Glidemet

NB Yes, the bottom of this post (and some previous posts) is missing in IE6. I’ve tried to fix the issue but admit defeat. If you have the option, you really should use a modern browser (e.g. IE7, Firefox, Opera (my preference), or Safari) rather than IE6.

Stand-out flights from today have to be Dan Pitman’s and A.C. Wright’s 300s. It sounds like a couple of people were flying with Dan, and Mr Wright (Alistair?) did another 100 km after for good measure! Next longest flights on the ladder are around 160 km, which is a bit… odd.

The weather was very interesting as the north-south running front over western England unexpectedly began to wave (i.e. formed a wave-shape in plan view, not orographic wave) and then formed a cyclonic circulation through the day. This was cyclogenesis happening on our doorstep and was fascinating to watch on the sat loops, though it did mean that frontal cloud moved further east in a few places than expected, and the southwest was brighter. The low center formed over Carmarthenshire.

The reason for this was something of a kink forming in the tropopause, with dry stratospheric air being brought down to lower levels than usual. The following IDV image uses the 00z GFS visualised at 06z to illustrate the situation:

The grey isosurface is something called “isentropic potential vorticity”, with a value of 2 (representative of the “dynamical tropopause”, where the troposphere meets the stratosphere). You can see a finger of IPV poking down into the lower levels. The cross-section shows vertical velocity—-ahead of the IPV anomaly is red rising air (deepening the nascent depression); behind is blue sinking air. (You can also see more blue sinking air further east which was forms the capping inversion—-as the air sinks it compresses and warms (like how a bike pump becomes hot when inflating a tyre), so that rising thermals meet warmer air at height and can’t rise any further.)

The reason why IPV leads to vertical motion is fairly horrible but you can read it here if you want. Jo, if you’re reading this, didn’t you study this recently? Perhaps you can explain it  .

Tomorrow (Friday), unfortunately, the large mass of cloud just east of England at the moment is forecast to move westwards bringing cloud and patchy rain to eastern England. Showers are likely to develop ahead of the front over the northwest and Midlands into Wales. Best soaring is likely to be south of the Thames—Bristol Channel line and into southwestern England, where bases should go up to 4,000 ft. Winds surface 5 knots from west through to north, at height 10-15 knots from northwest.

There’s still mixed messages for Saturday, and ironically the models have reversed their positions somewhat (i.e. those forecasting bad now looking good and vice versa). It basically depends on whether or not the front which will hamper the east on Friday moves back eastwards or not, but either way western parts should have a good day. Sunday isn’t a certainty either as although the surface pressure pattern will be a ridge, a frontal system comes rather close to the west. Hopefully it won’t get too far.

Monday looks fairly reasonable.

Tuesday morning
I was fed duff info about what happened over central southern England on Tuesday morning, so want to set the record straight. While a circulation did form it a) wasn’t at the surface and b) was an end-product of another process, not the cause of the persistent rain itself. The reason why there was no surface low on the analysis chart was because it didn’t exist! What the actual cause was this:

That’s a drawing of the height of the 500 hPa pressure surface on Tuesday morning. The big blue dip in the background is an “upper low”, but running towards the viewpoint, over southern England, is an “upper-level trough”. Below on the surface you can see the rainfall rate (no scale I know, I’ll come back to that) and MSL pressure.

The next adds a PV isosurface:

As I had to stretch the vertical scale to illustrate the topography of the pressure surface, the PV isosurfaces are massively exaggerated! But you can see one bit going straight through the area of rain over southern England. The problem with the forecasts that morning was that the models—-all of them—-totally under-estimated the strength of the PVA. The result was that the warm, moist air in the front lying over England was lifted much more than forecast, so much more rain fell than models predicted (so it doesn’t matter that there’s no scale on the image above!).

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