Model Output Discussion Autumn 2024

[Frigid]

The synoptics this month would've made for a pretty cold month in winter it has to be said. Hints of yet another northerly plunge at the end of September. 

[northwestsnow]

EC is very chilly ,probably cold for parts of Scotland next week.

Certainly scope for temps of - 3/-4..

[Addicks Fan 1981]

After the arctic outbreak of weather next week it looks then as things will back westerly, but according to AIFS 0Z it won't always be settled.   The chart for 10 days time, 

and the 850 hpa, 

still in the cold air the uk is however we have to look to the SW as the SW winds will take the N winds place.     

[northwestsnow]

 

 

 

By day 7-10 the EC mean has  W Europe well below average !

Scope for an early start for the Scottish ski industry ?

Edited September 19, 2024 by northwestsnow

[Methuselah]

 northwestsnow

Or a 1974-75 redux? 

[northwestsnow]

Following on from previous Exeter now progging much cooler showery weather into the NW end of next week.

These airstreams will certainly do no harm ref ssts and soil temps and I'd say at this juncture scope for some white stuff for the highest hills further North..

[Addicks Fan 1981]

 northwestsnow if I were you buddy I would use the AIFS more as it has better verification stats.    

[Mcconnor8]

Truly beautiful chart, if it was December - March

Edited September 19, 2024 by Mcconnor8

[viking_smb]

 Mcconnor8 i'm happy with that, hate the heat

[northwestsnow]

Genuine cold uppers into N Scotland at day 9

[Andy Bown]

GFS showing a long fetch Southerly and mid 20s on the last few days of the latest run! 

[WYorksWeather]

 Andy Bown Holy outlier though!

Interesting how it goes from the colder side of the ensemble and then flips back to a warm outlier. Of course we shouldn't expect this to verify.

I'm actually surprised by how quiet the forum is lately - though in a way is kind of understandable given that autumn often produces the most 'boring' weather for a lot of people's preferences. But certainly not at the moment - we're seeing some big temperature swings.

Will be interesting to see if this carries on for a while, or if we eventually see a return to more typical autumnal conditions. My assumption is that it might be linked to the current extremely weak polar vortex. If we see this strengthen as forecast, then perhaps mid-October will see more typical patterns establish themselves. Of course, who knows if this polar vortex forecast will verify - it performed very poorly last winter.

[Bricriu]

 WYorksWeather

It would  be fantastic if this current  pattern  lasted over the next few months, but we all know the polar vortex will more than likely get its act together sooner rather  than later.

[WYorksWeather]

Tonight I actually have time, so my EC clusters post is below.

Days 3-4

Low pressure moving up from the south, and high pressure retrogressing, leaving a cool north-easterly and fairly unsettled by Sunday.

Days 5-7

Cluster 1 (27%) has high pressure pulling out further to the west at day 5, introducing a direct northerly. By day 7, a deep low, perhaps stormy, is centred over Scotland. Regardless of storm development, very unsettled and cool.

Cluster 2 (24%) is very similar, but has a larger area of low pressure rather than a distinct low centre. No storm risk, but still cool and unsettled with winds from the north. Cluster 3 (20%) is broadly similar.

Cluster 4 (16%) has the cool northerly air taking a longer sea track, perhaps more of a relative polar maritime airmass than a direct northerly. Still cool, but somewhat less so. Potentially becoming stormy by day 7 with deep low pressure just to the west of the UK.

Cluster 5 (14%) is also cool and unsettled, with a direct northerly.

The OP was in Cluster 1.

Days 8-10

Cluster 1 (31%) keeps the blocking going, continuing to bring a northerly at day 8, though this does gradually flatten off to a north-westerly at day 9 and more of a westerly by day 10. Becoming less cool, but still on the cool side of things, and continuing unsettled conditions.

Cluster 2 (25%) reverts to a UK trough. The northerly flattens off to a north-westerly, but still on the cool side at day 10.

Cluster 3 (24%) keeps the northerly flow at day 8, but gradually flattens it off and we have a return to a much more typical Atlantic westerly pattern by day 10. Still unsettled.

Cluster 4 (20%) has the blocking signal retreat further north and west, more towards North America. This allows a much milder south-westerly flow to be introduced by day 9 and 10. Still a wet pattern though with low pressure generally in control.

The OP was in Cluster 2.

Days 11-15

The extended range patterns are highly uncertain, particularly if tropical storm activity comes into the mix. Regardless, here goes.

Cluster 1 (41%) has south-westerly winds at day 11, and this pattern broadly continues, with a deep and potentially stormy low crossing the UK from the west or south-west at day 13, and the pattern remains westerly and unsettled at day 15.

Cluster 2 (37%) has high pressure to the west at day 11, with a cool north-westerly. The high then progresses eastwards, and eventually sits to the east of the UK by day 15, with low pressure stalled to the west. This could create a plume setup, with a brief warm (or, at a stretch, very warm) spell possible.

Cluster 3 (22%) has a generally flat westerly with a NW/SE split. Warm-ish and dry in the south (not going to be especially warm by early October), and cooler and wetter further NW.

Summary

Overall, a pretty clear pattern. Modelling is now pretty firm on our currently easterly being exchanged for a northerly by early next week, bringing much cooler and more unsettled conditions. This broadly continues through day 5-7 as well.

Thereafter, there is some model disagreement on timing, but most options seem to go for a flattening off to a wind direction more towards westerly or north-westerly, which would allow temperatures to recover slightly, but likely still cool. Regardless of this, remaining unsettled, perhaps very unsettled.

In the extended range, perhaps a few more options open up, but really not much of a signal. Roughly equal chances of a NW/SE split, a generally mild but unsettled south-westerly pattern, and a brief plume setup. One of the hallmarks lately has been that the extended range has seen very weak signals, so I really wouldn't worry too much about the start of October yet - almost no option would surprise me at this stage.

 

 

[Weather-history]

Some of the 168hr rainfall totals are eye opening

 

The 168hr UKMO chart looks nasty

Edited September 20, 2024 by Weather-history

[Cambrian]

Some very striking charts appearing for the last few days of the month. 

We often speak in the winter months of the connection between an Aleutian / Alaskan trough and a UK and Ireland / Scandinavian trough. This year we have an autumn taster. But with a difference compared to the usual. 

Often when we see an Aleutian trough it is accompanied by a ridge over Western Canada, which can include the Pacific coast, and typically followed by a trough running south through the east of the US, before an Atlantic ridge and Scandinavian trough, part of a 4-wave hemispheric pattern.

This time the ridge is positioned further east with a plume of warm air heading up through Canada, here at day 6 on the 0z GFS op, the 20 degree T850 isotherm is further north over central Canada than the 0 degree isotherm is through the UK and Ireland. A 20 degree difference for the same latitude, and look at those anomalies! So it’s not just its positioning but also the extent of the warmth.

 

If we look at the 0z GEFS mean at day 7, we see our troughs, joined as one great slab of low heights running up over the Arctic with a preference for the Siberian side, and the anomaly chart shows an emphatic 3-wave pattern, troughs diving a long way south through the mid latitudes - at the Pacific coast, our northwest Europe trough and the third one heading down from Siberia into Mongolia. 

 

However, a closer examination finds a slight incursion of the polar low heights into the Canadian Arctic with an associated upper trough running down into eastern Canada and the eastern US, the “lost” fourth trough just where we’d expect it.

It’s visible too as a dip in the height contours to the south of the Great Lakes on the heights anomalies. It’s a measure of the magnitude of the Canadian warmth that it has largely concealed a hemispheric longwave trough.

So for now, it looks like we’ll continue with the theme of the same broad potential trough zones (Alaska, Hudson Bay, Scandinavia, east Asia) vying for the two or three available portions of cold air off the pole, and when not supplied, the zones taking it in turn for a more disengaged role with a period of relative quiescence, with transient bisecting ridge or perhaps a cut-off mid latitude low, until the next projection reaches down from the north. 

It is likely it’s only a matter of time before the PV will ultimately go through its annual autumn throes of pan-Arctic establishment, and a return to an overt 4-wave pattern may well be something to look out for in the first stages of this process.

However, I don’t go with the idea that seeing this 3 or 4 wave pattern relenting in the next while would mean we won’t see it return later in the autumn and winter, any more than if we were seeing unrelenting mobility and a charged-up jet stream with troughs galore that we would be stuck all winter with that either. Each year is its own animal. 

On that note, this year has been notable for the extended duration of slow moving meridional patterns. There is the occasional interlude of something a bit more mobile but the default for many months now is for a swift reinstatement of meridional and blocked patterns. This is shown nicely in the zonal wind analysis for 2023-24, the blue line in the chart below, running well below the long term average a good deal of the time. As we know, for the UK and Ireland, this can mean being frequently stuck near or under a trough for protracted spells, like the coming week.

And it doesn’t look like we’ll see a change from this general pattern any time soon, at least until the block of anomalously warm air over North America has dissipated (properly and sustainedly) and there are no clear signs as yet of this happening, here on the 0z GEFS days 12-16 mean T850s, warmth still pumping up into the Arctic, to meet over the pole with a similar effort from the Eurasian side, continually diluting the core vorticity of the PV, a fairly substantial spoke in the wheel.

So it’s no surprise that the SPV is currently markedly weaker than the ERA5 average and is forecasted to remain weaker than average. From the 0z GEFS mean, the zonal wind at 10 hPa 60N was 6.2 m/s, weakest for this date is: 5.9 m/s (2014), strongest is 13.2 m/s (2005). Only 1 of 31 members have a stronger vortex than average for the time of year. 

With the latest couple of forecasts showing it either flatlining altogether after the 25th or making erratic upward progress through to the end of the first week of October, it looks like the spoke remains in the wheel for the foreseeable.

[damianslaw]

Big contrasts from week to week at present. Next week we revert back to a much more unsettled and much cooler airstream. Indeed Scotland will jump from high summer to deepest late autumn, with places such as Aviemore seeing high single digit maxes after reaching the mid 20s this week.

GFS and ECM show a long draw northerly mid week, akin to same timescales last week. 

A very topsy turvy and far more interesting September this year than many the case. 

 

 

 

[Downburst]

Such a great read Cambrian. That zonal wind analysis is very informative indeed. I noticed, and this is FI on the GFS, that all the jet activity is pacific side and into North America, slightly south. In any case the North Atlantic sector is bereft of the usual powered up Jet for time of year. So perhaps after an unsettled week next week we may see a quieter Atlantic again, but totally based on this run and a hunch due to the unusual setup with the PV. 

[northwestsnow]

6z is noticeably cooler than average day 7 onwards with repeating NW airstreams ..

Early season frosts across the North ..

Likey !

[northwestsnow]

Something happening in the strat ??

[Derecho]

32 minutes ago, northwestsnow said:

6z is noticeably cooler than average day 7 onwards with repeating NW airstreams ..

Early season frosts across the North ..

Likey

Indeed, all these northerly topplers are reminding me of the early 2000s....

It's been quite a meridional September. Early October sees a stronger attempt at blocking sticking to our NW on the GFS 06z. Would be a cold opening to the month if that came off.

Edited September 20, 2024 by Derecho

[Derecho]

Thankfully this is at T168 but this UKMO looks nasty. If we can get the cold air across us cleanly hopefully we'll avoid a potent storm like this.

It is on its own at the moment but something like this can't be ruled out.

Edited September 20, 2024 by Derecho

[Mcconnor8]

 Derecho UKMO going a bit OTT with the wind speeds I think lol

[northwestsnow]

Certainly one would assume repeated Arctic sourced air attacks will help lower ssts locally.

I hate seeing high sea temps in Autumn / winter.

Then again I've vowed to never give north westerlies in winter a second look ever again after repeated let downs,often associated with sea temps in the Irish sea.

I'll have a nosey in a few weeks ...

Edited September 20, 2024 by northwestsnow

[FetchCB]

 Mcconnor8 Those windspeeds are kmh so 150 kmh relates to 93mph...nothing too exceptional for the areas shown