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)

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

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)

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

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

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

Hardly, Coverack was certainly 8" (Radar estimates 200mm, 170mm measured - admittedly based on buckets.

Sorry, that should read 6"

Graham

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.

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

In article ,
says...

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).


Just east of you in Denmead my Vantage Vue recorded 19mm for the 09:00-
09:00 period with a peak rain rate of 0.6mm/minute at 01:10 I didn't
consult the rain radar but happy to accept it wasn't predicted.


--
Kev
NNNN