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

Published 18 April 2017.

1. Potential influences.

Changes in sea temperature.

anomnight.4.17.2017

The Sea Temperature Anomaly (above) shows a little cooling in the western parts of the North Atlantic and an expansion of the warm area in the East Pacific. The Mediterranean and areas around Spain look to have increased warm anomalies.

El Nino predictions suggest a slight strengthening as shown by the NMME and CFS2 forecast plots below, although only a minority suggest a strong El Nino this summer.

nino04

Nino 3.4 Forecast. Left NMME 08Apr17 and Right CFS2 18Apr17.

The influence of an El Nino has on rainfall in the SW of the UK according the statistical output produced by IRI 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:

The polar vortex has been completely warmed out as shown by the ECMWF 30hPa plot.

ecmwf30a12

B: Hydrological Summary – see March 2017 summary PDF

Due to higher rainfall in March river flows across SW England were near normal and in the north somewhat above normal. October 2016 to March 2017 rainfall was below normal and the groundwater levels in the SE of Devon and into Dorset were below normal. (Parts of this area relies on water extraction from underground sources). Reservoir levels in the SW Water area stand at about 87% of capacity which is about 10% below the level prior to  the summer 1995 drought levels. April to date has been a drier than normal month.

C: Troposphere:

Forecast data from CFS2 and NMME show above normal 200 hPa heights through the Summer and into Autumn suggesting reduced westerly jet winds compared to a typical year. Pressure may be higher than average during the May to October period.

cMJJA0417

May to August CFS2 200hPa monthly mean (top) NMME anomaly (middle) CFS2 anomaly (lower)

cSON0417

Sept to November 200hPa

 

D: Surface outlook.

There is good support from international seasonal model output (April 2017 data) for temperatures to be above normal as illustrated by the NMME temperature anomaly plots (shown below). BCC China hints at the next colder than normal month may be December although this is not supported by other models. There is no significant indication of a colder than normal summer but note that the models have not been good at picking out a colder month in a sequence of mild months.

MJJA

May to August NMME temperature mean anomaly (top) and rainfall rate indicator (below)

SON

September to November NMME temperature anomaly and rainfall indicator.

Rainfall has been less reliably forecast and forecasts are not consistent between models or between new runs of the same model. The developing El Nino statistically leans towards to near normal rainfall over at least parts of the SW through summer although the El Nino may not become strong enough to be sure of this relationship holding true.

NMME signals are quite weak for SW England suggesting a wetter June and possibly August although May could well start drier and cooler than normal. There is some support for November being the next “wet” month,

Verification Recent results summary for 2016.

Rough and ready assessment of temperature and rainfall from various models  three month seasonal forecasts.  Note how forecasts issued in November to April had better results than other months. The left hand column is the total out of a potential max score of 36.

Temp2016

Temperature. Most models gave favourably results over 50% of the time.

Precip2016

Rainfall. Fewer models got favourable results over 50% of the time. 

There are hints that multi model ensembles may detract from a single models better performance.

2. Forecast.

Remainder of Spring (2017 May) :

Temperature probably near normal in May after a colder than normal start, becoming milder than average later. Chilly nights may be compensated for by warm daytime sunshine.

Mixed rainfall signal but probably near normal although parts of South and East Devon, Somerset and Dorset may be drier than normal.

Climate: 1981-2010 average period. May mean temperature 11 or 12°C but a few degrees cooler over the moors.  Spring average rainfall 200 to 300mm lowest values in E Devon and over parts of Somerset,  May rainfall typically 60 to 80mm but as low as 40mm E of Moors and in Somerset and parts of Dorset and more than 100mm over the moors.

Summer 2017  (June July August):

Consistent indication for temperatures to be warmer than 1981 to 2010 average although some western coastal areas may be nearer normal. July may be further above average then August. Increased chance of well above normal temperatures (anomaly more than 1 deg C) during the period. There are no indications for a cooler than normal summer.

Rainfall, probably drier then average in terms of number of dry days and possibly also rain total. Uncertain as to which month might have the highest number of wet days but some  hints that June and August could be a little wetter than July but not by much.  Indication of lower pressure than normal over France and the South of the UK may imply increased risk of thunder showers in August leading to highly variable rainfall totals.

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.

Autumn  2017  (September, October and November) :

Warmer than normal especially in September with temperatures mostly above normal throughout the season.

September and October probably near or below normal rainfall but followed by a wet November.

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.

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

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s