Kasim Awan

Some worried about this 4-5 days ago and were skeptical about the colder charts being posted. It now seems a fair possibility that a trend to mixing due to low expansion could verify and make things even more marginal. Though we will keep a trend for renwed amplification.

You need to be creative with meteorology. If you look closely the UK is actually exceptionally cold.

We'll be under a pizza slice by the 00z lol

We are seeing a push west in some output. Could be an emerging trend hope not.

Xmas eve snow potential. Overall quite poor. 6 am to 8am. Relatively stable inversion present below ~700ft. This would allow wintry conditions to 50-100m early on across SE Scotland & Northumberland. Precip rates of 2-3mm/ hour locally producing a good dusting down to 20-80m. As the convective flow pushes southwards early morning the surface inversion erodes due to maritime influences. See the retreat of the freezing line in the pics attached between 7 and 3pm. Between 12pm and 4.30pm showery precipitation will be gathering in the east. The surface flow is NNE but the 650hpa flow is due NE in the trough. This will fall as sleet and wet snow down to 0-50m with the settling snow line likely elevated to 200-250m. Therefore significant accumulations on the southern side of North York Moors ~7cm. A localized 0.5cm for around 10% of an area encompassing the east coast & ~25 miles inland though showers could reach the East Midlands due to the trough visualised nicely via the 850thetaE values. This producing localized heavy precip and therefore the risk of localized low level accumulation (1-2cm) during this time. The base settling snow line will steadily fall not far from 100m after about 5pm. More % of the precip will be snow. Dew point 0c line roughly 5 miles inland & at 100m asl. This should produce isolated coverings of 0.5-2cm over 110m in Lincolnshire. Subsequently below ~ 100m altidude settling snow will be partially limited to rates >3mm/hour, nontheless dustings up to 1cm are possible from inland Norfolk, boundaries of the wash and central Lincolnshire. Also note the possibility of flakes inland East Anglia later as the trough dives SE. One thing to note is that the precip is too mostly light to erode the maritime warm layer. Therefore I fully expect coastal parts to stay the wrong side of marginal throughout.

Good explanation of the role of heights. An added bit from me for those newer to snow forecasting, on winds. The thermal gradient through the atmosphere means that temperatures cool by around 2/3C for every 1000ft gained vertically. In winter however, there is a strong tendency for this to be "cancelled out" near the surface and cooler temperatures to be parked up here. In a situation with lower heights, winds are much lighter. This effectively allows the surface cool layer to remain in situ without this "mixing out". In the event of mixing in stronger winds, the non-inverted air aloft becomes mixed with the surface inversion cancelling it out. It also prevents adiabatic cooling - the process responsible for inversions. This means the snow when nearing the surface will melt as the air gets warmer. Let's take an example with -5C uppers at 300ft. This would give a surface air temperature of 3.75C. Too mild. Now. Imagine winds are lighter. Adibatic cooling of the surface has been allowed to develop. There is a cold layer up to 1000ft which has not been mixed out either. Temperatures between 0 and say 600 metres are 1C dew points 0C. A weather front with uppers of -3C pushes in. The layer between 600m and 1500m (uppers level) is saturated due to cloud. This means that the temperature only increases by 0.5C per 100m due to "saturated lapse rates". So at 600m our air temperature is 0.5C. So it's wet snow at 600m.. Now the snow hits the inversion. Stays as snow. Add in a bit of evaporative cooling and the air is easily below 0 throughout So you can start to see how -3C uppers can produce snow and -6C uppers rain depending on synoptics

GFS 06Z is much weaker with the development of shortwaves / slow movers, and resultantly drives down the snow risk slightly 27th 28th. Mainly because the low centre is placed over the midlands, and here is slightly less unstable. Only before a surface easterly warms the air 0 to 960hpa. This will be a factor to an extent it may be that we see snow chances on both the more unstable southern and northern margins of the low centre as it drops south as here these factors are reduced. The best few frames are 27th for northern hills and 28th is optimal for central UK as a light surface easterly develops reducing local low core warming & onshore wind impacts, this under a very unstable airmass before easterly winds produce a surface moderation. We then slowly build a more decent surface easterly and if clean enough could drive in conducive conditions again later 29th. Given the GFS' tendency to perform ok as well as this synoptic evolution being common sense in this set up this is a fair shout. What would otherwise be zonal energy is being forced north cooling only to dip over the UK again. I've seen these repeated patterns before and this is one. The chances with this next northerly are even greater. 11th Jan 2013 springs to mind. That was a good trough disruption / dive SE.

So ultimately as a next best we look for the toppling energy to now take on more of a negative alignment (NNW to SSE), the trough energy then becoming isolated by continued wave breaking / amplification. This cutting off helping to develop an easterly component. Plus the trend now for heights to lower around Iberia and suddenly the GEM seems a reasonable shout. Certainly a predisposition for this amplification atm given the teleconnections etc etc

Set up is optimal for very slow moving shortwave lows. These can outdo snow forecasts by some amount. I think this needs to be our concern / interest.

Groundhog day -in response to Mike's post. Renewed bouts of amplification seem credible.

I then think, given the synoptic options on the card, the focus turns from localized snow events to either convective NErlys or slider developments. These are synoptically credible evolutions following the 27th / 28th charts. GFS 18z takes the convevtive route - how this delivers depends on higher resolution dealing of the air mass nearer the time. However, I can envisage cold digging into precip and streamers etc as having potential. The slider option could produce frontal snow..

Depends on the position of the low. The way the GFS models it there would be a fair bit of shortwave / mesocyclonic precipitation. The parameters currently prescribed would support mixed precip; as uppers are moderated around low cores and surface warm layers where onshore winds form. This would support potentially prolonged yet fairly localized heavy wet snow events in areas with both heavy precip and optimal wind dir. Marginality being very significant in the areas noted above. I do think, just using a statistical law, that some low ground place will get lucky and have precip at the right time, i.e. before the surface inversion and bag a few days of snow cover as surface inversion feedbacks develop. They can do in slack systems to. This set up is more interesting to me than the BFTE.

GFS has a shallow trough in the east on Xmas morning. Not a completely unrealistic outcome.

Precip

Only right at the end. It's very well supported otherwise.

ECM signal for polar maritime energy + GFS signal for amplification = very cold moist feed / snow. A halfway house solution.

Smaller lows: less distance between cold air to North & moisture to South, promoting convergence & better shortwave development as a result. Nontheless, a dartboard low as per ecm still has noteworthy snow event potential.

216 BOOM.

The NW-SE jet axis has been plauged by models for a few days. It certainly isn't a negative given the mid Atlantic ridge is signalled to reattempt amplification.

Yeah, ukmo and ecm are like this.. Mixing is a valid worry more so with a larger low. Don't forget the gfs has been leading the way here. It could well be right in it's smaller low.

The trough would help push it inland. Only taking the odd cm though.

All ingredients there for it to develop.

-7 at 850. Freezing level ~260 metres.. That's potential.

EC makes a good effort at an Xmas eve North Sea trough

Enough said. Potential is there and we have established at what level the potential is. Pros will leave it at that until more data is available rather than becoming emotionally attached to upper air temperatures.