Experimental Long Range Forecast for SW England. February 2018 issue.

Published 18 February 2018.

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1. Influences.

Changes in sea temperature.

anomnight.2.15.2018

Sea Temperatures have cooled relative to average around the UK and coasts of western Europe probably due to the periods of fairly strong winds from a NW direction. Elsewhere the anomalies are not very different to last month with North Atlantic anomalies still above average. Sea temperatures in the tropical Atlantic area have cooled and are forecast remain slightly below average for the next few months as shown by the UK Met Office plot below.

tropatl0218

The La Nina in the Pacific is well established (as shown by the sea temperature anomaly chart at the top of the page) with a large area of below normal sea temperatures in the eastern Pacific near the equator. The CFS2 forecast for the La Nina suggests it may continue as a week feature through summer and autumn, as shown by the graphic below (right panel). This is supported by UKMO forecast ENSO 3.4 plume but like last month the ECMWF forecast is more like the NMME consensus forecast shown below (left), which warms out the La Nina conditions.

ENSO3.4

IRI published a statistical relationship between La Nina and UK rainfall and this is shown below for March to May. The Summer comparison is not shown due to likely weakening of the La Nina.

laNinaMAMppn

IRI Spring rain probabilities during a La Nina

The plots above suggest that SW England has slightly higher probabilities of Normal to Dry conditions for Spring.

Another statistical relationship which may be playing a part in the early Spring season is the Quasi-Biennial Oscillation (QBO). The QBO is linked to conditions over Western Europe during late autumn and early winter through an influence on the phase of the North Atlantic Oscillation (NAO) at the surface. An easterly phase of the QBO tends to moderately increase the chances of a negative phase of the NAO, which in turn increases the chances of below-average temperatures.

QBO0218

QBO monitoring data (shown above) indicates that the easterly (negative) phase of the QBO has extended towards the 50hPa level and may be extending to lower levels.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been in the positive phase for some time but recent predictions suggest that a negative phase may develop soon.

NAO0218

For more information see Met Office NAO information.

Changes in the upper atmosphere.

The next relationship to consider is the development of sudden stratospheric warming. A major stratospheric warming episode has started and is likely to lead to a reversal of the stratospheric winds over the East Atlantic/Europe.

30hpaEC0218

30hPa ECMWF 16/1200UTC and foecast for 26/1200UTC

What is not yet clear is whether this will lead to the complete warming out of the polar vortex ending the winter mode. Forecast models do suggest that an extended anticyclonic period over parts of Northern Europe now seems more likely with impacts on temperatures across the UK.

The temperature rise at 30hPa over the N Pole is quite dramatic but the plot for 50hPa zonal mean is not outside the extremes of previous events.

Analysis and Numerical (model) output.

A: Recent Climatology.

recentclimate0218

Temperatures across the SW of England have been slightly above normal in the period November 2017 to January 2018 and it was looking as if Spring would be early. Up to mid February however, temperatures have been running at around 0.8C below the long term average.

Rainfall in parts of the region has been above normal but large areas of Devon recorded values closer to the 30 year average in the November 2017 to January 2018 period. The number of days with over 1mm was well above normal.

Recent reservoir levels indicate above normal storage for the time of year and river flows in the region also show above average values.

riverflow

The  full summary is available from the Hydrological Summary  –  January 2018 summary PDF 

An experimental product from the Global Flood Awareness System suggests no enhanced flood risk, compared to the average risk, for the SW of England for the next 4 months. This is based on the early February monthly ECMWF seasonal forecast.

0218glofas

B: Upper Troposphere

CFS2 forecast for 200hPa suggests very slightly above normal heights across the UK but and possibly reduced jet strength in Spring and perhaps hints at increased troughing in summer.

200hPa0218

C: Lower Troposphere:

Comparing monthly data from the NMME (7th Feb), NCEP ensemble 10 day mean (14th Feb) and ECMWF (12th Feb) seasonal forecasts for Spring 2018.

Top row of each set, temperature anomaly. Lower row indication for above or below normal rainfall (reds are above normal in NMME and CFS2 but in EC indicates below normal).

NMMEMAM0218

March April May 2018  NMME

NCEPMAM0218

March April May 2018 NCEP mean

ECMAM0218

There is a marked lack of agreement between the forecasting systems for Spring or Summer. Graphics for the summer months and a text review of other models can be seen at www.weather-info.co.uk/wxsvc/seaslatest.html

D: Recent results/comments:

Seasonal models were slow to pick up a trend to colder temperatures for February and possibly March 2018, despite some at least being able to resolve the atmosphere into the stratosphere. A short assessment for the November 2017 to January 2018 period of seasonal forecasts available in October 2017 can be seen at www.weather-info.co.uk/wxsvc/VerifNDJ2017.html

2. Forecast.

Spring 2018 (March April and May):

Temperature for the season is likely to be near the long term average due to a milder May despite a colder than average March and near normal values in April. The risk of air frosts continues into April, although probably not very frequent.

Rainfall probably near normal or below normal early in the season but above normal later especially in western areas. Parts of the south and east of the region may well end up below normal for the season. There is a chance of a longer drier period early in March before the weather turns more changeable either later in March or in April. Month to month detail lacks consistency between models and is often unreliable.

Some snowfall may occur early in March, chiefly over the hills and moors. The timing of the end of colder than normal temperatures is uncertain and could be as early as the first week in March limiting the snow risk.

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 2018  (June July August). Limited model data:

The temperature is most likely to be near or only very slightly above normal for the season. August looks to have the best chance of above average temperatures.

Rainfall for the season probably near or a little above the long term average but with a chance that July might be a little drier and August wetter. Summer rainfall is often difficult due to showery nature leading to large variation in totals over the region. There is little agreement between the seasonal models with regards to which months might be wetter or 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  www.weatherservice.co.uk

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

 

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