09h41 - 4 September 2012
Numerous model runs (up to 00Z on Tues 4th) indicate that an upper trough (moving up from the south) will bring instability over the eastern half of South Africa (Edit: starting on Wed 5th, and intensifying) on Thurs 6th and Fri 7th.
Heavy rain is possible in places, particularly along the coast of KZN, where a cutoff-low could form (depending on the position of the developing high pressure cell over the Indian Ocean).
I don't have a lot of detailed experience with this kind of large and complex system (nor of the areas involved), so I can't give too many details.
Two obvious factors that stand out in the charts are the cold upper trough, and the high levels of moisture along the KZN coast.
This custom GFS chart shows the 500 hPa temperatures at 12Z on Thurs 6th, showing the position of the upper trough (indicating the potential for significant upper instability). The upper winds will blow clockwise around this trough.
The rain is likely to occur over a fairly wide area, and a prolonged period, so it is hard to give a simple summary.
Edit: The NCEP GFS animations give a nice overview of the daily and 6-hourly rainfall during the period from 18Z on Wed 5th to 12Z on Fri 7th:
www.cpc.ncep.noaa.gov/products/african_d...rica/24h_precip.html
www.cpc.ncep.noaa.gov/products/african_d...tl/safrica/mslp.html
This custom GFS chart shows the average rainfall (in mm/day) for the 60 hour period from 12Z on Wed 5th to 24Z on Fri 7th. Cumulative totals of up to 100mm are possible in places along the KZN coast.
Compiled by Gordon Richardson - Stormchasing SA
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