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

Published 18 December  2019.

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1. Influences.

Changes in sea temperature.

161219SSTA

North Atlantic and North Pacific sea temperature anomalies continue to show above average values although the North Atlantic roughly between 40 and 60 degrees north shows some well mixed areas due to Atlantic storms with cooling in eastern parts to the west of Europe.

tna_anom_20191201

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

In the Pacific neutral or weak El-Nino conditions are present and forecasts suggest conditions are likely to remain similar over winter then trend cooler from about late Spring 2020 – see CFSv2 and NMME multi model ensemble forecast below.

1219nino

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been positive since early December 2019 – implying a milder, wetter, winter 2019/2020. However ensemble forecasts show a trend to a negative phase in about a weeks time which if maintained could lead to colder weather types for the UK.

1219nao

NAO background information see the Met Office NAO information.

Recent Climatology

The first half of December has seen above average rainfall across SW England, except perhaps in the far West of Cornwall were values have been near or even slightly below average. Temperatures have also been slightly above average, again except for the far West of Cornwall which has been near average.

Temperature and rainfall anomalies for Autumn 2019 are shown below. A more local summary for the Teignmouth and Dawlish area is available here.

1219clim

Europe anomalies for Autumn 2019 are shown below.

map_SON_anomaly_Europe_ea_hydro_201909-201911_v01

 

River flows and groundwater in November reflect the much wetter conditions though it is interesting to see that groundwater in eastern England in November had still not recovered to normal. Details can be found in the November 2019 Hydrological summary PDF 

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The reservoir levels in the SW of England (15th December) show 82% storage, which is above normal for the time of year.

151219reslev

Global Flood Awareness System December forecast through to March 2019 largely reflects the well above average rain in November and early December.

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Atmosphere: Predictions.

A:  1: Stratosphere

The North Polar stratospheric vortex shifted east (relative to 00Deg W), as shown by the ECMWF 50hPa and 10hPa charts for 1200UTC on the 8th December. This led to a reversal of the stratospheric winds over North America and may be contributing to the recent colder types in that region. The vortex has started to edge back towards the pole as shown in the plot for the 17th December 2019, with an associated strengthening of the stratospheric winds and of the vortex.

Further warming is occurring and this may lead to another reshaping and displacement of the vortex over the next few weeks. This suggests there is some risk of a sudden warming leading to a change in European weather types in mid to late January. Current seasonal forecasts do not suggest a change persistent colder types at present.

1812196strat

Plot of 10hPa GFS model zonal wind history and forecast (Source @HannahAttard)

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A: 2: Upper Troposphere

Combined NCAR and CFSv2 200hPa averaged monthly mean height contours and anomalies (below).

Period January to March 2020. Indication of above normal heights throughout with marked ridging in February and hints at stronger than average Atlantic jet in March.

z200JFM1219

Period March to May 2020. Indication of above normal heights throughout but with hints at stronger than average Atlantic jet in March and April.

z200MAM1219

B: Lower Troposphere:

A selection of January to March 2020 solutions using December 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 super ensemble (BoM, CMC, Moscow, DWD, CPTEC, UKMO, Seoul, Washington, Tokyo, Toulouse, ECMWF) Tokyo, Beijing and Pretoria missing but Tokyo and Beijing data is available on their respective web sites.

WMOJFM122019

WMO super ensemble

UKMOJFM122019.png

UKMO

Further examples from WMO are available at the weather-info site.

ECEJFM122019

ECMWF  Jan to March 2020

nmme12JFM

NMME Jan to March 

E3JFM

CFS2 E3 (mean of 10 days model output) for January to March 2020

NASAJFM1219

NASA January to March

Spring 2020

ECEMAM122019

ECMWF for Spring 2020

NASAMAM1219

NASA for Spring 2020

nmme12MAM

NMME for Spring 2020

Summer 2020

NASAJJA1219

NASA for summer 2020

nmme12JJA

NMME Summer 2020

For SW England there remains a consensus for above normal temperatures and for above normal precipitation through the remainder of Winter and into Spring though less consensus for summer rainfall.

C: Comparing how well the models did for the Autumn, September to November 2019.

Poor temperature forecasts. BoM and perhaps Tokyo hinted at cooling trend but not colder than average. Most models failed to get colder signal. Rainfall signal for wetter in south was good but models in general were poor with only a few hinting at the drier Scotland.

For graphics and details see Verification review Autumn 2019 from August forecasts.

2. Forecast. SW England.

Remainder of Winter  (January and February 2020) 

Increased uncertainty due to changes in stratosphere.

The indication for a milder than average winter as a whole remains the main signal. Some hints that February could be nearer normal.

Precipitation for the winter as a whole is likely to be above average with some hints that parts of January or February could be less wet but no consistent indication. Given that the first half of winter has been wetter than average it seems likely that a drier period is on the cards although this could be temporary.

Risk of snow mainly for the moors and temporary in nature.

The signal for windier than usual conditions continues but with pressure near normal or even above normal close to the south of the UK this may be a feature of the weather further north and may be less evident in January.

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 Winter 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) 

Above average temperature for the season but in some models May continues to be indicated as being a less mild month (relative to its average). Precipitation uncertain but probably near or a little above 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 2020  (June July August)  limited data

Slightly above average temperature but possibly nearer normal in August. Rainfall uncertain but indications for near or perhaps slightly above average for the season overall.

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

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