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Summer 2018 update 2

Forecast update


09/05/2018

This is our second look at what the seasonal forecasting models and background signals are suggesting about the weather prospects for summer 2018. The first summer 2018 forecast indicators update issued on 7th April remains online to read. The TWO summer forecast will be issued in late May.

Sunflowers

By by simonwakefield [CC BY 2.0], via Wikimedia Commons

Seasonal model overview

Most of the seasonal forecasting models are not showing large precipitation anomalies for the meteorological summer (June, July and August) taken as a whole but the temperature data is more mixed. It is important to remember that the skill level (accuracy) of seasonal forecast models is low for the UK.

Model Temperature Precipitation
GloSea (UKM) Above average Above average slightly more likely
ECM Below average in the northwest No strong signal, poss slightly below average in the south
Meteo France Below average No strong signal, poss slightly below average in the south
CS3 No anomaly No anomaly
Jamstec Below average Slightly below average in the north west
CFSv2 Close to average Close to average
Table 1: Forecast temperature and precipitation anomalies, June, July and August

 

A number of the models are continuing to suggest the possibility of below average temperatures in at least parts of the UK. The exception is the GloSea which favours above average temperatures. Another point of note is that despite the possibility of cooler conditions several of the models also suggest below average levels of rainfall.

A cooler and drier than average summer in the UK would be quite unusual. Pressure anomaly charts suggest that periods of cooler northwesterly winds may occur more frequently than average during the June, July and August period and that is a possible explanation for the lower temperatures.

Misleading anomalies?

As the first update highlighted it may be the case that the fairly bland anomalies are misleading. They are for the three month period taken as a whole and there is a chance that extreme swings are cancelling each other out. For example hot and dry periods are matched by very cool and wet ones to give a close to average outcome overall.

Some of the model charts are suggesting higher than normal pressure to the north of the UK. That could easily lead to cool and wet periods as low pressure pushes up from the south. However it could also bring hot spells if the high pressure cells extend southwards.

ENSO conditions

The state of the El Niño Southern Oscillation (ENSO) has significant impact on temperature and rainfall patterns around the globe. Nonetheless its link to the UK summer is considered to be weak. Forecasts suggest ENSO neutral conditions through the summer months, therefore it is not expected to favour a particular outcome.



1

Atlantic Hurricane season

According to the Colorado State University Tropical Meteorology Project the 2018 Hurricane Season is expected to be slightly more active than usual. In total 14 named storms (average 12), 7 hurricanes (average 6) and 3 major hurricanes (average 2) are forecast. This could increase the chance of more unsettled weather in the UK towards the end of the meteorological summer.

Summary

1) Seasonal models suggest weak precipitation anomalies.

2) With the exception of GloSea there is some support for average or even below average temperatures.

2) Neutral ENSO conditions do not offer support for any one particular outcome.

3) The latest 2018 Atlantic Hurricane season forecast is for a slightly more active than usual season. This would possibly favour more unsettled weather towards the end of summer.

In conclusion the likelihood of a cooler than average summer is considered higher than it has been during recent years. However the likelihood of major variations is thought to be high and there is a possibility of spells of hot weather more than offsetting cooler and unsettled periods. If that happened the summer could make it into the warm or even very warm category.

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