Thursday, 27 September 2012

Update: Cold-front, trough & cutoff-low 26-29 September 2012


The last few model runs (GFS and NOGAPS up to 06Z on Wed 26th) have shown small but significant differences in the outlook beyond Fri 28th, and conditions seem unfavourable for the development of a cutoff-low. Many of the other elements will still be in place, and it will be very cold, wet and windy at times.

Edit: The morning's NCEP GFS 500hPa ensemble shows the upper trough pushing much less far inland than previously predicted, and exiting more rapidly via the east coast on Sat 29th: www.cpc.ncep.noaa.gov/products/african_d...nsm_spa_hgt500a.html


The surface low currently to the SW of Cape Town is predicted to intensify and persist, which will prevent the High Pressure cell from moving in, and keeping the south-easterly winds far offshore. Development of a cutoff-low would require the feeding of warm and moist surface air underneath the cold upper trough.

Rainfall along the Wild Coast (Eastern Cape/KZN border) will be enhanced by the interaction of the upper trough with warm and moist surface air. Intense thunderstorms are possible over the central interior, due to strong jet stream winds.

Edit: The NCEP GFS animation of the 6-hourly precipitation and surface winds and pressure gives a very nice overview of the development of the system over the next 72-84 hours: www.cpc.ncep.noaa.gov/products/african_d...tl/safrica/mslp.html


This custom GFS chart shows the likely surface winds at isobars at 00Z on Fri 28th.



Compiled by Gordon Richardson - Stormchasing SA

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