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24-12-2010, 19:58   #1
M.T. Cranium
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Thaw and possible flooding 26th-27th -- forecasts and discussion

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.

Last edited by M.T. Cranium; 24-12-2010 at 20:03.
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24-12-2010, 20:05   #2
homolumo
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Would you consider this an exceptionally quick temp rise MT? Or just a regular swing from very cold to mild?
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24-12-2010, 20:09   #3
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I foresee a lot of property being damaged from burst pipes.
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24-12-2010, 20:12   #4
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is this system approaching very very quickly, and is there a possibility we could get nailed again sow wise?

Thanks
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24-12-2010, 20:19   #5
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Quote:
Originally Posted by homolumo View Post
Would you consider this an exceptionally quick temp rise MT? Or just a regular swing from very cold to mild?


Quote:
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.
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24-12-2010, 20:26   #6
homolumo
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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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24-12-2010, 20:36   #7
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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
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24-12-2010, 21:07   #8
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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 !!
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24-12-2010, 21:09   #9
M.T. Cranium
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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).
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24-12-2010, 21:16   #10
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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.
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24-12-2010, 21:18   #11
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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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24-12-2010, 21:37   #12
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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
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24-12-2010, 21:46   #13
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Quote:
Originally Posted by downwiththatsor View Post
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!!
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24-12-2010, 22:22   #14
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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
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24-12-2010, 23:10   #15
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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.
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