Cold Spell - Feb 7th to 13th 2021 - Technical Discussion

Gaoth Laidir · 12-02-2021 10:44pm

Met Situation Valid at 0000 UTC
A GEN STRONG SE FLOW ACROSS THE REGION, LCA VERY STRONG IN FAR W.
A SLOW-MOV TROUGH LIES BRAEMAR TO ARBROATH TO N5600 W00100.
AN OCCLUSION LIES N5700 W01100 TO CASTLEBAR TO LIMERICK, MOV NE AT 10KT.

Strong wind warning
IN N AND E: WDSPR MEAN 15-20KT, GUSTS 25-30KT. OCNL, MAINLY SEA COT N, MEAN 20-25KT GUSTS 30-35KT.
IN W: WDSPR MEAN 25-30KT GUSTS 35-40KT. OCNL, MAINLY SEA COT, MEAN 30-35KT GUSTS 40-45KT. ISOL MEAN 40KT GUSTS 50KT.

Freezing Level
GEN SFC, BUT 1000FT FAR W.

Weather Conditions
Zone 1
SW OF A LINE SLIGO TO WATERFORD, MOVING NE AT 10KT:
GEN 15 KM IN OCNL RA, WITH 3-6/8ST 1000FT/1500, 6-8/8CUSC AND ACAS 1500FT/12000.
WDSPR 7KM IN RA OR RADZ, WITH 5-7/8ST 1000FT/1500, 6-8/8CUSC AND ACAS 1500FT/16000.
ISOL 3000M IN HVY RA, OR FZRA, WITH 6-8/8ST 500FT/1500, 6-8/8CUSC AND ACAS 1500FT/18000.
OCNL, ISOL LEE MON, 2000M IN RADZ OR BR, WITH 6-8/8ST 400FT/1500, 6-8/8CUSC AND ACAS 1500FT/16000.
ISOL, 200M IN HVY SN, WITH 6-8/8ST 200FT/1500, 6-8/8CUSC AND ACAS 1500FT/16000.

WRNG: HILL FG. MOD ICE AND MOD TURB IN CLD. SEV ICE IN FZRA. SEV ICE IN CLD BTN 2500FT AND 6000FT. MTW MAX VSP 800FPM AT 7000FT. MOD, OCNL SEV, TURB BLW 6000FT. SN MON ABOVE 2500FT

Zone 2
NE OF ZONE 1 AND SW OF A SLOW-MOV LINE N6000 W01000 TO DUNDALK TO PEMBROKE:
GEN 20KM WITH 5-8/8SC AND ACAS 2500-4000FT/10000.
ISOL 7KM IN RA, WITH 6-8/8CUSC AND ACAS 1500-2000FT/8000.
ISOL, 4000M IN RASN OR SN, WITH 3-6/8ST 400-800FT/1500 AND 8/8CUSC AND ACAS 1500FT/8000.

WRNG: HILL FG. MOD ICE AND MOD TURB IN CLD. WDSPR MOD, ISOL SEV, TURB BLW 6000FT. MTW MAX VSP 600FPM AT 5000FT.

Nqp15hhu · 12-02-2021 10:44pm

It’s from 4am to 10am.

Gaoth Laidir · 12-02-2021 11:03pm

Simulated IR satellite, showing a sharp frontal passage tomorrow morning.

Gaoth Laidir · 12-02-2021 11:14pm

Valid for 0200-1100 UTC
Met Situation Valid at 0600 UTC
GEN STRONG, BUT VERY STRONG IN W, SE FLOW ACROSS THE REGION. A FIRST OCCLUSION LIES FROM N5830 W01130 TO N5600 W01000 TO SLIGO, MOV ENE AT 10KT. A SECOND OCCLUSION LIES FROM N5330 W01100 TO DINGLE, MOV ENE AT 20KT.

Strong wind warning
OCNL, WDSPR SEA COT, MEAN 15-20KT GUST 25-30KT. OCNL SEA COT, WDSPR W AND N, MEAN 25-30KT GUST 35-40KT. OCNL SEA COT W, AND ISOL SEA COT N FROM 06Z, MEAN 35-40KT GUST 45-50KT.

Freezing Level
GEN SURFACE, BUT 500FT FAR N. 3000FT FAR W WITH OCNL 1000-2000FT SUB-ZERO LYR.

Weather Conditions
Zone 1
SW OF A LINE N5630 W00830 MALIN HEAD TO NEWRY TO PEMBROKE, MOV ENE AT 10KT IN N:
GEN 15 KM IN OCNL RA, WITH 3-6/8ST 1000FT/1500, 6-8/8CUSC AND ACAS
1500FT/12000.
WDSPR 7KM IN RA OR RADZ, WITH 5-7/8ST 1000FT/1500 AND 6-8/8CUSC AND ACAS 1500FT/16000.
ISOL 3000M IN HVY RA OR FZRA, WITH 6-8/8ST 500FT/1500 AND 6-8/8CUSC AND ACAS 1500FT/18000.
OCNL, ISOL LEE MON, 2000M IN RADZ OR BR, WITH 6-8/8ST 400FT/1500 AND 6-8/8CUSC AND ACAS 1500FT/16000.
ISOL, 200M IN HVY SN, WITH 6-8/8ST 200FT/1500 AND 6-8/8CUSC AND ACAS 1500FT/16000.

WRNG: HILL FG. MOD ICE AND MOD TURB IN CLD. SEV ICE IN FZRA. SEV ICE IN CLD BTN 2500FT AND 6000FT. MTW MAX VSP 800FPM AT 8000FT. MOD, OCNL SEV, TURB BLW 6000FT. SN MON ABOVE 2500FT.

Zone 2
NE OF ZONE 1, AND SW OF A SLOW-MOV LINE SOUTH UIST TO ISLAY TO ISLE OF MAN TO ANGLESEY:
GEN 20KM WITH 5-8/8SC AND ACAS 2500-4000FT/10000.
ISOL 8KM IN RA OR RADZ, WITH 6-8/8CUSC AND ACAS 1500-2000FT/8000.
ISOL 4000M IN RASN OR SN, WITH 3-6/8ST 500-700FT/1500 AND 8/8CUSC AND ACAS 1500FT/8000.

WRNG: HILL FG. MOD ICE AND MOD TURB IN CLD. WDSPR MOD, ISOL SEV, TURB BLW 6000FT. MTW MAX VSP 600FPM AT 6000FT.

Meteorite58 · 13-02-2021 12:00am

Posted this over in the other thread by mistake, posting the charts here so can find it easier later to compare with actual results.

Will see how well AROME does this time, that is cumulative amounts, would expect much less settling than that, maybe about half or so.

Sat Pic highlighting the pronounced back edge of the front

Gaoth Laidir · 13-02-2021 12:10am

It's as if it's just for spite that north Kildare gets nowt at all!

Gaoth Laidir · 13-02-2021 12:18am

Snow warning now out for Belfast Aldergrove

Snow
Issued at: Fri 12 Feb 2021 - 22:45 UTC
Valid from: Sat 13 Feb 2021 - 09:00 UTC
Valid to: Sat 13 Feb 2021 - 14:00 UTC
A PERIOD OF SLIGHT TO MODERATE SNOWFALL (OR RAIN AND SNOW MIXED) IS LIKELY TO OCCUR AT THE AIRFIELD.SLIGHT TO MODERATE SNOWFALL IS FORECAST, WITH ACCUMULATIONS OF AROUND 2CM. SNOW TYPE WET/DRY. VISIBILITY IS EXPECTED TO FALL BELOW 600M IN THE SNOWFALL, WITH A MINIMUM VISIBILITY OF 400M. WINDS ARE EXPECTED TO BE GREATER THAN 15KT WITH NOTICEABLE DRIFTING.

Gaoth Laidir · 13-02-2021 12:20am

Rain now being reported at Gurteen and Athenry. Gurteen's wbt is up to 0.9 C. Still ok north and east of there for now.

Danno · 13-02-2021 12:29am

Gaoth Laidir wrote: »

Rain now being reported at Gurteen and Athenry. Gurteen's wbt is up to 0.9 C. Still ok north and east of there for now.

Hanging in here with a WB of -0.4c. Air temp now +0.2c

Gaoth Laidir · 13-02-2021 12:30am

Sligo Airport TAF just issued. Light sleet overnight, with a slight chance of light snow from 2 am.

TAF EISG 122300Z 1300/1309 12020G32KT 9999 SCT030 BKN050
TEMPO 1300/1309 14025G42KT 4000 -RASN BKN012
PROB30 TEMPO 1302/1309 1800 -SN BKN008=

Casement. Similar, with slightly better chance of light snow from 4 am.

TAF EIME 122300Z 1300/1309 13015G27KT 9999 FEW025
TEMPO 1300/1306 14020G35KT
TEMPO 1303/1309 4000 -RASN BKN012
PROB40 TEMPO 1304/1308 1200 -SN BKN004
BECMG 1306/1308 14025G40KT=

Dublin Airport. Sleet, with slight chance of light snow from 5 am.

TAF EIDW 122300Z 1300/1324 14020G33KT 9999 SCT020 BKN050
PROB40 TEMPO 1300/1312 15025G40KT
BECMG 1303/1305 BKN014
TEMPO 1304/1310 4000 -RASN BKN009
PROB30 TEMPO 1305/1309 1200 -SN BKN004
BECMG 1320/1322 16022G38KT=

Belfast Aldergrove. Light snow from 10 am - 2 pm, with slight chance of moderate from 10 am - 1 pm.

TAF EGAA 122258Z 1300/1324 12017KT 9999 SCT022
TEMPO 1300/1310 13020G32KT
BECMG 1306/1309 BKN012
TEMPO 1310/1314 14022G35KT 3000 -SN
PROB30 TEMPO 1310/1313 0400 SN BKN006
PROB30 TEMPO 1314/1324 7000 -RADZ BKN008
BECMG 1314/1317 15022G35KT=

Gaoth Laidir · 13-02-2021 12:53am

Snow warnings also out for Derry and Belfast City airports now. Both say the same thing.

Snow
Issued at: Fri 12 Feb 2021 - 23:32 UTC
Valid from: Sat 13 Feb 2021 - 06:00 UTC
Valid to: Sat 13 Feb 2021 - 12:00 UTC
SNOW (OR RAIN AND SNOW MIXED) IS LIKELY TO OCCUR AT THE AIRFIELD.SLIGHT TO MODERATE SNOW IS FORECAST, WITH ACCUMULATIONS OF 2CM. SNOW TYPE WET. VISIBILITY IS NOT EXPECTED TO FALL BELOW 600M IN THE SNOWFALL, WITH A MINIMUM VISIBILITY OF 800M. WINDS ARE EXPECTED TO BE GREATER THAN 15KT WITH THE POSSIBILITY OF DRIFTING.

Mount Vesuvius · 13-02-2021 1:19am

https://twitter.com/MetEireann/status/1360369827127394306?s=19

Gaoth Laidir · 13-02-2021 2:09am

Met Éireann's 06Z aviation chart has just one occlusion lying NW-SE through Aran Islands - Killarney - Bandon, moving NE at 15 kts. No other front at all drawn on the chart. No sign of the warm sector present at 00Z.

Nqp15hhu · 13-02-2021 2:12am

Gaoth Laidir wrote: »

Sligo Airport TAF just issued. Light sleet overnight, with a slight chance of light snow from 2 am.

TAF EISG 122300Z 1300/1309 12020G32KT 9999 SCT030 BKN050
TEMPO 1300/1309 14025G42KT 4000 -RASN BKN012
PROB30 TEMPO 1302/1309 1800 -SN BKN008=

Casement. Similar, with slightly better chance of light snow from 4 am.

TAF EIME 122300Z 1300/1309 13015G27KT 9999 FEW025
TEMPO 1300/1306 14020G35KT
TEMPO 1303/1309 4000 -RASN BKN012
PROB40 TEMPO 1304/1308 1200 -SN BKN004
BECMG 1306/1308 14025G40KT=

Dublin Airport. Sleet, with slight chance of light snow from 5 am.

TAF EIDW 122300Z 1300/1324 14020G33KT 9999 SCT020 BKN050
PROB40 TEMPO 1300/1312 15025G40KT
BECMG 1303/1305 BKN014
TEMPO 1304/1310 4000 -RASN BKN009
PROB30 TEMPO 1305/1309 1200 -SN BKN004
BECMG 1320/1322 16022G38KT=

Belfast Aldergrove. Light snow from 10 am - 2 pm, with slight chance of moderate from 10 am - 1 pm.

TAF EGAA 122258Z 1300/1324 12017KT 9999 SCT022
TEMPO 1300/1310 13020G32KT
BECMG 1306/1309 BKN012
TEMPO 1310/1314 14022G35KT 3000 -SN
PROB30 TEMPO 1310/1313 0400 SN BKN006
PROB30 TEMPO 1314/1324 7000 -RADZ BKN008
BECMG 1314/1317 15022G35KT=

Is the front going to stall? It is already nearly at NI. So can’t imagine it snowing here tomorrow afternoon sadly.

Gaoth Laidir · 13-02-2021 11:05am

I think the reason for this ice pellets overnight was due to a layer of warm air aloft causing melt, with refreezing occuring again in the cold surface layer. The 23Z Camborne sounding (near Land's End) shows this layer nicely and a strong south-southeasterly low-level jet capable of advecting this layer up towards eastern Ireland later.

Red=temperature
Dark blue= wet-bulb temperature
Cyan=dewpoint.

Gaoth Laidir · 13-02-2021 2:39pm

Nice snow sounding from Castor Bay at 11Z today. A deep saturated layer at around -2 to -5 °C should be producing small flakes of plates or needles and columns. Not great for the more dentritic-type flakes, which need a saturation at below -10 °C.

Surface temp/wbt/dewpoint were 0.1/-0.1/-0.4
500-1000 hPa thickness: 535 dam
850-1000 hPa thickness: 129 dam

DOCARCH · 13-02-2021 2:45pm

Gaoth Laidir wrote: »

That diagram is clearly not correct as it is missing dinner plates! Ask Aunty!

Gaoth Laidir · 13-02-2021 2:57pm

DOCARCH wrote: »

That diagram is clearly not correct as it is missing dinner plates! Ask Aunty!

13-02-2021 4:49pm

DOCARCH wrote: »

That diagram is clearly not correct as it is missing dinner plates! Ask Aunty!

Neddyusa · 13-02-2021 7:47pm

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

Kaybaykwah · 13-02-2021 7:56pm

Neddyusa wrote: »

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

In spite of the relatively occurence of "snow events" in Ireland, I feel safe in the thought that Irish forumers can find more words than the Inuit to intuit every type of snow imaginable.

snowgal · 13-02-2021 9:03pm

Neddyusa wrote: »

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

Agree it’s an interesting question for regular followers and I’d like to know too. As you say not in any argue way but purely out of interest. Also agree re charts...

Oneiric 3 · 13-02-2021 9:14pm

Neddyusa wrote: »

When a "snow event" doesn't materialise as per the forecasts of the OP, posters either disappear or these threads tend to either die or descend into childish bickering.

I think many of the forum regulars would actually appreciate a technical/model review of why the weather didn't turn out as predicted.

Purely technical anaysis and not an "I told you so" type thing.

Any takers?
Eg - why did Thursday and Fridays fronts not follow the track or intensity predicted even at T-6.
Or
Which models performed best etc

My humble theory is that Thursday's front, coming up against a dry and relatively cold easterly, was regularly 'enhanced' (given its near stationary status) by "orogenic uplift' as it pushed up against the eastern side of the western coastal uplands, which I think may explain why it was regions close by got the greater snow totals, given what little energy this front contained was constantly being forced into that particular corner of the country for hours on end.

Analysis map for midnight Thursday might better help explain:

The pattern, in hindsight, is actually pretty straight forward, but many of us, myself included and even Met Eireann themselves didn't see it for what it was in the lead up to it.

snowgal · 13-02-2021 9:30pm

Oneiric 3 wrote: »

My humble theory is that Thursday's front, coming up against a dry and relatively cold easterly, was regularly 'enhanced' (given its near stationary status) by "orogenic uplift' as it pushed up against the eastern side of the western coastal uplands, which I think may explain why it was regions close by got the greater snow totals, given what little energy this front contained was constantly being forced into that particular corner of the country for hours on end.

Analysis map for midnight Thursday might better help explain:

The pattern, in hindsight, is actually pretty straight forward, but many of us, myself included and even Met Eireann themselves didn't see it for what it was in the lead up to it.

Thanks for that. Do you mean the front didn’t have as much energy as we thought initially?

Oneiric 3 · 13-02-2021 9:33pm

snowgal wrote: »

Thanks for that. Do you mean the front didn’t have as much energy as we thought initially?

Not really, more that the front itself wasn't a particularly active one. If it was, then this may have gone down as something historic.

snowgal · 13-02-2021 9:40pm

But obviously something showed up initially where everyone thought it was, like Met Eireann etc. Did it just come off weaker than first predicted?

Oneiric 3 · 13-02-2021 9:47pm

snowgal wrote: »

But obviously something showed up initially where everyone thought it was, like Met Eireann etc. Did it just come off weaker than first predicted?

No, as I tried to explain, it's slow movement allowed totals to build up in regions where the front was subject to near continuous orogenic uplift.

snowgal · 13-02-2021 9:55pm

Ok thanks

Oneiric 3 · 13-02-2021 10:25pm

snowgal wrote: »

Ok thanks

This radar for Thursday evening will explain better than I can. Front was most active in and around the hills of western Connacht as the lower air mass from the east was continually pushing up against it:

Even still, I think the highest total (rainfall equivalent) was about 18mm in Newport, which isn't an extraordinary total for a front that lasted over 12hrs, hence why I said it wasn't an active frontal feature by any means.

Oneiric 3 · 13-02-2021 11:24pm

Regarding last night, this is what the Apegge model was showing for 3am with regards Theta-E, which I know nothing about except that I think it shows the air mass type at roughly the height of the boundary layer.

Seems a little 'warmer' than I would have thought? Compare, for example, what the same model was showing during the height of the event on the 11th:

I'll leave greater minds than I to explain, if they so wish, what might have gone on here regarding this particular parameter and its influence, if it had any at all, on last night's frontal passage.

Edit: and here at the 850hpa temp equivalence for the two charts above (as per the ICON)

3am last night:

9pm on the 11th:

Cold Spell - Feb 7th to 13th 2021 - Technical Discussion

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