Archives For warm sector

2015-06-20_07-45-37

June has been a cool month so far and is on target to be the coolest June since 1991, or certainly close.  It has also been dry with just 28% of monthly rain so far for SE England.

In contrast to the June trend, mostly due to the Azores HIGH lodged firmly to the SW and dragging in a cool dry NW flow (bit unusual for a summer regime), today a humid moist warm sector crosses Reigate and this will bring a minor hiccup to the dry weather of the last week when there has been no rain at all in Reigate.  Humid air is due to arrive in the warm sector and this is indicated by modestly raised dew points (high teens) and PWAT (precipitable water) exceeding 30mm, showing potential for some heavy rain around but this is hit n miss. (update: metoffice weather warning issued in morning for heavy thundery rain)

Of interest are surface winds which appear to converge in a zone, associated with a cold front, across the south later today (spot the twisty wind barbs below): convergence is where winds arrive quicker than they leave a region and, as winds “pile up”, this often promotes LIFT (upward air motion) that encourages convection and storms. But it’s a slack scene and by no means a classic stormy picture. Scattered slight risk of heavy showers is likely to be about it.

converging winds

converging winds

Skew-t charts (cross-sections through the atmosphere showing temperature, wind speed, direction and humidity etc) also show reasonably high CAPE >600j/kg (convective available potential energy) and some negative lifted index in central southern England. These values are indicators of instability: which means air is free to rise to a great height, condense into tall clouds possibly forming cumulonimbus.  Note the change of wind speed with height, such wind shear also acts to duct air from the surface.  Well, the ingredients are there for heavy showers later today but they were also present in the much heralded plume last week and that came to nothing, catching out professional forecasts as well as amateur enthusiasts. There were a few notable heavy thundery outbreaks last week but many convective forecasters and storm enthusiasts were stung by the lack of activity and model predictions appeared to founder.  The scenario today is decidedly less “stormy” so storm fans should not get excited either!

2015-06-19_21-44-47

Heathrow Saturday unstable showery chart.. dry-ish mid level slot too?

Of note is a dry slot at 700hPa mid levels (shown well on the chart below) that can induce evaporative cooling.  Evaporative cooling is caused when moisture evaporates and reduces temperature.  This reduced temperature at mid levels of the atmosphere can create higher CAPE as warm parcels excitedly find themselves rising through ever cooler environmental air. If sunshine heats the surface this can promote heavy showers and possible thunderstorms, albeit scattered and only a slight risk today. In addition a weak jetstream moves to a position later pm where any showers will find themselves on the left-exit region of the jetstream.. this is area known to further lift air from the surface, like a hoover dragging air upwards into divergent air aloft.

The forecast is for scattered showers, some thundery later.  So, as usual, some places could miss them though it’s a moist air flow so some rain is likely most places.  Rainfall totals will vary between almost nothing to possible >10mm.

2015-06-20_08-04-35

more unsettled to end June

Sunday looks cloudy but mostly dry as showers clear off tonight. Next week and towards the end of June there is a threat of more rain, possibly pretty heavy on Monday as Atlantic LOWS nibble away at the Azores high that has dominated our weather recently.  Monday sees a frontal wave low sticking wet conditions across the south which could yield high rain totals. Thereafter, mid week sees the Azores HIGH ridging back in with pleasant warm and dry conditions but this looks temporary as Atlantic LOWS nibble away with wetter westerlies always trying to edge back in.

2015-06-20_11-36-22

Azores ridges back in, but looks temporary

The outlook is therefore occasionally unsettled, especially in the north further from any ridges, though with the risk of heavy showers at times in the south.  Gradual warming trend into July with possibility of a brief warm or hot SE flow to start the month, as shown below on the 850hPa temperature GEFS chart: note the “plume” (oh no) with regressed Azores HIGH and thermal heat LOW over Spain. Way off, but worth watching 🙂

2015-06-20_12-26-56

brief hot spell to start July?

Sleet and you shall find” is the likely scenario for Monday in Reigate! Deep and crisp and even snow is not looking on the cards for us on Monday despite it snowing almost everywhere else right now (see map) and despite low temperatures overnight and tomorrow morning.
Light snow might lay for a while am but temperatures are due to creep above freezing for Monday as a warm front pushes “warmer” air across the region. This will deliver sleety rain on a southerly wind rather than snow through the morning. Nevertheless, snow is notoriously tricky to forecast and the cold front following closely behind might still deliver moderate snow showers as temperatures fall Monday pm and overnight to Tuesday heading in on an increasingly brisk and cold Northerly / NE.  Why is snow so difficult to forecast accurately?  There are good reasons for this… but this is a question to tackle later.  Meanwhile, latest model runs suggest cold will hang on for at least a week, snow still very much on the cards… so perhaps “ask and it will be given to you” might still hold true for snow lovers? (Matthew 7:7)

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.

27-12-2012 22-58-26 dry spell in january