Currently, we are at the beginning of a very interesting weather period that is starting with a pronounced north atlantic cyclogenesis.
I intend to examine it by analysing satellite imagery and then forecasting its further development by some weather maps. Of course,
the weather maps taken from GFS 12z will decay....however, the fundamental aspects of that rapid cyclogenesis will be the same.
Several surface low pressure systems are displayed which can be identified by coiling frontal systems or thicker cloudiness bands. In the Southeast of Greenland, you can see a broad cloudiness field , the so-called "barocline cloudiness field" . It consists of homogenous deep cloudiness marking the warm front and lower clouds indicating the cold front which extends to the west up to Newfoundland. To the south of Greenland, there is a long , unstructured cloud band, the occlusion front. Therefore, remembering the origin of the cyclone, we should not forget that it already had been a mature cyclone. In opposite to cyclones developping from frontal waves.
The next picture shows the developping severe cyclone to the left. The black arrow indicates a dry intrusion (see below). The structure of the barocline cloud shield is remarkable because the edges are very sharp bounded. It seems like a triangle moving with the upper flow southeastward. In my opinion, this "triangle" is a good metaphoric illustration for what is happening in reality. The warmer air masses are advected northeastward while the colder air masses moving southeastward. As a consequence, the cyclogenesis continues with coiling frontal systems.
In the south of Iceland, there is a weaking comma low with coiling deep-moist convection bands. The cold front of the large low pressure area in the east of Iceland crosses Germany with little rain inside. A weak Genua cyclone has moved eastward affecting most parts of the central and eastern alpine region. A wave with partly stronger precipitation is situated in Western France. It gradually moves northeastward affecting Benelux and Germany.
The WV Imagery shows a starting dry intrusion in the rear of the cold front and to the south of the occlusionfront. The dry intrusion brings high values of isentropic potential vorticity (IPV) into lower levels. Having small IPV-Anomalies one can use the following equation :
That is, decreasing static stability will lead to increasing absolute vorticity (consisting of relative vorticity and planetary vorticity). Now, you have high static stability in the layer between stratosphere and troposphere (tropopause) with respective high potential temperatures (high IPV values ). Descending flow lead to dry adiabatic warming and conservation of the high IPV values. However, when the dry intrusion crosses the trough axis, the static stability will strongly decrease. As a result of the aforementioned equation, you will have a strong augmentation of absolute vorticity. Neglecting the planetary vorticity (relatively constant along a certain latitude) , the increasing relative vorticity leads to stronger upward motion and pressure fall at the surface. Thus, one can explain the strong pressure fall with rapid cyclogenesis (> 24 mbar in 24h) by the presence of a dry intrusion.
The upper flow is strongly meandering in the mid-latitudes with a broad long-wave trough from Iceland over Ireland up to the Azores. UK is situated ahead of the trough axis in a weak southwesterly flow increasing over North France , Benelux and Northern Germany. The Northwest Atlantic is dominated by a striking jetstream with a negative tilted trough axis extending from Southern Greenland to Newfoundland. As displayed by the white cirles , you can see that the analyzed surface cyclones have different stages of development. The comma low in the south of Iceland directly lies in the trough axis, thus no further deepening could be expected, as well as with the strong surface cyclone in the east of Iceland. The wave in the Gulf of Biscaya is situated ahead of a small-scale trough axis. Resulting cyclonic vorticity advection is not very well pronounced - thus, the further development in the charts below shows no deepening.
Our area of interest will be the white circle to the south of Greenland where the developping cyclone is situated ahead of the striking trough axis. The combination of strong cyclonic curvature vorticity advection and shear vorticity leads to high values of cyclonic vorticity advection.
I marked the trough axis pink. The location of the low pressure center is directly ahead of the trough axis with strong cyclonic curvature vorticity advection. Additionally, the decreasing distance between the isolines indicates the exit region of the jetstream. The low pressure center lies in the left exit region where upward motion predominates. Therefore, this cyclogenesis benefits from both cyclonic and shear vorticity advection. Aside from it, the cold front transform to a warm front over Newfoundland where a new high-energetic wave is oncoming.
12 hours later, the low pressure center continous its position ahead of the trough axis. However, it is shifted more towards the center of the upper level trough. The occlusion front is no longer linear but coils up. How could we interprete this indications ? The rapid cyclogenesis has reached its mature stage. The smaller the distance between trough (axis) and low pressure center and the more the occlusion front coils up, the faster it will reach its culmination.
The last map shows the location of the low pressure system directly below the upper-level trough center. That is, the axis between between both systems in height and at the surface will be normal. No further advection both temperature and vorticity advection will be possible. The system will have its culmination with around 960hPa. The frontal system will affect all part of the British Isles with severe precipitation and partly damaging wind gusts (850hPa average wind exceeding 35 m/s !).
Update:
Severe cyclone has reached 960hPa at 12 UTC, now you can see the occlusion front wrapping around the low pressure center. In the trough sector, very cold and dry air is advected southeastward. The cold front shows a weak wave delopment near Ireland.
Look at the WV Imagery :
Very impressive dry intrusion that overruns the low pressure center indicating that culmination will be soon. A second branch ejects along the cold front where the wave development takes place. What does that mean? The cold front spreads over GB in the night hours with intense precipitation. Most of it should be linear convection. Average winds in 850hPa of 70Kn (35 m/s) suggest that widespread damaging wind gusts (>30m/s) can be expected. In the rear of the cold front numberous showers (partly electrified) are expected to occur, always still with a threat of severe wind gusts, especially in Ireland, Scotland and Northern England. Convective activity will continue the next days but with gradually weaking winds.
© Felix Welzenbach