Extra Tropical Transition

General information: 5 August 2004 1200 UTC
Alex developed from a low pressure system off the northeast Florida coast and became the first tropical storm of the 2004 season on August 1. Alex intensified into a hurricane while brushing the Outer Banks of North Carolina on August 3. It then intensified into a major hurricane while tracking northeastward near the northern edge of the Gulf Stream on August 5. Alex was an atypical tropical cyclone in that it became the most intense hurricane north of 38°N when it reached its maximum intensity of 105 knots. (William M. Gray1 and Philip J. Klotzbach 2004)

300 hPa contours
Anticyclone was positioned over western Atlantic and American Case board. A broad zonal mobile flow extended across the Atlantic north of 45 Deg N. An Omega block was evident over Greenland and the Labrador straits. A confluent trough lay over the Mid Atlantic.

300 hPa winds
A jet stream with core speed 115 KT lies to the north of the Hurricane.

300 hPa isotachs
A jet stream with core speed 115 KT lies to the north of the Hurricane.

500 hPa contours
A broad zonal mobile flow extended across the Atlantic north of 45 Deg N.

Thickness
Warm tropical air extends south of Nova Scotia, with thickness values in excess of 5820 evident. Overlay of thickness fields and contour fields show warm advection occurring over the Atlantic. The cirrus shield on the IR indicating the extent of the warm advection this verifies well with the model fields.

850θw
High values of θ W of 20-22 Deg C extend from the Hurricanes source region. weak frontal band is evident over the Labrador straits, further frontal systems are evident over Europe.

Water Vapour 6.2 µm
In the far right of the image over New Foundland we see a dark elongated shape. This area indicates where a jet lies to the north of Alex. The relative dark contrast indicates that dry air high PV is being drawn down from the stratosphere. An upper trough lies over the central Atlantic extending from Greenland with an old low also evident to the west of the UK.An area of cloudiness lies to the north of the jet over the Labrador straits, this area of cloudiness is an occluding frontal system and weak baroclinic zone.

Water Vapour 7.3 µm
In the far right of the image over New Foundland we see a dark elongated shape. This area indicates where a jet lies to the north of Alex. The relative dark contrast indicates that dry air high PV is being drawn down from the stratosphere to relatively low levels . An upper trough lies over the central Atlantic extending from Greenland with an old low also evident to the west of the UK. An area of cloudiness lies to the north of the jet over the Labrador straits, this area of cloudiness is an occluding frontal system and weak baroclinic zone.

Latitude and Longitude grid.

PV at 300 hPa
A PV maximum is seen to the north of hurricane Alex associated with the jet stream with a baroclinic zone extending from Nova Scotia. We also see a PV maximum extending from Greenland to the west of the UK. Both these are relative to dark areas seen in the satellite imagery and signify entrenchment of dry stratospheric air on the cold side of the jet steam. We also see minor PV anomilies extending over the Atlantic.