December 19 2004 Wind Storm over Central Alberta

A Winter Season Convectively-Induced Wind Event

Prairie and Arctic Storm Prediction Centre (PASPC)

Winnipeg, MB

1830Z (11:30 am MST) images from Carvel RADAR 40 km west of Edmonton. Left image is 1.5 km CAPPI rain rate showing convective band of showers and embedded thunderstorms from Slave Lake (YZH) to north of Brazeau (VBZ) approaching RADAR site from the west. Bowing structure evident in band indicative of strong winds being brought to surface. This band was racing east at 100 km/h and would move into the Edmonton city area within 45 minutes. Right image is lowest angle Doppler radial velocity image showing wind max of 85 kt (160 km/h) over the Mayerthorpe area (EV5) at 2400 feet above surface. It is suspected that a large part of this wind max was mixed to the surface in this area based on local damage reports.

INTRODUCTION

On Sunday December 19^th, 2004 a rare wintertime convectively-induced wind storm caused considerable damage throughout central Alberta along with at least one fatality as powerful wind gusts of 100 km/h or greater blew across the area. The winds were associated with a squall line ahead of a band of showers and thunderstorms that had developed along a maritime cold front moving over western Alberta during the late morning. Such events are not uncommon in Alberta during the summer and fall months; however, it is quite rare to see such significant convection in the winter in this area. The convective band tapped 60-70 kt winds aloft and brought them to the surface over a widespread area with a peak wind gust of 63 kt (117 km/h) reported at Violet Grove southwest of Edmonton. It is possible that some localities northwest of Edmonton recorded even stronger wind gusts of 70 kts or more (130 km/h) based on damage reports. The squall line raced eastward across central Alberta at speeds up to 120 km/h over the next several hours, moving into southern Alberta and southwestern Saskatchewan by the end of the day. Most of the damage reports with this event were north and west of Edmonton which is the location on which this study will focus. The study will look at the synoptic situation of this event, as well as some RADAR images from the Carvel Doppler RADAR which was in the general area of the strongest winds with this event.

TIMELINE

DAMAGE REPORTS

METEOROLOGICAL ANALYSIS

RADAR IMAGES

PASPC RESPONSE

CONCLUSION

TIMELINE

1630Z - 1700Z (9:30 - 10 am MST) area of convection develops over eastern BC into the Grande Cache area with lightning strikes noted on lightning detector
1719Z (10:19 am) PIREP reports line of thunderstorms west of Edson with lightning, hail, rain and snow noted. Max tops on RADAR over 8 km (25,000 feet)
1800Z (11:00 am) convective line crosses Edson and Whitecourt giving 51 kt (90 km/h) gusts with lightning, heavy rain, hail and ice pellets observed
1805Z (11:05 am) Special weather statement issued for areas west of Edmonton highlighting strong winds from convective line
1820Z (11:20 am) line crosses Mayerthorpe area with damaging winds over 100 km/h estimated, radio tower bent in half, roofs damaged, power outages
1827Z (11:27 am) Wind Warning issued for areas west and north of Edmonton for wind gusts to 100 km/h
1850Z (11:50 am) Wind Warning extended for Edmonton City and regions east for wind gusts to 100 km/h
1850Z (11:50 am) strong wind gusts cause fatality in Alberta Beach as kiteboarder is blown into an abandoned church by strong winds
1905Z (12:05 pm) line moves into west Edmonton area; planes flipped at Villeneuve airport
1915Z (12:15 pm) line crosses the city of Edmonton giving 44-51 kt wind gusts to Namao and Edmonton City Centre Airport
1930Z (12:30 pm) 54 kt wind gust reported at Edmonton International Airport
2000Z (1:00 pm) line crosses Camrose with 49 kt wind gust

Map showing progression of squall line across Central Alberta in 10 minute intervals between 11 am and 1 pm MST Dec 19 2004 based on RADAR and surface reports.
The line took only 2 hours to travel the 200 km distance between 18 - 20Z. Note that most of the wind damage and strong wind reports (highlighted in yellow)
were received in areas northwest of Edmonton where Doppler RADAR indicated winds aloft of 85 kt (160 km/h) during the event

DAMAGE REPORTS

Some of the more significant damage reports received with this event included..

LOCALITY	TIME	DAMAGE REPORT
Mayerthorpe (120 km NW Edmonton)	11:18 am	Radio tower bent in half, shingles blown off roofs, flying debris, power outages
Alberta Beach (Lac Ste. Anne) (60 km W of Edmonton)	11:50 am (est)	FATALITY - kiteboarder on Lac Ste. Anne blown into church building by strong winds
Onoway (60 km NW Edmonton)	11:50 am (est)	160 ft X 40 ft hay shed destroyed by strong winds, forty 6 x 6 supports broken
Villeneuve (10 km NW Edmonton)	12:05 pm (est)	Small planes flipped over at Villeneuve airport
Morinville (20 km N Edmonton)	12:10 pm (est)	Flat metal roof ripped off maintenance shed, shingles off, other damage in area
Fedorah (40 km N Edmonton)	12:10 pm (est)	Roof pieces flying off, tree limbs falling
Edmonton	12:15 pm (est)	112 glass panels at City Hall damaged by flying gravel from nearby rooftops; $250K damage
Millet(40 km S Edmonton)	12:40 pm (est)	Unoccupied mobile home flipped over

METEOROLOGICAL ANALYSIS OF EVENT

SYNOPTIC PATTERN

The synoptic pattern associated with this event was typical of a cold frontal destabilising situation common in the convective summer months over Alberta. A low pressure system moving across northern Alberta pushes a cold front across central and southern regions of the province, marking the leading edge of a cooler airmass moving in from the northwest. This airmass is also very unstable as strong cooling aloft destabilises the airmass while surface cooling is minimal. As a result, strong winds aloft can be mixed down to the surface in this unstable situation. This is not an unusual occurrence over Alberta during the warm convective season; however, it is quite rare for such an event in the middle of December in this part of the world, especially with snow on the ground! This case demonstrates that convectively-induced severe weather can also occur in the winter months over Alberta if the conditions are right.

	12z surface analysis, Dec 19 2004 985 mb low pressure centre over northern BC with warm front extending into southwestern Saskatchewan and maritime cold front slicing southward into southern BC. It is along this cold front that the convectively-induced wind event would develop. Deeper arctic air is found east and north of arctic frontal boundary to the east of maritime warm front. An extensive area of snow is occurring east of arctic warm front with band of freezing rain immediately along warm front over Saskatchewan. Very mild airmass over most of Alberta with 12z temperatures above freezing over most places. Edmonton is +3°C while Calgary is +10°C with a brisk westerly chinook

	15z surface analysis, Dec 19 2004 The cold front associated with this event started moving across the BC-Alberta border around 1500z (8 am MST). Temperatures ahead of the front in the maritime warm sector were in the +7 to +10°C range with dewpoints around -1°C. Behind the front, temperatures fell quite sharply with rain showers changing to snow. An observation from Grande Cache (elevation 1200m, just inside the Alberta border) at 1624Z indicated zero visibility in heavy snow and blowing snow, west winds of 40-45 knots and a temperature drop from +11°C to 0°C in an hour and a half.

	18z surface analysis, Dec 19 2004 At 1800Z, the main low was analysed over northwestern Alberta northwest of Peace River with a central pressure of 988 mb. A maritime wave was now analysed near Slave Lake southeast of the main low as the warm air was beginning to occlude out of the system. The associated maritime cold front was extending southwestward over west-central Alberta having just moved through Edson and Whitecourt at 1750Z with peak wind gusts of 51 kt (95 km/h) from the northwest at both locations. The front was also accompanied by a mixture of rain, snow and ice pellets with Whitecourt reporting thunder and lightning at 1800Z. Temperatures drop behind the maritime front but still remain near or above freezing. Colder arctic air is found north of the arctic cold front pushing through northern BC.

	00Z surface analysis, Dec 20 2004 By 0000Z the main surface low had moved across northern Alberta into the Fort McMurray area with a central pressure of 986 mb. Southeast of the main low was a maritime wave centred near Saskatoon with an associated cold front over southwestern SK into southern AB. This front was still producing wind gusts to 90 km/h with wind warnings still in effect at 00Z associated with the frontal passage. This front marked the leading edge of cooler air over Alberta but the deeper arctic air was still confined to the far north behind the arctic cold front which was moving through the Peace River area behind the main low at 00Z.

12 hr surface prog Valid 00Z (6 pm MST) Dec 20 2004

12 hour surface prog from CMC's regional model shows 982 mb low over Fort McMurray area with maritime wave over central SK. 1000-500 mb thickness pattern suggests maritime cold front over southern Alberta with cooling aloft noted behind front.

Arctic cold front lays farther north over northern Alberta extending westward from main surface low. This front marks the leading edge of the true arctic airmass.

Prog has done quite well with positions of maritime and arctic features although main low over northeastern AB is 4 mb too deep.

UPPER AIR PATTERN

The upper air pattern on this day was strongly baroclinic, with a strong upper ridge over Alberta giving very mild air to the province while deep arctic air was found over the eastern Prairies. The 12Z 850 mb analysis showed a sharp warm front over eastern Saskatchewan separating unseasonably mild +12°C air at 850 mb over central Alberta from -18°C air over eastern Manitoba. The mild airmass over the west helped set several record highs across Alberta for Dec 19th, including a +8.0°C reading at YEG (Edmonton International). (see 12Z Stony Plain tephi)

At 700 mb a 70 kt southwest jet was observed on the 12Z sounding at YXS (Prince George), central BC, just ahead of an upper trof through BC. The low level wind prog from CMC was also indicating that a 60-70 knot low level jet was forecast to move into Alberta during the day. Thus, winds aloft were strong enough to support downdrafts of 100 km/h or more with any strong instability. This was noted by the PASPC night shift which issued a WIND WARNING for the Hinton-Grande Cache, Grand Prairie and Slave Lake regions at 4.22 am for the development of strong westerly winds of 60 gusting 80 km/h over the warning area that morning. Thus, the development of strong winds was anticipated for areas of west central Alberta for this particular situation due to the strong winds aloft and unstable conditions.

Increasing instability was supported by strong cooling aloft as noted by the 500 mb charts seen below. The 12Z 500 mb analysis shows an upper ridge with very mild air across Alberta with -18°C at 500 mb over Stony Plain. To the west, however, an upper trough was moving over northern BC with 500 mb temperatures of -32°C at Annette AK on the Pacific coast. This trough was forecast to move over eastern Alberta by the end of the day, putting much of northern and central Alberta into a destabilising situation during the day with strong cold advection aloft. The 00Z 500 mb analysis shows the much cooler air aloft that moved over BC and Alberta during the day with Edmonton dropping from -18°C to -32°C in 12 hours, and Prince George dropping from -23°C to -36°C.


12Z 500 mb analysis	00Z 500 mb analysis

500 mb charts for Dec 19th at 12Z (left) and 00Z (right). Charts show significant cooling aloft over northern and central Alberta as 500 mb trough moves through by end of the day. 500 mb temperature at Stony Plain drops from -18°C at 12Z to -32°C at 00Z. Similar cooling was observed at Prince George (YXS) in central BC.

Upper Air Soundings

12Z Stony Plain (WSE) sounding

12Z sounding from Stony Plain shows unseasonably warm airmass over central Alberta with +14°C temperatures at 900 mb and a freezing level above 700 mb (10,000 ft). Edmonton would set a record high of +8°C by noon before the maritime cold front sent temperatures lower in the afternoon.

Dramatic cooling of airmass noted by dashed line showing dry bulb temperatures 12 hours later at 00Z. 850 mb temperatures drop from +12°C to -5°C while 500 mb temperatures drop from -18°C to -32°C in 12 hours.

(00Z WSE sounding)

12Z Prince George (YXS) sounding)

This sounding represents the airmass just ahead of the maritime cold front as it was moving across interior BC at 12Z. Warm air at 850 mb would have prevented any surface-based instability along front. However, sounding shows potential instability above 700 mb given enough lift and moisture advection at this level. Colder air moving in aloft from the west would produce even steeper lapse rates and instability from this sounding.

(see 00Z YXS sounding)

Note very strong winds throughout column with 50+ knot winds from 800 mb and above with 70-80 knot winds from 700 to 500 mb.

Cold Frontal Squall Line Progression

The cold front associated with this event started moving across the BC-Alberta border around 1500Z (8 am MST) as noted by the 15Z surface analysis. Temperatures ahead of the front were in the +7 to +10°C range with dewpoints around -1°C. Behind the front, temperatures fell quite sharply with rain showers changing to snow. An observation from Grande Cache (elevation 1200m, just inside the Alberta border) at 1624Z indicated zero visibility in heavy snow and blowing snow, west winds of 40-45 knots and a temperature drop from +11°C to 0°C in an hour and a half. This in itself is not terribly unusual for an elevated site like Grande Cache behind a cold front in the winter. However, it was showing the strong baroclinicity associated with this cold front.

By 1700Z (10 am) the cold front was spawning a line of convection over eastern BC and west central Alberta with lightning strikes noted over the Grande Cache-Hinton region into eastern BC.This line of convection was just starting to appear on the western edge of the Carvel RADAR range at 1700z showing max tops above 8 km (25,000 ft) and maximum dBz values of 40 dBz along the line. A PIREP at 1719Z over Fox Creek west of Edson reported a line of CBs "as strong as any summer storm" with extensive lightning, hail, rain and snow noted.

By 1800Z (11 am), the front was moving through Whitecourt and Edson approaching Edmonton from the west at 80 km/h. The line of convection gave 51 kt (95 km/h) wind gusts to both Edson and Whitecourt between 1750 and 1800Z, along with ice pellets and rain. Whitecourt also noted lightning in cloud and thunder with the cold frontal squall line. At this time, RADAR was beginning to show lowering tops and weakening echoes as the line was moving east; however, the fast moving squall line and convective instability were still producing damaging wind gusts. Thus, a weakening trend on RADAR was not indicative of a reduction in winds being brought to the surface.

By 1820Z (11:20 am) the line was moving though the Mayerthorpe/Sangudo area about 120 km northwest of Edmonton. At this point, Doppler RADAR was indicating extremely strong winds aloft with an 85 kt (160 km/h) wind max noted at 2400 feet above the surface. Considerable wind damage was reported through the Mayerthorpe area including shingles off roofs and damage to a 60 foot radio antenna tower which was bent in half by the force of the winds. Although no surface wind speed reports were available from this area, it is suspected that winds of at least 130 km/h (70 knots) must have occurred in this area based on damage reports.

By 1850Z (11:50 am) the line was moving through the Lac Ste. Anne/Alberta Beach area causing a fatality when a kite-boarder on Lac Ste. Anne was caught by the wind and blown into the side of a church at a speed estimated over 100 km/h.

By 1910Z (12:10 pm) the cold frontal squall line was moving into the western suburbs of Edmonton at speeds of 100 to 120 km/h. Shortly after noon, the line produced wind gusts strong enough to flip small planes at the Villeneuve airport just to the west of the city. By 19:16Z (12:16 pm) the squall line was moving through the city of Edmonton with a peak wind gust of 51 knots (95 km/h) from the northwest recorded at Edmonton's City Centre airport (YXD). Small hail and thunder were also observed over parts of the city as the front was going through. The winds were strong enough to pick up rooftop gravel from high rise buildings and propel them into numerous glass panels at City Hall, causing at least $250 000 in damage.

The squall line continued to move southeast across the city at speeds of 100 km/h, moving through Sherwood Park by 1920Z and the International Airport south of the city by 1927Z (12:27 pm). A peak gust of 54 kt (100 km/h) was recorded at Edmonton International airport at 1933Z (12:33 pm). Photos taken of the squall line as it was approaching Edmonton International airport below show a distinct shelf cloud formation with the convective line, almost as well developed as that of a summer convective storm.

Photos showing shelf cloud formation of the squall line as it was approaching Edmonton International Airport around 12.25 pm. The temperature was +8°C at the time with a dewpoint of 0°C and winds out of the southwest at 14 kt (25 km/h). The convective squall line came through the airport at 12:27 pm with a peak gust of 54 kt (100 km/h) out of the northwest at 12:33 pm.

The convective line of showers and associated strong wind gusts continued to move rapidly east and south reaching Camrose by 2000Z (1:00 pm) with a peak gust of 49 knots (89 km/h). Thus, the line was continuing to produce severe wind gusts even though RADAR echoes were weakening. The squall line continued to produce wind gusts of 45 to 55 knots over eastern and southern AB as well as southwestern SK, but few additional damage reports were received from these areas. It appears the most significant damage was confined to areas north and west of Edmonton with this event which suggests the strongest winds were concurrent with the 85 kt low level jet max observed from Carvel Doppler RADAR between 1800 and 1900Z.

RADAR IMAGES

The following are RADAR images from Carvel RADAR (west of Edmonton) between 1700Z and 1930Z showing the progression of the squall line through central Alberta including Edmonton. Echo tops, 1.5 km CAPPI, and lowest angle Doppler radial velocity images are included. For animations of the RADAR images with this event, click here.

Note that upstream RADAR data were not available with this event since Spirit River RADAR (located northwest of Carvel RADAR range) was experiencing hardware problems.

1700Z (10:00 am MST) - Convection moving into western Alberta
At 1700Z, a narrow line of convection was just starting to appear on the western edge of the Carvel RADAR range, showing max tops above 8 km (25,000 ft) and maximum dBz values of 40 dBz along the line. Lightning detectors were showing some strikes along this line into east-central BC. A PIREP at 1719Z over Fox Creek west of Edson reported a line of CBs "as strong as any summer storm" with extensive lightning, hail, rain and snow noted. An earlier PIREP from Grand Cache at 1624Z indicated zero visibility in heavy snow and blowing snow, winds west 40-45 knots and the temperature having fallen from +11°C to 0°C in an hour and a half. Clearly the band of convection was developing along the maritime cold front and had crossed the Grand Cache area heading east. However, at this point, there was no indication that any severe weather was occurring with this line, or behind it.

1800Z (11:00 am MST) - Squall line moving through Edson and Whitecourt
By 18Z the cold frontal squall line was crossing Edson and Whitecourt producing wind gusts of 51 kt (95 km/h) at both locations along with rain and ice pellets, as well as thunder and lightning at Whitecourt where the strongest reflectivity values were being detected. Tops in the line were now down slightly to 5-6 km with max tops of 7 km. The line was approximately 150 km west of Edmonton and moving rapidly eastward.

1830Z (11:30 am MST) - Squall line moving east of Mayerthorpe
The squall line has now advanced east 50 to 60 km/h in 30 minutes indicating forward speeds of at least 100 km/h. In addition, a bowing structure can be seen in the line of stronger reflectivities northwest of the RADAR site having just moved through the Mayerthorpe area (EV5). The Doppler radial velocity image indicates an 85 kt (160 km/h) wind max at about 2000 feet above ground in this area. It is suspected that a large portion of this wind max was mixed down to the surface in the Mayerthorpe/Sangudo area based on damage reports received.

1850Z (11:50 am MST) - Squall line moving through Alberta Beach
The squall line has advanced eastward another 40 to 50 km in 30 minutes with bowing structure still evident in the strongest reflectivites just north of the Carvel RADAR site. Doppler image clearly shows the leading edge of the gust front approaching the RADAR site with 70 knot (130 km/h) winds observed within the 20 km RADAR ring, i.e. less than 650 feet above ground. Given the instability and speed of the front, it is most likely that most of this wind was mixed down to the surface in this area. Note that the kiteboarder fatality occurred around this time as the squall line was moving through the Lac Ste. Anne/Alberta Beach area. Precipitation minimum developing behind squall line makes it more difficult to assess the strongest low level winds from Doppler; however, area of grays near Mayerthorpe indicates 75 to 80 kt (140 km/h) winds at 2500 feet above ground. Note that precipitation minimum behind squall line is likely an indication of strong subsidence associated with descending jet max.

1910Z (12:10 pm MST) - Squall line moving into Edmonton
Just after noon, the squall line started to move into the western parts of Edmonton, with wind gusts strong enough to flip small planes at the Villeneuve airport. The bowing structure appears to have dissipated within the squall line, although some RADAR attenuation is possible at this time since the strongest reflectivites are passing right over the RADAR site. Doppler continues to show 75 to 80 kt winds (140 km/h) aloft at 3500 feet. Edmonton City Centre airport would record a wind gust to 51 knots (90 km/h) six minutes after these images causing local damage within the city.

1930Z (12:30 pm MST) - Squall line moving south and east of Edmonton
The squall line continues to advance rapidly eastward, moving 120 km in the past hour. At this time, the line has just moved over Edmonton International airport giving a wind gust to 54 knots from the northwest at 1933Z (12:33 pm). Echo tops at this time have lowered to about 5 km, with no lightning observed; however, the speed of the front and the convective instability continue to produce severe wind gusts of 50 knots or more along the squall line.

PASPC RESPONSE

Sunday Dec 19th was a particularly busy day at the PASPC Winnipeg office with significant weather occurring over much of the Prairies. Heavy snowfall warnings were in effect for central SK and central MB, snow and blowing snow were expected over southern Manitoba, freezing rain warnings were in effect for portions of northern Alberta and central Saskatchewan, and wind warnings were in effect for west-central Alberta and southern Alberta in the Crowsnest area. Most of the significant weather that day was due to adverse winter type weather which has a considerable impact on the travelling public and transportation industry. Thus the focus for the day shift was primarily assessing and forecasting the evolution of adverse winter weather across the Prairies including freezing rain and snow. (It should be noted that two people were killed that day south of Fort McMurray in a car accident attributed to slippery road conditions from freezing rain.) Strong winds were also anticipated over portions of west central Alberta on this day because of instability mixing down strong winds aloft behind the maritime cold front. What was difficult to anticipate, however, was the strength of the wind associated with an out-of-season convective squall line moving rapidly across central Alberta. Thus, this event was more of a situational awareness issue than a meteorological forecast problem.

The automated forecast for Edmonton that morning was indicating winds "becoming west 20 km/h late in the morning". Wind speeds were increased considerably by PASPC forecasters who realised that instability and strong winds aloft would result in stronger winds. The official 5 am Environment Canada forecast for Edmonton that day called for wind speeds increasing to 40 gusting 60 km/h by afternoon. Thus, strong gusty winds were anticipated and forecast for central Alberta that day, with wind warnings bing issued upstream over west-central Alberta. (It should be noted that the kite-boarder killed in this event had warned his spouse that morning not to go out because of high winds in the forecast.)

As the convection developed over western Alberta and rare December thunderstorms began to develop along the line with locally strong winds and hail, the PASPC issued a special weather statement to highlight the unusual situation. At 11.05 am the following special weather statement was issued for areas west of Edmonton to advise the public of locally strong winds occurring with this developing line of convection. As the statement was being prepared, wind gusts to 90 km/h (51 kt) were recorded at Whitecourt and Edson around 1800Z (11.00 am MST).

SPECIAL WEATHER STATEMENT ISSUED FOR..
SPRUCE-GROVE-MORINVILLE-MAYERTHORPE-EVANSBURG
WESTLOCK-BARRHEAD-ATHABASCA
WHITECOURT-EDSON-FOX CREEK-SWAN HILLS. 

..LINE OF SHOWERS AND LOCAL THUNDERSTORMS PRODUCING STRONG WINDS..

A LINE OF SHOWERS AND RARE DECEMBER THUNDERSTORMS HAS DEVELOPED OVER WEST
CENTRAL ALBERTA FROM EDSON TO WHITECOURT. THE LINE HAS PRODUCED WIND GUSTS 
TO 70 TO 80 KM/H ALONG WITH SMALL HAIL AND REDUCED VISIBILITIES IN RAIN AND
SNOW. THIS LINE IS MOVING EAST AT 70 KM/H AND WILL AFFECT AREAS FROM EVANSBURG 
TO BARRHEAD AND AREAS EAST OVER THE NEXT HOUR.

It was initially thought that the line of convection would weaken as it headed east away from the Rockies since RADAR was indicating that cloud tops were falling and RADAR echoes were weakening as of 1800Z. However, as the line continued to move rapidly eastward, PASPC started to receive watcher calls about very strong winds occurring with this line. Based on RADAR signatures and watcher reports, a WIND WARNING was issued at 11.27 am for SPRUCE-GROVE-MORINVILLE-MAYERTHORPE-EVANSBURG and WESTLOCK-BARRHEAD-ATHABASCA regions noting that wind gusts to 100 km/h had been reported with the line of convection and could be expected over the warning area. Note that even though the winds were associated with a line of convection with embedded thunderstorms, a synoptic scale WIND WARNING was issued instead of small scale convective warnings. A synoptic scale warning was a much easier way of highlighting a strong wind event over a large area even though it was convectively induced.

After additional watcher reports were received of strong winds continuing with this line of convection, the WIND WARNING was extended to the City of Edmonton and surrounding regions including Fort Saskatchewan, Drayton Valley, and Leduc regions at 11.50 am. The warning indicated wind gusts to 100 km/h were expected by early afternoon over the warning area due to a line of showers and thunderstorms. Wind warnings would continue to be extended into eastern and southern Alberta into southwestern Saskatchewan during the course of the afternoon into the evening as the squall line continued southeast.

There was considerable media interest in this story as the afternoon progressed and damage reports came in. By late afternoon, PASPC issued a storm event summary to highlight the most significant reports received with this event.

CONCLUSION

The Dec 19th 2004 wind event over central Alberta was the result of a common convective process that brings strong winds aloft to the surface along a line of showers or thunderstorms. What makes this case so unique is the time of year in which it occurred. Severe winds from convection are not uncommon in the summer or fall; however, it is quite rare for convectively-induced severe winds to occur in late December in central Alberta. However, this case shows that it does occur, and can be a significant severe weather event if strong winds aloft are expected over an area. Wintertime convection, although weak by summer standards, may produce severe wind gusts since winds aloft are usually much stronger. Thus, even weak convection in the cold season may mix potentially damaging winds to the surface, especially in a cold advection unstable situation.

Note that even though this event was mainly due to a convective process, its effects were widespread. Virtually every reporting station registered wind gusts near or above the warning criterion of 90 km/h as the squall line went through. Thus, this event was best described as a convectively-induced synoptic scale wind event. Due to the speed and areal extent of this event, the fastest and easiest way of warning the public of strong winds was with a synoptic scale WIND WARNING as opposed to smaller scale convective warnings. Even though the strongest winds did not last more than an hour or so at most localities, the impact of a sudden gust to 100 km/h or more warranted such a warning to the public.

The main purpose of this study is to highlight a rare cold season convective event and its significant impact on the public. Although convection is not normally considered a high-impact phenomenon in the cold season, forecasters should be aware of those cases when it may occur, and its potential to produce severe weather. In this case, an unseasonably mild airmass over Alberta was being replaced by cooler air from BC with sharply falling temperatures aloft. The leading edge of this cooler airmass was marked by a cold front that triggered a line of showers and rare December thunderstorms that in itself was a signal of strong instability. Even though the convection "weakened" within a couple of hours with lower tops and less intense RADAR returns, the associated squall line along the cold front continued to produce extreme wind gusts due to very strong winds aloft. Thus, even weak convection can still produce severe wind gusts if winds aloft are sufficiently strong. Situational awareness of events like this may help forecasters give more lead time in similar situations in the future.

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