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

Published 23 August 2019.

GDPR statement: Please note that the author of this blog does not set cookies but the host “wordpress.com” sets multiple cookies which are outside of the authors control. You can view their cookie policy here. 

1. Influences.

Changes in sea temperature.

2019augssta

Except for an area between 10 and 40 degrees West, just south of 50 deg North the Atlantic continues to warm above the long term average and the tropical area is forecast to remain slightly above average through the turn of the year into 2020.

tna_anom_20190801

In the Pacific weak or neutral El-Nino conditions have become establish and are likely to remain similar through the Autumn and Winter period – see CFSv2 and NMME multi model ensemble forecast below.  The latest forecast from Tropical Storm Risk (University College London) along with NCEP have slightly increased the number of expected Atlantic tropical storms for the remainder of the period.

201908nino

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been in a negative phase for much of the period since April 2019. This translates for the summer period to a colder/wetter phase for the UK as indicated by the wet June. Recent NAO forecasts have moved to a positive phase and are forecast to remain positive at least in the short term suggesting drier and warmer types are more likely but if a positive phase was maintained it would lean towards a milder wetter winter?

201908nao

NAO background information see the Met Office NAO information.

Recent Climatology

August so far (23rd) has been wetter than average, slightly warmer but with less sunshine –  leading to a warmer than average summer. The temperature and rainfall anomalies from Spring onward are shown below.

201908temp

201908rain

The picture for rainfall is more mixed but many places will probably end up with above average rain for the the summer season. A few parts of the SW may be close to average depending on the rain total in August –  see the June + July anomalies below.

201908jjrain

River flows and groundwater in July reflect that drier month. More details can be found in the July 2019 Hydrological summary PDF 

201908riverflow

201908ground

The reservoir levels in the SW of England (18th August) show 65% storage which is close to or slightly above normal August level.

201908resev

Looking ahead (Aug to Nov) the experimental product from the Global Flood Awareness System (Version 2) suggests reduced flood risk for the SW over the next few months although local flash flooding would not be included in this.

201908glofas

Atmosphere: Predictions.

A:  1: Stratosphere

Stratosphere is in summer mode as shown by the ECMWF 50hPa chart for 1200UTC 22nd August 2019.

20190850hpa

A: 2: Upper Troposphere

CFSv2 200hPa monthly mean forecast height (contours) and anomaly (lower row) for the period September to November 2019, based on 8th August data, show above normal heights throughout the Autumn but with hints at strong Atlantic jet (on average) developing for November.

z200SON0819

B: Lower Troposphere:

A selection of September to November 2019 solutions in low resolution from WMO using August data. Three month season and individual months. Top row temperature anomaly and mid row precipitation anomaly.

White areas are area where the probabilities of below/normal/above are similar hence there is no overall signal for this period from the WMO super ensemble which includes DWD, BoM, Seoul, CPTEC Brazil, UKMO, Washington, Pretoria, Tokyo, ECMWF,  and Toulouse seasonal models. Moscow, Beijing and CMC missing.

WMOSON082019

Some examples from individual centres are shown below. Full list can be seen at the seasonal forecast page.

Data from NASA

NASASON0819

UKMO

UKMOSON082019

ECMWF

ECLWMOSON082019

JAPAN

TOKSON082019

NMME Autumn 2019

08NMMESON

NMME Winter 2019/20

08NMMEDJF

For SW England, data from  Japan Canada and Germany suggest near normal temperatures but all other models suggest above normal temperatures this Autumn but rainfall forecast are much more varied.

C: Recent results for (May to July 2019):

Temperature forecast for a warming trend to above normal for the season were good although some output may be a little warm. Many precipitation forecasts were poor but a trend to wetter types was suggested although not specifically the much wetter June.

Comparison graphics can be seen at the verification page for May June July 2019

2. Forecast. SW England.

Autumn 2019  (September October November)

Overall the season is likely to see above normal temperatures, again hints at being slightly above normal in September then much milder later in Autumn.

Rainfall forecasts slightly favour above normal rainfall for the season, perhaps with a less wet start in September and a wetter end of Autumn but this is not clear. Some linger drier periods are suggested but no agreement as to when (may straddle a month and be missed by monthly anomalies)

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 2020 January February)  limited data

The indication for a milder than average winter remains the main signal. Precipitation is likely to be near or above average although there are hints that parts of the S and E parts of the S and E of the region may have normal or below average values. Below average snowfall is more likely than above average.

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.

Spring 2020  (March April May) very limited data

Above average temperature for the season but May could be a colder month. Precipitation near average or below but May could be wetter than average,

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.

 

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