Aviation weather thread

Gaoth Laidir

Carnacalla wrote: »

Dublin's Temperature has been fairly jumpy!

20:00 0c
20:30 -3c
21:00 -1c
21:30 -3c
22:00 -2c

A bit strange. The synops gave a steady cooling trend as the sky cleared. I don't think those half-hourly METARs are correct. Very unlikely to get such fluctuations in these conditions.

18:00 2.1 (2/8 cloud)
19:00 0.9 (1/8 cloud)
20:00 0.1 (1/8 cloud)
21:00 -1.0 (0/8 cloud)
22:00 -1.9 (0/8 cloud)

JCX BXC

Reporting -4 now!

Gaoth Laidir

Carnacalla wrote: »

Reporting -4 now!

And yet only -2.8 on the 23Z synop. The metars are wonky.

smurfjed

synops

Where are the synops measured?

Gaoth Laidir

smurfjed wrote: »

Where are the synops measured?

The same as the metars. Both are manually reported. Just not sure why the metar temps seemed off on the half hours, though the 23.30 one looks more in line now.

JCX BXC

FZFG in the forecast for NOC,ORK and DUB for the morning.

Gaoth Laidir

Carnacalla wrote: »

FZFG in the forecast for NOC,ORK and DUB for the morning.

And Casement now too.

TAF EIME 242000Z 2421/2506 28005KT 9999 SCT025 SCT045
TEMPO 2423/2506 4000 BR
PROB30 TEMPO 2503/2506 0800 FZFG FEW004=

bronn

Carnacalla wrote: »

FZFG in the forecast for NOC,ORK and DUB for the morning.

Sounded like you sneezed there. :pac:

BMJD

Gaoth Laidir wrote: »

And yet only -2.8 on the 23Z synop. The metars are wonky.

They aren't wonky. There can be a few minutes' difference between the metar and synop reading hence the apparent fluctuation. The M04 could well have been a reading of -3.6 (rounded to -4) so the difference could have been 0.8 degrees, which isn't huge.

Gaoth Laidir

BMJD wrote: »

They aren't wonky. There can be a few minutes' difference between the metar and synop reading hence the apparent fluctuation. The M04 could well have been a reading of -3.6 (rounded to -4) so the difference could have been 0.8 degrees, which isn't huge.

The wild fluctuations to -3 only on the half hour and back up again on the hour on an otherwise steadily decreasing trend in clear and calm conditions are highly unlikely. On the hour they agreed with the synops, but the half-hourlies were anomalously low at -3 (even if that was -2.6) e.g. jumping from 0.1 to -2.6 to -1.0 to -2.6 to -1.9 every 30 minutes. I just don't buy it at all. This could be possible in different conditions of higher wind or variable cloud cover, but not last night.

18:00 2.1 (2/8 cloud)
19:00 0.9 (1/8 cloud)
20:00 0.1 (1/8 cloud)
20:30 -3 METAR
21:00 -1.0 (0/8 cloud)
21:30 -3 METAR
22:00 -1.9 (0/8 cloud)
22:30 -4 METAR
23:00 -2.8 (0/8 cloud)
23:30 -4 METAR
00:00 -3.5 (0/8 cloud)

BMJD

So you're saying it's impossible?

Gaoth Laidir

BMJD wrote: »

So you're saying it's impossible?

I underlined it above. It is highly unlikely in the conditions we had that night. Nigh on impossible. I think human error is vastly more probable. I'm running out of quantifiers here...

JCX BXC

Currently Foggy with a his of 400m at SNN. Ironically it's currently the only foggy airport, and last night it was the only major airport without fog in the forecast.

Gaoth Laidir

Carnacalla wrote: »

Currently Foggy with a his of 400m at SNN. Ironically it's currently the only foggy airport, and last night it was the only major airport without fog in the forecast.

Slightly up on its 300 m earlier this morning. It had been reporting 12 km with shallow fog for a few hours.

BMJD

Gaoth Laidir wrote: »

I underlined it above. It is highly unlikely in the conditions we had that night. Nigh on impossible. I think human error is vastly more probable. I'm running out of quantifiers here...

For human error to have occurred, that would mean that somebody overrided an automatic weather system 3 or 4 times for no reason. If it was a computer error, which it couldn't because they have alarms for large variations, as well as back-up systems in case of instrument failure. ATC would probably have requested a temperature check too.

It's unusual but obviously not impossible.

Gaoth Laidir

BMJD wrote: »

For human error to have occurred, that would mean that somebody overrided an automatic weather system 3 or 4 times for no reason. If it was a computer error, which it couldn't because they have alarms for large variations, as well as back-up systems in case of instrument failure. ATC would probably have requested a temperature check too.

It's unusual but obviously not impossible.

The reports are coded manually so human error is indeed the most likely reason.

BMJD

Gaoth Laidir wrote: »

The reports are coded manually so human error is indeed the most likely reason.

They aren't.

Gaoth Laidir

BMJD wrote: »

They aren't.

The metars and synops are 100% manual observations at DUB, 24/7/365.

BMJD

Gaoth Laidir wrote: »

The metars and synops are 100% manual observations at DUB, 24/7/365.

Temperature, wind and pressure values are all automated; the only human input is to review and send the data. The only way human error can occur is if those values are manually changed.

Gaoth Laidir

BMJD wrote: »

Temperature, wind and pressure values are all automated; the only human input is to review and send the data. The only way human error can occur is if those values are manually changed.

My point exactly.

Human input also involves visibility, cloudbase, type, coverage, present/past weather, etc. All collated together with the other data and sent off in coded format.

BMJD

Gaoth Laidir wrote: »

My point exactly.

Human input also involves visibility, cloudbase, type, coverage, present/past weather, etc. All collated together with the other data and sent off in coded format.

I'm not sure how that validates your point about this:

Gaoth Laidir wrote: »

I think human error is vastly more probable.

I will say it again: those temperature values were not inputted manually. At no stage of the production of a metar or synop are they manually entered or coded - it is automated. Before they are transmitted they are reviewed. That is the only human input regarding the temperature/dew point.

Gaoth Laidir

BMJD wrote: »

I'm not sure how that validates your point about this:



I will say it again: those temperature values were not inputted manually. At no stage of the production of a metar or synop are they manually entered or coded - it is automated. Before they are transmitted they are reviewed. That is the only human input regarding the temperature/dew point.

And I'm saying that this review was erroneous. It gets back to simple physics. In years of studying meteorology I have never seen consistent fluctuations such as those highlighted above happening in the conditions in which they did. There is no atmospheric process whereby this can happen in such conditions. Therefore it is down to instrument or human error. If it's instrument error then the human review did not pick it up. Either way, those half-hourly metar temperatures were "wonky".

BMJD

And yet Belfast Aldergrove showed a similar, though less pronounced, series of variations that evening:

METAR EGAA 231720Z 34005KT 310V010 9999 FEW045 04/M01 Q1022=
SA 23/02/2016 17:50->
METAR EGAA 231750Z 33004KT 290V360 CAVOK 03/M02 Q1022=
SA 23/02/2016 18:20->
METAR EGAA 231820Z 35003KT CAVOK 02/M02 Q1022=
SA 23/02/2016 18:50->
METAR EGAA 231850Z 00000KT CAVOK 01/M03 Q1022=
SA 23/02/2016 19:20->
METAR EGAA 231920Z VRB02KT CAVOK 01/M02 Q1022=
SA 23/02/2016 19:50->
METAR EGAA 231950Z 00000KT CAVOK M01/M03 Q1022=
SA 23/02/2016 20:20->
METAR EGAA 232020Z 00000KT CAVOK M01/M03 Q1022=
SA 23/02/2016 20:50->
METAR EGAA 232050Z 00000KT CAVOK M02/M04 Q1022=
SA 23/02/2016 21:20->
METAR EGAA 232120Z 15003KT CAVOK M03/M04 Q1022=
SA 23/02/2016 21:50->
METAR EGAA 232150Z 19003KT CAVOK M01/M03 Q1022=
SA 23/02/2016 22:20->
METAR EGAA 232220Z 19002KT CAVOK M02/M03 Q1022=
SA 23/02/2016 22:50->
METAR EGAA 232250Z NIL=
SA 23/02/2016 23:20->
METAR EGAA 232320Z 16004KT CAVOK M01/M03 Q1022=
SA 23/02/2016 23:50->
METAR EGAA 232350Z 19003KT CAVOK M02/M03 Q1022=
SA 24/02/2016 00:20->
METAR EGAA 240020Z 18003KT 9999 FEW042 M00/M01 Q1021=
SA 24/02/2016 00:50->
METAR EGAA 240050Z 19004KT 9999 BKN042 M00/M02 Q1021=
SA 24/02/2016 01:20->
METAR EGAA 240120Z 20004KT 9999 BKN046 00/M01 Q1021=

Gaoth Laidir

Well in this case the difference is even worse! Absolutely no correlation with the synop values, so I'm not sure if they actually have different sensors for different types of reports or what.

The metars are 10 minutes before the hour and half hour, so every hour there are 3 readings plotted in the space of 30 minutes, e.g. 19:50, 20:00, 20:20. The chances of natural fluctuations up and down between such close metar and synop reports is zero. This is even more proof that there is some other reason, be it instrumental or human, here too.

Cloud cover from the hourly synops is also shown below.



Here's Dublin again in clearer form.

Gaoth Laidir

Recent sleet has now turned to light snow at Cork at 6 pm.

METAR EICK 261800Z 36016KT 4000 -SN BKN003 BKN006 02/01 Q1000 NOSIG=

JCX BXC

From what I've learned, should snow not drastically decrease the visibility?

Negative_G

-SN (Light snow) would not drastically decrease visibility.

SN OR +SN would see a larger reduction.

Growler!!!

Carnacalla wrote: »

From what I've learned, should snow not drastically decrease the visibility?

9999 in the metar means in excess of 10km met vis. A decrease to 4km is a good drop but not an issue for approaches. Biggest issue with snow is contaminated runways, taxiways and it will affect hold over times when de-icing.

Gaoth Laidir

Up to 3 cm of snow forecast at Cork tonight.

EICK AD WRNG 01 VALID 261756/270300 SNOW FBL 1 TO 3CM FCST =

At 7 pm it was reported at <1 mm.

Gaoth Laidir

Back to light rain at 8 pm.

JCX BXC

Back to snow again, moderate snow at 10pm.

JCX BXC

Snow was removed from the TAF of Cork at 11pm, yet its been snowing since.
Fairly poor forecasting.

Growler!!!

Carnacalla wrote: »

Snow was removed from the TAF of Cork at 11pm, yet its been snowing since.
Fairly poor forecasting.

Careful now, read it again:

TAF: EICK 262300Z 2700/2724 02017G28KT 9999 SCT005 BKN010 TEMPO 2700/2708 3000 -RA SCT003 BKN006 PROB40 TEMPO 2700/2703 1500 RASN SCT001 BKN003 BECMG 2702/2704 04013KT BECMG 2721/2723 35005KT

40% chance of rain and snow from midnight to 3am.

JCX BXC

Rain snow being sleet?
There's a distinctive difference between Sleet and Snow.

At least that's how In understand it. When you have RASN reported in a Metar its sleet.

Growler!!!

Carnacalla wrote: »

Rain snow being sleet?
There's a distinctive difference between Sleet and Snow.

At least that's how In understand it. When you have RASN reported in a Metar its sleet.

Haha you're being pedantic now. Is it not snowing? Between midnight and 3?

JCX BXC

Growler!!! wrote: »

Haha you're being pedantic now. Is it not snowing? Between midnight and 3?

Not really though.

RA
RASN
SN
All different things, of course I don't have the qualifications to say how much the difference between RASN and SN is important, but still there is a fairly large difference. Sleet is almost depressing, whereas snow is fun!

Gaoth Laidir

Carnacalla wrote: »

Not really though.

RA
RASN
SN
All different things, of course I don't have the qualifications to say how much the difference between RASN and SN is important, but still there is a fairly large difference. Sleet is almost depressing, whereas snow is fun!

In Table 4678 of the WMO's official Manual on Codes 306 there is no official code for "Sleet" as we know it here (a mix of rain and snow or melting snow). In the US their sleet is our Ice Pellets (PL), which are translucent balls of ice. The NWS in the US does officially use the word Sleet for Ice Pellets.

RASN in a METAR is used to mean a mix of Rain and Snow, both occurring at equal probability. Yes, this is what we colloquially call Sleet.

So if a TAF calls for RASN then individual METARS during that period could show either RA, SN or RASN (a mix of rain and snow).

Anyway, it did turn out to be more snow than rain at Cork as the synops reported 2 cm of lying snow between 2 and 4 am.

Pat Dunne

Gaoth Laidir wrote: »

In the US their sleet is our Ice Pellets (PL), which are translucent balls of ice. The NWS in the US, however, does officially use the word Sleet for Ice Pellets.

Are Ice Pellets, what we refer to as Hail / Hail Stones in this part of the world?

Gaoth Laidir

Pat Dunne wrote: »

Are Ice Pellets, what we refer to as Hail / Hail Stones in this part of the world?

No, hail is opaque and given the code GR (diameter >5 mm) or GS (<5 mm). Hail forms by a totally different process.

Ice Pellets form when there is a warm layer a couple of thousand feet up, with a sub-zero layer below it down to the surface. Snow from above the warm layer semi-melts in the warm layer but then refreezes in the surface layer.

calnand

Currently sitting on a plane at cork, waiting for the runway to be cleared of snow, they said 20mm is remaining after two sprays they're attempting one more before deciding to scrape it.

Growler!!!

No take offs or landings for over 2 hours, now one in the hold. All down to snow and ice. Why hasn't a SNOCLO been issued in the metar? Or even a snowtam?

Gaoth Laidir

Growler!!! wrote: »

No take offs or landings for over 2 hours, now one in the hold. All down to snow and ice. Why hasn't a SNOCLO been issued in the metar? Or even a snowtam?

I was wondering that myself. At least one or the other or runway contamination details on the METARs.

Strang, though, that the 7 and 8 am synops only reported <1 mm of lying snow.

pepe the prawn

I take TAF's to be an indication of what weather conditions may exist in an area over a particular period of time. Weather forecasting is a dark art, and so if a taf says RA SN, then I'd be planning for rain, sleet, snow, and any other associated conditions.

I remember last year when the weather was CAVOK at ORK, WAT and SNN, yet instructors wouldn't let students off to do solo navs because the visibility was so poor with haze, yet it wasn't in the METARS, vis was actually stated as being 9999. Even though realistically it was probably around 5-6k at best.

It must be remembered that forecasts are just that, forecasts. They are not definitive statements of the exact conditions that will be present at a particular time, they are predictions. And just because it says RA SN in the TAF, doesn't mean that you will only get sleet and no snow.

JCX BXC

I understand the bit about them only being forecasts, which are in essence only predictions. Its the fact that when the forecast was made it was snowing, and was for hours before and after. Yet this was not reflected in the TAF

Growler!!!

Carnacalla wrote: »

I understand the bit about them only being forecasts, which are in essence only predictions. Its the fact that when the forecast was made it was snowing, and was for hours before and after. Yet this was bit reflected in the TAF

Ah here, you're going to be SLEETING on about this all day!:D:):D

I'll get my coat...........................

Gaoth Laidir

Carnacalla wrote: »

I understand the bit about them only being forecasts, which are in essence only predictions. Its the fact that when the forecast was made it was snowing, and was for hours before and after. Yet this was bit reflected in the TAF

I agree. Although past weather does not guarantee future returns, the TAF should have at least mentioned PROB40 SN. There was a half a degree either way between snow and rain/sleet, call it whatever, but there were several hours of SN reports overnight and as such this led to delays this morning.

Not that a different TAF would have made one blind bit of difference to the actual weather that happened. If only!!

JCX BXC

Currently Snowing at BFS.

Snow in the TAF's for DUB,NOC and BFS.

Gaoth Laidir

Gust of 60 kts in the last hour at Casement.

PsMETAR BALD 020700Z 28041G60KT 9999 -SHRA SCT015 OVC026 02/01 0993 MSL=

Gaoth Laidir

3 mm of wet snow on the runway at DUB this morning. There seem to have been a few delays

Gaoth Laidir

Severe frost warnings out for DUB and BAL up to 09Z. May cause deicing delays at DUB.

●EIME AD WRNG 01 VALID 052100/060900 FROST SEV GND AND MOD AIR DEPOSITIONS LIKELY FCST =
●EIDW AD WRNG 01 VALID 052000/060900 FROST SEV GND AND MOD AIR DEPOSITIONS LIKELY FCST =

Not as bad at the other airports.

●EINN AD WRNG 01 VALID 052100/060800 FROST MOD GND AND FBL AIR DEPOSITIONS LIKELY FCST =
●EICK AD WRNG 01 VALID 052100/060900 FROST MOD GND AND FBL AIR DEPOSITIONS LIKELY =
●EIKN AD WRNG 01 VALID 052100/060900 FROST FBL GND AND FBL AIR DEPOSITIONS LIKELY FCST =