Archives For heavy rain

Reigate is at risk from heavy rain moving north across the SE during Friday pm through to Saturday. 24 hour rainfall totals could exceed 30mm for this period. This marks the start of an interesting period of weather seeing temperatures taking a big drop from Sunday through to Monday. Prepare for cold temperatures and icy conditions in Reigate by Monday through to Tuesday especially. Snow is a risk but exact details remain uncertain: the main snow risk for Sunday is north of London, Midlands and north of the M4 corridor but temperatures fall overnight easily low enough for any rain round Reigate to increasingly fall as snow. It will certainly be chilly and Monday night could be extraordinarily cold for March (where’s Spring?)! 

It looks like February could see a return to winter conditions for Reigate (2 small colour charts) and maybe even cold enough for some snow; though with the sun being that bit stronger it is unlikely to be as chilly as last week. The jetstream, currently right over the UK and bringing our stormy conditions and unseasonably high temperatures (12°C today at midday in Reigate), is likely to be shifted south during the second week of February. A blocking upper ridge in the Atlantic and building HIGH pressure over the North Pole will combine to swing winds round to a cool northerly direction over the UK. A LOW over Europe will add to the chill by bringing in periods of cold Easterly winds over the UK as well and it is these which often bring significant snow to the South East.
Meanwhile, this week’s extraordinary weather is not over yet: a deep depression (see big synoptic chart) is forecast to rapidly swing up out of the SW on Friday. Depending on it’s precise track over the UK it could bring heavy rain and strong winds or EVEN some snow. The models disagree by a critical hundred miles or so: MetOffice has the LOW tracking south of Reigate= easterly winds = snow; GFS has it tracking north of Reigate = westerly winds = rain. (Update: UK Met Office model is now on it’s own with snow in the south; all other models take the track of the LOW further north leaving strong SW gales and rain on Friday but no snow for Reigate – snow will fall further north across the UK; so the synoptic chart above is probably WRONG already!!).  In both scenarios the back of this LOW will see nippy northerly winds bringing temperatures right down on Saturday, but at least it should be bright.

28-12-2012 09-21-42 scotland

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.

25-12-2012 17-24-45 xmas day 12pm 2012

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.

25-12-2012 08-17-20 xmas day lightning and hail storm

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.

Prepare for a SOGGY Christmas for us in Reigate! In fact, we could get a whopping 100mm (forecast update Tuesday suggests up to 70mm – a lot still!) of rain between now and finishing the turkey on Christmas Day.  This is approaching the TOTAL rainfall we had in Reigate during the entire month of November (79mm) … expected in the next week!
Reason: a big LOW pressure sits out in the Atlantic for the entire period, barely moving, but holding our position on the “left exit region” of the jetstream which will give birth to several vigorous baby depressions. These depressions will INTENSIFY (falling pressure , causing stronger winds and more rainfall) as they move across the UK. Rain is expected in Reigate on Wednesday pm, HEAVY rain Thursday am, Friday will be a respite before a wet weekend heralds a really wet Christmas (update Tuesday: cooler and dry for Xmas Day). AFTER the festivities there are signs of a prompt cooling off period for us all. Meanwhile, think of people in Russia – temperatures there are down to -30C at least!

mother LOW december 17 2012

This weekend sees two powerful storms tracking rapidly NE across the southern part of the UK: the first to hit will bring strong winds and heavy rain to Reigate over Saturday afternoon and night – it will track across and clear by Sunday morning, bringing colder clearer NW winds in its wake.  A brief interlude on Sunday is likely to be followed by a big low tracking in from a westerly direction which will bring more strong winds and heavy rain.  It will clear by Tuesday and THIS will herald the arrival of those increasingly cold and brisk NE winds for the rest of next week.  Prepare for increasingly wintry weather!

Saturday – intense low out of Biscay

Sunday: brief high pressure ridge between two monsters!

An unusual weather pattern exists right now: High pressure in the Atlantic is stopping our normal south/westerly winds and allowing cool north and north westerlies to dominate.  Once the rain on Monday shifts from the South East, Tuesday will be our best day – especially out west where a ridge of high pressure briefly flirts with the UK.  After that an ugly low pressure system swoops down from Iceland and the Low will centre over NW Scotland.  Strong winds and heavy rain will affect much of the country, especially the western half.

However, a tricky beast of a storm (a wave depression) might rapidly develop to the SW of the UK and rip across the south rather quickly sometime around Friday / Saturday.  Look at the jetstream chart below for next Saturday and see how the jetstream really picks up over the South during this time, dragging the storm along on the surface, like an unruly dog on a lead! This “dog” has a pretty big bark and a big bite and could deliver a deluge of rain and strong gales over the south.  Let’s wait and see: some bonfire nights might be wet and windy.