Hydrological Outlook UK. February 2015 HYDROLOGICAL OUTLOOK UK. Period: From Feb Issued on using data to the end of January 2015

Transcription

1 Hydrological Outlook UK Period: From Feb 2015 Issued on using data to the end of January 2015 SUMMARY The outlook for river flows and groundwater levels across the UK for February indicates some variability. Above normal river flows are the most likely outcome in north-west Scotland; for the remainder of Scotland, Northern Ireland, Wales and most coastal areas of England normal to above normal river flows are most likely. For most of central England the most likely outcome is for river flows to be in the normal range. During February groundwater levels are most likely to be in the normal range. There are, however, parts of the east coast of England above Norfolk, and north-east Scotland in which, while normal flows and groundwater levels are most likely, there is the possibility of below normal flows and groundwater levels. The outlook for both river flows and groundwater levels from February to April is for normal conditions. Rainfall: Latest predictions for UK-mean precipitation favour near- to above-average rainfall for February, although there is a large degree of uncertainty. For February-March-April, predictability is low and the forecast does not differ significantly from climatology, with above-average and below-average precipitation equally probable. The probability that UK precipitation for February-March-April will fall either into the driest or wettest of our five categories is around 20% (the probability for each of these categories is 20%). River flows: River flows in January reflected the marked variation in observed rainfall with higher than normal flows in western parts in each of Scotland, Northern Ireland, Wales and northern England, and also in the far south east of England. Conversely low flows were seen on the Derwent (Yorkshire) and Ythan (north-east Scotland). The pattern of higher than normal flows is likely to persist given the likelihood of above average rainfall; the expectation of higher than average rainfall suggests that rivers in which low flows were observed in January are likely to return to normal. Looking ahead to the coming three months, there is no strong signal towards above- or below-normal flows. Above average river flows are likely in north west Scotland in February. River flows and groundwater levels are largely expected to be within the normal range in most of the UK for the next three months. HYDROLOGICAL OUTLOOK UK Groundwater: Groundwater levels during January were generally in the normal to above normal range across the UK, with the exception of the Permo-Triassic sandstones which recorded above normal levels. Groundwater levels are likely to remain at or slightly above normal in most aquifers in both February and the coming three months. Along the east coast of England lower than normal rainfall in January makes it unlikely that high groundwater levels will occur even with high rainfall. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website: During February, normal or above normal river flows are most likely in southern Scotland, north-west England, and Kent Shaded areas show principal aquifers

2 Hydrological Outlook UK About the Hydrological Outlook UK About the Outlook: This document presents an outlook for the UK water situation for the next 1 3 months and beyond, using observational datasets, meteorological forecasts and a suite of hydrological modelling tools. The outlook is produced in a collaboration between the Centre for Ecology and Hydrology (CEH), British Geological Survey (BGS), the UK Met Office (UKMO), the Environment Agency (EA), Natural Resources Wales (NRW), the Scottish Environment Protection Agency (SEPA), and the Northern Ireland Rivers Agency (RA). Data and Models: The Hydrological Outlook depends on the active cooperation of many data suppliers. This cooperation is gratefully acknowledged. Historic river flow and groundwater data are sourced from the UK National River Flow Archive and the National Groundwater Level Archive. Contemporary data are provided by the EA, SEPA, NRW and RA. These data are used to initialise hydrological models, and to provide outlook information based on statistical analysis of historical analogues. Climate forecasts are produced by the UKMO. Hydrological modelling is undertaken by CEH using the Grid-to-Grid, PDM and CLASSIC hydrological models and by the EA using CATCHMOD. Hydrogeological modelling uses the R-groundwater model run by BGS and CATCHMOD run by the EA. More information is available from the Outlooks website: Disclaimers: This document aims to provide an indicative outlook for the water situation using the most comprehensive and up-to-date hydrological data, and modelling techniques. The Outlooks are intended to provide guidance on the likely water situation in the UK over the coming months, and should not be used in isolation, but alongside other sources of information such as flood warnings and meteorological forecasts (see links right). Some of the features displayed on the maps contained in this report are based on the following data with permission of the controller of HMSO. (i) Ordnance Survey data. Crown copyright and/or database right Licence no (ii) Land and Property Services data. Crown copyright and database right, S&LA 145. (iii) Met Office rainfall data. Crown copyright. All rights reserved. Unauthorised reproduction infringes crown copyright and may lead to prosecution or civil proceedings. Further information: For more detailed information about the Hydrological Outlook, and the derivation of the maps, plots and interpretation provided in this outlook, please visit the Hydrological Outlook UK website. The website features a host of other background information, including a wider range of sources of information which are used in the preparation of this Outlook. Contact: Hydrological Outlooks UK Centre for Ecology & Hydrology Wallingford Oxfordshire t: OX10 8BB e: enquiries@hydoutuk.net Reference: Hydrological Outlook UK, 2015, February Centre for Ecology and Hydrology, Oxfordshire UK, Online, Other Sources of Information: The Hydrological Outlook should be used alongside other sources of up-to-date information on the current water resources status and flood risk. Hydrological Summary for the UK: provides summary of current water resources status for the UK: Environment Agency Water Situation Reports: provides summary of water resources status on a monthly and weekly basis for England: Flood warnings are continually updated, and should be consulted for an up-to-date and localised assessment of flood risk: Environment Agency: Scottish Environment Protection Agency: UK Met Office forecasts for the UK: HYDROLOGICAL OUTLOOK UK

3 Met Office 3-month Outlook Period: February - April 2015 Issue date: Fig P1 3-month UK outlook for precipitation in the context of the observed annual cycle The forecast presented here is for February and the average of the February-March-April period for the United Kingdom as a whole. The forecast for February will be superseded by the long-range information on the public weather forecast web page ( starting from 6. This forecast is based on information from observations, several numerical models and expert judgement. SUMMARY - PRECIPITATION: Latest predictions for UK-mean precipitation favour near- to above-average rainfall for February, although there is a large degree of uncertainty. For February-March-April, predictability is low and the forecast does not differ significantly from climatology, with above-average and below-average precipitation equally probable. The probability that UK precipitation for February-March-April will fall either into the driest or wettest of our five categories is around 20% (the probability for each of these categories is 20%). CONTEXT: As already mentioned in the temperature section, there is a fairly consistent signal from computer models for the positive phase of the North Atlantic Oscillation (NAO), which has prevailed through the winter so far, to continue into February. Usually, this atmospheric circulation pattern is associated with above-average precipitation and this is reflected in figure P2, which shows a shift towards wetter than average conditions. Similar to January, the weather in February is likely to alternate between spells of northwesterly winds, bringing drier, colder conditions and southwesterly winds, which often bring mild, wet conditions. There is a signal in computer models that during the more unsettled spells of weather, northern parts of the UK are at highest risk of strong winds and heavy rain. For February-March-April as a whole, although near- to above-average precipitation is slightly favoured, uncertainty is large; this is highlighted in figure P2, where there is a broad range of outcomes. There is disagreement between models over which atmospheric pattern will dominate, although there is a slight preference in the majority of models, for below-average pressure near the UK, which is generally associated with wetter-than-average conditions. Precipitation - 3-month UK-average accumulation (mm) NDJ DJF JFM FMA MAM AMJ MJJ JJA JAS ASO SON OND NDJ DJF JFM FMA 3-month periods Observations Average Observation outlook: Feb-Apr 3-MONTH OUTLOOK Fig P2 Precipitation monthly UK-average accumulation (mm) 1-month and 3-month UK outlook for precipitation in the context of observed climatology February Precipitation 3-month UK-average accumulation (mm) February-April Fig P3 Average daily precipitation (mm/day) month and 3-month UK outlook for precipitation in the context of recent climatology: year-to-year and within-season variability 1-MONTH OUTLOOK 3-MONTH OUTLOOK 20 0 Observations Average Observations outlook: Feb Feb-Apr 100 * 1mm/day =28mm accumulation in Feb =89mm accumulation in Feb-Apr Years Observed averages: Feb-Apr Feb Mar Apr 2015 outlook: Feb Feb-Apr This Outlook provides an indication of possible temperature and rainfall conditions over the next 3 months. It is part of a suite of forecasts designed for contingency planners. The Outlook should not be used in isolation but should be used with shorter-range and more detailed (30-day, 15-day and 1-to-5-day) forecasts and warnings available to the contingency planning community from the Met Office.

4 Met Office 3-month Outlook Period: February - April 2015 Issue date: The forecast presented here is for February and the average of the February-March-April period for the United Kingdom as a whole. The forecast for February will be superseded by the long-range information on the public weather forecast web page ( starting from 6. This forecast is based on information from observations, several numerical models and expert judgement. SUMMARY - TEMPERATURE: For February near- to above-average temperatures are more probable than below-average. For February-March-April as a whole above-average temperatures are more likely than below-average. Overall, the probability that the UK-mean temperature for February-March-April will fall into the coldest of our five categories is around 15% and the probability that it will fall into the warmest category is around 30% (the probability for each of these categories is 20%). CONTEXT: The tropical Pacific Ocean overall remains warmer than average. However, sea surface temperatures in the eastern Pacific have cooled in the past month, with the strongest positive anomalies remaining across central and western parts of the tropical Pacific Ocean. This change in the pattern of sea surface temperature anomalies suggests that we are now moving away from a possible weak El Niño event, with near-neutral conditions the most probable outcome in the coming months; this will offer little predictive value for conditions across Europe during the period of this forecast. In the Arctic, sea ice growth has slowed and overall extent is now below average, particularly across the Bering Sea and Sea of Okhotsk. This factor is not expected to offer any useful predictability for Europe in the next three months. The Quasi-Biennial Oscillation (QBO), an oscillation of the equatorial zonal wind in the stratosphere, is now firmly in an easterly phase. Typically, an easterly phase is associated with a weaker polar vortex. A weaker polar vortex can lead to a greater incidence of blocking patterns over the northern hemisphere in winter, which would increase the probability of cold weather across northern Europe. However, this is a transitional time of year with the QBO probably exerting some influence at the beginning of the season, but providing little known contribution by the end. For February, there is a reasonably strong signal, from several models, for a continuation of a positive phase of the North Atlantic Oscillation, with winds blowing frequently from the west or northwest. Overall this pattern typically brings near- to above-average temperatures but, as seen in January, this does not preclude occasional spells of colder weather with snow. The left-hand graph in figure T2 shows a shift towards milder conditions and the chance of a prolonged, severe spell of cold weather is reduced compared to climatology. For February-March-April as a whole, computer models begin to diverge; consequently no clear signals emerge and predictability is considered low. The right-hand graph of figure T2 reflects this, with the forecast curve more closely resembling climatology than the curve for February. Nevertheless, there is still an increased probability of above-average temperatures. Fig T1 Average UK temperature (Celsius) 3-month UK outlook for temperature in the context of the observed annual cycle NDJ DJF JFM FMA MAM AMJ MJJ JJA JAS ASO SON OND NDJ DJF JFM FMA 3-month periods Observations Average Observation outlook: Feb-Apr 3-MONTH OUTLOOK Fig T2 1-month and 3-month UK outlook for temperature in the context of observed climatology Average UK temperature (Celsius) February Average UK temperature (Celsius) February-April Fig T3 Average UK temperature (Celsius) month and 3-month UK outlook for temperature in the context of recent climatology: year-to-year and within-season variability 1-MONTH OUTLOOK 3-MONTH OUTLOOK Years Observations Average Observations outlook: Feb Feb-Apr Observed averages: Feb-Apr Feb Mar Apr 2015 outlook: Feb Feb-Apr This Outlook provides an indication of possible temperature and rainfall conditions over the next 3 months. It is part of a suite of forecasts designed for contingency planners. The Outlook should not be used in isolation but should be used with shorter-range and more detailed (30-day, 15-day and 1-to-5-day) forecasts and warnings available to the contingency planning community from the Met Office.

5 Outlook based on hydrological persistence and analogy Period: January 2015 March 2015 Overview Issued on using data to the end of January SUMMARY The outlooks for February and for February to April are for normal to above normal flows across much of the United Kingdom, with local areas of lower flows mainly in the northeast. Note that forecasts are unavailable for large parts of western United Kingdom. RIVER FLOW ANALOGY 1-month flow outlook Outlooks from hydrological analogues are based on a comparison of river flow during recent months with flows during the same months in previous years at a set of approximately 90 sites from across the UK. These sites are depicted on the two maps. Years with observed flows that most closely resemble current conditions are identified as the best analogues and the outlook is based on extrapolating from current conditions based on these analogues. It is, however, often the case that a simpler forecast based on the persistence of river flow provides a better forecast than provided by analogy. This is particularly true for slowly responding catchments associated with aquifer outcrops. Both methods are considered at each site and the forecast from the method with the higher confidence is presented. A simple classification of flows is used (high, medium and low) as indicated by the colours of the dots, with the confidence 3-month flow outlook of the forecast being represented by the size of the dot. A tag on the dot indicates which method has been used in each instance. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

6 These figures provide insight into the hydrological analogue methodology for a set of sites from across the UK. In each of the time series graphs the bold black line represents the observed flow during the past nine months. The grey band indicates the normal flow range (the normal band includes 44% Outlook based on hydrological persistence and analogy Period: of observed flows in each month). The selected analogues are shown as thin lines and the trajectories that flows took in the following month are also shown. The forecast is shown as the dashed red line, and in each plot it states whether this has come from the analogues or has been generated on the basis of persistence. Site-based: 1 month outlook Issued on using data to the end of January RIVER FLOW ANALOGY The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

7 These figures provide insight into the hydrological analogue methodology for a set of sites from across the UK. In each of the time series graphs the bold black line represents the observed flow during the past nine months. The grey band indicates the normal flow range (the normal band includes 44% Outlook based on hydrological persistence and analogy Period: April 2015 of observed flows in each month). The selected analogues are shown as thin lines and the trajectories that flows took in the following three months are also shown. The forecast is shown as the dashed red line, and in each plot it states whether this has come from the analogues or has been generated on the basis of persistence. Site-based: 3 month outlook Issued on using data to the end of January RIVER FLOW ANALOGY The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

8 Outlook based on modelled flow from historical climate Overview Period: January 2016 Issued on using data to the end of January SUMMARY For the next 6 months there is a lower likelihood of below normal flow in Central England, parts of East Anglia and Southeast England, with Southeast England also showing increased likelihood of above normal flow. RIVER FLOW FROM HISTORICAL CLIMATE This outlook is based on monthly ensembles of historical sequences of observed climate (rainfall and potential evapotranspiration) that form input to hydrological models. The outputs are probabilistic simulations of the average river flow over the forecast horizon (1 to 12 months ahead), at each location. The simulations are generated by conceptual rainfall-runoff models from CEH (PDM and CLASSIC) and the EA (CATCHMOD) calibrated on observed or naturalised flows. The graphs show variation over time of the number of simulated river flow, in each monthly ensemble, that fall within each the seven categories: exceptionally low, notably low, below normal, normal, above normal, notably high and exceptionally high. The categories represent cumulative flow conditions, e.g. For 3-month, the simulated total 3-month flow simulated compared to the historical 3-month flow distribution. Bar plots show the outlook distribution for 3 time horizons: 3, 6 and 12 months. Stack diagrams show the variation over time of the outlook distribution for a number of individual catchments The monthly variations can be compared to the long-term average distribution of river flow (shown as columns on the left and right of each timeline graph). This outlook is based entirely on historical sequences and therefore, this is not a forecast. It does not contain any knowledge of the state of the atmosphere and ocean. It is hence possible that some of the historical sequences used might be inconsistent with current large-scale atmospheric conditions and would therefore be unlikely to occur in the next few months. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

9 Avon Wye Severn Outlook based on modelled flow from historical climate Overview Lune RIVER FLOW FROM HISTORICAL CLIMATE Ure Trent This outlook is based on monthly ensembles of historical sequences of observed climate (rainfall and potential evapotranspiration) that form input to hydrological models. The outputs are probabilistic simulations of the average river flow over the forecast horizon (1 to 12 months ahead), at each location. The simulations are generated by conceptual rainfall-runoff models from CEH (PDM and CLASSIC) and the EA (CATCHMOD) calibrated on observed or naturalised flows. The graphs show variation over time of the number of simulated river flow, in each monthly ensemble, that fall within each the seven categories: exceptionally low, notably low, below normal, normal, above normal, notably high and exceptionally high. The categories represent cumulative flow conditions, e.g. For 3-month, the simulated total 3-month flow simulated compared to the historical 3-month flow distribution. Bar plots show the outlook distribution for 3 time horizons: 3, 6 and 12 months. Stack diagrams show the variation over time of the outlook distribution for a number of individual catchments The monthly variations can be compared to the long-term average distribution of river flow (shown as columns on the left and right of each timeline graph). This outlook is based entirely on historical sequences and therefore, this is not a forecast. It does not contain any knowledge of the state of the atmosphere and ocean. It is hence possible that some of the historical sequences used might be inconsistent with current large-scale atmospheric conditions and would therefore be unlikely to occur in the next few months. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

10 Outlook based on Modelled Flow from Rainfall Forecasts The regional maps illustrating the regional river flows for five members of the Met Office ensemble of rainfall forecasts give some indication of the range of possible river flows in the coming months. As noted previously, the actual flows could be more extreme than the flows generated by either the lowest or highest members of the rainfall ensemble. The bar charts (below) give further insight into the range of river flow forecasts by considering all members of the forecast rainfall ensemble. The regional bar charts show the percentage of ensemble forecasts falling in each of the flow categories as generated by the monthly-resolution water-balance model. As before results are averaged by region then ranked in terms of 49 years of historical regional flow estimates ( ). SUMMARY: This month, following a wetter than average January there is a >60% chance of river flows being Normal or above (Above normal, Notably high or Exceptionally high) in all regions. There is a >25% chance that river flows will be Above normal, Notably high or Exceptionally high in all regions (except Anglia). Conversely, there is a >20% chance that river flows will be Below normal, Notably low or Exceptionally low in many northern and easterly regions (A,N,Y,FR,NER,TWR). Over the next 3 months In all regions there is a > 45% chance of Normal flows. There is a >30% chance of Above normal flows or higher in SR. Conversely, there is a >30% chance of Below normal, Notably low and Exceptionally low flows in N, NER and TWR. example Period: April 2015 Issue date: month 3-month 1- and 3-month variability SCOTLAND HR Highlands Region NER North East Region TR Tay Region FR Forth Region CR Clyde Region TWR Tweed Region SR Solway Region ENGLAND N Northumbria NW North West Y Yorkshire ST Severn Trent A Anglian T Thames S Southern W Wessex SW South West WALES WEL Welsh NORTHERN IRELAND This method cannot currently be used in Northern Ireland RIVER FLOW FROM RAINFALL FORECASTS The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

11 Rockley (SU17/57) Heathlanes (SJ62/112) Skirwith (NY63/2) Outlook based on modelled groundwater from historical climate Period: January 2016 Issued on using data to the end of January SUMMARY :For the responsive aquifers, especially the Chalk, the trend towards a climatological probability will continue. The legacy of the wet winters of 2012/13 and 2013/14 in maintaining high groundwater levels in some Permo Trias aquifers is finally fading, and levels may approach more normal ranges over the next 12 months. Dalton Holme (SE94/5) Washpit Farm (TF81/2A) GROUNDWATER FROM HISTORICAL CLIMATE This outlook is based on monthly ensembles of historical sequences of observed climate (rainfall and potential evpotranspiration) that form input to hydrological models. The outputs are probabilistic simulations of the average groundwater level over the forecast horizon (3 to 12 months ahead), at each location. that fall within each the seven categories: exceptionally low, notably low, below normal, normal, above normal, notably high and exceptionally high. The monthly variations can be compared to the long-term average distribution of levels, which are shown as columns on the left and right of each graph. Little Bucket Farm (TR14/9) knowledge of the state of the atmosphere and ocean. It is hence possible that some of the historical sequences used might be inconsistent with current large-scale atmospheric conditions and would therefore be unlikely to occur in the next few months. The graphs show variation over time of the number of simulated groundwater levels in each monthly ensemble, This outlook is based entirely on historical sequences and therefore, this is not a forecast. It does not contain any The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

12 Outlook based on modelled groundwater level & climate forecast Period: April 2015 SUMMARY: Groundwater levels are likely to remain at or slightly above normal in most aquifers. Along the east coast of England it now appears unlikely that high groundwater levels will occur even with high rainfall. NB. The number of groundwater models used for this outlook has been increased, and represents a wider range of aquifers than in previous outlooks. Lowest rainfall forecast 1 st quartile Median 3 rd quartile Highest rainfall forecast Overview Issued on using data to the end of January These forecasts are produced by running five members of the Met Office ensemble climate forecast through groundwater models of observation borehole hydrographs at 42 sites across the country. The sites are distributed across the principal aquifers. Based on the distribution of observed historical groundwater levels in a given month, seven categories have been derived for each site: very low, low, below normal, normal, above normal, high, and very high. The forecast groundwater level is assigned to one of these seven categories depending on where it falls within the distribution of the historically observed values. Key Percentile range of historic observed values for relevant month Exceptionally high levels > 95 Notably high levels Above normal Normal Below normal Notably low levels 5-13 Exceptionally low levels < 5 GROUNDWATER FROM CLIMATE FORECASTS 1-month outlook Lowest rainfall forecast 1 st quartile Median 3 rd quartile Highest rainfall forecast 3-month outlook The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website: