Model Output Discussion

[nick sussex]

Well nice to see the models still going with a cold snap for the end of the week, theres generally good agreement upto 120hrs, the UKMO though looks better at 144hrs with the upstream pattern and there could be some interesting possibilities down the line.

[Thundery wintry showers]

The potential reload after the weekend also looks interesting but at this range it is prone to considerable uncertainty. Breakdowns from the NW after a northerly outbreak rarely bring significant snowfalls, but there are exceptions. We had a breakdown from the NW following a long northerly spell in February 1970 and this had relatively cold air behind the front as well as the very cold air ahead of it, and so many places saw snow from it:

http://www.wetterzentrale.de/archive/ra/1970/Rrea00119700217.gif

Also the 12th January 1999 provided surprisingly widespread snowfalls, especially as the preceding northerly of the 8th-11th had been pretty sluggish:

http://www.wetterzentrale.de/archive/ra/1999/Rrea00119990112.gif

And of course there were 3 inches of snow in Norwich from a breakdown from the NW on 23 November 2008 and snow fell widely also in E Scotland & NE England. So, while a snowy breakdown on Sunday/Monday is an outside bet I wouldn't rule it out at this stage.

The airmass behind the front bringing the breakdown will be heavily modified (see the pools of >-5C 850hPa air floating around) and so I would expect a general thaw and just rain, hail & sleet showers. However, a lot then depends on whether high pressure can build to the NW again, or better still for cold/snow lovers, generally across to the NW- the former would give us a renewed northerly with temperatures dragging back down again, while the latter would give us a northerly followed by a very cold easterly.

[Lewis]

I must say, i'm very happy with the 12z, not just from a personal point of view, but almost nationwide.

Of course things change all the time, but i'm certainly liking the evolution.

To me the 12z shows snow showers for Eastern areas, NE England, Scotland (where they will be more frequent and heavier) and also EA (Norwich area always does well with these setups) TWS will probably confirm but i'm almost certain that Norwich gets 99% of their thunder snow and highly convective showers from these type of setups.

Again a long way to go, so troughs and disturbances will be hard to predict, but I'm pretty sure they will be some in this type of unstable flow.

2 possible break down events, the first on Friday with the LP, giving some very impressive snowfall (especially for eastern areas, and the East Midlands) After this a re-load from the North as the LP sinks Southwards, bringing more snow showers, with the Western Isles looking more favored (and N Ireland) of course Eastern and NE coastal areas will continue to see snow showers, although these becoming confined to the extreme coastal fringes come Saturday night.

Sunday, possible snowfall event again with a break down from the west, with some frontal snow possible on the leading edge across Eastern Areas where the colder air sticks around much longer.

Lovely run, i suppose that is the confirmation of this Northerly, how it pans out upgrades/downgrades is something we need to keep an eye on.

Key things to keep an eye on over the coming 48 hours;

1. We need the HP to stay more or less the same or spread westwards, keep checking for any eastwards movement, this is important to just how unstable the flow will be.

2. 850'S and thickness relating to the LP on Friday, isotherm and dew points, although at this resolution i tend to wait until nearer the time frame for the NMM model.

3. Winds at the surface and upper/important with regards shower activity.

Staying cold i think until at least the 7th Feb.

Edited January 25, 2010 by Storm Force Lewis

[SP1986]

Cold spell fairly locked on now, although in all likeliness there is a breakdown date to it, which may occur a few days later/earlier than currently shown. Good snow prospects although the GFS continues to struggle with coastal temperatures I feel and perhaps temperature are too low overall, although given it's degree, it's not surprising.

Difficult to know if there are any snow events yet, as usual it will be a case of seeing come the time.

[Thundery wintry showers]

I must say, i'm very happy with the 12z, not just from a personal point of view, but almost nationwide.

Of course things change all the time, but i'm certainly liking the evolution.

To me the 12z shows snow showers for Eastern areas, NE England, Scotland (where they will be more frequent and heavier) and also EA (Norwich area always does well with these setups) TWS will probably confirm but i'm almost certain that Norwich gets 99% of their thunder snow and highly convective showers from these type of setups.

The Norwich area is prone to vigorous North Sea convection with winds vectored anywhere between NNW and E, provided that the upper air is cold enough to produce large scale instability over the North Sea. Most people from this area comment that easterlies are more reliable for snow than northerlies but this is mainly because easterlies have a shorter track over the sea down here and so they tend to be colder- but as long as it is cold enough a full-on northerly will usually deliver a significant snow cover. Thundersnow was reported from parts of East Anglia on 17/18 December and again at intervals between 5 and 8 January, though only the former produced any reports specifically from the Norwich area.

The GFS 12Z, as it happens, largely kills off the cold/snow potential after the weekend, with the modified flow being followed by a build of high pressure and settled weather, so we can't assume that the reload will bring any further snow events at this stage. It might, but it might not.

[Guest North Sea Snow Convection]

There is a fair chance that following the northerly there will be a pressure build close to the UK which will keep the remaining northerly air surface cold locked in and then after that a pressure rise out of the arctic into the next cold phase with the ridge being sucked northwards or north eastwards.

UKMO most especially shows this 'lock in' with a more pronounced surface ridge further northwards than the GFS which is probably being typically over progressive with the atlantic advances, especially as the lower resolution is rapidly advancing by that time.

Edited January 25, 2010 by North Sea Snow Convection

[Lewis]

thanks for the info TWS, totally agree with you, regarding the re-load there is so much uncertainty to;

1. If it happens.

2. How potent it will be

3. If it delivers in terms of precipitation

Edited January 25, 2010 by Storm Force Lewis

[thegreat316]

The UKMO and GFS 12z both have much more of a N/NE flow compared with the 6z with more of a N/NW flow

GFS 6Z

GFS 12Z

UKMO OH

UKMO 12H

Still early doors but nice trend

Edited January 25, 2010 by thegreat316

[The PIT]

I wouldn't say nailed on yet as the return of the easterly looked possible at times in the last few weeks but never got going.

Plenty of cold air in the Northerly set up and secondary feature certainly needs watching. The cold air seems to stick around a while as well.

[Thundery wintry showers]

There is a fair chance that following the northerly there will be a pressure build close to the UK which will keep the remaining northerly air surface cold locked in and then after that a pressure rise out of the arctic into the next cold phase with the ridge being sucked northwards or north eastwards.

UKMO most especially shows this 'lock in' with a more pronounced surface ridge further northwards than the GFS which is probably being typically over progressive with the atlantic advances, especially as the lower resolution is rapidly advancing by that time.

A fair point, even the breakdown from the NW and subsequent reload isn't set in stone yet, because although the GFS and ECMWF are both going for it at the moment, at this range I think there's still a chance that both of them could be building high pressure too far south. Just a small chance IMHO, but it's there, and the UKMO at T+144 certainly looks different to the GFS/ECM so far with the northerly still going strong at midday Sunday.

[SP1986]

I wouldnt say a change to a more east of north airflow was a new development, in fact when this northerly first appeared it was showing this north-northeast air fetch. Isobars sometimes look like they're from the north though and more often the wind is from the northwest. I do think the wind will be largely from the NNW/north west rather than the northeast.

[Guest North Sea Snow Convection]

A fair point, even the breakdown from the NW and subsequent reload isn't set in stone yet, because although the GFS and ECMWF are both going for it at the moment, at this range I think there's still a chance that both of them could be building high pressure too far south. Just a small chance IMHO, but it's there, and the UKMO at T+144 certainly looks different to the GFS/ECM so far with the northerly still going strong at midday Sunday.

There were a lot of colder solutions in this mornings ECM than the operational, and I do think there is a reasonable chance that the high pressure build is too far south

Interesting to see which way the 12z trends

Edited January 25, 2010 by North Sea Snow Convection

[Tim Bland]

Certainly an interesting evolution progged by the GFS, a low forms and in this flow its really not all that unlikely either...

A very rainy day for the west and south, but very good snowfall for East Anglia NW towards the NE of England, areas west/south of that probably just rain and sleet and maybe some backedge snow.

For a countrywide event you need the low to develop a little later, the early Jan event was nearly perfect with the timings in that respect.

I would be supprised if the precipitation makes it to East Anglia. The western extent on most models is the Midlands. This will change allot over the comming days but I dont expect it to reach as far as here (Herts / Beds boarder) for example.

[J10]

The front pushing south into Friday does indeed looks too mild for western areas, however the NNW wind into Saturday, looks very conducive tor the potential of heavy overnight snow showers.

[Snowyowl9]

I wouldnt say a change to a more east of north airflow was a new development, in fact when this northerly first appeared it was showing this north-northeast air fetch. Isobars sometimes look like they're from the north though and more often the wind is from the northwest. I do think the wind will be largely from the NNW/north west rather than the northeast.

Agree.

XC has winds ranging from SW wednesday morning to NW late that day,As far as NNW thursday then GFS shows a low coming south so all directions friday to north/NE then back to north then NW saturday better than recent N-lys which haved turned into surface west/NW-lys.

UKMO would give a good NNW wind.

http://www.wzkarten.de/pics/Rukm961.gif

[Cheese Rice]

Gfs run looks very good to me, so much for "winter is over". The main interest for most on here is how Friday morning shapes up, previous runs showed the PPN alot further west but now the latest run shows the precipitation further east. Good for people in the east, bad for people in the west. For me personally I would prefer the PPN to be slightly further east so I have a definite chance of snow. Anyway the areas that look prone to some sizable snow (10cm), are the East/North Midlands, the South East, West/Central Scotland and parts of North West England. Places like West midlands and Wales are likely to see rain turning to snow, but this doesn't look like it would lead to anything significant.

Another area if interest is Monday Morning, this could be another snow event, but a very marginal one at that. Southern areas have very little chance of snow, but certainly northern England and Scotland and Possibly Northern Ireland could well see a snow event. It would be very marginal, but we would already have cold air in place and cold air coming in behind, and the front moves in very quickly, this could give less chance for the warmer air to dig in. Perhaps a Snow to rain to Snow event which wouldn't be too bad and the rain wouldn't last for too long.

Also places like Wales/South West and North Scotland and Norwich look to receive some sizable snowfall totals over the weekend with showers merging together to give prolonged heavy snow, certainly one to watch.

It does look like everywhere in The UK will receive some snowfall in the coming days.

Edited January 25, 2010 by 10123

[aspire27]

Shouldnt we be looking at the Northerly (good or bad for different regions) as the starter before the main course? Isnt it into FI we should be looking for trends as the Northerly disapates?

[D KARLSON]

I would say there is more uncertainty here than from our usual northerlies ,as we have areas of low pressure moving south down the north sea much as we did last year.

Anywhere could get surprise snowfalls from this and the northerly could end up being a nne easterly or nn westerly ,there is too much scope for change as these areas of low pressure dumbell around each other.

Certainly exciting as once again we are being spoilt rotten here and i must admit i was sick of seeing snow here in yorks ,but very quickly ive got my cravings back and its all conjecture at the mo perhaps wednesday will give a better insight as for after that further cold spells are always likely as the siberian high throws further ridges out.

Edited January 25, 2010 by D KARLSON

[Thundery wintry showers]

Shouldnt we be looking at the Northerly (good or bad for different regions) as the starter before the main course? Isnt it into FI we should be looking for trends as the Northerly disapates?

I'm not sure about that, especially as there's no guarantee that we will get a "main course" and some parts of the country, particularly near north-facing coasts, are potentially in line for a fair dumping of snow this weekend.

[Snowstorm1]

I'm not sure about that, especially as there's no guarantee that we will get a "main course" and some parts of the country, particularly near north-facing coasts, are potentially in line for a fair dumping of snow this weekend.

Near north facing coasts does that cover areas such as northeast england for example

[aspire27]

I'm not sure about that, especially as there's no guarantee that we will get a "main course" and some parts of the country, particularly near north-facing coasts, are potentially in line for a fair dumping of snow this weekend.

Im talking about a potent Easterly that people here say might develop out of this setup.

[Cheese Rice]

Near north facing coasts does that cover areas such as northeast england for example

The wind won't push the showers inland, so no, we need a NE wind if we want the showers to push anywhere near inland.

Edited January 25, 2010 by 10123

[Snowstorm1]

The wind won't push the showers inland, so no, we need a NE wind if we want the showers to push anywhere near inland.

so people are suggesting a nnw wind and others are suggesting a northerly and other nne wind is this so hard to forcast because of the margins of error?

[Matty M]

ECM keeps it very cold.

[Duane S.]

ECM 12Z an improvement so far compared to the 00z +144

Edited January 25, 2010 by s4lancia