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uk.sci.weather (UK Weather) (uk.sci.weather) For the discussion of daily weather events, chiefly affecting the UK and adjacent parts of Europe, both past and predicted. The discussion is open to all, but contributions on a practical scientific level are encouraged. |
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#1
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Last night here in west Fareham we had two non-thundery torrential
downpours as the rain area crossed between 00:30 and 02:00 BST. The first lasted 40 minutes and peaked at ~45mm/hour at 00:45 then tailed off to stop briefly before resuming at 01:30. The second downpour ended about 02:10 BST and peaked for a minute or two at 100 mm/hour at 01:40 BST, roof gutters overflowed and it looked and sounded as torrential as a thunderstorm would produce (no hail though). The above is from my VP2 bucket tips but I did empty the rain gauge at 3 am and 21mm fell in those two 40 minute periods, just over half in the second burst. However... None of this was apparent on the (Netweather) 5 minute 500m rainfall radar before, during or after the rain. The maximum rate shown for here was (is still) ~8 mm/hour and the radar 24 hour total currently for my location and nearby stands at ~10 mm (true total is ~24mm). I assume that the heaviest part of the downpours were too brief to register on radar - can anyone tell me if this is a common problem, as I haven't noticed it occurring to this extent previously. Or is it likely to be a rainfall radar calibration issue? -- Dave Fareham (W) |
#2
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On 30/07/2017 16:41, Dave Ludlow wrote:
Last night here in west Fareham we had two non-thundery torrential downpours as the rain area crossed between 00:30 and 02:00 BST. The first lasted 40 minutes and peaked at ~45mm/hour at 00:45 then tailed off to stop briefly before resuming at 01:30. The second downpour ended about 02:10 BST and peaked for a minute or two at 100 mm/hour at 01:40 BST, roof gutters overflowed and it looked and sounded as torrential as a thunderstorm would produce (no hail though). The above is from my VP2 bucket tips but I did empty the rain gauge at 3 am and 21mm fell in those two 40 minute periods, just over half in the second burst. However... None of this was apparent on the (Netweather) 5 minute 500m rainfall radar before, during or after the rain. The maximum rate shown for here was (is still) ~8 mm/hour and the radar 24 hour total currently for my location and nearby stands at ~10 mm (true total is ~24mm). I assume that the heaviest part of the downpours were too brief to register on radar - can anyone tell me if this is a common problem, as I haven't noticed it occurring to this extent previously. Or is it likely to be a rainfall radar calibration issue? This site is run by a retired meteorologist type, St Denys Southampton. , same 2 bouts of heavy rain this morning http://www.seatern.org.uk/WCstdenys/WC/customgraph6.jpg Evening of 18 July 2017, at 17:10 it read 120mm/hr for one spot reading , but the radar image on this site showed no more than 25mm/hr for the St Denys pixel at that time https://max.nwstatic.co.uk I assumed that very heavy rain blocks radar signals ie hiding even heavier rain , just as it does visible light. Does anyone know how to interpret , for a given road with a given slope, the depth of rainwater running in the gutter, to a mm/hr measure? other than cross-comparing with a local met site during various events. Anyone know where to find better info than 2.82 inches for Southampton, 31 Oct 1953 , reported in the local press an event something like the recent Coverack event, a wall of water coming down from higher ground, with overloaded storm drain system |
#3
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On Sun, 30 Jul 2017 19:23:21 +0100, N_Cook wrote:
On 30/07/2017 16:41, Dave Ludlow wrote: Last night here in west Fareham we had two non-thundery torrential downpours as the rain area crossed between 00:30 and 02:00 BST. The first lasted 40 minutes and peaked at ~45mm/hour at 00:45 then tailed off to stop briefly before resuming at 01:30. The second downpour ended about 02:10 BST and peaked for a minute or two at 100 mm/hour at 01:40 BST, roof gutters overflowed and it looked and sounded as torrential as a thunderstorm would produce (no hail though). The above is from my VP2 bucket tips but I did empty the rain gauge at 3 am and 21mm fell in those two 40 minute periods, just over half in the second burst. However... None of this was apparent on the (Netweather) 5 minute 500m rainfall radar before, during or after the rain. The maximum rate shown for here was (is still) ~8 mm/hour and the radar 24 hour total currently for my location and nearby stands at ~10 mm (true total is ~24mm). I assume that the heaviest part of the downpours were too brief to register on radar - can anyone tell me if this is a common problem, as I haven't noticed it occurring to this extent previously. Or is it likely to be a rainfall radar calibration issue? This site is run by a retired meteorologist type, St Denys Southampton. , same 2 bouts of heavy rain this morning http://www.seatern.org.uk/WCstdenys/WC/customgraph6.jpg Evening of 18 July 2017, at 17:10 it read 120mm/hr for one spot reading , but the radar image on this site showed no more than 25mm/hr for the St Denys pixel at that time https://max.nwstatic.co.uk I assumed that very heavy rain blocks radar signals ie hiding even heavier rain , just as it does visible light. Thanks for that, it's useful to have corroboration of my observations... and St. Denys is 6 miles NW of here so it wasn't a very small local effect. Both events being completely missed by the rainfall radar is in my experience here very unusual, unique in my experience even in torrential rainfall. The 18 July event was similar but the divergence seems to have been much less extreme in scale. I wonder if others have experienced this kind of anomaly? -- Dave Fareham (W) |
#4
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On 30/07/2017 16:41, Dave Ludlow wrote:
Last night here in west Fareham we had two non-thundery torrential downpours as the rain area crossed between 00:30 and 02:00 BST. The first lasted 40 minutes and peaked at ~45mm/hour at 00:45 then tailed off to stop briefly before resuming at 01:30. The second downpour ended about 02:10 BST and peaked for a minute or two at 100 mm/hour at 01:40 BST, roof gutters overflowed and it looked and sounded as torrential as a thunderstorm would produce (no hail though). The above is from my VP2 bucket tips but I did empty the rain gauge at 3 am and 21mm fell in those two 40 minute periods, just over half in the second burst. However... None of this was apparent on the (Netweather) 5 minute 500m rainfall radar before, during or after the rain. The maximum rate shown for here was (is still) ~8 mm/hour and the radar 24 hour total currently for my location and nearby stands at ~10 mm (true total is ~24mm). I assume that the heaviest part of the downpours were too brief to register on radar - can anyone tell me if this is a common problem, as I haven't noticed it occurring to this extent previously. Or is it likely to be a rainfall radar calibration issue? 24 hour rainfall ending 30/0900 Z in Romsey was also 24mm, and some of the rain was most certainly torrential. Five significant rainfall events of 19mm or more have occurred this July, (only one accompanied by thunder), leading to 132mm for the month, the wettest July by some margin (20mm) in my 43 year record. Nigel (Romsey, Hampshire) 40m amsl |
#5
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On Sunday, 30 July 2017 16:41:28 UTC+1, Dave Ludlow wrote:
I assume that the heaviest part of the downpours were too brief to register on radar - can anyone tell me if this is a common problem, as I haven't noticed it occurring to this extent previously. Or is it likely to be a rainfall radar calibration issue? I notice that your local radar (Dean Hill) is off the air at the moment. Maybe it was at the time of the heavy rain too? If so, you would've experienced some quality degradation in your area. It is possible for radar to "miss" heavy cells - this depends on the spatial and temporal resolution of the data. At 5 minute temporal and 500 metre spatial, I don't think a lot will be missed - especially in a frontal situation. -- Freddie Fishpool Farm Hyssington Powys 296m AMSL http://www.fishpoolfarmweather.co.uk/ https://twitter.com/FishpoolFarmWx for hourly reports |
#6
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![]() Anyone know where to find better info than 2.82 inches for Southampton, 31 Oct 1953 , reported in the local press an event something like the recent Coverack event Hardly, Coverack was certainly 8" (Radar estimates 200mm, 170mm measured - admittedly based on buckets. Here's a calculation https://coverack.org.uk/Info17/Rainf...hallenge .pdf I've recorded 4" in 2 hours. Graham Penzance |
#7
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![]() Hardly, Coverack was certainly 8" (Radar estimates 200mm, 170mm measured - admittedly based on buckets. Sorry, that should read 6" Graham |
#8
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On 31/07/2017 09:25, Graham Easterling wrote:
Anyone know where to find better info than 2.82 inches for Southampton, 31 Oct 1953 , reported in the local press an event something like the recent Coverack event Hardly, Coverack was certainly 8" (Radar estimates 200mm, 170mm measured - admittedly based on buckets. Here's a calculation https://coverack.org.uk/Info17/Rainf...hallenge .pdf I've recorded 4" in 2 hours. Graham Penzance Sorry, I intended better in the sense of better resolution, ie hourly totals. |
#9
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![]() Thanks for that, it's useful to have corroboration of my observations... and St. Denys is 6 miles NW of here so it wasn't a very small local effect. Both events being completely missed by the rainfall radar is in my experience here very unusual, unique in my experience even in torrential rainfall. The 18 July event was similar but the divergence seems to have been much less extreme in scale. I wonder if others have experienced this kind of anomaly? I've noticed recently that GFS/0.25deg is seriously underpredicting precipitation rainfall rates, again too small scale events? 20mm/hr for countywide and no hints of more like 50mm or more/hr |
#10
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