Penellype's Weather Channel

[Penellype]

Question for you Penellype?

We have an old chap (93) over the allotment who's a bit of believer in old sayings....he is convinced February's weather is going to be very cold....any indications he may be right?

Its possible. As I say, the position of high pressure is crucial.

Here are a couple of charts from today's midday GFS run (one of the colder members of the ensemble in this run).



This chart shows high pressure ridging towards Greenland on Sun 4th Feb, with a strong wind from the north. The upper air temperatures for the same time show the -10 isotherm down as far as the midlands - this would be very cold probably with snow showers:



2 days later the high has collapsed over us and brings the wind in from the north east, again very cold:



If this high were to continue to ridge towards Scandinavia (which doesn't happen on this run, but easily could) this could lock in cold weather for much of February.

I must stress that this is only one of the runs of the GFS model and others do not back this scenario up. The ensemble chart shows a lot of scatter, with several members very cold (including the thick green line for the operational run above) and some really very mild:



It is too soon to say which is right, or it could be a mixture of warm and cold spells as we have had so far this winter. What does look likely from the ensemble is that after tomorrow's rain it will generally become somewhat drier under the influence of higher pressure.

[Penellype]

Getting very windy, and here comes the squall line, right on schedule:



Radar picture from https://www.netweather.tv/live-weather/radar

Edit: The temperature here before the squall arrived was 13.8C (at 7.30am)! After the squall (7.45am) it was down to 12.2 and it will drop a lot further after the cold front passes (to the west of the main rain band).

[Chippy Minton]

The squall line passed over here at precisely half way between home and the train station

Could do with a spell of dry to let the water soak away, want to get a few early peas in and need to continue the dig over and need to get into the new area at the plot.....

[Penellype]

Dry for the next couple of days but more rain due on Sat and early next week I'm afraid.

[Penellype]

Following up on my previous post about a cold February, this morning's GFS run goes to town on this idea. The high that is currently to our south and keeps ridging north, giving us a few days of alternate warm and cold starts to move north, with huge southward loops in the jet stream either side of the high, so by 7 Feb we have this:



The high is joining up with another area of high pressure, currently over asia, which is also moving north. The winds are moving round to the east.

By the following weekend (as far as this run goes), we are well and truly locked into a very cold easterly wind, with a huge high pressure to the north:



This is going nowhere fast, and could lead to a bitterly cold month.

THIS IS ONLY ONE SOLUTION AND AT THE MOMENT THERE IS A LOT OF UNCERTAINTY.



Note that the thick green line which corresponds to the above charts is the coldest at the end of the run. However there are several other members of this ensemble which are also very cold (and some which are very warm).

My point is that a very cold pattern for this month can't be ruled out.

[KevinM67]

Great !!!

[Penellype]

Just to show how volatile these models are at this range, this is the equivalent chart for 10th Feb from the 6am run of the GFS (the previous post was from the midnight run):



As you can see, this is completely different - there is no high pressure to the north in this run. This shows why you need to look at the whole ensemble, preferably over several runs, to see whether there is consistency in the forecast or not. Clearly the answer here is that there is not.

[Cadalot]

I have a question this morning outside at 6:00 and 0 degrees at 9:00 with lowest temperatures recorded in the greenhouse of between -2.2 to -0.2 over night from the 10 monitoring locations and that was with the terracotta pot candle heater.



Looking at the weather forecast for the next 10 days I think the Terracotta pot candle will be used most nights in the next 10 days



What I really don't understand is why is the temperature dips a degree after the sun has gone at 16:48 but from 23:00 or 11pm until 3am the following morning is rising by 4 degrees.

[Penellype]

I have a question this morning outside at 6:00 and 0 degrees at 9:00 with lowest temperatures recorded in the greenhouse of between -2.2 to -0.2 over night from the 10 monitoring locations and that was with the terracotta pot candle heater.

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Looking at the weather forecast for the next 10 days I think the Terracotta pot candle will be used most nights in the next 10 days

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What I really don't understand is why is the temperature dips a degree after the sun has gone at 16:48 but from 23:00 or 11pm until 3am the following morning is rising by 4 degrees.

What is happening here is that last night we had high pressure with light winds, allowing the cold air to sink towards the ground with nothing much to mix it up. The sky was clear so heat radiated out into space, and you had a frost. During the day these conditions persisted, but the sun warmed the air so it got a bit warmer. Once the sun goes down the heat again starts to radiate into space - a clear or partly clear sky is showing up to about 8pm. After that it starts to cloud over as the pressure falls and this stops the heat radiating into space so fast. Also notice that the wind starts to pick up. This mixes the air and prevents the cold air sinking so the temperature at ground level actually rises.

[Penellype]

The models have been really struggling recently, with most runs trying to build a high over Scandinavia and attempting to turn the wind into the east, with varying degrees of success.

The current mild spell (although those in the south who had a sharp frost last night may dispute that description) is coming to an end with cold fronts moving down across the country tonight into tomorrow associated with a low pressure moving through and to our east, turning the wind into the north. There will be snow behind this in Scotland and parts of northern England - how far south it gets is uncertain. There are yellow warnings of snow and ice for parts of Scotland tomorrow and Thursday.

The rest of the week will be colder and windier with frosts most nights, although it will briefly be milder over the weekend as another low moves through. After that there is good agreement that it will become colder next week, but not on how long this will last. The picture is very confused:



What strikes me as odd about this ensemble (and the general forecasts for the first part of February) is that most of them mention high pressure, and indeed the rainfall total is low, but the temperature profile is the typical up and down wave pattern associated with the warm and cold sectors of low pressure systems. If the models are confused, so am I.

The cause of all this confusion is nicely summed up in the currently predicted chart for Monday:



Here high pressure is ridging through the country attempting to join up with a high (1030mb) over Scandinavia. The wind is nearly but not quite getting into the east, as illustrated by the 1020 isobar which turns south half way across the north sea. The current temperatures forecast for York on this day are a frost early and late, reaching 5 degrees at mid day, 10 by 3pm, and dropping to 5 again by 6pm (!).

The fly in the ointment is that huge purple area over Greenland. That is the polar vortex, and it is winding itself up far too close to us to allow the high pressure to form a block. Basically the lows run over the top of us, pull the wind into the north as the high behind tries to ridge towards Greenland, only for the high to be steamrollered away by the strength of that low pressure.

My guess, for what it is worth, is that we will continue to have cold snaps with warmer spells in between for some time, until that polar vortex weakens enough to allow a blocking high to form. If/when that happens the position of it will be crucial in determining what happens next, particularly with regard to the temperature.

[1Bee]

We had a bizarre moment on (I think) Sunday night when it suddenly built up to what were clearly gale force winds, but the only weather warning was for rain in the highlands somewhere... even Metcheck only mentioned it being "a bit breezy" in our neck of the woods.

It felt a bit twilight zone...

[Penellype]

Models have changed things a bit today, with next week looking really very cold now. There is significant potential for several periods of snow, starting as early as Saturday, as fronts move into cold air feeding in from the continent, and get stopped in their tracks by a pool of cold air under high pressure over Scandinavia.

As I said before, it really doesn't take much of a change in the predicted positions of highs and lows for us to find ourselves in very different conditions in this sort of pattern. It is possible that the models will flip back to milder conditions again, but for the moment the first half of February looks potentially cold, although some of the members bring the milder westerly atlantic weather back towards the end of next week:



Note that any lines below the -10 line are basically predicting that the temperature will remain below freezing all day.

Because this cold air is coming from the east, the coldest areas will be the south east and east. It will generally be less cold, but possibly snowier in the north and west as atlantic fronts stall.

I will update this thread if anything changes significantly, otherwise expect warnings of snow and ice to appear at metoffice.gov.uk/public/weather/warnings probably starting on Saturday. (There are actually yellow warnings tonight for snow and ice in Scotland and ice in parts of England)

[Greenleaves]

Question for you Penellype?

We have an old chap (93) over the allotment who's a bit of believer in old sayings....he is convinced February's weather is going to be very cold....any indications he may be right?

I wonder?....

[Penellype]

I wonder?....

I certainly wouldn't completely dismiss the older ways of predicting the weather. This guy is interesting WWT - Weather Without Technology - Welcome - his February forecast (cold) is here: http://www.weatherwithouttechnology....ruary_2018.pdf.

He gives an easy method of predicting the weather for your region using the phases of the moon and the exact time that those phases happen (this is different depending on your location). As an interesting exercise I wrote down the moon predictions for each "quarter day" (new moon, 1st quarter, full moon, 3rd quarter) for the whole of 2017 for York. I then looked at my own notes for the weather during those periods and decided whether the predictions were good, nondescript or definitely wrong. Bear in mind that this is not a simple yes or no answer. The prediction is for the whole of the time between the quarter day and the next quarter day, just over a week. So a prediction may say "showery" and I would rate that good if there were showers on more days than not, nondescript if there were showers or rain on a couple of days, and wrong if it was dry all week. And so on.

My results were:
Good 48% of the time
Nondescript 35% of the time
Wrong 17% of the time.

This is not the 90% accuracy that he claims on his website, but it isn't actually at all bad, given the range of possible weather. Occasionally the predictions were spectacularly right, for example:
13th September - prediction "Wind and rain". Actual weather - 13th September "storm Aileen" then changeable for the following week.

So my verdict is that while you can't rely on this method to give an accurate prediction, you should not be surprised if it is right more often than you would predict if it was simply random.

Today's full moon, Exactly full at 13.26 in York, predicts snow and rain. The forecast for the next week often mentions snow and rain, although some of the days may be mostly dry.

The interesting thing about this is that you can do it a long time ahead. So for example taking a date at random, say 30th June, looking at the nearest quarter day which is 28th June, the full moon occurs in York at 05.53, and the prediction is "rain". In fact the whole of the prediction for spring and summer this year is fairly grim.

We shall see.

[Lumpy]

Penellype here is, what I suspect to be a very daft question.

Can moonlight create a rainbow effect?

In the garden Monday night, moon in the East and I'm sure that the arch below it was a moonbow or I was delusional after watching too much sci fi. It was a perfect arch just like the daytime but without the colours.