Penellype's Weather Channel

[Nicos]

Wow....Canada has turned a nasty shade of purple

[Penellype]

Wow....Canada has turned a nasty shade of purple

Yes, that's the polar vortex.

[Nicos]

I have to keep reminding myself of the meaning of these meteorological terms

I just spotted a new term I don't recall seeing before....

"explosive bombogenesis"

Good innit?

[Penellype]

..

"explosive bombogenesis"

Oh dear. The correct term is explosive cyclogenesis, but I suspect that's too hard.

[Nicos]

^^^^... bombogenesis is hard enough to say!...is that a silent b in there

[Thelma Sanders]

Oh dear. The correct term is explosive cyclogenesis, but I suspect that's too hard.

They seem to use bombogenesis on American sites
eg

According to the NOAA's National Ocean Service. a bombogenesis forms when "cold air mass collides with a warm air mass". The formation of this weather phenomenon is called "bombogenesis" and the resulting storm is known as a "bomb cyclone".

American meteorologist Ryan Maue explained in a blog post that the "bombing" effect happens when there is a rapid decrease or drop in surface barometric pressure of at least 24 millibars in 24 hours.

Maue also said that a bomb cyclone shares several characteristics with tropical storms such as hurricane force winds.

[Penellype]

Thanks for that Thelma. Nice to see there is actually a sensible definition of a bomb cyclone.

[bikermike]

cyclogenesis = Phil Collins on a Bicycle...

we had quite a flurry of snow on the north circular on the way home last night at c7pm. cold enough to stick on the cars, but not the ground. And then freeze overnight.

With macro-scale weather predictions of the sort you are looking at, does it tend that the whole of the UK is affected in the same way (ie South may be warmer than north, but they are both cooled by the same amount by the sorts of weather events we are talking about) - or do different bits end up in different weather systems?

[chris_m]

Up here in South Cumbrial, yesterday was "interesting" weatherwise. We had just about every type of weather we could; sun, cloud, drizzle, rain, hail, sleet, snow and spells of strong winds - then, to cap it all, in the evening we had thundersnow. Apparently the road to the east was closed in three places because of the conditions, with cars (including police cars) either stuck on the hills or sliding down them.

Just about the only weather condition we didn't get was a heatwave ;-)

[Lumpy]

It was -7 here last night and we are only in Lincolnshire.

We live in what they call one of the cliff villages which means there is a steep valley between us and Newark On Trent. The weather can be much warmer just 3-4 miles closer to Lincoln. I've actually seen it where Lincoln has rain whilst here has quite heavy snow.

Obvously, the land formations also make a huge differance to the weather.

[Penellype]

With macro-scale weather predictions of the sort you are looking at, does it tend that the whole of the UK is affected in the same way (ie South may be warmer than north, but they are both cooled by the same amount by the sorts of weather events we are talking about) - or do different bits end up in different weather systems?

Absolutely not. What happens is critically dependent on the positions of highs and lows, fronts and air masses. You can easily get a situation where the east is freezing cold in an easterly wind, while the west is warm, wet and windy. You can get a situation where the south is in an easterly wind but the north is more westerly and therefore warmer.

The weather is rarely simple!

[Penellype]

After a cold start in many places today sees the beginning of a pattern change. Wet and windy weather will move in from the west from a low pressure system to the north of Scotland. As this moves away into tomorrow high pressure starts to build from the south, bringing drier weather and lighter winds (although Scotland will be wetter at times). Depending on how much fog there is in this one (which I find impossible to assess) this could lead to dull, foggy days or bright and sunny ones. In prolonged sunshine the temperatures could reach double figures during the day early next week, but it will be much colder if it is foggy. Nights will be chilly with frost possible under clear skies.

What happens next is still very much open to question, and the models keep changing their minds. Most of the solutions now appear to be cold or very cold, although the odd mild one remains. This morning's GFS ensembles have trended much colder in the last couple of runs, with some very extreme solutions towards the end of the run:



There is plenty of time for this to change.

To give an example of the differences between models, by day 10 (24th Feb) the GFS looks like this:



Note that this solution has high pressure over Greenland and low over Scandinavia, dragging the air in from the north east, which is bitterly cold.

The upper air temperatures associated with this show a huge block of cold air over Europe poised to head straight for us:



If this solution came off we would be stuck in a cold pattern for weeks, with low pressure systems running underneath the blocking high and probably dumping a load of snow on us at times. There would also be frequent snow showers in the east.

The ECMWF has this situation by day 10:



This is quite different - there is high pressure over Scandinavia and low pressure over Greenland. This would be cold, as the air is coming off the continent, but it is nowhere near as cold as the GFS because there is a south easterly wind which does not have the northerly component associated with the GFS solution. The corresponding upper air chart looks very different:



The very cold air is way back over Russia and some of it is being pushed down into eastern Europe, but it is much less cold over us.

Where the ECM would go after this is open to debate. I think it likely that the high would move north over Scandinavia which would turn the wind more into the east, becoming colder. However there are plenty of purple colours to the west and this could mean that there is enough energy in the atlantic to flatten the blocking high and keep it more to the south, stopping the very cold air from coming this way. Compare this to the GFS where the high has blocked off the atlantic, allowing it to move north and force the low pressure to run underneath it.

I think the models are struggling as a result of the current SSW (as discussed earlier) which is proceeding in a somewhat unusual manner (I think). I don't have much experience of these things, but my understanding is that the warming starts at around 10hpa, high in the stratosphere, and gradually propagates down into the lower layers. However, this time (and I don't know how unusual this is), it is already much more dramatically warmer at 30hpa (compared with a few days ago ago) than it is at 10hpa. To me this is odd, but then the pattern throughout this winter has been odd, so maybe that should not be too much of a surprise!

So, my call on this is that the models are still totally confused. Enjoy the slightly milder weather over the next few days (by no means warm), but don't think that winter has gone away, because there seems an increasingly high chance that it hasn't.

[Aunty Social]

Day before yesterday on our Welsh mountaintop, we had a totally unexpected snowbomb. Brown-out - I thought my new electric propagator was faulty and had to be sent back - then full power cut with no heat on the coldest day of 2018 so far. Cue coal, kindling, and fireplace!

Normal weather services now resumed.

[Thelma Sanders]

Enjoy the slightly milder weather over the next few days (by no means warm), but don't think that winter has gone away, because there seems an increasingly high chance that it hasn't.

Just about sums up what I've been thinking

I've certainly put early sowings on hold 'til the cold makes up it's mind where it's going

[Penellype]

The last 2 runs of the GFS model have come up with Operational runs that are very much at the milder end of the spectrum. The Operational is the one that most weather apps use, and therefore these will be showing much warmer temperatures at their longer range than previously. Don't be fooled - although it is perfectly possible that the Operational is right, there are plenty of ensemble members that still put us in a bitterly cold easterly or northerly flow for the last week of February. Even the Operational run takes a nosedive at the beginning of March.