Review and Experimental Long Range Forecast for SW England. December 2018.

Published 17 December 2018

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1. Influences.

Changes in sea temperature.

sstanom131218

The sea temperature in the North Sea remains above normal and temperatures in the N and E Atlantic (west of the UK) slow slightly less of a cool anomaly compared to last month. The sea temperature remains above normal south of 50 deg north and west of 20 deg West across to Florida but is cooler near the equator.

tna_anom_20181201

UKMO December 2018 Tropical North Atlantic SST forecast

The tropical North Atlantic SST has been below the forecast value from last month but is forecast to return to near average. In the Pacific the El Nino is well established (see graphic below) and is forecast to remain through to Autumn 2019.

171218elnino

The dominant IRI statistical predictor for UK rainfall based on the Pacific sea temperature anomalies for the winter season is shown below. Probabilities shown are for “near normal” precipitation, which is the category which has higher probs than either below or above normal.

elninonormal

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been swapping between a positive and negative and is forecast to continue to oscillate over the next month but with no strong signal for a negative phase. See below.

171218nao

For NAO background information see the Met Office NAO information.

Recent Climatology

In the SW of England data to December 16th shows temperatures above average by over two degrees, above normal rain and well below average sunshine. November also saw well above average temperatures although earlier months were nearer average. Rainfall in November was above average but the longer term rain total remains lower than normal (see below).

171218clim

Recent rainfall has boosted soil moisture and the forecast (GFS) is for more rain to come in December hence increases in soil moisture.

171218soilm

Rover flows in November reflect the reflect the wetter November and December although rivers in the east of SW England were still lower reflecting the longer term rainfall deficit.

161218riverflow

Ground water has improved to near normal see details in the November Hydrological Summary which can be viewed here – November 2018 summary PDF 

The reservoir levels in the SW of England (9th December) show 69% storage which is a big increase since last month due to the wetter November and first half of December.

161218res

The experimental product from the Global Flood Awareness System (Version 2) based on early December ECMWF seasonal forecast for December to  March. To date this system has not be a useful guide. The current EC seasonal forecast does not suggest well above average rain hence no or very low flood signal.

161218glofas

Atmosphere predictions.

A:  Stratosphere and Upper Troposphere

The stratospheric polar vortex was well establish with temperatures in line with the normal variations but in the last week or so marked warming has begun leading to a forecast re-positioning of the polar vortex and a forecast stratospheric wind reversal over the USA an d Pacific. It is not yet clear if this reversal/weakening of the stratospheric wind flow will extend to the UK area but it at least seems probable. THIS MAY HAVE STRONG IMPACT on the forecast for the remainder of the winter but as yet models only suggest N America surface temperature cools markedly in January compared to earlier forecasts (to well below normal) and that N/NE Europe and Russia becomes colder than normal in February.

151218ecstrat

ECMWF 50hPa forecast data 15th December 1200UTC

Note not all seasonal models cover events in the stratosphere but UKMO CFS2 and ECMWF certainly have layers into the stratosphere. A sudden stratospheric warming is more often than not (but not always) followed by a change of type for the UK weather to a more blocked pattern with possibly colder easterly types.

CFS2 200hPa 8th December 2018 data for the January to March 2019 period continues to suggest above normal heights across the S of the UK and  imply enhanced Atlantic jet flow in January suggesting disturbed weather.

091218cfs2z200JFM

CFS2 Jan to March 2019 mean 200hPa height and anomaly forecast.

091218cfs2z200MAM

CFS2 March to May 2019 mean 200hPa height and anomaly forecast.

For the Spring period there is a hint of below normal 200hPa heights in March over NE Uk may imply more cyclonic N or NW flows then above normal heights return.

B: Lower Troposphere:

Graphics for January to March 2010 –  comparison of ECMWF NMME and CFS2(E3 version). Top row are temperature anomaly and lower row rain rate anomaly. Red/oranges are above “model hindcast” normal and blue below.

ECJFM122018

ECMWF December data monthly forecast temp and rain anomaly JFM 2019

 

E3JFM1218

CFS2 December data monthly forecast temp and rain anomaly JFM 2019 

NMMEJFM1218

CFS2 December data monthly forecast temp and rain anomaly JFM 2019 

In most of the output there are indications for above normal temperature and rain in the SW of UK for the three month period though with differing ideas of which month could be drier.

For the March to May period comparison graphics are shown below:

ECMAM122018

ECMWF December 2018 data for March to May 2019

E3MAM1218

CFS2 (E3) December 2018 data for March to May 2019

NMMEMAM1218

NMME December 2018 data for March to May 2019

Near or slightly above normal temperatures are indicated and some agreement that April could be drier than normal.

C: Recent results:

Typically models over forecast the temperature positive anomaly and have poor matches to rainfall distribution over the UK. A look at how “good” seasonal forecasts have been can be seen at the review page.

2. Forecast.

Remainder of Winter (January and February 2019).

161218 – Caution marked stratospheric warning occurring/forecast to occur next two weeks which might impact the remainder of winter forecast and change to colder easterly types.

Main consensus is for near or above average temperatures for January but nearer normal in February and perhaps even slightly colder than average.

January is likely to be wetter than average with average snow risk. February probably nearer normal rainfall but with an increased snow risk especially for the moors.

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall;  January 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 2019  (March April May) based on less data.

Possibly near normal or a colder than average March then recovering to milder than average temperatures for the season, with May probably the month with the strongest signal for above normal temperature.

A wetter than average season is likely with increased snow risk for March, mainly over the hills. April could be the drier of the months with May 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.

Summer 2019  (June July August) based on very limted data.

Another warmer than average summer with slightly above normal rain although the number of wet days may be below average and there is some indication that June could be drier.

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.

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

ECMWF seasonal monthly data from weather.us

Stratospheric Diagnostics from Free University Berlin

Climate data from The Met Office UK and NCEP USA.

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