Mid morning snow in SE catches Met Office on the hop

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xmetman

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Jan 21, 2018, 6:04:09 AM1/21/18
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RonB

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Jan 21, 2018, 7:55:08 AM1/21/18
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This is the third time that the Met Men have been caught out this winter in the Southeast.
Settling nicely now here in Woodford Green and probably all over the Southeast

RonB

Dave C

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Jan 21, 2018, 7:55:12 AM1/21/18
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It's East of that map as well. Here in S.E Essex chucking it down - heaviest of the winter. Settled on most surfaces except road. Temp 1.1C DP -0.2C
Dave

xmetman

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Jan 21, 2018, 8:10:16 AM1/21/18
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A big contrast in temperatures from west to east with Exeter 12.6°C and Kenley just 0.5°C at 12 UTC

George in Edinburgh

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Jan 21, 2018, 8:18:25 AM1/21/18
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Shows up nicely on the North Weald Airfield (Epping) webcam
https://www203.mangocam.com/c/NorthWealdAirfield

George in Swanston, Edinburgh
www.swanstonweather.co.uk

Richard Dixon

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Jan 21, 2018, 8:26:21 AM1/21/18
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A pleasant surprise snow for an hour and a half even in Blackheath. Was starting to settle right before it turned abruptly to rain.

Richard

Julian Mayes

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Jan 21, 2018, 8:53:08 AM1/21/18
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For the KIngston area and adjacent north Surrey close to the Thames, heavy sleet only - just verging on proper snow briefly around 11am - was as exciting as it became.  Now raining, as Richard said.  Now the third snow event this winter which has affected Herts, Essex, Bucks but not the lowest lying areas of west London / Thames valley.  Here in Molesey, no hint of snow lying so far this winter. 

The road weather sensors in much of Scotland show much midday ploughing and gritting going on - as well as impressive snow depths, though much of it has been on the ground for 3-4 days of course up there. Gritter drivers and highways staff will be grateful for a rest in a  few days as the milder weather arrives, a well-deserved break.   

Julian 

Molesey, Surrey   

Jack Harrison

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Jan 21, 2018, 1:59:52 PM1/21/18
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Here in my location Nairnshire, very little snow this past week although most of what did fall hasn't melted yet.
My rain gauge has presumably thawed as it shows 0.2 mms rain today which translates to 2 mms snow that fell just before dusk.  Non event.

Jack

Freddie

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Jan 21, 2018, 3:27:05 PM1/21/18
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Are you saying they've been caught out because they didn't forecast it, or was it because they hadn't put out a yellow warning?

--
Freddie

xmetman

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Jan 21, 2018, 5:40:33 PM1/21/18
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Freddie

We've had this conversation before about what constitutes a warning. In my opinion the original warning area didn't extend far enough south, so the update when it came at 0820 UTC was too late to be called a warning because the event had already started. They were simply issuing its laid down in the procedures that they should. As it happened it snowed for an hour or two across most of southern England and the southeast but only put down 3 or 4 cm as far as I can see from th SYNOPs. A very similar thing occurred on the 11th of December 2017 when High Wycombe had 17 cm of snow, this time the warm air saved their blushes.

Bruce.

Paul Kendall

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Jan 22, 2018, 2:24:33 AM1/22/18
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As regards High Wycombe. There was indeed moderate snow for a couple of hours, Snow settled well on grass and on cars but nothing worse than slush on the roads. Soon after midday it turned to rain. Biggest problem was the amount of standing water on the roads.
Paul Kendall

Freddie

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Jan 22, 2018, 4:57:42 AM1/22/18
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I was only asking why you thought they got it wrong.  I agree the forecast was wrong - but I don't agree that a warning was required.  Right up until the last minute the air looked too warm for snow in both the models and the actual ascents.  There must've been an error of 1 degree at most - which would've passed unnoticed had the temperature been 2 or 3 degrees either way.  It was a very marginal situation, and I would hated to have been the guy in the hot seat making the decisions.  The precision required at the moment in time was too great for either human or computer.

--
Freddie
Fishpool Farm
Hyssington
Powys
296m AMSL
http://www.fishpoolfarmweather.co.uk/
https://twitter.com/FishpoolFarmWx for hourly reports

xmetman

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Jan 22, 2018, 11:57:12 AM1/22/18
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Freddie

It was an unusual Sunday morning. 

The warm front was pushing eastward across the southwest and rain was moving E'SE just ahead of it, some of the rain across wales was 4-8 mm/hr with a few red pixels 8-16 mm. 

I think I'm right in saying that they issued two warnings:- 

(1) A yellow one for heavy rain and the reported flash flooding that was occurring in North Devon 
(2) A yellow for snow to extend the area south of the existing warning. 

Perhaps the indications from the 06 UTC mesoscale suggested that there might be moderate or even heavy snow for a couple of hours across the Cotswolds and Chilterns and that's what prompted it.

I've done a little digging into the flash flooding at Combe Martin and I still can't really explain it from the weather radar.

Bruce.



Jack Harrison

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Jan 22, 2018, 12:21:03 PM1/22/18
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There's a great deal of analysis and sensible comments here.  What sort of post mortem would the Met Office hold?

Bruce.  You often post some excellent synops charts (such as at the beginning of this thread).  What is the source?   I can't find anything.

Jack

Freddie

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Jan 22, 2018, 1:38:08 PM1/22/18
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I think the warning would probably had been issued once there was a likelihood of the snow penetrating to low levels. I don't think the impact of snow over the Chilterns and Cotswolds would warrant a warning in itself.

The Combe Martin area is comprised of "fast response" rivers that typically flood quite quickly and die down equally quickly. If Exmoor has already experienced heavy rain in the preceeding few days then the response is even quicker and more marked. Having said all that, the radar does look quite uninspiring, so maybe the local radar was unfortunately off the air at the time.

--
Freddie

Scott W

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Jan 22, 2018, 2:43:23 PM1/22/18
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On Sunday, January 21, 2018 at 1:53:08 PM UTC, Julian Mayes wrote:
For the KIngston area and adjacent north Surrey close to the Thames, heavy sleet only - just verging on proper snow briefly around 11am - was as exciting as it became.  Now raining, as Richard said.  Now the third snow event this winter which has affected Herts, Essex, Bucks but not the lowest lying areas of west London / Thames valley.  Here in Molesey, no hint of snow lying so far this winter. 

The road weather sensors in much of Scotland show much midday ploughing and gritting going on - as well as impressive snow depths, though much of it has been on the ground for 3-4 days of course up there. Gritter drivers and highways staff will be grateful for a rest in a  few days as the milder weather arrives, a well-deserved break.   

Bit late to this but I experienced yesterday's snow in an area from Leavesden, near Watford, to Wanstead. At Leavesden, site of the Warner Brothers Harry Potter studio, the snow began to fall lightly at first at 10.30am before coming down in thick flakes. When I saw it falling on Privet Drive through the cafe window I initially thought it was the studio's special effects. It gradually got heavier and started covering all surfaces, leaving a layer of slush on local roads, I approached a local petrol station a bit too fast and had to take evasive action when my wheels locked - I think the local gritters must have been caught out. The snow gradually turned to pellets, sleet and then rain. By the time I reached Wanstead at 1.30pm there was just a thin layer of slush on car windscreens.

Len W

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Jan 22, 2018, 3:07:27 PM1/22/18
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I agree Bruce.
I was mystified why there was flooding in N Devon.
Nothing to indicate on the radar during the day.

Radar is not usually challenged in that area?

Len
Wembury

xmetman

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Jan 23, 2018, 1:45:19 AM1/23/18
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If you look at the OS map that I included in my blog the stream that runs through Combe Martin doesn't originate on Exmoor as far as I can see...

xmetman

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Jan 23, 2018, 1:52:42 AM1/23/18
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Devon was in warm air by then so I don't think it was some bright banding effect. All of the radars were present, but I have a feeling that the Predennack radar was under reporting.

xmetman

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Jan 23, 2018, 1:58:38 AM1/23/18
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Jack

I wrote that SYNOP plotting software in 1998 when I was a programmer with the Met Office, I download and parse the obs from OGIMET. I can go back 6 hourly to 1973 and plot a chart for just about anywhere.

Bruce.

Jack Harrison

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Jan 23, 2018, 2:58:13 AM1/23/18
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I fully understand Bruce that it's your personal baby and so not generally available.  Your charts are excellent and far easier to understand than raw synops.

As pilots we had to be able to decode station circles for licence purposes but as soon as we had passed the exams, that ability tended to fade somewhat:-)

Pity you can't go back as far as my birthday, 7th December 1938:-)  Met office archives are of some help but only give part the story. 
My dad once told me that there were snow showers at the time I was born (in Great Yarmouth on the east coast).

I have to say that you must have been an extremely good programmer Bruce.

Jack

xmetman

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Jan 23, 2018, 4:09:27 AM1/23/18
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Jack

Thanks for the compliment!

Believe me there are some very clever programmers out there, and I'm sorry to say that I'm not one of them - I do it all by dogged persistence. 

Data is king though - because if you don't have a data source for the latest observations you don't have a program.

Bruce.

Freddie

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Jan 23, 2018, 4:38:05 AM1/23/18
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On Tuesday, 23 January 2018 06:45:19 UTC, xmetman wrote:
If you look at the OS map that I included in my blog the stream that runs through Combe Martin  doesn't originate on Exmoor as far as I can see...
When I said "If Exmoor has already experienced heavy rain in the preceeding few days..." I actually meant North Devon.  The high ground isn't confined to the national park, as I'm sure you're aware.  The river catchments in north Devon are typically small but include much high ground.  Looking at Combe Martin in particular, it is especialy small.  Therefore you could quite safely deduce that the flooding was a result of either very localised heavy rain in the catchment, or possibly the release of dammed water when some sort of obstacle shifted.

Len W

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Jan 23, 2018, 5:00:13 AM1/23/18
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Swimbridge and Muddiford were two other places in N Devon that were affected by flooding.

Len
Wembury

Freddie

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Jan 23, 2018, 5:10:12 AM1/23/18
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On Tuesday, 23 January 2018 10:00:13 UTC, Len W wrote:
Swimbridge and Muddiford were two other places in N Devon that were affected by flooding.

So it sounds as if there genuinely was heavy rain as opposed to a watercourse being obstructed then unblocked.  Bearing in mind the locations of the flooding, it looks as if the area to the north and north east of Barnstaple  was affected by heavy rain that the radar missed.

xmetman

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Jan 23, 2018, 5:32:35 AM1/23/18
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Yes you're right, that does point to a more widespread area of heavy rain that a blocked stream.
I'm hoping David Smart see's this and pops up with all the answers!

Smartie

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Jan 23, 2018, 6:55:13 AM1/23/18
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I've had a quicklook at the 1km Nimrod data and the fax. This shows the warm front quasi-stationary over the area during the morning with overall flow from the ~NW or ~WNW. Embedded in the wide frontal rainband are narrower bands with echoes suggestive of convective rainfall.
Snapshot attached.
This suggests the presence of  Conditional Symmetric Instability (CSI) ie upright circulations embedded in slantwise ascent. Looping of the imagery also suggests some training of stronger echoes over the coast and higher ground in the bands. Such short wavelength circulations can be poorly represented  even in the UKV, but I'll have a look at it with the 2.5km WRF.
I see the catchment has received repeated inundations since December with the Umber reaching increasingly higher peaks so the area was well prepped in the medium term.

I 'll follow up, see if he CSI idea stands up.
Message has been deleted

Smartie

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Jan 23, 2018, 8:05:33 AM1/23/18
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Another complication may have been shallow seeder-feeder. The precipitation beneathe the radar beam may have been considerably enhanced and not detected. At Barnstaple 0845 rain rate peaked at ~15mm/hr after the surface wind veered westerly and started to pick up. The 1km radar  shows only upto 8mm/hr in the area at the same time.Coombe Martin is tthe circled dot.



xmetman

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Jan 23, 2018, 9:26:42 AM1/23/18
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Is that enough to produce the flash flooding we saw in images even if the ground was already saturated?

Smartie

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Jan 25, 2018, 9:03:14 AM1/25/18
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Added a post with a brief analysis here-

Questions and comments welcome.

Smartie

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Jan 25, 2018, 9:58:04 AM1/25/18
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Just to note further the response of the Umber at Combe Martin to heavy rain appears typical for winter, but note the increasing mean level and increasingly higher peaks this winter.
Snapshot from flood warnings website-


xmetman

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Jan 25, 2018, 12:02:30 PM1/25/18
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Excellent worthy of the old Met Mag - I particularly like the cross section!
Did you see the classic line convection on the cold front yesterday?

Smartie

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Jan 26, 2018, 3:51:41 AM1/26/18
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Thanks Bruce.
Most striking thing about the CF I thought  was the extent of the stratiform rain behind the line.
David

Dave C

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Jan 28, 2018, 11:34:22 AM1/28/18
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There was no need for a warning. Hardly a terrible error but if they were perfect they might have mentioned snow being more widespread I guess. Wouldn't have even raised an eyebrow 40 years ago.
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