forecaster

Recent conditions get the hearts of aviation forecasters beating rapidly; no problems staying awake there! The sort of action weather shown by some models can sometimes be less troublesome by comparison...

High to the west and low to the east => negative mountain torque event.

Cloudy. Rain later. Max 11C. Min 6C. Just based on what might happen if the current pattern does not shift.

We saw these signs popping up in the models in early November prior to the mid-November brief cold snap, but they came to nothing. Will be interesting if repeated failures form a pattern this year, or if they manage to get it right? Can't find the exact run I was thinking of, but some examples in this thread of promising output which turned out very mundane in the end: http://forum.netweather.tv/topic/78340-model-output-discussion-1st-nov2013/page-75

Sounds about right, based on this (NCEP reanalysis of 300mb scalar wind* mean in December from 1948): The jet not only steers depressions, but sections of the jet are favourable for upper divergence and convergence, with enforce particular surface patterns. The poleward exit region of a jet favours surface low pressure, so you can see why climatologically we often see depressions in the region around Iceland and Greenland and NE Canada. The area around the Azores (roughly) lies near the equatorward exit of the polar jet and the poleward entrance of the sub-tropical jet. Both these regions favour surface high pressure, hence the semi-permanent Azores High. * I'm assuming they take the mean wind vector and take the magnitude of that. It looks like a sensible result anyway.

Very pleasant today. Reached 12C with unbroken sunshine from 10 or 11AM. I wouldn't complain if we saw more of this.

Southern Hemisphere? That image looks like it does't even go as far as the northern hemisphere tropics!

No, fog is probably the hardest thing to forecast, or low cloud. Both could be an issue under such situations. Were this set up to occur in summer though, it would be relatively straightforward.

Did that happen at the end of winter 11/12?

But, it's still only November! Oh. Wait. Sorry.

I think the gradient might be enough across the SE by the end of the weekend to have accomplished significant mixing there. Td at EGLL currently +2C, but would expect that to be more like +7C (ish) by Monday. Light drift from the S would be very slow to mix in drier air again next week. So I would expect persistent fog to be more an issue than frost and freezing fog, in the SE anyway. Could be very boring in the SW with an Sc layer instead. I think the chances of a decent wintry continental high were dealt a blow a few days ago when models started showing this weekend introducing a westerly flow rather than dying northerly or post-northerly ridge. The higher dewpoints I think make a more foggy autumnal, but not notably cold, spell more likely.But....I'm quite accustomed to getting it wrong.

Yeah I think there are all sorts of tricky criteria to consider with this. Free air temperature is the base, but the radiative near surface inversion above a snow pack is another offsetting factor. Also, there is the balance between what will help melt snow faster: sunshine or cloudy conditions? You'll get longwave radiation from cloud, whereas sunshine may not have a great impact due to the high albedo of snow.Not straightforward, but at least you can be fairly safe in betting that an increase in the snowpack is not likely......

Model freezing levels above 3000m in anticyclonic conditions doesn't look too good for them does it. There would be a strong subsidence inversion, with cold weather in the valleys, but once you're into the free air, it would be relatively warm. Could be challenging to sustain a widespread snowpack under those conditions.

Pdo, please clarify. Clearly not pacific decadal oscillation?

That's maximum temperature occurring in the 00Z-06Z overnight period. Doesn't look cold even in the SE!

Part of the problem is that there's no true definition for "zonality". It doesn't strictly mean anything, therefore can mean all things to all people, hence we end up in discussions where a member says "return to zonality" then the endless "I don't see anything like zonality, in fact just the opposite!" follow ups. The term "zonal" itself - with respect to wind direction - could mean winds with a predominant westerly or easterly component. But that's obviously not what's meant by "zonality" since no one calls an easterly flow "zonal" even though it really is. So does "zonality" mean frequent depressions tracking across the country from the Atlantic? In terms of the global circulation model, that's just called "westerlies".

We've been hearing a lot about how next week's high might still lead to cold surface temperatures in the SE. This seemed a sure bet for a while, but I'm now less convinced. The westerly flow from the weekend should enable maritime air to mix into the boundary layer. Any anticyclone, even with a continental drift, following this will not displace that air. It would remain near average at the surface with overnight fog likely. To prevent that from happening, and to get a genuine cold high with continental drift, you would have to hope that the stability is such that on the weekend the Atlantic air does not mix down to the surface across the entire country. I don't think this is the "form horse" at the moment. Looking at the GFS 06Z run: Here we have a possible continental drift for much of S England, with the maritime S/SW flow affecting only the fringes of the UK. So what would be the source of the air were this to happen? The red line is a back trajectory starting at Heathrow at 100m and the blue line at 1500m. You can see the different origins of the air at both levels, but the surface air still has substantial maritime modification. Not a cold anticyclonic spell by any means. Maximum temperatures in the SE would push near the low double figures. The caveat of course is that this is all based on one GFS run. But since the "continental drift" is going to continue to be a subject of debate we have to remember a few things: 1) If you already have maritime near-surface air, a continental drift is not going to displace that. 2) If you want a cold anticyclonic spell, you have to prevent the maritime air from mixing in to the boundary layer to begin with. This looked likely up until a few days ago, now it seems less likely. 3) Once the anticyclonic spell gets going, whatever you have at the surface is pretty much what you're stuck with. Although there is a radiation imbalance at this time of the year, your problem is that the dewpoints will be quite high. This leads to issues with overnight fog, which prevents significant cooling. You could get cold (ish) daytime highs in areas only where the fog lingers - but these highs would likely still be around 7C. 4) Stratocumulus layers are the other issue. Only at short range can you figure out whether it will be fog or Sc, but both ultimately lead to similar regimes of low max-min variation. Don't think people should be getting their hopes up too much regarding this high for next week, unless they live in places like Benson.

Scotland on Saturday shouldn't be especially cold apart from in the morning: Temperatures heading back towards average for most people. Also, a pretty major "downgrade" occurred for Wales and the southwest. Maximum temperatures on the weekend will be close to average. This is about 5C warmer than what some earlier model runs would have led to.

Looking at the UKMO FAX for Sunday: I think there may be enough gradient there to mix in mild air for most of the country. The very far SE might escape, but given the trend we've seen over the last few days, I couldn't rule out further changes. If this westerly flow does manage to replace the boundary layer air with Atlantic sourced air, then that would be the prevailing story into next week. If the S/SE becomes anticyclone dominated into next week, then instead of cold, continental surface air we might be left with milder, muggier maritime air at the surface. That could lead to average daytime temperatures, persistent fog, but a lack of cold nights. What I'm saying is that a light continental drift, if it sets up next week, is going to have a hard time displacing surface mild maritime air if that has managed to mix in during the weekend.

There will be a cross isobaric component to the flow (H->L), and also a general outflow from the high. So you might have a source from the Med, but this would be after that pattern persists for a few days. Isobars are not trajectories and figuring out the trajectory of the air is not as straightforward as eyeballing one isobaric map.

Almost all my career has been looking at the 0hr to 48hr range. It's really not dull or mundane. There's always something. Apart from the times when there's not.

What exactly is "DECIDER"? Very interesting. Did you cover the globe or just Europe? Was this a dissertation? Is it available anywhere?

I often thought that myself, but they seem to have done pretty well for themselves in some years, proving me wrong.

Depends on the definition you use. But probably too late I would think. As for the chart shown by shed, that's bad news for Scottish ski resorts, having to put up with temperatures several degrees above freezing for a few days. Will there be a snoweating foehn effect happening?

The model output on the UKMO page really shows that Saturday/Sunday is turning out like almost none could have expected 3-4 days ago: Overnight lows, will be hard to get right, but no widespread frosts "across the snowfields" there! Max temps on Saturday, which for a while was looking like a cold and crisp day: Winds light, but from SW quarter.