The yo-yo thread, for whence and whom it may concern.

[harryc]

Way too early to say if this cold spell is a short snap of cold weather for a couple of days. The models are having difficulty beyond the initial cold blast to say if there is a reload of cold or not.

H

[stephenprudence]

Its not deep rooted easterlies and greenland highs im worried about, its high pressure, persistent radiational frosts and ice days due to strong inversions that areBI going to do any damage in gardens

[Nigel]

Way too early to say if this cold spell is a short snap of cold weather for a couple of days. The models are having difficulty beyond the initial cold blast to say if there is a reload of cold or not.

H

The air that is hitting us is from the arctic, its not a sustained blast , its a block of cold air that will dissipate, so we can be certain its a short spell. i am not saying it will get warm afterwards , I am saying the arctic blast will disspitate and temperatures will move back towards normal. After that its uncertain,but there is no further blocks of arctic air at least for a few more days.

[Tom2006]

I dont know why there is a need to play a cold warning down. Whilst I see what your saying Nigel temperatures are looking like heading well down on the averagw and all the signs are there that blocking will take hold. Its then just a matter of time before we get badly hit with cold so well worth posting something like this. Its looking more than a snap to me.

[Nigel]

Tom, its a cold plunge,they happen every winter, the first cold plunge almost always happens at this point in November. In fact its so predictable an occurrence at this point in November that I was even able to predict it at the end of October before there was any hint of it in the models.

I recorded 62 air frosts in one of the mild years. I cant imagine what this board would have been like that year. Yet I had Butias thriving in my garden.

Before you get too carried away with worries about the models, check out the jet stream position and forecast. It is not entrenched south of us , nor is it forecast to move there. As long as the jetstream holds we can breathe easily.

if you dont believe me, then this is todays Met Office forecast... note emphasis is on temperatures just below average rather than just above , so i am not the one doing the cold ramping, a little cold yes, exceptionally cold for time of year no.

The Met Office said there is a chance temperatures will sink below average over the next 30 days with snow on the way.

Its 30-day outlook states: “Conditions are not expected to be as mild as recent weeks, instead, temperatures are thought more likely to be just below average than above.

“Snow is not unusual during late November and early December.

“So, with temperatures more likely to be just below average, the chance of seeing snow across the UK increases relative to recent weeks.”

[cheshirepalms]

Actually Crosby is a well known frost hollow when Northeast winds occur. You watch, Crosby will record the lower temperatures on Merseyside in the next two weeks.

I'm sure it will, but its not in the bracket of being one of the coldest places in the UK, even in this upcoming cold spell, alongside the Scottish Highlands or deep inland middle England.

[stephenprudence]

well yes its not the coldest but it certasinly has potential to be the coldest in England given a particular setup. In 2010 Crosby was the second coldest location after Benson, with Southport not far behind!

[stephenprudence]

I think whilst looking for Greenland Highs and exceptional cold atmospheric air masses, I think the point may have been missed somewhat. We will not need cold airmasses, or a Greenland High to deliver cold. What we will likely have, is a northerlies devlivering Arctic air, then high pressure trapping that air, creating very cold surface conditions, meaning widespread ice days and severe frost at night. Of course the models do not often pick up on inversion situations very well, indeed I've seen the models forecast 7C when the next day, in reality it was 0C, so the models won't tell you anything about how cold it will be in this situation. The cold, or indeed perhaps damaging cold, will come from surface pooling of cold under high pressure conditions, not airmass driven cold from a colder source.

[cheshirepalms]

well yes its not the coldest but it certasinly has potential to be the coldest in England given a particular setup. In 2010 Crosby was the second coldest location after Benson, with Southport not far behind!

I just think somewhere inland will suffer more, and the sea is a lot warmer now than it was in December 2010.

[stephenprudence]

The models are now starting to firm things up, and it looks now like there will be some rather severe frosts, especially for western parts (plus coastal areas). Widespread -5C with sheltered areas/frost hollows possibly getting -8 or -9C, so it looks like a harsh cold spell coming up at ground level.

I have no doubt now that this will not be a normal cold spell at ground level, with potentially long lasting, severe and damaging cold temperatures, more especially for western parts. Whilst it won't feature cold air masses with air from a cold source, the potential for prolonged surface cold are very high, and I feel this spell could be on a par with 2009 easily. 2009 was also a spell defined by surface cold, and inversions, which brought exceptionally cold air to many parts of the UK. It also brought Cornwall some serious cold, even to coastal areas.

The west, will take the brunt of this as suggested earlier, with the east still getting frost, but not as harsh, nor as numerous as places in the west.

This will be a difficult and testing time for many gardeners, especially those still recovering from Dec 2010.

Again the culprit being cold pooling air from relatively cool direction, but exacerbated by radiation loss at night.

[harryc]

I wouldn't possibly disagree with the weather guru of the forum and it is looking a bit ominous but with the way the models can not agree on the outcome post the initial cold plunge I think it is too early to suggest what the outcome is likely to be post 7 days.

H

[stephenprudence]

To be honest, there's no other way but a decline into settled conditions, there is no other potential... milder weather is 99% out of the question, almost no chance - that I can be sure of. The issue is, I feel, the jetstream has not only been diverted next week but is in pieces, also the vortex may attempt to split.. if it splits, we really are in serious trouble (to the tune of 2010), but at the moment, it doesn't look like the split will happen. However almost certainly the following weather will consist of quiet, high pressure or col type whether with cold air trapped in there, with no escape. Whilst the air will not be very cold, the potential for rapid radiation loss at night will be high, so low temperatures, or extreme low temperatures (for the UK), may occur.

Under the conditions suggested by models now, I would expect severe frosts in the realms of -6C in my location, so you can understand how maybe sheltered Cheshire, or other frost hollow areas, may be as low as -10C.

The worst thing is there is absolutely nothing to shift this lingering cold pooling.. so it hangs around, gradually getting colder as it does.. vicious circle. I hope we get some kind of miracle, or a loophole from which we can avoid severe surface frosts, but I cannot see how we can escape, at this current moment in time.

[Nigel]

Severe frosts at end November do happen from time to time. It happened several times in the 90,s . Radiational freezes on clear still cold nights are a fact of winter. I still dont see long term severe cold , the jet stream is still too favourable to allow a long term cold scenario such as 2009. The significant feature in both 2009 and 2010 and this spring, was a jet stream embedded much further south than normal.
It is possible the jetstream forecasts are wrong and the jetstream re-establishes to the south after this cold plunge , but I still dont see long term cold developing out of this situation, or a greenland block under these circumstances.
As long as the jetstream remains where it is , whether its strong or weak , it simply will not support an elongated severe cold event, because the air above us will remain warm, allowing only night time radiational freezes with daytimes several degrees above zero. For me this type of cold is not damaging , it is the type of cold where daytimes remain at or below freezing that do the serious damage.

[Dave Brown]

In my experience Arctic plunges are common in November, and in many years see a freezing Guy Fawkes, and Remembrance. I was in a Scout Band in the late 1960s and snow on the Remembrance Parade was about 50/50. Nigel has had similar experience and both of us have successfully grown palms and exotics, which is why maybe we don't see major problems with -6C or lower in November..... it happens, so you just have to be prepared in advance to swing into action with just a few days, or less, notice

I was exotic gardening with only the 24 hour forecast being 75% reliable, so you always had to be ready from mid October onwards. It was also the 25% getting it wrong, even on the day, that sharpened your senses about the feel of the weather, and learning how your location would be affected.

[stephenprudence]

Wednesday/Thursday looks the main cold day this week, at the moment the models are not in agreement where the coldest night is going to be, however, this run it showed it being coldest in southeast, however, I suspect that will end up being the west that get the lowest temperature rather than the east when it's settled.

Still going for widespread prolonged night frosts thereafter.

Mean minimum temperature for Manchester in next two weeks around -1.5C.. the only saving grace, if we don't get inversions (at least localised inversions will come into play), is daytime temperature for the majority should recover above freezing.