Would love to hear from Darren/Retron tomorrow am. This is looking good though early days yet. Darren let us know your thoughts please.
Originally Posted by: Sasa
I've been pretty quiet recently (largely due to going back to work), but suffice to say things are going as expected. The pattern change on the 9th is still there and, I must say, was an excellent spot by the ECM-15 almost two weeks ago. Sustained mild weather was never an option (beyond 5% or so) after the change and as mentioned last week the choice then became whether we'd have an average spell or a colder one.
The only change since the weekend is that the colder option has become more and more likely, with it now being the odds-on favourite. There's still a small but not ignorable chance of it staying near average, but for the time being I'd go for the cold option.
There are a few things to watch out for now that cold is looking likely:
- How long will it last? Nobody knows, but a spell of several cold days is now looking probable.
- Will it snow? Almost certainly, but of course it's too far out to be specific. In the meantime, the GFS op has a prolonged rain-to-snow scenario for southern England, which would be interesting for us southerners!
- Will the pattern be self-sustaining? The models are undecided but going on recent years we'll need to watch out for trough disruption to the west. (There are several informal terms for this, "sliders", "lows going under the block" and so on). Every trough that disrupts to the west gives an extension to the cold.
- The upper high is part of the puzzle. It's related to the trough disruption (which is itself related to the jet) but for a quick glance the location of that upper high is important. If it sinks SE'wards it becomes harder - but not impossible! - to sustain a cold spell.
- The 528 line is less useful than usual. When we get a feed from the east during winter the lower levels tend to cool faster, meaning you get a cold, low-dewpoint "wedge" of air advected ahead of the colder upper air. This only serves to increase the likelihood of snow rather than rain. The effect of course lessens the further north you are as the sea has a greater modifying effect.
So, to sum up - watch for trough disruption (or sliders, lows under the block, lows getting shredded etc). Watch for the upper high near Scandinavia. And don't be at all surprised to see the models "wobble" more than once, as features too small to be resolved at the moment get picked up nearer the time (aka "spoiler lows", "shortwaves" etc).
The overall pattern is one which should excite cold weather fans and it will if nothing else bring a completely different feel to things than we've had so far this winter.
And as a final thing, it's the ECM which did the best with this. First to pick up the pattern change around a fortnight ago and the ensembles have been more consistent than its rivals. GEM deserves a mention too, as it's been pretty good at picking up the signal for an easterly. G(E)FS has performed relatively poorly and it's no wonder that work is currently being rushed along on a new version of the model. We'll see the results of that in April. It's also a reminder that although ensembles are the way to go, they're only as good as the base model driving them - if the base model is performing poorly, you'll get massive swings from the whole ensemble suite. Much as we've seen over the last 48 hours from GEFS!