Snow and Winter Precipitation Discussion

Welcome to the snow discussion section of snowchat4um 🙂 Here you can find snow forecasts from myself, these will be detailed & I will be answering questions about the snow forecast. Any other meteorological chat is welcome in the forum.

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@smith54, there is a risk of back edge snowfall late Wednesday / Thursday. Upper air temps of -4C, a surface pressure of 950mb means the effective adiabatic 100m snow line 850hpa temp is -4.5C. So, conducive conditions are possible however the condition of intensity is still required at and around this snow line to secure accumulations. Need to watch hi res output for this intensity however at present > significant & disruptive snow > 250m possible with risk of few hours of heavy wet snow to lower levels.

Interested in weekend too

GFS is too widespread with the precip >> occulled front more localized.

About pinpointing the low level 1-3cm area, Pennines almost nailed on now. Arpege goes for Yorks & Lincs. This magnitude most likely though any reduction in modelled wind speeds would increase low level accumulation so > potential is there for 4-8cms below 250m locally.

• This reply was modified 3 years, 10 months ago by Kasim Awan.

The pivot on Wednesday night’s system is interesting too.

I’ve seen similar pivots with winds under 15mph produce 4-6″ at 300ft. Wind speed is working against us here, so the evap cooling thermodynamic gradient will be shallower (will take longer to develop conducive conditions at low levels). This however does not discount possible evap cooling from optimal pivotting reducing the snow line under 300m which would give 3-6cms locally. Possibly 20cm locally above this height in the central / northern Pennines under an ECM/GFS solution.

The concern is 1) faster eastwards movement and 2) collapsing of the front in the NWP – it is over 72 hours out & these Polar Maritime set ups are highly mobile i.e. subject to change as seen on ICON (very little snow) so it is a high risk high potential reward set up. Granted when isolated the ICON is not favoured.

We need to watch for upgrades in the size of the front at 72-78 hours this will improve the thermodynamic gradient. A stronger low core is preferred here.

Ultimately the snow risk doesn’t stop here I’d envisage more shortwave development in the Wed-Sun time frame on the -4 gradient which is preferred from a snow risk perspective.

I’m also watching the weekend as Scott will agree the models often under estimate the abilities of surface cold to stall fronts >> snow risk. Arpege spins up an interesting feature SW of us…

SouthYorks wrote:

Overnight runs from Icon, GFS and ECM all take the low on Thursday on a NE track, deepening as it goes, into the North Sea. Ppn associated with it therefore take a more northerly easterly route and any snow from the front misses most of Yorkshire and instead shows more over NE England/Southern Scotland.

Arpege still keen on snow overnight into Thursday for more of the Yorkshire region, with it taking the ppn on more of an Easterly route existing the UK.

Good morning all. A positive set of 00z hi res runs, certainly in terms of scope for possible improvements which may occur when hi res get to grips with the fronts. A cluster of output (ec, icon, wrf) have the main occlusion & evap cooling over NE England / S Scotland, however the cold undercutting (into a separate front) still occurs at a reasonable speed across “central” Northern England.

Granted this solution does produce much stronger accumulation base north of us, and limits the snow line here to 200-300m asl. Patchy coverings >> below 250m and 2-5cms for 50% between 250 and 250 and 3-9cm widely above would be feasible from this solution.

A more south easterly track is possible as per Arpege would produce much more favourable conditions. Whilst this is is not favoured, yet, it reflects what would be expected in normal adjustments. The result on Arpege is stronger precipitation lead cooling, a moderate undercut and a surface snow line of 50-100m, so this needs a watch. It is also possible for the front pivot to move into the south of England & the occluded front to then move into Central North (best outcome). >> This is possible however at present I would favour the “middle ground” Icon / Gfs solution

So >> idea of a front undercut over our region with a possible thermodynamically lead snow line to 150m with a base of 350m producing 2-6cms largely above 250m & the occluded front largely to our N. Watch for straying precip embedded in the adiabatically cooled air early Thu which may produce some localized accumulations.

Room for sway here esp with the occluded front which where it falls could produce substantial low level snowfall. As well as with the main pivot and undercut to the south of this which could upgrade and at present is our main focus.

• This reply was modified 3 years, 10 months ago by Kasim Awan.

Kasim experimental Model