Short range model discussion - into 2018

[sawan]

10 minutes ago, knocker said:

Further to the above the occlusion is over Ireland by Tuesday 00 a belt of rain/sleet/snow is lying down England, Scotland Wales having cleared N. Ireland. This is another marginal scenario vis snow but certainly quite possible in northern areas particularly on high ground.

Wind on 24 hours and 1200 Wednesday has a very deep depression south of Iceland with the associated fronts bringing strong winds and rain to all parts of the UK but quite likely sleet and snow preceding the occlusion in places. Firming up on the detail of  possibly tomorrow.

 

 

Hi Knocks, going through your posts the most abundantly used word is 'marginal'. Do you think this winter has been on the boundary line set up? I can't remember any 'knife edge' winters like this since I have developed this odd passion about the weather

[knocker]

9 minutes ago, sawan said:

Hi Knocks, going through your posts the most abundantly used word is 'marginal'. Do you think this winter has been on the boundary line set up? I can't remember any 'knife edge' winters like this since I have developed this odd passion about the weather

I think it more likely it's the way the dice have fallen and we have often been under the influence of a cold Pm airflow which lends itself to marginal snow events.I can't really comment on previous winters as I'm not much of stats/records person and I'm afraid my memory is failing these days

[knocker]

Still no agreement between the anomalies this evening although I'm not sure the differences are as great they appear.

Upstream they are in good agreement with the Siberian vortex lobe. Aleutian ridge into the Arctic and still an intense Canadian vortex with a strong upper westerly leaving the eastern seaboard across the Atlantic.  But downstream the ecm takes a slightly different stance and doesn't make so much of the east European ridge, and in particular the positive anomalies and ridging in the Franz Joseph and Svalbard regions as the GEFS and some extent NOAA. Thus with the vortex/Iceland trough continuing to be quite influential the flow across the Atlantic remains quite zonal with the high pressure suppressed. .Whereas the former two initiate some ridging of the Azores in the vicinity of the UK. It would not take a major correction to get more agreement on this, The det runs will need to pick the bones out of it but taking a punt I would go for unsettled becoming more settled later on in the ten day period, particularly in the south with temps remaining below average.

 

[davehsug]

I still can't equate the predicted temperatures of 5-6 on BBC weather, tomorrow to Wednesday, with the risk of any snow. Are the new providers deliberately inflating temperatures? Given uppers around -8, 1-3 is surely where we should be at?

[The Enforcer]

7 hours ago, knocker said:

Further to the above the occlusion is over Ireland by Tuesday 00 a belt of rain/sleet/snow is lying down England, Scotland Wales having cleared N. Ireland. This is another marginal scenario vis snow but certainly quite possible in northern areas particularly on high ground.

I have not been expecting much from these continual polar maritime frontal attacks, even with the colder than usual uppers on offer. However, I am surprised that the Tuesday front isn't producing strong forescasts/warnings for a widespread snow-rain-snow event. This is because:

1. there is the current PM air ahead of the front;

2. the mild sector is pretty narrow (compare with yesterday and this Wednesday) and tame, as evidenced by the occlusion (but it's there hence the 'rain' in the sandwich)

3. The front then appears to stall as it is still shown over central uk on the 12pm T+48 hours Fax;

4. there is more PM air undercutting on the front's western flank; and

5. The precipitation intensity appears to be there unlike last Tuesday (when otherwise a snow event would have occurred);

Hopefully there will be some micro-scale adjustments to help make this a reality.

Edited February 11, 2018 by The Enforcer

[knocker]

Today will start with a reasonable severe frost in many places compounded by ice in many western and northern areas where snow showers have persisted overnight. These showers will continue during the morning in these areas, more particularly west Wales, N. Ireland, NW England and Scotland whilst most other areas will be quite sunny if a little cold. The showers will become less frequent later as the surface wind backs south westerly with arrival of the occlusion west of Ireland associated with a depression 972mb to the NW

By midnight the front is over Ireland with the triple point over Cork and the band of rain, sleet and snow will move steadily east overnight and through Tuesday. There could well be some significant snow falls in Scotland and perhaps the north, mainly on the higher ground but not exclusively, with some quite strong winds in the south. Clearer weather behind as the front clears to the east with some wintry showers mainly in the north west

But by 0600 on Wednesday there is a deep low 944mb SWS of Iceland with the associated fronts just to the west of the UK with the wind once again picking up from the south west. By midday the frontal systems are over the UK along with the associated precipitation.

The question is once again will the precipitation be of snow in places and the answer is probably some preceding the front in the north but in general the temps/dew points will be too high and rain will be the order of the day.

Once all of this has cleared to the east a strong westerly wind prevails with frequent wintry showers in western Scotland with cold Pm air dominating the Atlantic By Friday quieter weather will have arrived in the south as the westerly abates with the arrival of a little high cell

 

Edited February 12, 2018 by knocker

[The Enforcer]

8 hours ago, The Enforcer said:

I have not been expecting much from these continual polar maritime frontal attacks, even with the colder than usual uppers on offer. However, I am surprised that the Tuesday front isn't producing strong forescasts/warnings for a widespread snow-rain-snow event. This is because:

1. there is the current PM air ahead of the front;

2. the mild sector is pretty narrow (compare with yesterday and this Wednesday) and tame, as evidenced by the occlusion (but it's there hence the 'rain' in the sandwich)

3. The front then appears to stall as it is still shown over central uk on the 12pm T+48 hours Fax;

4. there is more PM air undercutting on the front's western flank; and

5. The precipitation intensity appears to be there unlike last Tuesday (when otherwise a snow event would have occurred);

Hopefully there will be some micro-scale adjustments to help make this a reality.

I thought of a sixth factor:

6. The air ahead of the front is being dragged up over France, which is a very cold landmass at the moment.

[knocker]

7 minutes ago, The Enforcer said:

I thought of a sixth factor:

6. The air ahead of the front is being dragged up over France, which is a very cold landmass at the moment.

Is that strictly true as the colder air to the south is quickly shunted east as the warmer  (relatively speaking) air from the south west moves in? As far as I'm aware some quite widespread snow in northern areas is expected.

[The Enforcer]

16 minutes ago, knocker said:

Is that strictly true as the colder air to the south is quickly shunted east as the warmer  (relatively speaking) air from the south west moves in? As far as I'm aware some quite widespread snow in northern areas is expected.

Well I thought about that, but looking at Euro 4 there is snow forecast from the front in northern France right up to the coast, which suggests otherwise?

[knocker]

6 minutes ago, The Enforcer said:

Well I thought about that, but looking at Euro 4 there is snow forecast from the front in northern France right up to the coast, which suggests otherwise?

You may well be right but it's not that obvious to me looking at the forecast sounding for the south.

Edited February 12, 2018 by knocker

[The Enforcer]

23 minutes ago, knocker said:

You may well be right but it's not that obvious to me looking at the forecast sounding for the south.

I can't pretend to fully understand that chart, but it still strikes me that there is an inconsistency in the modelling, where it shows similar uppers over northern France and Central Southern England (arguably they remain lower in the SE corner of the latter). If the colder air source from northern France is swept away so as modelled it doesn't snow inland at all in CSE, I can't fathom how it can then still be a major snow event on the northern coast of France?

[knocker]

3 hours ago, The Enforcer said:

I can't pretend to fully understand that chart, but it still strikes me that there is an inconsistency in the modelling, where it shows similar uppers over northern France and Central Southern England (arguably they remain lower in the SE corner of the latter). If the colder air source from northern France is swept away so as modelled it doesn't snow inland at all in CSE, I can't fathom how it can then still be a major snow event on the northern coast of France?

Well I can't really add any more but if you take two samples one in France in the heavy snow the is temp 1.7C and dew point 0C and same time, 1200, over central southern England it is 5.2C and 2,6. This will vary as you move the position slightly. Thus the colder surface air in situ may swing it and that's always assuming the model is correct which is not a given in marginal snow events.

Edited February 12, 2018 by knocker

[knocker]

The midday geostationary (courtesy Dundee Satellite Receiving Station) showing clear skies over much of England and the next frontal systems on there way,

And the fax update for 06 with developing wave just to complicate matters

[The Enforcer]

10 minutes ago, knocker said:

Well I can't really add any more but if you take two samples one in France in the heavy snow the is temp 1.7C and dew point 0C and same time, 1200, over central southern England it is 5.2C and 2,6. This will vary as you move the position slightly. Thus the colder surface air in situ may swing it and that's always assuming the model is correct which is not a given in marginal snow events.

Thanks for the reply. I totally get why France is snow and CSE is rain under those conditions, but I am not getting why there are such big differences in these conditions. The undercut of PM air from the north-west arrives in the UK before France as well. What is especially suspect about the particular chart you posted is that it is even showing snow over the channel at 0m ASL, but at the same time nothing at all over the highest ground of East Sussex and Kent, where surely upper air, 2 metres and dew points conditions would all be more favourable.

[The Enforcer]

21 minutes ago, knocker said:

And the fax update for 06 with developing wave just to complicate matters

At a very basic level, does the wave development cause the front to the south of the low to have a greater negative tilt (NW to SE rather than N to S), thereby introducing a more continental airflow and increasing the chances of snowfall?

[knocker]

1 minute ago, The Enforcer said:

At a very basic level, does the wave development cause the front to the south of the low to have a greater negative tilt (NW to SE rather than N to S), thereby introducing a more continental airflow and increasing the chances of snowfall?

I'm afraid the dynamics of the frontal wave are a tad above my pay grade but yes that's possible as the front slows and tilts but whether that increases the chances of snowfall is another matter, and if it did it would more than likely be in the northern sector of the wave. But really this is more in JH's area of expertise and if he spots it he might help out

[johnholmes]

As any front moves in then the winds ahead of it BACK, (you call it giving the flow a -ve tilt), yet another buzz word that has developed on here. Not your fault but I do wish folk would be less prone to these phrases.

As to will it bring colder air in from France or further east. If it stalled, little movement, then it could but I doubt that will occur sufficiently to cause that. 

Another thing that 'might' happen is evaporative cooling 'if' the rate of rainfall is high enough. Best to wait to watch the radar in the morning for that item.

Hope that helps a bit.

Sorry for sounding off, not your fault.

[johnholmes]

1 hour ago, The Enforcer said:

Thanks for the reply. I totally get why France is snow and CSE is rain under those conditions, but I am not getting why there are such big differences in these conditions. The undercut of PM air from the north-west arrives in the UK before France as well. What is especially suspect about the particular chart you posted is that it is even showing snow over the channel at 0m ASL, but at the same time nothing at all over the highest ground of East Sussex and Kent, where surely upper air, 2 metres and dew points conditions would all be more favourable.

Is that what the model shows for T and Td?

[The Enforcer]

2 minutes ago, johnholmes said:

As any front moves in then the winds ahead of it BACK, (you call it giving the flow a -ve tilt), yet another buzz word that has developed on here. Not your fault but I do wish folk would be less prone to these phrases.

As to will it bring colder air in from France or further east. If it stalled, little movement, then it could but I doubt that will occur sufficiently to cause that. 

Another thing that 'might' happen is evaporative cooling 'if' the rate of rainfall is high enough. Best to wait to watch the radar in the morning for that item.

Hope that helps a bit.

Sorry for sounding off, not your fault.

Thanks for your reply and no offence taken. What do you make of the back winds not producing any snow over higher ground in the SE corner, closest to the continent, but showing snow at 0m ASL over the channel? Surely an error?

[The Enforcer]

3 minutes ago, johnholmes said:

Is that what the model shows for T and Td?

Sorry, I don't know what T and Td are?

[Paul]

5 minutes ago, The Enforcer said:

Sorry, I don't know what T and Td are?

Temperature and dewpoint  

[swebby]

1 hour ago, johnholmes said:

As any front moves in then the winds ahead of it BACK, (you call it giving the flow a -ve tilt), yet another buzz word that has developed on here. Not your fault but I do wish folk would be less prone to these phrases.

As to will it bring colder air in from France or further east. If it stalled, little movement, then it could but I doubt that will occur sufficiently to cause that. 

Another thing that 'might' happen is evaporative cooling 'if' the rate of rainfall is high enough. Best to wait to watch the radar in the morning for that item.

Hope that helps a bit.

Sorry for sounding off, not your fault.

Ian Fergusson has flagged this in a tweet.  He's mentioned small low pressure centres embedded in the front (the largest being the wave Knocker mentioned a few posts above) slackening the flow in phase with the heavier ppt = possibly wintry. I assume this is going to be one where looking out of the window will turn out to be the most reliable option.

link to tweet https://twitter.com/fergieweather/status/962824679814811650/photo/1

Edited February 12, 2018 by swebby
added link

[johnholmes]

1 hour ago, The Enforcer said:

Thanks for your reply and no offence taken. What do you make of the back winds not producing any snow over higher ground in the SE corner, closest to the continent, but showing snow at 0m ASL over the channel? Surely an error?

I don't think so, air for over the Channel is being sourced from a colder area of the continent, its passage over the Channel may well be just sufficient to change the bottom 1-2,000 ft or so of the air to cause rain or sleet rather than snow.

Predicting snow is extremely difficult, other than in large continents and mountain areas.

[johnholmes]

8 minutes ago, swebby said:

Ian Fergusson has flagged this in a tweet.  He's mentioned small low pressure centres embedded in the front (the largest being the wave Knocker mentioned a few posts above) slackening the flow in phase with the heavier ppt = possibly wintry. I assume this is going to be one where looking out of the window will turn out to be the most reliable option.

link to tweet https://twitter.com/fergieweather/status/962824679814811650/photo/1

Notice in that Arpege prediction there is little backing occurring in the wind flow ahead of those minor lows.

To me the change, for a short time, from rain to snow, is the evaporative cooling occurring, due to heavier rain causing the air to cool. A well known phenomena but hard to predict more than a few hours ahead.

Edited February 12, 2018 by johnholmes

[chrisbell-nottheweatherman]

11 minutes ago, johnholmes said:

Notice in that Arpege prediction there is little backing occurring in the wind flow ahead of those minor lows.

 

Indeed, John - there's a pronounced veer behind the front but no obvious backing ahead of it; is this unusual, and do you have any idea what might cause it?