Big Change Monday Onward, much colder, ice and snow later

Deep Easterly

maquiladora wrote: »

Interesting ECM in FI, I cant tell if that would be rain or snow 168-192?

I would say sleet on the coast and snow for the hills along the coast. I would be surprised if showers penetrated very much inland with the incredibly slack airflow.

:mad:

On a brighter note, just watched Evelyn's forecast on TV just now. She highlighted the upcoming drop in temps with "severe frosts" rather than snow.

Supercell

Deep Easterly wrote: »

I would say sleet on the coast and snow for the hills along the coast. I would be surprised is showers penetrated very much inland with the incredibly slack airflow.

:mad:

On a brighter note, just watched Evelyn's forecast on TV just now. She highlighted the upcoming drop in temps with "severe frosts" rather than snow.

Pressure so low there will be more than showers, I would bet on troughs rotating around that.

maquiladora

http://www.meteociel.fr/modeles/ecmwf/run/ECM0-168.GIF?15-0

Looks ok for snow?

Yeah the amount of precip will be the thing, but still, fairly deep in FI and will probably look quite different in another 48 hours...

Deep Easterly

Supercell wrote: »

Pressure so low there will be more than showers, I would bet on troughs rotating around that.

Low pressure does not make the man. There may be troughs imbedded in the flow, but there is very little sign of sharp veering in the time frame spoken off in those ECM charts. Result, showers by the coast, which may be heavy, thundery and wintry, but mostly if not completely dry further inland.

On the other hand, charts at that range are not good at picking up squall potential so plenty of hope yet, we may see something developing nearer the time.

Supercell

New 12z parallel ensembles show pretty good agreement.
The fridge is here for the foreseeable.

krobo

Seems to me Met.e are closer to agreement with the posters on here than they let on. They just really dont like to play around with the s word.

maquiladora

Supercell wrote: »

New 12z parallel ensembles show pretty good agreement.
The fridge is here for the foreseeable.

Kinda worrying that it seems to have had little support for that -10 air over us around the 18th? But also good to see it didnt have much support for a rapid breakdown around Christmas Eve.

Deep Easterly

ECM is showing a low pressure generated easterly at 240hrs for the 2nd run in a row:



That would make for a very raw feeling Christmas Day! :pac:

Supercell

maquiladora wrote: »

Kinda worrying that it seems to have had little support for that -10 air over us around the 18th? But also good to see it didnt have much support for a rapid breakdown around Christmas Eve.

Yep , interesting wobble there, overall it is consistent though. If it is accurate on the 17th then it would be wintry precip on mountains >350 metres only probably.
These situations are so rare it really a now cast and best guess scenario, its not boring that's for sure!.

maquiladora

Supercell wrote: »

Yep , interesting wobble there, overall it is consistent though. If it is accurate on the 17th then it would be wintry precip on mountains >350 metres only probably.
These situations are so rare it really a now cast and best guess scenario, its not boring that's for sure!.

Only a couple of days away from the period of radar watching, dp watching and lamp post watching expect with festive music in the background and a tin of quality street. :pac:

Deep Easterly

As I suspected, the ECM not showing those showers pushing much inland on Sunday & Monday next:

For me, dry as a bone, whereas Belmullet looks to have rainy cold showers, while Letterkenny may see a snow flake or 2.

Too far off to be set in stone though.

Su Campu

I don't buy it lads. It's one run of one model. The ECM and UKMO don't go near as crazy as the GFS, the ECM has too zonal a H500 flow over north America to allow the North Atlantic High retrogress westward enough for the real cold pool to reach us from the east. At the moment it looks like the northerly jet will be too far east for the lowest 1000-500 thicknesses to take effect. If only we were 300 miles further to the east.....:rolleyes:

For polar lows to develop we need a good pool of sub -40°C H500 temperatures, which looks dodgy at the moment. I stills ay what I said yesterday - that there will be wintry showers for some, most likely on north facing coastal areas.

Danno

Right, 48 hours to go for this one. According to todays Midday GFS output there is a strong chance of snow showers in the southeast for Thursday evening...

Pressure will be 1022hPa and falling, Upper air temperatures will range from -25c at 500hPa to -8c at 850hPa, dewpoints will be 0c or lower everywhere in Ireland excluding the extreme coastal locations of the SE and SW. Air temperatures will generally be 3c to 5c at 6pm, dropping to 0c to -2c later.

I think with SST's (Sea Surface Temperatures) at 8c or 9c in the Irish Sea, with a very cold airmass overhead, that the neccessary ingredients are in place for snow showers over the SE, areas I feel will get hit will be counties... Dublin, Kildare, Wicklow, Carlow, Wexford, Waterford, Kilkenny, Laois, E Offaly and E Tipperary.

sdanseo

Ok so I'm still only learning to read the charts but from what I've learned, am I right in saying that the main factors needed for snow are good precip, 500hpa values of ~522dm or below, in low pressure with an easterly wind to see snow on the east coast?

Now, at the risk (don't care) of making myself look really thick:

Can someone tell me in relatively simple terms how to calculate wind direction from a chart? Like, is the below a northwesterly or a southeasterly? Or, as my hunch goes, is the reason snow won't fall because there's just not enough wind from the cold air towards us? (I know the bit about tightly packed isobars=faster wind speed and that the bars are indicative of direction; but each isobar has two potential directions and therein lies my confusion)

Just as an example:



Plenty of precip of the associated chart.

Supercell

Su Campu wrote: »

I don't buy it lads. It's one run of one model. The ECM and UKMO don't go near as crazy as the GFS. I stills ay what I said yesterday - that there will be wintry showers for some, most likely on north facing coastal areas.

Maybe not as crazy in FI..but that IS called FI for a reason.

Meanwhile back in the not so FI, I present for the east:-

http://www.wetterzentrale.de/pics/brack1.gif
http://www.wetterzentrale.de/pics/brack1a.gif

Elmo5

I can't claim to understand charts too much either, but I can't see why the mood here has changed to a more down beat one in terms of snow potential.

The charts are still looking fairly good (i think?) for sun/mon/tues of next week and that is still too far away for Met Eireann to have an opinion on.

I'd love to even see a dusting this close to Christmas!

maquiladora

Elmo5 wrote: »

I can't claim to understand charts too much either, but I can't see why the mood here has changed to a more down beat one in terms of snow potential.

A lot of that upbeatness was as a result of the 12Z GFS model run which was an upgrade on the previous one within the reliable timeframe, giving more potential for snow showers Thursday/Friday & now the UKMO Fax charts for the same period are hinting at something similar.

Su Campu

sdonn wrote: »

Ok so I'm still only learning to read the charts but from what I've learned, am I right in saying that the main factors needed for snow are good precip, 500hpa values of ~522dm or below, in low pressure with an easterly wind to see snow on the east coast?

Kinda right, though it varies a lot. Firstly you mean H500-H1000 THICKNESSES of less than 522dm (this refers to the thickness of the layer between the 1000hPa and 500hPa pressure surfaces, which is directly related to the average temperature of the layer (or airmass) - the colder the airmass, the denser it is, so the closer those two pressure levels are to eachother. So with sub 522dm thicknesses, the airmass is cold enough through its volume to promote frozen precipitation.

For the east coast yes, a cold easterly wind over the Irish Sea will cause lake-effect snow if the wind speed, sea run, etc are right.

Now, at the risk (don't care) of making myself look really thick:

Can someone tell me in relatively simple terms how to calculate wind direction from a chart? Like, is the below a northwesterly or a southeasterly? Or, as my hunch goes, is the reason snow won't fall because there's just not enough wind from the cold air towards us? (I know the bit about tightly packed isobars=faster wind speed and that the bars are indicative of direction; but each isobar has two potential directions and therein lies my confusion)

Just as an example:



Plenty of precip of the associated chart.

Above the boundary layer, wind direction is generally aligned with the isobars (or more correctly, the isohypes or height contours) - anticlockwise around lows, clockwise around highs. On a surface chart, however, the wind is at an angle to the isobars inwards towards the low pressure - over sea this angle is around 15°, over land it's up to about 30°, due to frictional decceleration.

In the chart above, the 500hPa height contours between Ireland and Iceland run northwest-southeast, and are ALIGNED with the isotherms at that height. This means that the wind is not advecting (blowing) the cold air towards us, rather skirting along the edge of it. What you want to see are the height countours crossing the isotherms, preferably at 90°, for optimimum cold advection.

sdanseo

Su Campu wrote: »

Kinda right, though it varies a lot. Firstly you mean H500-H1000 THICKNESSES of less than 522dm (this refers to the thickness of the layer between the 1000hPa and 500hPa pressure surfaces, which is directly related to the average temperature of the layer (or airmass) - the colder the airmass, the denser it is, so the closer those two pressure levels are to eachother. So with sub 522dm thicknesses, the airmass is cold enough through its volume to promote frozen precipitation.

For the east coast yes, a cold easterly wind over the Irish Sea will cause lake-effect snow if the wind speed, sea run, etc are right.



Above the boundary layer, wind direction is generally aligned with the isobars (or more correctly, the isohypes or height contours) - anticlockwise around lows, clockwise around highs. On a surface chart, however, the wind is at an angle to the isobars inwards towards the low pressure - over sea this angle is around 15°, over land it's up to about 30°, due to frictional decceleration.

In the chart above, the 500hPa height contours between Ireland and Iceland run northwest-southeast, and are ALIGNED with the isotherms at that height. This means that the wind is not advecting (blowing) the cold air towards us, rather skirting along the edge of it. What you want to see are the height countours crossing the isotherms, preferably at 90°, for optimimum cold advection.

Explained nice and simply, but without treating me like a total n00b either. Just what I needed, cheers

Elmer Blooker

I'm surprised I'm the first to comment on Evelyn Cusack's forecast after the 9 o clock news. She said showers would become increasingly wintry on the Irish Sea, temperatures not much above freezing during the daytime on Friday (1 or 2c) and sleet, snow showers for the weekend. We're getting there, slowly but surely!:)

munsterlegend

Elmer Blooker wrote: »

I'm surprised I'm the first to comment on Evelyn Cusack's forecast after the 9 o clock news. She said showers would become increasingly wintry on the Irish Sea, temperatures not much above freezing during the daytime on Friday (1 or 2c) and sleet, snow showers for the weekend. We're getting there, slowly but surely!:)

yeah bit of an improvement alright though still a northern/eastern coastal event per met eireann so far. we live in hope down south!

maquiladora

18Z GFS has very little or any changes from the 12Z so far at 90 hours.

Its nice to see a chart with ( a little ) snow shower potential 48 hours out and not in FI. :pac:

maquiladora

:pac:

pp_me129

patneve

Great snow potential for next week according to the latest GFS with lovely low pressure flirting with our island:D

(somehow it's always 'next week' LOL)

darkman2

It's looking good if you like snow.


18z GFS is a very cold run for Ireland with plenty of snow opportunities over the weekend - and Thursday and Friday too. Firstly the risk is in the East. This then transfers North on Saturday - Saturday night will see the risk countrywide as a cold front moves South. With cold air ahead and behind this system it is highly likely to see snow in most places overnight. The South coast may struggle a little though. Sunday looks like widespread snow showers and freezing conditions as the LP to the North comes very close or even over Ireland. Mild air will try to come in from the Southwest at times but this actually improves the snow chances even further because we need it to create the trough lines we need for more widespread snow.

Just that short post from me tonight. UKMO and ECM are very good aswell. All very promising for the Weekend and into the early days of next week.

Nite all!

forkassed

18z = Snowfest

Elmo5

Now that's better!

I do believe that Evelyn smiled too when she mentioned Christmas? anyone else notice?

MUSEIST

WOW, Another amazing run.
I keep waiting for the downgrade but it just keeps looking better and better way out to FI.:D

maquiladora

Lol, impressive 18Z, several snow days, brings the low right over Ireland, White Christmas and pushes the breakdown back to 348 hours....just missing the kitchen sink.