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

Published 19 September 2017.

1. Influences.

Changes in sea temperature.

anomnight.9.18.2017

The Atlantic to the W of the UK changed to a positive anomaly for a time but the colder area to the SW has expanded probably due to recent strong winds in this area. Overall, as can be seen in the graphic above, much of the Atlantic remains above normal for the time of year. In the Pacific conditions are moving towards La Nina as illustrated by the NMME forecast graphic below.

Statistically (according to IRI) La Nina conditions might imply near normal rainfall over much of UK in the winter period (Dec to Feb) but with the NW being wetter and the S possibly drier. The La Nina may not be strong enough for this effect to be noticeable.

SSTNINO

Left: NMME forecast for sea temperature in the Nino 3.4 region. Right: Tropical north Atlantic forecast sea temperature UKMO

AO

The North Atlantic Oscillation index (shown by a 500hPa index in the image above) turned positive in June and more or less remained in a positive phase until the third week in July before reverting to a negative phase which remains the dominant phase in the latest analysis. See Met Office NAO information.

Analysis and Numerical (model) output.

A: Rainfall during August was near or perhaps slightly above normal across much of SW England, in terms of measured rain, and together with the higher totals in June and July this has made up for the deficit from previous months.  The number of “dry” days was near or a little above normal for August but for the summer period as a whole was close to the average despite well above normal rain totals. This adds to the evidence for rain events to be “heavier” when they occur rather than there being more “wet” days.

Recent reservoir water levels have recovered and are now well above normal for the summer months. River flows and groundwater in the region should be available from the Hydrological Summary (Web says it has been published but pdf file was missing as of 19 Sept2017) –  August 2017 summary PDF

Sunshine was below normal for the summer despite temperatures being near or slightly above normal – largely due to warmer June offsetting near normal or cooler values in July and August.

B: Upper Troposphere:

Average monthly 200hPa heights are forecast to be above normal right through until February in the CFS 2 data although increased zonal flow is also indicated later in the Winter period which is somewhat later than earlier forecast output.

z2001

CFS2 200hPa height and anomaly Oct Nov Dec 2017

z200DJF

CFS2 200 hPa mean and anomaly Dec 2017 Jan and Feb 2018

C: Lower Troposphere:

NMME data continues to indicate strong positive temperature anomalies into Autumn and through Winter, although October and November values are nearer normal in SW England possibly due to the cooler than average sea temperatures just west of the UK. There remains strong agreement between most available models that temperatures will be above average through winter. Note rainfall anomaly is indicated in the form of an index for above (oranges) or below (blues) based on daily rain rates average over a month.

NMME1

NMME Temperature anomaly and rainfall index October and November 2017

NMMEDJF

NMME Temperature anomaly forecast and rainfall index. Dec 2017 to Feb 2018

D: Comment: 2016 Seasonal forecast rainfall data available in August and September were generally poor and the 2017 output for Summer 2017 were also quite poor.  Temperatures were generally better forecast although for the summer months there seems to be a warm bias in many models. Verification summary October to December last year.

2. Forecast.

Remainder of Autumn  2017  (October and November) :

Temperatures are likely to be near or perhaps slightly above normal perhaps with November seeing slightly higher positive anomalies than October.

Rainfall probably near normal, perhaps a little below normal in October and hence slightly above normal in November although month to month detail lacks consistency  between model output .

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) :

A mild or very mild winter continues to be indicated by almost all model output. This does not rule out some colder spells but suggest they will be short lived. Individual station plots indicate brief cold snaps in late December and in the first half of January although this detail is unlikely to be reliable.

Probably windier than a typical winter with near normal rainfall in the S and E of the Region but probably above normal in the N and W of the Region. Snow risk reduced compared to average, mainly in late December or first hale of January 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.

Spring (2018 March April and May)  limted data:

Temperature probably above normal for the season with rainfall near or a little below normal although some agreement that April could be wetter than average.

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

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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