Rapid cyclogenesis

The Low pressure system crossing the South Tasman Sea and Foveaux strait today is well positioned to encounter rapid cyclogenesis.

In other words the central pressure of this system is expected to “drop” from 987 hPa at 18/00UTC (1pm Wednesday NZDT to 962 hPa by 19/00UTC  ( 1pm Thursday NZDT).   


Forecast Central pressure every 3 hours  (e.g. 987 18/00=  987hPa at 00UTC on 18 Jan) of the passing Low as forecast by Meteo France and displayed in EXPEDITION software  Map is for 18/21UTC= 10am Thursday NZDT


This is a classic textbook case for explosive cyclogenesis –

The system:

1)  is moving from warm water/air near Australia to cooler water/air near NZ –destabilising the surface air  (Warm advection)

2) and  is underneath the pole-ward exit region of a wide and intense jetstream today—this helps remove air from the top of system faster than it can be replaced, sucking air up much like a hose sucks a garden  spray, and quickly lowering its surface pressure. (needed for rapid cyclogenesis) .

3) and then is expected to encounter the apex of a passing upper trough – this helps it twist and spin.  (cyclonic vorticity advection)

4) All this is happening near the Southern Alps , so the flow around the system is being disrupted by the terrain, and energy is being added by the release of heavy rain as air is blown up and over the western slopes.



The  surface low L  shown on a map of the passing upper trough at 200hpa level as seen on Windy


This is a classic illustration of what happens when individual terms in the  OMEGA equation combine together.

To read more about Omega equation see https://en.wikipedia.org/wiki/Omega_equation)

When the central pressure of a low drops 24hPa in 24 hours (when corrected to latitude 60south) that’s the definition of 1 Bergeron  or 1B, taken by some meteorologists as the threshold for a “meteorological Bomb” see https://en.wikipedia.org/wiki/Explosive_cyclogenesis

We live in interesting times.