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This is the forecast for Lasham on 20th June 2017:
This forecast was last updated 20/06/17 07:47 GMT
Forecaster: David Masson
Synopsis: Around the edges first:
- In the SE it may only blue to ~5000ft, although a temp of 32C might give cu base 6000-7000ft+
- In the SW blue to 28-30C would only give blue to to 3000-4000ft under
- In Mid Wales across to E.Anglia rather a lot of top cover and humidity and towering cu and risk of CB's. I would also discard S.Wales.
That leaves an usable area bounded from 51N to 52N and 2.5W to 0.5W. With the best area something like Lasham-Yeovil-Gloucester.
At Lasham a trigger temp of 28C reached by midday gives cu base 4500-5000ft. At max of temp of 30-31C we would cu base 5500ft+. In the M4 corridor a max temp of 30-32C and dewpoints of 16-17C there is the start of towering cu tops to 10,000ft and then the the risk of thundery showers base 5500-6000ft+ or large CB tops to 38,000ft.
Good for 200-400km?
--- figures changed, last nights text unchanged ---
A weak slowly moving front across northern England. Much more humid air in central areas and between than that and to the north of about the M4 (eg: dewp 18-20C). Only S of the M4 still has a bit drier air with dewpoints of 15-16C.
At Lasham maybe weak blue early, but will be will really want we want more of a a trigger temp of 26C reached by about midday giving a cu base of 4500-5000ft.
In the drier air around here in W.Sussex-Hampshire-Wilts-Dorset we could have shallow cu base 5000-5500ft+.
In the M4 corridor towering cumulus base 3000-3500ft/10,0000ft with the increasing risk of HEAVY thundery showers with tops to 38,000ft.
Good for 200-400km but rather a confined area maybe like Yeovil-Wells-Chippenham, but I think I'd try to stay south of the M4.
Temperatures: Max/Min -> 31C / 15C, Dewpoint -> 16C
Wind strength/direction: 8 knots / 110 degrees
Cloud base/thermal top: 5600 feet
Thermal strength: 3 knots
Thermal start/finish: 1200/1800
Visibility: 20000 metres
Outlook: Wed: wind 180/15-18ks maybe tenps of 27C not high enough give thermals in the south, but temps of 28-29C and dewpoints 20C inland which give cu base 3000ft+.
Thu: much fresher after a cold front. Wind 300/14kts and shallow cumulus base 3000-3500ft+ in the M4 corridor, much still higher temps in the southern counties with cu base 4000-4500ft+.