9
Volume 7 ,
Atmosphere
LA
RIVIERE
1969
TORNADO, JULY 20, 1968
E. H. V. Dexter
M e t e o r o l o g i c a l S e r v i c e of Canada
INTRODUCTION
Tornadoes a r e a s e v e r e weather phenomena of t h e s o u t h e r n Canadian
P r a i r i e s . Because of i n f r e q u e n t r e p o r t s of t h e i r o c c u r r e n c e , roughly one
p e r summer s e a s o n s i n c e t h e t u r n of t h e c e n t u r y , and a l s o b y . v i r t u e of
t h e i r d e s t r u c t i v e power, t h e r e p o r t of a t o r n a d o i s a newsworthy e v e n t .
On t h e evening of J u l y 20, a d e v a s t a t i n g t o r n a d o r i p p e d a m i l l i o n
d o l l a r p a t h of d e s t r u c t i o n a c r o s s s o u t h e r n Manitoba. F o r t u n a t e l y t h e r e
was no l o s s of l i f e . The p a t h of t h i s s t o r m was i n a r e a d i l y a c c e s s i b l e
a r e a of t h e Province. I t p a r a l l e l e d a main w e s t - e a s t highway and c r o s s e d
a r e s o r t town where a p r e s s photographer was v a c a t i o n i n g . It was a l s o
s i g h t e d by s e v e r a l t e c h n i c a l l y t r a i n e d i n d i v i d u a l s . Thus i t s p r o g r e s s
and many d e t a i l s of t h i s p a r t i c u l a r s t o r m a r e w e l l documented.
This a r t i c l e w i l l a t t e m p t
t h i s e v e n t f u l evening.
t o d e s c r i b e some
of
t h e happenings
on
THE SYNOPTIC SITUATION
The s y n o p t i c c h a r t f o r J u l y 2 1 a t OOOOZ i s shown i n F i g u r e 1. T h i s
c h a r t shows a low p r e s s u r e a r e a a s s o c i a t e d w i t h t h e Maritime f r o n t movi n g e a s t a c r o s s c e n t r a l Saskatchewan. A t t h e same time a wave on t h e
P o l a r f r o n t i s moving e a s t w a r d a l o n g t h e I n t e r n a t i o n a l Border and was
c e n t r e d a t P i l o t Mound. The i n i t i a l development of t h e t o r n a d o took
p l a c e a t t h e c r e s t of t h i s wave.
The p r e d i c t i o n of s e v e r e weather o v e r t h e s o u t h e r n P r a i r i e s h a s
been c l o s e l y s t u d i e d o v e r t h e p a s t few s e a s o n s by t h e P r a i r i e Weather
C e n t r a l a t w i n n i p e g ( l ) . These s t u d i e s have developed a s e t of c r i t e r i a
f o r t h e o c c u r r e n c e of s e v e r e w e a t h e r and r e f e r e n c e . t o F i g u r e 1 w i l l
i l l u s t r a t e some of t h e s e f a c t o r s .
The S h i l o e v e n i n g r a d i o s o n d e o b s e r v a t i o n was missing.
However, a
s o u t h e r l y wind of 40 k n o t s was r e p o r t e d a t Winnipeg, and Fargo a l s o r e p o r t e d a wind of t h e same magnitude a t 5000 f e e t . T h i s i n d i c a t e d t h e
development of a pronounced s o u t h e r l y low l e v e l j e t .
This r e s u l t e d i n
f o r c i n g a narrow tongue of m o i s t a i r northward w i t h t h e a x i s d i r e c t e d
towards t h e s c e n e of t h e occurrence. Dew p o i n t s i n t h i s moist tongue had
i n c r e a s e d s t e a d i l y and had reached a s h i g h a s 74OF a t Grand Forks. Maximum t e m p e r a t u r e s i n t h e warm s e c t o r had exceeded 100°F over South Dakota
and reached w e l l i n t o t h e n i n e t i e s i n North Dakota. I t w i l l b e s e e n
from t h e diagram t h a t t h e maximum temperature r i d g e r u n n i n g from D e v i l ' s
Lake t o Aberdeen i s l o c a t e d west of t h e low l e v e l m o i s t u r e tongue. There
i s a l s o a s t r o n g i n t r u s i o n of dry a i r from w e s t e r n North Dakota where
dew p o i n t s have dropped s h a r p l y t o a s low a s 290F a t Dickinson. I t h a s
been recognized t h a t t h e placement of t h e low l e v e l h o t t e r and d r i e r a i r
a d j a c e n t t o t h e moist tongue and a wind flow t h a t p r o v i d e s a s t r o n g grad i e n t from d r y t o moist i s an e s s e n t i a l parameter i n tornado development.
The Showalter Index i n t h e warm s e c t o r a i r mass was -2.
T h i s would
be g e n e r a l l y accepted a s l e n d i n g weak t o moderate s u p p o r t t o t o r n a d o development.
A t upper l e v e l s t h e j e t s t r e a m extended eastward a c r o s s t h e a r e a of
i n t e r e s t from a maximum of 110 k n o t s o v e r Montana. The tornado o c c u r r e d
a t t h e r i g h t e x i t , g e n e r a l l y considered a f a v o u r a b l e l o c a t i o n .
Radar s c a n n i n g from t h e P r a i r i e Weather C e n t r a l had i n d i c a t e d t h e
development of a s t r o n g l i n e of echoes from n o r t h t o south-west
of Lake
Manitoba t o t h e wave c r e s t . Tops were r e p o r t e d t o 52 thousand w i t h an
a r e a of maximum t o p s t o 60 thousand i n t h e s o u t h e r n end of t h e l i n e .
THE PATH OF THE STORM
The tornado t r a v e r s e d a p a t h roughly 25 m i l e s i n l e n g t h o v e r a
p e r i o d of approximately two hours. The p a t h and p o i n t s of p r i n c i p a l
known damage a r e shown i n F i g u r e 2.
The f u n n e l was f i r s t s i g h t e d by M e t e o r o l o g i c a l T e c h n i c i a n B a l l e g e e r
a t approximately 23302 n e a r t h e j u n c t i o n of Highways 3 and 34 e a s t of
P i l o t Mound.
( i i n Fig. 2) By d r i v i n g a l o n g Highway 3 eastbound t h e p a t h of t h e s t o r m
was followed. I t was s e e n t o touch down i n a hay f i e l d n o r t h of t h e
highway and r a i s e a l a r g e column of d u s t . S w i r l i n g a l o n g i t c r o s s e d a
farmyard on t h e brow of t h e h i l l l e a d i n g t o t h e Pembina River Valley ( i i
i n Fig. 2).
The s t o r m t h e n swept down t h e narrow b u t s t e e p gorge l e a d i n g t o t h e
r i v e r . Large a r e a s of t r e e s were l e v e l l e d , a park d e s t r o y e d and extens i v e damage i n f l i c t e d on a farmyard. Dave Bonner of t h e Winnipeg Free
P r e s s , who was v a c a t i o n i n g i n t h e a r e a took photographs a s t h e s t o r m approached o v e r t h e c r e s t of t h e h i l l . From t h e r e i t continued down i n t o
t h e v a l l e y , c r o s s i n g t h e a r e a of t h e s k i lodge and c a b i n s on t h e s o u t h '
s i d e of t h e v a l l e y ( i i i i n Fig. 2 ) .
I n F i g u r e 4 t h e t r a i l e r h a s been o v e r t u r n e d , t h e t e n t t h a t was n e a r
t h e c a b i n and t h e f e n c i n g s u r r o u n d i n g i t have been blown away. But more
d r a m a t i c a l l y , t h e s m a l l w h i t e b u i l d i n g t o t h e r i g h t of f i g u r e h a s been
completely d e s t r o y e d .
, F o r t u n a t e l y , t h e s t o r m crossed t h e s o u t h s i d e of t h e v a l l e y and
w h i l e t h e r e was c o n s i d e r a b l e roof damage and t h e top of one of t h e g r a i n
e l e v a t o r s i n town was blown o f f , t h e town was s p a r e d major damage. A s i t
l e f t t h e e a s t s i d e o f t h e v a l l e y t h e f u n n e l appeared t o be l i f t i n g
and o n l y s l i g h t t r e e damage and a few downed t e l e p h o n e p o l e s were v i s i b l e i n t h i s area.
However, t h e s t o r m s t r u c k w i t h renewed f u r y on t h e p l a i n s two m i l e s
n o r t h e a s t and above t h e town of La R i v i g r e ( i v i n Fig; 2). S e v e r a l farmy a r d s were d e s t r o y e d i n t h i s a r e a . From t h e r e i t continued on a n e a s t n o r t h e a s t t r a c k , damaging s e v e r a l more farmyards a s i t passed some t h r e e
m i l e s n o r t h of t h e town of Manitou ( v i n Fig. 2 ) .
Reports of damage become s k e t c h y beyond t h i s p o i n t b u t a f u r t h e r
A t this
s i g h t i n g was made j u s t w e s t of Miami a t 01302 ( v i i n Fig. 2 ) .
time t h e f u n n e l was c r o s s i n g an open f i e l d and was observed t o l i f t from
t h e ground and draw back i n t o t h e cloud. A p a t h of heavy r a i n continued
on a n e a s t e r l y c o u r s e c r o s s i n g Highway 75 n e a r S t e . Agathe l a t e r i n t h e
evening ( v i i i n Fig. 2 ) .
EYEWITNESS REPORTS
A s i n a l l such c a s e s t h e r e were many eyewitness r e p o r t s and det a i l s of p e r s o n a l a d v e n t u r e . There i s a r e s i d u a l s c e p t i c i s m about t h e
o c c u r r e n c e s of t o r n a d o e s i n t h i s a r e a and many of t h e s e r e p o r t s helped
t o d i s p e l t h i s f e e l i n g and confirm t h e p r e s e n c e of a tornado.
The P o l i c e Chief i n t h e town of ,Manitou r e p o r t e d t h a t " A t about
c l o u d s began t o form and c i r c l e clock-wise. The c l o u d s gained
momentum and a cone dropped down t o t h e g r o u n d , l i f t i n g and t o u c h i n g down
i n a zig-zag p a t t e r n . "
Another w i t n e s s r e p o r t e d , "I s e e n h e r coming, a w h i t e s t r e a k and b l a c k ,
about 150 f e e t a c r o s s , s h e was j u s t a-boiling."
7 p.m.the
There were s e v e r a l r e p o r t s of t h e accompanying n o i s e . One l a d y des c r i b e d i t a s a " t e r r i f i c r o a r i n g " w h i l e a n o t h e r w i t n e s s s a i d i t approached w i t h "a huge sound l i k e a magnified furnace".
S e v e r a l r e p o r t s i n d i c a t e d t h a t i t looked l i k e a p i l l a r of d u s t and
smoke. One d e s c r i p t i o n added t h a t i t was " l i f t i n g b a l e s of hay, t r e e s ,
and o t h e r t h i n g s , d i s - i n t e g r a t i n g them and spewing them o u t . I t was
d a r k and looked l i k e an atomic bomb mushroom. I t was i n no h u r r y . There
was a b i g b l a c k cloud h o v e r i n g o v e r i t . A f t e r t h e t o r n a d o , r a i n f e l l " .
Among t h o s e b e l i e v i n g i t t o b e a cloud of smoke was a m o t o r i s t . He
r e p o r t e d , "We went t o have a look. The c a r began t o sway from s i d e t o
s i d e . The c a r was swerving, t h e n i t was going o v e r and over. "The c a r
landed u p s i d e down i n t h e d i t c h and t h e t h r e e occupants were hospitalized.
Other r e p o r t s such a s "I h e a r d a mighty r o a r - i t was b l a c k o u t s i d e .
Our e l e c t r i c c l o c k stopped a t 7:50 p.m." h e l p e d
t o p i n down t h e p a t h of t h e storm.
I went t o t h e basement.
One farmyard was comThere were a l s o t h e " f r e a k occurrences".
p l e t e l y d e s t r o y e d w i t h t h e lumber smashed and t w i s t e d and d r i v e n i n t o
t h e s u r r o u n d i n g t r e e s . The o n l y t h i n g s remaining were t h e cement foundat i o n and a h o r s e . The l a t t e r was unharmed even though i t was i n t h e
b a r n when i t was demolished.
I n a n o t h e r farmyard, f o u r b u i l d i n g s were i n a row. The f i r s t and
t h i r d escaped damage. The o t h e r two were completely demolished. S t e e l
g r a n a r i e s , i n a n o t h e r i n s t a n c e , were picked up and d e p o s i t e d a s t w i s t e d
wreckage more t h a n a q u a r t e r of a m i l e away w h i l e metal r o o f i n g
was
wrapped around t r e e s a t t h e same d i s t a n c e .
Another f a m i l y r e p o r t e d almost unbearable p r e s s u r e on t h e i r e a r
drums a s t h e s t o r m passed over t h e i r house. They had t a k e n s h e l t e r i n
t h e basement. Suddenly t h e house l i f t e d a few i n c h e s and s e t t l e d back
on i t s f o u n d a t i o n , smashing windows and doors. A t t h e same time t h e r e
was immediate r e l i e f from t h e p r e s s u r e .
THE FORECAST PROBLEM
On t h i s p a r t i c u l a r day t h e p o s s i b i l i t y of s e v e r e s t o r m a c t i v i t y was
recognized a s e a r l y a s 2015002 and c o n s u l t a t i o n s were h e l d w i t h t h e Cent r a l Analysis O f f i c e i n Montreal and w i t h t h e Severe Weather Centre i n
Kansas C i t y . Reference t o t h e development of s e v e r e weather was made i n
t h e guidance i s s u e d by t h e P r a i r i e Weather C e n t r a l a t 2015452.
However, i t was n o t p o s s i b l e a t t h i s s t a g e t o p i n p o i n t t h e a r e a of
a c t i v i t y and f o r e c a s t s f o r t h e a r e a d i d n o t c a r r y tornado ' w a r n i n g s . A
f u r t h e r d e t e r r e n t under such circumstances i s t h e i n a b i l i t y t o c o n t r o l
t h e communications media, whose " e f f o r t s " on s e v e r a l p r e v i o u s o c c a s i o n s
have r e s u l t e d i n p u b l i c a n x i e t y b o r d e r i n g on p a n i c .
The a p p l i c a t i o n
tremely w e l l i n t h i s
fidently stated that
c o n d i t i o n a l upon i t s
of s e v e r e weather f o r e c a s t i n g t e c h n i q u e s worked exc a s e and w i t h a d d i t i o n a l e x p e r i e n c e i t can be cona worthwhile s e r v i c e t o t h e p u b l i c can b e p r o v i d e d ,
c o n t r o l l e d d i s s e m i n a t i o n and p r o p e r use.
10
-
continued on page 27
ACKNOWLEDGEMENTS
While t h e w r i t e r d i d spend some time i n t h e a r e a subsequent t o
s t o r m , much of t h i s material h a s been gleaned from p r e s s r - e p o r t s
other sources.
the
and
A deep d e b t of g r a t i t u d e i s owed t o M r . Dave Bonner o f t h e Winnipeg
F r e e P r e s s f o r t h e u s e o f h i s photographs and h i s comments; a l s o t o Mr.
W.Crowley and t h e Canadian F o r c e s Base a t P o r t a g e l a P r a i r i e f o r supplyi n g a e r i a l views of t h e a r e a ; t o Mr. A r t h u r B a l l e g e e r , a M e t e o r o l o g i c a l
T e c h n i c i a n a t t h e P r a i r i e Weather C e n t r a l who f o l l o w e d t h e s t o r m by c a r ;
and t o M r . R.A. Cauwenberghe, a p r o f e s s i o n a l e n g i n e e r who s u p p l i e d a
comprehensive r e p o r t of h i s s i g h t i n g o f t h i s storm.
REFERENCES
( 1 ) . On t h e F e a s i b i l i t y of I n c o r p o r a t i n g P r o c e d u r e s and F o r e c a s t s of
S e v e r e Summer Weather Storms and Winter Storms W i t h i n t h e P r a i r i e
Weather C e n t r a l .
(Unpublished submission by S.V.A. Gordon)
.
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- c o n t i n u e d overleaf