Nick F

Yep, quite an active cold front coming through, just been hit by a blob of heavy rain here near Heathrow ahead of the main convective rain band which is producing the odd sferic.

What I find more intriguing was this unusually long-tracked supercell, which dropped a destructive tornado early on, that crossed nearly 600 miles over several states on Tuesday ahead of the same cold front that was further west over U.S. Plains back then

So, the 18z GFS singing from the same hymn sheet as EC day 10. Ties in with the z500 ENS mean from both models shifting the mean upper trough from the west of the UK to the east of the UK by day 10. Now will it still be there in the morning, won't hold my breath ... but has support from London EPS for turning colder toward mid-month

A look at the stats showing that winter 2016-17 was dry and mild for the UK overall, though there were some regional differences for each month View the full blog here

Like the warmer parts of the year, looks like northern France and Belgium stole all the storms again, despite pretty cold uppers and relatively mild/moist surface airmass. French storms triggering along cold front which cleared southern UK overnight. A look at the skew-ts and 12z radiosonde ascents though suggest airmass is rather moist all the way up to the tropopause across much of the UK today - which doesn't favour electrification of convection. So sferics been rather sparse today and convection messy. Another chance on Tuesday for some convection with hail and the odd rumble of thunder in the cold polar NWly flow destabilised by weak surface heating in sunny spells.

Thought I'd start a new thread as other one was rather long and started back in September. Monday looking rather lively across many areas convective-wise, as a cold vortex originating from southern Greenland drops SE over UK and creates rather steep lapse rates atop of moist maritime flow which will be weakly heated in the increasingly strong sunshine as we head into early Spring. Enough CAPE and wind shear forecast for me to warrant a marginal risk of severe weather toward SE England and E Anglia - primarily for isolated strong/damaging convective gusts in heavier showers/storms, but a brief isolated tornado can't be ruled out with veering winds and increasing strength with height. Storm & Convective Forecast Issued 2017-02-26 23:12:52 Valid: 27/02/2017 00z to 28/02/2017 00z CONVECTIVE / STORM FORECAST - 27TH FEB 2017 Synopsis Upper low/cold vortex, that broke away from Polar Vortex over Greenland over the weekend, moves SE across the U.K. During Monday, steepening lapse rates and creating an unstable airmass across the U.K. A surface low, just NW of EIRE at noon Monday, will drift east across S Scotland in the afternoon / evening, a cold front will clear E Anglia and SE England early afternoon followed by troughs moving through in the increasingly cold but unstable SWly veering Wly flow. ... EIRE, N. IRELAND, ENGLAND, WALES, W SCOTLAND... Increasingly cold air spreading east aloft (500mb temps <-35C across western U.K.) will create steep 850-500mb lapse rates (31-32C fall with height) atop of moist maritime airmass which will be weakly surface heated in sunny spells. As a result, models simulate 200-400 j/kg CAPE by early afternoon / peak warmth, this will support heavy showers and scattered thunderstorms which, given low heights / freezing levels, will produce hail, sleet and snow over hills (and lower elevations in heaviest cells) and also strong convective gusts. A low risk of localised severe weather does exist on Monday in association with more organised areas/band of storms that develop along cold front moving east across England and Wales during the morning and also one or two troughs following into the afternoon, particularly across the southern end where stronger winds aloft near jet and veering with height may allow forced convective updrafts along to organise. More organised bands of convection/storms would be capable of drawing strong winds aloft down toward the surface as isolated damaging gusts - perhaps 50-60mph inland and 60-70 with exposure in heaviest showers/storms. Also, winds veering and increasing with height may allow updrafts to rotate into a few funnel clouds or even isolated brief tornado/waterspouts. Therefore have issued a MARGINAL risk for southern counties + E Anglia where overlap of CAPE, wind shear / stronger winds aloft are enhanced. Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2017-02-26 23:12:52 Valid: 27/02/2017 00z to 28/02/2017 00z CONVECTIVE / STORM FORECAST - 27TH FEB 2017 Click here for the full forecastCheck the full storm forecast here

Parts of a post I made last Monday looking ahead into early March and although model guidance is still fumbling around with the surface synoptics in the medium range they are generally converging on the upper/500mb pattern with the +ve height anomaly over Greenland and Iceland with troughing underneath as I highlighted from the EPS/GEFS mean 6 days ago. The lag effects of MJO P8/1 and the stratosphere warming / PV displacement IMO likely the precursor to the +ve heights around Greenland and Iceland. Although I have lost the appetite now to hunt for snow and cold + the operationals aren't overly keen on lowering heights / developing troughing further east across central/eastern Europe, which keeps low pressure to the SW and a milder S to SWly flow at times, there is scope for pushing low pressure systems further east into Europe to our south in the medium range, 00z EC high res attempts too days 7-8 before trough disruption stops it. 00z EC high res, at day 10 keen to extend Azores ridge NE toward UK, however the EPS z500 mean and the EPS mean and spread suggest perhaps emphasis towards lower heights to the SW than the high res In the shorter-term, upper low dropping SE across UK and EIRE Monday / Tuesday brings quite low heights across the UK. Fax chart for 12z Monday shows the 528 dam line moving into the west, steep lapse rates with the low heights/cold air aloft, so will be some heavy showers with hail, thunder and even some sleet and snow in the heavier burst across the north and west - as dew point lower from the west in the chilly Pm westerly flow 00z ECM on weatherbell indicating snow for western hills on Monday (Wales, NW England, western Scotland), then perhaps a more organised area of snow moving SE across southern Scotland and far N + NE England Tuesday morning which could be tied in with a trough / front moving S /SE as winds turn more Nly for a time So don't be surprised to see some snow flakes early next week across the north and west, though settling snow moslty confined to hills daytime, though some spots in the north may see brief settling to lower elevations Monday night.

Will be turning quite chilly in the west too on Monday, with low heights and dew points dropping to 0C and below in the brisk Pm westerly flow - could be some wintriness in the showers too with snow over hills in the west.

81 mph gust at Weybourne in Norfolk, unusually strong gusts for this far south

78mph gust at Aberdaron recently T+00 fax chart for 06z this morning

Pretty impressive pressure drop in last 3 hours in NW Ireland, some 7-8mb?

Quite a well-developed cloud head on the low now looking at most recent airmass imagery, indicative perhaps of the low now moving into the left exit of the jet streak and now starting RACY?

METEOSAT airmass imagery frames for today show nicely the development from frontal wave into a depression, with baroclinic leaf forming as dry air intrusion (in red) originating from the lower stratosphere overruns the baroclinic zone and warm conveyor. The dry air descending from the lower stratosphere is characterised by higher potential vorticity (PV) values, which induces cyclogenesis as can be seen by geopotential contours. This descending dry air my also be responsible for a sting jet on the SW flank of the deepening low as it crosses central UK and out into the N Sea, near backbent occlusion The upper shortwave trough digging SE over the Atlantic, seen as kink in the jet immediately behind the low, also plays its part on Doris's RACY, by pushing it on to the poleward (cold) side of the jet stream and also into the left exit as it crosses EIRE and UK, which allows rapid deepening too. Left exit of the jet (along with right entrance) is a developmental area for lows, because this where upper air diverges - causing air to converge at the surface to fill the void and thus lower pressure.

Storm Doris, the fourth-named storm since beginning of last Autumn will cross northern UK on Thursday, bringing damaging 60-80mph wind gusts to central parts and snow to Scotland and higher ground of northern England View the full blog here

IMO we are beginning to see the models finally latch on to the upper flow amplification effects of MJO passing through P8-1 ... having passed through P8 at an unusually high amplitude. Also the troposphere will eventually start to reflect what's happening in the stratosphere too end of month and early March, i.e. a warming at the pole and displacement of the sPV across Scandinavia and NW Russia So it's inconceivable that there won't be extra-tropical impacts from both the lagged colder 8-1 phases of MJO and the stratospheric downwelling. The typical MJO lag is about 10 days, so P8-1 impacts in a window from 28th Feb/1st March through into the first week of March + the tropospheric upper flow pattern beginning to reflect the stratosphere too. What looks most likely, looking at medium range and extended EPS and GEFS is height rises near or just east of Greenland while the Candian trop PV sinks south and the jet + trop troughing sinks south over the Atlantic. So we are likely to see increased chance of colder air leaking south then perhaps eventually SW across the UK from high latitudes as we enter early March. Obviously, this could go wrong for the south, with the low pressure systems perhaps not tracking far enough south - which means the cold air stays over northern UK while the south stays mild. But a lot of water to go under the bridge until we can firm up on this IMO likely pattern change away from low heights dominating to our north like now. But, don't think today's temps reaching 17-18C in the southeast marks the end for deep cold and snow potential this season ... yet. Remember, the models guidance for the trop can often play catch up with what the MJO and stratosphere precursors are indicating ...

Spring on Monday, with mid-teens over Midlands and E England ... to winter next Friday, as a N/NWly flow brings temps struggling to get higher than low single figures Midlands north. 00z EPS z500 mean indicating support for the brief N-NWly end of next week, as upper flow amplification moves out of NE N America downstream over N Atlantic. Unfortunately, good agreement, the flow switches back to SWly thereafter, as a trough digs SE from N Canada out into the NW Atlantic

There is typically a lag time of around 10 days for an MJO-induced Rossby Wave to reach high latitudes (Hoskins, B. J., and D. J. Karoly, 1981: The steady-state linear response of a spherical atmosphere to thermal and orographic forcing. J. Atmos. Sci., 38, 1175–1196) - phases 8 more favourable than 7 for UK so like you say around the 25th, but not because of the subsequent return to phase 7 shown on the GEFS, but from the current phase movement through 8 and 1. The GFS forecast for it move back into phase 7 around the 24th may not have an impact until into early March. For example, MJO went into phase 7 on the 9th Feb, 10 days on from then we can compare the phase 7 composite for amplitude >1 with the forecast GEFS z500 mean for the 19th Feb, not too dissimilar upstream other then the lower heights downstream over Scandi Feb MJO composite for phase 8 with amplitude >1 (below), 10 days from current phase movement into 8, takes us to around the 25th, IMO movement from 8 into 1 more favourable than 7 into 8, but obviously MJO propagation in 8/1 and the resultant increased amplification of Rossby waves into higher latitudes does not automatically = blocked/cold/snow for us unfortunately, as more mesoscale changes in the atmosphere can ruin prospects. However, we do also have the wave number 1 warming helping to displace the sPV over toward NW Russia / northern Scandi after its current return to northern Greenland / N Pole area. This appears to have the immediate response of increasing the 500mb zonal flow over far N Atlantic into Scandi, but with time height rises / amplification transferring east over N America under a stratospheric high developing and moving east towards Greenland may allow height rises over N Atlantic and perhaps into Greenland to end the month. Of course, I could be completely wrong, the atmospheric patterns don't always follow the historic composites of oscillations and/or there is disconnect between the trop and the strat. But I think there is some positive indications from both the MJO movements and next sPV displacement to give tentative signs of winter returning at the end of this month and into early March. Not ideal, as snow tends not hang around as long as Dec-Jan, but don't be fooled by the warm up and early Spring feel over the next 10 days or so.

Another winter storm about to hit New England, from summer heat back to winter chill in the southern U.S. High Plains and the Australian heatwave. View the full blog here

Both GFS and ECM displace the sPV back to NW Russia/northern Scandi by around T+120, currently its centred over N Greenland/ N Pole. If you remember last time this happened in late January we saw the zonal flow in the troposphere strengthen over the Atlantic, with the NWP churning out zonal charts out to day 15, something similar at the moment it looks too. But IMO we could eventually see mid-Atlantic ridging building north as the core of the trop PV moves over Svalbard and Norway while more amplification works downstream, certainly at day 10 the flow is looking more amplified over N America on 12z EC.

Moderate fall of big fat fluffy flakes in SE25, but struggling to settle, pffft

Quite an impressive area of precip moving into E Anglia and earmarked for Thames Estuary, hopefully it doesn't turn too marginal when it arrives in London area - dew points holding out for now.

Chucking it down with a fine snow here near Croydon, settling on cars and fences and now trying to on grass but not roads/pavement. Not sure we will even reach a Kentimetre though.

Yes, coldies better not look at the 18z GFS for next weekend, really is a horror show with a SWly flow sourced all the way the other side of the Azores with +10C T850s above the UK.next Sunday. Though it does redeem itself in FI. Is rather at odds with the 12z ECM for noon on Sunday 19th, which has an prominent upper low still over Iberia. 12z EC for 12z Sun 19/02 18z GFS for 12z Sun 19/02 On the 12z EC and GFS operational runs we saw differences between the two models on how quickly they exit the upper low over NE N America and also trough disruption SE towards Iberia in the 8-10 day range So certainly the models are not unanimous with the upper flow pattern in a weeks time over the Atlantic or even with to the north - with regards to how far down the low heights extend from the trop PV over the arctic.

Getting a measly dusting now on cars and grass here in sarf London, keeps up might even be enough to make a snowball in the morning.