"Norman Lynagh" wrote in message
...
I'm a bit surprised at the minimum temperatures forecast on BBC1 at 6.30
p.m. this evening. Single figure mins were predicted for Scotland but
elsewhere double figure mins were predicted. I haven't looked into it in
much detail but for this part of the country the McKenzie method
predicts a min of around 7-8C, assuming light winds and only small
amounts of cloud. This would be quite a bit colder than the TV forecast
suggested.
I have noticed for some time now that the forecast minimum temperatures
seem to err on the high side on radiation nights. I'm not certain but I
think that this apparent systematic error started at the time of the
introduction of the new graphics. It may be a presentation problem
rather than a forecasting problem.
Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles 85m a.s.l.
England
You're all very lucky you can use McKenzie!
Try forecasting the min. at Haytor! McKenzie doesn't work as we rarely get
radiation nights. There are always patches of cloud to surprise you, always that
subtle increase in wind speed either due to the low-level jet or the effect of
the hills, always that subtle change in wind direction going from a land
direction to a sea direction causing the temperature to rise and vice-versa, and
of course in winter that surprise snowfall as the wet-bulb freezing level falls
in precipitation and the temperature plummets rapidly.
I ignore all TV and radio forecast mins as they will never apply to my locality
(nor should they, as hardly anyone lives here). Instead I use my judgement as
best I can using available data and also climatology. Wind direction seems to be
an important variable. With a wind speed 5 knots or more from a close sea
direction in November and December you can forget a frost at Haytor, mins will
normally then be in the range 0-2 degrees C on a clear night.
So in my area forecast mins are sometimes too high and sometimes too low and
occasionally right,
Oh the joys of weather forecasting :-)
Will.
--
" Visit Dartmoor National Park "
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