Penellype's Weather Channel

[Penellype]

SPRING.

Every single long range forecast model I have seen is going for a milder than average spring. Many, but not all, are also going for wetter than average, with low pressure the dominant weather type.

WARNING - this does NOT mean that we will have continuous warm weather or that the risk of frost has gone - it hasn't.

Currently we are somewhat stuck under a complex low pressure system which is not particularly deep, so we are seeing bands of rain and showers with some drier intervals but often fairly light winds. The flow is generally from the atlantic so it is not particularly cold, but where skies clear at night it is still cold enough for a touch of frost. This situation looks likely to persist over the weekend before the low gives way to a more normal ridge-trough-ridge-trough altlantic scenario next week. This should lead to somewhat less continuous wet weather particularly in the south, although there will still be rain at times.

What happens next is the big question, and this is where the shorter term models are not agreeing with the long range ones. Beyond the reliable timeframe, and therefore subject to change, many of the model runs are seeing a ridge of high pressure building in the atlantic, towards Greenland. This would turn the winds into the north, making it much colder around the weekend of 11th/12th March. There are several possible developments if this happens. The ridge could stay towards Greenland, blocking the atlantic and leaving us in several days of cold northerly winds which could be accompanied by snow. The ridge could collapse over us and allow the atlantic back to give a return to the ridge-trough-ridge-trough scenario favoured by the long range models. The ridge could drift eastwards over us and give us a nice taste of warm, dry and spring like weather. Or it could collapse east over Scandinavia putting us in a cold and possibly extremely persistent easterly wind. The various runs of the models are divided between these options and it is impossible to say which will happen. However there is a good chance of a significant cold spell in the middle of March, as shown by today's ensemble chart for York:

[Penellype]

If they say it often enough - perhaps one day they will be right! You'd think they would know better by now

It sells papers - they don't really care if it is right or not! But yes, sooner or later we will have a cold and snowy winter.

[Penellype]

Time for an update.

Things are warming up this week, although there will still be quite a bit of rain around. This is the ensemble chart for York:



You can quite clearly see the rise in upper air temperature today and the large rainfall spike associated with a front moving in from the west. Notice the uncertainty creeping in as early as tomorrow as far as the temperature goes - this is quite unusual.

The remainder of the week is likely to be mild, with little risk of frost away from high ground and northern Scotland. Things cool down again at the weekend and a wider risk of ground frost is then possible where skies clear.

The rise in temperature is caused by warm air advection, with low pressure in the atlantic drawing warm air over us and towards Scandinavia. By Friday we have this:



which has a very mild south westerly flow. Unfortunately it is also a damp one, so there is likely to be a lot of cloud and some drizzle with this, with the best of any brightness in the east. Temperatures of around 15C would be possible in the south in any sunshine.

The uncertainties get worse over the weekend, with a low pressure moving to the north of the country and fronts crossing us at times. When and exactly where is hard to pin down at present.

The difficulty with what happens next is quite nicely illustrated by Monday's chart:



On the face of it this looks great - warm upper air and high pressure. But look at the 1030 isobar. The high building up from the south has joined forces with high pressure over Scandinavia. To the north the flow is coming from the warm south west, but to the south it is coming from the east. It would not take much of an adjustment (moving the high slightly further north and west) to put us into a cold easterly flow, with night frosts and wintery showers possible. Most of the models get very close to this situation without actually getting there next week, but the latest run of the GFS (which does not match the ensemble chart above) has another go later in the month:



This is, of course, well beyond the reliable timeframe, but it does show what could happen.

So, what can we make of this lot? It is clear that the rest of this week is going to be warm and unsettled at times, although there could be some very pleasant sunny spells in amongst. Timing will depend on where you live as the weather fronts bringing the rain move across the country. At the weekend it is likely to become cooler, while staying unsettled. Next week there seems to be a general drying trend (note the reduction in rainfall spikes on the ensemble chart) as high pressure sets in, but the positioning of this will be crucial in determining whether we see some very pleasant warm spring sunshine, or a bitterly cold easterly wind, or something in between.

For a more in depth explanation of the complexities of this situation see Matt Hugo's blog: Model Guidance - Unsettled & Mild But The Devil Is In The Detail - Blog by Matt Hugo - Netweather.tv

[Penellype]

Regarding my comment in the previous post about the uncertainty "as early as tomorrow".

This was yesterday's ensemble from midnight, which was the latest available when I wrote my post:



Note the position of the white average line in the forecast upper air temperatures directly above the large rainfall spike on the left.

This is today's midnight run - look at the difference:



The sharp drop in temperature was forecast by a couple of the members yesterday but most didn't see it at all.

With this sort of uncertainty creeping in at only 24hrs after the model starts, its not surprising that there is no definite agreement about what happens next week!

[burnie]

I don't think we've finished with winter yet, fresh snow on the lower hills round us again

[Penellype]

I agree, there is still plenty of potential for frost and snow. Remember, white Easter is more common than white Christmas! That does not mean I think we will have a white Easter this year, but it shows that there is still wintery potential for another month or so.

[Penellype]

Finally some agreement about what happens next week. Far from being cold and snowy as looked possible a week or so ago, it now looks like being very pleasant indeed.

The ensemble for York shows a good deal of dry weather (except for tomorrow) and warm upper air temperatures which should be reflected at the surface with similar temperatures to at present, quite warm if the sun comes out.



Things look to cool down by next weekend with rainfall spikes coming back, but given the amount of scatter at this point I'd be treating that with caution. The very cold runs for the next couple of weeks have disappeared, but at the end of this run a couple of members are going really quite cold, reminding us that cold weather is still possible at the end of March.

[Penellype]

After a very warm day today, with temperatures reaching 18 degrees in parts of the south, things are about to cool down a bit.

This is today's chart:



You can see that the upper air temperature is warm and with high pressure giving clear skies (away from northern and western Scotland which has been nearer to low pressure and rather cooler, cloudier and wetter in places), warm south westerly winds have lifted the temperature well above average for mid March.

The ensemble chart for York looks like this:



This shows a sudden dip in upper air temperature associated with a cold front that is due to move south across the country into Friday. There is not much rain on this, although it will be wetter in the west. By Friday the yellow colours have gone from much of the country:



This signals a change to cooler, more unsettled and windier weather - note the reappearance of rainfall spikes along the bottom of the ensemble chart. There will be more rain for western and northern areas, with the south always drier, but not completely dry.

The chart shows an almost immediate rise in temperatures again, but this is a more westerly pattern and will not be as warm as it has been this week, with more cloud and rain around. Sunday's chart shows the difference well:



Although the upper air temperature is not that different, the flow is coming from the north west rather than the south west, so it will be cooler, although by no means cold. The uncertainties creeping into the ensemble chart reflect uncertainty about the exact track and timing of low pressure systems passing to the north of Scotland, one of which is the remnants of "storm Stella" which has given blizzards and huge falls of snow in eastern America recently. It will NOT be giving us a similar pasting having weakened considerably in its journey across the Atlantic.

There is good agreement for a drop in temperatures in the middle of next week. This is the chart for Wednesday 22nd:



The jet stream has moved south of us and low pressure is moving down across the country, dragging in some northerly winds in this particular run of the GFS model. Details differ between models and runs, but some colder weather with a northerly or easterly wind for a couple of days seems likely. This would lead to some wintery showers over hills and night frosts where skies clear.

There is as usual considerable uncertainty about what happens next, although the most likely scenario at present is a return to higher pressure, probably drier conditions and pleasantly warm days. Nights could be cold if the sky is clear, and amounts of cloud are uncertain at this distance.

[Penellype]

You may have heard that it is going to get colder this week, and yes, it is. This is today's ensemble chart for York:



You can clearly see the drop in upper air temperatures, and with this dropping below the -5 line and some rainfall spikes showing too, there could be some sleet or snow around especially over northern hills. There is still some uncertainty about how long this lasts, with the thick green line of the operational model one of the coldest runs today.

Translating this into what is happening, the chart for Wednesday shows low pressure sinking southwards, with the blue colours showing the drop in upper air temperature:



The winds are turning into the north, making it feel much colder. Frost can be expected where skies clear and it will feel chilly in the daytime with temperatures in single figures.

By Saturday high pressure has moved in with an increase in the upper air temperature:



The rainfall spikes have disappeared and all looks very nice. However, compare this with the chart on 15th in post #188. There the air was coming up from the south west across Scotland and the north, with the centre of the high further south. Its not much, but it makes a difference. While daytime temperatures will be very pleasant, with clear skies and light winds, frosts could be a problem at night. This for example is the forecast min temperatures for Saturday morning:



This is still some way off, but it wants watching - basically clear skies and light winds at this time of year can mean damaging frosts. Be prepared!

[Penellype]

Quick update:

The high pressure which is giving most of the country dry, warm and sunny days and cold, potentially frosty nights will gradually slip away to the east over the next few days. This will increasingly drag the air up from the south west, reducing the frost risk by mid week but also introducing the likelihood of rain moving in from the west. Low pressure is then set to bring more changeable weather at the end of March and into the first few days of April.

As always there is disagreement between the models as to what happens next, with some going for a return to high pressure and a very similar setup to the current one, while the GFS model resorts to its default setup of a more westerly (low-high-low-high) regime. I'd go with the return to high pressure being more likely as that appears to be what the atmosphere "wants" to do at the moment.

There is currently no sign of anything particularly cold on the horizon, although it is by no means too late for wintery weather. A return to high pressure would bring back the possibility of more over night frosts, while a low in the right place could still pull down a northerly wind and snow, which is actually what one of the red ensemble lines seems to be trying to do right at the end of this morning's run:



The steep ups and downs (eg the thick blue line) after about 3rd April are charactaristic of the typical westerly pattern that this model tends to revert to. As I mentioned, this is not really backed up by the other models at this stage, and is in any case too far ahead for any sort of certainty.

[Penellype]

After some notably warm days and nights more akin to May or June than late March, the weather is set to cool down to something nearer normal over the weekend. Saturday will be a day of sunshine and showers, with not everyone getting wet. Sunday should be drier and still very pleasant. Nights will be chillier than recently and frost could be an issue on Sunday morning especially in the north and west - don't get caught out!

The risk of night time frost persists for the remainder of the week where skies clear and winds are light. Keep an eye on the forecasts for warnings of frost in your area as it is not possible to be precise at this range.

The GFS has persistently hinted at a colder snap around next weekend, with this morning's midnight ensemble chart looking like this:



This shows the cooling to nearer average over the weekend, with the peaks next week being due to high pressure. Huge uncertainties start to appear around 7th April, with the green operational run one of the ones going quite cold. This is linked to high pressure forming over Greenland leading to the flow coming from the north:



If this were to happen there could be some damaging overnight frosts and wintery showers - it is not too late for snow! These are the minimum temperatures for the morning of Sunday 9th April from the midnight run:



The latest run (6am today) backs off this idea, with fewer of the runs going cold, and the green operational run now one of the warmer ones. It is fair to say that the ECMWF model does not see the northerly wind at all. This is the 6am ensemble chart:



Note that the next week or so is fairly dry due to the high pressure, but the rainfall spikes return as the uncertainty increases. This will be due to some of the runs reverting to the default westerly pattern, so this is not to be relied upon at this stage.

So in summary, a lot of mostly dry weather coming up with pleasant days but chilly nights possible. Uncertainty towards the end of the week with the possibility of a cold snap developing with potentially damaging frost in places, although this may not appear at all. At this time of year it pays to keep the fleece handy just in case.

[Penellype]

2 weeks away, so thankfully in fantasy land for now, but the GFS wants to bring winter back with a vengeance on 15th April! These are the forecast charts for min temperature and precipitation type for midday:





As you can see from the ensemble chart below, the thick green line is one of the coldest at that point, so lets hope it is just an April fool.

[Alison]

Well so far we've not had winter yet as it's been an annoyingly warm one, not come enough to kill anything unfortunately

[Penellype]

Yes its been another mild one, although with a couple of cold snaps in January. I have quite a few aphids around already.

[Penellype]

Still a lot of uncertainty about whether or not we are going to get a cold spell around Easter. This is this morning's ensemble chart:



Currently things are well above average for the time of year after a very warm March. Temperatures could reach the 20s quite widely over the weekend before a drop back to near normal which will feel quite chilly.

On this particular run of the GFS a lot of the members are well below the seasonal average from 13th April right through to the end of the run. The white mean line is also below average and hovering around the -5 line at times. Only a couple of days ago the white line stayed above the red line almost all the way.

This is a nightmare for gardeners as if today's run is right there could be some quite sharp frosts and the possibility of snow, which had disappeared from the charts recently. The culprit can easily be seen from this chart for midnight on Easter Sunday:



Here the isobars trace back a long way north, which would definitely be cold. This is still 10 days away and a lot could change.