Model output discussion - heading into 2018

[stodge]

Afternoon all

We're not even a third of the way through winter and the usual old moaners and miseries are on complaining about the absence of 20 foot snow drifts outside their back door, storm force easterlies and minus 20 uppers at T+6 which will no doubt get downgraded..

I'm nowhere near as pessimistic even though it's entirely possible I've had my snow for the winter here in lowland East London.

As I keep saying, the route to very cold starts from very mild and there may well be a period of pain to endure before the result so many on here want. The GEFS shows a (to me) growing cluster offering heights to the NE into the second week of January and this was the time scale advertised initially by those who argued for a typical Nina winter.

I hoped for a 10-14 day milder interlude but have always suspected it could be nearer 28 days before the pattern changed or relaxed sufficiently to break the Atlantic and let other influences gain some traction.

The build of heights back into Europe and the evolution of a blocking HP (favourably located in time) seems that route while the continuing pattern of a southerly jet and colder outbreaks of PM air will no doubt deliver transient snow to many (and rather more to hose with altitude in the north) it's not, I suspect, what many really want.

The synoptics to deliver a 10-15 day nationwide very cold spell with snow don't happen often - the fact we remember the times when they do speaks volumes - but they've happened more often further into winter than now. To seemingly right off the season before we're even one third of the way through and especially when many have seen snow lying seems curious. 

The 12Z suite yesterday was excellent for cold weather fans - let's see if this continues and no doubt someone will jump in at T+96 and declare the run useless because one LP is 50 miles further north.  

[bluearmy]

13 minutes ago, stodge said:

The 12Z suite yesterday was excellent for cold weather fans - let's see if this continues and no doubt someone will jump in at T+96 and declare the run useless because one LP is 50 miles further north.  

does that tell us how high the bar currently is ?

we aren't past the point that one of the models pulls a rabbit out of the hat, given the upcoming amplified polar ridge. However, I would say today's 12z will take us beyond and then coldies are looking for either the flow to remain flat and therefore the upper trough to be on average, south of the uk allowing for runners in the base or possible sliders or for a decent amplification in week 2 of the Azores ridge which could evolve from a MLB to something more interesting. given the forecast strengthening of the strat flow, I think the latter isn't a great option although it could lead to cold settled which some would prefer. 

[ACCOUNT DELETED]

UKMO has the rain moving into Wales later on Christmas Day to allow a dry morning with sunny spells, similar on Boxing Day, dry and sunny next Wednesday. Great news for those getting bikes, scooters etc from Santa 

Edited December 23, 2017 by Draig Goch

[Paul]

3 hours ago, johnholmes said:

Best you ask Paul but Net Wx outputs are based on GFS data with software belonging to Net Wx as far as I know?

 

The NetWx models use GFS as starting data but have their own physics etc, and run at 3km / 9km so will show quite different output to the GFS. 

In terms of the NMM, is worth bearing in mind that there are many flavours of it out there, the one on Meteociel for instance will use different physics to ours, so again the output can be quite different. We've done a fair bit of work tuning for the UK. 

[ACCOUNT DELETED]

GFS ensembles 192hrs for Porthcawl near Swansea follow UKMO again and indicate a colder but drier midweek with a ridge of high pressure, so I'm slowly gaining more confidence in this and after this dull, dark week with frequent fog & drizzle it will come as a welcome relief for many to see drier, brighter and indeed sunnier weather.

[Man With Beard]

1 hour ago, johnholmes said:

I'm not sure about having slack westerlies based on looking at 500mb anomaly charts. All look to have a fairly strong westerly flow into the UK and Europe in the next 2 weeks. Some surface lows are going to be pretty deep with gale force winds associated with them, especially in the NW'ern third of the country. But, yes, ridges behind these systems may give 24-48 hours of colder conditions, even with snow in places, largely higher ground in the northern half of the UK but some might get lucky elsewhere. There is no sign, to me, though of anything that could lead to any deep cold of several days or more at the moment.

I think I really meant to say "slacker than before" rather than "slack" ... Always getting into trouble with you, aren't I

[fromey]

30 minutes ago, Draig Goch said:

GFS ensembles 192hrs for Porthcawl near Swansea follow UKMO again and indicate a colder but drier midweek with a ridge of high pressure, so I'm slowly gaining more confidence in this and after this dull, dark week with frequent fog & drizzle it will come as a welcome relief for many to see drier, brighter and indeed sunnier weather.

That also is what Ian f has just tweeted( his tweet is on the Twitter thread) temps falling fairly below avg tues/wed/Thursday rain/snow 

[shaky]

Gfs slightly further south again!!man these sliders take it out of u lol!!

[booferking]

Sliders full of rain & slush 

Sorry for those confused slush is very wet snow that melts quickly & rain is just plain old rain.

Edited December 23, 2017 by booferking

[bluearmy]

Meteociel ukmo is currently  faulty before anyone comments ....

[SylvainTV]

10 minutes ago, bluearmy said:

Meteociel ukmo is currently  faulty before anyone comments ....

Yep, sorry about that, I'm regenerating charts.. But maybe all data won't be there.

 

72-144h data Z500-PRMSL is ok now. For some reason, I got data from last month...

Edited December 23, 2017 by SylvainTV

[booferking]

UKMO working fine on wetterzentrale so no data issue.

[tight isobar]

Gfs 12z has shunted slider LP further south...

And we know the gfs is a little wobbly with precip rates/types..

However if its anywhere near on the money' there is some very notable falling snow rates as the system heads east into the north sea...

And east midlands right down across home counties and across east anglia through to kent may get a dumping...

Again all speculative atm but coming into better range now.

Edited December 23, 2017 by tight isobar

[knocker]

[Chesil View]

22 minutes ago, booferking said:

UKMO working fine on wetterzentrale so no data issue.

Indeed Boofer and at T96 ukmo near enough has a fabled crosspolar flow. Shame it doesn't last

[Guest]

SLIDERGATE 2 VERSUS SLIDERGATE 1

I know that there has already been considerable discussion/debate/explanations/agreement/disagreement from many on here regarding how much if any wintry precipitation might be achieved out of next week's predicted slider low. While no two similar set ups are ever identical we can learn a lot with the benefit of hindsight and then look for the differences. Unfortunately, some of these may well be critical as a few posters have already alluded to some of them. Now, everyone who reads my posts knows that I'm a "coldie" and I love snowy weather too and I do not wish to dampen any excitement and Christmas spirits. Let's have a much closer look at conditions during the "slider 1" event and then compare these to the forecast conditions for the "slider 2" event. At least this way, we'll have a better idea of what we might need to expect for next week (assuming that the slider 2 LP does develop and cross the UK on one track or another as predicted). 

SLIDER 1:

Surface Pressure:   

         Saturday  Dec 9th 0050                        Sunday  Dec 10th 0050                         Monday  Dec 11th 0050

                              

It's useful to remind ourselves of the path of the slider low. I show the position just prior to the event, in the middle of it and towards the end.

Surface Temperatures:     

          Saturday  Dec 9th 0050                      Sunday  Dec 10th 0050                          Monday  Dec 11th 0050

                              

I am using the "Europe" view maps as these are highly relevant to why we managed to achieve a fairly widespread snow event. Remember, following a series of Arctic outbreaks, the northerly that pushed across the whole UK during the course of Thursday, December 7th, was rather more potent than its predecessors. There were increasingly sharp and widespread frosts (away from exposed coasts) from Friday to Sunday night. Some favoured/exposed areas (such as northern Scotland, Northern Ireland, north-west England through the Cheshire gap into the north and west Midlands as well as parts of northern and western Wales) already had some decent snow accumulations from the snow showers from late Thursday, through Friday until they gradually died out during Saturday. The ridge ahead of the slider produced calmer conditions and many places had a harder frost (especially those under any deeper snow cover). On early Sunday, milder air moved into the south-west but did not encroach any further. The slider was sucking in some of the colder air ahead of it, not just from central and eastern parts of the UK but also from the nearby continent. 

Surface Dew Point Temperatures:     

          Saturday  Dec 9th 0050                      Sunday  Dec 10th 0050                         Monday  Dec 11th 0050

                                  

When we are looking for snowfall rather than rainfall we know that conditions in the UK can often be extremely marginal.  We look for the upper temperatures, the sub 528 dam lines as well as surface temperatures. In many set ups these can be the most important  prerequisites but as @johnholmes frequently reminds us all, with slider events and undercuts, dew point and wet bulb thermometer temperatures can be critical and sometimes snow events can occur with much higher uppers as the LP increasingly engages with the colder air ahead of it (there is a link somewhere to an extremely useful archived part of this forum which John often directs us to but I cannot locate it right now - it provides an excellent account of the required conditions for marginal snow events; perhaps John or someone else could post this again). I cannot find wet bulb readings for the key dates (perhaps someone else can?) but I show the dew points above. These were well below 0c over much of England on Saturday and Sunday (ahead of the slider LP's arrival) and slightly below 0c on the nearby continent such as the Benelux countries. These sub 0c dew points were really key to turning the precipitation to snow over quite a wide area.

850 Upper Temperatures:   

        Saturday  Dec 9th 0100                        Sunday  Dec 10th 0100                       Monday  Dec 11th 0100 

                                   

Here I've used the ECM archived charts with "T+0" hours to get the then "current" position. On Friday into Saturday much of the country had been under -6c 850s with small parts under sub -8c 850s. By early Sunday you can see the 0c and higher 850s approaching the south-west of the UK on the milder side of the slider LP. This just got into the extreme south-west for a few hours during Sunday but "slid" away south-eastwards in association with the track of the LP (hence the name) usually happens when the LP is engaging the cold air in front of it . I did a post on the "Profile of a Slider" just before this event on the recently closed model thread. The sub -6c 850s are not only hanging on (and there are still parts with sub -8c 850s) but they are starting to pivot over central parts of the UK and return south-westwards. There is a little mixing out of both the upper and surface air within the circulation of the LP system and by early Monday, southern England is closer to -2c 850s, with much of the UK under sub -4c and some parts still under sub-6c 850s.

Precipitation - Rainfall and Snowfall Radar:

           Saturday  Dec 9th 0100                      Sunday  Dec 10th 0100                  Sunday  Dec 10th 1300                       Monday  Dec 11th 0100    

                                    

This demonstrates the progress of the precipitation - green for rain and pink for snow with the darker colours representing the heavier precipitation. The remaining snow showers were dying out during Saturday and by early Sunday, rain had moved into the south-west and just starting to turn to snow on its north-eastern edge. With a standard LP approaching from the west or south-west on a north-east trajectory the milder air would push across much of the country with (given the same temperature profile) some transient snowfall on the leading edge but progressively turning back to rain. On the other hand with a slider LP (assuming it maintains its generally south-eastward trajectory) far more of the rain "can" turn to snow (again under conditions which are similar to those described above). This can produce some quite widespread snowfall as the Sunday1300 chart shows. The slider also brings back the cold air in its rear and you can see how it returned southwards leaving rather more of southern England with snowfall than had been forecast even at T+0! Marginal rain/snow, with so many factors involved, is almost always one of the most difficult events to forecast accurately. I will not show the snow accumulation charts or provide any snow amounts IMBY accounts as that's not appropriate for this thread. There are other factors - i didn't show the 528 dam lines, although these can be rather less important with slider events where the near to surface conditions are key (of course you need a cold enough general set up in the first place). The intensity of the precipitation is also very important. Parts of London and the south-east saw bouts of heavier rain which quickly turned to snow - as it eased it often turned back to rain again. Slower moving events with very light winds can allow for "evaporative cooling" and steady rain at the surface with temperatures as high as 6c or 7c can steadily fall towards 0c over the course of several hours. The snow at the higher levels (say 1000 + metres) gradually cools the layers as it melts and evaporates and eventually quite heavy snow "can" fall at the surface. Philip Eden wrote a great article on one of the best recent examples of this type of event back in 2004 (here's the link: https://www.weatheronline.co.uk/reports/philip-eden/Absorbing-latent-heat.htm ). A few of you already know that Philip and I used to research frost hollows and we set up a weather station (mostly to record temperatures) in the famous Rickmansworth frost hollow (in the back garden of a friend of mine) just as the renowned meteorologist "Eric Hawke" had done back in the the 1930s and1940s (he was the first ever president of the Royal Meteorological Society). The station was closed when my friend moved house and then I moved down to Devon and lost touch with Philip. Sadly, I've heard that he has been suffering from a very debilitating form of dementia for the last few years which explains his disappearance from the media and the public eye. I think that "slider" events are usually a little too mobile (not fast but they make steady progress) and this would probably rule out most (if not all) of the potential for any evaporative cooling to last long enough to make an impact.

SLIDER 2  

Now that  I've diagnosed and dissected the "slider 1" event, I hope that this gives a better idea of what to look out for should the generally predicted synoptic pattern for next week materialise and produces the second slider event of this month (I remember well over 10 of them during the epic winter of 1962/63). I will not look at the pressure patterns as the final track will probably not be nailed down until around T+24 - I'll leave that up to the reat of you who are interested in this. "Slider 2" may quite literally end up as a bit of a "damp squib". Much of the precipitation may well fall as rain or perhaps sleet with any snow likely to be confined to high ground. One thing that I've learnt in my many years of following the weather is not to rule anything out, particularly in very marginal set ups and I would love to proven wrong. Remember that we ideally need cold air ahead of the system both over the UK and the closer continent. Will there be sufficient cold air in place? Will the dew points be low enough? I'll just show a handful of charts to give an idea of what is currently indicated. I'll use two time frames of T+72 and T+96 (from 0z output, adjusted to T+66 and T+90 for 6z output) which, depending on the model, would appear to be around the period the LP is expecting to track somewhere across the UK. 

850 Upper Temperatures:   

ECM Tuesday Dec 26th 0100 ECM Wednesday Dec 27th 0100 UKMO Tuesday Dec 26th 0100 UKMO Wednesday Dec 27th 0100   GEFS 6z Ensembles      

       

Just sub -2c to sub -4c 850s moving into the UK replacing values that were over 0c. I also show the GEFS ensemble chart for the West Midlands (deliberately a central location) and the mean 850s do fall to -5c early on Dec 27th (probably the only marginally more positive indicator).

 2m Surface Temperatures:   

GFS - Tuesday Dec 26th 0100  GFS - Wednesday Dec 27th    UKMO - Tuesday Dec 26th 0100  UKMO - Wednesday Dec 27th          GEFS 6z Ensembles     

             

These surface temperature charts in this format are not available on Meteoceil for ECM and UKMO (I will rely on Karl, @Frosty. to provide his excellent detailed charts for all the finer detail - if he hasn't already while I've been preparing this post!). So instead I show GFS charts (slider taking a more northerly route on their 6z but with another LP following closely behind further south) and GEFS charts (slider closer to ECM and UKMO solutions). These are 1 am charts and 4c to 6c generally looks too high - just closer to 0c in Scotland No cold air over the nearby continent. The GEFS ensemble West Midlands chart shows the surface temperatures falling to 3c early on Dec 27th but only falling towards 0c after the slider has passed. This is an indication that the slider is introducing rather colder air behind it. I'm not sure if this might provide an outside chance of some back edge wintry precipitation - one to watch.

Surface Dew Point Temperatures: 

Weerplaza - mid Netherlands ensembles:

I found it difficult to find any meaningful dew point forecast charts for 3 to 4 days out but, after digging a little deeper, I found this for The Netherlands and this is useful for checking if any sub 0c dew point values are available to suck into the slider LP circulation.  .

      

The dew points are above zero during the predicted slider event and only trending down below zero the following day. This is an ensemble chart and a few members do drop below zero on Dec 26th and Dec 27th.

That's more than enough. On all this current evidence, I would be very surprised if we manage any lowland snowfall in central or southern districts but I'll leave it up to the regular "snow chasers" to monitor any moves in the right direction in the forthcoming model output.. 
 

Edited December 23, 2017 by Guest
Correct typos and check charts

[knocker]

Will it snow? (thank the lord for the retired creme de la creme from the METO)

https://www.netweather.tv/forum/topic/23729-will-it-snow/

Edited December 23, 2017 by knocker

[snowbunting]

20 minutes ago, knocker said:

Think these images say it all. It really is winter on a knife edge. The charts keep things very unsettled with a strong NWly output theme remaining strong.  

[Daniel*]

I feel too much emphasis is put on the need for embedded cold, it isn’t essential however with convective PPN I’d agree what I see there is a reasonable chance of back edge snow, across southern/eastern districts as the low pulls away into the near continent ushering in a much colder air mass for wintry precipitation.

Ideally we want the low to be as shallow as possible this makes it more conducive for evaporative cooling also I feel timing will be important.

Edited December 23, 2017 by Daniel*

[Daniel*]

@Bring Back1962-63

I’d much rather trust the data fed into bbc weather app than GFS / GEFS control temperatures projections I’m from a quite inner London location and I have a high of 4C on Wednesday. Same last time and there was some decent snowfall for my location albeit not much settled, however it did rain for many hours. This will change..

[snowray]

This is the sort of little feature that gives snowfall to many of us when easter is in April, could snow just about anywhere I think next week including late on Christmas day.

[comet]

1 hour ago, bluearmy said:

Meteociel ukmo is currently  faulty before anyone comments ....

Again very happy how things (including how the models may play out ) for next week with a good possibility of snowfall for a number of locations albeit rather Temporary.

[Guest Durham Weather]

11 minutes ago, snowray said:

This is the sort of little feature that gives snowfall to many of us when easter is in April, could snow just about anywhere I think next week including late on Christmas day.

 

 

Nice little patch of pink there over Durham and North Yorkshire. In my experience we’d need an onshore N-Easterly for that to come about. Is that likely do you think?

[snowray]

1 minute ago, Larger than average Hobo said:

Nice little patch of pink there over Durham and North Yorkshire. In my experience we’d need an onshore N-Easterly for that to come about. Is that likely do you think?

No its a Northwesterly thats bringing the colder temps in, a N Easterly coming in from the sea would produce more in the way of rain/sleet I would have thought because this is pretty marginal stuff.

[johnholmes]

45 minutes ago, knocker said:

Will it snow? (thank the lord for the retired creme de la creme from the METO)

https://www.netweather.tv/forum/topic/23729-will-it-snow/

gawd lad you will have some spilling their coffee reading that about me!

It is worth looking at though folks and does work 'in your backyard' IF the model is showing ppn over it, also factor in your height, it can make a difference, finally obshore winds around lows tend to give sleet or rain rather than snow on the coast but this changes as one moves inland even without elevation to wet snow at leat.

also sorry MWB nothing personal.

It will be worth watching how the Fax charts, for the same time, change over the next 24 hours and of course the actual on the day.

 

Edited December 23, 2017 by johnholmes