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

Published 19 April 2018

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1. Influences.

Changes in sea temperature.

anomnight.4.16.2018

Due to the cold March and start of April sea temperatures around the UK are generally below normal, in particular North Sea temperatures show a strong negative anomaly.  This continues to have potential impact on temperatures across the UK during the remainder of April and into May and may also result in increased sea fog or haar. Elsewhere the anomalies are not very different to last month with North Atlantic anomalies marginally lower. 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).

NINO0418

Left: UKMO Tropical North Atlantic Sea Temperature forecast.                                                      Right: CFS2 Tropical Pacific Nino 3.4 area Sea Temperature Forecast

The La Nina in the Pacific although well established  (as shown by the sea temperature anomaly chart at the top of the page) but is showing signs of weakening. The consensus from the models (see graphic below) is for a move to neutral and then weak El Nino conditions later in the year.

figure1

ECMWF and multi model comparison for ENSO conditions (April output)

A consequence is that the IRI relationship between ENSO and UK rainfall is likely to be unreliable and is not reproduced in this document.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been rather variable over recent weeks but is forecast to return to a positive phase after a short negative dip during the second half of April.

nao170418

or more information see Met Office NAO information.

Changes in the upper atmosphere.

The stratospheric polar vortex is now history with a generally slack pressure pattern as shown by the 50hPa ECMWF analysis below.

ecmwf50a12

Recent Climatology.

march2018

The Met Office stated that for March 2018 “The provisional UK mean temperature was 3.8 °C, which is 1.6 °C below the 1981-2010 long-term average, but it was significantly less cold than March 2013.”

March was a very unsettled month with Low Pressure dominating the weather patterns across the United Kingdom. In addition, there were two periods of snow across SW England which is very unusual. Rainfall measurements during snow can be a problem but looking at the water equivalent and adding this to the other rainfall brought rainfall totals in the Dawlish and Teignmouth area, for example, to between 170mm and 250mm for the month. This was roughly three times more rainfall than an average March. There were at least eighteen wet days which was well above normal.

Rainfall was well above average across the UK except in the NW where totals were below normal. Looking at the October 2017 to March 2018 rainfall totals these have not been far from average and if anything a little below normal across northern areas.

River flows for March reflect the wetter month (see below) and reservoir levels are  above normal storage for the time of year. The full Hydrological Summary for February is available from this link  – March 2018 summary PDF 

riverflowsmarch2018

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 next 4 months. This is based on the early April monthly ECMWF seasonal forecast and represents a marked change from last month experimental forecast.

glofas0418

The left hand graph and probability tables for the River Severn illustrate the change in forecast which now has high probs of a flood risk compared to last months forecast.

Atmosphere predictions.

A: Upper Troposphere

CFS2 data (as supplied to NNME data set April 2018)  for 200hPa suggests above normal heights across the UK and possibly reduced jet strength through Summer but continues to hints at increased troughing in summer or at least relatively reduced anomalies.

z200JJA0418

200hPa mean height June July August with anomalies below.

In the Autumn, October looks to have a risk of enhanced jet flow with reduced flow over UK in September and November.

z200SON0418

200hPa mean height September October November with anomalies below.

B: Lower Troposphere:

Model data is not in good agreement but in general shows a warming trend relative to normal through the summer but hints at cooler start to summer across the SW of the Uk.

Not much agreement with regards to rainfall with ECMWF in particular at odds with other models with regards drier August a;though some support from NMME.

NMMEJJA0418

NMME temperature anomaly forecast (top) and rain rate (below) May June July and August

CFS2E3JJA0418

CFS2 (mean of 10 days output) Temperature anomaly forecast (top) and rain rate (below) for June July and August

120418SEASONMtoSECMWF

ECMWF Temperature anomaly forecast (top) and rain rate (below) for May  June July and August

Top row of each set, temperature anomaly (+ oranges or in EC green). Lower row indication for above or below normal rainfall (reds are above normal in NMME and CFS2 but below normal in EC).  (Note NCEP is a part of the NMME multi model ensemble).

Full size graphics and a text review of other models can be seen at www.weather-info.co.uk/wxsvc/seaslatest.html

C: Recent results/comments:

Looking at the verification summary for January to March models generally failed to get the colder types but got some idea of wetter than average but not the month to month variation

2. Forecast.

Remainder of Spring 2018 (May):

Temperature during May probably near normal for the region as a whole but perhaps slightly cooler than average in the west and a little above inland in the east.

Conflicting indications for rainfall but main indication is for near or above normal rainfall the above normal values possibly in the S and W of the region.

 

May climate: 1981-2010 average mean temperature roughly 11 or 12°C but a little warmer over parts of Somerset and nearer 9°C over the Moors. Average 1981 to 2010 rain 80mm to 100mm but 60mm to 80mm in parts of Somerset, the Exe Valley and East Dorset but over 100mm across the Moors.

Summer 2018  (June July August). 

 

Temperature: near normal for the season possibly with cooler than normal start and warmer than normal August.
Rain: Indication for normal or below normal rainfall for the season but monthly data uncertain about the wetter month(s) though possibly August the wettest (compared to average) with the higher rain rates. Note CFS and ECMWF pretty well opposite signals. CFS preferred but most models were poor with 2017 summer rain rate anomalies. 

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. 

Temperature near or slightly above average. Rainfall near normal but may start the season drier than average making up for the rain totals later in the season.

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) minimal data. 

Milder start to winter but trending nearer average values during January and/or February. Wetter than average early in the winter but perhaps a drier February. Near normal snowfall so little if any snow on lower ground but some snow 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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