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

Published 21 April 2019.

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1. Influences.

Changes in sea temperature.

180419ssta

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 or 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.

tna0419

In the Pacific weak El-Nino conditions have continued, see CFS2 and  multi model ensemble forecast below but current forecasts are mostly slightly less warm.

nino0419

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) returned to slightly positive (warmer wetter phase for UK) from mid April after the first negative phase period for some time.

noa0419

NAO background information see the Met Office NAO information.

Recent Climatology

In the SW of England, main station SYNOP data to April 1800 20th shows the average temperature anomaly to be around +0.7 C, but zero in the east. Rainfall between 60 and 90% of average with about 55% of normal sunshine hours.

tempsummary

The mean temperature over the last six months has been above average except for January (which was near average). February was exceptionally mild and followed on from a warmer than average summer 2018.

Rainfall was above average in SW England during March (mainly until 17th). For the six month period October to March south parts were slightly above average but northern parts remained below the long term 1981-2010 average.

rainfall

The recent wetter periods helped river flow recover from the lows in January but river flows were reported as slightly below average. Ground water in the east of the region remains normal but eastern parts of the UK have less good water resource levels.

riverflow

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

The reservoir levels in the SW of England (14th April) show 88% storage which is slightly below the normal April level.

reslevel14

The experimental product from the Global Flood Awareness System (Version 2) based on early April ECMWF seasonal forecast for April to July 2019. The system does not suggest an enhanced flood risk for SW England.

Glofas0419

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.

npole0419

Temperatures have recovered and it looks like the polar vortex will transition to “summer” mode by early or mid May as seen by the ECMWF forecast (dated 18th April) and shown below.

strat0419

A 2: Upper Troposphere

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

z2000419

Some indication of more troughing just west of the UK but overall weaker than normal jet flow may be lead to more blocked patterns. Uncertain location of higher pressure which may be further east for periods.

B: Lower Troposphere:

Selection of May to July 2019 solutions in low resolution from WMO (not all data available at date of issue) . Top row temperature anomaly and lower row precipitation anomaly. Above normal orange/reds below normal in blue shades.

mjjsum0419

Three month average anomalies sourced from WMO up to 20 April 2019

Above normal temperatures are indicated but data from Australia and Russia are the exception with a below normal temperature forecast. Rainfall patterns less clear cut with the cooler wetter output (2) slightly outnumbered by the near normal/uncertain (6) and drier solutions (3).

MtoAugE3

CFS2 (E3 data) 1st April for May to August

NMME140419JJA

CFS2 (E3 data) 124th April May to August

Although temperature forecasts from CFS2 (E3 data) show consistent positive anomalies the rainfall anomalies are in less good agreement between model runs.

NMME0419MJJA

NMME data 9th April for May to August

C: Recent results for the period January February March:

Temperature mostly fair indication (scale no indication poor fair good). 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 was fair at best and many models were poor. Month to month forecasts were poor. Pressure was slightly 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  ( May) 

Milder/warmer than average temperature is forecast by most models May in the SW of England.

Rainfall forecasts suggest than May could be drier than average (few models suggest wetter).

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly  May 11 or 12°C. Average spring 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset. May roughly 80-150mm over the moors, 60-80mm elsewhere but 40-60mm locally to the East of Dartmoor and over large parts of Somerset and Dorset.

Summer 2019  (June July August) 

A better than average summer is more likely than a less good summer.

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

Models are mixed at which month could be drier or wetter. Overall near 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. Higher temperatures may lead to shorter periods of heavier rain but fewer wet days which has been a pattern in recent years. Indications of some linger dry spells.

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) limited data. 

Warmer September then milder than normal, monthly and for the season.  Could start drier in September then  end with a wetter November hence overall average rain for the season.

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 data. 

Milder than average throughout. Wetter start to winter then ending below average. 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

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