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

23 September 2012: I may as well post some charts for this system, but with the caveat that there is still considerable uncertainty in the position, timing and intensity. A 'ball-park' accuracy estimate at this duration (6-7 days or 144-168 hours in advance) would be within 200-300km in position, 12-18 hours in timing, and 50-200% in amount of rainfall.

This custom GFS chart (based on the 12Z GFS run on Sat 22nd) shows the likely average rainfall (mm/day) during the 36 hour period from 12Z on Fri 28th to 24Z on Sat 29th. Cumulative rainfall totals in excess of 100mm are possible in places.

Note: The NOGAPS model (and SAWS forecasts) all show somewhat different outcomes, so this chart must not be taken literally. Forecasts close to the time should give better accuracy, but cutoff-lows are inherently unpredictable.

22 September 2012: Numerous model runs (GFS and NOGAPS up to 06Z on Sat 22nd) indicate a connected sequence of events driven by a major frontal trough over most of South Africa between Wed 26th and Sat 29th. The first will be a cold front brushing the SW-Cape on Wed 26th and Thurs 27th, bringing some rain, and cooler temperatures (again).

The (cold cored) upper trough associated with the front is likely to push far inland on Fri 28th, and develop into a cutoff-low as the surface high pressure pushes in behind. The cutoff-low is likely to intensify on Sat 29th, as upper instability interacts with surface moisture and heat from the tropics, producing widespread heavy rain over the interior of SA.

It is difficult to give details of the position and intensity of this system (particularly at the longer durations), but there is no doubt that this will be a significant weather event. Cutoff-lows have the potential to produce rainfall in excess of 50-100mm/day, so this system should be watched carefully.

Rather than posting a specific prediction (which will soon be out of date), the GFS ensemble of the 500hPa isolines gives an idea of the intense disturbance in the upper circulation, as well as the uncertainty at 6 days (144 hours) ahead: www.cpc.ncep.noaa.gov/products/african_d...nsm_spa_hgt500a.html

This NCEP GFS ensemble chart shows the heights of the 500hPa isolines at 00Z on Fri 28th, as the trough develops over the western interior of SA:

Compiled by Gordon Richardson (Stormchasing SA)