By Derek van Dam
As a meteorologist, I incessantly look at weather charts and climate information to determine my forecasts. When I am involved in a particular event that is weather dependent, I become fanatical with the weather outlook. Cape Town is a special place for a weatherman. Mother Nature has a tendency of simply not caring what Derek Van Dam has to say on his e.TV & eNCA weather bulletins (forgive me for speaking in the 3rd person). Since weather forecasting is an “educated guess” (yes, I may have just been the first meteorologist to admit that to you) I thought I would share my forecast for this Saturday’s Two Oceans Marathon and also share with you how I come to the conclusions that I do.
When looking at weather charts, it is critical to consider surface, mid and upper level variables to help formulate an accurate forecast. With that said, I am going to focus on a few of the most important variables for race day. First of which is temperature; undoubtedly one of those “make or break” variables for runners, especially if it is too hot. The parameter that I pay close attention to is the 850mb (/- 1500m altitude) temperature and wind speed. This helps me look for cold or hot air advection (fancy weatherman talk for “movement of air masses”) within an area and it also helps identify stable and unstable air masses. Here’s an example of this variable across South Africa, valid for 2pm on race day:
Next, I will focus on wind. Often in Cape Town, we sacrifice one extreme for another. If it’s not hot, then there’s a good chance the SE’r could pump. I investigate this weather variable by looking closely at the Mean Sea Level Pressure (MSLP) over the Southwestern Cape Peninsula. This is a surface based variable that can tell me if there will be a “pressure squeeze” over the peninsula and whether the high & low pressure systems are positioned in such a way that stronger winds will begin to form. Here is an example of this variable valid for 2pm on race day:
Next, I want to determine if there will be cloud on the day. This is done primarily by a variable known as 700mb (/- 3000m altitude) Relative Humidity. It can be inferred (of course, this is not always true) that clouds form around this altitude. If you want to see if cloud is going to be present on a particular day, then you can see how saturated that level of the atmosphere is using Relative Humidity. Here’s an example of that variable valid for 2pm on race day:
And lastly…let’s see if rain will be present on race day. For this, I look to a variable known as the “Quantitative Precipitation Forecast” or “Q.P.F.” This is part of a numeric model that determines precipitation maximum and minimums. Here is an example of Q.P.F. valid for a 24-hour period from midnight Saturday to Midnight Sunday (notice there is no colouring over Cape Town-which represents dry weather):
Now that I’ve filled your brain to the max with weather terminology and information, it’s about time for my official weather forecast for the Old Mutual Two Oceans Marathon. Without further adieu, here you are:
Start of Race (6am/6:30am): Dry with cloud on the mountains. Fresh SE wind gusting between 25-35 km/h. Temperature of 15 degrees Celsius.
Finish/Midday (11-2pm): Windy (Fresh SE between 30-40 km/h) & dry with partly cloudy skies. High temp: 22 Degrees Celsius.
Enjoy everyone. I hope to see you out there. I’ll cheer you on if you cheer me on!
Yours in Weather,
Derek Van Dam