Potential Vorticity A strong PV anomaly clearly coincides with the dry slot between the cloud head and the cloud band. The isolines of PV=1 reach down to the surface. The influence of this PV maximum on the entire development is evident.
Isotachs
The isotachs show a similar distribution to the situation of six hours before. Jet streams are found southeast of the cloud head as well as on its poleward edge. The cloud head itself is not superimposed by high wind speeds.
Temperature Advection The field of temperature advection shows a maximum of WA within and in front of the cloud head, and CA directly behind it. A strong maximum of CA near the surface is found within the cloud band of the cold front.
Vorticity Advection The distribution of vorticity advection is similar to the situation six hours before. PVA in all layers is associated with the dry tongue while PVA in the region of the cloud head is restricted to upper layers.
Relative Humidity The distribution of relative humidity at 18 UTC shows an impressive image. The dry slot is clearly revealed by the model. Low values of relative humidity reach down until almost 800 hPa in a very narrow zone.
