A distinctive cold front showed up on the rainfall radar today 29 December as it tracked east across the UK and brought a sharp period of heavy showers. The thread-like line of (red=heavy) showers on the radar opposite shows the cold front very distinctly. A drop in temperature also occured as the front passed over. In Reigate, temperatures dropped rapidly from steady highs of 11°C in the morning to 7°C within hours of the front passing through. Blustery winds of 30mph also accompanied the front and then dropped off quickly as pressure rose after the front passed over. The satellite photo shows the line of frontal cloud and the bubbly cumulus clouds bringing showers in the polar-maritime air behind the front. A classic text-book passage of a cold front!
Archives For December 2012
A snap-shot of weather today reveals how diverse and extreme and fascinating our European weather actually is! The east – west split is obvious: freezing temperatures on the continent with everywhere east of Germany below freezing. To the west we are bathed in warm balmy Atlantic air which is over 10°C. Northern Italy is enjoying almost spring-like weather in the Po Basin, possibly due to air sinking and warming from the Alps.
Meanwhile, Iceland is due to be hammered by hurricane force winds over 76mph (>34m/s)with an intensifying LOW pressure bringing extremely high winds to the NW of the country on Saturday. This LOW pressure will have a central pressure of 942mb: which is extraordinarily low. Winds blow from HIGH to LOW pressure (albeit deflected to the right by the coriolis force – more later on this). The bigger the difference between HIGH and LOW pressure, the higher the winds. See the weather map below and spot the isobars placed close together over NW Iceland: this indicates ferocious Force 12 winds. The UK will be breezy, located on the edge of this LOW, but no where near as windy as Iceland, fortunately. Heads-up for a “heat-wave” in the UK next week :-)!!
Check the heavy relief rainfall over NW Scotland! Relief rainfall is happening here because warm moist air from the SW is being forced to rise over the Scottish mountains, where it cools, condenses, forms clouds and then rains…heavily! The warm front which passed over Scotland overnight brought the moist SW tropical airstream and the hills of NW Highlands are continuing to “wring-out” the moisture. Locations on the west coast are 10°C while those on the east coast are a chilly 5°C or less. The Scottish west coast is warmed by the North Atlantic Drift (Gulf Stream), a warm ocean current. Winter temperatures up on the NW Scottish coast are frequently warmer than those we get in SE UK which is often influenced by chilly easterly continental winds in winter. At the moment, for example, Reigate is 8°C and the Isle of Skye is 10°C.
Look at the video and spot the obvious blob of relief rainfall in NW Scotland: what’s amazing is how the heaviest rain consistently matches the highest ground over the NW Highlands and the rainfall area also matches the outline of the coast almost exactly. Each mountain seems to do its own “orographic job” uplifting the moist air (which is inherently stable, so not happy to be forced up, it prefers to sink). The glens (valleys) also seem to appear as areas of lower rainfall. A remarkable example of relief rainfall. The relief rainfall also appears to “stay-put” while the frontal rainfall has drifted off into the North Sea. This highlights the different mechanisms by which the two areas of rain are being formed: frontal air masses move, while mountains do not! In contrast, us down in the SE UK saw only drizzly light rain as the fronts passed through. We have no significant high ground so no relief rainfall. On the rainfall radar you can also spot the RAINSHADOW effect over eastern Scotland where less rain is falling and there is even a dry spot. In brief, this is because the air sinks, warms and dries out in the lee of the mountains, leaving a clearer, drier area -sometimes literally a “hole” in the clouds.
A text book depression / low pressure system approaching the UK from the west might signal the beginning of the end for this long run of bad weather. It’s a big low pressure system tracking NW rather slowly and will take all weekend to clear off but most of the rain will fall in the west and not much will reach Reigate. The warm front is due to pass over Reigate around midday on Friday bringing some brisk SW winds and cloud and some light rain. The “warm sector” following the warm front comprises an air mass called “Tropical Maritime”, bringing balmy +10°C temperatures to Reigate over night Friday through to Saturday morning. The cold front is due to pass over Reigate after lunch on Saturday when temperatures will drop by several degrees to 3°C overnight . Despite the slow movement of the depression as a whole, winds will be pretty strong especially as the cold front passes. The cold front will bring gusty winds, possibly up to 50mph on Saturday morning, and heavier showers on a cooler NW wind. The air mass following a cold front is called “polar martime” and will feel significantly cooler in the wind, even in sheltered Reigate.
There are signs that a HIGH pressure will build up over the south of the UK by the new year week bringing drier weather, at last! The ensemble forecast below shows a drier spell to start off 2013. An ensemble forecast is one which combines several computer weather forecasting model “runs” and sees how well they match. The closer the match, the more confident the forecast and more probability that it will be correct. Each “model run” is “perturbed” which means tiny differences in starting data are used to simulate the error and vagaries of real chaotic weather systems.
A heavy band of thundery showers caused hail, thunder and lightning over Reigate at 8.30am Christmas morning. A trough passed over the region and brought intense rainfall reaching 93mm/hr at 8.08am. Temperatures then dropped suddenly during the showers from 8C to 5.7C and winds gusted to over 25mph. Winds veered from Southerly to Westerly as the LOW pressure tracked north and out into the North Sea. The blue dashed line on the charts shows what happened as the trough passed over. The map shows the synoptic situation and the chart at the bottom shows lightning strikes occuring over Reigate at the time the storms passed over.
Over 16mm of rain fell on Christmas Day bringing the total for December to nearly 90mm. With further rain forecast it is likely that Reigate rainfall will top 100mm by the end of the month: an “extreme” monthly rainfall for our area. Our wettest day this half of the year was September 23 with 27mm of rain in one day.
Winds in the North Sea exceeded 60 mph today, creating waves over 30 feet high. On the Beaufort Scale winds topped out at violent storm force 11. The strongest winds preceeded a frontal system attached to the same LOW pressure which brought the heavy rain to the UK yesterday. Every region of the country has flood alerts and almost every sea area surrounding the UK has been issued with gale warnings.
With serious flooding elsewhere in the UK it seems that Reigate and the South East fortunately escaped any serious problems from this storm. The River Mole has burst its banks but this seems rather trivial in comparison! The low pressure and unsettled conditions are set to continue.
Temperatures reached 12C in Reigate yesterday and stayed above 10C overnight. This is very warm for December! The reason is shown in the latest NOAA high resolution satellite taken this 10.15am Sunday. The bank of cloud shows the warm funnel of air brought from the SW by a vigorous jetstream. North of this stationary front are blobs of cumulus cloud shown catching the sunshine. South of the front is a big High pressure giving Spain clear conditions. Also, spot the snow on the Pyrenees. The jetstream turns South over Europe and is bringing cold conditions over places like Greece where snow has fallen over Mount Pelion on the Aegean coast.
Wind speeds measured today, Saturday 22 December, at 8am on the Greenwich Lightship (in the middle of the English Channel) topped 40mph (Force8) and are increasing as the low passes over to the north. In contrast, wind speeds in Reigate remain at a 10 minute maximum average of 13mph. Wind speeds are usually higher out at sea because there is less friction at the surface (fewer hills, trees etc) to slow it down. The map shows NOAA data buoys located all over the world and used by meteorologists to forecast weather. The graph is for the Greenwich Lightship this morning: note the big wind increase since the “calm before the storm” at 20:00 GMT.
Reigate should prepare for 24 hours of rain, starting Saturday 6am and continuing pretty much non-stop until Sunday morning. Rainfall totals for Reigate could be over 20mm which will add to the 18mm we got on our wet Wednesday this week. Totals before Christmas set to hit the 60 or 70mm mark which, as predicted, approach the total rainfall received in Reigate during the whole of November!
Interestingly, during the heaviest rain tomorrow the cloud above Reigate will extend over 9000 metres in height! This is as thick as Mountain Everest is high (8848m). In addition, temperatures will go from 12C at ground level (warm for December) to -50C at 9,000 metres. Winds in Reigate will be around 20mph while at 9,000 metres they will be blowing at around 120mph.