On Apr 28, 12:20*pm, "Jon O'Rourke" wrote:
"Dawlish" wrote in message

...

For the main trough and its developing wave to drift so far North,
took forecasters by surprise; as did the intensity of the
precipitation that developed around the centre of the developing low.
So: not a great forecast, MO, late on the weather warning, but
entirely understandable. Its possibly down to data grid sizes and the
small area occupied by the "wet" part of that wave. The forecast
models just didn't have enough data to produce accurate forecasts of
precipitation intensity and location.

I don't think that's how or why it happened. The main frontal zone was split
with elevated storms tied to the forward upper front, effectively in the
warm sector, with further storms developing to the rear of this in and
around the shallow moist zone/near the surface front. Hence the wave, as far
as I can tell, wasn't the key player that it was originally expected to be
and didn't really develop over the UK. Also, given that much of the forward
precip. was falling into a relatively dry boundary layer surface totals were
likely to be low and not necessarily likely to trigger flash warnings.
Either way all the models that I'd seen struggled with the complexity of it
all and it looked like a very difficult day for those tasked with getting
the detail right.

00Z ASXXhttp://www.wetterzentrale.de/archive/2008/brack/bracka20080428.gif
showing the split system.

Jon.

I agree, Jon. It certainly wasn't easy! I'm not sure why you say that
the wave didn't develop over the UK. Did you mean "England" It
certainly developed over the UK, though not so much over central
England, as was forecast Friday/Saturday. Instead it developed over
the far NE of England and finally gave some high rainfall totals over
SE Scotland, triggering the eventual weather warning. It ran along the
trough, pressure fell as it did so and a clear circulation developed,
with a warm sector, which is shown to be occluding on the present FAX
chart. Thank you for your analysis of the front over England though.
Yes, there were clear pre frontal troughs, consistent with the front
splitting, producing pre-frontal instability and the air was dry. In
England, high rainfall totals only occurred along these lines of
destabilisation and even then were not enough, as you say, to have
triggered a weather warning. I've read reports of rain being seen at a
distance, but very little being recorded at ground level. A lot of
evaporation may have occurred in the precipitation from some of those
lines of showers.

Much of England, Southern Scotland, Wales and Ireland appears to have
chicken pox today with some beefy showers, one of which is presently
falling on Dawlish!

With the stalled Atlantic I talked about, we're likely to have the
unusual circumstance of a retrogressing low. It looks like it will
move W to E, bringing us into the coold Northerlies that I'd forecast
for the 1st May, but that now looks unlikely, with it heading into the
Atlantic and allowing some warm air to spread across the UK for next
weekend. Another possible, plume, 21C+, possible thundery breakdown
and showers/bands of rain/strong sunshine in-between, all this week
too! Suprb weather watching!

Off out over the Haldon Hills soon, on my way to the gym. Looking
forward to seeing some big Cu+/Cbs. I'll wave at Haytor from little
Haldon, Will!

Paul