Experimental Long Range Forecast for SW England. July 2017 issue.

Published 17 July 2017.

1. Influences.

Changes in sea temperature.

anomnight.7.13.2017

The sea temperature remains warmer than average near and to the southwest of the UK as well as in the Mediterranean. Much of the North Atlantic remains below average and this colder area has extended towards Ireland.  The colder North Atlantic sea temperatures increases the risk of cooler temperatures this summer with winds from the West or NW but winds from other directions flow over warmer than normal seas.

El Nino is expected to remain in a neutral phase as shown in the CFS2 and NMME ensemble forecast below. A developing  El Nino typically acts to reduce Atlantic Hurricanes hence this effect is not likely for the coming season.

nino3.4

Tropical Atlantic temperatures are above normal as shown by the UKMO graphic below and the North Atlantic Oscillation (shown by a 500hPa index in the image below) turned positive in June and has more or less remained in a positive phase. See Met Office NAO information.

NATL

Analysis and Numerical (model) output.

A: Rainfall during June was above normal across much of SW England but this has not made up for the deficit from previous months.

rainfall

In addition most of the rain in June fell on just two days and reservoir water levels remain below average. River flows and groundwater in the region show mixed results with moorland rivers showing increased flow as can be seen in the Hydrological Summary –  June 2017 summary PDF

res

SW Water Resevoir Levels

B: Upper Troposphere:

There is some agreement between models that pressure in the south of England may be above average through the Autumn. Average monthly 200hPa heights are forecast to be above normal right through until February in the CFS 2 data.

z200SAON0717

CFS2 mean monthly 200hPa heights and anomaly August to November

z200DJF0717

CFS2 mean monthly 200hPa heights and anomaly December to February

C: Lower Troposphere:

NMME data continues to indicate strong positive temperature anomalies through the remainder of summer, into Autumn and through winter. There is good agreement between most available models that temperatures will be above average through winter.

Autumn

August to November NMME monthly temperature anomaly (upper row) and (lower row) rainfall rate indication for below in blue or above in orange 

Copernicus and CFS2 three month means differ in the temperature anomaly and rainfall but this may be due to smaller anomalies during this period.

soncomp

Rainfall forecasts are the least reliable (as shown in the verification section below) but the strongest anomalies for the Autumn are in September and October. For the winter there is a strong signal for an unusually mild and wet season across much of the UK.

winter

NMME temperature and rainfall anomaly forecast for Dec 2017 to Feb 2018

Verification. Recent results June 2017.

NMMEPPN

The image above shows June rainfall anomalies based on forecast data available in March April and May and the associated observed anomalies (right) in CPC and Met Office data. The most recent 3 month verification is available at this link.

2. Forecast.

Remainder of Summer 2017  (August):

Above normal temperatures are most likely though perhaps not averaging out with a large anomaly. There is a chance that Easterly winds may be more frequent than normal, due to higher than normal pressure over the UK. Probably drier than normal, reduced number of wet days if not rain total. Rain total may be modified by shower/thunderstorm risk which can bring locally high rain totals but not for the whole region.

August 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. Average 1981 to 2010 rain 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  2017  (September, October and November) :

There remains a consistent indication for a warmer/milder than normal season. Possibly well above normal in October but perhaps only slightly above normal in other months.

Rainfall probably near normal or a little below normal for the season but the month to month detail is inconsistent and at least one month may be see above normal rain. The trend is for increasing rainfall later in the Autumn.

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 (2017 December, 2018  January and February)  –  limited data:

A mild or very mild winter is likely which does not rule out some colder spells. Individual station plots indicate brief cold snaps in late December and again in January and February. February may be slightly less mild than the other months, compared to average.

Probably windier than typical and rainfall is likely to be above normal for the season. Snow risk reduced compared to average, mainly in late December then little if any snow, except perhaps 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 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 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

 

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