Exceptional / possibly record rains in Stornoway

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Exceptional / possibly record rains in Stornoway

Postby Eddy Graham » 2015 Mar 08, 22:33

Hi Everyone

As I've just said in another post, I haven't seen an end to winter/ start to March like it since 1990 (if I am correct), and it all seems to be down to this intense Azores high that's lasted for months now:

In the past 3 weeks (16th Feb-7th March inclusive), we've had 226.4mm of rain here in Stornoway town. The airport (which usually "records" less rainfall than in town*) is at 177.2mm for the same period. In a daily record for Stornoway (combination of town/airport sites since 1931), the previous highest values for this period (21-day running sums) are:
187.1mm (8th March 1990)
199.8mm (8th March 1997)

Furthermore, on Friday/Saturday last (6th/7th), we had a 48-hr total of 49.6mm (42mm at the airport). The highest in the Stornoway series is 45.1mm on 14th/15th March 1990.

In addition, before 2013 the absolute daily max for March in Stornoway was 28.3mm (I know, rather low, but indicative of cooler seas at this time of year, and more frequent high pressure). But last year (2014), I measured 32.2mm on the 15th. And on Friday 6th last, I measured 30.3mm in 24hrs at my station in Stornoway town.

The Dec, Jan, Feb total at my station was an enormous 603.5mm, and we've already had another 77mm since 1st March...

And as I type, another violent storm is bearing down upon us, with forecasts of mean speeds of 65mph (touching violent storm force 11) for tomorrow night... (see separate post in "Daily Discussion")

E.

*N.B. In my view, it is difficult to make direct quantitative comparisons / analysis with the current state of the Stornoway Airport precipitation record for the following reasons: In my own studies from 2009-2015, it routinely reports 20-25% less rain than my station in Stornoway town. However, this was not always the case - I have read unpublished documents which show turf walls were built around the gauge in the 1950s-1970s in order "make it agree more closely" with neighbouring gauges in the Stornoway region (due to suspected wind loss). Since automation, the now tipping-bucket automatic gauge is now near the top of a sand dune (with no turf wall surrounding it).
Eddie Graham
Stornoway, Na h-Eileanan an Iar (Outer Hebrides), Scotland.

Stornoway town COL station, 20 AMSL, local records back to 1873.
Hebridean Weather Blog: http://bit.ly/1IFAPJa
Twitter Weather Feed: https://twitter.com/eddy_weather
Eddy Graham
 
Posts: 82
Joined: 2012 Jul 18, 17:09
Location: Stornoway, Scotland

Re: Exceptional / possibly record rains in Stornoway

Postby Dave Evans » 2015 Mar 09, 11:21

Is Stornoway airport a site reporting to the Met' Office? If so it does make you wonder about the station quality control if they will accept data from a gauge near the top of a dune in such a windy and exposed area of Britain.
Dave Evans
 
Posts: 32
Joined: 2012 Feb 22, 13:58
Location: Middleton Moor, Derbyshire Peak District. 321 m

Re: Exceptional / possibly record rains in Stornoway

Postby Eddy Graham » 2015 Mar 09, 13:30

Hi Dave,

Yes 'Stornoway Airport' (SYY, 03026) is the official Met office site, now fully automated since ~ year 2000. It has been in present location (on a small dune above the main airport buildings) since sometime in the 1980s (when the Met Office occupied there own building there for many years - apparently there was once a full staff of 12!). I understand that there were several previous sites, lower down in the hollow near the runway (some of which had 'turf walls' surrounding the rain gauge).

I don't want to get into an argument over reliability here, but I'm sure the Met Office do as best they can given the location/circumstances - although like most airport sites, the focus has always been on aeronautical safety and operational weather forecasting (and not long-term climatological) needs. Suffice to say, that the long-term precipitation dataset for Stornoway (based on a combination of town & out-of-town sites [including the airport] from the 1850s to present) has never been homogenised. And with precipitation differences typically of the order of 20-25% between the airport and town, I think homogenisation would be a first key step towards better understanding Stornoway precipitation.

But certainly, whatever the reasons for the "lower" measurements of rainfall at SYY 03026, we are still in an unusual (possibly very unusual) precipitation regime at present here.

E.
Eddie Graham
Stornoway, Na h-Eileanan an Iar (Outer Hebrides), Scotland.

Stornoway town COL station, 20 AMSL, local records back to 1873.
Hebridean Weather Blog: http://bit.ly/1IFAPJa
Twitter Weather Feed: https://twitter.com/eddy_weather
Eddy Graham
 
Posts: 82
Joined: 2012 Jul 18, 17:09
Location: Stornoway, Scotland

Re: Exceptional / possibly record rains in Stornoway

Postby Eddy Graham » 2015 Mar 15, 22:04

OK - the rains have stopped, and talk-about "jekyll and hyde" style weather! (as we' ve just had 3 glorious days in a row now, it looks like Spain today...)

But to put the recent rains in context:
In 27-days (16 Feb to 14 March inclusive), I recorded 267.3mm at my station in Stornoway town
In the same 27-day period, Stornoway Airport recorded 206mm (from Ogimet)

I have only once recorded a higher total within 1 month (Dec 2013).
At Stornoway Airport (based on Met Office monthly data), only the month of March 1990 equals such a value (206mm)
For Februarys, only the Februarys of 1990,1997 and 1998 exceed 206mm (in a record back to 1873).

Thus, regardless of the recent rains being split between 2 rainfall months (and which both will probably figure as "unremarkable" in the long-term stats), this was a highly unusual precipitation episode, but one that is becoming more common through recent decades.

I'll sum this up in two figures (using Stornoway Airport data only, based on daily Met Office records from 1931-2015)
Figure 1: 48-hr March precip totals over past 85 years; 7-8th March 2015 had 2nd highest amount
tn_Slide2.PNG
tn_Slide2.PNG (71.61 KiB) Viewed 505 times

Figure 2: 21-day running totals ending any day in Feb up to 14 March in any year: 2015 had the 3rd greatest amount (185mm), after 1990 and 1997. Note the increase in recent decades (esp. since the NW European 'step' change in climate ~1988) + the increase in variability (n.b. March 2013 actually had the LOWEST 21-day total in the record!). The X-axis is years from 1931 to 2015:
tn_Stornoway-March-21day-Precip_16Feb-14Mar.png
tn_Stornoway-March-21day-Precip_16Feb-14Mar.png (48.03 KiB) Viewed 505 times


E. Graham, Stornoway Town, 15/3/2015
Eddie Graham
Stornoway, Na h-Eileanan an Iar (Outer Hebrides), Scotland.

Stornoway town COL station, 20 AMSL, local records back to 1873.
Hebridean Weather Blog: http://bit.ly/1IFAPJa
Twitter Weather Feed: https://twitter.com/eddy_weather
Eddy Graham
 
Posts: 82
Joined: 2012 Jul 18, 17:09
Location: Stornoway, Scotland


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