Review and Experimental Long Range Forecast for SW England. March 2020.

Published 20 March 2020.

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1. Influences.

Changes in sea temperature.

160320ssta

Noticeable changes in the Atlantic include a much larger area of cooler than average North Atlantic sea temperatures to the west of the UK and Eire also a much warmer area near the equator between Africa and South America. The North Sea, Baltic and Med remain above normal.

tna_anom_20200301

The tropical north Atlantic is forecast to remain very slightly warmer than average through Spring and Summer 2020 (See Met Office graphic above).

180320nin34

In the Pacific neutral or weak El-Nino conditions are present but forecasts suggest conditions are likely to trend cooler during Summer and into early Autumn 2020 – see CFSv2 and NMME multi model ensemble forecast above.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been mostly positive since early December 2019 and is forecast to remain slightly positive over the next week before trending negative at the end of March which might hint at a colder start to Spring.

190320nao

NAO background information see the Met Office NAO information.

Recent Climatology

March, to the 19th, has been slightly cloudier, milder and wetter than average.

Winter, as can be seen in the graphics below, has been very mild and in most places wet due to a very wet February.

190320djf

190320winter

River flows in February 2020 reflected the very wet conditions with high river flows in most areas apart from Aberdeenshire. It is perhaps a surprise that groundwater in eastern England is still below average. Details can be found in the February 2020 Hydrological summary PDF 

1903river

The reservoir levels in the SW of England (15th  March) showed 97.5% storage. The less than 100% value is due to Colliford as the the other main reservoirs are at 100% capacity.

190320res

Global Flood Awareness System March forecast (high) risk through to June 2020 largely reflects the well above average rain in February and early March. The risk reduces from late April or early May.

Atmosphere: Predictions.

A:  1: Stratosphere

A second phase of significant warming is occurring at 10hPa and to some extent at 50hPa but from unusually low temperature values.

190320strat1

ECMWF 10 and 50hPa 18/1200UTC data shows that despite ongoing warming the polar stratospheric vortex is reluctant to be eroded completely.

190320strat2

A: 2: Upper Troposphere

CFSv2 200hPa contours for April to August 2020 are shown below (top row) along with the anomalies from CFS (middle row) and NMME (lower row). Suggestion of  enhanced Atlantic jet for April towards or just north of the UK imply unsettled types but possibly higher pressure further south.

z200AMJja0320

B: Lower Troposphere:

A selection of April to June solutions using March 2020 data are shown below. Three month season and individual months, top row temperature anomaly and lower row precipitation anomaly.

White areas are where the probabilities of below/normal/above are similar hence there is no overall signal for this period from the WMO data. Problems with the WMO web site in South Korea has made access to this data difficult.

APRIL TO JUNE 2020

UKMOAMJ032020

UKMO

CPTECAMJ032020

Brazil Met Service

DWDAMJ032020

German Met Service

MONTAMJ032020

Canadian Met Service

BOMAMJ032020

Australian Met Service

If they become available further examples from WMO will be added to the weather-info site.

ECEAMJ032020

ECMWF

NASAAJM0320

NASA USA

190320cfs

CFS2 USA

190320nmme

NMME USA

SUMMER JUNE TO AUGUST 2020

ECEJJA032020

ECMWF

NASAJJA0320

NASA

190320CFS2JJA

CFS2

190320NMMEJJA

NMME

C: Comparing the output of seasonal models for December 2019,  January and February 2020, based on November data.

Observed Temperature. Well above average for the season and each month.

Forecast temperature:  Most models forecast the above normal temperatures but perhaps not the detail.

Observed Rainfall. Above average all areas thanks to an exceptionally wet February. January was the relatively less wet month.

Precipitation Forecast: Above average rainfall was forecast but the much wetter February was not well forecast with some models suggesting a less wet month.

Observed Pressure. Below average but near or slightly above in far South.

Pressure Forecast was correct.

For graphics and details see Verification review 2019 Dec 2020 Jan Feb from Nov data.

2. Forecast. SW England.

Spring 2020  (April May) 

Some models suggesting near normal temperatures for April and perhaops early May and then above normal but models seem not to be able to reliably predict colder months. The near average signal may imply a colder than average April.

Precipitation is uncertain but probably a drier than average period in April and possibly early May before a return to near normal rainfall amounts.

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 2020  (June July August)  limited data

Near or perhaps only slightly above average temperatures are expected for the summer season with June suggested as the month least likely to have above normal values. Very mixed rainfall patterns but an overall indication for a drier than average summer, although August could be wetter than average.

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 2020 (September October November) limited data.

Overall above average temperature and rainfall is suggested for the season but  September could be drier than average.

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 2020 (2020 December 2021 January February) very limited data.

Another milder than average winter in indicated with near average rainfall resulting in below average snowfall (snow chiefly over the moors).

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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