icedust

I must apologise to many of you who regularly post on this forum. Until today, I have been thinking 'Why are all these people posting about 10 day forecasts as if the synoptics will bear any resemblance to these forecasts in 10 days time'. In fact , until today I would look at T+240 charts to find out what was NOT going to happen. How wrong I have been! And now all that has changed. In my tier 4 lockdown this morning I have done some verification from the Meteociel site. I chose the past 2 significant changes of types that involved cold weather over the UK. First the major development of the upper trough over W Europe on Dec26-Dec28. Best of the major models by far was the GFS OP. signalled at T+264 and nailed at T+240. second the ECMWF which suggested the change at T+240 but a bit too far east with that diving depression. Then good from T+216. but it threw a wobbly at T+192 Third the GFS para which was OK but not until T+216. Next event was the easterly which started today and will likely last a few days. best again by far was the GFS OP suggesting it at T+264, nailed at T+240, but it threw a wobbly at T+192, although it went off track at T+144 Second best was the GFS para, not too bad at T+216 and T+240, again it had a wobbly at T+192. Third best the ECMWF which was poor at T+216 and T+240, then good from T+192. I am having trouble uploading my working Word Doc table so the above summary will have to do. So finally I attach the latest 2 GFS OP (the best model?) forecasts at T+240 to perhaps see what the future holds. Yes,I know some runs are better than others and these may not verify etc etc. and it has bad runs I could go on and on. But I am now not as dismissive as I used to be. They both show a mid atlantic ridge with the northerly over the UK, OK one has a depression over us but the signal is clear, Cold.

Just had 6cm here in Macclesfield in an hour. The snow moving across the Pennines to much of NW England was completely not forecast this morning. So being an ex-met office forecaster I have had a look at the models based on midnight data to see what they should have forecast. Basically they failed, in spite of data being available to enable the snow to have been predicted. In short it was a warm front development in deep unstable air I show the 850mb and 700mb chart for 1500Z, from the Euro4 model using the midnight forecast data today. Preceded by the 500mb chart for 2400Z today (note the long finger of sub -34C air streaming from the northeast). So at 1500Z (a snapshot) there is a pool of cold air at 850mb and 700mb over the northwest. With a northeast airflow behind it. Now there is a 500mb depression over us at 1500Z and a trough at both 850mb and 700mb. The warmer air upstream of this cold pool is rising above the cold pool and it contains very unstable air to NorthSea temperatures - a North Sea Streamer-. These streamers often do not make it over the Pennines, without a stronger gradient, but the frontal development encouraged and enhanced the streamer for our side of the Pennines. One can see that the ingredients for this snowfall were predicted by the Euro4, but not the actual snowfall event. There is plenty of work for computer modellers to do , which no doubt over the years to come will occur. Good use of the very expensive new computer promised for the Met Office. I leave you with the precipitation forecasts, put out by the Euro4 and based on midnight data. An essentially dry day for the northwest!!!!!!.. Room for improvement. Incidentally I have been unable to find an accurate forecast of the snow, based on midnight data and therefore useful to the public, from any available numerical model. Perhaps someone has had more success. And the Arpege model: The few zeros is all that it could manage

Hi all 06Z Euro4 looks interesting for us Cheshire Folk, and nearby. Possible polar low development in the Liverpool Bay area tomorrow night. Note the very cold 500mb temp of -36C. A pre-requisite for Polar lows. But Euro4 was very poor last night for this morning's accumulations. So we will see.....

Yep, extreme rainfall here in Macclesfield, so heavy that visibility was 50m briefly. A notable line squall moving east over the last few hours. Manchester Airport reported thunder and a gust of 75kn (86mph) at 1120, which I did not believe until the radar revealed the squall, so I am sure it was real.

I was marooned in Devon (Hemyock) in February 1978. It snowed for 40 hours non-stop with an easterly gale. 20ft-30ft drifts. over 2ft level snow (but it could not be measured due to drifts). Said to be the worst blizzard of the 20th century for Devon (surpassed by 1891). All roads in Devon were said to be closed. M5 was empty. Princetown cut off for a week. And that was followed within 2 days of its ending with a rapid thaw. Let's hope this one is not as bad.

Model chaos indeed. ECMWF, EURO4 and GFS are all predicting 850 temps above zero edging in to the SW and far south of England at around T+ 42 to T+48 . BUT at the same time the surface and 950mb temps well BELOW zero.: This is a forecasters' nightmare and could be (as our North American friends say) an 'ice storm' or freezing rain. It depends how deep that warm nose is forecast to be whether the snow melts into rain as it falls through this nose. Luckily all is back to some sort of normal by T+54 (by T+72 for ECMWF due to its only 24h read-outs)with all levels sub zero again and back to snow. This is model prediction and may not come to fruition, let's hope so.

There is a cruel irony in the answer - if you are into cold and snow. So often the Azores High is public enemy number one in preventing cold easterlies reaching the UK. But in 1991 it was an Azores High that was our friend and ridged northeast, after that major cold pool that gave snow in 1991 had passed , and prevented it from moving too far west. Now this year, and lets just assume for the moment that the forecasts are correct, there is a major upper LOW and surface low over - yes the Azores, quite the opposite to 1991. Our cold pool, forecast to move west over the next few days, is expected to merge with this and the resultant vortex will be to the west of the UK. Winds at all levels are expected to blow around this and we will be receiving winds from the south ( we'l see whether they actually make it!). Coupled with this the block to the north is moving west rapidly, driven by the strong east winds at all levels, to its south. The amazing thing this year is that a 1050 anticyclone over Norway tomorrow evening is forecast to completely disappear (by moving west) and lose 30mb within 48h. Scandi Highs used to be pretty resillient in late winter. The good old Azores ridge kept this going in 1991. As a footnote I blame global warming. Given any excuse all that warm air to the south will blow north. I am sure the area north of the 546 thickness line is much reduced in our winters these days, making warming up a lot easier. The Arctic is certainly warming (see the post from ?1962-63 a few days ago) . Also a good website is http://nsidc.org/arcticseaicenews/

Who cares about Friday - yet - when there is all that potential energy you show above to come out before then. All snow. Help!

That's a very interesting forecast tool. Not come across that before but it seems to have some physical sense, in that low dew points encourage cooling by evaporation, or sublimation, hence maintaining any snow that falls and reducing the melting into rain. As a forecaster for many years one obviously had to consider the many variables that affect snow or rain to occur in the UK. The hardest one to get right is the intensity of precipitation. In marginal conditions this makes all the difference. Heavy snow cools air as it melts and falls further down before turning all to rain. So that is what usually embarrasses the professionals when unexpected heavy snow blocks roads. It was supposed to be rain/sleet but was so heavy it turned back to snow. So using your forecast tool above when say, A=40% the 60 occasions of rain, could be light to moderate ppn and the 40% when it snowed may be due (mainly, the usual caveats used in forecasting) when it was heavy. Also very difficult is the freezing rain scenario but the Americans have this more than us and will know all about that. I only came across it once in my 25 years as a forecaster and it was forecast well using model guidance with a good nose of above zero air at 900mb to 850mb. For the end of the week, lets hope the professionals have some decent model output regarding that hazard nearer the time.

Hi John, I remember you from my days in the Met Office. at Manchester (Andy Day). Just discovered this site. You are of course correct. 510dm lines are rare these days (Jan 1996 was a good one here in Macclesfield and 1991 (over 20cm)). Looking at the excellent graphics on weatheronline I see the phenomenal low temperatures at 500mb due for tomorrow. -46!!. and a sub 504 cold pool for Scotland. That has to mean thunder snow and horrendous conditions for some places, with the eastern side right up the UK getting really bad in many places.

Polar Low? Re the possible polar low overnight/early tomorrow. Investigating the upper air it seems the trough is under a minimum of 500mb temperature (about minus 36) which is moving west under the otherwise undisturbed baroclinic flow. Baroclinic to those not in the jargon means thickness lines being advected, in this case cold advection slightly wobbling, maybe wobbling enough to generate the surface trough, not evident higher up. In short, a small area of increased instability moving west to southwest will pep up and merge the showers in this trough (may become a very small centre). This instability is being extra invigorated by the surface trough dragging in warmer surface air ahead of it from the north. So a polar low? well may be a case of semantics, but it is definitely away from frontal zones and will be unstable; so yes Polar Low will do.

Well what an interesting set of charts is developing. The duration of the upcoming cold spell seems to hinge on developments over the Tropic of Cancer! Watch how models develop a very low latitude low over the Bahamas in the next 24-48h and move this northeast over the next week. Where this ends up is crucial to where the Atlantic High develops. If this Low gets to the Azores this will allow a decent ridge to anchor north to Greenland and not slip south. On the other hand the T+240 ECM keeps the low far west possibly resulting in some decent northwards warm advection over the Atlantic and causing a good ridge north to Iceland/Greenland and block eastward flow south of those islands. On yet the other hand the whole development of this low latitude low may fizzle and we get back to a boring westerly breaking through again. Incidentally, I for one am 100% convinced our record breaking December was caused by 'global warming' with that very persistent warm block out of Africa and across Europe for the whole month. Is this forecast Tropical low also due to 'global warming' or an El Nino feature?

ANOTHER POLAR LOW Hi all You probably remember the polar low forum at the end of January that went 'viral' on that night. The interest then was in how much snow people would get and the low track in a general N-NW flow. Well look at the satellite images for today and the observations over Ireland. Another polar low is tracking east, just leaving the Dublin area as I speak. Looks set to give a good dump over the Peak district this afternoon, though many places will see just rain/sleet. Just goes to show how common these features are. Any old unstable airmass to depth will do, with a bit of PVA at height, just like today. The models have picked this up quite well over the last 12h or so, especially Euro4, latching onto the differential wind flow at height, and developing the feature. Don't know quite where to post this but I'll also post it in the model section as I have mentioned the model output above.

ANOTHER POLAR LOW Hi all You probably remember the polar low forum at the end of January that went 'viral' on that night. The interest then was in how much snow people would get and the low track in a general N-NW flow. Well look at the satellite images for today and the observations over Ireland. Another polar low is tracking east, just leaving the Dublin area as I speak. Looks set to give a good dump over the Peak district this afternoon, though many places will see just rain/sleet. Just goes to show how common these features are. Any old unstable airmass to depth will do, with a bit of PVA at height, just like today. The models have picked this up quite well over the last 12h or so, especially Euro4, latching onto the differential wind flow at height, and developing the feature. Don't know quite where to post this but I'll also post it in the model section as I have mentioned the model output above.

Hi all. Had great fun following this brilliant forum last night. Just to introduce myself; I was senior forecaster at Manchester Weather Centre for 20 years, now retired. I think that it was definitely a polar low. It showed all the characteristics on the sat pics. Also, 'mature' polar lows do acquire frontal characteristics, there are many examples. As circulation develops around the low, warmer air is entrained, with cold air to the north coming down the west side and a sort of warm front/cold front pair develops. Hence the areas of rain last night were in the warm sector, with snow to the north and east of the low - exactly as happens to the north of a low up the channel. Another thing. Polar lows are not dependent on particular temperatures at 500mb on any other pressure level. They simply form (exact mechanism is up for debate) when sea temps cause instability to sufficient depth and a circulation develops - like hurricanes. Both hurricanes and polar lows need some upper encouragement to start, easterly waves/troughs for hurricanes and I think yesterday's polar low was encouraged to start with the old trough/occlusion extending west from the parent low or by some small PVA (see below). But it was all unstable to sea temps hence the polar low. This can happen in any season and I certainly saw them outside winter. I'm in Macclesfield and we got 7cm snow from it overnight and Manchester airport closed twice (look at the METARS for EGCC) during the night to clear the snow. Another thing. MetO forecasters that produce the 'official analysis' are obsessed with putting fronts on charts along lines of cloud. The Norwegian frontal theory, still used for mslp analyses is still rampant, because it is a useful tool in forecasting , but don't take all those occlusions you see on charts too literally, many of them did not originate by a cold front catching up a warm front. They simply mark troughs or just lines of cloud from satellite imagery, originating from various mechanisms Positive Vorticity Advection (PVA) being one of them. PVA is a term from fluid dynamics and is an area of differential flow in the atmosphere at any level. Of interest to the forecaster are areas of differential flow at high level (usually 500mb or above). Jet streams are classic examples where a change of wind flow from high speed to low speed, coupled with a change of direction in the flow is an area of vorticity (spin). This changes the pressure in that place (just like flow over an aircraft wing). Positive VA causes lower pressure and Negative VA causes an increase in pressure. The atmosphere being 3 dimensional (4D if you are picky) then air moves vertically as well as horizontally to try and equate the pressure gradient. PVA creates lowering pressure beneath it. NVA causes descending air away from the increase in pressure at that level and highs develop along with, usually, clearing cloud. Anyway I've bored you enough now just though you needed another input.