56

Fig.  6.16.1.  Pressure  pattern  over  Europe  and  wind  field  over  the  Baltic  Sea  on  11  April
1997,  18  UTC
On  10  April,  successive  troughs  of  low  pressure  moved  across  Scandinavia  and  the  Baltic
Sea,  causing  a  southeastward  flow  of  warm  air  and  an  inflow  of  cold  Arctic  air  in  their  rear.  In
front  of  the  trough  and  in  its  southern  part,  a  westerly  gale  followed  by  a  westerly  storm  of  8-9
  Bft  developed  over  the  eastern  North  Sea,  Kattegat  and  the  Western  Baltic  Sea  in  the
afternoon.  Around  midnight  and  in  the  early  hours  of  11  April,  wind  speeds  in  front  of  the
advancing  cold  front  occasionally  exceeded  10-11  Bft  over  the  western  basins,  with
directions  backing  temporarily  west-southwest.  The  storm  was  less  severe  in  the  eastern
basins  of  the  Baltic  Sea.
Around  09  UTC  on  11  April,  the  storm  behind  the  cold  front  veered  NW  and  reached  8  to  9
Bft  in  gusts.  At  about  15  UTC,  a  secondary  cold  front  crossed  the  coast,  with  winds  veering
NNW  -  N  and  reaching  10  Bft  in  gusts.  In  the  afternoon  and  during  the  night,  the  winds  from
northerly  directions  were  still  gusty  but  decreased  gradually  to  7-5  Bft  in  the  early  hours  of  12
April.
In  the  course  of  12  April,  a  high  pressure  ridge  began  to  develop  over  Scandinavia,  which
was  later  destroyed  by  a  southward  moving  low-pressure  trough  on  13  April.  In  front  of  the
warm  front  which  crossed  the  southern  coast  at  18  UTC  on  13  April  and  became  stationary
until  16  April,  SW  winds  increased  to  about  6-7  Bft  between  09  and  15  UTC  on  14  April.
Wind  directions  on  14  April  generally  veered  from  WSW  to  W  -  NNW,  but  in  the  central  part
of  the  Southern  Baltic  Sea  they  oscillated  locally  over  a  rather  wide  range  due  to  squall-like
disturbances  generated  in  the  pre-frontal  zone.
After  the  front  had  crossed  the  coastline  at  about  18  UTC  on  14  April,  the  northerly  air  flow  of
force  6-7  Bft,  which  reached  8  Bft  in  gusts,  established  itself  along  the  eastern  edge  of  the
high  pressure  ridge,  which  became  stronger  over  Scandinavia.  As  the  “old”  quasi-stationary
trough  did  not  retreat  (Fig.  6.16.2.),  the  pressure  gradient  zone  with  its  northerly  winds
remained  over  Scandinavia  and  the  Baltic  Sea  until  16  April,  decreasing  gradually.