Archives For showers

2015-05-02_06-58-07

analysis chart shows HIGH edging out with moist Atlantic winds ready to pounce

The analysis chart above shows a weakening ridge of HIGH pressure over the UK being edged out north by a slow moving Atlantic LOW to the W/SW.  Reigate is still currently (Saturday am) in cool dull easterly winds generated by the HIGH but a significant switch in wind direction will take place over the next 12 hours into the bank holiday period as a wholly different mild and humid Sub-Tropical Atlantic air mass, with a source region round the Azores, takes hold from the SW.

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weather cross-section

A mild moist S/SW wind drives in from the Atlantic as the LOW edges north east tonight. The first mass of rain is edging onto radar from the SW and is expected to arrive in Reigate by around mid-late pm today.  Most rain is likely for places further north and west but the SE is still likely to pick up plenty of wet weather overnight with low cloud and rain into Sunday morning when it could turn heavy and showery for a while in the early morning as the trough passes directly overhead and pressure continues to fall. Things are expected to clear to brighter conditions later in the afternoon as pressure rises and winds turn more westerly. Cloud cover will hopefully break and cloud height will lift during the afternoon becoming more cumuliform.

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trough and fronts migrate north, showers follow

If the sun comes out then there could be a low risk of an odd heavy shower Sunday afternoon, possibly thundery, but these are more likely further north of our area where more unstable air makes progress across the Midlands and East Anglia.

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During Sunday winds will be occasionally blustery with moderate convective gusts possible, especially on hills and nearer the coast, and make the mild temperatures Tmax 16C feel considerably cooler. Temperatures overnight could hold up to a balmy 12-13C.

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Overnight Saturday-Sunday rain could linger as showers through the morning

Winds turn from SW to more southerly through Monday and pressure should up-tick slightly giving a mostly dry and warm day and less windy as things stand currently.  Troughs could progress east during Monday and build cloud and produce some showers.  More importantly there is a looming threat of something special for later Monday-Tuesday night.

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As the northern block (high over Greenland) holds on, the Atlantic LOW just west of Ireland will usher in a mild and moist S/SW flow of air from the continent.  An unstable LOW brewing in the topical Atlantic today (Saturday) is forecast to sweep up and intensify from Biscay later Monday and into Tuesday and this might bring heavy rain and winds to the south and SE and a possible thundery episode later Monday but more likely overnight into early Tuesday for SE.  The jetstream is dipping well south and is forecast to perk up and approach the UK from an unusually southerly direction by Tuesday.  If this happens the jetstream could deepen this low considerably, as modelled by some charts (latest UKMet shows 980mb).

Depending on the evolution we could find ourselves in the unstable left exit region of a jet where divergence aloft enhances convective action and creates heavy rain.  Warm air from the south will also contain more moisture.  A dry slot at mid-levels might also enhance instability (rising dry air cools more quickly increasing lapse rates and CAPE, enhancing lift).  High dew points near the surface temperature also encourage condensation and indicate extremely moist warm surface flows.

So all these ingredients stirred up could be a good recipe for some briefly moderate-severe weather in our region especially some briefly torrential rain, though totals are unlikely to amount to more than 10mm.  Gusty winds and gales near coasts could also accompany this system.  Latest metoffice chart shows pressure dipping to 980mb in the North Sea which is significantly LOW pressure for the time of year.

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coastal gales and convective gusts inland

However these episodes have a habit of tracking across Holland and merely clip Kent with thundery showers and miss us entirely.  Models also generally exaggerate these early on and then things flatten out nearer the time considerably.  Nevertheless, it is worth watching this develop as our first potential “warm plume” of the year.  If we take a direct hit the SE could have some heavy rain.

The GEFS summary below clearly shows the two main rainfall spikes tonight and Monday night.

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GEFS 850hPa temperatures and rainfall London

Later mid-week the LOW is expected to drift east across the UK bringing in a more westerly pattern so unsettled showery weather is likely for a while. Thereafter, a rise in pressure from a developing Euro high pressure may then take place from the south and settle things down for us in the SE, though this might only make faltering progress.

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2015-05-02_07-05-01

faltering pressure rise later next week

Slightly edgy stuff for the weekend for fans of airmass battles. Current weather is in a BLOCKING pattern with a HIGH NE of the UK over Scandinavia blockiing the passage of a LOW pressure cyclone which has been cut-off down off the west coast of Spain all week. It’s gradually filling and has been wafting a lot of nice warm upper air our way from the South, hence our mild temps for much of the past week and the stagnant weather… not much happens when the jetstream abandons the country!  This usually means the weather (good or bad) stays much the same for ages until the BLOCK moves or the JET returns.  

However, the weekend heralds a change, of sorts, for next week. The HIGH over Norway is building and will increase the pressure gradient as the LOW sneeks up the Channel. A brisk easterly wind generated by the pressure gradient will push the relatively drier continental air (from the HIGH) right into the warm, moist upper air arriving from Spain. The result will be lots of rain in the SW and for the Channel. Some of this will be heavy and possibly thundery on Saturday and any slight change in the location of this marriage of airmasses could bring rain anywhere to the south.
The fronts are not now expected to push much up from the coast or out of the SW.  However, whilst a dry mostly cloudy day is forecast for the weekend in the SE, with so much action nearby, we shouldn’t rule out the possibility of a shower. This is especially the case on Sunday afternoon when, despite a good general forecast, the fronts do wiggle closer up the Channel and might deliver some showers in the afternoon.  The SE and Reigate are on the dry side of the front but humid showery air is never far away.
Next week looks more interesting as a new LOW, invigorated by a refreshed jetstream, is due to deepen and stay out west due to the blocking high to our east.  It is set to deliver some waves of warm unstable air across the UK sometime midweek. On a stiff warm southerly wind this might generate heavy rain mid-week, some possibly thundery with plenty of interesting convection. A plume of so
rts.

end sept 30

HIGH cape and low LI indicate poss storms mid week

HIGH cape and low LI indicate poss storms mid week

An unseasonable depression will bring significant rainfall to much of the UK on Monday.  The SE will escape the worst of the heavy rain crossing the rest of the country further NW in a deepening LOW moving NE across the country through Sunday night and Monday.  Many parts of the UK will experience large rainfall totals exceeding 40mm and with flooding in places.  The SE is blessed by the late arrival of any fronts (weakening too) and the movement of the system to the NE driven by a brisk southerly jetstream 20,000 feet overhead.  It simply passes us by.  Nevertheless, Reigate could see 5mm or so of rain, mostly arriving in the form of showers in the afternoon, some heavy: so there is a twist in the “tail” of this depression.

The sounding shows the instability of the air as the front crosses.  Any warm parcels of air lifting through the atmosphere (dashed line) as the cold front approaches will be warmer than the surrounding incoming cool airmass (red line) so will continue to lift and this is a so-called “unstable” situation likely to form tall cumulonimbus clouds and heavy rain and possibly fast moving thunderstorms.  The dew point (blue line) is also very near to the air temperature so the atmosphere to a great altitude is humid / near saturated and likely to condense large amounts of water vapour into cloud droplets.  Moderate rainfall totals on a brisk air flow are possible.

frontal sounding shows instability

frontal sounding shows instability

The far corner of the SE may see no rain at all during most of Monday but an area of instability and rapid upward velocity (lift) associated with the passage of a cold front approaching from midday could spark off heavy showers and thunderstorms in the SE and Reigate.
Temperatures in the SE will be higher than anywhere else in the UK as we enjoy a warm southerly flow of upper air in the warm sector of this depression with a strong southerly jetstream starting the day over Reigate (>70mph). Temperatures at 10,000 feet will fall below freezing over Reigate during the course of the day as the cold front brings in much cooler NW air flow across the UK.  The rest of the week looks less unsettled, temperatures recovering to normal though with more rain at times. Typical August!

showers over east coast 03-02-2013Today was bright and clear with a chilly northerly breeze, as forecast. Clouds developed throughout the day over Reigate but remained benignly flat anf fluffy, eventually joining up to form a thin broken blanket of stratocumulus through which the sun set spectacularly. Typically, in a northerly winter Arctic airflow, snow showers reach the East coast but fizzle-out and disappear inland. Today was an excellent example of this: watch the video. Places near the Kent coast and East Anglia saw wintry showers whilst places further from the sea, like Reigate, saw none at all.  A simple explanation goes like this:
Clouds form when air rises, cools and water vapour condenses into water droplets (clouds are tiny water droplets and/or ice crystals, not cotton wool!). The main driver of rising air is the heating of an airmass by a relatively warm surface. The North Sea is still quite “warm” at a balmy 7°C (note the nearby higher surface temperature in Margate 4.5°C) and this was sufficient to warm the chilly Arctic airmass travelling across it.  This warming caused convective uplift of warm bubbles of air (thermals) which formed shower clouds over the North Sea.  Think of the bubbles formed when a cold pan of soup is heated on the stove: they rise up, just like thermals rising through the atmosphere.  Rising thermals initiated cloud formationover the North Sea produced the shower clouds visible on the video marching across the East coast. The showers died-out inland because the land in January is colder than the sea (note the lower surface temp in Reigate 2.5°C) so warming is reduced and thermals are much less frequent.  Fluffy clouds may form but do not rise to any great height: these are called “fair weather cumulus”.   Showers and clouds tend to fade away in the evening because thermal activity is reduced.  (n.b: April showers occur for similar reasons: the stronger Spring sunshine warms the ground quickly and sets off rising thermals through airmasses which are still cool from the winter.  We must wait a few more months for the sun to get powerful enough for showers of this nature to kick-off inland: something to look forward to!)  Temperature changes with height are called lapse rates. Lapse rates control cloud formation and precipitation.  Try this site for more info.

The Arctic air mass will be replaced by warmer, wetter westerly winds tomorrow.  But don’t despair, if you enjoyed the fluffy cumulus clouds and outstanding visibility today then you will be happy to learn there is more to come this week!  Models agree that another long cold snap is due to return, initiated by an Arctic airmass on Wednesday.  This cold snap could last into mid-February with the possibility of an especially cold dip in temperatures next weekend.