The inaccuracy in the forecast for today has led to alot of criticism of
the Met Office on this newsgroup. However I feel that it highlights an
important issue in weather forecasting; the chaotic nature of the
atmosphere.

The atmosphere is a chaotic system and as such predictions of its future
state at a given time are always prone to uncertainty. The extent of
that uncertainty depends on the uncertainty in your initial conditions
which you feed into your numerical model.

Forecasts relating to areas of rapid development are very sensitive to
the initial conditions. A slight change in the initial state will lead
to large differences in the forecasts. The storm today is good example
of such a sensitive region, as was the 1987 storm.

The best way to improve forecasts in these situations is either to 1)
improve your estimate of the inital state of the atmosphere at that time
-this can be done through an improved data assimilation system or by
targetting extra observations to be taken in this region. or 2) to take
account of the uncertainty through using an ensemble forecast.

In 1987, the storm was not forecast to develop as much as it did, todays
forecast overestimated the development (and also produced a slight error
in the track which unfortunately led to a big difference in the weather
experienced on the ground). However, an ensemble of forecasts would have
revealed the large uncertainty associated with these forecasts. Extra
observations would have helped reduced the spread (uncertainty) of the
ensemble and given us a more accurate forecast.

The point I am trying to make (in a very long winded way) is that a
single forecast will not yield as much information as an ensemble in
these sensitive conditions. People saying "the GFS model did better than
MO" are missing the point. They are both to all intents and purposes
very similar models giving in effect individual ensemble members. The
fact that they are different merely represents the high uncertainty of
the situation, not that one is "better" than the other.

The best way to have forecast this storm more accurately would have been
to take more observations from the sensitive region. This is an exciting
new area of research. People interested in this should look at the
THORPEX program:

http://www.mmm.ucar.edu/uswrp/programs/thorpex.html

Hopefully, within the next 10 years the results of this should improve
our forecasts dranatically for situations like these...