Review and Experimental Long Range Forecast for SW England. July 2019.

Published 19 July 2019.

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1. Influences.

Changes in sea temperature.

150719ssta

Due to the recent settled and at times very warm weather, sea temperatures around the UK (and in the Mediterranean) have strongly recovered being above normal in the latest data. Sea temperatures around Greenland and Alaska are notably above average.

Tropical Atlantic has continued to warm and is forecast to remain slightly warmer than average – see Met Office graphic below:

tna_anom_20190701

In the Pacific weak El-Nino conditions have become weak and may now become neutral or even slightly La Nina this Autumn – see CFSv2 and NMME multi model ensemble forecast below. This may impact on the number of Atlantic tropical storms which are reduced during El Nino years. The latest forecast from Tropical Storm Risk (University College London) call for near normal numbers of Atlantic storms but stress the uncertainty with regards El Nino changes.

nino340719

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been in a negative phase since later April 2019 which for the summer period translates to a colder/wetter phase for the UK and June reflected the wetter type in many areas but with temperatures near normal or slightly above. In July this has also been the case in Scotland but not further south. Recent NAO forecasts suggest a (possibly temporary) change to a positive state with drier and warmer types more likely.

NAO0719

NAO background information see the Met Office NAO information.

Recent Climatology

Despite a colder first half of June the warmer period at the end of the month led to near normal mean temperatures for most parts of the SW. Up until the 18th of July it has been dry with only about 1% of typical July rainfall. Data from the main SYNOP station show above average sunshine but a mean temperature of 17 Celsius which is near average, despite some very warm days. Probably parts of the SW have had warmer than average values once all the data is available early in August.

temp

Met Office mean temperature anomalies Autumn 2018, Winter, Spring and June 2019

PPN0719

Met Office rainfall anomalies Autumn 2018, Winter, Spring and June 2019

Because of the June rainfall 12 month totals to end of June are close to average although but temperature is above average as shown by the Copernicus graphic below.

map_12month_anomaly_Europe_ea_hydro_201906_v01

riverflows

River flows in June reflect the wet month although annualised rates are below average more details including ground water can be found  in the  June 2019 Hydrological summary PDF 

Soil moisture content was still below normal over parts of the SW at the end of June and the dry first half of July is not likely to have helped.

sm0719

The reservoir levels in the SW of England (14th July) show 73% storage which is slightly close to a normal July level.

resevoir

Looking ahead the experimental product from the Global Flood Awareness System (Version 2) suggests reduced flood risk for the SW over the next few months although local flash flooding would not be included in this.

GLOFAS0719

Atmosphere: Predictions.

A:  1: Stratosphere

Stratosphere is in summer mode as shown by the ECMWF 50hPa chart for 1200UTC 18th July 2019.

50hpa0719

A: 2: Upper Troposphere

CFSv2 200hPa monthly mean forecast height (contours) and anomaly (lower row) for the period August to November 2019 based on 7th July show above normal heights throughout and it is not until February that some below normal heights are indicated over the north of Scotland.

z200ASON0719

CFS2 200hPa mean and anomaly August to November

z200DJF0719

CFS2 200hPa mean and anomaly December 2019 to February 2020

B: Lower Troposphere:

A selection of August to October 2019 solutions in low resolution from WMO using July data. Three month season and individual months. Top row temperature anomaly and mid row precipitation anomaly.

White areas are area where the probabilities of below/normal/above are similar hence there is no overall signal for this period from the WMO super ensemble which includes DWD, CMC, BoM, Moscow, Seoul, (CPTEC missing data), UKMO, Washington, Pretoria, Tokyo, ECMWF, Beijing and Toulouse seasonal models.

WMOASO072019

WMO super ensemble August to October

The super ensemble has an indication for above normal temperature but no agreement in the precipitation detail.

Some examples from individual centres are shown below. Full list can be seen at the seasonal forecast page.

UKMOASO072019

UKMO ensemble August to October

WASHASO072019

Washington NCEP WMO ensemble August to October

NMMEASON

NMME multi model ensemble August to November

NMMEDJF

NMME multi model ensemble December 2019 to February 2020

There are a few models that suggest colder and wetter periods in the August to October period but not the majority.

A summary for SW England from all the models for August to October looks roughly:
Temperature: 70% above normal, 20% normal 0% below normal and 10% no signal.
Rainfall:  15% above normal, 30% normal, 20% below normal and 35% no signal.

No signal = similar probs for above/normal/below

Additional model graphics can be seen at the seasonal forecast summary

C: Recent results for (April to June 2019):

Some good pointers to temperature anomaly and cooler signal in some models but the overall above average forecast for the season was correct. Some good indications from some models for a wetter June in England and in May for Scotland but idea of drier end to season was not correct, although the very end of June was dry and warm.

Most precipitation forecasts were poor.

Comparison graphics can be seen at the verification page for April May June 2019

2. Forecast. SW England.

Remainder of Summer 2019  (August) 

August is likely to see near or above normal temperature. There are slightly more solutions in favour of a drier than average month than those for a wetter one but thunderstorm rain rates could lead to some very wet days in a few places.

Higher temperatures may lead to shorter periods of heavier rain but fewer wet days which has been a pattern in recent years.

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 2019  (September October November) limited data. 

Overall the season is likely to see above normal temperatures, possibly nearer normal in September and much milder later in Autumn.

Rainfall is likely to be below normal for the season, especially in the S and E of the region perhaps nearer normal elsewhere. Little agreement as to which month could be wetter but some hints that it could be October.

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 2019  (December January February) limited data. 

The indication for a milder than average winter remains the main signal. Rainfall is likely to be above average although there are hints that December and / or February may be less wet in parts of the S and E of the region. Below average snowfall more likely than above average.

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  here.

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

International seasonal monthly data from WMO

Stratospheric Diagnostics from Japan Met Agency and Free University Berlin

Climate data from The Met Office UK and NCEP USA