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Platinum Jubilee weekend

Weather prospects


23/05/2022

The Queen's Platinum Jubilee

The Queen's Platinum Jubilee bank holiday period stretches from June 2nd to 5th. The meteorological summer begins on June 1st, but the weather can be very mixed at this time of the year. In fact, the terms "European Monsoon" and "return of the westerlies" are often used through the month. They describe the trend for westerly winds to become more frequent again during June and July.

Platinum Jubilee

When winds are coming from a westerly direction in the summer the weather in the UK tends to be quite changeable.

Computer model guidance

A range of computer models predicting the medium term weather prospects for the UK are available. However, when looking more than about 5 days ahead it is very much about using them to identify trends and probabilities rather than specifics. So what do they suggest at the time of publication?

General picture

Most computer models are suggesting that high pressure will become centred to the north and west of the UK through the last week of May. This should lead to a good deal of dry periods, but with winds coming from a northerly quadrant temperatures are likely to be on the low side, particularly in the north. The warmest conditions could be expected in sheltered parts of the south. 

The forecast chart below is generated using data from the global ECM model. It shows pressure patterns on June 2nd. 

ECM 00z pressure chart, init 23rd May 2022

By the start of the Jubilee period there are indications that the risk of showery rain will be increasing as pressure begins to fall over the UK. 

Rain risk assessed

Deterministic computer models which provide a single outcome for a given time are very useful when looking at short ranges. However, at the time of publication the Jubilee period is 11 days away and at this range ensemble models which provide a range of solutions are preferred. 

The computer model is run many times with the starting conditions varied slightly to help account for uncertainty in the present state of the atmosphere. If most of the runs show a similar solution at a given time the chance of it being correct is higher than if they go off in different directions.

The chart below for London is generated using data from the global ECM ensemble model. Each line represents the rain forecast from one run.  

There is clear signal for mostly dry weather between May 25th and 29th. Beyond that the number of spikes increases, with each one meaning that rain is forecast by an individual run at the given time. The horizontal axis shows the datetime and runs out to June 7th. 

ECMWF ENS 00z, London precipitation, init 23rd May 2022

Plots for locations in other parts of the UK tell a similar story.

So can we expect rain?

The chart above points towards a risk of rain during the Jubilee period, but how great is it? The ECM ensemble model contains 51 runs, so even if a minority of them are forecasting rain the chart can quickly become cluttered. 

To break things down further it is worth looking at the rain data table from a different ensemble model, the GEFS. It shows the percentage of runs in the model which are forecasting it to be dry or wet at a given time. The light grey shows it to be completely dry and the dark grey suggests light rain. The other colours also show heavier rain as can be seen on the key.

What this chart suggests is a moderate risk of rain through the days and a lower one through the nights. It indicates that rain is most likely to be coming from showers which bubble up during the daylight hours and then dissipate in the night. 

GEFS 00z, London rain data table, init 23rd May 2022

Based on the current data the good news is that it doesn't look like a washout. Data tables for other UK locations tell a similar story. The bad news is that a completely dry Jubilee period doesn't appear to be the most probable outcome either. 

Will it be warm?

As discussed, the last days of May are not looking particularly warm due to the likelihood of winds coming from the north. The forecast maximum temperature GEFS data table for London gives some pointers as to how things could develop.

The majority of runs are in the 16C to 20C orange category. Later on the number falling into the 21C to 25C darker orange category increases but they stay in the minority. There are also a few cooler runs as well as one or two which go into the very warm or hot red category.  

London GEFS 00z data table showing maximum temperatures
 

Data tables for locations further north show lower temperatures on the whole. That is often the case, although there is a caveat. Sometimes when high pressure builds to the north areas of low pressure push up from continental Europe or the southwest and bring unsettled conditions to southern Britain. If that happens the coolest and wettest weather could well be in the south.

Summary

At this stage it is too early to be confident about the details of the weather during the Platinum Jubilee period. The signal is for high pressure to be centred to the north and west of the UK. That leads to the possibility of showery rain and rather cool conditions for the time of the year.

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