Archives For severe weather

Heads up, Reigate!  Or rather… wrap up, stay in and keep warm: SE weather for the end of this week looks awful and, whilst nothing approaching the US Mid-West, it could be described as potentially severe for the SE.

Pick words from this list and repeat and then remind yourself it’s nearly June! “COLD, heavy persistent rain, windy, wet snow, sleet, hail, heavy thundery showers, freezing wind chill, 35 mph northerly gusts… at the end of May!

Thursday: bright start but with cold northerly winds +20mph and heavy showers building in the afternoon.  Feeling chilly at only 10ºC max.  Overnight temperatures fall to 3ºC and any showers could even fall as icy sleet or even wet snow on the Downs. 

Friday: things actually look worse than earlier this week: COLDER northerly winds will drag temperatures right down to 3 or 4ºC in heavy showers and a maximum of  8 – 10°C with temperatures falling lowest in showers.  The centre of a nasty deepening LOW pressure will track directly across the SE on Friday and we can expect persistent rain, some heavy at times and accompanied by really cold winds (for the time of year).  Later in the day heavy thundery showers have the potential to bring hail and sleet.  Significant instability is forecast (*CAPE >350 j/kg; before the Oklahoma tornado CAPE measured 5000 j/kg), currently the highest this year in the SE, which means there is a fair chance of severe weather events in the form of thunderstorms.  Watch out for mountainous cumulonimbus cloud formations extending to the height of Mount Everest and possible funnel or mamatus clouds under dark bases.
Freezing levels will be down to 600m and during any heavy showers evaporative cooling could potentially cause heavy rain to turn to sleet as snow forms above 1000m and heavy rain draws down colder air.  Evaporative cooling happens when enough of the heavy rain evaporates on the descent to take heat out of the air and freeze supercooled droplets or sleet into proper wet snow.  Strong winds gusting 35mph will make it feel truly nasty.  This all sounds ridiculous for May… but the potential is there..let’s hope it’s not quite this bad!

Friday night will be a chilly and wet experience outside.  Temperatures will fall to 4ºC but feel near freezing in northerly winds. Not good for camping.

The weekend recovers reasonably well with a weak high pressure ridge bringing bright and dry spell Saturday behind the Friday storm. The fine weather  could hang on through Sunday but showers could threaten as pressure falls away as a deep LOW approaches from the NW to threaten Monday with rain.  The rest of half term looks unsettled but a bit warmer and drier at the end and best in the NW of the UK where HIGH pressure is likely to ridge across.

*CAPE = convective available potential energy: how buoyant a parcel of air is and this determines speed of vertical extent of convective cloud formation.

In the end: Max 8.4C; Min 4.3C; Rainfall 8mm; gust 24mph NW; So, in the end, pretty foul and unpleasant but fortunately no observed wet snow or sleet anywhere.  Timings of light rain onset were good; heaviest rain pm and cold temps made it feel more like January. No thunderstorms heard but lightning was recorded in SE on ATD lightning detector; CAPES fell to just over 100j/kg in the last run of GFS in the morning, which is a little disappointing for extreme weather enthusiasts!

What makes a thunderstorm?

The conditions for lightning occur when powerful updrafts in cumulonimbus clouds force water droplets and ice crystals to rub against one another, creating massive amounts of positive- and negative-charged particles. The updrafts cause these two types of charged particles to separate, with the top of the thundercloud usually becoming positively charged as the lower part becomes negatively charged.

Here are the ingredients which formed the heavy “April showers” and first thunderstorm (TS) of 2013 over Reigate on Friday afternoon, 12 April 2013.  Whilst only a minor storm, it possibly still contributed to a multi-vehicle non-fatal accident on the M25, closure of the motorway for several hours, poor visibility, local flooding and hail across the area. Convective isolated rainfall events like these are important but tricky to forecast accurately: predicting exactly how much rain will fall, what type and precisely where and when isolated showers and thunderstorms will take place has a lower success rate than other elements of forecasting, like temperature predictions, for example.  Here is a round-up of the key indicators that enabled @RGSweather to issue a forecast for possible thundery activity more than 4 days before and monitor it’s development thereafter and issue a local forecast warning of a thunderstorm risk with very marginal low level tornado risk on the morning of 12 April.

A single relatively minor thunderstorm developed mid-afternoon with hail and lightning crossing north of Reigate on April 12, following a line roughly along the M25 between Leatherhead and Reigate.  The morning saw scattered and heavy showers but little organised severe weather.  Skies darkened over Reigate by 3pm under thicker cumulonimbus cloud and this thunderstorm caused some hazardous driving conditions on the M25 and a non-fatal multi-vehicle accident coincided exactly with the time the storm passed over the M25 which was closed clockwise for several hours thereafter.

Synoptic Situation: An upper trough over the UK and surface low (pictured) across southern UK moved slowly east during Friday, with an unstable airmass building cumulonimbus and increasingly heavy showers during the day as April sunshine heated the surface and increased instability. Here are the figures for yesterday and brief analysis…

LAPSE RATES: +29°C: Cold temperatures at 500mb heights and warming at the surface in the April sunshine caused steep LAPSE RATES of 29ºC. Lapse rates are the drop in temperature with height usually measured between 850hPa (1500m) and 500hPa (5000m). Steeper lapse rates indicate an unstable airmass where parcels of warm air heated at the surface in spring sunshine will rise rapidly and remain warmer than the environmental air surrounding them. Such air parcels will condense, releasing latent heat, which causes further rapid uplift and potential for the formation of cumulonimbus clouds given the absence of any inhibiting factors, like a cap (inversion or isothermal layer… see link below).

CAPE and Lifted Index: 378j/kg; LI -1: Convective Available Potential Energy (CAPE) is a measure of the energy in the atmosphere for convection (j/kg).  Figures in MidWest USA approaching 6000 j/kg cause tornadoes. Here in the UK, CAPES above 300j/kg can cause thunderstorms.  Lifted index is the difference in temperature between the environmental air at 500hPa and a parcel of air lifted to that height: a negative LI indicates buoyancy in rising air parcels and instability and significant convection.

Vorticity at 700hPa: upper air velocity at mid levels means that air is rising. April 12 has UVV: upwardly mobile air at mid-levels.

PWAT: 20mm: Precipitable water is the amount of water that would fall to the surface if all the moisture in the atmopshere rained or hailed out.  Relative humidity is a measure of how saturated the air is at various levels in the atmosphere.  100% means saturated: most levels were at least 80% RH.

TTI index

Total Totals Index (TTI): 60: this is a forecasting index used to measure potential storm strength.  It is calculated using the difference in dew point and temperature between 850hPa (1500m) and 500hPa (5000m).  TTI’s in the MidWest of >60 can yield severe tornadic supercells.

Wind shear: this means change in speed and direction of winds with height.  April 12 saw little deep layer wind shear: winds were blowing at similar strength and direction throughout the atmosphere so little rotation or organisation into severe storms could develop.  Nevertheless, slow moving storms deposited a lot of rain locally and caused minor localised flooding.

WAA: warm air advection: introduction of warm air at the surface increases lapse rates and can increase likelihood of severe TS: 12 April saw little WAA and this inhibited the development of any organised severe weather.

Towards the evening warmer air moved in aloft and, along with the removal of surface heating as the sun set, rapidly reduced lapse rates and inhibited convection causing towering cumulus clouds to melt away leaving a clear night.

So, several ingredients were present to create a marginal storm risk but the absence of some other critical factors like WAA and wind shear kept a lid on the severity and distribution of thundery activity yesterday.  Hopefully, this brief round-up of key storm indices relevant for SE England should help in predicting more severe weather in our region in the future.

Tornado Titans posted this on the CAP and skew-t charts. v helpful. If this is all too much then watch this instead…!