GFS Rainfall Forecast

Please note that:

  1. I am not a forecast meteorologist/forecaster and this is only my opinion/interpretation and
  2. predictions made by numerical models (such as GFS used here) beyond 3/4 days can be inaccurate, particularly when it comes to precipitation and temperatures at specific locations.

After an interesting introduction to the Spring season, with the devastating hailstorm that hit Lichtenburg and the Pilanesberg National Park hailstorm the day afterwards, it’s been a rather quiet few weeks since. Some people in KZN might argue otherwise. 🙂

It shouldn’t come as any real surprise because generally the thunderstorms over the Highveld/interior only really start in earnest from about 10 Oct.

But this “boring weather” appears to change from this weekend with the passage of an upper trough!

The GFS model runs are showing rainfall for most of the FS, NW, NC and EC as well as parts of GP, MP and KZN. Looking at the corresponding instability maps it seems there should be quite a few thunderstorms around this weekend.

Thunderstorm activity should start over the N/NE’ern FS, parts of GP and stretch across into NW today (Sat 19/09).

On Sun (20/09) storms should shift to the W with most of the activity being over the central and E’ern half of NC, FS, central and W’ern NW and high-lying areas of EC.

By Mon (21/09) the majority of storms should stretch from Botswana through central NW and FS into the EC and S’ern KZN.

Tues (22/09) should see most of the thunderstorms occurring in the E’ern FS, GP and into N’ern KZN and S’ern and central MP.

What intrigues me about this weekends set up is the combination of specific atmospheric characteristics often associated with severe thunderstorms (particularly on Sun and possibly Mon):

  1. Low level moisture – no thunderstorm can exist without at least some moisture,
  2. A dry line and/or surface convergence – a sharp change in dewpoint temperature over a relatively short distance (often acts as a trigger from thunderstorm development),
  3. Instability – an unstable environment means thunderstorms can develop relatively freely, and
  4. Wind shear – the change in both wind-speed and -direction with an increase in height (aids in thunderstorm development and can hold ice particles [hail] aloft for longer periods than ordinary thunderstorms).

Saturday (19/09)

At the time of writing this, the high pressure has already started ridging, feeding in low level moisture onto the Highveld (airflow shown by solid arrows). The will result in N’erly/NE’erly airflow near the surface. With the formation of an upper air low wind aloft is forecast to be W/WNW’erly (dashed arrows) – a good change in wind direction with height.

The strongest surface convergence is marked by a blue ellipse.

Although the moisture, dryline and wind shear correspond reasonably well over the NW and GP, most of the instability (CAPE) and strongest convergence appears as though it will be located over Botswana, with some instability stretching down in NW.

Sunday (20/09)

Sunday is a different story. All the ingredients/conditions that are favourable for severe thunderstorm development are forecast for the NE’ern EC.

This does not mean there WILL be severe thunderstorms, or that ALL the thunderstorm that develop will be severe, but rather there is the possibility for severe thunderstorm development.

If I didn’t have to be at work on Mon (and Tues) I would be off to the area where the NC, FS and NW meet.

The effects of the high ridging, feeding in low level moisture, will stretch farther W compared to Sat which will shift the dryline in a similar way. CAPE is forecast to be between 1000 and 1500 J/kg and this overlaps very well with the low level convergence. All of this occurs in a region where there is good wind shear.

At this stage, it looks possible that severe thunderstorms (including the possibility of supercells) could develop over this area (W’ern NW and FS, central to E’ern NC).

Monday (21/09)

Monday becomes rather tricky.

With the progression of the surface trough (associated with the upper trough) from W to E, it will shift the dryline more to the E. Although the dryline does not appear as though it will be all that close to the area where thunderstorms are expected, surface convergence over central NW and FS should be a sufficient trigger for thunderstorm development.

Low level moisture is still forecast to be present, while instability should increase (compared to Sun) – larger area of CAPE > 1250 J/kg with a small area forecast to have 1500 – 1750 J/kg. Wind shear also remains favourable.

The problem is that a significant amount of high- and mid-level moisture (assumed to be cloud) is forecast for the morning (possibly throughout the day). This could, depending on how thick and/or persistent it is, reduce the heating effect of the sun and prevent convection.

In the past this has often been the case, especially for low- mid-level cloud. But I still feel that it’s still possible that severe thunderstorms could develop over the central NW and FS. It’s possible that surface convergence could be a sufficient trigger for thunderstorm development.

Bear in mind that it is also a few days ahead and things could change between now and then.

Tues (22/09)

Although there should still be thunderstorms over tFS on Tuesday, there is a significant decrease in CAPE (compared to the forecast for Monday) in the region where wind shear and surface convergence looks good. There is also a drop in moisture and the cloud cover forecast for Monday appears to continue through to Tuesday.

For N’ern KZN it actually looks very good for decent thunderstorms. Thunderstorms should develop along/near the escarpment due to orographic effects and then they should move E into more unstable regions (higher CAPE).

Again, this is a number of days away and thing will likely change between now and then.

 

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