 H 22 55 1 -21 25 23 37 213 22 39 205 -2 16 229 +16 192 57 240 1032 28 -17 -1 +2 +21 37 -10 42 0 9 21 273 35 226 1028 19 211 5 5 -6 19 248 26 4100441 10804 1013 46054 48 11703 -11 -10 16 25 251 23 23 235 25 21 279 18 248 331 213 L 14 61 234 0 35 276 +5 12 25 233 -24 27 40 215 +21 64 41 225 -7 28 272 -17 -13 -4 +9 59 252 -5 -8 H +4 4 3440 193 -2 16 12 12 9 27 19 26 30 23 2 4 14 25 268 -7 +22 K3L3 11001 28 -3 10702 MKYJ851820000 60 239 63 16 36 234 40 4100842 19 350603 -10 234 25 260 28 17 +3 213 16 000701 46047 11603 -3 30 244 -1 34 60 239 21 243 29 246 23 28 +12 16 001603 -7 52 -9 15 18 28 260 -3 230 25013 -6 16 41 0 43 -6 15 1028 29 270 31 252 -13 38 17 5 30 -9 -12 15 201 9 32 259 18 40 188 143 11603 6 25 271 12 -8 37 210 62 162 71 33 234 14 32 231 216 35 0 0 -4 19 -6 14 31 249 33 -5 +16 26 -11 -8 -7 TNF1 69 20403 45 192 211 8 19 247 247 15 0 28 0 4100960 1060 -5 -7 37 248 15 21 49 176 23 130804 65 231 +2 +1 32 -7 15 24 000901 -17 66 13 277 +2 51 0 250 15 0 -2 36 7575 75 120 0 8 H 12 13 33 52 180 42 0 36 256 235 0 14 35 34 240 +2 49 196 DHKW51 20302 -3 -4 24 +4 0 52 153 SPGP17370 20402 35 277 201 4203947 0 C6FN568 360302 225 25 +716 22 0 35 042 10602 55 157 62 0 17 0 10502 9 26 0 30 256 12 21 21 50 76 139 14 -7 35 220 LUML126 VENF152 147 43 DIHE40 63 5 +12 14 262 34 265 8 180 +10 17 34 +7 73 -3 28 25503529 10502 61 25 23 11 16 30 12 0 0 14 56 164 24 77 68 25 -14 68 146 48 200 66 155 +9 ZCDG869 23 -60 4204151 0 68 1 63 3235 226 10604 -5 54 CGYE45 20608 23 4200358 10504 68 132SMK=168 28 36 -1 66 48 PTAT225 22 360506 26 77 144 67 145 WABU66 20402 133 25 70 140 0 -10 75 34 -20 27 010802 20401 -10 10704 ZCBU274 64 179 WCX533362 205044200162 75 27 26 000000 32 219 20 050805 27 70 79 43 36 260 +2 64 138 68 = 29 0 72 34 251 0 75 0000Z SURFACE ANALYSIS 27 4200260 10907 64 DATE: SAT DEC 25 2004 34 24 72 ISSUED: 0114Z SAT DEC 25 2004 66 153 41 24 79 125 BY HPC ANALYST ZIEGENFELDER 34 79 46 1001 1003 34 79 +5 COLLABORTING CENTERS: HPC, TPC, OPC 68 166 30 72 HBNE73 20302 1003 28 29 100502 70 123 L 79 131 7979146 75 37 45 66 100 50 178 75 ELWX57175 -30 76 125 79 112 1008 = o -15 28 ZCDF849 45 V2AJ170 201 82 125 24 360603 59 24 010800 1012 0 MCZN274 -120 -115 -110 -100 -95 -90 -85 -80 -75 -10552 62 75 120 26

Fig. 8. United States Hydrometeorological Prediction Center surface analysis valid at 0000 UTC 25 December 2004. Star symbol marks the location of the soundings in Figure 10, and the bold, dashed line represents the cross section shown in Figure 11. Low-latitude thundersnow 325

In the lower troposphere, the cold-core signature of this cyclone became more clear with the broad shortwave trough at 700-hPa (Fig. 9c) beneath the ones found at 300 and 500-hPa and the coldest air found along 700-hPa trough axis. To the east over the Texas Gulf coast, only the southernmost tip of the state was above freezing at this level, but a broad thermal gradient was also observed along and parallel to the coastline. A similar thermal pattern was found along the Texas Gulf coast at 850-hPa (Fig. 9d), with the 0°C isotherm immediately along the coastline. At the surface (Fig. 8), a broad region of high pressure was positioned across northern Texas and northeastern México. The cold front that had moved through south Texas previously was analyzed at this time as a stationary front well south into the Gulf of México. An inverted trough was oriented perpendicular to the Texas Gulf coast in the vicinity of Corpus Christi, where near-freezing temperatures, wintry , and a moderate northerly breeze were observed. a) b) 10 14 14 10 18 14 10 18 14 18 10 14 18 14 14 10 10 10 10 10 14 14 8400 14 14 5040 5400 10 510018 8880 8520 14 8640 10 5160 8760 10 5220 9000 5460 5340 5280 10 40 40 5520 10 9120 10 10 10 18 18 18 44 18 80 18 10 10 30 4030 9120 5640 5580 5820 70 9240 36 10 14 9360 564018 10 5700 9480 32 60 5760 20 9600 20 28 5820

50 24

20 40 -120 -110 -100 -90-80 -120 -110 -100 -90-80 16 041225/0000V000 300MB Heights, ISOTACHS (m/s) 041225/0000V000 500MB Heights, ABS VORTICITY (1/S*10-5) !0 c) d) 1350 1260 10 2670 30 1290 30 2700 1320 50 50 13501320 2730 25 2880 5 -10 30 1380 2910 27902760 -30 2820 1410 -5 2940 2850 0 1440 2970 -5 40 -20 40 15 3000 10 10 1470 15 10 10 5 -5 1500 1530

3150 1560 30 5 3030 30 1530 15 -5 1530 1530 1500 3060 15 5 3090 0 5 10 1470 15 20 10 2015 10 3120 20

1500

-120 -110 -100 -90-80 -120 -110 -100 -90-80

041225/0000V000 700MB Heights, TEMP (C) 041225/0000V000 850 MB Heights, TEMP (C)

Fig. 9. As in Figure 2, but valid over North America at 0000 UTC 25 December 2004. 326 D. Dolif Neto et al.

3.2 Mesoscale analysis Cold, post-frontal air was present in the lowest ~250 hPa of the sounding from Corpus Christi, Texas (KCRP; latitude: 27.77°N and longitude: 97.50°W), flown at 0000 UTC 25 December 2004 (Fig. 10). Indeed, surface winds were northerly, becoming easterly with height and finally switching to a more southerly flow at and above the frontal inversion, demonstrating a clear profile of veering with height. While this would usually suggest the presence of warm advection (WAD) in the layer, diagnostics at 850 hPa failed to show a significant region of WAD at that level. Yet, the sounding revealed significant mid-tropospheric instability above the frontal inversion (Fig. 10). The 700-500-hPa value of 7.3 K km-1 translated into a Lifted Index of -13 and a CAPE of 155 J kg-1, both for a parcel originating at 700 hPa; these values are in line with those found by Market (2006) for central US thundersnow events. This plot suggests a region that approached the criteria for a moist absolutely unstable layer (e. g. Bryan and Fritsch, 2000) and conducive to the development of upright convection.

RAOB - 20041225_0000_TEXAS_KCRPUPA_WMO.TXT -- 72251 - KCRP CORPUSNM CHI FT (x1000) mb 100 TROP Lvl : 13508 m AGL FRZG Lvl : 116 m AGL 16 cclEL Hgt : 5330 m AGL lfcEL Hgt : 7196 m AGL 50 LFC Hgt : 3854 m AGL 15 CCL Hgt : 5253 m AGL LCL Hgt : 3117 m AGL Water : 1.74 cm 150 Hail : ---- cm 14 T2Gust : -- kt 45 WindEx : 11 kt SWEAT : 97.0 13 CAP : 0.3 Boyden : 84.1 40 200 S {TT} : 27.9 12 KO : 28.2 LJ : -1.3 TT : 34.7 Kl : 15.4 11 250 Tc : 37.7 °C 35 Storm : 266/29 0-6km 10 s-rH : 732 0-3km 300 s-rH : 268 0-2km s-rH : 42 0-1km 30 9 CAPE+ only : 155 J/kg CAPE 0-3km: 0 J/kg CIN total : -4 J/km 8 DCAPE 6km : 1059 J/kg 25 400 VGP 0-4km : 0.2 EHI 0-2 km : 3 7 MVV : 18 m/s BRN : 1 6 20 500 LFC Lift / LPL 700 mb

FOG FSI : 36.8 5 Threat : 7.5 15 600 Point : 4.2°C 4

700 3 10

800 2 850 5 900 1 925 Stn Elev: 13 m 1000 QNH = 1023.3 mb 1050 00 DA: -598 m, ISA -40 -30 -20 -10 0 10 20 30 °C MSL KNOTS

Fig. 10. Skew-T log-p plots of radiosonde data valid at 0000 UTC 25 December 2004 from Corpus Christi, Texas, United States, which is represented by the star in Figure 8.

Cross section analysis (Fig. 11) revealed the presence of potential instability over a broad region above the frontal inversion. Moreover, this layer resided south of and above a region of mid-level frontogenesis (>1.0 K 100 km-1 3 h-1), where the upward vertical motions were already in excess of 5 μb s-1. However, it is important to note that the occurrence of thundersnow went on well beyond 0000 UTC, and while the generation of upright convection dominated early on, at least one investigator (Morales, 2008) has shown that the stability regime eventually evolved, with slantwise convection becoming the dominant mode. Morales also showed that the magnitude and duration of the snowfall was promoted by a quasi-steady flow of dry, northerly air near the surface. This paper differs from that of Morales in that we emphasize the thundering nature of these events, and thus the sounding structure that makes the conducive to lightning generation. Low-latitude thundersnow 327

300

334

400 330

326 70 -2 -3 -4 500 70

372

318 100 314 310 700 90

306 80 850 298 302 290 294 286 925 282 70 1000 SAT Dec 25 2004 0000V000 GFS thinner Pressure Vertical Velocity North SET Dec 25 2004 0000V000 GFS 211 Wind Vectors South SAT Dec 25 2004 0000V000 GFS 211 Equivalent Potential Te mperature SAT Dec 25 2004 0000V000 GFS 211 Relative Humidity

Fig. 11. Cross section from the GFS initial fields for the Texas Gulf coast valid at 0000 UTC 25 December 2004 along the line shown in Figure 8. Winds are plotted in station model format (in knots), with relative humidity (shaded every 10% above 70%), equivalent potential temperature (solid, every 2 K), and pressure vertical velocity (dashed, every 1 μb s-1).

The satellite imagery appeared deceptively benign with the coldest cloud tops, associated with the larger cloud mass, found well off the Texas coast (Fig.12). Small nascent cloud bands were beginning to emerge over southern Texas with cloud top temperatures of ~ -40°C. Banding was present in the precipitation field, as rendered by radar (not shown), and lightning activity (as revealed by the North American Lightning Detection Network) attended these bands, the full history of which is shown in Figure 13. This event occurred over a much longer time period, so data for a full 24-hour period are supplied. Of the 706 cloud-to-ground flashes shown over the 24-hour period, 511 (72%) were of negative polarity, and 195 (28%) were positive. However, only 16 of the flashes shown in Figure 13 occurred onshore during this time. Therefore, the flash density and frequency were quite low for those onshore flashes that occurred primarily with snowfall. No real trend was apparent in the onshore flashes, although the period of peak activity was clearly between 1800 UTC 24 December 2004 and 0000 UTC 25 December 2004 (the blue dots). Of the 16 flashes that occurred well onshore, only one was positive and had a peak amplitude of +45.4 kA. The remaining 15 cloud-to-ground flashes that occurred well onshore had a mean peak amplitude of -24.2 kA (±17.2 kA); the median value was -17.2 kA. Being deeper into the cold than the Southern Hemisphere case, it comes as no surprise that both the flash density and the flash frequency are lower with this event. Freezing and other wintry precipitation were already occurring right on the Texas coast (Fig. 8). Yet, that lightning was occurring is in line with the sounding profile from KCRP at that time (Fig. 10), with the most unstable parcel originating with a temperature warmer than -10°C, and its vertical excursion extending nearly 4 km, with an equilibrium level at ~7 km AGL and a temperature of nearly -30°C. These elements of the vertical thermal 328 D. Dolif Neto et al.

Fig. 12. GOES-12 infrared satellite image 2345 UTC 25 December 2004. The color table is used to denote the temperature scale, shown at left.

Dec 24, 1200 - 1800 UTC Dec 24, 1800 - Dec 25 0000 UTC Dec 25, 0000 - 0600 UTC Dec 25, 0600 - 1200 UTC

Fig. 13. Cloud-to-ground flashes from the North American Lightning Detection Network for the 24-hour period 1200 UTC 24 December to 1200 UTC 25 December 2004. Color coding breaks flashes into 6-hour subperiods. Low-latitude thundersnow 329

profile are all in line with the experimental findings of Takahashi (1978) as well as later field investigations (Takahashi et al., 1999). Specifically, this thermodynamic profile allowed for a positively buoyant parcel through a deep layer. Also, such a convective cloud would cross over both the -10°C level (a known charge reversal temperature) and -20°C, a level known for active particle charging processes (provided adequate moisture; Takahashi et al., 1999).

4. Summary Two cases of thundersnow are examined from the low latitude regions of both the Northern and Southern Hemispheres. The event over southern Brazil was marked by onshore and upslope flow near the surface, while the Texas coast event had no comparable topographic relief, and an offshore surface flow regime. The idea that terrain played a role in the Brazilian event is supported by the nearby soundings, both of which originate from close to sea level. Indeed, the post-frontal sounding from SBPA (Fig. 4a) is cold enough to support snowfall down to 1510 m MSL (and below), yet above, the profile is roughly isothermal about freezing up to ~700 hPa. Moreover, the surface conditions evince an atmosphere that is quite unstable, suggesting that surface-based convection was likely. In conjunction with the other regional sounding (Fig. 4b), the Brazilian case appears to have been one of forced ascent over orography, with snow falling from a larger, parent stratiform cloud but with embedded convection. The event over the Texas coast resulted from the release of elevated instability. It is true that the near-surface flow was offshore, and topographic relief of the kind in the Brazilian case does not exist in coastal Texas. Moreover, the Brazilian case had the benefit of a frontal zone in the immediate area, having passed through the region with thundersnow only hours prior. By contrast, the Texas thundersnow occurred north of a cold frontal zone that had moved through two days prior. More importantly, the thundersnow represented the release of elevated potential instability above the frontal zone. To summarize, both of the cases were the product of significant and well-defined synoptic-scale weather systems at low latitudes. A strong baroclinic zone was also present in both cases, with a well-defined upper-tropospheric jet structure aloft in support of a lower-tropospheric front. In each case, cold air of appreciable depth and areal extent was drawn much closer to the equator than is typical. Yet, important distinctions existed between these cases on the smaller scale. In both events, a moist neutral thermal profile existed in the mid-troposphere, but external, low-level forcing for ascent was of much greater importance in the Southern Hemisphere event. That forecasters at low latitudes (and lower altitudes) find themselves in a situation where snow is likely or already occurring clearly indicates an anomalous event. It is not unreasonable, then, that such events may also possess dynamics sufficient to create heavy snowfall, and perhaps thundersnow. Should the synoptic and mesoscale situation appear conducive to snowfall, then one may also assess the likelihood of thundersnow. On the larger scale, forecasters can anticipate such anomalous snow events by looking for these features: 1) significant and well-defined synoptic-scale weather systems at low latitudes, 2) a strong baroclinic zone with a well-defined (≥60 ms-1) jet structure aloft, 3) cold air of appreciable depth and areal extent drawn much closer to the equator than is typical, and 4) a moist neutral to conditionally unstable layer above the frontal zone. On the smaller scale, forecasters can anticipate a thundersnow event best via sounding or cross section analysis. It is desirable for the local atmospheric thermodynamic profile to: 1) be sufficiently cold (≤0°C) throughout, 2) be unstable to vertical or slantwise perturbations, often above a stable layer 330 D. Dolif Neto et al.

(especially in the absence of orography), 3) have the most unstable parcel originate from a level where the temperature is warmer than -10°C, and 4) have the equilibrium level of the parcel in number 3 above extend to a level where the temperature is colder than -20°C.

Acknowledgements The authors would like to thank the two anonymous reviewers whose efforts produced an improved manuscript. This work is supported by the National Science Foundation (NSF), Award No. ATM- 0239010. Any opinions, findings, conclusions or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the NSF.

References Bryan G. H. and M. J. Fritsch, 2000. Moist absolute instability: The sixth static stability state. Bull. Amer. Meteor. Soc. 81, 1207-1230. Crowe C. C., P. S. Market, B. P. Pettegrew, C. J. Melick and J. Podzimek, 2006. An investiga- tion of thundersnow and deep snow accumulations. Geophys. Res. Lett. 33, L24812, doi: 10.1029/2006GL028214. Guinan P. E., 1988. A case study of an intense south Texas snowstorm during January 12-13, 1985. M. S. Thesis, University of Illinois at Urbana-Champaign, 103 pp. Lupo A. R., J. J. Nocera, L. F. Bosart, E. G. Hoffman and D. J. Knight, 2001. South American cold surges: Types, composites and case studies. Mon. Wea. Rev. 129, 1021-1041. Market P. S., C. E. Halcomb and R. L. Ebert, 2002. A climatology of thundersnow events over the contiguous United States. Wea. Forecast. 17, 1290-1295. Market P. S., A. M. Oravetz, D. Gaede, E. Bookbinder, A. R. Lupo, C. J. Melick, L. L. Smith, R. Thomas, R. Redburn, B. P. Pettegrew and A. E. Becker, 2006. Proximity soundings of thunders- now in the central United States. J. Geophys. Res. 111, D19208, doi:10.1029/2006JD007061. Morales R. F., 2008. The historic christmas 2004 south Texas snow event: Diagnosis of the heavy snow band. Nat. Wea. Dig. 32, 135-152. Takahashi T., 1978. Riming electrification as a charge generation mechanism in .J. Atmos. Sci. 35, 1536-1548. Takahashi T., T. Tajiri and Y. Sonoi, 1999. Charges on and snow crystals and the electrical structure of winter thunderstorms. J. Atmos. Sci. 56, 1561-1578.