Precipitation



Rainfall in Lake District tops 150 mm

Figure 1

I reckon that rainfall accumulations in the Lake District in the last 24 hours have topped 150 mm on the highest fells (fig 1). The wettest place I can see from my list is probably close to Ambleside (fig 2), with around 90 mm of rain since 18 UTC yesterday evening, although these are rough and ready estimates derived from the 5 minute coarse data that’s available on the Met Office website. A pixel is a long way at this resolution, and you only have to be one or two out to return a value from over high ground rather than lower down.

Figure 2

Wet in Lake District

Figure 1 – Estimated overnight accumulations since 18 UTC on 10 October 2017

A thoroughly wet night and morning across the Lake District and Snowdonia. Estimates from weather radar show purple pixels which equate to rainfall accumulations of over a 100 mm in the last 15 hours or so across higher ground (fig 1). It was well anticipated and forecast by the Met Office in their yellow warning of yesterday morning. The one thing that does puzzle me about yellow warnings for heavy rain like that is, the much higher accumulations that I’ve seen in western Scotland never seem to get a mention, perhaps the impact there is lessened because by the sparsity of the population, or maybe it’s more likely that they don’t want to get caught out by another mega-orographic rainfall event in the Lake District like we saw in December 2015? The more persistent heavy rain, should clear the area with the passage of the cold front early this afternoon (fig 2).

Figure 2 – Courtesy of the Met Office

Drier than average Northern Scotland

Figure 1

Rather surprisingly, Northern Scotland currently has the lowest annual rainfall anomalies for the whole of the UK, with an annual running accumulation to the end of September of just 90.9% of the 1981-2010 long-term average. The spell started with a very dry October and November in 2016, and the gap has only started to narrow in the last couple of months. It’s rather strange to think that somewhere can be seen as drier than average, when it’s seen rainfall totals of 1487.1 mm (58.5 inches) in the last year, but that’s the wonder of statistics for you.

Last nights rain and strong winds

Rather surprisingly the heavy rain of yesterday and last night never merited a yellow warning in its own right. I suppose we should expect 24 hour accumulations of 100+ mm in the Lake District at this time of the year, but heavy rainfall warnings have been issued for a lot less, and the Met Office approach to use of yellow warnings in my opinion are not at all inconsistent. Here is my best estimate of the accumulations for the 24 hour period ending 06 UTC this morning from the weather radar:

Figure 1

I must admit in the cold light of day my estimates, which are usually fairly close, look way too high for yesterday. That could be due to the fast-moving nature of the developing low Xavier as it tracked eastward, here are the real values (fig 2).

Figure 2


Looking at the overnight strong winds across the country produced by Xavier, here’s a chart of the maximum gusts above gale force from 20 UTC yesterday to 08 UTC this morning (fig 3).

Figure 3

The Met Office yellow warning was a little bit on the small side areally (fig 4), and didn’t cover all the gusts of 40+ that occurred in parts of northwest England, southwest Scotland, South Wales and southern Midlands. Forecast gust speeds used the: 40-50 locally 60 ploy, and were generally in good agreement with reported values, apart from the 76 mph at Snod Hill on the Fylingdale* moors (268 M) which again was just outside the yellow area.

Figure 2 – Courtesy of the Met Office

*I never realised that Fylingdales now looked like that! The last time I saw it was over 40 years ago now, was when it resembled a giant golf ball, which at least was more attractive than it is now. Don’t you just love their motto “Vigilamus” which translates to “We are watching”. By the way www.vigilamus.com is not available as a URL but it would have been a good one.

Another missed warning?

Figure 1

Surely there should have been out a warning for heavy rain as well as for strong winds today? Area’s of red pixels are now appearing on the estimated accumulations from weather radar since 06 UTC this morning. The inset graph shows that’s it’s been raining heavily since 07 UTC on the coast of Ayrshire. At the moment the heaviest rain is over the Lake District where red pixels, 50 to 75 mm, are also evident, and the developing wave feature is still away to the west.

Rainfall accumulations – 1st October 2017

Figure 1

Plenty of rainfall across the Western Highlands of Scotland yesterday (Sunday, 1st October 2017), with estimated accumulations from the weather radar in excess of 100 mm over the mountains (fig 1). Scattered areas of light blue pixels (16-24 mm) south of the Forth Clyde valley, but small totals from the wet spots in the Lake District, Snowdonia and the west of Wales, well justifies the Met Office’s cancellation of the dual yellow warning for heavy rain and strong wind they issued on Friday morning (fig 2).

Figure 2 – Courtesy of the Met Office

All the heavy rain is in Norway

If you had wondered where all the heavy rain was overnight, you’ll notice from the 12 hour totals to 06 UTC this morning (fig 1), it had been falling over large parts of southern Norway.

Figure 1

The heavy rain was from the cold front that cleared the UK on Friday, but stalled as it came up against that record-breaking anticyclone that’s been dominating the weather over northeast Russia for the last week now (fig 2).

Figure 2 – Courtesy of Met Office

Ex-hurricane Maria has got to get her skates on if she is to bring rain to the southwest tomorrow, but by the look of it (fig 3) she is about to get a piggy-back on a 100 knot westerly jet stream to help her on her way.

Figure 3 – Courtesy of NetWeather.tv

Large variations in September rainfall

Figure 1

There are large variation in rainfall accumulations from east to west so far this September, with a total of 19.9 mm of rain at Shoeburyness in Essex, to as much as 329.6 mm at Capel Curig in Snowdonia (fig 1). The table below is split into two, the lower grid showing the driest places, and the upper grid showing the wettest places, it kinda works, but was a bit awkward to program (fig 2). SYNOP reception of rainfall stats was pretty good through the month.

Figure 2

17 Sep 2017 – estimated rainfall accumulations

I had a look back at rainfall yesterday (17 Sep 2017), for the 17 hour period between 12 UTC to 05 UTC this morning, just to see what the estimated accumulations from the weather radar looked like (fig 1). The wettest place that I could find was Westbury in Wiltshire with 39.7 mm of rain, and although it was wet in Ceredigion and Cornwall, it wasn’t quite as wet as it had been on Friday, so that was good news. Hopefully the showers will be lighter and scattered over the next few days and give these places a chance to dry out. The yellow warning issued for yesterday was certainly justified for Westbury  in the torrential rain the town saw during yesterday afternoon (see the inset hyetograph in figure 1).

Figure 1

Here are the 24 hour totals till 06 UTC this morning from the SYNOP stations for completeness.

Figure 2

Amazingly, Liscombe in Somerset remained dry throughout the 24 hours, which is more than can be said of Jersey.

 

Showers being restricted from the west

Figure 1

I would say, writing as a non-expert, that the showers over West Wales and the Celtic Sea have lost their intensity from earlier this morning (fig 1), due to encroaching upper and medium cloud that’s streaming south across Southwest Wales and the extreme west of Cornwall (fig 2), from an upper warm front that was lying across Western Scotland at 06 UTC (fig 3). I reckon warmer air aloft is stabilising the atmosphere and limiting the convection. The showers certainly haven’t lost any of their potency across the south of Devon at the moment though, and I don’t think I’ve ever seen the sky so full of towering cumulus and CB, or lines of CB, as I can see looking out of the window at the moment. The showers are sharp, but have been relatively brief so far today.

Figure 2

Figure 3 – Courtesy of the Met Office