Review and Experimental Long Range Forecast for SW England. March 2019.

Published 15 March 2019.

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1. Influences.

Changes in sea temperature.

140319sst

The sea temperature around the UK and across much of the North Atlantic is similar to last month being fairly close to the seasonal normal, a shade warmer/cooler in places.

The Met Office diagnostic for the tropical Atlantic (shown below) was near normal with a trend to above normal values in the majority of the ensemble solutions.

150319tropatl

In the Pacific weak El-Nino conditions have continued, see CFS2 and  multi model ensemble forecast below.

140319ninot

An interesting feature is the sea temperature anomaly to the west of Panama. The cold tongue is caused by up-welling of colder salty water which is probably due to the spread west of lower salinity water by persistent easterly winds.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been slightly positive (warmer wetter phase for UK) since early January and the index is likely to remain in a positive phase over the short term. Longer term there are signs of a disruption in this pattern.

150319nao

For NAO background information see the Met Office NAO information.

Recent Climatology

In the SW of England, March main station SYNOP data to 0600 15th shows the average temperature anomaly to be around +1.5 C, rainfall between 100 and 110% of average with about 40% of normal sunshine hours.temp150319

Temperatures over the last six months have been near or above average with February exceptionally mild and follows on from a warmer than average summer 2018.

Rainfall was above average in SW England during November and December 2018 otherwise the nine months from June 2018 has seen normal or below normal rainfall for most parts.

climrain1503

The wetter period in the first half of February 2019 helped river flow recover from the lows in January but river flows were reported as slightly below average.

river1503

Full details can be found in the  February 2019 Hydrological summary PDF 

The reservoir levels in the SW of England (10th March) show 87% storage which is below the normal winter level.

resevoit1503

The experimental product from the Global Flood Awareness System (Version 2) based on early March ECMWF seasonal forecast for April to June 2019. The system does not suggest an enhanced flood risk for UK currently and suggest lower than normal river flows are possible.

150319glofas

Atmosphere predictions.

A:  1: Stratosphere 

The stratospheric polar vortex re-established and became unusually strong. Much colder than normal temperatures were recorded, especially at 10 and 30hPa.  30hpaRecent forecasts (ECMWF shown below) suggest a new area of warming is likely to develop and this may well be the final warming prior to the change to a “summer” pattern. This may fit in with some forecasts of more blocking patterns and the lower than average river flow forecast shown by Glofas.

50hpa

A 2: Upper Troposphere

CFSv2 200hPa monthly mean forecast height (contours) and anomaly (lower row).

150319cfs200Z

Some indication of enhanced jet in April with a SSW jet possibly steering lows further N over or to N of UK on average. More generally ridged pattern for May and June but less of a signal for July and August.

B: Lower Troposphere:

Selection of Spring (April May June and some July August) solutions. Top row temperature anomaly and lower row precipitation anomaly. Above normal orange/reds below normal in blue shades.

1503ECj

ECMWF monthly

CFS2E30319

CFS2 E3 monthly

NMME0319

NMME monthly 

Selection of models UKMO, Australia, Russia, France, Canada and Germany. Left image season April to June then separate months. Top row temperature anomaly and lower row precipitation anomaly.  Russia and Australian colder and wetter most others warmer drier after a wetter start and possible end of the period April to August. Other models available at web link.

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C: Recent results for the winter period December January February:

Temperature mostly fair indication (scale no indication poor fair good)  but signal for the milder February was confused by the colder indication due to a sudden stratospheric warming which although blocked patterns developed the flow remained mild rather than colder easterly. Rainfall good indication for December but poor overall for the season being wetter than average. Pressure was above average so at best only a partial signal. Better models seemed to be NMME and Japan.

2. Forecast. SW England.

Remainder of Spring 2019  (April May) 

Milder than average temperatures continue for both for April and May in the SW o England.

Rainfall totals normal to wetter than normal in April but trending to drier than average in May.

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

Summer 2019  (June July August) 

Warmer than average summer although temperatures in August may be closer to normal.

Overall near or below normal rainfall seems likely but uncertain as to whether it is the number of dry days or the rain total that will be lower. Higher temperatures may lead to shorter periods of heavier rain but fewer wet days which has been a pattern in recent years.

Several models suggest a drier than average June and possibly July although some suggest August rather than July to be drier. Probably an increased risk of at least normal rainfall later in the summer. Overall monthly details is inconsistent but the idea that the start of summer could be the drier period seems well supported although this is different to February data.

Chance of reduced river flows and reservoir levels causing some concerns.

Summer climate: 1981 to 2010 average daily mean temperature 14 or 15°C in many areas to 16 or 17°C in main urban areas also locally east of the moors and more widely in Somerset. Maximum temperatures average 19 to over 21°C in similar areas. July often warmer than August. Average 1981 to 2010 rain 300mm over the moors, typically 200 to 250mm in many coastal and eastern areas. June often drier than July and July drier than August.

Autumn 2019  (September October November). 

Warmer September then milder than normal, monthly and for the season.  Could start drier in September then  be a wetter than average start to Autumn in the SW but then near or even below normal rainfall possible for for October or November.

Autumn climate: 1981 to 2010 average mean temperature 11 or 12°C but nearer 10°C in rural areas. Maximum temperatures average 14 to 16°C. November normally colder than October which is colder than September. Average 1981 to 2010 rain 300 to 400mm, 600 to 800mm over the moors but east of the moors and in lowland Somerset more like 200 to 300mm. September often drier than October or November.

Winter 2019  (December January February) minimal. 

Milder than average throughout. Wetter December then below average rainfall hence a drier than average winter. Below average snowfall.

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall; Dec and Jan typically wetter than February. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor. Snow climatology less than 5 days lying snow over lowland areas 5 to 10 hills, say hills around 200 metre elevation – one in three years have no lying snow.

3. Caution.

Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary can be seen at  here.

The attempt at a Regional Forecast for SW England aims to test whether such a forecast of temperature and rainfall variation from average can be made using numerical model data available on the internet. The forecast should not be used for any other purpose. A brief verification summary for the UK and Eire is routinely published at http://www.weather-info.co.uk/wxsvc/Verification.html or Click here for the Teignmouth and Dawlish summary

4. References.

SST anomaly NOAA Remote Sens. 2014, 6(11), 11579-11606; doi:10.3390/rs61111579

IRI statistics: Mason, S.J. and L. Goddard, 2001: Probabilistic precipitation anomalies associated with ENSO. Bull. Amer. Meteor. Soc., 82, 619-638.

UK climate details see: http://www.metoffice.gov.uk/public/weather/climate/

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS (http://www.globalfloods.eu/) of the European Commission Joint Research Centre and the European Centre for Medium-Range Weather Forecasts.
Reference: Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F.: GloFAS – global ensemble streamflow forecasting and flood early warning, Hydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.

‘Copernicus Products’  as listed in the C3S or CAMS Service Product Specification or any other items available through an ECMWF Copernicus  http://climate.copernicus.eu

International seasonal monthly data from WMO

Stratospheric Diagnostics from Japan Met Agency and Free University Berlin

Climate data from The Met Office UK and NCEP USA