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This is the forecast for Lasham on 17th July 2017:
This forecast was last updated 17/07/17 07:27 GMT
Forecaster: David Masson
Synopsis: Update 8:10am
Not as good as said last night:
- GFS and the actuals suggest blue except to our SE where this is more humidity (and with cumulus this might be the best bit now)
- GFS and the sat pics show quite a lot of top cover and altocu streaming from the west and this looks like it could be a problem later in the day
Best guess for thermals now comes from Larkhill which says a trigger temp of 19C gives blue to 3000ft+ which could be reached quite early (10am, 11am). Then only slowly improving to ~4000ft with a max temp of 23C later in the day.
GFS say maybe blue to 4500ft+ or shallow cu base 4500ft+ later in the day to our west, but that is questionable.
Now suggest either tasks staying in the south - maybe 200-300km either to SE with a greater chance of cu (base 3000-4000ft+) or to the west.
PS. Thursday is looking quite good now.
--- figures changed, last night's text left (but less relevant) ---
A weak front near the south coast. This gives a range of stability and humidity and is likely to give a big range of conditions. Hopefully it will be a bit more clear in the morning?
Most areas have some some top cover which might cause a problem at times. This will be moving from 260/30-40kts.
At Lasham a trigger temp of 21-22C gives blue to 3000ft+ or cu base 3000ft+ by about midday??? By mid afternoon somewhere between good cu base 3500-4000ft+ OR shallow cu base 4000-4500ft+???
For now the best line for the PM looks like from Cambridge to Reading to Devon. To the NW of that line more likely to be blue (eg: blue to 3000ft+ early PM and blue to 4000-4500ft+ or shallow cu base 4000-4500ft+ mid PM).
To the SE of that line (especially E.Hants + Sussex where it dewpoints are higher) rather lot of cloud, say 5-7/8s CuSc 3000/6000ft.
I'd probably try to task to either to the NE and then WSW or vice-versa depending which way looks best on the day. Probably good for 250-350km+ in the PM. If you get the task right and it streets then maybe 350-500km+. (eg: something like LAS-OXF-GRW-CHA-LAS or vice-versa)
Temperatures: Max/Min -> 23C / 12C, Dewpoint -> 10C
Wind strength/direction: 7 knots / 70 degrees
Cloud base/thermal top: 4000 feet
Thermal strength: 3 knots
Thermal start/finish: 1100/1800
Visibility: >21000 metres
Outlook: Tue: the front in near the south coast moves north slowly. Wind 100/20kts, shallow cu base 3000ft+.
Not quite clear of the timing yet, but after that a thundery low moves up from Biscay and may gives thundery showers on Wednesday.
Other information: Thanks Hugh and all the paragliders and hangliders for your support - I wondered how many of you there might be there :-)