Photos: Stormchasing: Kwazulu Natal (4 November 2012)

Thought I'd share some storm chasing photos I've taken so far of this years storm season. All photos taken in KZN, from Dundee, Washbank, Howick - Sheldon Strydom

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

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)

Upper trough & rain Eastern-RSA 5-7 Sep 2012

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

Update: Cold-fronts and more snow SW&S-Cape 31August 2012 - 01September 2012

Not a lot to report this morning (Tues 28th), with various model runs 'wobbling' quite a lot in the last 24 hours, but gradually starting to converge. As mentioned yesterday, the details will only become clearer in the next 12-18 hours, once the surface low has developed.

Light to moderate snow is likely over the SW-Cape ranges between 1400-1600m during the morning of Sat 1st (with more at higher elevations 1600-1800m), and spreading to the southern Cape mountain ranges during the day.

Recent model runs suggest less rainfall than initially predicted, due to a shift in the upper trough (which would have created upper instability). The latest SAWS forecast (11am on Tues 28th) has reduced the rainfall for Cape Town on Fri 31st from 40mm to a more realistic 30mm, but this is still a lot of rain in a 24 hour period!

Gale force winds (35 kts) are likely along the SW-Cape coast at times during Fri 31st, moving to the southern and Eastern Cape during Sat 1st. Moderate swells (4m) are likely (the relatively small size of the low will limit further development).

The front predicted for Sun 2nd and Mon 3rd seems likely to have minimal effects on Cape Town (though it will remain rather cold), but may have an impact on the southern Cape coast.


Edit: This afternoon (15Z on Tues 28th) the SAWS ship synoptic chart and EUMetsat Antarctic sector Airmass animation both show the developing low:
metzone.weathersa.co.za/images/articles/ma_sy.gif
oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/ANTARCTIC/

Edit: The low is currently positioned more than 3000km due west of Cape Town, and still has a very long time to evolve before landfall (72 hours or 3 days time). The models should now become a lot more consistent, with this detailed input data available.

Compiled by Gordon Richardson - Stormchasing SA

Cold-front and possble snow SW&S-Cape 30-31 August 2012

Recent model runs continue to show a severe frontal system (though with slightly later arrival) on Fri 31st, and snow extending into Sat 1st. Spring Day will definitely not be on the 1st day of September this year!

Numerous model runs (GFS and NOGAPS 12Z, 18Z and 00Z on Mon 27th) indicate that the freezing level will drop to 1400m over the SW-Cape, with some runs showing the level down to 1200m early on Sat 1st. The freezing level is likely to remain below 1800m for most of the weekend, but lack of moisture will limit snow to the southern Cape coastal mountain ranges (rather than further inland) during Sat 1st. The site snow-forecast.com shows this nicely: www.snow-forecast.com/maps/dynamic/southafrica

The weather over Cape Town is likely to remain unsettled until at least Sun 2nd and Mon 3rd, when another (weaker) cold front may brush the S-Cape coast. After that front, the GFS ensemble (9-12 day outlook) shows a dry spell for the Western Cape from Tues 4th until Fri 8th.

This custom GFS chart shows the possible extent of snow accumulation and freezing-level in the period up to 12Z on Sat 1st. The exact details are likely to change, since this chart is 132 hours (5.5 days) in advance. Note that the GFS model has a rather large grid-size, and very limited terrain detail of the Cape coastal mountain ranges.

Compiled by Gordon Richardson - Stormchasing SA

Update: Upper-trough and frontal-low SW-Cape 25 August 20

The storm arrived over Cape Town in the early hours of this morning (03Z on Sat 25th), almost exactly as predicted by the models. Strong wind gusts and heavy rain showers are noticeable at dawn (7am).

The Cape Town radar is working, and shows a few isolated storms passing directly overhead. I just saw flashes of lightning in the direction of Simonstown, and the CT radar confirms a storm cell with intensity > 50 dBZ passing over that area (0440Z).

The main front has already passed over Cape Town (03Z on Sat 25th), but heavy showers are likely at times for most of today, as the upper trough moves overhead.

The weather in Cape Town should start to clear on Sun 26th, as the low moves rapidly away to the east (offshore).

- Gordon Richardson - Stormchasing SA

Update: Cold-fronts and snow SW-Cape 19-20 and 21-22 August 2012

This morning's GFS model runs (00Z and 06Z on Sun 19th) are much less favourable for snow (below 1400-1600m) on Wed 22nd. This is frustrating, but not too surprising given the relatively weak overall size and intensity of these frontal systems (compared to the ones we had earlier this month).

The outlook for Fri 24th is still highly uncertain, with several recent model runs and forecasts all giving completely different scenarios (from a powerful cold-front to a weak cutoff-low).

The only constant seems that daytime temperatures for the SW-Cape are likely to remain low (close to 15C) for this whole week.

Compiled by Gordon Richardson - Stormchasing SA

Cold Fronts SW-Cape 10-12 Aug & 14-16 Aug 2012

Just a preliminary thread post about the upcoming storms (since people are already posting on another thread). I haven't been able to produce custom charts yet, due to problems with the NCEP nomad servers. The approaching systems can be seen developing on the SAWS ship synoptic chart, and EUMetsat Antarctic sector Airmass animation:
metzone.weathersa.co.za/images/articles/ma_sy.gif
oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/ANTARCTIC/

Numerous model runs and forecasts (GFS and NOGAPS up to 00Z on Wed 8th) indicate two separate frontal systems reaching the SW-Cape between Fri 10th and Thurs 16th. Both systems look quite intense, with the possibility of strong winds, heavy rain, and snow.

The NCEP GFS ensemble of the 500 hPa isolines give an idea of the intensity of the systems, as well as the uncertainty at the longer durations (5-6 days ahead). www.cpc.ncep.noaa.gov/products/african_d...nsm_spa_hgt500a.html

Edit: The first NCEP GFS ensemble chart shows the position of the 500 hPa isolines at 12Z on Sat 11th, and the second at 00Z on Tues 14th.

Edit: When the lower of the two 500 hPa isolines (5550m) is over land, the freezing-level is likely to be below 1500m, and snow is likely over the mountains.

Compiled by Gordon Richardson - Stormchasing SA

Re: Cold-front, upper-trough and more snow 5-9 Aug 2012

This afternoon's Aqua Modis overpass (Wed 8th) shows the massive amounts of widespread and thick snow on and around the Drakensberg and Lesotho: rapidfire.sci.gsfc.nasa.gov/imagery/subs...rg.2012221.aqua.500m

I was so surprised that I checked the false-colour image, to confirm that this is really snow (pale blue), now just low cloud (white). rapidfire.sci.gsfc.nasa.gov/imagery/subs...012221.aqua.721.500m

Edit: I used an image editor to extract the colour cyan (negative red), giving this view of the snow, with lingering low clouds masked out (grey).

Images: lance-modis.eosdis.nasa.gov + Google Earth.  Edited by Jboyx989

Compiled by Gordon Richardson (Stormchasing SA)

UPDATE: Cold-front, upper-trough and more snow 5 - 9 August 2012

I wasn't going to post any more charts on this thread, but the 12Z GFS run (Tues 7th) shows a shift in the peak rainfall towards land, with very heavy falls (> 100mm in 24 hours) along the southern KZN coast.

This custom GFS chart shows the likely rainfall during the 24 hour period from 12Z on Tues 7th to 12Z on Wed 8th. The peak roughly covers the coast from latt 32S 29E (E-Cape Wild Coast) to 30S 31E (Durban), and as far inland as 30S 29E (near Underberg).

Compiled by Gordon Richardson - Stormchasing SA

Update: Cold-front, upper-trough and more snow 5 - 7 Aug 2012

With (most of) the custom GFS models updating, and the outcome of this event coming closer, it is time to post a few charts. Freezing-level charts and likely snow can be seen on the site snow-forecast.com: www.snow-forecast.com/maps/dynamic/southafrica

The NCEP ensemble charts should also be available soon.

This custom GFS chart (based on the 00Z run on Fri 3rd) shows the temperature at the 500 hPa level (roughly 5000m elevation) at 00Z on Tues 7th (96 hours ahead). The extreme disturbance from normal conditions can be seen, with temperatures about 15C below the seasonal average (-30C vs -15C).

This disturbance is likely to have two effects, the first obviously being very cold conditions near the surface, with widespread snow. The second is instability, due to cold air moving over (relatively) warm and moist air near the surface. This could lead to rapid intensification of the surface low (although the position of this low is not clear yet).

Possible outcomes include heavy rain showers, and the development of a cutoff-low along the southern Cape coast (not KZN) on Tues 7th, although it is not possible to make detailed predictions at this stage (4-5 days in advance).

Compiled by Gordon Richardson - StormchasingSA

Update: Cold-front, low & possible snow 31 July 2012 - 01 August 2012

It is very cold over the southern half of SA this morning (Wed 1st), although conditions for snow are marginal due to limited moisture. Snow is likely along the southern Cape mountains, and over the high ground of the Eastern Cape.

The custom GFS charts have not been updating till now, but this morning's chart shows the likely precipitation and freezing level during this morning (00Z to 12Z on Mon 1st). The site snow-forecast.com has a much more detailed terrain map showing the mountain ranges: www.snow-forecast.com/maps/dynamic/southafrica

Compiled by Gordon Richardson - Stormchasing SA

Cold-front, low and possible snow 31 July 2012 - 01 August 2012

The latest model runs (GFS and NOGAPS up to 00Z on Sun 29th) have wobbled slightly, but continue to show modest amounts of rain, together with extremely cold upper air, over the southern Cape on Tues 31st and Wed 1st.

Strong winds are possible in places along the Eastern Cape coast on Wed 1st, as the low intensifies and pressure gradients are enhanced by the following high pressure cell.

The site snow-forecast.com shows the freezing level below 1200m along many of the coastal mountain ranges, with widespread (but light) snow likely, though not reaching as far as the Drakensberg: www.snow-forecast.com/maps/dynamic/southafrica


Edit: Due to problems with the NCEP custom GFS server, the only freezing-level charts currently available for this event are those on the snow-forecast.com site. For interest I am posting the GFS chart of the 850 hPa temperature (1500m elevation) at 06Z on Wed 1st, which shows the very cold air over the southern Cape coast, and parts of the interior of SA:

Compiled by Gordon Richardson - Stormchasing SA

SAWDOS Storm Chasing: Bloemfontein (21 - 26 March 2012) Part 2

Images: SAWDIS (Click on images for larger view.)

SAWDOS Storm Chasing: Bloemfontein (21 - 26 March 2012) Part 1

Images: SAWDOS (Click on images for larger view.)

Unfortunately not ideal conditions for chasing storms in the Free State over the above period. However I caught a few isolated small storms over the period. Please note that the images were taken while the vehicle was moving at 120km/h. So the quality is not good, but either that or no images.