Kasim Awan

Strong surface inversion developing on 18z now. Some places getting ice days.

Lots of negativity popping up now.. I would be surprised if the magnitude of blocking modelled doesn't manage to produce some decent low level snow in the next 2 weeks. Also, I would stay away from analyzing 198-240h too much right now. The evolution before this is fantastic, and gives us a good chance of decent solutions afterwards, combine this with a strong signal for a highly unusual pattern anyway.

Thanks. What I was looking at uses a different algorithmn that produced 1-3cm. Nevermind. The shortwave development 28th/29th is now almost a guarantee. The set up is more marginal I think than some are making out.. Good wind direction & precip intensity are key. At the mo we have a trend for the low centre to slide across southern UK from 28th PM which puts the interest for that precip area Sheffield & south. Disregarding Scotland atm. The surface marginality for the shortwave is below 250-300m (which is a rough base dewpoint 0C line with uppers of -4C & surface pressure of 960mb). This slice could be fairly easily eroded under a lengthy spell of precip and given the low speed etc could produce >10cm locally. The shortwave intensity is far from resolved. A weaker shortwave as shown on the Arpege would limit precip significantly. Reasonable shortwave intensity is favoured now tho - so encouraging signs. By Boxing day when the models will have a better grasp of the low intensity (which influences the location of the shortwave precip and the likelihood of a snow event for many), a more firm assessment can be made. At the moment I shall stand firm on the above uncertainties mentioned.

Where did you get that from?

Amazing 12z so far.

Morning of 28th does look to be a potential prime-time from a parameter perspective.

ICON pulls the low to the South East again 28th..

I love it when things go right. Snow down to sea level this morning parts of the NE as hinted at in my earlier snowfall assessment for today. Anyway, enough banging my own drum. Cracking 00z output too.

I'm sure Winter 1962 started around a similar time...

ICON total snow accumulation out to 120h. Merry Christmas all. I think somewhere could do really well.. Note: this is not to be taken literally at all. It merely highlights that the Hi Res are in agreement with the snowfall assessment, in that there is *potential* for significant snowfall to modest elevations. The main factors here are time of day, elevation, wind speed/dir and precipitation dynamics. Get just a couple of those optimal and you will bank. The reason why the ICON is like this is because it pulls a shortwave low off to the south east of the UK (Dover). Air is always coldest on the northern side of the low. This is somewhat credible given the ECM / GFS also forecast it to head south. In terms of a probabilistic forecast, I'd say this scenario has around a 10% chance of verifying. A weaker low with a less optimal position would be more favored in statistics, and this would still produce a few cm locally with the added dose of marginality present. Those are my thoughts anyway.

Lets see how many people throw the towel in just because the *pub* run has pressure 2mb lower over Greenland

Heights over the Atlantic are stumbling further east on this run. Low NW of Iceland is also a little stronger. These are both positives as the deeper low will contribute to the cold pool as it dives south east, with the high then likely to build back strongly over Greenland.

It's not a classic west based -NAO. There is a Negative NAO profile on the eastern side of the "Griceland" high around Ireland so it is more a central based negative NAO imo. The positive out of the low over GL is that it is programmed to reconnect to the low near the Azores and in that process it will likely dive south and produce a northerly. This is now a recurring pattern in the output.

58764th Northerly incoming

Ladies & gentlemen we have a full house.

There's no point fixating on a particular T850 value as the hallmark for snow. There are around 25 factors that influence the T850s required for snow. For example, low heights, which move the altitude of 850hpa down by about 200 metres. This automatically drops the T850 requirement by 1-2C. For example, a -4C 850 temp in high heights would produce a 1500m temp of -4C, however in low heights T850 is near 1300m so a -4C T850 would produce a Temp at 1500m of -6C.. In the set up modelled for 27th-29th, surface inversions will form a cold layer up to 2000ft asl. This reduces the T850 requirements, as they do not need to be at the depth of cold needed to ensure the entire troposphere is freezing due to the presence of an inversion. So uppers of -2/-3C can suffice if this permits the temp to remain below 0 from the T850 level to the top of the inversion! Given we have both low heights & light winds 27th-29th, these two mechanisms will work to reduce the T850 requirements. 4th March 2016. Low lying Cheshire got 4". T850s of -2C, evap cooling sets up a superficial inversion (the temps don't decrease near the surface, but the top-down lapse rates drop to near 0 permitting -2C uppers to be just tidy indeed). Even 0C uppers can suffice if evap cooling drops the top-down lapse rate to 0 so that effectively the air does not warm as the snowflake falls near the surface). So theoretically uppers of 0C and a surface inversion could produce heavy icy snow / blizzard. But... Marginality - precip too light and you will fail to get the evap cooling needed to reduce the top-down lapse rate, keeping the surface warm. Strong winds, shore wind, the Fohen effect, urban heat island, and about a gazillian other factors can do the same. So hopefully this will clear up the T850 debate once and for all. It is a lot more complex than a single figure. Ultimately the potential is there for "high" T850s to produce amazing events. It is up to meteorologists to determine the contribution of ALL these factors and how this will impact the precip state.

Looks like an MJO phase climatological still lol

Vast majory of GEFS members support the renewed bout of amplification at D7-9. Low energy plunging south under a nice surface inversion. This solution is both logical & now well supported.

Try to focus on T144-210 for trends. And the trend is amazing.. Plus T850s become less important once surface inversions develop, it is possible to have high 850s and v cold surface temps.

Boom.

GEM joins the club. Such a strong signal for an unusually weak Atlantic. So, high chance / confidence that a cold, blocked pattern will verify at D7-10.

Wow. ICON and GFS more or less identical at 180-190h. Much better tilt on the renewed bout of amplification. Cold packing in.

Those totals shown on the ecm are ridiculously overdone. Software in the website is used to extrapolate the accumulations. Not raw data from the ECMWF. This uses a poor algorithm.

The snow line will reach sea level for a few places at some point 27th-29th.

-5 will be ample for pretty much all ppn in this set up with light winds. This is akin to -8 in a showery sea run feed.