Glidemet

First, Saturday. Thanks for the feedback from everyone which is very useful. Saturday looked quite cloudy on Wednesday, pretty decent on Thursday, and finally cirrus-y and blue on Friday! You always have to wonder with high cloud bases—-often they need only be a little higher and it’ll be blue.

Your feedback was invaluable as there’s no other way of determining how high or strong the thermals were on a blue day—-no METAR reports to look at. Forecasting the height of climbs on blue days is a little tricky as while the top of the boundary layer is a known, you can never climb that high—-only as high as the point at which the strength of the updraft falls to that of your sinkrate. The original RASP sink rate was set at 225 fpm which in my opinion is too high for gliders but about right for paragliders. I reckon around 175 fpm is more appropriate for gliders (although of course there’s a bit of a range from an empty Open Class to a tanked 15 m, and you’ve got to allow for control surface waggle too), and Paul Scorer added a new parameter to UK RASP over winter set at that value. Unfortunately, I completely forgot to look at it on Friday so used my usual “look at the BL top and guess” method…

I reckoned on “as high as 4,500 ft in a region Cotswolds—the Wash, while to the south of that area it will be blue to around 3,000 ft or a little more. East Anglia should be white to some 5,000 ft”. To be honest East Anglia looked pretty blue too on satpics. In the south it sounds like some climbs eventually went up to around 3,500 ft but were generally around 3,000 ft for the most part. There was also a fair amount of wave about, no doubt due to the lower layer of stable air forming the inversion.

Sunday was a right mess, starting with a surprise “elevated” thunderstorm coming ashore from the Channel over Hampshire and heading northwest towards Buckinghamshire (though steadily weakening by this point) and I think it eventually went out to sea over Norfolk. This wasn’t forecast by any weather model or any human forecaster for that matter. “Elevated” refers to the fact that the storm wasn’t feeding off surface boundary layer air, but an aloft layer of warm, moist air overtopped by warm but dry air. These are often highly electrified and can produce significant hail too.

Later on Sunday surface heating in the Dorset—Birmingham area, ahead of a weak cold front over Wales and away from the messy cloud east, leading to severe thunderstorms initiating. Strong 0-6 km windshear meant that the precipitation was falling well away from the updraft allowing the storms to maintain themselves for some time. One persistently produced rain rates of over 200 mm/hr for about 45-60 minutes (probably quite large hail rather than heavy rain), moving over a fairly sparsely populated area from south of Stoke-on-Trent into the southern Peak District. There was also another powerful storm near Leicester which probably broke a few greenhouses…

Behind the track of the early elevated storm it seems that the air was quite stable to low levels and there was a little wave over Lasham, an interesting little phenomenon, especially as the air was so unstable just a little wset. There was a nice gentle non-turning wind gradient with height on the Herstmonceux balloon so it was probably very localised wave triggered off local topography. Elsewhere there were patches of thermal soaring to low heights. Easily the most complex day I’ve seen for a very long time.

Early outlook
Tuesday is starting in the south and in East Anglia, but quite heavy frontal rain is pushing into the southwest and this will spread north east to everyone by late afternoon.

Tomorrow (Wednesday) is another complex day with a low center tracking up through south central England. Either side of the low’s track, i.e. East Anglia and the west and Wales, will probably be quite wet with central areas marginally drier, but generally it’ll be a poor day.

Thursday will be another day of showers, while Friday looks cloudy and rainy at times. Saturday sees a frontal system moving in from the west bringing cloud and rain. Sunday doesn’t look, at the moment, any better.

First, let’s deal with Friday’s forecast for Saturday. I was thrown by the Met Office T+24 chart which greatly accelerated the progress of the trough south. Considering that these charts are the result of considerable expert human-input, I sided with it even though it was out of step of previous charts, and against model guidance. “Oops”. That said, people did still have a good day on the South Downs, as forecast.

Saturday’s forecast for Sunday was a lot better—-in fact I’m very pleased with how it turned out. Sunday was a complex day but even from the night before, the forecast was perfect, with many pilots from Dunstable (and at least one from Lasham) making very good use of the great conditions London—Lincolnshire (Wyton METAR reporting cloudbases of 6,000 ft!). In the Midlands—southwest showers did pop up getting in the way or slowing people down.

The earlier forecasts for the weekend in the week were also good. The models handled the weekend’s weather well.

Mind you I have a broken watch and it’s right twice a day

Early Outlook weather forecast
Tomorrow (Tuesday) looks generally quite good. Best parts will be along the south coast (inland a bit) where cloud bases should top 4,000 ft. North of that to the latitude of the Chilterns bases will be a bit lower (~3,500 ft), and there’ a risk of the cumulus growing quite tall and a risk of showers, but on the whole it should be a fairly decent day in that area. North again, into East Anglia, and into Wales and down across to the southwest, the clouds will likely grow tall enough to shower. In between though bases should still be over 3,000 ft, and the showers quite well seperated. Winds will be very light from the southwest.

Wednesday looks similar but drier; again a generally good day. Wind now around to the west but light (sub 10 knots). Thursday should be good in the east but in the west a trough will spoil play. Friday will likely see one or more troughs about, so a generally more cloudy and showery day for many, but nicer bits in between.

Saturday and Sunday look very much like Friday. The synoptics are interesting for the latter half of the week—-on Wednesday a low develops north of Scotland and moves west, and kinda stays near Northern Ireland right through the weekend, giving us an unstable southwesterly of air taking a curving track around from the colder air to the north.

On Sunday the UKMET is alone in moving the low east; the GFS and ECMWF leave it where it is.

In Other News
Mr Sea brings news of a new version of the Google Earth IGC replayer. This version is particularly tasty with the ability to run fullscreen and replay data to a PDA too.

A “review” is warranted this week for the Strange Case of the Missing Showers on Sunday, which has been mentioned in ladder reports and was looked at in more detail by Mike Rubin in Saturday’s comments. This was a similar situation to one that’s already happened once before this year, although that was in the context of wave, not thermal. However this bit is the same:

“-if the air is rising at a few centimetres an hour, it’s cooling and thus becoming unstable, if it’s sinking at that rate it compresses and warms, becoming stable, perhaps enough for wave. On Thursday the GFS saw Sunday’s air as doing the former, while by Friday it had settled into forecasting the latter. A small change can make a big difference to outcome.”

So it was on Sunday. Running up to Sunday the mid-level vorticity had been pretty neutral, with a little bit of positive down on the south coast (hence “Sunday will be fairly cloudy with showery rain mostly confined to the south coast area” on Friday). Positive vorticity can lift and destablise air, while negative vorticity leads to subsidence and inversions forming due to adiabatic compression.

Various models, the Met Office and their UK meso in particular but also the GFS, were keen to develop showers across the south on Sunday. While forecast sounding data isn’t available for the UKMET models it was probably similar to the GFS, which was showing an unstable airmass. However it was only just unstable, and it only needed a small amount of subsidence for a useful inversion to form. Recognising the finely balanced situation on Thursday I wrote “Sunday will be a lot better; brighter with less wind. There’ll likely be showers in the west but the east could be quite soarable” (you’d expect the subsidence to be close behind the departing front). On Saturday evening the forecast cloudbase for Sunday was around 4,500 ft (it actually took a long time to reach that) and while there was potential for showers, with such weak upper level dynamics they were going to be few and far between. As it was there were showers towards Wales and a big one popped up on radar over Sussex in the afternoon, but that was it.

Worth noting that the “on the day” forecasts were good. The Dunstable forecast was bang on, and David Masson’s forecast late on Saturday was also correct from what I can remember. That’s why those forecasts are linked over on the left . My forecast on Friday left something to be desired—-the “fairly cloudy” bit was technically correct, but I should have investigated further and flagged that the cloud was convective.

Early Outlook forecast
Tomorrow (Tuesday) will have 15-20 knots at the surface and more like 30 at height, but blowing along on those brisk winds will be plenty of cumulus at about 3,500 ft in post-occlusion air.

Wednesday sees high pressure becoming dominant. However a warm front will be in the general vicinity so it’ll be a fairly cloud day for most, though (depending on exactly where the front ends up) the east could be nicely soarable to 3,500 ft odd. Wind a gentle 10-15 knots from the northwest, but lower towards the Bristol Channel.

Thursday, despite the high becoming established south of Ireland, still looks quite cloudy as bits of Wednesday’s warm front are still hanging around. Friday will probably see that warm front clear but we’ll be in the resulting warm sector airmass, which is far from ideal for soaring. Might just be another fairly cloudy day, but could be useful for soaring. We’ll see. What does seem more certain is a lack of wave through all this—-winds are rather light, only a few knots at the surface rising to 20 or so at height.

Weekend starts of looking interesting as the flow becomes more northerly and will probably strengthen on Saturday. Sunday’s charts don’t look all that different to Saturdays, so probably more of the same. Whatever that is  .

At the weekend it rained. A lot. Brize saw 40 mm, and several other sites over the Cotswolds way saw similar or higher totals. Last Tuesday’s summary of the weekend’s prospects was “horrible!”, which summed it up pretty well.

Early Outlook weather forecast
Ending with a slight Scottish feel to it for those who’re going to Portmoak for Easter and the following week…

Tomorrow (Tuesday) will be similar, with the best weather (bases ~4,500 ft) again in the west (i.e. Wales and the southwest), and perhaps the cloud from the North Sea won’t make it quite as far west as it did today. For the rest, another grey day with dampness nearer the eastern coasts. Wind for the South Downs will also be largely the same—-about 10 knots from the NNE.

Wednesday looks a stronger hope for both thermal and South Downs ridge. There should be less cloud around, and the wind into Sussex should be a little stronger than the previous couple of days.

Thursday sees it all go westerly again with a low moving east north of Scotland, bringing brisk winds, cloud and rain. Not a great day really, best further south. Good Friday sees said low hang a right and head towards Germany, dragging cold air around it’s northern flank into Scotland, leading to the possibility of heavy snow in Scotland (see the Met Office advisories on the left—-I think I was bit harsh on them before…). For the rest of the country, including the forecast area, it’s going to be a cold, windy, damp and possibly snowy day.

The weekend starts with a whole bunch of snow showers pushing into eastern England, while further west it should be drier and brighter. Scotland should escape the worst with just general cloud. Sunday sees a new low develop over the northern North Sea, bringing more cloud and precipitation across the UK (on the other hand, if the dissenting model—-the UKMET—-is correct, there’ll be no such low and the weather much more settled).

Into the week after next, more sustained snow looks likely in Scotland and northern England while southern England and Wales is drier and brighter. After that, things should get a little quieter.

The weekend went as forecast—-cloudy with warm front on Saturday, showers on Sunday. I probably should have mentioned something about the cloud base lowering through Saturday and accessory clouds below the main cloud deck.

Sunday was interesting as a small jetstreak crossed the country, leading to enhancement of the showers on one side and their suppression on the other, giving a some soaring for a while (e.g. at Booker). This was forecast ahead of time, but what with being at the BGA conference on Saturday and taking forever to find a petrol station in Banbury (are they illegal there?!) on the way home, I didn’t get the chance to do an update. That said, pretty much everywhere got a shower at some point on Sunday, and all the forecasts for both days leading up to the weekend were correct. (Amusingly someone said to me last week “[we] can’t predict the weather”! Erm, yes we can  .)

Early Outlook
Tomorrow (Tuesday) sees more cloud and rain clearing to a brigher afternoon, 15 knots southwesterly wind. Wednesday will be a brighter drier day, and would probably be quite nice if it wasn’t for more strong winds coming in. They’ll be a little less that Monday morning, and will be strongest in the top of the forecast area, with the south being comparatively calmer, but still seeing some 25-30 knots westerly.

Thursday will be the best day for a long while, bright with less wind than of late, and the strong possibility of wave over Wales. Friday however sees it all going back down the Gary Glitter with yet more rain, possibly from a rather persistent stationary front.

The GFS differs significantly from the Euro models for Saturday but none look exactly great, all being variations on the theme of cloud and damp. There’s too much variation between the models at present to write a forecast for Sunday.

There was a bit of all three to be had these last few days. Here’s Phil King’s 280 km wave flight on Sunday:

Nice. There was also great ridge flights from Nymspfield and Booker in the northwesterlies on Saturday. Nympsfield also saw some cracking height gains on Saturday too, like this 11,000 ft climb by Alison Mulder. Shobdon saw lots of people reach FL145 (the bottom of the airway over the site), but the high flying winds made getting away cross-country difficult that day.

Forecast-wise I don’t think the day-befores could have been better. The earlier forecasts were maybe a bit slow to spot the potential of the weekend, but that’s why there commenting is possible on this blog—-so people like Phil and Bruce Cooper can poke me when I need it! As it was, Saturday’s potential was flagged a little late, only by Thursday, while Sunday’s wave wasn’t forecast until Friday. The latter was interesting—-if the air is rising at a few centimetres an hour, it’s cooling and thus becoming unstable, if it’s sinking at that rate it compresses and warms, becoming stable, perhaps enough for wave. On Thursday the GFS saw Sunday’s air as doing the former, while by Friday it had settled into forecasting the latter. A small change can make a big difference to outcome.

Today’s (Tuesday’s) thermal activity was good too, though the wind at height was higher than ideal, and I should have forecast that. A few people did 170-200 km, which is nice. The South Downs also worked very well, with several people flying what must be pretty much every inch of the ridge. Laurie Gregoire was out making an Astir go unnaturally fast once again, while in Wales Martin Pingel eschewed thermals for the interesting northerly wave to go and do 87 km.

Not the Early Outlook
Well, it is Tuesday! More the “normal forecast” by now really. Still. Tomorrow (Wednesday) will be a mostly bright day though steadily thicker cirrus from the top of a warm front will edge across the country during the daytime. Not good for soaring, great for training though. Wind 10-15 from the WSW going west during the day. That wind will also be bringing much warmer air than the last couple of days.

Thursday will be a generally cloudy day with 15 knots from the west. A bit dull after the excitement of the last few days really. Friday will be brighter again, a bit showery but maybe soarable depending on how things play out in post cold-front air.

Saturday will be cloudy and damp with a strong (20 knots) southwesterly flow. Sunday maybe brighter.