Thursday, 18 October 2012

Update: Cut-Off Low appears to be forming along the Southern Cape Coast (Period: 18 - 21 October 2012)

The 12Z GFS model run (Wed 17th) has again increased the intensity of the cutoff-low, and shifted the position of the peak rainfall closer to Cape Town. The latest GFS model prediction is quite close to the current SAWS forecast (attached by Cousen).

Do not mock this system, it will be a serious weather event! The nature of this event is likely to change over the 4 day period, from an inital cold-core upper trough and cold-front, to a warm core cutoff-low as it advects moisture from the Agulhas current.

The individual model runs fluctuate a lot, but the potential for heavy rain remains high. The 12Z NOGAPS model run (Wed 17th) is now quite close to the GFS prediction (although it has been through a number of detail differences).

Edit: Recent model runs (up to 12Z on Wed 17th) indicate that the freezing level will be below 1800m for a short time during the morning of Fri 19th. Light snow is possible on the SW-Cape mountains, though it is likely to melt during the day, as warmer rain falls during the afternoon.

This custom GFS chart shows the likely average rainfall (in mm/day) during the 60 hour period from 00Z on Fri 19th to 12Z on Sun 21st. Cumulative totals in excess of 100mm are possible in places. Orographic effects and onshore moisture flow over the mountain ranges in the Overberg could produce even higher totals in places.

Compiled by Gordon Richardson - Stormchasing SA

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