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

Published 17 January 2018.

1. Influences.

Changes in sea temperature.

SSTanom150118

Sea Temperatures are slightly cooler than average around the UK and coasts of western Europe but elsewhere the anomalies are similar to last month with Atlantic anomalies still above normal. Sea temperatures in the tropical Atlantic area are forecast to remain slightly above normal, as shown in the UK Met Office plot below.

160118tropnatl

The La Nina in the Pacific is well established, with a large area of below normal sea temperatures in the eastern Pacific, south of 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 (left). This is supported by UKMO forecast ENSO plume but ECMWF is similar to the NMME consensus forecast shown below (right), which warms out the La Nina condithions.

nino160118

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

Probability for Dry/Normal/Wet based on a La Nina being active

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

Another statistical relationship which may play a part in the remainder of the Winter and for the 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.”

QBO

QBO monitoring data (shown above) indicates that the easterly (negative) phase of the QBO remains above 50hPa and has had little impact this winter.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been in the positive phase for some time and ensemble predictions suggest that a weak positive phase will continue, maintaining the signal for a  milder/wetter remainder of winter.

160118NAO

For more information see Met Office NAO information.

The next relationship to consider is the potential for the development of a sudden stratospheric warming. Should this occur there can be an impact on the Jet stream causing it to weaken and allowing High pressure to develop over Europe. This could bring colder temperatures further west and possibly into the UK. To date there has been periods of warming over the N Pacific and into northern Canada. The stratospheric polar vortex was displaced eastwards but then redeveloped further west resulting in a drop in North Pole 30hPa temperatures (shown below).

30hpa

Over recent weeks there have been a series of minor warming event across the N Pacific and into Canada with some model runs splitting the polar vortex, which might have been a prelude to a change away from the mobile westerly types currently affecting western Europe/Uk area. More recent ECMWF model runs have moved away from this idea and maintained a strong vortex although again being displaced further east away from the pole.

ecmwf30a12

ECMWF 30hPa T+00 +120 and +240 hours from 16th Jan 2018

Stratospheric temperatures in the North Polar vortex have been low enough for the formation of Polar Stratospheric clouds which are implicated in Ozone destruction. Should the vortex be maintained this could become an item of interest this Spring.

50hpa

Left Total Ozone. Right 50hPa minimum temperature plot.

 

Analysis and Numerical (model) output.

A: Recent Climatology.

temp

Temperatures across the SW of England have been slightly above normal and for many areas the last four months have been slightly drier, despite a wetter December.  January in SW England so far is running slightly milder than average, wetter in the west but drier in the east (based on limited data).

Recent reservoir levels remain above normal for the time of year and river flows in the region have returned to near average values.

Decriver

The  full summary is available from the Hydrological Summary  –  December 2017 summary PDF 

An experimental product from the Global Flood Awareness System suggests the flood risk areas for the next 4 months based on January ECMWF seasonal forecast.Glofass

For the SW of England the river flood risk does not appear to be elevated compared to normal with the main risk areas across Eire/N Ireland and N England/S Scotland.

B: Upper Troposphere:

z200

CFS2 forecast for 200hPa suggest above normal heights across the UK but still with a signal for enhanced jet across the Atlantic for a time in Spring.

C: Lower Troposphere:

Comparing the CFS2 mean of 10 days forecasts and the NMME output for Spring 2018. Top row of each pair, temperature anomaly. Lower row indication for above or below normal rainfall (reds are above).

mamcomp

NMME forecast for Summer 2018.jjacomp

The forecasts tend to be a little warm and the rainfall output is often poor with month to month detail. The below average rainfall in the S of UK does fit to some extent with the IRI probabilities when experiencing a La Nina in the Pacific.

D: Subjective model scores for temperature and rainfall for the seasonal forecasts issued December 2016 to September 2017 valid for the following three months. (e.g. September forecast valid for Oct Nov Dec). Left hand column is total of all months where there is a potential max score of 30. Scores for rainfall show only a few good forecasts.

verif

2. Forecast.

Remainder of Winter ( 2018 February) :

There are strong indications for a milder than average February in the SW of England.  This does not rule out some colder spells, probably with North or NW winds rather than easterly, but suggests that the cold spells will be short lived.

Mobile westerly patterns look like continuing to dominate the weather leading to above normal  rainfall especially in Cornwall and in areas to the west of the Moors. Elsewhere near normal or slightly above normal rain totals are most likely. The snow risk remains reduced compared to average with little if any snow for lower ground but some snow for the moors.

February Climate: 1981-2010 Temperature; average temperature values for lowland areas 5 or 6°C but over 6°C in West Cornwall. Rainfall;  February. 1981-2010 average 80-100mm mm lowlands but 60-80mm in areas to E of Dartmoor and 40-80mm East of Exmoor. Over the Moors 100-250mm. 

Spring 2018 (March April and May):

Temperature for the season is likely to be near or a little above the long term average. There are, however, hints of a colder spell which is more likely in April than the other months. Risk of air frost continues into April though not very frequent.

Rainfall probably near normal but in the south and east of the region rain totals may well be below normal. Although a mostly unsettled theme is likely there is a chance of a longer drier period sometime later in March or in April. Month to month detail lacks consistency between models and is often unreliable.

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 average temperature is most likely to be near or only slightly above normal for the season. During August there is the best chance of above average temperatures.

Rainfall for the season probably below the long term average (in terms of number of wet days, if not actual rain totals) but with a chance that August could see above normal rainfall. Summer rainfall is often difficult due to showery nature leading to large variation in totals over the region.

 

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 and brief verification of previous Long Range Forecast summaries can be seen at   http://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

Stratospheric Diagnostics from Free University Berlin

 

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