Cold Spell Late Feb/ Early March Technical Discussion only MOD NOTE POST #1

Villain

Harmony out to 18:00 Friday now and it has Rain south of Carlow after 16:00 Friday but as others has mentioned it often gets that the snow/rain split wrong.

sdanseo

UKMO have issued red warnings for Scotland. Very rare to see that since their impact-based criteria for red is so much higher.
The comments here sound positively scary

Elmer Blooker

Showers now developing around Anglesea which is due east of Dublin. (apologies for imby post) Worth keeping an eye imo.
0.5c now and no thaw really worth talking about, the sun doesn't stay out for long with all the cumulus about.

lucernarian

M.T. Cranium wrote: »

When the core of cold air gets out over the Irish Sea during daytime convection period, all hell is going to break loose, expect blizzard conditions in many parts of Leinster and near the south coast sometimes spreading from those regions further west. Eastern and central Ulster will also have heavy snow at times.

There are some remarkably cold temperatures at the surface now in England and Wales, for example -7 C at B'ham, -8 C at Bala, Wales, and a stunning -9 C at Southend-on-Sea. Have seen it as low as -11 C in Oxfordshire during the night.

With a stronger gradient developing, conditions will be ripe for maximum sea effect production rates perhaps even surpassing the best we saw in late 2010 which is saying something. It goes without saying that these streamers will be discharging lightning at a prodigious rate.

If these conditions were over the Great Lakes I would be thinking 50 to 80 cm potential, not sure if salt water reacts less vigorously and the Irish Sea is a bit less than ideally sized or shaped so let's say 30 to 60 cm potential in these streamers later today. That won't happen all over Leinster but some places will get amounts that may further complicate what's left of road travel.

Was that an expectation for just today? From what's already fallen so far and extrapolating from the current radar echoes, it seems more like 20cm tops is what's gonna materialise by midnight tonight and more widely maybe 15cm or so (the Pale, southwest of the Pale and Wicklow).

adam240610

Some of the clouds currently rolling in over bray as viewed from greystones are enormous in height, similar to the thunderclouds I recall from my visits to Florida.

Looking at the netweather radar, I'm hoping the showers over western wales roughly due east of Dublin and Wicklow make their way over here by 6 with a nice reload. One can hope anyways.

Gaoth Laidir

Dewpoints now down to -13 °C through southern England. Crazy stuff, it's not even over snowfields.

Villain

The Netweather High Res GFS from 06z has some very heavy precip tomorrow night:

TheMilkyPirate

Villain wrote: »

The Netweather High Res GFS from 06z has some very heavy precip tomorrow night:

Hopefully because it's been a complete let down here in Wexford so far. Have had one decent shower so far over the two days. Maybe from tonight on we will fare better

MetLuver

lucernarian wrote: »

Was that an expectation for just today? From what's already fallen so far and extrapolating from the current radar echoes, it seems more like 20cm tops is what's gonna materialise by midnight tonight and more widely maybe 15cm or so (the Pale, southwest of the Pale and Wicklow).

Sorry for the long quote but could you tell me what time MT posted this and where please. Also, looking at netweather radar, does it look like a lot of activity is going to leave Bangor, Wales and head for Dublin/North Wicklow?

KingdomRushed

While it is livening up a bit again the Irish Sea hasn't produced a whole amount of shower activity since morning. Many of the weather models had it as being quite active, and I even saw a post here predicting 30 - 60cm from shower trains.

So the question is, what are the factors which are resulting in this lull in shower activity? Wind, temp gradient, moisture? How is it so notoriously hard to predict?

sdanseo

MetLuver wrote: »

Sorry for the long quote but could you tell me what time MT posted this and where please. Also, looking at netweather radar, does it look like a lot of activity is going to leave Bangor, Wales and head for Dublin/North Wicklow?

Click on the arrow...

PukkaStukka

KingdomRushed wrote: »

While it is livening up a bit again the Irish Sea hasn't produced a whole amount of shower activity since morning. Many of the weather models had it as being quite active, and I even saw a post here predicting 30 - 60cm from shower trains.

So the question is, what are the factors which are resulting in this lull in shower activity? Wind, temp gradient, moisture? How is it so notoriously hard to predict?

Met Eireann mentioned in a radio forecast earlier that they expected this to happen with the showers during the day, but expected a ramp once it has gotten dark.

lucernarian

KingdomRushed wrote: »

While it is livening up a bit again the Irish Sea hasn't produced a whole amount of shower activity since morning. Many of the weather models had it as being quite active, and I even saw a post here predicting 30 - 60cm from shower trains.

So the question is, what are the factors which are resulting in this lull in shower activity? Wind, temp gradient, moisture? How is it so notoriously hard to predict?

Upper wind direction has been fluctuating a lot over today (even a bit more than I expected). You can see how it's different from 10.45am to 1.45pm on the rainfall radar today (attached)

highdef

I think the wind has gone back round to just about north of east, hence the activity beginning to pep up again. I'd expect snow showers further south that recently on the east coast in 30 - 40 minutes. Perhaps northside of the city initially.

Gonzo

KingdomRushed wrote: »

While it is livening up a bit again the Irish Sea hasn't produced a whole amount of shower activity since morning. Many of the weather models had it as being quite active, and I even saw a post here predicting 30 - 60cm from shower trains.

So the question is, what are the factors which are resulting in this lull in shower activity? Wind, temp gradient, moisture? How is it so notoriously hard to predict?

hopefully it will pick up later. Parts of Dublin and Wicklow have done very well but many other parts only have single figure cm readings so far, there are even parts with green fields. The IOM and Welsh shadows have had a major impact so far in the snow distribution, a much bigger impact than they had during 2010. I think this is the main reason why there isn't 15cm+ widely across much of Leinster right now.

Will be interesting to see how the streamers perform over the next 24 hours before Storm Emma arrives. I have a feeling the bulk of the snowfall in the shadowed areas will be from Emma. Interesting 36-48 hours ahead.

Irish Steve

lucernarian wrote: »

Upper wind direction has been fluctuating a lot over today (even a bit more than I expected). You can see how it's different from 10.45am to 1.45pm on the rainfall radar today (attached)

So that begs the question, what causes those significant swings in the upper winds, given that there's nothing apparent on the forecast charts to indicate such a change. Is it related to the different upper air temperatures, which sets up movements across the general flow, or is it something else? What's been very apparent is that some of these swings are significant, and not just minor disturbances in the flow pattern.

dak

MetLuver wrote: »

Sorry for the long quote but could you tell me what time MT posted this and where please. Also, looking at netweather radar, does it look like a lot of activity is going to leave Bangor, Wales and head for Dublin/North Wicklow?

Posted 9 hours ago in this thread. If you click on any user name you can find all posts by that person from a drop down list!

Supercell

Latest metar confirms shift in wind to 080 again, this direction delivered the goods here on south Co Dublin last night:

EIDW 281530Z 08023KT 9999 VCSH FEW012 SCT016CB BKN030 M01/M08 Q1016 R10/190095 TEMPO 1200
EIDW 281500Z 10017KT 9999 FEW012 SCT016CB BKN026 M01/M08 Q1017 R10/190095 TEMPO 1200

MetLuver

dak wrote: »

Posted 9 hours ago in this thread. If you click on any user name you can find all posts by that person from a drop down list!

Thanks dak.

Gaoth Laidir

Irish Steve wrote: »

So that begs the question, what causes those significant swings in the upper winds, given that there's nothing apparent on the forecast charts to indicate such a change. Is it related to the different upper air temperatures, which sets up movements across the general flow, or is it something else? What's been very apparent is that some of these swings are significant, and not just minor disturbances in the flow pattern.

The swing in wind direction has only been around 30 degrees, so nothing major, but it's had a major influence due to the proximity of Wales. Such changes are common and are due to small disturbances at levels other than on the surface chart.

cherryghost

Not much change in GFS this afternoon, if anything the precip maintains some intensity in eastern areas til sundown Friday evening

Elmer Blooker

PukkaStukka wrote: »

Met Eireann mentioned in a radio forecast earlier that they expected this to happen with the showers during the day, but expected a ramp once it has gotten dark.

Looking east over the Irish Sea I am certain that there will be reports of thunder snow in the next few hours. One big Cnb to the south east of me is heading in the Wicklow direction I would think?

The Specialist

Noobie question lads but what model does the Met Eireann "short range" graphic compile from? Is it accurate in regards to rainfall and timing?

PukkaStukka

Elmer Blooker wrote: »

PukkaStukka wrote: »

Met Eireann mentioned in a radio forecast earlier that they expected this to happen with the showers during the day, but expected a ramp once it has gotten dark.

Looking east over the Irish Sea I am certain that there will be reports of thunder snow in the next few hours. One big Cnb to the south east of me is heading in the Wicklow direction I would think?

TS activity was also forecast earlier and I am aware anecdotally of one in the Cavan area over lunchtime, so the probability of more is certainly there.

Oneiric 3

Estoinian Hirlam for tomorrow night:



Snow (blue) over pretty much all of Munster & Leinster, and in accordance with the latest Met warming.

On another theme, a dew point reading of -10.5c in Valentia.

Oneiric 3

The Specialist wrote: »

Noobie question lads but what model does the Met Eireann "short range" graphic compile from? Is it accurate in regards to rainfall and timing?

Usually pretty accurate. The model itself is based on the 'Hirlam' output, which is one of the best hi-res models available. As far as I am aware, it updates every 6 hours on the Met site, usually around 4pm, 10pm 4am & so on.

AloKildare

I'm interested to know what people's thoughts are, on the Wicklow shadow that often affects precipitation in Kildare? Will it affect precipitation here again tomorrow night seeing as winds are forecast ESE?

cherryghost

HIRLAM upgrading snow precip over the entire country by Friday lunchtime. Could be over 20cm in places and 40+ in localized areas in the next 48 hours, over Munster and Leinster

lucernarian

Gaoth Laidir wrote: »

The swing in wind direction has only been around 30 degrees, so nothing major, but it's had a major influence due to the proximity of Wales. Such changes are common and are due to small disturbances at levels other than on the surface chart.

30 degrees and back again in a relatively short period.

Oneiric 3

lucernarian wrote: »

30 degrees and back again in a relatively short period.

When you have cold air from land (in this case, the UK) spilling in over relatively warm waters, it is going to cause subtle pressure changes which will affect small scale wind flows. Also, I would be pretty sure that the Pennines/Welsh Mts would cause some sort of 'eddying' effect to.

Edit, here is the Met Éireann hourly analysis between late last night and 3pm today, Gives a good idea of the small scale tweaks that occurred throughout the period.