Glidemet

Tomorrow (Saturday) will be drizzly wet all day in the south (kinda south of Gower—Wash). North to the border will be cloudy, but Northumberland should brighten up later. Wind about 15 knots from the north east, cloud bases around 1,500 ft (though likely some scruffy stuff around below that).

RASP is showing a good Helm Wave tomorrow, and it is also forecasting a clear slot. Needless to say feed back (and some photos if possible) would be nice from the edensoaring guys. Apologies I was so negative in my mini-forecast in the comments to yesterday’s post (though of course RASP could be wrong—-hence the feedback request!). I don’t see any other obvious wave areas.

Sunday should be brighter and will possibly be soarable, though the airmass looks a bit damp. Spready-outy? We’ll see. RASP is showing a signal for wave downwind of Dartmoor which will be of interest to the Brentnor folk. That’s from a 12 km forecast and I’ve never noticed it do that before, so will be interesting to see how that turns out. No such indication for Helm Wave but I wouldn’t read anything into that—-the 12 km model’s topography is simplistically coarse.

Monday looks bright the west and cloudy in the east; Tuesday just looks cloudy.

(And yes, I did say “north to the border” above. I am trying to push the forecast out to all of England and Wales, but no promises that it will last—the workload can be quite high sometimes!)

In Other News
Hertfordshire Scouts have their own Falke which they use to do “air days” for Scouts. I know the guys behind this well and they’re an excellent bunch, offering a very well thought-out package to Scout groups all over the country (theory of flight and model making as well as the actual flying), and they’re extremely popular. They need extra MGIR or SLMG instructors to help with the flying so if you can help out please talk to Gransden on

Sorry, but a Javascript-enabled browser is required to email me.

The cloud and rain has come to something of a halt over southern Britain—-I hear it’s still quite nice in the Peak District! Unfortunately the cloud and occasional rain is going to stay put for all of tomorrow (Friday). Just a light wind from the northeast and a cloudbase of about 1,500 ft, so winch launches for training just be possible at most sites.

Saturday now looks rubbish on the charts—-rain rain and more rain for the south. Bleurgh. That said North Wales and East Anglia should stay dry. Sunday and Monday dry but still cloudy.

Where to find RASP
It’s been pointed out to me that I forgot to link to UK RASP in my post on Tuesday—-d’oh. You can find it here: UK RASP. There’s also a “slimline” viewer I use linked from the sidebar on the lower left but it’s a bit out of date (I really need to sort that out), so that link might change in the near future.

Tomorrow (Thursday) rain, heavy at times, pushes north through the morning turning showery later. Pretty cloudy behind the rain, wind low teens from the northeast. Sadly this will bring to an end a period of lovely weather—-London Heathrow has recorded twelve straight days of 20+ °C, which I think is by some way the longest such period since August 2006!

Friday again cloudy and wet at times, Saturday brighter and quite probably some decent soaring to be had. Sunday looks cloudy and a bit wet again.

Sorry this is so late—-round a friends using their laptop, plus it’s a complex situation (when isn’t it!).

Well there was cumulus “southwest of the Chilterns today”… forming at about six o’clock! It took the surface-based convection a looong time to push the inversion high enough to let rising thermals reach the condensation level (this process was well-forecast by the new “on the day” RASP, though it was a little too keen to produce cumulus). On the other side of the country North Sea stratus did push in as far as Tibenham and even as far as Husbands Bosworth judging by their webcam (now in focus again having cooled down ). I took a look at Chilbolton Lidar around noon and it was showing blue to 4,000 ft (I’ve known about the Chilbolton Lidar—-it’s not just the great big weather radar they have there—-for years, but this is the first year I’ve remembered to check it routinely!). It also showed a 4 knot thermal at one point, but the thermals, as discussed in previous forecasts, were quite weedy with limited vertical extent.

Now that North Sea stratus has pushed inland as far as the Midlands at the moment, and will hang about overnight, but during the daytime tomorrow (Wednesday) it should dissolve in the morning, before creeping back in during the afternoon. Lincolnshire and the East Midlands should be affected more than East Anglia, but nowhere is safe. Elsewhere we should see much less high-level cirrus (hurrah), and the day will be mostly blue as the inversion is pushed up by daytime convection. Expect blue climbs to above 3,500 ft by early afternoon on over much of the forecast area not affected by North Sea cloud. There might be some cumulus later south of the Thames Valley, but warm continental air will advect in killing thermals and making conditions very conducive to thunderstorms over the south and southwest—-be warned.

Surface winds 10-15 knots from the east, and there should be easterly wave over Edensoaring.

Thursday currently has a Weather Advisory out on it—-see the bottom of the sidebar. It’s for 25 to 50 mm of rain over the western Thames Valley and the Cotswolds, and that amount of rain may well cause some local flash flooding as well as unpleasant driving conditions. The culprit is a dynamically-driven occlusion which might become slow-moving, but the event is not a certainty. Outside the wet south of England and Wales it’ll be a generally cloudy day.

Friday through Sunday looks cloudy and wet at times, also a lot cooler than recently.

In Other Good News
The ladder is back! Already people are uploading flights which is great to see.

Your webcam is out of focus. Could somebody fix it please? It’s a very useful cam.

If you go down to the UK RASP website today you’re in for a big surprise. Paul Scorer (nephew of the “father” of wave science, R.S. Scorer) has put the upgraded RASP system live. I’ll take a few moments to talk through the changes Paul has made, but first I want to publically say thanks to Paul for all the work he does for us. Paul has taken RASP and pushed way beyond what the original developer, Dr Jack Glendening, envisaged. Wrangling cutting-edge meteorological models, ones exactly the same as used by the National Weather Service in the US, ain’t easy!

The changes
The most notable change is to the forecasts you see in the morning, the so-called “on the day” run. This was originally 12 km for the UK and 4 km for an area of eastern Scotland and northern England. Well now it’s 4 km for the whole of the UK! It’s worth noting that this equals the resolution of the Met Office’s own mesocale model, so it really is something.

The 4 km has been shown to forecast areas of wave activity well (John Williams used it to know that his famous “North Sea 1” turnpoint was reachable, which demonstrates the faith he has in the system). The resolution is not sufficient to forecast individual wave bars, but what you will see on the vertical velocity at 850, 700, and 500mb charts are what look like slightly fat “wave bars”. The wavelength of these plotted bars is much larger (2-3x usually) than what you’ll find in the air, but what they WILL show you is the position, extent, and timing of wave systems. For the first time, Wales and all of northern England will have accurate wave forecasts.

This 4 km grid takes some 9 hours to compute, even with custom-compiled (iFort) executables running on a 4 GiB Q6600 Quad Core 2. If we started this run with 00z data you wouldn’t see the charts until much too late, so instead we’re initialising the run from 18z “day before” data. This is a calculated exchange between resolution and accuracy of data: even though the initialisation data is six hours older, the 3x increase in the resolution of the model’s grid more than compensates. In the US an identical model is run for severe weather (tornadoes etc.) forecasting and the scientists concerned have found that even though they have to start the model twelve hours earlier than their 12 km one, their 4 km run is far more useful for forecasting.

As it stands, you’ll be seeing the output from our 4 km model at about 4am local. This was actually a request from John Williams so he could plan the longest flights possible using all the daylight he can!

You’ll also see an upgrade in the “next day” charts you see at around 7pm in the evening onwards. The resolution of these charts-for-tomorrow has been pushed to 5.1 km, starting from 12z data. The slightly larger resolution means the run only takes about three hours. We haven’t had a wave event during the time that the test system has been running in parallel with the operational but I believe it will be suffient for wave forecasting all over the UK.

All the remaining days—-from, +2, the “day after next”, through to +6—-will remain at 12 km. This is because fine-scale details lose their predicitability at these timescales—-no-one in the world runs a 4 km beyond 48 hours.

Nerdy bit
Skip this part if you want but do read the “caveats” section beneath.

You may wonder how the resolution of a model is chosen. There’s two factors—-the “nest ratio” from the parent model, and the meteorological impact of the resolution. In the case of RASP the parent/”driving” model is the GFS, which has a global resolution 36 km. A model “nested” within this parent will have a grid resolution defined by an integer ratio. A typical value is 3, which is why a common RASP resolution is 12 km—-36 / 3. The old “window” run was nested within the first RASP run, again with a ratio of 3, giving a resolution of 4 km. What Paul has down with the new RASP is push the first grid ratio to 9. This is steep, but is actually exactly what NOAA does with their high-resolution models I mentioned earlier (we actually consulted with the chief NOAA 4 km model-wrangler, Matthew Pyle, on this). For the “next day” forecasts the grid ratio is 7, giving the 5.1 km grid.

The other part is the meteorological side. A global model, running at a grid scale of 25-40 km, can’t see individual cumulus clouds—-they’re too small. Instead the model uses something called “parameterisation”, which is a kind of sub-model which approximates the effects cumulus has on the rest of the atmosphere without actually modelling them themselves. This is called, erm, cumulus parameterisation. This is generally used down to a resolution of 12 km. At scales of 5 km or less, the models can actually start beginning to model individual cumulus clouds. Between 12 km and 5 km is the “cumulus gap”, where parameterisation is too an inaccurate approximation, and the grid is too large to model convection explicitly, so you’ll rarely find models running in this middle ground (I think the Germans have one though. Those crazy Germans, eh?).

Now when I say “cumulus” I should really say cumiform—-we ain’t talking about individual little puffy clouds here (that would need sub 1 km grids), we’re talking full-on cumulonimbus storm clouds. In the US the 4 km models are used, with considerable success, to forecast supercell storms (the type that cause tornadoes—-and tornadoes have killed 98 people in the US so far this spring, the worst toll for ten years—-and the “season” runs until the end of June…). Here in the UK true supercells are rarity but we certainly get big storms, and 4 km RASP will model these. They’re visible as little diamonds (the intersections of the grid lines) on insolation charts, or little blue holes on boundary-layer top plots (the latter because the storm’s downdraught reduces the BL to zero height).

Notably the new UK RASP appears to be modelling cloud streets (technically “horizontal convective rolls”). There show up as closely spaced with-wind lines of different cloud bases, or strips of vertical motion. RASP is a little too eager to forecast these sometimes, but is by and large quite good with it.

Caveats
This is an important bit, so pay attention 007. In the US, the high-res models are used to forecast the “mode” of convection (supercell, multicell, line squall etc.) and the area where such storms can be expected. What they won’t do—-well actually they do do, but not often enough to be reliable—-is forecast the exact positions of individual storms. Same goes with the new UK RASP. It will forecast areas of wave, but it won’t be kilometer-perfect, nor will it show individual wave bars. It will often indicate areas of horizontal convective rolls, but it again won’t be perfect in where it puts them. It will show sharp changes in things like dewpoints and inversion heights, and those boundaries will be out there, but not exactly where RASP shows them. (One thing that it does do with remarkable accuracy is show sea-breeze fronts. It’s uncanny with those).

I hope this little run-down has been interesting and informative, and you’ll all be making best use of the new UK RASP :-).