Attempt at a Regional Forecast for SW England based on long range experimental forecasts – 16 Sept 2015

This is an attempt at a Regional Forecast for SW England based on long range experimental forecasts for Autumn 2015 / Winter / Spring 2016. Prepared using data up to 16 September 2015 only to test if any usable forecast can be made. This forecast should not be used for any other purpose.

Main ongoing uncertainties are the El Nino in the Pacific and the colder than normal sea temperatures in the North Atlantic as shown on the NOAA sea temp anomaly chart below.

SST140915

Fig 1 NOAA night time satellite derived Sea Surface Temperature Anomalies. 14 Sept 2015.

Statistical data from the International Research Institute (IRI) suggests precipitation anomalies during  Autumn (Fig 2) and Winter (Fig 3). Broadly favouring normal or above normal precipitation in the South and drier in North in Autumn and normal or above normal in many areas during winter.

Fig 2  IRI precipitation probabilities for drier, wetter or normal, during El Nino conditions AUTUMN

Fig 2 IRI precipitation probabilities for drier, wetter or normal, during El Nino conditions AUTUMN

Fig 3  IRI precipitation probabilities for drier, wetter or normal, during El Nino conditions WINTER

Fig 3 IRI precipitation probabilities for drier, wetter or normal, during El Nino conditions WINTER

Forecast  for SW England based on a review of Experimental Long Range Forecasts issued by National Meteorological Services. September 2015 data shows fairly good agreement between models with regards precipitation but less agreement with about temperatures.

Remainder of Autumn 2015 (Oct Nov) :

Overall a very changeable period is likely possibly windy at times. This is typical of Autumn and is likely to lead to near or milder than normal temperature (1981-2010 Average 11 or 12°C for lowland and coastal areas in October and 8 or 9°C in November). Main chance of above normal values is in eastern parts and below normal in the west (related to cooler than normal Atlantic Sea Temperature).

Forecast rainfall is predicted, by most available seasonal forecast models, to be above normal. (1981-2010 Autumn average 300-400 mm lowlands but Teignmouth, Dawlish, Exeter areas 200-300 mm). This forecast fits fairly well the El Nino statistical values for wetter than normal shown in the central panel of Fig 2 above.

Winter 2015/16:

Perhaps less frequent low pressures crossing Atlantic towards UK in latter part of period compared to the unsettled start of winter. Temperature near or above normal especially in December and parts of January, though still with a risk of slightly lower than average values in western areas. Temperatures by February near normal rather than above. (Typical winter average 1981-2010 values for lowland areas 7°C in the West and 5°C in the East).

Precipitation probably above normal in many areas to start winter but nearer normal later in winter, hence overall above normal. (Dec and Jan typically wetter than February in any case. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor). Snow risk near normal (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). Average snow lying days over moors January, February below normal December.

Spring 2016:

Hints at a colder than normal start to spring recovering to above normal into May. Rainfall/snowfall near normal becoming wetter than normal in May. (Average 1981 to 2010 rain 200 to 300 mm. Temp 9 or 10°C, roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C).

Caution experimental long range forecasts do not have a good success rate. The data used for the above forecast summary and brief verifications of previous summaries can be seen at http://www.weatherservice.co.uk

Fig 1 NOAA Remote Sens. 2014, 6(11), 11579-11606; doi:10.3390/rs61111579  http://www.mdpi.com/2072-4292/6/11/11579

Fig 2 and 3 ref: Mason, S.J. and L. Goddard, 2001: Probabilistic precipitation anomalies associated with ENSO. Bull. Amer. Meteor. Soc., 82, 619-638. doi: http://dx.doi.org/10.1175/1520-0477(2001)082<0619:PPAAWE?2.3.CO;2

See http://www.metoffice.gov.uk/public/weather/climate/ for UK climate details.

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