Experimental Long Range Forecast for SW England August 2016 issue.

Published 18 August 2016.

Potential influences that might affect the forecast:

SEA TEMPERATURE.

anomnight.8.15.2016

NOAA/NESDIS night time sea temperature anomaly 8th and 15th August 2016

wctan1

8 to 13 August Temperature anomaly

Sea temperatures by early August had risen with the cold anomaly in the North Atlantic much reduced but by the 15th cooler than normal sea temperature had returned to waters around Scotland.

The colder temperatures in the Mediterranean seems strange but may be linked to colder air that pushed south over Europe bringing cooler than normal temperatures to many areas. Despite sunny days night time cooling is probably responsible.

In the top graphic the fairly weak La Nina can be seen in the Pacific. The La Nina is likely to last into the start of the winter but there is less agreement after this.

ps2png-atls12-95e2cf679cd58ee9b4db4dd119a05a8d-XZGtuQ

Some differences between ECMWF, UKMO and NCEP are shown for the NINO3.4 area.figure1-2

Overall a return to neutral conditions during the winds of 2016/17 seems to be the consensus in the early August CPC/IRI probability forecast for Nino 3.4.

 

 

A consequence of  this consensus is that any statistical relationship between La Nino and UK rainfall becomes even more unreliable during the winter. The graphics below show this relationship for the Autumn and Winter.

SONnina

Sept to Nov. IRI probability for drier/wetter/normal precipitation during  La Nina

The suggestion of a drier than normal Autumn in parts of England and Wales is reflected in some  of the model output bu the La Nina is fairly week so this may not be reliable.

DJFnina

Dec to Feb. IRI probability for drier/wetter/normal precipitation during La Nina

For the winter period La Nina conditions may be even weaker or trending to neutral conditions with uncertain impact. Hence the relationships shown by the graphic are even less reliable than normal.

MODEL OUTPUT.

A text summary of several seasonal model outputs can be seen in the UK and Eire summary at www.weatherservice.co.uk

200hPa heights are predicted by CFS2 and NMME to be above normal but hints at slightly enhanced zonal pattern in September and November. This is a change from last month when October rather than November had stronger zonality.

SON200Z0816

Sep to Nov. CFS2 forecast mean 200hPa heights with NMME and CFS2 anomalies.

DJF200Z0816

Dec to Feb. CFS2 200hPa heights with CFS2 and NMME anomalies

Some indication of above normal heights but also increased Jet flow across the Atlantic during Jan and possibly also Feb though this is less noticeable in the NMME output.

Looking at the NMME output for temperature and rainfall, shown below, it can be seen that above normal temperatures are predicted although the anomalies are small until later in the winter. Precipitation is less clear cut although the NW of UK and parts of Eire  look more likely to see above normal precipitation this is less clear elsewhere. For SW England 4 out of 6 months show up as below normal.

 

NMMESON0816

Sep to Nov NMME temperature anomaly (top) and precipitation indication (lower). Key blue below/red above ensemble normal.

NMMEDJF0816

Dec to Feb NMME temperature anomaly (top) and precipitation indication (lower)

There is some support from other models for below normal precipitation in the S of the UK for periods in the Autumn and perhaps early December but also hints at some much wetter months. Previous indications of a wet start to winter and a less wet Feb seems to have been push back to see a wetter than normal period starting later in December and lasting into February. Month to month detail often proves unreliable with regards precipitation.

Forecast:

 

Autumn  (September, October and November) 2016:

Fairly good agreement for the temperature during this season to be above normal. September and October near or a little above normal although November is expected to be especially mild. There is a chance of some very mild spells during this season.

Rainfall probably near normal or a little below normal for the season as a whole especially in eastern parts of the region. September possibly near or slightly above normal, October possibly drier than normal but with a wetter than normal November at least in Cornwall, N and W Devon but perhaps not in SE and E parts of the Region.

 (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  (2016 December, 2017  January and February) :

 

A mild winter is most likely (does not rule out some frost), although early December may see temperatures closer to normal. In January temperatures probably well above normal on average with February near normal or above normal.

Near normal or above normal rainfall across the SW is most likely for the season as a whole, especially in the N and W but with a chance that SE and E parts of Devon and Dorset might be a little drier than normal for periods in December and possibly February too. During parts of January and / or February, however, above normal precipitation totals may occur generally across the Region. Below normal snowfall is more likely than above normal which means just a little snow accumulation over the moors but not much elsewhere.

(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)

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

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/

Stratospheric images prepared by Freie Universität Berlin using European Centre for Medium Range Weather Forecasting data supplied by Deutscher Wetterdienst

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info

 

 

 

 

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