As discussed and predicted yesterday, Invest 98S has formed quickly
September 10, 2016 at 7:10 pm #93948
Several long-range forecasts indicate an intense weather event reaching the SW-Cape on Fri 16th. This has the potential to bring strong winds, heavy rain, very cold-conditions, and the possibility of widespread snow during Sat 17th.
As usual there is significant uncertainty so far ahead (6 days or 144 hours), but recent GFS model runs (up to 12Z on Sat 10th) have shown growing consistency in the outcome.
The following charts give a taste of the likely outlook for this event, though the details will certainly change closer to the time:
Attachments:September 11, 2016 at 1:02 pm #93952
Recent GFS runs (up to 06Z on Sun 11th) continue to look very encouraging for this event. The following charts show a likely scenario, though the details are likely to change closer to the time.
One notable feature is that the front will arrive overnight on Thurs 15th, and the snow will commence early on Fri 16th, and fall throughout the following day or two, with the freezing-level well below 1500m for most of this period. This means that snow will fall during the daytime, but that it will remain mostly cloudy until Sat 17th, when the best viewing opportunity should occur.
Attachments:September 12, 2016 at 10:27 am #93955
Today’s medium-range model charts (5 days or 120 hours) are showing some additional details, with broadly the same overall scenario for this upcoming event,.
The wind-direction will be south-westerly during the front (unlike the normal north-westerly), and this will bring extremely cold polar air to the SW-Cape during Fri 16th. Daytime temperatures will be below 12C over the entire Western Cape province and much of the central interior, and below 4C over the high ground of the SW-Cape.
Snow is highly likely, with falls down to 1200m possible over the SW-Cape, spreading to the southern Cape and Drakensberg later.
Attachments:September 12, 2016 at 8:10 pm #93958
The latest GFS model run (12Z on Mon 12th) shows significant deepening of the surface low (1004hPa) before landfall on Fri 16th. This would produce gale-force (50km/h+) NW winds over the SW-Cape, with the potential to cause moderate storm damage to trees, etc.
Note that there is still significant uncertainty in the exact details this far (84 hours or 3.5 days) ahead. Also note that the surface low will have minimal effects on the cold-core upper-trough, and the prediction for widespread snow remains valid.
Attachments:September 13, 2016 at 9:50 am #93960
Further recent GFS model runs (up to 00Z on Tues 13th) show phenomenal deepening of the surface low (1000hPa) as it passes Cape Town early on Fri 16th. There is still some uncertainty 3 days (72 hours) ahead, but expect strong to gale-force winds (60-70km/h) at times!
The outlook for snow remains very favourable, with most model runs showing in excess of 30cm over the highest peaks in the SW-Cape during Fri 16th.
Edit: The development of the surface low will cause a slight delay in the arrival of the rain and snow, to the afternoon of Fri 16th (instead of the morning as predicted previously).
- This reply was modified 7 months, 1 week ago by Gordon Richardson. Reason: Timing
Attachments:September 15, 2016 at 12:35 pm #93964
The distribution and amount of rain and snow have changed significantly for this event, as the models develop closer to landfall. I will post detailed updates closer to the time.
Something not in doubt is that it will be extremely cold over most of SA by Sun 18th! This chart shows daytime temperatures below freezing (0C) over the high ground of Lesotho, and very cold elsewhere.
Attachments:September 15, 2016 at 6:53 pm #93966
This afternoon’s EUMetsat image (15Z on Thurs 15th) shows the surface low developing north-west of Cape Town. This is quite an unusual sight at any time of year, but particularly so in early Spring!
Attachments:September 16, 2016 at 8:44 am #93968
This morning’s EUMetsat image (06Z on Fri 16th) shows the weather system making landfall on Cape Town. However, it looks like a hybrid cutoff-low rather than a classic cold-front.
This has several consequences on how the system will unfold during the next 12 hours:
The surface low did not intensify as much as shown by some earlier predictions, and overnight winds were relatively light (certainly not gale-force!)
Rainfall comprise heavy showers and persist for several hours, rather than a sharply defined frontal boundary.
The cold upper air (shown as reddish brown) is still far offshore, it is still relatively mild, and snow will not being until much later today (Fri 16th).
Once the suface low has passed over Cape Town, events will unfold more in line with earlier predictions. Widespread light snow is likely along the southern Cape mountain ranges early tomorrow (Sat 17th), and spreading to the Drakensberg by Sun 18th.
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