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Snow Showers and potential Accumulations - Ulster (Northern especially)

  • 29-12-2020 5:47pm
    #1
    Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    I thought, I'd go ahead and create a thread for our snow risk as it is a different synoptic situation to the system affecting the southern half of the island.

    I will post screenshots below of peak parameters and the required parameters for snow to give an insight into the expected conditions. It is going to be relatively cold here over the next 24-30 hours.

    850hPa Temperatures (<-5c at least)

    11pm Tonight:

    U2u6UJK.png

    Midday Tomorrow:
    u5vPSNp.png
    Freezing Level (must be <500 metres):

    11pm Tonight:
    KKl5fxy.png
    Noon Tomorrow:
    EcB4YQH.png

    Wet Bulb Zero Level (must be <300 metres):


    Midnight:
    K8bZsoz.gif

    3pm Tomorrow:
    hOVdJT0.gif

    Max Temps of 2-3c:
    yXUsWBg.png

    Snow showers will hug the north, northwest and western coasts from 10pm:

    pS7iJbE.png

    Soundings:


    Derry at midnight (-0c):
    AS4mfgl.png
    There is a small warm layer, but this is not thick or warm enough to prevent snow.

    Letterkenny at midnight (1c):
    RyEASUF.png

    Midday Derry (1c):
    GXT9W0L.png

    Midday Letterkenny (3c):
    Aqb3HOD.png

    Red Line = Air Temperature
    Blue Line = Wet Bulb Temp (<1c for snow)
    Green Line = Dew Point Temp

    You follow the red line to along the diagonal hatched lines and read across to get the freezing level. I referenced a warm layer as you can see the red line moves to the right after passing the freezing level for a short while.

    Summary:

    So far us in Ulster, especially the North and Northwestern regions, we have a very high chance of seeing snow tonight or tomorrow. It is unlikely that snow will fall in places such as Enniskillen, Belfast or Cookstown but if you live in Letterkenny, Derry etc you have a very high chance.

    Whether you get a covering will depend on the location of the showers. It is not clear what the snow line will be, but perhaps there could be coverings at sea level under persistent or frequent showers.

    But it will be a cold day tomorrow not getting above 2c in most places.

    Please be aware that temperatures will not start to drop to 'snow territory' until later this evening. So don't be expecting snow yet.

    Met Office Warning: https://www.metoffice.gov.uk/weather/warnings-and-advice/uk-warnings#?date=2020-12-30&id=f9cbf32d-fa49-4c06-883d-5714329b09d8

    Met.ie Warning: https://www.met.ie/warnings/tomorrow/donegal


«13456

Comments

  • Registered Users Posts: 32 polarhullar


    Thanks for this! Great to have someone who knows what they're talking about in the North West and from my area (10 miles away).


  • Registered Users Posts: 12,113 ✭✭✭✭ joujoujou


    Weather warning issued for (almost) entire Ulster as well as Sligo and Leitrim.

    ww.png


  • Registered Users Posts: 110 ✭✭ J Cheevor Loophole


    Just a kilometer away from the sea here in Donegal, so it should be an interesting watch. Many thanks for this thread.


  • Registered Users Posts: 54 ✭✭ booferking


    The NI Weather & Flood Advisory Service has issued the following weather warning.

    Warning of Snow & Ice has been issued for Northern Ireland.

    Warnings valid from 18:00 29/12/2020 until 23:00 30/12/2020

    Showers are expected to develop move in and affect parts of NI (especially the North & West) as it becomes colder showers will turn increasingly to sleet & snow. (Rain/sleet around coastal areas)

    Showers heaviest and most frequent within the snow warning area (North & West), showers likely to make it further east but likely to be less heavy (Warning may be amended)

    Where snow falls, accumulations of several cm's possible, especially over the hills and where showers merge to give longer spells of sleet/snow. However these may be very localised as they are in the form of showers.

    Ice will be an additional hazard with temperatures expected to drop well below freezing. Showers & snow melt will leave surfaces wet and may wash grit off roads & footpaths. The public should take extra care overnight and during tomorrow morning.

    NB: Although our snow warning covers much of NI, not everywhere will have snowfall while other areas have snowfall, as it will be in the form of showers, please bare this in mind if travelling overnight & tomorrow morning as conditions could change from place to place. Rain/sleet most likely around coastal areas

    The public should be aware of :-
    Accumulations of snow, especially on high level routes along with Ice which will lead to disruption to travel overnight and during tomorrow morning.


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    3.1c now.. shower incoming would be temped to drive to 300m but cba.

    For anyone enquiring about surface temps, use this site. It’s already subzero surface on the higher routes.

    http://www.trafficweather.info/roadWeather/rwisMap.jsp

    The dew points aren’t right though.


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  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    Icy rain now


  • Registered Users Posts: 2,023 ✭✭✭ Donegal Storm


    I'm right on the coast in Donegal for the week so doubt we'll see anything here, better chance back in Letterkenny. Lots of hail showers these past few days and plenty of snow on the hills already so hopefully someone gets lucky


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    I Don’t like the SE trajectory of the showers.. they mostly end up in north Antrim or weaken over the Donegal mountains. I would prefer a straight southerly trajectory.


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    http://www.meteociel.com/cartes_obs/sondage_display.php?id=3918&map=2&date=1609239600&map2=2. Surface air temp/wbt (Theta W)/dewpoint were 4.6/3.6/2.3 C, with 850 hPa temp -3.7 C. Wet-bulb zero height of ~700 m, snowline ~600 m.

    It will be launched again at 00Z, so we'll see how things have cooled down by then.

    To get the sounding in tabular form, click here, click on the station and then select "Tableau de valeurs" from the drop-down list bottom right.

    537546.PNG


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    Thanks! That’s a great site and is interesting. Temp here is static at 2.7c.

    Not sure if dew points are supposed to drop.


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  • Registered Users Posts: 12,113 ✭✭✭✭ joujoujou


    Some snowfalls seen a few minutes ago, just a couple of miles south of co. Sligo border.


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Some lying snow reported at a few stations in England, mostly 200-300 m, but one lower one near Manchester (Rosethorne, 35 m amsl) has 2 cm.

    537566.png

    Wet-bulbs are perfect throughout much of Ireland tonight but n'er a drop of precip to be had, of course.

    537567.png


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    My temperature is on the way down. It is extremely marginal. Surprising.

    We have rain right now.


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    Some lying snow reported at a few stations in England, mostly 200-300 m, but one lower one near Manchester (Rosethorne, 35 m amsl) has 2 cm.

    537566.png

    Wet-bulbs are perfect throughout much of Ireland tonight but n'er a drop of precip to be had, of course.

    537567.png

    I took a nap but it looks like it’s rained a few times here in the last hour.

    The temps are not that warm though. I hope it becomes a bit less marginal.

    And if you looked at most of those stations the temps would probably rise above zero with altitude.


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    This is a higher altitude than me (but not immediately near me) and it is slightly warmer but also equally terrible dew point.

    Just to show that temps aren’t at least dropping with altitude.

    https://www.wunderground.com/dashboard/pws/ILIMAVAD8


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    Soundings for midnight and 6am... again worried about that warm layer., and it is gone by 6am but will that be too late?

    Such a waste of cold air. I really hope I see snow (at my house).


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Nqp15hhu wrote: »
    This is a higher altitude than me (but not immediately near me) and it is slightly warmer but also equally terrible dew point.

    Just to show that temps aren’t at least dropping with altitude.

    https://www.wunderground.com/dashboard/pws/ILIMAVAD8

    Latest wet-bulb temperatures. I see Lough Fea (225 m) hasn't been reporting for the past few hours. At 7 pm it was temp 1.6, dp -0.3, wbt 0.8. Just a little too high. Need wbt 0.5 or below.

    https://meteologix.com/ie/observations/northern-ireland/wet-bulb-temperature/20201229-2200z.html


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    Latest wet-bulb temperatures. I see Lough Fea hasn't been reporting all day.

    https://meteologix.com/ie/observations/northern-ireland/wet-bulb-temperature/20201229-2200z.html

    Wet bulb temps are useless if there is a warm layer above. Which there appears to be.


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Nqp15hhu wrote: »
    Wet bulb temps are useless if there is a warm layer above. Which there appears to be.

    No they're not necessarily. If the wbt is not warmer above then snow will remain at the wbt as it falls.


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    No they're not necessarily. If the wbt is not warmer above then snow will remain at the wbt as it falls.

    I need the warm layer to disappear


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  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Nqp15hhu wrote: »
    I need the warm layer to disappear

    Don't we all!!


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    Would just like to know when it will become productive for snow because it’s a waste of time sitting waiting now.

    These warm layers are hopeless and happened here before. No snow is coming in the next few hours.


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Nqp15hhu wrote: »
    Would just like to know when it will become productive for snow because it’s a waste of time sitting waiting now.

    These warm layers are hopeless and happened here before. No snow is coming in the next few hours.

    925 hPa (650 m) temperature should fall a degree or two overnight, getting to -2 C by 6 am.

    https://meteologix.com/ie/model-charts/rapid-euro/ireland/temperature-925mb/20201230-0000z.html


  • Registered Users Posts: 5,951 ✭✭✭ munsterlegend


    925 hPa (650 m) temperature should fall a degree or two overnight, getting to -2 C by 6 am.

    https://meteologix.com/ie/model-charts/rapid-euro/ireland/temperature-925mb/20201230-0000z.html

    Could you not just go up there GL and do something quicker?


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    925 hPa (650 m) temperature should fall a degree or two overnight, getting to -2 C by 6 am.

    https://meteologix.com/ie/model-charts/rapid-euro/ireland/temperature-925mb/20201230-0000z.html

    Ok well hopefully that straightens things out.


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Nqp15hhu wrote: »
    Would that warm layer be present beneath that though? It appears to run up to 900 hPa.

    Check Castor Bay sounding after midnight to get an idea.

    It's snowing and sticking in Dungiven at the moment, apparently. Glenshane Pass too.
    Station : A6 Glenshane Pass
    Organisation : DFI Roads

    Time : 29.12.2020 23:00 Europe/Belfast

    Relative Humidity (1) 98 %

    Rain State (1) l.snow

    Wind Speed (1) 3.9 m/s

    Surface Temperature (1) -1.3 °C


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Nqp15hhu wrote: »
    Note the very high humidity so must be marginal

    Not if the temperature is below zero, which it probably is up there.

    Magilligan No. 2, north of you, on the coast, is currently temp 2.7, dewpoint 1.2, wbt 2.1 C. The marine layer is just keeping things too marginal. Just depends on how far back towards it the coast it can retreat from you.

    AAXX 29234 03907 45682 /2807 10027 20012 30014 40022 50001
    333 55300 20000 81/37 86/58 85/68
    555 7/098=


  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    Not if the temperature is below zero, which it probably is up there.

    Magilligan No. 2, north of you, on the coast, is currently temp 2.7, dewpoint 1.2, wbt 2.1 C. The marine layer is just keeping things too marginal. Just depends on how far back towards it the coast it can retreat from you.

    AAXX 29234 03907 45682 /2807 10027 20012 30014 40022 50001
    333 55300 20000 81/37 86/58 85/68
    555 7/098=

    Perhaps it is yes. Wet bulb is 1.1c here. But why would a marine layer cause an inversion at altitude?


  • Registered Users Posts: 8,202 ✭✭✭ Gaoth Laidir


    Nqp15hhu wrote: »
    Perhaps it is yes. Wet bulb is 1.1c here. But why would a marine layer cause an inversion at altitude?

    It's not causing the inversion, it's the surface cooling underneath it that is, forming a very shallow cold layer underneath the warmer marine air.


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  • Banned (with Prison Access) Posts: 2,254 ✭✭✭ Nqp15hhu


    It's not causing the inversion, it's the surface cooling underneath it that is, forming a very shallow cold layer underneath the warmer marine air.

    Ok hard to visualise.. would think the sea air wouldn’t affect what’s going on at 200-300 metres but will just keep watching temps.

    Edit; raining again at 1.9c... bit icy


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