wrong #Reigate forecast today… a possible reason…

February 28, 2014 — Leave a comment

Weather is chaotic and numerical weather models are not perfect. The forecast for Reigate today went rather awry, though not completely.  It was forecast to rain heavily, perhaps on and off, but the forecast was for heavy rain more or less throughout the day. Check UKMO forecast from yesterday below.  Some models brought 24 hour totals of 20-30mm to SE at points on the lead up to the event.  The cause of the forecast deluge: a small scale low tracking NW to SE with a tightly wrapped occluded front crossing the area once, then lingering nearby to deposit more rain during the day before drifting off southeast. Once the front had passed through early am, it turned out to be a splendid day with sunshine and bright spells throughout, until rain later.  So what went wrong/right?

The front passed over as forecast during early am dropping 6mm on Reigate before 8am.  It then sat N of London most of the day while further south convection over Sussex caused significant Cb clouds and showers (some thundery) to spark off from midday.  For us in Reigate, we had a splendidly bright day with glorious sunshine by 8am and bubbly cumulus clouds thereafter, the odd spot of rain but nothing significant until early afternoon when the front migrated south east.  So for most daylight hours Reigate was dry, quite the opposite of the forecast.

The photos above and graphics below suggest a possible reason for this.  Reigate sat in a sort of “Goldilocks Gap” between the persistent frontal rain further north and convective rain nearer the LOW further south. It is notable that the convective showers built mostly over the land, showing almost April-shower tendencies to build on warmer land surfaces than the now-cooler sea. The occluded front sat close to Reigate, frontal wave clouds and cirrus were visible above and to the north for most of the day.  This may have helped suppress convection.  As warm tropical air is lofted over an occluded front it spreads out and forms a cirrus veil, this often suggests a broad inversion of warmer air aloft that effectively suppresses uplift of thermals: the cirrus acts like a lid.  So cumulus clouds over Reigate and the N Downs stayed small and harmless.  Not far south, in Sussex, thundery downpours developed as the buoyant air lofted uninhibited by any inversion.  You can see this on the radar image below.

Reigate was therefore dry for most of the day perhaps because of our location in a sort of Goldilocks Gap (our word) that was just far enough from the occluded front to avoid persistent rain and just near enough to benefit from the inversion to prevent convective showers. Met-Magic!  The graphics and photos try to explain this further.

This is just one possible reason why slight changes in the tack of a LOW will render a forecast completely wrong, even in the middle of a LOW pressure when all hope of a nice day might be thought lost.  Further ideas are most welcome to extend this.

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