Model output discussion - here comes the beast!

[Youcan'tbecirrus]

56 minutes ago, nick sussex said:

It may or may not turn milder by next weekend, before that there is some quite exceptional cold shown for this late in the winter, and plenty of snow chances.

Theres also the possibility of a big snow event as the low to the sw moves ne, the margin for error even if every model agrees at day 5 is enough to markedly change the conditions on the ground for parts of the UK.

So my advice just wait and see what the trend is over the next few runs. 

With regards possible temps at the end of the week Nick, what are your thoughts about temps where there is lying snow ? Are those numbers factored into the models so could milder temps be suppressed at the surface ?   

[mb018538]

I think the 6z ensemble just shows this is a long way from being sorted out.....all options including milder and staying cold on the table for the end of the week. More runs needed!!

[Snowjokes92]

gfs 6z mean, bit of an upgrade compared to recent output! Those thinking of a mild breakdown, might think differently, still plenty time to change as this won't ne nailed yet  snow showers popping up again i would imagine as well. Im not sure how far north the low will go, but alot of the models seem to take the northern extent of the low towards central northern england. Could be plenty snow for northern england from this.

[Eastnorthwest]

I'll repeat the drum bang this morning

Beyond 5 days in the world of model watching is any  forecasters  nightmare 

2 days ago the cold coming was going to be a spell, this morning it's a snap,later today or tomorrow ?,who knows, interesting or frustrating that's how it is 

If in the next 24 hours they flip back to a return to a spell ,then I won't be surprised cos that's how it goes with these models 

Fact is enjoy the next 4 days and hope by Wednesday the charts for the following 4/5 days will be what you hoped for

[ukpaul]

The trend certainly seems to be to bring the first low further north on all the major models, similarly they are also modifying the higher uppers shown yesterday to remain colder, at least for most of the country. The two together are very positive allowing a snow event moving to the far north on Thursday and Friday whilst giving us a fighting chance of those colder uppers returning south.

That is the likelihood at this present moment but the trend to a northwards drift of the low and less mixing out of colder uppers is a generally positive one.

There is also more of a consensus of what the second low does. ECM on its south west to north east track ovr the country allows for those colder uppers to be dragged south after it moves through.

One final trend which is more worrying and, has me concerned is the likelihood/possibility of freezing rain on Friday. Already an ECM chart has been posted but here’s WRF showing it as well. That’s the mother of all disruptive weathers available for this country right there.

. 

 

Edited February 25, 2018 by ukpaul

[bluearmy]

Either mogreps has jumped this morning or Exeter are unhappy to back it against all the other modelling. 

so we are where we are re next weekend with modelling uncertain how far north the breakdown gets. What we have lost is Exeter’s opinion that it was likely to stay cold pretty well countrywide out to tues/wed 

it wouldn’t be the first time that mogreps has been a run or so later to the eps and to be fair to the gefs, they arrived there on yesterday’s 06z run. 

EDIT: I note the automated ukmo data for Friday has changed the predicted snowfall for my location from heavy to light and the winds remain easterly whereas earlier they had become lighter southerly in the afternoon. Do they use ukv for this?  

Edited February 25, 2018 by bluearmy

[Mokidugway]

Never ceases to amaze how quickly epic cold can be eroded by our friend the Atlantic ,still some good chances of epic snow for some to come though ,enjoy 

[Philbill]

As a scientist I can see why the arguments are occurring over just where this low pressure is going on Friday. Since most of the models are showing a breakdown to warmer air over the SW on Friday its a reasonable assumption. Also the ensembles would all be in agreement if the models are showing the same for most/all perturbations. However, these are extreme circumstances and we are 4/5 days away as more data is fed in closer to the time I expect them to correct South as I believe they are being overly progressive in pushing the cold air back. A 50-100 mile correction further South would mean blizzards for the South the like that have not been seen for a while. I don't think even with the model output we see now that anyone can say its "game over for the cold spell". Also its not as though the jet has changed and an Atlantic depression was being forecast. I think this is more difficult to call and the models are struggling a little. Interesting few days ahead.

Edited February 25, 2018 by Philbill

[Mike Poole]

3 minutes ago, ukpaul said:

 

One final trend which is more worrying and, has me concerned is the likelihood/possibility of freezing rain on Friday. 

 

Freezing rain also shows up on the GFS 6z, in Central Southern England - this certainly is not something we could do with, it could cause chaos:

The purple hatched area is freezing rain.  Obviously it's still 5 days away, but it's in the mix of possibilities.  

Personally, I still think this low will be further South than the models are showing currently, we'll see.

[Derecho]

Right so I noticed my earlier post was greeted with a few confused reactions earlier so I thought I'd clear things up. Here is all 22 members from the GFS 06z, time period is generally late Friday. I've marked the boundary between where precipitation falls as rain or snow. Anywhere southwards of the boundary will see rain. Anywhere north will see snow. Areas in the black line its hard to call but this will be where the heaviest precipitation is.
* P3 is an odd one out because it moves north embedded in the cold surface air.


OP                                                                Control                                             p1                                                             p2
         p3* - p6   p7 - p10       p11 - p14     p15 - p18
 

                       P19                                                  P20

Main thing to note here is that T850s are not always the best guide in these situations, the boundary between airmasses is much better. Areas north of the black line will still be very cold at the surface. Runs which I class as disappointing in my graphs come saturday are those which bring the the low pressure too far north. The ideal scenario would be for the low to stall its northwards movement and then get pushed eastwards keeping us in the cold air.

So that is the rationale behind my graphs, I've recounted them and here is another post to give the picture for mid-saturday:



We need to see the low fail to make inroads into the UK.

Hope that clears things up.

 

 

Edited February 25, 2018 by Quicksilver1989

[Catacol]

OK - chores done and head clear after yesterday's rugby drama.

Let's dissect the later week - amazing to think that the next 72 hours are going to be very cold and snowy... and we arent even arguing about that! What about Friday?

Everyone is focusing on the low - but it isnt about the low. It's all about the block and related to that the cold in place that has been put there by the circulation pattern created by the block. If the block moves too far west it lowers pressure to the north and allows lower heights from the south in. But is the block being correctly modelled? How often have we seen a block poorly modelled? Often. Take a closer look at ECM for the key period - first the starting point at T+72

Very clear block to the north. near straight easterlies. 24 hours on -

swift draining on those heights to Greenland with door being gently opened. 24 hours on

Heights to the north gone completely. The block has meanwhile headed out west of Greenland. 

If Mogreps is in disagreement with this then I would suggest it has nothing to do with the low and everything to do with the speed of transition of that block. When I looked at this SSW last week I kept having a feeling that the strength of attack would SPEED patterns up - that the cold would come in a bit early. As we get closer to zero hour that theory has been blown out the water - as tends to happen with tradition west to east modelling the timescale has been dragged out, and where we were once seeing the very cold air arrive Sun into Mon it now will hit more Mon into Tues. 24 hours difference. The "jam tomorrow" scenario that frustrated posters often moan about as models are initially over quick at moving this on.

So - what about the speed of that block movement? I have no experience in an SSW like this - others are better and might chip in later... but I have rarely seen a block move this fast. We know that the Canadian vortex has taken a pounding and we have a pattern in reverse... but think about the cold air - it has arrived a little later than initially modelled. not unusual. If this block is being modelled to move west as little as 12 hours too fast at 144 hours range then the whole scenario changes. The low cannot go so far north and will have to slide more east. Is this what Mogreps is seeing better than the other models? Or are the Met so snowed under (ha!) by the next few days that they are spending scant time thinking or worrying about Friday. One can only guess at the requests for info that must be flooding in from major agencies in the SE alone. 

Experience suggests many things. Blocks often move slower than forecast. Breakdowns are often pushed back. GFS in particular often over models low pressure in the atlantic. Blah blah. Pattern nailed for next weekend?? Absolutely not. Absolutely in the balance. I'd quickly refer to the later text of the Met forecast also - the longer range doesnt even sniff at "mild" apart from the very brief reference to SW up to 10th March. The longer term solution presented by GFS of a low filling over western europe and dragging in SW air just doesnt seem to be a factor in their thinking. Again - wish we could see Mogreps... but I have to assume that a gently blocking signal is retained to the north to hold the track of the low pressures over south. If not - then that text forecast is going to change dramatically and quickly.

What about other factors? Attard SSW chart this morning is great

SSW to end a bit early, but recovering to paltry levels before dropping down again with a forecast second reversal (may or may not happen, but 10hpa forcing all but gone through the month) Looking at the laters throught the atmosphere we have a clear block sat over Europe at 10hpa with accompanying east to west frictional forcing on layers underneath

 

and further down at 100hpa clear signs of a retained gentle blocking signal over Greenland, storm track south and shattered and weakened vortex remnants leaving Canada, moving to northern Russia and introducing potential for reload cyclonic NE air

 

and what about at 2m level? Note the arctic today - above freezing!!

 

All the cold therefore firing out sideways, as inspired anyway by the vortex condition, with the result that any easterly airstream we might tap into further down the line is going to be more January than March in terms of feel, enhanced by convective activity on the rise thanks to strengthening sun.

In some respects we are getting a bit too sidetracked by Friday/Saturday. For those like me in the south it is hugely distracting because it represents the potential shattering of a blizzard based dream... but it ignores the near term significance of Tues-Thurs, ignores the longer term suggestions going through March, and ignores the reality that Friday could still see the 0 isotherm sit over northern France and a snow to snow event for all... if blocking is being overmodelled in speed of movement.

I'd pay a lot of money for winter synoptics like this every year. Let's try to enjoy them, even if rain sticks its unwelcome nose through the door at times.

 

EDIT - just noticed a change in MetO text for the weekend and beyond. Bah. That's not good. :-( Somerset cursed again?

Edited February 25, 2018 by Catacol

[swfc]

29 minutes ago, Catacol said:

OK - chores done and head clear after yesterday's rugby drama.

Let's dissect the later week - amazing to think that the next 72 hours are going to be very cold and snowy... and we arent even arguing about that! What about Friday?

Everyone is focusing on the low - but it isnt about the low. It's all about the block and related to that the cold in place that has been put there by the circulation pattern created by the block. If the block moves too far west it lowers pressure to the north and allows lower heights from the south in. But is the block being correctly modelled? How often have we seen a block poorly modelled? Often. Take a closer look at ECM for the key period - first the starting point at T+72

Very clear block to the north. near straight easterlies. 24 hours on -

swift draining on those heights to Greenland with door being gently opened. 24 hours on

Heights to the north gone completely. The block has meanwhile headed out west of Greenland. 

If Mogreps is in disagreement with this then I would suggest it has nothing to do with the low and everything to do with the speed of transition of that block. When I looked at this SSW last week I kept having a feeling that the strength of attack would SPEED patterns up - that the cold would come in a bit early. As we get closer to zero hour that theory has been blown out the water - as tends to happen with tradition west to east modelling the timescale has been dragged out, and where we were once seeing the very cold air arrive Sun into Mon it now will hit more Mon into Tues. 24 hours difference. The "jam tomorrow" scenario that frustrated posters often moan about as models are initially over quick at moving this on.

So - what about the speed of that block movement? I have no experience in an SSW like this - others are better and might chip in later... but I have rarely seen a block move this fast. We know that the Canadian vortex has taken a pounding and we have a pattern in reverse... but think about the cold air - it has arrived a little later than initially modelled. not unusual. If this block is being modelled to move west as little as 12 hours too fast at 144 hours range then the whole scenario changes. The low cannot go so far north and will have to slide more east. Is this what Mogreps is seeing better than the other models? Or are the Met so snowed under (ha!) by the next few days that they are spending scant time thinking or worrying about Friday. One can only guess at the requests for info that must be flooding in from major agencies in the SE alone. 

Experience suggests many things. Blocks often move slower than forecast. Breakdowns are often pushed back. GFS in particular often over models low pressure in the atlantic. Blah blah. Pattern nailed for next weekend?? Absolutely not. Absolutely in the balance. I'd quickly refer to the later text of the Met forecast also - the longer range doesnt even sniff at "mild" apart from the very brief reference to SW up to 10th March. The longer term solution presented by GFS of a low filling over western europe and dragging in SW air just doesnt seem to be a factor in their thinking. Again - wish we could see Mogreps... but I have to assume that a gently blocking signal is retained to the north to hold the track of the low pressures over south. If not - then that text forecast is going to change dramatically and quickly.

What about other factors? Attard SSW chart this morning is great

SSW to end a bit early, but recovering to paltry levels before dropping down again with a forecast second reversal (may or may not happen, but 10hpa forcing all but gone through the month) Looking at the laters throught the atmosphere we have a clear block sat over Europe at 10hpa with accompanying east to west frictional forcing on layers underneath

 

and further down at 100hpa clear signs of a retained gentle blocking signal over Greenland, storm track south and shattered and weakened vortex remnants leaving Canada, moving to northern Russia and introducing potential for reload cyclonic NE air

 

and what about at 2m level? Note the arctic today - above freezing!!

 

All the cold therefore firing out sideways, as inspired anyway by the vortex condition, with the result that any easterly airstream we might tap into further down the line is going to be more January than March in terms of feel, enhanced by convective activity on the rise thanks to strengthening sun.

In some respects we are getting a bit too sidetracked by Friday/Saturday. For those like me in the south it is hugely distracting because it represents the potential shattering of a blizzard based dream... but it ignores the near term significance of Tues-Thurs, ignores the longer term suggestions going through March, and ignores the reality that Friday could still see the 0 isotherm sit over northern France and a snow to snow event for all... if blocking is being overmodelled in speed of movement.

I'd pay a lot of money for winter synoptics like this every year. Let's try to enjoy them, even if rain sticks its unwelcome nose through the door at times.

 

EDIT - just noticed a change in MetO text for the weekend and beyond. Bah. That's not good. :-( Somerset cursed again?

but the extended period ete looks good

[johnwirral]

27 minutes ago, Philbill said:

As a scientist I can see why the arguments are occurring over just where this low pressure is going on Friday. Since most of the models are showing a breakdown to warmer air over the SW on Friday its a reasonable assumption. Also the ensembles would all be in agreement if the models are showing the same for most/all perturbations. However, these are extreme circumstances and we are 4/5 days away as more data is fed in closer to the time I expect them to correct South as I believe they are being overly progressive in pushing the cold air back. A 50-100 mile correction further South would mean blizzards for the South the like that have not been seen for a while. I don't think even with the model output we see now that anyone can say its "game over for the cold spell". Also its not as though the jet has changed and an Atlantic depression was being forecast. I think this is more difficult to call and the models are struggling a little. Interesting few days ahead.

As a scientist why you are using the term "correct" to indicate your favoured direction of travel for Thurs/Fridays low, models do not correct they are merely reacting to the input fed into them. 

I not sure I agree with your synopsis that due to unusual circumstances the models must be wrong , when there has been a clear trend for the low to push north over the last few runs. We won't know until nearer the time, so may be it can "correct" north,,,

[Frosty.]

Well I can't remember the last time we had such a dangerously exciting wintry spell to confidently look forward to but it's about to become very real!!..the 00z / 6z show severe winter just around the corner, snow showers start piling into the east tonight and then snow becomes increasingly disruptive through the week ahead climaxing in a potential almost nationwide EPIC blizzard later in the week and I haven't even mentioned the Easterly gales, severe drifting, severe wind chill and penetrating frosts..oh i just did!!..strap yourselves in for a spectacular week!❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄

Edited February 25, 2018 by Frosty.

[Philbill]

I did not say the models "must be wrong" I am saying its too early to tell which is exactly what you are saying. I just don't think the push North will be as much as shown. Of course you are also entitled to your opinion which is what is great about a forum. It promotes healthy debate as long as some people don't turn it into something personal

Edited February 25, 2018 by Philbill

[weirpig]

What a wonderful week of weather coming up. Even if we take the models at face value. We have very good uppers bitter windchill copious amounts of snow for some then the mother of all blizzards the end of the week then a gradual thaw possibly. I for one am amazed at the output  and to get this so late in the season is surely a bonus.  Wonderful. If the cold is going to breakdown what a evolution to have 

[johnwirral]

Nothing personal but one of my pet hates is the term "correction" it implies that it is correct for the low to move south but not north, when at this stage it's too early to tell.

The GFS overdoes the precipitation, but can be good at spotting a breakdown, I seem to remember is was one of the first ones to spot the end of the 2010 cold spell, although I stand to be corrected. 

Edited February 25, 2018 by johnwirral

[Summer Sun]

Updated NAO showing a rise in early March looks like the peak will be -2 on the 1st

[Philbill]

1 minute ago, weirpig said:

What a wonderful week of weather coming up. Even if we take the models at face value. We have very good uppers bitter windchill copious amounts of snow for some then the mother of all blizzards the end of the week then a gradual thaw possibly. I for one am amazed at the output  and to get this so late in the season is surely a bonus.  Wonderful. If the cold is going to breakdown what a evolution to have 

Yes its hard to believe when you look at the weather being predicted. I think on the whole the models have done quite well in their predicted outcomes for this cold spell.

[winterof79]

AO hardly recovering to +ve

[stokepa31]

Right, time to see what ive learned over the last few weeks of watching this develop. Ill make a statement below and you can tell me if im being overly simplistic with my thoughts.

Going back a week  we had charts that showed the greenland high in close proximity to out NW shores, indeed some were concerned that it was too close and would force the beast to track too far south with us missing the closest air. Over the last few days it would seem the GH has changed position and alignment allowing the beast to score a direct hit but at the same time progress westerly at a faster pace. This fast pace of travel may now lead to an easier ingress of low pressure warmer air into the south as the main thrust of cold air has accelerated through into the atlantic leaving a chink in the flank so to speak. 

That could be tosh but it will help me to get some feedback. 

Edited February 25, 2018 by stokepa31
sp

[Catacol]

Hmmm - this ensemble mean from Friday (thanks to @MetmanJames) seems a long way away at the moment - how fast have the models turned around?!!!!!

 

Edited February 25, 2018 by Catacol

[Catacol]

9 minutes ago, stokepa31 said:

Right, time to see what ive learned over the last few weeks of watching this develop. Ill make a statement below and you can tell me if im being overly simplistic with my thoughts.

Going back a week  we had charts that showed the greenland high in close proximity to out NW shores, indeed some were concerned that it was too close and would force the beast to track too far south with us missing the closest air. Over the last few days it would seem the GH has changed position and alignment allowing the beast to score a direct hit but at the same time progress easterly at a faster pace. This fast pace of travel may now lead to an easier ingress of low pressure warmer air into the south as the main thrust of cold air has accelerated through into the atlantic leaving a chink in the flank so to speak. 

That could be tosh but it will help me to get some feedback. 

Absolutely spot on - but westerly block movement, not easterly... Heights to the north draining away too fast which leaves the flank exposed.

Edited February 25, 2018 by Catacol

[stokepa31]

2 minutes ago, Catacol said:

Absolutely spot on - but westerly block movement, not easterly... Heights to the north draining away too fast which leaves the flank exposed.

sorry yes i meant east to west faster not east. DOH

[mike Meehan]

Looks like an interesting week coming up - the lucky ones in the east can probably look forward to streamers from off the channel and the North Sea, which often give rise to an unpredictable amount of snow in some areas.  

Sometimes I have seen the forecasters caught out when the amount exceeds expectations but the conditions have to be just right for this. 

In my case being at Watford I am in hopes of a Thames Streamer but it depends a lot on wind direction to gain the full benefit and it can make the difference between just a covering and several inches.

We will see what we will see. Should it happen hoping for the powdery stuff,  but it needs the temps to stay below freezing

As it is a beautiful blue sky today - temp 3.2C with a 55% humidity.