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Thaw and possible flooding 26th-27th -- forecasts and discussion

  • 24-12-2010 7:58pm
    #1
    Registered Users Posts: 14,268 ✭✭✭✭M.T. Cranium


    I hate to post this thread, looking at all the lovely snow pictures on the forum, but I think there will be significant and possibly some locally severe impacts from a brief thaw and potential flooding.

    All the models are now more or less in agreement on the timing and we can be fairly confident that a widespread thaw will begin in the west early 26th while across the east it will come more gradually through the later 26th into the 27th. With this thaw, there could be as much as 10-30 mms of rain. Now it's possible that some snow and some sleet, freezing rain will fall ahead of this rain across parts of the east and north, but that won't change the outcome, it will only add more potential for the eventual mess to come. To give some idea of the pace of change, expect -15 C Christmas morning, -5 C Christmas afternoon, zero by midnight, +5 C by mid-day 26th, and +7-9 by mid-day 27th in most parts of Ireland. There will be local variations during the warming period as cold air gets trapped in valleys. Somewhere like coastal Kerry would warm up a lot faster than somewhere like a valley in eastern Mayo.

    The operative word here is mess -- few places will get a quick, no-nonsense thaw. Lower elevations of the southwest and west that have snow cover may get this, a quick return to "normal" Irish winter conditions with little more than some local flooding. Further up in elevation across the southwest and large parts of Connacht, Donegal, west Ulster, I would expect this thaw to take most of the 26th and the night of the 26th-27th to remove all but high elevation snow, and that with the potential impacts of road flooding, stream flooding, and mudslides in a few places, cutting off some roads.

    Further east, where there is substantial snow, the arrival of milder air will be delayed by the slow retreat of the arctic high, and the main impacts will be delayed to the 27th although they could start during the night of 26th-27th. Given the depth of snow in some parts of Leinster and Ulster, everyone in a flood-prone location probably needs to be on high alert for locally severe flooding. It will all depend on how much snow melts in higher reaches of your local drainage, or how much rain adds to the snow melt in low-lying drainage areas. Just like the snow streamers, this will be largely a wait and see situation but better safe than sorry, I wanted to sound a fairly strong alert here and hope it won't be needed, since many readers of the forum might have other things to do tomorrow and Sunday.

    Of course, the deeper snow well up the slopes of the local hills and mountains will likely not melt, at least not very much, and yet there could be snow slides in a few places so I would caution against expeditions into the hills on skis or any other form of transportation during this unstable period.

    A potential hazard that may not be so obvious is that of roof damage or collapse. Flat roofed buildings, especially where snow has built up, can start to strain under the added weight of saturated snow and cave in. This seems especially true for modern-style office buildings and recreational buildings, most houses with peaked roofs should just see their snow sliding off when it thaws. Larger buildings tend to have a more resistant form of engineering design, the sort of building to monitor is where you have half a dozen shops or offices or residential town houses in a relatively new building that is 1-2 storeys and flat-roofed. In Canada, we've had problems with arenas (rinks) caving in during similar weather sequences, so any buildings like that would be worth checking.

    This thread can become a meeting place for those with observations about the thaw and flooding, and those who want to offer specific alerts and predictions that are likely to be much more useful to you than my very general long-distance early warning.

    For those thinking of travelling late 26th or 27th, expect delays due to fog, road slush, road flooding or ponding, and in hilly areas, possible mudslides blocking roads. Before that period starts, there could be problems with ice from freezing rain or sleet.


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Comments

  • Registered Users Posts: 1,080 ✭✭✭homolumo


    Would you consider this an exceptionally quick temp rise MT? Or just a regular swing from very cold to mild?


  • Banned (with Prison Access) Posts: 5,737 ✭✭✭MidlandsM


    I foresee a lot of property being damaged from burst pipes.


  • Users Awaiting Email Confirmation Posts: 99 ✭✭Royal Dub


    http://www.sat24.com/gb

    is this system approaching very very quickly, and is there a possibility we could get nailed again sow wise?

    Thanks


  • Registered Users Posts: 407 ✭✭AfterDusk


    homolumo wrote: »
    Would you consider this an exceptionally quick temp rise MT? Or just a regular swing from very cold to mild?

    :rolleyes:
    To give some idea of the pace of change, expect -15 C Christmas morning, -5 C Christmas afternoon, zero by midnight, +5 C by mid-day 26th, and +7-9 by mid-day 27th in most parts of Ireland.


  • Registered Users Posts: 1,080 ✭✭✭homolumo


    homolumo wrote: »
    Would you consider this an exceptionally quick temp rise MT? Or just a regular swing from very cold to mild?

    Just to clarify MT as others are eye rolling, for a thaw is this one of the faster ones you have seen?


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  • Banned (with Prison Access) Posts: 25,234 ✭✭✭✭Sponge Bob


    I reckon the main snow masses that will thaw on the 26th will be Connemara and Kerry. Most of Mayo and Sligo will drain to the sea on the 27th. The carrying capacity of the Corrib is suspect, all the gates are closed right now more or less and will need to be opened within 24 hours to dump some water before the thaw.

    The interesting ones will be the inland snow masses that drain into the suck and shannon ...which could mean flooding around Athlone and points south by the 28th or so and indeed on the Clare river in Galway.

    The lock and weir keepers on the rivers will all be off next week ...God help us ...and God help Claregalway next week ....just for the record :(


  • Registered Users Posts: 5,146 ✭✭✭Morrisseeee


    I just can't get my head around this weather 'carry-on' & this current 'change' thats going to happen, ie. we've had a month of sub-freezing conditions, then in one day there's going to be a 20'C swing !! .....and in that time (record temps, polar lows, snowmageddon) we've only had 2 (shortish) snow showers down South !! .......and now in one fell-swoop the rain is just going to come in and sweep across the country !!!.............I just don't get it !! :eek::o


  • Registered Users Posts: 14,268 ✭✭✭✭M.T. Cranium


    I consider it fast enough, let's say, especially in Kerry and west Connacht, I think it will be perceived as very fast and I thank the poster above for the specific info about the Corrib, what worries me here is that the impacts come over a holiday weekend. Even the Boards weather forum is likely to be pretty quiet tomorrow, you would think.

    Not such a sudden event for the east perhaps, but could catch some by surprise. I've had several PMs already asking me about this situation, and I'm generally saying hope for the best but plan for the worst (or the bad at least, maybe not the worst).


  • Moderators, Science, Health & Environment Moderators, Society & Culture Moderators Posts: 60,014 Mod ✭✭✭✭Wibbs


    At this stage its almost redundant, but thank you again MT, not just for your efforts and remarkably accurate forecasts, but the addition of useful and practical information with them.

    Rejoice in the awareness of feeling stupid, for that’s how you end up learning new things. If you’re not aware you’re stupid, you probably are.



  • Registered Users Posts: 11,134 ✭✭✭✭maquiladora


    Met Eireann in the TV forecast after the 8:30 news stressed the risk of flooding / dangerous road conditions for Sunday & Monday.


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  • Registered Users Posts: 908 ✭✭✭downwiththatsor


    I personally think that it will be treacherous on both road and pavement when the sleet / rain arrives here in the south west. The River Maine at Castlemaine has frozen over, this is tidal and at the moment with the full moon the tides are nearly 4m and as you can see it has still frozen.
    It is an exceptional and almost unprecedented freeze here.
    Not alot of people have running water anymore and many are looking forward to a thaw.
    regards all


  • Registered Users Posts: 177 ✭✭lyndonjones


    I personally think that it will be treacherous on both road and pavement when the sleet / rain arrives here in the south west. The River Maine at Castlemaine has frozen over, this is tidal and at the moment with the full moon the tides are nearly 4m and as you can see it has still frozen.
    It is an exceptional and almost unprecedented freeze here.
    Not alot of people have running water anymore and many are looking forward to a thaw.
    regards all

    What an amazing picture!!! Thanks for uploading, it is rare to see a river in such a condition!!


  • Registered Users Posts: 347 ✭✭isle of man


    So the heat is coming in nearly as fast as the cold did.

    some fubarded weather we having.

    prob lucky as we only have 2 weeks worth of slat left here


  • Registered Users Posts: 14,268 ✭✭✭✭M.T. Cranium


    For the Isle of Man and for anyone else who may happen to read this in parts of southwest Scotland or Wales, the cold air is likely to hang in quite a bit longer and the thaw may only be partial when it does come. The effects will be even more gradual in eastern England where they may not get much rain and only reach about 4-5 C before colder air sets in again.


  • Registered Users Posts: 48,124 ✭✭✭✭km79


    here in east galway we had only one night of snow - last friday night. nothing major really . are we still likely to see flooding???


  • Registered Users Posts: 347 ✭✭isle of man


    For the Isle of Man and for anyone else who may happen to read this in parts of southwest Scotland or Wales, the cold air is likely to hang in quite a bit longer and the thaw may only be partial when it does come. The effects will be even more gradual in eastern England where they may not get much rain and only reach about 4-5 C before colder air sets in again.

    cheers for the imput,

    we have had 2-4degrees in the day time here for past 2 days.

    biggest thaw has been today, you could see the snow going, and water running off the roofs,
    -4 now

    in fair ness its not been that slippy, but now we are into the thaw cool bit everything is like a sheet of ice.
    frost down again tonight.

    I would think we could well hit the 7-8degrees here, as that sea has some warming effect on this small island


  • Closed Accounts Posts: 88,978 ✭✭✭✭mike65


    I just can't get my head around this weather 'carry-on' & this current 'change' thats going to happen, ie. we've had a month of sub-freezing conditions, then in one day there's going to be a 20'C swing !! .....and in that time (record temps, polar lows, snowmageddon) we've only had 2 (shortish) snow showers down South !! .......and now in one fell-swoop the rain is just going to come in and sweep across the country !!!.............I just don't get it !! :eek::o

    Hardly much to understand, our position means we straddle different climatic zones, the swing is just what happens when one (the Atlantic) beats the other after it being in charge so long. Bring it on baby (no flooding will happen here).


  • Registered Users Posts: 9,580 ✭✭✭Birdnuts


    After another cracking week of epic winter weather, next week looks positively dreary and despressing:(


  • Moderators, Science, Health & Environment Moderators Posts: 16,668 Mod ✭✭✭✭Gonzo


    with temperatures around double figures next week it's gonna be a shock to the system after nearly 5 weeks of temperatures below 6C - most days being close to 0 or below.


  • Registered Users Posts: 50 ✭✭paulclan


    Some water got into our house during partial thaw yesterday.Got up on roof and cleared out the valleys, several, and shifted maybe a ton of snow lodged between ridges.Problem is during thaw, the outlets will block and the water level will rise over the flashings unless cleared beforehand.

    link:http://chestofbooks.com/architecture/House-Construction/1-Flat-Roofs-Continued.html

    Many properties would have had snowboards fitted in the past, but climate change meant they rotten away or were not recognised for what they were.
    In the absence of snowboards to keep a drainage channel open the results will not be pretty as water cascades down the inner walls of properties.
    Please pass on this information to those with complex/old roof structures who may be affected.Sorry to bring this up durint a festive few days!


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  • Registered Users Posts: 9,580 ✭✭✭Birdnuts


    Thaw already hitting the South Coast with both Valentia and Sherkin already up to the dizzy heights of 6C:eek:


  • Registered Users Posts: 1,461 ✭✭✭Musicman2000


    Enjoy the thaw, It will be a short one like the last time


  • Registered Users Posts: 11,134 ✭✭✭✭maquiladora


    Weather Warning

    Issued at 25 December 2010 - 16:40

    Severe Weather Warning.

    Staying very cold tonight with widespread ice and freezing fog.

    Dry but icy at first on St Stephens Day, but a spell of wet and windy weather, moving across the country from the Atlantic during the day, will fall as snow or freezing rain for a time. This freezing rain is likely to cause black ice and may cause driving and walking conditions to deteriorate.

    Heavy rain in all areas overnight and again at times on Monday, along with a significant snow melt, will lead to some flooding.

    Valid until 1200 27 Dec 2010

    http://www.met.ie/forecasts/warnings.asp


  • Registered Users Posts: 3,516 ✭✭✭irish1967


    As a lover of extreme weather I am looking forward to this thaw. I mean what better way to finish off this period of extreme cold. A normal drippy half baked thaw would have been such an anticlimax. I for one will be watching the radar as it collides with us.


  • Closed Accounts Posts: 5,234 ✭✭✭thetonynator


    Gonzo wrote: »
    with temperatures around double figures next week it's gonna be a shock to the system after nearly 5 weeks of temperatures below 6C - most days being close to 0 or below.


    wouldn't count on double figures in all locations, cold (not as severe) should be setting in soon again.


  • Registered Users Posts: 5,946 ✭✭✭TheMilkyPirate


    irish1967 wrote: »
    As a lover of extreme weather I am looking forward to this thaw. I mean what better way to finish off this period of extreme cold. A normal drippy half baked thaw would have been such an anticlimax. I for one will be watching the radar as it collides with us.

    Bring it on baby I love snow but I love any weather really ha. Hope it fcukin pisses down the whole night few nice gusts too please!


  • Closed Accounts Posts: 5,550 ✭✭✭Min


    I hope no one gets their house or business flooded.

    Will be nice when the sky starts urinating again instead of defecating.


  • Registered Users Posts: 706 ✭✭✭Tae laidir


    I believe we have a two significant issues that could lead to unusual flooding and possible avalanches on steep snowfields:
    Firstly, after the last slight thaw, the ground became saturated with water. I believe this has now frozen under the latest snow. In fact, I looked in the local rivers and streams, and they are flowing very slowly, showing there has been little recent run-off from the last thaw.
    Secondly, the lying snow is very powdery, and hasn't thawed and refrozen. It therefore contains a lot of air. 'As every schoolboy knows', air has less latent heat than ice, and therefore the snow will easily be melted by rain at anything above zero.


  • Closed Accounts Posts: 11,001 ✭✭✭✭opinion guy


    Tae laidir wrote: »
    I believe we have a two significant issues that could lead to unusual flooding and possible avalanches on steep snowfields:
    Firstly, after the last slight thaw, the ground became saturated with water. I believe this has now frozen under the latest snow. In fact, I looked in the local rivers and streams, and they are flowing very slowly, showing there has been little recent run-off from the last thaw.
    Secondly, the lying snow is very powdery, and hasn't thawed and refrozen. It therefore contains a lot of air. 'As every schoolboy knows', air has less latent heat than ice, and therefore the snow will easily be melted by rain at anything above zero.

    Weather forecast on 9 o clock news said that the ground was frozen to a depth of 30cm


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  • Registered Users Posts: 585 ✭✭✭Chicken Run


    Weather forecast on 9 o clock news said that the ground was frozen to a depth of 30cm

    Can believe that... Had to move an electric fence today and might as well have tried to put the posts into concrete !!!

    Bring on the thaw, I'd like my running water back please... And my thermals badly need a wash before the next cold spell !!


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