Mon, 5 June 2017

A day out and this storm looks set to rival 1984 and 2001 in Cape Town with a real chance of severe storm tides and wind and flood damage as 50kt gales drive 60ft swells ashore.


DIK WEDNESDAY: The eye of the storm jams right up against the coast. Photo 1stweather

We've been saying Winter is Coming for so long now, the words have rearranged themselves into a question, but the behemoth about to batter us silly brings something beyond the pale. The naysayers had better buck up and listen because the models have not wavered in a week. In fact, they have upgraded. The rain is coming, and the wind, and the waves: big time.

The low pressure system behind the storm is holding steady, and is currently (10h00 Monday) situated 1200 nautical miles west of Cape Town and heading towards us at a speed of about 40 knots (about 75km/h). Winds freshen tomorrow, slowly building all afternoon.

However, the wind, waves, and rain arrive all at once, from midnight into Wednesday morning, each getting more and more severe. The first bits of rain set in from about 10pm Tuesday, with heavy rainsqualls blasted sideways by midnight. By 8am, this first wave of moisture is dumping a lot of rain across the SW Cape and has spread to the mountain catchment areas. Solid rain.

safr sea_19

DYNAMIC FETCH: The surf could be bigger than model due to this phenomenon. Photo Stormsurf

The wind pumps all day Wednesday, but the worst of it looks set to come between 11am and 5pm, with gusts approaching hurricane-force (2 x galeforce), especially over the southern parts of the Peninsula. After initial heavy downpours, it rains on and off all day, including up in the mountains, but a second 'wave' of rain becomes even more severe in the late afternoon, and it's bucketing down in the catchment area for 8-10 hours from late Wednesday into the early hours of Thursday. It should really make a difference to dam levels, but there will be widespread flooding.

It should really make a difference to dam levels, but there will be widespread flooding The biggest portion of the swell also comes on Wednesday afternoon when the original large long period swell from the storm's deep ocean peak (it began pulsing from the storm when it was 2000 miles west of Cape Town on Saturday and has been travelling for 4 days) makes landfall, but with a whole lot of mess riding over it. You'd normally call it a 15 foot swell at 17 seconds - the kind of awesome surfing swell that would bring barrelling 20 foot sets to Dungeons - only the storm has been grinding over the moving swell the entire four days, so the phenomena of dynamic wind fetch will come into play.

This will probably mean gigantic 65 foot swells - larger than modelled due to the exponential gain exerted by the wind on the already existing storm swell. The stormsea will be a gigantic melting pot of swell right across the spectrum - from tiny capillary waves seconds old through many heights, volumes and intervals right up to massive groundswells four days old.


WIND FIELD: Expect a major storm tide as gales and waves push ashore over the incoming tide.

There are worrying factors, apart from the damage potential from the gale and rain. The incoming tide, almost at spring tide (Friday), will coincide with the peaking stormsea as it strikes the shoreline. Tide and waves run over the top of a slight rise in sea level due to the moderate proximity of the eye of the storm (less atmospheric weight on the sea surface = higher sea). It won't be as pronounced as the surge created by hurricanes that strike the Americas. The combined eye, wind and swell of these hurricanes can combine to surges of 20 feet. This eye passes close by, only 250 miles or so.

We may have sea water pushed well past the normal shoreline When you add atmospheric surge to astronomical tide to the shoreward surge created by 60 foot swells and 50 knot westerlies pummelling the coastline, you'd be correct in suggesting we may have sea water pushed inland well past the normal shoreline. The combined effects of the above (tidal surge plus shoreward storm energy) is called a storm tide, and it could exceed 10 feet or more.

The tempest originally formed due west of Cape Town off southern Brazil on Friday. The initial cyclogenesis saw it deepen to 985 mb as it barrelled across the Atlantic towards us. The storm maintained this pressure and has not downgraded like other storms we've had in this drought cycle, which have been pushed south by a stronger-than-forecast South Atlantic High.


MAIN RAIN: Torrential downpours will result in flooding - good news for the dams.

This time, High pressure cells all around the storm are keeping it fed and bloated, with one piece of High sinking into a bizarre band 1,700 nautical miles long and extending so far deep, the southern end parks on 65 degrees south, not far from the ice shelf.

This storm tracks right over the top of that southern High, and because of this the charts are holding steady. Other areas of high pressure to the north, west, and east have served to lock the storm into its track. What's more, by tomorrow night as it nears us, instead of downgrading, this storm has strengthened to 975mb, though the wind changes from NW to WSW to SW, and then SSW as it moves east.

The giant west swells will also follow this trend, though more slowly. By Wednesday evening they are mixed with a new, more local SW to S storm swell that starts to fill up False Bay as the storm tracks below us, with Thursday indicating a strong southerly direction to the swell.

We're in for a helluva ride.


+2 #6 BearCaramello 2017-06-06 07:38
This article was so well written :) Thanks for making it fun to read! :)
0 #5 MsTish 2017-06-05 21:45
Never seen so much excitement over a 'storm' :lol:
Tez Mac
+1 #4 CaptainTez Mac 2017-06-05 19:32
Watch the Storm water drains and be careful of flash floods around Cape Town. Cape Town drivers will slow down even more. Chappies will be closed too.
Land rover Defender owners, get yer wellies on and ready your beasties for some fun, towing out stranded vehicles. :P
-2 #3 ConcernedSajjaad 2017-06-05 13:51
Are there any concerns for people working in Seapoint, Camps Bay area?.
+8 #2 Okehenri 2017-06-05 13:37
I'm just a bit worried about one thing. The high to the northeast is stronger than the high to the south and it seems the eye of the storm had already begun to move southwards. It has already partially dropped down below 60 degrees south, going by the latest synoptic chart. I would frankly, prefer the big guy hitting us, under the circumstances. Best regards!
Sian Williams
-5 #1 MsSian Williams 2017-06-05 12:55
Stay indoors, keep off the roads. That's wise.
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