Another Wintry Blast heading our way

[dru`s haven]

lovely gillykat ,they sound fab hope you will show some pics if we ever get some sunshine for them to grow

[Lumpy]

Tinned and frozen veggies are beginning to be really appealing.......................

[Bal]

I've had my toms in the green house for about 2 weeks now. They seem to be doing OK. I cover them with several layers of fleece at night. Will that be enough if the weather gets worse? They survived this weekends snow, hail and frost.

[Containergardener]

Looks like another frost tonight ...as they say. Ne'er cast a clout til may is oot

[gringo]

What's with all these negative waves?

It's gonna be a mother beautiful summer, no freezing nights, and the bridge WILL be there in the morning!

[Containergardener]

I really hope it is a good one gringo.

[Gillykat]

It WILL be - I've ordered my heatwave!!!!! And coming from someone who absolutely HATES being too hot but is prepared to suffer for the sake of her melons that's saying something!

[Gillykat]

To be honest, I know I won't be very popular for saying this, but I wouldn't mind a bit of rain over summer...

I was going to say rain is ok...as long as it falls during the night....but then I thought ''hold on - that'll bring out all the slugs and snails!''

[Containergardener]

By eck gillys getting her melons oot

[Containergardener]

I don't want anymore rain !
I can make pottery from this clay at the moment, no wonder the Romans rocked up ere .

[Penellype]

Now that we are a little nearer to the forecast cold spell I thought it might be interesting to show what is currently forecast. The screenshot below is the predicted 850mb temperatures (at about 5000 feet) and precipitation forecast for Manchester from midnight last night. The temperatures at the surface will be warmer, it is the trends we are looking at. I've put some notes on the screenshot to help explain it.



There are 20 individual coloured lines with slightly different starting conditions to allow for experimental error in the initial measurements. The white line is the average of all of the runs and the red line is the 30 year average for this time of year. The closer the coloured lines are together the more likely the scenario. As you can see there is very good agreement between all 20 lines that the temperature will nosedive over the weekend. What happens after that, and how far the temperature falls is less certain.

The stronger green line is the GFS operational - this is the most modern update to the model, with the highest resolution (ie there are more measurements closer together than for the other lines). The stronger blue line is the control run - this is the run which was the operational before the most recent update. Interestingtly the first dip well below the -5 line is forecast by most of the runs, but the later dip around 26th April shows the operational run as one of the coldest. This could be an "outlier" and it may well correct upwards as that date gets nearer.

Personally I would believe this forecast upto the start of 23rd April, and expect the details of anything beyond that to change to some degree. It is therefore too early to say if any of the rainfall spikes are accurate, and whether they will fall as rain or snow if they are. However as we are already getting night frosts with the temperature near or above the red line, night frosts at the weekend and during next week look a certainty, particularly if skies are clear and winds are light.

Original screenshot from http://www.wetterzentrale.de/pics/MT...hester_ens.png. Charts are also available for London and Aberdeen - simply substitute the other location for Manchester in the URL.

Edit: I should have made clear that many of the forecasts you see on websites use the GFS operational forecast (the thick green line on the screenshot).

[Lumpy]

I really wish I could read graphs - it's very pretty. Ta for the explanation!

[Penellype]

Sorry, its easy to assume that people understand things like graphs. This is actually 2 graphs with 2 different scales on the same screen, and I can see why it could be confusing.

Fairly obviously, the dates are along the bottom of the graph, so the further right you go the further into the future (and the less reliable) the data is.

The temperature scale is on the left side of the graph, running from -30 to +30 centigrade and this refers to the lines in the middle of the graph. This would cover most eventualities for the UK. All of the coloured lines show the temperature rising to about 5C and staying there until Friday, when there is a sudden drop to around -5C. The reason why 850mb temperatures are used instead of sea level ones for instance is that at 850mb you don't get the day and night variation so it gives a more accurate picture about what is actually happening. As a very rough guide, to get the surface level maximum temperature you add about 10-15 to the 850mb temperatures, depending on whether the air is fairly dry (as now) or wetter. The drier the air the higher the amount you add. I don't know how to calculate the expected minimum temperatures though, except that the colder the upper air is the more likely it is to be cold below.

On the right hand side is another scale, and this is mm of precipitation (usually rain). This refers only to the lines at the bottom of the graph. 25mm of rain is nearly an inch, so this would be a huge peak on this scale, running into the temperature lines.

The white line gives you the average of all the coloured lines, so it is going to be somewhere in the middle. Any single coloured line that is a long way from the others is an outlier and can potentially be ignored. For example around 30th April there is a large green peak which is way bigger than any of the other runs at that time. If you look carefully in the temperature area you will see that at the same time there is a sharp rise and then fall in temperature in the same green. This particular run appears to be forecasting warm and then cold fronts causing steep rise and fall in temperature and accompanying rain. This would be typical of a low pressure system as these often contain a warm, muggy segment in between the 2 fronts. The other runs are mostly not seeing this at this time, although one of the orange runs sees something a bit similar slightly earlier. The spread of different solutions at this point illustrates the uncertainties that creep in the further ahead you look.

The way to use these things is to look at the general picture, and assume that anything beyond about 5 days is liable to change which can lead to a very different possible outcome. Use the average temperature (white line) as a guide to whether we are likely to be above or below average for the time of year, and the spread of the coloured lines as a measure of the likely accuracy. Use the precipitation spikes as a general idea of whether it is likely to be wet or dry, and only pay attention to the actual timing in the next 3-5 days.