Heavy Snow - 4-5th Jan - Amber warning issued

[Ball Lightning]

 HarvSlugger Dew points above zero increase snow melting but it doesn't really affect what's falling out of the sky.

Anyway, my radar is showing wet snow/sleet is about you hit you any minute now, but precipitation type radars are notoriously incorrect.

A

[ANYWEATHER]

1 minute ago, Homogenous Nucleation said:

Dew points above zero increase snow melting but it doesn't really affect what's falling out of the sky.

You won’t get much snow at all if dewpoints are in positive numbers! You will however get snow at three or four degrees above freezing, when dewpoints are below freezing. Typically happens in spring! 

[Jordan S]

The 6z ECM below..

In the north Midlands/north Wales after precipitation primarily turns to rain with a thaw to varying degrees during the day, the precipitation *could then turn briefly back to sleet snow late tomorrow as it all clears east and colder temperatures follow, but not as significant as further north by then.

Also thought I'd add the amount of precipitation that could fall whilst being freezing rain below, just a guide especially with showing what areas are likely at higher risk, this freezing rain risk begins during the early hours and risk remains high to mid morning primarily for the areas south of northern England, precipitation remaining as snow for many parts of northern England northwards, for many in the republic of Ireland especially the areas highlighted before.  Those in northern Ireland, particularly southern/southeastern part of northern Ireland, seeing some snow too, repeating myself from previous updates ofcourse so apologies.

[festivalking]

Dew points taking a dive here before the main intense ppn makes itself known. Looking forward to lots of cold rain to be replaced by lots of lovely warm rain

[HarvSlugger]

 Homogenous Nucleation yes my dewpoint is currently 0.5c on edge currently. Could start off as rain/sleet then it will turn to snow once the heavier precipitation hits

[ANYWEATHER]

6 minutes ago, Homogenous Nucleation said:

Dew points above zero increase snow melting

You’re on about sublimation. The process of snow evaporation with temperatures well below freezing. One of the only times that water ……ie snow can transfer into a water vapour/gas with out condensation.

[ANYWEATHER]

 HarvSlugger heavier precipitation won’t make it snow in this case. The dewpoints, that’s the parcel of air way above in the atmosphere needs to be negative or below freezing firstly . And if the surface temperature is above freezing and too warm for snow , the process of Evaporative Cooling , where heavy precipitation drags the cold air from way above and replaces the surface with colder air , thus turning the rain to snow. The key in this instance is for the heavier precipitation, the more chance of seeing snow fall and settle…..

[Met4Cast]

26 minutes ago, AltonMatthew said:

Anyone see on the radar that the recipe over France is Snow? Surely that would mean the  air moving north would be colder than expected?? Perhaps more snow than rain tomorrow??

Reports on the ground suggest nothing is falling, seems to be the radar being janky.

[Gypo]

Going to see Nosferatu at 8pm but if the UKV is accurate then I'll be lamppost watching instead!

[HarvSlugger]

The Hermon pembrokeshire snow shield 

Only joking, it's just because I'm a slightly lower elevation than all around me

Meaning the snow level here is currently 750 feet+

[festivalking]

PPN has pepped up here. All rain mind  and before someone says you weren't due any.....I know!

Edited January 4 by festivalking

[bluearmy]

For anyone expecting the system to do weird stuff, it looks bang on track on its journey towards tip of s Ireland  

[Ciderwithrosie]

Heavy (ish) rain here now and according to my weather station 3.3C and falling- however the ground is wet so I can't see anything would settle should it turn to snow.

 

[V for Very Cold]

 bluearmy probably move north again

[snowstorm27x]

 bluearmy looks like it's fragmenting quite alot so it's really hard to tell. Midlands looks like some heavy snow for around 7 hours before the rain. 4am Snowball fights in Brum lol  

[Ball Lightning]

 Ciderwithrosie Careful it could be freezing rain by the looks of the sounding ascent in Cornwall earlier, looks like milder air is getting entrained aloft

[festivalking]

Just driven up to Princetown where it's lightly snowing unsurprisingly it's near freezing up here

[Don Corleone]

 Gypo I'll send you a photo of my ex mother in law, it'll be cheaper, and last longer!!

[smichling]

 

A forecast sounding shows the freezing rain risk quite well, this is actually for central London at midnight tonight and you can see the layer of air between about 850mb to 750mb that is above freezing, meaning that snow falling through this layer melts to rain. There is also a layer beneath 850mb which returns to below freezing, supercooling the rain so that it would freeze on impact with the surface. Both the sub-zero surface layer and warmer layer above are needed here, but the GFS shows the risk is there even further SE than I've heard mentioned. If anyone can pull forecast soundings from other models it would be great to see if the GFS has any support here for the London area.

[Ball Lightning]

 smichling AROME for same place same time:

[A.J]

 Don Corleone just spat my coffee out.....ahahah !

 

dew points hovering around 0C here, and temps around 3c.....some areas of the NE should do very well out of this event, but alas just a mile or so from the coast it'll likely be a wintry mix here, the north sea influence and all that.....hope springs eternal though !

[smichling]

 Homogenous Nucleation  Thanks, interesting! What website did you get that sounding from may I ask?

[Ciderwithrosie]

 Homogenous Nucleation Thank you for the  information much appreciated. I'll warn my adult children to be careful if they are going out though.In fact i'll advise them to stay in .

it's stopped now and down to 2.8c.

[Ball Lightning]

 smichling

https://www.meteociel.fr/modeles/sondage2arome.php?mode=0&lon=-1.8&lat=51.2&ech=1&map=10

set lon= to longitude and lat= to latitude. ech=t+hours from start of run.

Edited January 4 by Homogenous Nucleation

[Gypo]

Given the pivot on the warm front approaching, some areas could see prolonged snow and keep those cold 2m surface temps