Experimental Long Range Forecast for SW England. February 2017.

Published 17 February 2017.

1. Potential influences.

Changes in sea temperature.

anomnight-2-16-2017

Apart from some relative cooling in Biscay and around Spain and Portugal, the sea temperature anomaly in the North Atlantic is similar to those in mid January. The La Nina in the mid Pacific has decayed with the current neutral conditions likely to slowly return to a weak El Nino state during the 2017 summer. This transition can been seen in the CFS2 and NNME output shown below which is perhaps a little quicker and stronger indication than last months forecast.

Should an El Nino develop more quickly the influence on rainfall in the SW of the UK (according the statistical output IRI statistical output) is such that there is an increased probability of near normal summer rainfall for the N and W of the Region but less impact in the S and E.

Analysis and Numerical (model) output.

A: Stratosphere:

A major stratospheric warming event took place in early February with a splitting of the Polar Vortex and a temporary reversal of the stratospheric wind in the UK area. Temperature at the North Pole rose sharply at 30hPa but has since returned to normal, as shown in the graphic below.

pole30_nh

Tokyo issued at major stratospheric warming start message on the 3rd and an end message on the 10th.

startstopstrat

Subsequently the western part of the vortex has declined with a new strong vortex developing to the north of Russia during the second half of February.

contstrat

It is not clear whether this will aid the development of high pressure to the west of the UK and more cyclonic patterns further east hence an increased risk of chilly NW types for the UK?

B: Troposphere:

200hPa CFS2 and NMME monthly mean anomalies hint at slightly stronger jet flow towards the S of UK in March but with above normal heights forecast to develop later in Spring.

z200mam0217

March April May 2017 CFS2 200hPa mean heights (top) anomaly (mid) and NMME anomaly (bottom row) Data 6FEB2017.

z200jja0217

June July August CFS2 mean heights anomaly and NMME anomaly

For the summer months positive anomalies continue but with differences between CFS2 and he other models making up the NMME group of ensembles.

C: Surface NMME output:

mam0217

NMME Mar Apr May temperature anomaly (top) Rainfall indication for above/below normal.

There is good support from the various seasonal models for above average temperature or at worst normal values. Rainfall patterns are less clear but other output does suggest a wetter March in the west.

jja0217

NMME Jun Jul Aug temperature and rainfall indication.

There is a consistent signal for above average temperature (up to plus 1 C anomaly) across the UK  the rainfall is less consistent but at least a hint of drier overall in places.

Recent results.

UK: Verification for the November 2016 to January period showed that the temperature for the season was a reasonably guide for the UK but month to month detail was unreliable. Precipitation detail is often poor mainly due to dry spells being under forecast.

SW England:

2. Forecast.

 

Spring (2017 March April and May) :

Average temperature probably above normal for the season. March less clear could be nearer normal, with chilly NW or N winds for periods although if rainfall correct then milder nights may tip the balance to above normal. April slightly above normal with May possibly the mildest relative to average values.

Rainfall probably near normal for the season, perhaps above normal in the more exposed western parts of the Southwest but below normal in parts of South and East Devon, parts of Somerset and into Dorset. March could be the wetter month of the three.

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.

Summer 2017  (June July August):

Temperatures probably warmer than 1981 to 2010 average although some western coastal areas may be nearer normal. Rainfall, probably drier then average in terms of number of dry days and possibly also rain total. Drier types more especially for July and August perhaps after a wetter June. This is a change from last months forecast and may be unreliable.

Summer 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. July often warmer than August. Average 1981 to 2010 rain 300mm over the moors, typically 200 to 250mm in many coastal and eastern areas. June often drier than July and July drier than August.

Early indication for Autumn  (September, October and November) 2017 – limited data:

 

Possibly a drier and warmer start to Autumn then near normal or wetter with near normal temperatures.

(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)

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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