Review and Experimental Long Range Forecast for SW England. August 2020.

Published 21 August 2020.

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1. Influences.

Changes in sea temperature

ssta08

Atlantic equatorial sea temperatures look warmer than climatology as area area around the UK, in the N Sea, Norwegian Sea and Baltic. The North Atlantic west of Britain has seen a reduction the the cooler than average areas and the Artic Circle shows  very strong positive anomalies. Higher than average sea temperatures in the Gulf of Mexico may aid the development of current potential storms 13L and 14L.

Colder sea temperatures in the East Pacific just south of the equator show the La Nina has strengthened.

tna_anom_20200801

The tropical north Atlantic is forecast to remain slightly warmer than average through to at least January 2021 (See Met Office graphic above).

In the Atlantic, 2020 so far ( Wikipedia link for details. ) there have been 14 tropical depressions 11 tropical storms and 2 hurricanes resulting in 45 deaths and nearly six billion US dollars worth of damage. One tropical storm resulted in no electricity  to homes parts of New Jersey, USA, for a week due to trees falling over power lines.

21f3aca3b79ecb29985205d49291aed2

Time line above and tracks below

1280px-2020_Atlantic_hurricane_season_summary_map

In the Pacific sea temperatures continue to trend towards cooler / La Nina conditions which may now last until the mid Spring 2021 before returning to neutral conditions.

nino08

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) turned positive at the start of August and heralded the drier and warmer weather. The index turned negative in mid August a forerunner of more unsettled and cooler weather. The trend is for this to revert back to positive later in the month but for how long?

nao08

Recent Climatology –  SW England

August to the 20th: Temperatures have been running over two degree above the long term average across the SW thanks to the very hot spell in the first half of the month when temperatures reached into the mid thirties Celsius.

The month had been drier than average until heavy rain fell on the 14th and more especially the 19th. These two days accounted for a months average rainfall. The recent heavy rain with thunderstorms led to flash flooding in, for example, Exeter and Barnstaple. In addition unseasonably windy conditions brought high seas to coastal areas in recent days (19-21 August) in association with storm Ellen and its parent Atlantic depression.

The week 9 to 15th gives a reasonable indication of the extent of the hot weather.au9to1508

The earlier part of the summer (June and July) showed near average temperatures after a warmer June and cooler July, but mostly above average rain (except the SE of England)  despite a drier July in the south.

climsum08

River flows in July 2020 were near normal across the SW and groundwater levels were above normal in the eastern parts of SW England. Details can be found in the July 2020 Hydrological summary PDF 

river08

The reservoir levels in the SW of England (16th August) were close to average.

res08

Global Flood Awareness System. August forecast has near normal flood risk for southern parts of UK (Orange below normal Blue above).

glo08

Atmosphere: Predictions.

A:  1: Stratosphere

Stratosphere remains in summer mode as shown by the 50hPa chart for 20th August.

ecmwf50a12

A: 2: Upper Troposphere

CFSv2 200hPa contours for September to November 2020  shown in top row along with the anomalies from CFS (middle row) and the NMME anomalies (lower row). Forecast of above normal heights more especially in the western Atlantic and southern Europe implies more changeable weather for the UK especially the in the north.

z200SON0820

B: Lower Troposphere:

A selection of  September to November solutions using August 2020 data are shown below. Three month season and individual months, top row temperature anomaly and lower row precipitation anomaly. (WMO graphics not available at time of writing).

NMMEson08

NMME

cfsson08

CFS2

NASASON0820

NASA

ECESON082020

ECMWF

For the Winter period December 2020 to February 2021.

NMMEdjf08

NMME winter

cfsdjf08

CFSv2 winter

NASADJF0820

NASA winter

ECEDJF082020

ECMWF winter

If the WMO graphics become available they will be added to the weather-info site. 

C: Comparing the output of seasonal models for May June July 2020, based on April  2020 data.

For graphics and details see Verification review 2020 May Jun Jul

Temperature: Reasonable indication for above average values although signal for slightly below average July was missed except in NCEP and DWD models.
Rainfall: Rain forecast fairly poor. UKMO and DWD got some idea of the sequence at least in S of UK.

Scoring is for whole season and will attempt to state good, fair, poor or no signal (similar probs for above normal, normal and below normal).

1. Russia:                        Temp fair.    PPN fair.
2. USA – CFS2 :                Temp good . PPN fair .
3. UKMO Contingency: Temp good . PPN fair .
4. UKMO :                        Temp good . PPN good .  PMSL fair
5. USA – IRI :                    Temp fair.    PPN fair.
6. KMA APCC :                 Temp fair .  PPN mainly no signal .
7. JMA :                             Temp good. PPN fair . PMSL good
8. NMME :                        Temp good . PPN fair .
9. WMO :                          Temp good . PPN poor .
10. BCC :                           Temp good. PPN fair .
11. NASA :                        Temp good . PPN fair .
12. Brazil:                        Temp fair.    PPN fair .
13. CanSips :                    Temp good. PPN poor .
14. 15. Copernicus          Temp good. PPN fair . PMSL good
16. EC                                Temp good. PPN fair . PMSL good
17. MF                                Temp fair.  PPN fair. PMSL fair.
18. JAMSTEC: .
19: ECMWF (monthly)   Temp: fair.   PPN fair.
20: CMCC                          Temp fair.    PPN fair. PMSL fair.
21: DWD                           Temp good.  PPN fair. PMSL good.

 

2. Forecast. SW England.

Autumn 2020 (September October November) 

Temperature are likely to be above the 30 year average (1981-2010) for the season and may be above average for each month although September might be nearer average.

Rainfall above average rainfall is likely for the season although October in some models is drier than average in places but perhaps not the SW of England.

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 (2020 December 2021 January February).

A milder than average Winter in indicated as well as each month being milder than average. Caution models tend to be poor at picking colder types.

Mixed rainfall indications but Winter probably having above average rainfall and below average snowfall. Some hints that December and/or February could be less wet.

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  (2021 March April May) limited data.

Milder than average temperatures seem likely with near normal rainfall.

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 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

International seasonal monthly data from WMO

Stratospheric Diagnostics from Japan Met Agency and ECMWF via  Free University Berlin

Climate data from The Met Office UK and NCEP USA

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