Memorandum submitted by the Met Office
1. Exceptional summer precipitation was
well forecast. Individual rainfall events over this summer were
exceptional while the accumulated totals over the period were
unprecedented. The weather events were well forecast, indeed the
weather forecasts leading up to the major July flooding event
were the most accurate and detailed provided for any major flooding
event in the UK.
2. Current warning mechanisms and structures
have led to confusion. Responsibility for issuing severe weather
warnings lies with the Met Office, while river (fluvial) flood
related warnings are a matter for the EA. There has been some
confusion amongst user groups particularly when severe weather
events and consequent flooding occur at different times and places.
This communication issue has been identified in some regional
lessons learned exercises.
3. Improved weather forecasts will lead
to improved flood forecasts. Advances in probabilistic forecasting
techniques combined with the operational use of higher resolution
models will give higher confidence in the broader pattern of weather
and the precision in the detail. Although a very high resolution
model was used to significant effect during the July flood event,
this is not an operational system. Operational implementation
awaits enhanced supercomputer power.
4. Integrated models will increase the effectiveness
of warnings. The Met Office is developing its range of ensemble
based forecasting techniques to provide a probabilistic framework
within which to deliver enhanced decision support tools to government
agencies. Development of these tools will require close cooperation
Integration of ensemble weather forecast and river models would
provide more reliable river flood forecasts at extended lead times.
5. Climate change may mean we see more intense
severe weather events. Any specific individual weather event cannot
be attributed to climate change. Broadly speaking, climate change
in the UK is predicted to produce wetter winters and drier summers
with the potential for increasing intensity of severe weather
6. The past is no longer an adequate guide
to the future. There is significant uncertainty over the impact
of climate change on flooding events and the probability of them
occurring over any given period. As climate research develops,
and higher resolution models can be run, there will be increased
certainty about regional scale impacts and its effect on flooding.
This is important because, under a changing climate, the use of
(based on previous climatology) for deciding on national infrastructure
may not be the most appropriate mechanism; decisions should draw
heavily on the best available regional climate predictions.
7. The Met Office is a Trading Fund Agency
owned by MOD. It is a world leading organisation, both in the
field of weather forecasting and climate prediction, operating
on a 24/7 basis with the highest standards of operational resilience.
Responsible for providing forecasts on all timescales (from an
hour ahead to 100+ yrs), the Met Office is uniquely positioned
to support the UK's response to changing incidence of severe weather
events due to climate change. It is responsiblethrough
its Public Weather Servicefor providing the National Severe
Weather Warning Service for the whole of the UK.
8. Unlike many other nations in Europe
where weather and flood forecasting and warning are integrated
into a single meteorological and hydrological agency, the Met
Office has no direct responsibility for river or coastal flood
9. In England and Wales, river and coastal
flood forecasting are the responsibility of the Environment Agency
with whom the Met Office works closely, providing to them storm
tide warning services, weather forecast information (daily) and
weather warnings (6 hours before an event). Currently no agency
provides warnings of localised flash flooding (also known as pluvial
flooding), although the Met Office will give an indication of
such risks alongside its weather forecasts.
10. Similar responsibilities for river and
coastal flooding apply to SEPA in Scotland. There is neither a
system nor single delegated authority for issuing flood warnings
in Northern Ireland.
11. The Met Office routinely utilises three
atmospheric forecast models over land: a Global model at 40 km
resolution; a North Atlantic and European (NAE) model at 12 km
resolution, and; a fine scale UK model at 4 km resolution. The
regional models are deployable and can be run for any location
in the world in support of military and commercial customers.
12. In addition, the Met Office can also
run a model at 1.5 km resolution, which provided significant benefit
during the July flood. However, given current supercomputer power,
this must be restricted in frequency and carefully scheduled to
avoid impacting on both the capability required to run the operational
models and on computer time allocated to research in weather forecasting.
13. The Met Office has also introduced a
probabilistic system of forecasting using multiple model scenarios
to simulate and quantify uncertainty. This system currently has
a 24 km resolution.
14. The data used to initialise numerical
weather models comes from various sourcesincluding direct
observations of the atmosphere, oceans and land surface conditions,
as well as previous model runs. Both the Met Office and the EA
utilise information from rain gauges and from radar observations
in rainfall and flood modelling respectively. However, ownership
and responsibility of rain gauges across the UK is split between
the agencies and consequently not all of the information is available
to either party in real time.
15. The Met Office leads and hosts the National
Centre for Ocean Forecasting (NCOF) and utilises a suite of models,
including a surge model and a wave model, to provide forecasts
of tides and storm surges to Government agencies, particularly
the EA and Marine Coastguard Agency, as well as to commercial
16. The Met Office Hadley Centre, funded
through Defra and MOD, provides the best climate prediction science
in the world
which supports the UK Government's high profile policy role on
Climate Change issues.
17. The unusual pattern of weather experienced
this summer occurred as a consequence of the location and strength
of the jet stream
combined with unusually high Atlantic sea temperatures. For
much of this summer the jet stream was further south and stronger
than is typical, resulting in many weather systems crossing southern
and central parts of the UK. These depressions have been more
intense with some interacting with the very warm and moist air
to the south, generating exceptionally heavy rainfall events.
18. The cumulative rainfall total in the
UK for May, June and July 2007 was unprecedented; Met Office records
show that 414.1 mm of rain fell across England and Wales, making
it the wettest May to July since the England and Wales Precipitation
record began in 1766.
19. The exceptionally heavy rain culminated
in two severe, disruptive flooding events.
a. On 24-25 June, a deep and slow-moving
area of low pressure brought a prolonged period of heavy rain
causing widespread flooding in parts of Yorkshire and the Humber,
Derbyshire, Lincolnshire and Worcestershire. The flooding was
a result not only of the heavy rain during the 24-25 June, but
also the antecedent conditions of a wet May and June, especially
the heavy rain during the 12-14 June, which caused the ground
to be saturated and water levels to be high prior to this event.
b. The second flooding event was the result
of exceptionally heavy rain which moved northwards across the
UK from late on 19 July throughout 20 July, the heaviest being
reported at Pershore College (Worcestershire), with Met Office
data showing 157.4 mm recorded in the 48 hour period. Widespread
flash flooding occurred on the morning of 20 July across Southeast
England, and later in the day across the Midlands as the system
moved north-westwards, causing widespread disruption to the motorway
and rail networks. River flooding followed over the subsequent
days along the Severn and its tributaries in Gloucestershire,
Worcestershire, Herefordshire, Shropshire, and along the Thames
and its tributaries in Wiltshire, Oxfordshire, Berkshire and Surrey.
More detailed rainfall statistics are at Annex A.
20. The Met Office issued an Early Warning
through the National Severe Weather Warning Service (NSWWS) on
22 June which gave 3 days notice of potential disruption.
An update issued on 24 June focused correctly on the worst hit
areas and provided highly accurate estimates of rainfall totals
correctly forecasting that 50-100 mm of rainfall would fall within
24 hours of 2200 BST on Sunday with the worst affected areas being
Yorkshire and Lincolnshire.
21. Forecasting of the 19-20 July rainfall
event was perhaps the most detailed and accurate ever achieved
by the Met Office for a high profile severe weather event. To
be able to accurately forecast 100 mm of rain at county level
with 24 to 36 hours lead time is an indication of the process
wide improvements that have recently been made. Significant use
was made of multi model probability forecasting techniques, while
the "on demand" high resolution (1.5 km) forecast model
was run for parts of the affected area to give more detailed forecast
information. The potential for excessive amounts of rain on Friday
20 July was communicated to the EA and other customer groups,
together with the uncertainties. Confidence was considered high
enough to issue an NSWWS Early Warning on Wednesday 18 July.
More detailed timelines of Met Office activities
prior to the flooding events on the 24-25 June and 19-20 July
are at Annexes B and C.
22. During the most intense rainfall on
20 July, a technical fault arose with the radar rainfall measurement
system. The result of this was a degradation in the quality of
the quantitative rainfall estimates issued to the EA for the areas
of heaviest rain. The fault has now been corrected, and re-analysis
of the data is underway to confirm the scale of the impact, in
consultation with the Environment Agency and other users of the
23. As a Trading Fund the Met Office places
a strong emphasis on meeting customer requirements. Its core mission
is to use the best science to develop and tailor services that
allow them to make the best informed decisions possible. Due to
its role as the National Weather Service and the body responsible
for the National Severe Weather Warning Service it has very strong
links with UK's emergency response infrastructure, its customers
(both government and private sector) and the general public. It
has well established and trusted mechanisms for communicating
warnings in particular through its website and its partnership
with the BBC. During the flooding events the Met Office provided
support to numerous organisations including the EA, HPA, HA, Network
Rail, local authorities and Gold Commands, as well as attending
Civil Contingency meetings at COBR.
24. Although communication of warnings did
take place successfully, there was evidence that the underlying
message was confused. This occurred for a number of reasons:
a. individuals and agencies can confuse severe
weather warnings and flood warnings;
b. some organisations or individuals look
to one organisation for warningson occasions customers
expected flood briefings as well as weather warnings from the
c. although severe weather and flooding are
linked they do not necessarily occur at the same time (for example
the Met Office web site showed no weather warnings at a time when
parts of the UK were experiencing severe flooding).
25. Although lessons learned exercises undertaken
by regional authorities highlighted Met Office forecasts as being
of high quality, it was generally acknowledged that the fragmented
responsibilities for warnings hinder understanding and therefore
26. Research in the Met Office is focussed
on improving all aspects of our forecasts, primarily through improvements
to the performance of our numerical weather prediction models.
In line with our understanding of public and customer requirements,
work is currently focussed particularly on two major areas of
27. Higher resolution forecasting. The purpose
of high resolution modelling is to enable forecasts to become
more precise about the weather expected in any locality. The results
of our research are extremely promising. The graphic below shows
the potential benefit of higher resolution models by applying
them retrospectively to the major flooding event in Carlisle in
January 2005. The figures show how an increase in forecast model
resolution from 12 km (b) to 1 km (c) provides a much increased
improvement in accumulated precipitation when compared to observations
28. Probability forecasting. Due to the
chaotic nature of the atmosphere, the further in advance forecasts
are made, the more small scale weather features become unpredictable.
How severe this unpredictability becomes is dependent on the particular
weather pattern and location. Consequently, although we can provide
a "best" forecast it is not possible to indicate how
likely this really is. This is particularly important for severe
events where the chosen response is likely to vary with the confidence
associated with the forecast. In response to this problem we have
a research programme aimed at forecasting the probability of certain
weather events occurringthrough running "ensembles"
of individual forecasts. We are now looking towards combining
this approach with the high resolution model to look at forecasting
the probability of intense local rainstorms.
29. Both of these research advances will
lead to significant improvements in the information available
from Met Office forecasts. Greater use of probabilistic forecasting
techniques in principle allows more informed decision makingparticularly
for emergency response organisations. For example, a 25% chance
of an event occurring may require a different response to a 75%
chance. In reality, probability increases as an event gets closer,
so the customer response process should be able to react to changing
uncertainty. We will continue to work closely with customers and
other Government agencies to ensure the additional information
available is used to the best effect.
30. These research advances require substantial
increases in computer power before they can be implemented in
routine operational forecasting. In 2009 we expect to be able
to purchase a new supercomputer with sufficient power to implement
a UK scale 1.5 km high resolution forecast model and a small ensemble
model with a 12 km resolution. The combination of high resolution
modelling and probabilistic ensemble forecasting is very importantneither
approach alone will give both confidence in the predicted weather
nor the precision in the detailed location.
31. Further funding would be needed to fully
realise the benefits (better pinpointing of areas at risk) that
can be delivered through operational use of a 1 km ensemble.
32. With the recent severe flooding in the
UK this summer, climate change and what this may mean for extreme
rainfall in the future, is very much at the forefront of people's
mind. Although any specific individual event cannot and should
not be attributed to climate change, we are able to make statements
about the risk of such events altering as a result of climate
change. It seems likely there will be a greater risk of heavy
rainfall across the UK in the future, particularly in winter.
33. The rainfall that caused the flooding
in the UK this summer was as a result of weather systems occurring
as part of the natural variability of the climate. The overall
weather pattern is broadly consistent with conditions during previous
events, although with the low pressure situated slightly further
south. This weather pattern is not associated with climate change.
Nevertheless, the amount of rainfall could have been larger because
of climate change. In particular, warm sea surface temperatures
in the vicinity of the UK this summer probably contributed to
the high levels of atmospheric moisture.
34. A change in the proportion of summertime
rainfall falling as heavy downpours would be likely to have a
significant impact on flooding; short period intense events tend
to cause local flash floods. However, for flooding over larger
areas, longer period rainfall is required, with antecedent conditions
being very important. In general, models suggest that a possible
signature of global warming may be relatively greater increases
in rainfall for those extremes which are rarest and of shortest
duration (ie the most intense).
35. Although there is a general consensus
on the broad features of expected climate change, there are still
uncertainties, particularly when considering how the climate may
change locally. Although climate models capture the key processes
identified as important for climate change, it is not possible
to represent the full complexity of the climate system. Generally
we use the ability of a model to reproduce the climate of the
recent past as an indicator of its likely skill in predicting
36. Natural variability of the atmosphere
is a further source of uncertainty. Natural variability leads
to our familiar daily weather patterns and on longer timescales
to phenomena such as El Nino. Future projections will in part
reflect natural variability and it is important to distinguish
this from an underlying shift in the climate caused by increased
greenhouse gases. In addition, natural variability is a significant
factor when considering extreme events on a local scale.
37. Although we are able to make confident
statements about increases or decreases in extreme precipitation
in some regions, the magnitude of these changes remains uncertain.
This is particularly pronounced over central Europe and the UK
in summer as these areas are in a transition zone between increased
and decreased precipitation.
38. Under a warming climate the probability
of extreme heavy rainfall eventsand hence floodingis
likely to change, and this will become evident over the coming
decades. In an ideal world investment in flood defences would
be derived from accurate models of future climate. However currently
there is a great deal of uncertainty about the regional impacts
of climate changeparticularly at the local scale and with
regard to precipitation. For example, although we may be confident
of increases in extreme rainfall across the UK in winter, the
actual magnitude of these increases is uncertain. Understanding
how extreme rainfall may change in detail locally in the future
remains a key challenge for climate scientists. In time, with
the ability to run higher resolution models, improved science
and the investment of resources in large ensembles of models sampling
uncertainties, improved regional scale predictions will be available.
39. The development of a national flood
defence infrastructure has, to date, placed a heavy emphasis on
return periods derived from historical records. However it is
crucial to understand that the past is no longer an adequate guide
to the future; large-scale investment in flood defences and other
parts of the critical infrastructure would be much better derived
from improved regional scale predictions. In the context of recent
flooding events, increased supercomputing power beyond that currently
planned would also lead to improved weather forecasts, better
regional detail, better input to flood models and improved warning
40. Against this background, the Met Office
has prepared a proposal for increased supercomputing power for
consideration within the context of the Comprehensive Spending
Review (CSR). The proposal would provide around 20 times the current
computing capability by 2009 rising to around 40 times by 2012.
5 A number of studies are taking place to evaluate
options for expanding the scope of flood warning services as well
as using probabilistic and higher resolution forecasts. Back
Current flood modelling techniques rely on observed (rather than
forecast) precipitation, but rainfall forecasts could be used
to better effect, particularly for rapid response flooding events. Back
A return period denotes a recurrence interval. It is a statistical
measure of how often an event of a certain size is likely to happen.
For many of the recent rainfall events return periods were greater
than 200 years. Back
An independent review of the Met Office Hadley Centre from Risk
Solutions, commissioned by Defra and MoD, was published by Defra
on 15 May-it concluded that the Hadley Centre was at the pinnacle
of world climate science. The review is available on the Defra
website: http://www.defra.gov.uk/environment/climatechange/research Back
A ribbon of very strong winds in the upper atmosphere which largely
determines where the weather systems that bring rain to the UK
will develop and move across Western Europe. Back
As opposed to likelihood of rain-confidence of heavy rain time
of issue was 100%. Back
The opposite of El Nino, which occurs naturally every 3-7 years. Back