nick sussex

BREAKING NEWS!!! The UKMO and GEM discounted from the NOAA US forecast, outlier solutions this in the run up to the UK slider. A blend of the GFS/ECM solutions preferred.

Good agreement for the overall pattern for at least the next ten days. A couple of slider lows then a re-amplification of the upstream pattern, a deep low over the east USA helping to build a ridge to the nw ahead of it. A good example of how cold in the ne USA needn't mean mild mush in the UK.

At T96 hrs upstream the UKMO has an 985 low compared to the GFS shallow feature. Because of that deepening this low runs more ne inflating lower heights over Greenland. This dominos further and will effect the track of the next low into the UK. NOAA will definitely be commenting on this later as it effects the New England states.

In terms of the latest output to come out only one so far is the NAM. The 12hrs has altered the jet track out of the USA and we only get the upstream view but this looks more condusive downstream to a southwards correction compared to its earlier 06hrs. I'll stress though this is the mesoscale model normally just for the USA so treat with caution!

I see the slider continues to cause major drama! As Nick F just mentioned phase timing or no phasing with the trough to the east is critical, the normal effect of phasing is to pull the upstream system further north and east so no phasing or later phasing will alter the track. I wouldn't trust any output until the slider is within T72hrs range because of that. Later on the GFS tries to flatten the pattern but its view upstream isn't what NOAA expect, a blended solution with the ECM to address both biases should see the cold continue for longer.

Day 13 is an awfully long way out, I wouldn't don't take one set of extended ensembles as a fait accompli. If its still there in a few runs then that would be disappointing but I've seen the ensembles make marked shifts between runs.

How far ahead is that Feb, by ext do you mean past 15 days? The De Bilt shows a large spread developing from around the 20th. But theres still quite a few colder solutions. I really wouldn't worry about it , at that range they can easily flip between runs.

Good to see the slider is still on although still time for changes to the track. Whats complicating matters even more than normal is this slider phases with the troughing to the east, timing of that phase also makes a difference to where the snow line might be. Looking further ahead the next big event in terms of impacts for the UK is the development of a big eastern USA storm which will have a big effect on the downstream pattern, the ECM and GFS handle this quite differently. The GFS has the flatter system which is often the case in these scenarios. Looking at the Maine forecast is interesting re their thoughts on the troughing upstream: Flow wl turn twd the northwest on Sunday in wake of system mvg east into Canada with a brief ridge of high pressure expected on Monday. Next system wl dig south into the Great Lks on Tue with guidance indicating potential for trof to take on negative tilt by mid-week. Although details differ regarding timing, mid-week system looks to be significant at this time. A negative tilt to the upstream troughing would be good news for UK coldies so something to keep an eye on.

I'm glad I'm not in the high risk area for any snow I don't think I'd be able to sleep and would stay up biting my nails till the next run! The issue is effected by two key areas, near Iceland with any positive heights hanging on to help force the low se and upstream with shortwave energy spiralling around the main trough which gets ejected east into the Atlantic. Its really a nightmare forecast. Putting aside slidergate the GFS still re-amplifies the upstream pattern and both the ECM and GFS agree on a big storm developing over the eastern USA.

The GFS had a better start near Iceland with better heights there, slightly let down by a bit too flat upstream later but this won't get sorted out for a few days. Very complicated and rather messy set up but the chance remains for that to deliver some snow.

This looks a bit different with the shortwave break away earlier, could be interesting! Looks like a sw correction coming up with a slightly better upstream profile.

It looks like crunch time is approaching for the first slider low. So if you're in the high risk or high fun area time to start biting those finger nails. The good news is that past this slider the upstream pattern re-amplifies and even the underwhelming UKMO at T144hrs goes onto to take the next upstream low further se at T168hrs. It's not a case of miss this opportunity and then having to wait weeks for another.

Upstream flow re-amplifying at T168hrs on the ECM so interest should continue through to day ten.

The ECM looks a bit sharper with that low to the nw than earlier outputs and is more amplified upstream. Much now depends on phasing of shortwave energy upstream to develop a more amplified low to throw up a better ridge behind the possible slider.

Well good and bad news this morning! The ECM isn't interested in the favourable early slider however its later output is an upgrade compared to last night. The GFS delivers a couple of sliders and not as good as the ECM later. The UKMO looks a bit washed out upstream at T144hrs in terms of lack of shortwave development upstream and flatter at that point but somehow grabs victory from the jaws of defeat at T168hrs with the jet running south of the UK with another slider at that point. In terms of overall pattern little change positive PNA still there but the broad trough ridge pattern belies the uncertainty over the UK. Overall the NH pattern continues to look favourable for the UK to see further cold and snow. Its a pattern where you need some luck because the dividing line between cold and less cold is small on a global scale. Expectations can sometimes ride ahead of the type of set up, its not one which you can guarantee non-stop cold but it is one where you have chances for periods of cold and snow if that makes sense.

Of course some of the younger members here will have no idea what I'm talking about re the 1996 event. But I know some of the old timers in the se feel my pain ! The key thing with these sliders is some height rise to the ne to help angle the jet se'wards, you don't need a super strong block just even weak heights are sufficient. Its a juggling act of energy and that's why these set ups are a nightmare to forecast. They're great fun to watch unfold and it will be stress free this time as I'm a neutral observer!

I was living in Wimbledon at the time and watched in trauma as a few flakes blew through. What was remarkable if I put this chart up on first sight it looks a bit underwhelming. Feb 5th 1996 Next day The surface flow ahead pulled in some very cold air from the continent, the flow looks south on first sight but was actually se. It took weeks of counselling to get over the trauma!

Normally low pressure runs west to east or sometimes sw to ne so the normal zonal pattern in winter rarely engages any cold air over the UK as that's removed before the precip sets in. Here the low slides se through the UK or just to the west. With a chance it will engage cold air, we have seen of course those Channel Lows in the past but this is equally rare sliding se from near Greenland. One of the most infamous sliders was 1996 in February , I remember that because all I got in London was a few rogue flakes whilst further west people were sledging!

The latest MJO update has been issued by NOAA. The CPC velocity potential based and RMM MJO indices indicate an enhanced MJO signal over the Maritime Continent. Some eastward propagation has been observed over the past few days.  Dynamical and statistical forecast models depict fairly canonical MJO evolution over the next one to two weeks, with the enhanced phase shifting eastward over the West Pacific, and suppressed convection developing over the Indian Ocean.  Tropical cyclogenesis is possible in Week-1 over two regions where MJO-related convection is ongoing: the Bay of Bengal and the western North Pacific.  The current and forecast MJO evolution favors anomalous troughing near the Aleutians and over east-central North America throughout the remainder of the month. Given they expect the current Aleutian trough and central east USA trough pattern to last throughout December as long as you can maintain enough amplitude in that eastern USA trough and with help from weak height rises to the ne then the UK should continue to see chances for further cold and snow.

Some interesting changes in tonights outputs so far. The UKMO has increased its amplitude and now looks like bringing a slider low in . The GFS brings several sliders, and the positive PNA continues to be re-loaded which is great news for coldies. The detail though re these sliders is likely to change this far out. The difficulty at present is the track and how much cold air these will engage but overall it looks like at least the next two weeks will provide opportunities for snow and cold.

Not much change overnight. Still the same overall pattern for at least the next ten days, its just filling in the detail which is difficult and of course all important re the possible slider low around T168hrs. In terms of the UKMO which once again has become the attention seeker for the wrong reasons I'd say at this point its a clear outlier. Regardless of the differences downstream between the ECM and GFS they both keep more amplification in the east Pacific which is also supported by the GEM at T144hrs, the UKMO has the flattest solution over the west USA and Canada. Aswell as this given comments from NOAA in terms of where the Atlantic ridge is expected to be this throws further doubt into its placement of the ridge. TROUGHING WILL BE STUCK IN THE EAST AS UPPER RIDGING REMAINS IN PLACE OVER WESTERN CANADA/CONUS AS WELL AS IN THE NW ATLANTIC. Even with it being flatter at T144hrs looking further upstream it still looks like it will re-load the positive PNA pattern. And its T168hrs chart still has the jet to the south of the UK. So just as the other day not the best from it but certainly people shouldn't view its output as bringing the curtain down on cold prospects. In terms of the slider low still lots of uncertainty there but whatever happens although the outputs don't shout bitter cold and ice days the re-loading positive PNA keeps cold and snow options on the table until at least the middle of the month.

Yes I'd be less confident if the signal to re-load that positive PNA was inconsistent but its appeared for days now on every run. It's not often you see that and currently it looks like a very strong signal.

Some very unusual and welcome synoptics showing up. Great to see these trends in early December and very happy with tonights outputs. The PV blob has no chance as long as you get that re-load of the positive PNA as that pulls the PV lobe towards Canada and helps to separate the low tracking se from the PV mothership. Then positive heights to the ne can build sw.

The PV is more bloated here but before it gets ideas above its station a re-load of the positive PNA takes place with a some great amplification working in from the Pacific. Factor in some GFS bias and edge the pattern further west.

That's a rare set up as well but had a deeper level of cold air to draw on as the feed ahead of the fronts was from the east.