Lavers Et Al (2014) Fig 3c

This week the Met Office published their
three-month outlook
, giving a glimpse of likely weather for
this coming winter. The forecast says predictions “favour”
near-or above-average rainfall
between now and the end of
January.

This might not sound like good news for those affected
by last year’s
wettest
winter on record, which saw the Environment Agency
issue 14 severe flood warnings and evacuate 1,000 homes in
the Thames
catchment
alone.

But a crumb of comfort might be that scientists are
working on a way to predict the heavy rainfall that can cause
flooding further in advance. A new study says that forecasting of
heavy rain and storms could be happen as much as three days earlier
by tracking water vapour instead than rainfall.

Flood forecasting

The study, published in Nature
Communications
, suggests that examining water vapour provides a
more reliable way of predicting flood events than rainfall. The
researchers use the example of the widespread flooding in Europe
last winter to test their theory.

The UK has a range of organisations that track flood
risk. In England and Wales, responsibility for flood risk
information for the government and emergency services falls to
the Flood
Forecasting Centre
(FFC). It was established in 2009
following the
Pitt Review
of the summer
floods
of 2007, and is jointly managed by the Met Office
and the Environment Agency. These bodies are likely to be
interested in how this new work could improve their own forecasting
methods.

Water vapour

Water vapour can get transported around in the form of giant
atmospheric rivers, like the one shown in the figure below.

Water vapour transport across the Atlantic Ocean
in December 2013. Lavers et al. (2014)

These flows of water vapour tend to bring areas
of low pressure as they travel across the North Atlantic Ocean
towards Europe, and some of the vapour will eventually fall as
heavy rain.

Because these flows operate on a longer timescale than the
day-to-day variation of rain, streams of water vapour approaching
the UK can be tracked more reliably.

Overall, the study suggests that using water
vapour could allow scientists to predict heavy rain three days
earlier for much of Europe, including the UK.

A dark and stormy night

The researchers tested their approach by assessing the heavy
rain and flooding across Europe in last year’s winter.

Researchers analysed how early they would have
been able to predict the intense storm that hit Europe on Christmas
Eve last year. In the UK that storm disrupted the Christmas travel
plans of thousands of people, and left 75,000
people
without electricity.

The maps below show warnings of extreme weather
calculated using two different methods – water vapour transport and
rainfall. Forecasts of the storm are shown for seven days before
Christmas Eve (top) and one day before (bottom). Forecasts using
water vapour are on the left, and those using rainfall are on the
right.

 

Maps showing 7-day (a and d) and 1-day (b and e)
forecasts of an intense storm hitting Europe on 24th December 2013.
Forecasts are made using water vapour transport (a and b) and
rainfall (d and e). Lavers et al. (2014)

The scientists found a stronger predictor of
extreme weather using the water vapour method, shown as the orange
and pink band as the storm travels across the Atlantic Ocean. In
this instance, the researchers say, this is equivalent to receiving
a warning two days earlier than when using rainfall to make the
forecast.

The researchers argue that looking at water
vapour instead of rainfall is a more reliable way of predicting
heavy rainfall events, and can provide a better way of forecasting
floods in Europe.

With the Intergovernmental Panel on Climate Change
reporting
that extreme precipitation over most mid-latitude
areas are very likely to become more intense under climate change,
any improvement in our ability to forecast heavy rain is likely to
be welcomed both by those experiencing floods, and those responding
to them.

 

Lavers, D.A. et al. (2014) Extending medium-range
predictability of extreme hydrological events in Europe, Nature
Communications, 5:5382 doi:10.1038/ncomms6382 [this
journal is open access, therefore this article is free to
download]

Updated – To bring the language around the Met
Office forecast in line with the forecast itself – replaced
‘expects’ with ‘predictions favour’.

Via: http://www.carbonbrief.org/blog/2014/11/flood-warnings-could-come-three-days-earlier-study-suggests/