Severe Flood Threat For Friday-Saturday?

[Roger J Smith]

Severe flood threat for Friday-Saturday ??

_________________________________________

Looks to me as though a rapid thaw and locally heavy rain will combine on Friday and Saturday to produce a widespread flood threat in the UK and Ireland. I would leave the specifics of which river valleys to your better judgement but the weather parameters are developing quite consistently on all the models. It's very similar looking to mid-November with the monsoonal heavy rain, strong southerly flow and mild temperatures, but although the rain potential here is bound to be less (by half, hopefully) the ground is now of course frozen and widely snow-covered.

I don't feel that an extensive analysis is required here, it's a very straightforward situation, snow and cold to late Thursday, quick temperature increases followed by strong winds and heavy rains by Friday night into Saturday, especially in western parts of the UK and much of Ireland too (where snow depths have increased in places following Tuesday's rather complex storm).

That's about all I have to kick off a discussion on it. There may be many areas at risk of serious flooding, as temperatures soar towards 10 C or even higher in places. All but the deepest high mountain snow will quickly be gone under these conditions.

[ANYWEATHER]

Severe flood threat for Friday-Saturday ??

_________________________________________

Looks to me as though a rapid thaw and locally heavy rain will combine on Friday and Saturday to produce a widespread flood threat in the UK and Ireland. I would leave the specifics of which river valleys to your better judgement but the weather parameters are developing quite consistently on all the models. It's very similar looking to mid-November with the monsoonal heavy rain, strong southerly flow and mild temperatures, but although the rain potential here is bound to be less (by half, hopefully) the ground is now of course frozen and widely snow-covered.

I don't feel that an extensive analysis is required here, it's a very straightforward situation, snow and cold to late Thursday, quick temperature increases followed by strong winds and heavy rains by Friday night into Saturday, especially in western parts of the UK and much of Ireland too (where snow depths have increased in places following Tuesday's rather complex storm).

That's about all I have to kick off a discussion on it. There may be many areas at risk of serious flooding, as temperatures soar towards 10 C or even higher in places. All but the deepest high mountain snow will quickly be gone under these conditions.

Really cant see a major flood event. Perhaps 10c in the far southweat of the uk but generally still cold over the weekend for most with a slow thaw!

[Just Before Dawn]

Really cant see a major flood event. Perhaps 10c in the far southweat of the uk but generally still cold over the weekend for most with a slow thaw!

It's a complex situation though temps are crucial, as is the issue of frozen ground, which has a lower capacity to absorb surface water run-off. One to keep an eye on certainly.

[Guest North Sea Snow Convection]

Sounds a bit over dramatic to me.

If we were going into a period of sustained rainfall following this cold spell then certainly a bigger problem might arise. The thaw begins some 48 hours before the rain arrives (ie by tomorrow) and so a fair portion, albeit not all, of the snow will disappear before the rain arrives anyway. Also, snowfall apart, conditions have been dry for some time and river levels shouldn't be anywhere near the levels they were at the start of the winter

Not to be complacent about it of course, it needs to be watched - but a one off band of rainfall doesn't constitute 'monsoonal' rainfall imo, nor equate to the persistently grim conditions of November last year.

Edited January 13, 2010 by North Sea Snow Convection

[ANYWEATHER]

Sounds a bit over dramatic to me.

If we were going into a period of sustained rainfall following this cold spell then certainly a bigger problem might arise. The thaw begins some 48 hours before the rain arrives (ie by tomorrow) and so a fair portion, albeit not all, of the snow will disappear before the rain arrives anyway. Also, snowfall apart, conditions have been dry for some time and river levels shouldn't be anywhere near the levels they were at the start of the winter

Not to be complacent about it of course, it needs to be watched - but a one off band of rainfall doesn't constitute 'monsoonal' rainfall imo, nor equate to the persistently grim conditions of November last year.

Apart from the southwest and west Wales it looks still to remain preety cold over the weekend for many so a slow thaw, but im sure many picticularly in the north and east will hold on to something of a snow cover before the next freeze comes!

[Guest North Sea Snow Convection]

Apart from the southwest and west Wales it looks still to remain preety cold over the weekend for many so a slow thaw, but im sure many picticularly in the north and east will hold on to something of a snow cover before the next freeze comes!

Yes, it is just about possible that some areas may have some snow that survives through the milder interlude as you say!

I had about 2cms of snow on New Years night, some of which lasted until the snowfalls of the middle of last week - which in turn carried on and culminated in the heavy snow showers of Friday. Most of that (about 15cms) has lasted until the snowfall of this morning - which is about another 3cms!

Not expecting any of this to last after today through to the next possible cold spell next week - but it does go to show how slow thaws can be between snowfalls. On the basis that the extent of the milder temps has been downgraded today this should as you say, reduce the effects of sudden and very rapid thaws that might create a large problem in terms of any rain that arrives .

[CardiffStorm]

Im in no way an expert but I think if the ground remains frozen all this heavy rain during Friday/Saturday could cause problems with local flooding as the amount of surface run-off will be huge. I guess it just depends if the temperatures keep dropping which atm they are. I dont think the thawing snow will cause huge problems.

Edited January 13, 2010 by jamo_s

[johnholmes]

There is a fairly complex link between temperatures above zero C, snow melt and equivalent rainfall amounts. I am sure what we used to have to do manually is now part of a computer programme run in the Environment Agency along with forecast inputs of temperature, wind strength, and of course anticipated rainfall from the Met O.

Compacted snow as a VERY general rule and 25cm is about 2 inches of rainfall. So with 30-50cm on some of higher levels that immediately drain into the various river heads it COULD be a major problem along some of the major rivers in England and Wales IF the predicted temperature increase at those high levels occurs along with rainfall totals as currently being predicted. It is certainly yet another complication for the professional weather forecasters in an already very difficult week.

[Just Before Dawn]

There is a fairly complex link between temperatures above zero C, snow melt and equivalent rainfall amounts. I am sure what we used to have to do manually is now part of a computer programme run in the Environment Agency along with forecast inputs of temperature, wind strength, and of course anticipated rainfall from the Met O.

That's right John, they then share the outputs with Drainage Authorities and, based on experience and topography, make a flood risk determination.

[Gray-Wolf]

Many of the culverts lining the road down all the hill arounf here are under compacted snow (no grit for most routes) so like autumn leaves we may find problems being shed down the hillsides into the valley bottoms.

The extreme low temps must also have penetrated some of the culverts themselves and so ice blockage may be an issue if we face heavy rain over the next 48hrs.

All just a part of the 'even larger teapot' eh?

[Guest]

Many of the culverts lining the road down all the hill arounf here are under compacted snow (no grit for most routes) so like autumn leaves we may find problems being shed down the hillsides into the valley bottoms.

The extreme low temps must also have penetrated some of the culverts themselves and so ice blockage may be an issue if we face heavy rain over the next 48hrs.

All just a part of the 'even larger teapot' eh?

I have been shocked by how many burst water mains there have been locally recently. The most impressive one was on the M 8 motorway close to the Braehead shopping centre yesterday. The fountain was 50ft high!!!

[Roger J Smith]

Is there a product that shows current snow on the ground measurements? I have a fairly extensive set of anecdotal reports at my disposal both here and on two other forums including Ireland. What I'm gathering is that some higher elevations have 2 to 4 feet of snow which usually means that if the snow line is about 200-400 ft above sea level, then the snow cover increases about in a linear scale from zero to the max about halfway to summit level, and stays near max over the summits, then decreases in some different way depending on upslope or downslope, onshore etc, factors. So using those concepts and a variety of actual measurements reported, I would speculate that the major flood risks would occur in Devon, Somerset, southeast Wales, the Severn valley, and parts of Ireland near the Wicklow and Dublin mountains.

Other flood risks might be more subdued due to either less snow or less time in the mild air being advected in.

Those factors, more than lack of snow, might save Cumbria from major flooding. There must be some parts of southwest Yorkshire at risk, I would think, and to maybe a lesser extent, all of the drainage in eastern and southeastern England will at least approach bankfull conditions.

Then you would have urban flooding from clogged drains, snowbanks ponding water in parking lots and underpasses, not to mention that rainfall QPF values are being shown near 2 inches in places.

I would say expect an unholy mess by Saturday with scattered reports of serious flooding. Another hazard perhaps not being discussed much yet, would be dense fog over melting snowpacks, given that the dewpoints in this oncoming mild air mass will be higher than the likely temperatures produced by contact with the snow. To give an example, I would expect Plymouth to be reporting 11/10 temp dp sort of values, but up the hills in Dartmoor, this would cool and then supersaturate out to 5/5 with zero visibility. That sort of process is likely all over Britain and Ireland on Friday and in eastern regions Saturday. Also there could be snowslides in steep areas of the north, and a risk of mudslides in all steep terrain, given the volume of meltwater and rainfall and the frost coming out of the soil.

The one limiting factor I see is that the event is fairly short duration, but as the melt should start slowly tomorrow, there is a 48-72 hour window for melting combined with a 12-24 hour rainfall event. It has me concerned but I could imagine worse.

[Stargazer]

I live in an area that has flooded in the past in similar circumstances (in March 1947), but this time there is considerably less snow to melt, the thaw is very gradual and we're only expecting moderate rain. A representative of the Environment Agency advised on the radio yesterday that only 'flood watches' are expected in our area - although some western areas may be more severely affected.

Our house is in the Thames floodplain about 2.5m above normal river level but the floodplain is very wide in Reading and floods caused by torrential rainfall upstream tend to be attenuated before they reach Reading (for example, Osney Island in Oxford flooded severely after the torrential rain of 20 July 2007 but by the time the wave reached us some days later the river level in Reading only rose about 30cm). The EA estimates that water would reach our floor level about once in 150 years - the 1947 event is generally considered a once in 60-100 year event in our area that flooded about 1600 properties around Reading and just missed our house.

In the end I don't see widespread river flooding similar to 1947 as a likely outcome - in eastern parts anyway - but localised flooding caused by rapid snow melt and inadequate drainage may still occur.

Edited January 15, 2010 by Stargazer

[West Sussex Kate]

I thought you might like to look at this.......http://www.dailymail.co.uk/news/article-1243325/More-ice-warnings-thaw-gives-rise-flood-fears.html

[Stargazer]

So, according to The Daily Mail...

"Another forecasting organisation, Positive Weather Solutions, said winter could have 'a sting in the tail', with snow and low temperatures at the start of April leaving the North East and Scotland looking 'like a Christmas picture postcard'."

equates to

"UK to hit 10C this weekend but big freeze could last until April"

Oh, and house prices will crash again as usual

[Roger J Smith]

I agree that it can't be as bad as 1947 in general terms for this flood potential, but I do think some serious flood situations are possible especially in the southwest, near central Wales, and possibly in parts of Yorkshire, as well as eastern Ireland. There is considerable rain yet to come overnight and tomorrow morning, and an air mass saturated near 10 C is moving in over the snowpack (cooling like I predicted down to 3-5 C over the snow). The snow is reportedly melting very rapidly in the Dublin mountains by the accounts of Irish weather watchers. The peak of any flooding will likely come mid-day Saturday but conditions will not change that much for 2-3 days so a few other flood situations could develop more slowly over that time. I hope it isn't too dramatic, there was more than enough flood damage already. However, I wouldn't take it too lightly, there are some areas with a lot of snow to be melted.

[Stargazer]

Our area is now on 'flood watch', and many footpaths and some roads near the smaller rivers are now flooded. We went walking this afternoon and had to keep altering our course to avoid floodwater - one particularly low lying house next to the River Loddon near Twyford had water up to the doorstep and sandbags out, but the owners seemed unconcerned and the water level was apparently a fair way below July 2007 levels there. The water was over the top of the depth marker at the Land's End ford and starting to encroach on the pub car park. So far we have a roughly once in 3-5 year flood event round here.