Review and Experimental Long Range Forecast for SW England. May 2018 issue.

Published 18 May 2018

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1. Influences.

Changes in sea temperature.

anomnight.5.17.2018

Sea temperatures in the North and East Atlantic, to the west of the UK, remain generally below normal although further to the SW there looks to be a large area with above normal values. The colder than normal sea temperatures may impact UK temperatures when winds are from a Westerly point. Conversely due to a warmer than normal central Europe and more frequent winds from an Easterly point North Sea temperatures have recovered remarkably and show a strong positive anomaly. Elsewhere the anomalies are not very different to last month.

Sea temperatures in the tropical Atlantic are slightly below average but expected to recover over the next few months as shown by the UK Met Office plot below (left).

ENSO0518

Left: UKMO Tropical North Atlantic forecast. Right: Pacific ENSO 3.4 area forecast.

The La Nina in the Pacific continues to weaken  with models suggesting and move to neutral conditions then a warming to an El Nino state in the Autumn of 2018.

EC010518ENSO

Differences between ECMWF and CFS2 illustrate the uncertainty in a move to El Nino. Any statistical predictor for UK rainfall based on the Pacific Sea Temperature anomalies is unlikely to be of use.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been positive for some time but is predicted to move to a more variable perhaps negative state in the second half of May.

NAO0518

For background information see the Met Office NAO information.

Changes in the upper atmosphere.

The stratosphere remain in summer mode as shown by the 50hPa ECMWF analysis for 17th May 1200UTC,shown below:ecmwf50a12

Recent Climatology.

climat180518

Following a colder than normal March temperature recovered to above normal in April. Despite the N/NW of the UK remaining drier than normal elsewhere rainfall totals were above normal in much the same areas as in March. Away from the NW of the UK, the six month rainfall period changed from near normal/drier to the end of March to above normal. May 2018 though has been drier than normal so far except perhaps in the NW of the UK.

River flows for April reflect the wetter month (see below) and reservoir levels in the SW of England (13th May) show above normal storage for the time of year. The full Hydrological Summary for April is available from this link  – April 2018 summary PDF 

180518riverflow

Looking ahead, the experimental product from the Global Flood Awareness System suggests enhanced flood risk, compared to the average risk, for the SW of England for the four months starting 1st May 2048. This is based on the early May monthly ECMWF seasonal forecast and seems heavily biased to rainfall in March and April because EC output looks for drier than normal conditions for SW England.

180518glofas

Atmosphere predictions.

A: Upper Troposphere

CFS2 data (as supplied to NNME data set May 2018) for 200hPa continues to suggest above normal heights across the UK and possibly reduced jet strength through Summer.

z200JJA0518

CFS2 200hPa heights (top) and anomalies (below) June to August. May 2018 data.

z200SON0518

CFS2 200 hPa mean heights and anomalies September to November

During the Autumn despite above normal heights there are signs of increased Jet flow along 50 deg N in October and perhaps more troughs / blocking near the UK in November.

B: Lower Troposphere:

Summer:  June July August 2018. Caution rain rate colour scheme different / reversed in ECMWF output compared to NMME and CFS2.

Anomalies for temperature are less than 1 degree C in all output. For SW England there is some agreement for above normal values. Rain rate also indicate below normal rainfall for SW England although the agreement elsewhere in the UK is not so clear

100518NMME

NMME muli model ensemble output 7 May data. Top Temperature Bottom rain rate anomaly. 

100518NCEPE3

CFS2 10 day mean output 13 May data. Top Temperature Bottom rain rate anomaly (red wetter)

ECJJA052018

ECMWF Top Temperature Bottom Rain rate anomaly (note rain has blue for wetter)

Full size graphics including graphics for Autumn and a text review of additional seasonal models can be viewed at www.weather-info.co.uk/wxsvc/seaslatest.html

C: Recent results/comments:

The following images show a comparison between observed and forecast for one month ahead and illustrates the lack of reliability in any one system.

1: RAIN RATE ANOMALY

Forecast rain rate anomaly in January for February 2018 were mostly poor. UKMO Contingency issued after Stratospheric warming had occured captured the drier theme

February2018PPNverif

Data January 2018 for February 2018

Better Forecast in February for March rainfall but not UKMO Contingency

March2018PPNverif

Data February for March 2018 Precipitation

Data in March for April rather variable but NCEP had correct idea.

April2018PPNverif

Data March for April 2018 Precipitation

2: TEMPERATURE ANOMALY:

March2018verif

Temperature anomaly February for March

April2018verif

Temperature anomaly for March for April 2018

A brief review of each seasonal forecast can be seen at the verification index.

2. Forecast.

 

Summer 2018  (June July August). 

Most likely a warmer than average summer although June may be closer to normal but with warmer than average months to follow. Cooler values most likely in the west of the region with westerly winds due to cooler than normal sea temperature early in the season.

Drier than normal but some uncertainty for August with some indications that August could be wetter – uncertain whether this is wetter due to more wet days or higher overall rainfall (which can occur with less wet days in a month). Summer rain often uncertain due to increased convective elements which increase the variability of rain totals across regions.

Pressure is likely to be above normal across the UK which implies a better than average summer.

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 2018  (September October November) early indications. 

Milder than normal for the season as a whole but with larger anomalies later in the season. Rainfall starting near or below normal in September and possibly October but November wetter than average which may make the season slightly wetter 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 2018/9  (December January and February) very limted data. 

Slightly milder than an average winter possibly colder than average in January.Milder start to winter but trending nearer average values during January and/or February.  Precipitation very uncertain slightly more solutions suggest above normal. Near normal snowfall (hence mostly 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  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

Climate data from The Met Office UK and NCEP USA.

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