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WVI example

This led to high rainfall totals and 'damaging' winds in the NE Atlantic/NW European region in late autumn 2002 (The "Prestige" Storm).

 

Water vapour example & 300 hPa

 

 

 A  Dry slot of rapidly descending stratospheric air to the immediate rearward of the synoptic feature. Strongest gusts occurred as the leading edge of this feature encountered landfall over northern Portugal/NW Spain.
 B  Area of broad-scale ascent associated with the warm conveyor of the developing low.
 Other notes:  This magnificent image in the 'water-vapour' channel from the ESA/EuMetSat 'Meteosat' platform neatly captures the major development that occurred through the 13th November 2002, which in the next few hours was to produce high-winds and heavy rain over NW Iberia, with gusts reported to at least 60 knots. One ship in Sea Area 'FitzRoy' reported a mean wind of 65 knots as the leading edge of the 'dry slot' swung east. (Incidentally, this storm was responsible for the severe damage to the Tanker "Prestige", which subsequently broke up on the 19th, with consequent loss of its contents - a major pollution episode ensued.)

During the evening/night to come, the storm swept NE to bring heavy rainfall (with flooding) to many parts of NW Europe (including Britain), along with high winds. Squally winds were reported for the west, and later the northern Departments of France, and in the period 14/0300 to 14/0900 UTC, gusts of at least 60 knots were reported just inland of the central and eastern English Channel, with 'on-coast' gusts to at least 70 knots (possibly more in isolated spots).

 

 1  Note the implied accumulation of air just below the jet-core as the high-speed air on the NW'ly jet has to slow abruptly as it encounter the weaker & markedly cyclonically curved gradient in the trough axis: the air has to spread out and descend, encouraging the downward penetration of high-PV/stratospheric air. This action brings air having a high velocity to low altitudes, mixing with the already 'perky' low-atmosphere flow.
 2  If the air is going down sharply (1), then it must be going up somewhere, and this region, just forward of the highly diffluent trough and associated PVA-max region, is just one such, where heavy rainfall was reported.
 3  The equatorward 'tail' of the cirrus outflow shield, curling back around the dry intrusion, where in former times a classical occlusion might have been drawn, but is really an upper-air feature, and not necessarily frontal either. Various workers have found that the strongest gusts (possibly tornadic-enhanced) occur near/just in front of the 'tail' of this feature in the dark slot.