Norrly Topplers!

[summer blizzard]

Hi, i have opened this thread, not to complain at how northerlies are always downgraded, but to see how the modeling of a northerly changes from day to day as it is constantly upgraded/downgraded, to illustrate this, i have picked Sunday 2nd September 12Z as my timeframe, and have chosen the FAX charts as my source because these have an element of human skill involved...

Here is the first available chart for that timeframe...

Points of note from this are:

A central pressure of 983mb-986mb..

3 troughs in the flow and the cold front extending as far west as England..

Please discuss, and i will post the next chart tommorow evening...

[johnholmes]

first point are the 3 troughs?

Why 3, why not 6 or better still why not 1 to show what is really likely to happen, ie: the change of wind and the more wintry nature of any showers in the FAR north of the chart behind it.

I've never understood how there can be so many troughs on a chart so far ahead. 24 hours possibly, 12 hours perhaps more so but not at this range.

so much for me rushing to support the Met O!

[Nick F]

I guess the troughs are drawn on looking at the kinks in the computer generated isobars, or if upper troughs are indicated, drawn on looking at upper air flow/vorticity charts. Always wondered the worth of them being included so far out though ... as the chance of them coming off in that position is very remote!

[summer blizzard]

first point are the 3 troughs?

Why 3, why not 6 or better still why not 1 to show what is really likely to happen, ie: the change of wind and the more wintry nature of any showers in the FAR north of the chart behind it.

I've never understood how there can be so many troughs on a chart so far ahead. 24 hours possibly, 12 hours perhaps more so but not at this range.

so much for me rushing to support the Met O!

Thats a good point, i suppose that they think the positive vorcity, lapse rates and instibility only warrant three troughs, though i think that it is rare that there are more than four in a flow like that, which illustrates that this is quite a potent northerly for the time of year, and thus the atmosphere is unstable.

[johnholmes]

in my experience, with a short northerly flow, as this one seems likely to be, more than 1 is a feast,

3, nope I won't swear!

[summer blizzard]

Just because we will only get the northerly flow for a day, does not mean that there will be only one in the flow, only that only one will reach us.

[johnholmes]

I guess the troughs are drawn on looking at the kinks in the computer generated isobars or if upper troughs, drawn on looking at upper air flow/vorticity charts. Always wondered the worth of them being included so far out though ... as the chance of them coming off in that position is very remote!

yes as I've posted elsewhere, showing a trough, one, gives an indication of the change to a more Polar airflow, with increased shower activity and probably lower thickness values at both 850 and 500mb, with the obvious conclusions to be drawn from that.

At T+120 only one is sensible.

At T+24 probably two with however many patches of unstable cloud with supported wind changes from less than T+24.

Next time, that is if we get one in winter, just check on how many actual troughs appeared on the actual chart compared to those predicted?

Just because we will only get the northerly flow for a day, does not mean that there will be only one in the flow, only that only one will reach us.

nope what I posted is what I meant.

Edited August 28, 2007 by johnholmes

[summer blizzard]

yes as I've posted elsewhere, showing a trough, one, gives an indication of the change to a more Polar airflow, with increased shower activity and probably lower thickness values at both 850 and 500mb, with the obvious conclusions to be drawn from that.

At T+120 only one is sensible.

At T+24 probably two with however many patches of unstable cloud with supported wind changes from less than T+24.

Next time, that is if we get one in winter, just check on how many actual troughs appeared on the actual chart compared to those predicted?

I dont really watch the FAX charts unless we are seeing a warm plume or cold plunge, so i will take your word for it...

Indeed Jhon, i will do that, this is also one of the reasons i opened this thread, and if you would like to give your imput, we could do this on a more frequant basis when a cold plunge comes within 132 hours (FAX range), or perhaps we could alternate the models.

[johnholmes]

I dont really watch the FAX charts unless we are seeing a warm plume or cold plunge, so i will take your word for it...

check the current T+120 or 132 chart at that time, 12z Sunday to see how many they have on their actual Fax chart and also look at the infra red sat picc to see how many you think it should have.

There is also the upper air ascents from nearby sites that may or may not support the presence of a trough.

[Nick F]

At T+24 probably two with however many patches of unstable cloud with supported wind changes from less than T+24.

Next time, that is if we get one in winter, just check on how many actual troughs appeared on the actual chart compared to those predicted?

nope what I posted is what I meant.

Sometimes the actual faxes can get quite overcomplicated unless you know what you are looking at, look at all the minor shortwaves shown on the 18z fax tonight for example, particularly in association with the thundery low over Spain:

Then we have all the other more unfamiliar additions to the fax charts such as developing cold/warm fronts (frontogenesis) and weakening cold/warm fronts (frontolysis), upper cold/warm fronts, and even convergence lines - a speciality of Duty forecasters to use during winter Nerlies. All these symbols can be found here:

http://www.metoffice.gov.uk/weather/uk/gui...y.html#pressure

[summer blizzard]

Points of note from this are:

A central pressure of 983mb-986mb..

3 troughs in the flow and the cold front extending as far west as England..

Please discuss, and i will post the next chart tommorow evening...

Sorry, should of saved last nights chart as it has now changed...

Since last night, several developments have taken place:

1) Predicted central pressure has been changed to 988mb-991mb which has also allowed the Atlantic High pressure are to move further east than yesterday

2) Because of the weaker low pressure are, the northerly plunge is much weaker, while the cold front was into the channel yesterday, it is now stradling northern England

3) There is now just one trough in the flow, because the northerly plunge is now further north, there is less instibility due to cooller seas...

So the points of note to remeber next time, are:

High pressure are of 1029mb-1031mb..

Low pressure area of 988mb-991mb

Cold front as far south as northern England, only one trough in the flow..

[Lettucing Gutted]

http://85.214.49.20/wz/pics/brack4a.gif

Doesn't look much like a Northerly over Britain now. Not only downgraded but gone.

Edited August 29, 2007 by Craig Evans

[david16]

So is it just going to be the same old seemingly forever and a day constant mild muck sweeping in from the atlantic again this winter?

The MET Office pressure chart just looks the traditional autumn/winter pattern of recent times just in time for Autumn (and probably virtually the entire winter as well) that the low pressure will be with winds from the southwest again when it eventually reaches us.

The low pressure system sitting to the west of the high is worryingly too far north for comfort.

Edited August 30, 2007 by david16

[summer blizzard]

http://www.wetterzentrale.de/wz/pics/bracka.gif

Looking at the chart, the difference from what was first progged is profound, for instance, the front in reality had a warm sector, though there were two troughs, intensyfying an occluded front...

In terms of the pressure stats we have 1029mb and 986mb...

What this has tought me, is that at the 132 hour range, the models have latched on to the strength of the low and high pressure areas, it is simply the orientation due to shortwaves ect... which is decided upon, in most modern cases, the orientation becomes more westerly.

[stormforce 10]

we'd get constant northerlies if the earth pivoted on its axis and thats a possibility!