Review and Experimental Long Range Forecast for SW England. January 2019.

Published 18 January 2019

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1. Influences.

Changes in sea temperature.

170119ssta

The sea temperature around the UK and across much of the North Atlantic has seen a change to above normal values, although the Met Office diagnostic for the tropical Atlantic (shown below) was slightly below normal at the start of January. This follows a mild start to January for the UK and much of the North and West of Europe (but not the South and East).

tna_anom_20190101

Met Office Tropical North Atlantic analysis and forecast

 

The fairly modest El Nino in the Pacific is now forecast to be slightly less strong than previous forecasts and may not impact on the UK weather for the remainder of Winter and into Spring.

nino0119

ENSO Nino 3.4 area forecast CFS2 and NMME

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been slightly positive (warmer wetter phase for UK) since early January and there is only a weak signal for a negative phase in the NOAA CPC forecast for two weeks from 17th Jan.

180119nao

For NAO background information see the Met Office NAO information.

Recent Climatology

In the SW of England January data to the 18th shows temperatures above average 0.5C in the east to over 1C in the west. Rainfall and sunshine have been very low. Anomalies for Europe in week 2 are shown below.

180118612clim

Over the last six months or so temperatures have been above the long term average

 

180119climrecentt

and rainfall near or mostly only a little below average thanks to a wet December in the south.

180119climrecent

Rover flows and ground water levels in SW England during December reflect the reflect the wetter November and December but some northern and eastern parts of the UK continue to have reduced river and ground water levels..

180119riverground

Full details can be found in the  December 2018 Hydrological summary PDF 

The reservoir levels in the SW of England (13th January) show 78% storage which is below the normal winter level.

reslevel180118

The experimental product from the Global Flood Awareness System (Version 2) based on early January ECMWF seasonal forecast for February to April 2019. To date this system has not be a useful guide. The current EC seasonal forecast does suggest well above average rain in the southern half of UK for Feb and March but not April. Only a minority of the flood solutions suggest above normal river flow for London, slightly higher numbers for the western end of the Thames hence river flows in some SW parts may not be correct.

180119glofas

Atmosphere predictions.

A:  1: Stratosphere 

A sudden stratospheric warming (SSW) took place during December and JMA issued a STRATALERT on the 21st of December, the warming became MAJOR and has continued.

180118stratalert

Note not all seasonal models cover events in the stratosphere but UKMO CFS2 and ECMWF certainly have layers into the stratosphere. A sudden stratospheric warming is more often than not (but not always) followed by a change of type for the UK weather to a more blocked pattern with possibly colder easterly types.

A recent study by Karphechko et al. (2017) suggests that around 43% of SSW do not propagate downward to influence the troposphere. Simon Lee (PhD student studying the stratosphere and S2S forecasting, University of Reading) wrote on 27th Dec 2018:

Major stratospheric sudden warming events (SSWs) attract widespread attention because they are now known to have significant impacts on the tropospheric circulation (e.g. Baldwin and Dunkerton 2001, hereafter BD01). Anomalies in the stratospheric circulation (often expressed as the Northern Annual Mode (NAM) index, or polar cap geopotential height anomalies) propagate downwards through the stratosphere into the troposphere, rather like “dripping paint” (such as BD01 Fig. 2). A major SSW is associated with the development of a negative NAM in the stratosphere; the “typical” response is the development of a negative NAM (or the associated NAO/AO) in the troposphere ~10-14 days after the central date of the SSW (when the 10 hPa 60N zonal-mean zonal wind becomes easterly) which can persist for several months.

However, not all SSWs were created equal – and some SSWs do not strongly couple to the tropospheric circulation. A recent study by Karphechko et al. (2017) classified major SSWs as “downward propagating” (dSSW) or otherwise (nSSW) based on the 1000 hPa NAM index following the event, and found 43% were nSSW – i.e., not followed by a strong and persistently negative surface NAM. This is not a small fraction of SSWs, and the atmospheric evolution following the two types was found to be significantly different.

Our perception of SSWs in recent years has been highly influenced by a relatively unusual clustering of vortex-split, downward-propagating events (Jan 2009, Feb 2010, Jan 2013 and Feb 2018) which all had similar tropospheric impacts (all 4 of those events were followed by an outbreak of snow/cold in the UK, for example). The most recent nSSW occurred in Feb 2008. Thus, the announcement of a major SSW – particularly on social media – has become synonymous with a specific weather pattern.” 

Full text by Simon Lee December 2018

ECMWF data for 10 hPa and 150hPa suggest that full coupling has not yet happened. The latest forecast at 10 hPa shows the polar stratospheric vortex forming again,

180119strat

GFS 10 hPa zonal wind had shown a reversal but recent forecast suggest a return to climatology. Graphic below H Attard, Albany NY

u_65n_10hpa

CFS2 climate model has been forecasting a return of the 10 hPa polar vortex in February for some time. A recent 10hPa forecast for Feb Mar and Apr is show below.

cfs2ssw10

A 2: Upper Troposphere

z200fmam0119

CFS2 200hPa 8th January 2019 data for the Feb to May 2019. The below normal heights in February might imply a more cyclonic even easterly pattern for a time. But the above normal heights suggest less unsettled types for the S of the UK after February but with the chance of stronger than normal Atlantic jet flow suggesting disturbed weather especially in the North of UK.

B: Lower Troposphere:

B1: February to March seasonal graphic  –  temperature anomaly. Top row Met Office, lower row multi model ensemble (not ECMWF). Run from 1st of January.

130119t2mcombined

February to March seasonal graphic  –  precipitation anomaly. Top row Met Office, lower row multi model ensemble (not ECMWF)

130119ppncombined

Combined temperature and precipitation anomaly forecasts Feb to May seasonal forecast graphics: Red above and below below normal.

B2: NMME data 8th Jan 2019

nmmefmam0119

B3: ECMWF 14th January ecfmam0119

B4: GFS2 E3 version mean of 10 days output (00 06 12 and 18UTC) each with 4 runs of the model.

First version on the 6th of Jan

e3fmam0119

Second version on the 12th of Jan

e3fmam0119jan12th

We can perhaps get an idea that seasonal models run later in January have picked up a change to colder types, eg: the later version of the GFS and to some extent the Met Office output.

Another feature in CFS and UKMO is for the south the be see above normal precipitation as well as being colder, leading to an increased snow risk for southern half of UK. Below is CFS2 for February with data up to 16th of January.

160119cfs2

The main problem seems to be after a colder start to February how long into the month is this likely to last, 2 or perhaps 3 weeks. Data up to 18th suggests slightly less cold in week 2 and more widely in the week from the 15th February.

B5: For the summer models agree on normal or above normal temperatures but less clear on rainfall. The NMME graphic below shows July wetter in the South but little agreement in this detail.

nmmejja0119

C: Recent results:

A few models did quite well with the October to December period but typically models over forecast the temperature positive anomaly and have poor matches to rainfall distribution over the UK. A look at how “good” seasonal forecasts have been can be seen at the review page.

2. Forecast. SW England.

Remainder of Winter (February 2019).

A cold start to February is likely but probably returning to near normal by around mid month. Rainfall is likely to be above average across SW England and there is an increased risk of snow especially over the hills/moor but some snow cannot be ruled out elsewhere especially at the start of the month.

February Climate: 1981-2010 Temperature; average temperature values for lowland areas 5 or 6°C but over 6°C in West Cornwall. Rainfall;  February. 1981-2010 average 80-100mm mm lowlands but 60-80mm in areas to E of Dartmoor and 40-80mm East of Exmoor. Over the Moors 100-250mm. .

Spring 2019  (March April May) 

Possibly near normal temperatures in March then trending to milder than average temperatures for the season. May probably the month with the strongest signal for above normal temperature.

A wetter than average season is likely mainly due to rainfall in the first half of the season. The end of the season may well see nearer normal rainfall but there is little agreement in month to month rainfall detail with some models showing May wetter.

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 2019  (June July August) based on limited data.

Slightly warmer than average summer with near or slightly above normal rain although  there is some indication that June could be drier.

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 2019  (September October November) early indications only. 

Milder than normal. Possibly starting warmer in September and then trending near normal later in the season. Wetter than average season but monthly data trending nearer normal later.

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 can be seen at  here.

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

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS (http://www.globalfloods.eu/) of the European Commission Joint Research Centre and the European Centre for Medium-Range Weather Forecasts.
Reference: Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F.: GloFAS – global ensemble streamflow forecasting and flood early warning, Hydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.

‘Copernicus Products’  as listed in the C3S or CAMS Service Product Specification or any other items available through an ECMWF Copernicus  http://climate.copernicus.eu

ECMWF seasonal monthly data from weather.us

Stratospheric Diagnostics from Free University Berlin

Climate data from The Met Office UK and NCEP USA.

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