Re: Cold-fronts & cutoff-low SW-Cape 18-21 Oct 2012

This morning (Fri 19th) it is rather cold in Cape Town (even by winter standards), with persistent rain showers. It seems that more rain fell after the passage of the 'cold-front', with some lightning strikes about 100km offshore. The freezing level is currently below 1500m, so snow is highly possible this morning over the SW-Cape mountains.

Edit: Just after I posted this comment, I got an SMS from Xmesox confirming that there is snow on the Hottentots Holland mountains near Somerset West

Edit: Xmesox also mentioned snow at the Matroosberg. Their Facebook page just updated with a photo of a wedding couple surrounded by snow flurries!!
www.facebook.com/pages/Matroosberg-Priva...Reserve/447597915289

Edit #2: I didn't post a 'snow' chart on this thread yesterday, since conditions looked marginal. This morning's chart (00Z on Fri 19th) comes a bit late, but clearly shows the likely area of snow coverage:

Click on image to view full-size.



This custom GFS chart shows the likely rainfall (in mm/day) during the 60 hour period from 00Z today (Fri 19th) to 12Z on Sun 21st. Cumulative totals in excess of 75mm are possible in places, increasing over coastal mountain ranges (due to orographic effects).

Click on image to view full-size.


This current scenario is well illustrated by the EUMetsat Airmass animation and SAWS ship synoptic chart, which show a moderate intensity surface low (central pressure < 1004hPa) passing just south of Cape Town:
oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/SOUTHERNAFRICA/
metzone.weathersa.co.za/images/articles/ma_sy.gif

So far this is all normal for a late-winter cold-front, which will clear later in the day. What comes next is different, with the upper trough persisting (as described previously), and the eventual formation of a cutoff low on Sat 20th and Sun 21st. The SAWS has issued warnings for heavy rain tomorrow (Sat 20th) in the Overberg, parts of the Cape Winelands and the Eden District (but not yet for Sun 21st).

This morning's GFS model run (00Z on Fri 19th) again shows a difference in the prediced rainfall distribution. This is more likely to be a 'band' than a single peak, with heavy showers at first today (Fri 19th) over the SW-Cape. Heavy rain is likely over the southern Cape (near Port Elizabeth) on Sat 20th, with the peak shifting westwards to the Overberg overnight into Sun 21st. The cutoff-low will move offshore late on Sun 21st (a bit earlier than predicted).

This custom GFS chart shows the likely rainfall (in mm/day) during the 60 hour period from 00Z today (Fri 19th) to 12Z on Sun 21st. Cumulative totals in excess of 75mm are possible in places, increasing over coastal mountain ranges (due to orographic effects).

Click on image to view full-size.

Compiled by Gordon Richardson - Stormchasing SA