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

Published 18 July 2018

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1. Influences.

Changes in sea temperature.

170718

Sea temperature around the UK remains well above  normal. In contrast a large area in the northern North Atlantic remains well below normal and according to the Met Office June issue contingency forecast: “this pattern moderately increases the probability of high pressure over Northern Europe. In summer, high pressure is associated with above-average temperatures”.

The tropical Atlantic is showing signs of becoming warmer than average and the East Pacific is trending towards El Nino conditions, from the current neutral state.

170718sst

Left: UKMO Tropical North Atlantic forecast. Right CFS2 ENSO 3.4 forecast.

ENSOEC0718

ECMWF ENSO 3.4  SST forecast

GFS and ECMWF more or less in line suggesting the max anomaly is likely to be in November/December 2018 followed by some cooling (relative to average).

The IRI statistical predictor for UK rainfall based on the Pacific Sea Temperature anomalies is unlikely to be of use until the Autumn when there is a forecast EL Nino state. The graphic below shows the probs for normal precipitation during an El Nino this winter.

elninonormal

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been mostly positive for some time. The move to a negative state was shorted lived and the latest forecast suggests another short dip before returning to a positive suggesting weak or no Atlantic mobility in the next few weeks.

170718nao

For background information see the Met Office NAO information.

Recent Climatology

July so far in SW England has seen under half the 1981-2010 average rainfall, more sunshine than normal and much higher temperatures. Temperature anomalies running at over 3 deg C above the long term average.

For England and Wales an interesting scatter plot for JUNE 2018 was produced in the Hydrological Summary showing 1976 was warmer and 1921 and 1925 was drier.

scat0718

Anomalies for the UK as a whole, temperature shown are Spring and June with rainfall June and April to June 2018:

TandR0718

Following the dry weather water content in the top of the soil has reduced and is likely to further reduce in most areas. See GFS analysis and forecast below.

soil40718

Rover flows in June reflect the recent drier period. The full Hydrological Summary for  June is available from this link  – June2018 summary PDF 

river0718

Interestingly ground water levels for eastern parts of SW England were average or above average during June – see the full report for details.

The reservoir levels in the SW of England (15th July) show 75% storage but are following a path typical of a dry year and reducing sharply.

The 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 four months starting 1st July 2018 and based on ECMWF seasonal forecast.

glofas0718

Atmosphere predictions.

A: Upper Troposphere

CFS2 data (as supplied to NNME data set July 2018) for September to November at 200hPa continues to suggest above normal heights across the UK and possibly reduced jet strength until November.

z200SON0718

200hpa mean height and anomaly Sep to Nov 2018

z200DJF0718

200hPa mean height and anomaly December 2018 to February 2019

Enhanced jet possible in December but much less likely in February.

B: Lower Troposphere:

August to November temperature anomaly and precipitation indication for below/above model normal. Caution ECMWF uses opposite colour scheme for above normal rain.

ASONNMME0718

NMME top temperature bottom precipitation (orange above normal)

ASONE30718

CFS2 (E3 version 10 day average) Top temperature anomaly bottom precipitation anomaly (orange above normal)

ASONEC0718

ECMWF top temperature anomaly bottom precipitation anomaly (BLUE ABOVE normal)

Above normal temperatures suggested. Hints at a risk of increased rain in August but more especially in November.

December 2018 to February 2019.

 

DJFNNME0718

NMME DEC 2018 to FEB 2019 top temperature anomaly bottom above/below normal precipitation (blue below normal)

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

C: Recent results/comments:

UK Summary issued March for April May June – “Summary – 170318 – Temperature for the season starting normal or below normal ending above normal hence overall could be close to normal for the season. Sea temperature near N Sea may stay below normal helping hold back temperatures for parts of NE or E of England. Rainfall likely to be below average in the S and SW of UK and Eire but above in the north. Some indication that May could be the wetter month for the south and April the drier month for some western areas but in generally not much agreement between models for month to month detail.” 
Verdict on summary: Temperature Fair did get warmer. Rainfall below average signal for S good but below average was more widespread. April detail not good though some hints at locally wetter in S England in May.

Poor indication for the very dry June or the record warmth. One of the problem with ensemble means?

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

2. Forecast.

Remainder of Summer 2018  (August). 

Above normal temperatures are likely to continue through August but perhaps not with such a large anomaly as July.

August rainfall is not clear cut with mixed indications. Probably much nearer normal than of late and possibly wetter than average, perhaps due to thunder showers. It is uncertain whether wetter could be due to a greater number of “wet” days or higher rainfall rates typical of a thundery type. The latter has been a feature recent rainfall patterns and can occur with less “wet” days in a month.

Pressure is likely to be above normal across the UK, especially the north but less so in the south.

August climate: 1981 to 2010 average daily mean temperature 16 or 17°C in many areas, a little cooler over the N coastal areas of Devon and Cornwall  to 16 or 17°C  and a few degrees cooler over the moor. Locally over 17C in parts of Somerset.  Average rain in August typically 60 to 80mm, but 40 to 60mm in places especially East of the moors and in Somerset. Locally over 100mm across the moors.

Autumn 2018  (September October November) early indications. 

Milder than normal for the season as a whole and possibly each month individually but trending to smaller anomalies/nearer normal temperatures for November.

Rainfall near normal for the season but with a drier than average September and parts of October followed by a much wetter period into November.

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) limited data from 4 models. 

Temperature probably near or above normal in December, much milder than average in January but possibly near or below normal in February – making he season as a whole near average or slightly above.

Wetter than average winter chiefly due to wetter December and January and  despite below normal precipitation in February. Below average snowfall, though slightly increased risk during February compared to normal.

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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