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St Patrick's Weekend Snow Risk - Technical Discussion Only - READ MOD NOTE POST #1

Kermit.de.frog

MOD NOTE

This thread is for Technical Discussion Only - for everything else use the event chat thread Off topic posts will be moved there.

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Don’t say you weren’t warned!!

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M.T. Cranium

The current temperatures in Scandinavia are awe-inspiring given the calendar date:

http://www.meteociel.fr/observations-meteo/temperatures.php?region=su

this is the high pressure area that will drift down to Scotland and then Ireland over the next three or four days.

M.T. Cranium

giveitholly wrote: »

Any chance of getting a countrywide map from harmonie

What location are you interested in?

Looks like the streamers are predicted to run inland WSW from that guidance.

Anyway, it's likely to be all over by Sunday night and sea effect snow has a fairly quick settling rate so give it most of Monday and Tuesday to evaporate, should be largely gone by mid-week except in higher parts of the southeast where it may be falling on top of existing snow and a few places could see 50 cm new snow.

Mount Vesuvius

Harmonie showing great accumulations there Weathercheck.
Even taking away 5-10 of rain /sleet early Saturday still leaves loads of white gold.

giveitholly

Weathercheck wrote: »

MS paint of Harmonie accumed precip by 18z Sun

Any chance of getting a countrywide map from harmonie

ninebeanrows

MS paint of Harmonie accumed precip by 18z Sun

Mount Vesuvius

Nice one Gaoth Laidir.

Also latest Arpege shows the intense precip zone. Going to be lots of fun and games if these models are correct.

overview_027.jpg?2018031618

Gaoth Laidir

The real moneymaker in this setup is to be found at mid-levels, in the form of a concentrated pool of very cold sub-minus 20 air advecting low partial thicknesses westwards. It gets into the east by sunset Saturday, so this will mark the start of the heaviest showers. By midnight to 6 am we're in the core of it, so showers should be reaching their peak by sunrise Sunday, gradually losing intensity throughout the morning and afternoon. Note: the 850 hPa charts don't really show the same level of change, remaining within a degree or two of -10 °C throughout.

Gaoth Laidir

Artane2002 wrote: »

Is this fax chart showing the possibility of heavy, persistent snow clipping the south coast on Sunday? I could be reading this wrong but it looks like a convergence zone to me? This is where two air masses meet causing very heavy precipitation I think. The Boscastle floods of 2004 were caused by a convergence zone from what I've read on Netweather. Professional help would be appreciated. Credit to supernova on Netweather for the image.

Yes, that's a convergence line. The Arpege is picking that up too in the form of an enhanced band of snow running right through Manchester and Liverpool tomorrow night.

Mount Vesuvius

@Gabeeg
Position of trough in upper atmosphere at noon. These steer what's underneath so I'm expecting lots of activity especially around 6am Sunday. Instability over Irish sea helping to bolster up some streamers

Artane2002

Is this fax chart showing the possibility of heavy, persistent snow clipping the south coast on Sunday? I could be reading this wrong but it looks like a convergence zone to me? This is where two air masses meet causing very heavy precipitation I think. The Boscastle floods of 2004 were caused by a convergence zone from what I've read on Netweather. Professional help would be appreciated. Credit to supernova on Netweather for the image.

gabeeg

Mount Vesuvius wrote: »

This is the atmosphere profile for Sunday midday showing
the 500 hPa geopotential height (contours) and temperature at 850 hPa (shading) from the ECM

Curvier the better, right?

Mount Vesuvius

This is the atmosphere profile for Sunday midday showing
the 500 hPa geopotential height (contours) and temperature at 850 hPa (shading) from the ECM

Mount Vesuvius

Some places getting a good dumping from streamers. Probably should be Red Alert in areas but impossible to tell where yet untill they form.

highdef

kittyn wrote: »

@Weathercheck ....... So where roughly are these sweet spots?

In reality, these sweet spots are very much nowcast scenarios. They are virtually impossible to forecast with any sort of accuracy. I managed almost 20cm during daylight hours on the Wednesday of the recent snowy spell. Yet places less than 10km away for hardly anything. That was pure luck for me and could not be forecasted.

sdanseo

kittyn wrote: »

@Weathercheck ....... So where roughly are these sweet spots?

About as easy to forecast as the winning lotto numbers at this point.

kittyn

@Weathercheck ....... So where roughly are these sweet spots?

ninebeanrows

Harmonie showing some sweet spots getting 20-30cm, more generally Leinster 5-15. All streamer line dependent

Calibos

Kermit.de.frog wrote: »

Convective gusts? Pretty much confirms the drifting risk...

Ah Jaysus, dont tell me strong winds are going to both blow the streamers over me too quickly and carry salty sea spray aloft over me onto Bray Seafront and environs ruining any chance I have of more than 5cm of accumulation like they did with Emma?? Say it aint So?

Neddyusa

Kermit.de.frog wrote: »

Convective gusts? Pretty much confirms the drifting risk...

Was just going to ask:
What are forecast windspeeds for Sat night/Sun relative to the last time?

It was the drifting that caused all the disruption on roads here rather than the amount of snow on the Tuesday - Thursday of the last spell.

Kermit.de.frog

M.T. Cranium wrote: »

Gusts to 107 km/hr at Arkona (the northern point of eastern Germany).

Convective gusts? Pretty much confirms the drifting risk...

thomasj

I'm having problems with that link MT.

Edit: got it working on the desktop site seems to be problems from the old app

M.T. Cranium

North German radar loop gives an idea what Irish Sea might produce ...

http://www.wetterzentrale.de/de/reanalysis.php?map=34&model=rad&var=415&jaar=2018&maand=3&dag=16&uur=2125&h=1&nmaps=32

remember Baltic is close to 1 C, Irish Sea 6 to 8 C. North Sea now 2 to 6 C in the sector likely to be upwind from Ireland.

Gusts to 107 km/hr at Arkona (the northern point of eastern Germany).

The snow further south was produced by an eastward-moving wave on the arctic front and is not part of the forecast scenario.