In uk.sci.weather on Thu, 11 Sep 2003 at 06:03:33, Chris Handscomb wrote
:
Was just wondering if anyone could tell me how you could gauge how much rain
was likely to fall from a frontal system as it passes over the UK. Like this
cold front that will clear overnight tonight. I notice that the BBC are
forecasting very little rain from it except in the far north-west. But how
do they know? Is it purely from the results of all the
quasi-semi-cross-frontal-geostrophic thing equations in the computer models?
Is there a way of telling by hand by looking at satellite images, synoptic
charts etc?

Please help - I'm very curious!
My amateur weather prediction status is on the line!

Sometimes I think they just roll a dice.

AT least we finally had a decent amount of rain here - 7.7mm, the
highest for 6 weeks.
--
Paul Hyett, Cheltenham, England

In uk.sci.weather on Thu, 11 Sep 2003 at 06:03:33, Chris
Handscomb wrote
:
Was just wondering if anyone could tell me how you could gauge
how much rain
was likely to fall from a frontal system as it passes over the
UK. Like this
cold front that will clear overnight tonight. I notice that
the BBC are
forecasting very little rain from it except in the far
north-west. But how
do they know? Is it purely from the results of all the
quasi-semi-cross-frontal-geostrophic thing equations in the
computer models?
Is there a way of telling by hand by looking at satellite
images, synoptic
charts etc?

Please help - I'm very curious!

Not sure if this is of any help, but it is something I put
together a few years back which goes into some of the mechanisms
behind whether frontal activity is 'active' or 'weak' etc.

http://homepage.ntlworld.com/booty.w...o/frontdie.htm

It is possible using a thermodynamic diagram (actual or forecast)
to work out the amount of precipitable water that would be
produced, but of course, as others elsewhere have pointed out,
it's all done by computer models now, which integrate the
moisture variable, along with the dynamic/thermal forcing to
produce 'model' rain totals (usually apportioned between
'convective' and 'dynamic' components.

In my experience though, you have to be very careful with model
grid-box rain amounts and as with some other facets of the 'art',
if the analysis is in error, it is unlikely that the forecast
will be correct - in deed large errors can sometimes grow from
small errors in initialisation of the humidity field -
unfortunately it's not always (often?) possible to determine
whether the model is right when the rain belt is over the sea at
the edge of the radar envelope!

That's why you need human forecasters to try and tease out the
correct analysis patterns in cloud/PPN from the over-enthusiastic
ones! And why you get 'hand-waving' presentations in tricky
situations when it's not quite clear how much rain is going to
fall. Refreshingly vague our weather is sometimes ;-)

HTH

Martin.
--
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