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Interlagos,Ìý5-7ÌýNovember 2010 (Round 18)

(This forecast will be regularly updated. This entry: FridayÌý5 November, 15:00hrs GMT)

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Quick Links:

• Ìý(viaÌýCentro de Previsão de Tempo e Estudos Climáticos, CPTEC)
• Ìý(via Centro de Previsão de Tempo e Estudos Climáticos, CPTECÌý)
• (via Centro de Previsão de Tempo e Estudos Climáticos, CPTEC)
• (Regional Eta-20 model output from MCT/INPE/CPTEC)

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ÌýForecast Summary:

(PPN = Precipitation)

Emphasis:ÌýIncreasing confidence sees Friday to remain dry or largely so - possible arrival of some afternoon showers aside -Ìýbefore a thundery trough/cold frontÌýmoving north brings threat of wet weather overnight into Saturday AM. The remainder of Saturday sees further thunderstorms (possibly severe, with torrential downpours in places), showers or prolonged spells of rain likely during the morning; still a threat of showers (some heavy) continuingÌýinto the FP3/quali windows.ÌýCurrently good inter-model agreement for Sunday toÌýbe dry or largely so for the race, with a fairly lowÌý% chanceÌýof some showers forming later PM.

Friday:ÌýCONFIDENCEÌýHIGH: Sunny spells and dry for much of the day, before cloud increases later PM, bringing showers/t-stormsÌýpossibly by late afternoon but more especially into the evening and then overnight. Chance of PPN 25% until end of FP2. Hot; Max 29C.Ìý Breezier later. Wind NNW-NNE

Saturday: CONFIDENCEÌýMODERATE-HIGH (PPN totals subject to some doubt): Cloudy. Outbreaks of rain or showers/t-storms arriving overnight and continuingÌýinto the morning, heavy at times, some severe t-storms possible.ÌýÌýA continued periodic threat of some further heavy showers / spells of rainÌýappearingÌýthroughout the afternoon,Ìýyielding some very wet conditions on track. Chance of PPN 90%. Cooler. Max 21C. WindÌýlight-moderate, variable; mostly S.

Sunday:ÌýCONFIDENCEÌýHIGH: Perhaps more widespread early cloud, but this breaking to offer sunny spells for remainder of the day. Some deeper convective cloud build-up is likely later into the afternoon with a fewÌý(mostly light) showers possible, but the balance currently firmly favours dry conditions. Chance of PPN 25%. ÌýMax 23C.Ìý Wind light-moderate, SE-ESE.Ìý

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Synopsis & Forecast Evolution:ÌýÌý

ÌýFri. Update, based on 00 & 06UTC model output:

Pretty good inter-model agreement now for the next 24-36hrs, with the cold front and associated thundery band now very evident on IR satellite loop (see link above) steadily approaching from the SW. The resultant rainfall later Friday overnight and into Saturday has potential to be heavy at times, albeit the actual extremes of PPN accumulation remain open to uncertainty. Recent runs of GFS have turned to become one of the more extreme solutions, falling somewhat closer into lineÌýwith earlier output throughout the medium range from most other operational centres. However, the variation in rainfall modelling between models still needs emphasising; e.g., through the next 24hrs,ÌýBrazilian global output (RSPAS) offers fairlyÌýconservative rain totalsÌýacross much of São Paulo (<5mm), while their regional 20-km resolution model (ETA-20) concurs with GFS and some others (including UKMO and ECMWF) on significant accumulations of between 40-100mm. As explained in my update yesterday,Ìý there's ample potential for mass convergence and organisation of the thundery zone during it's northward passage and the potential for severe weather certainly cannot be discounted during Friday night and Saturday. Could we see a repeat of scenes from lastÌýyear's event? Very possible, but timing will be critical.

Meanwhile, the models still hold onto the notion of a dry (or drier)Ìýaffair on Sunday. The upper trough is expected to lose intensity, albeit some models vary their handling of prevailing flow at mid and upper levels, thus reaching different conclusions of the position and amplitude/phasing of the trough and associated PPN. Current consensus sees the rain further to the NE, with moisture convergence activated across the Amazon and across to the Atlantic coasts, yielding thunderstorms once again but not affecting São Paulo. Still subject to some flux, for sure, but for now the prospect of a dry race remaiins the stronger bet.

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Thurs. Update, based on 00 & 12UTC model output:

There's growing inter-model and inter-run continuity now, with little significant to add to the general theme described since the outset of this blog. Much of Friday will remain dry and fair, but with deeper convective development into the afternoon (perhaps even by FP2)Ìýand evening, heralding the arrival of the cold front from the south. This becomes a dominant feature overnight into Saturday, with most models tending towards clearance (of the main PPN band) by around midday on Saturday, but the finer detail of local rainfall threat shouldn't be taken too literally, given the varied handling of frontal PPN spatio-temporal extentÌýby different models. Some models - notably ECMWF, UKMO, CMC and Brazilian regional NWP output - have been bullish about the wet weather potential for Saturday throughout the medium range lead-up. Much like we saw in Korea, GFS has somewhat turned around to fall more into line with these solutions,Ìýbut with inevitable run-to-run oscillations re finer detail.ÌýI expect this to remain so right up to the wire: after all, ignoring the mesoscale continuity, a dozen miles will make a huge difference in terms of how theÌýthundery band clears (or not) ahead of the track sessions on Saturday, thus dictating fortunes on the circuit.ÌýRadars at the ready, folks...

There's a cocktail of critical upper and low-level atmospheric dynamics coming into play, including potential for localised mass convergence; import of moist air via low-level jet (LLJ) out of the vast Amazon basinÌýand consequently, aÌýprospect ofÌýseeing aÌýmore organised structure of any frontal/trough passage, offering a risk of heavy downpours in places. The fairly low pre-frontal pressure gradient is significant, however. GFS (and some other models) still prefer a somewhat discontinuous feature from Atlantic into the continental interior, making the local PPN accumulation even harder to reliably call, whereas others (including UKMO) suggest a more coherent band of heavy rain, with slower rate of clearance during Saturday. Of relevance here is how regional Brazilian modellling for Sao Paulo (i.e., ETA20, ETA40) tend to concur with UKMO and ECMWF in threateningÌýhigher amounts of PPN, in contrast to some other global outputÌý(e.g., GFS) - this all courtesy of differences in howÌýthe upper level flowÌýis synthesised.ÌýÌý

ÌýEither way, the potential for heavy rain (plus low but no less emphasisedÌýthreat of severe t-storms)Ìýat some stage early Saturday comes with high forecast confidence: a wet, damp or drying track being the likely outcome for FP3 and possibly quali too. Any cloud breaks PM will add insolation effects and bolster a threat of further localised showers, hence the % chance of some rain never really diminishes to any significant degree during the periods of track action. It could prove knife-edge, not least given the latent drama surrounding this race and it's importance for the championship tables...

Sunday still looks a far better affair, with a goood deal of dry and largely sunny weather prevailing and a broadly lowÌýÌýbut not insignificantÌýthreat of afternoon light showers (as exemplified by both UKMO and ECMWF output)). Further model runs are needed to lock-down the likely chance of these, but at least broad inter-model consensus offers a high chance for a dry or largely dry race.

Weds. Update, based on 00UTC model output:

A largely similar story to my original entryÌýand some better continuity - of sorts - is now emerging. The broadscale evolution has good inter-model agreement, with the upper trough dominant from overnight Friday and then throughout Saturday; a cold front moving north later Friday overnight into Saturday AM. Inevitably, each model handles the precipitation signal somewhat differently, i.e., the areal extent of showers and timing of frontal clearance northwards. In short, no model can be wholly trusted on this level of detail, not least at this rangeÌýinÌýa convective regime. Importantly, the new runs of ECMWF, UKMO and CMCÌýare a tad more progressive (albeit not by much) in easing the frontalÌýband of showers and t-storms northwards through SaturdayÌýmorning to possibly clear Interlagos by midday (or indeed earlier). GFS still remains the most progressive in this respect, with good continuity; similarly Eta-20 regional modelling now follows it's lead. It also hasÌýthe primary PPN signalÌýrestricted to overnight Fri-Sat, followed by further late afternoon/evening showers on a trough post-front. This sort of shortwave feature (plus any local diurnal shower development) inevitably keeps a threat of some showers still presentÌýinto the afternoon, but there's a fair chance of the wettest conditions having passed ahead of FP3 and quali, perhapsÌýby some margin. Sunday, meanwhile, continues to look dry or very largely so, with good inter-model support for this outcomeÌý(as earlier noted) and a dry race expected.

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(originalÌýentry follows)Ìý

So, are we ready for the drama? Interlagos seems to have an annual ability to offer heart-stopping moments - not least in the last three championship-deciding events there.Ìý And if there's a last weather-borne banana skin to await the 2010 title contenders, it will surely appear hereÌýat São Paulo,Ìýrather than at Abu Dhabi...

The potential is certainly there for Brazil's lively spring weather to playÌýsome part in the destiny of this year's title hopes. The key focus is on Saturday, when the possibility of (at least periodically) wet weather will be of most concern for teams and drivers as the early forecast evolves.

The key models employed for this forecast blog are the global models from UK MetOffice (UKMO); ECMWF, US-GFS, Canada's CMC, Japan's JMA, US-NOGAPS and the regional (20km resolution) output from Brazil's INPE/CPTEC.

During the later stages of Friday, an upper trough elongates northwards out of Argentina, with a surface lowÌýspinning-up offshore to the east. Up aloft, the polar jetstream windsÌýloop northwards to define the upper trough axis, offering potential for inclement weather to gather below. The subtropical jet lies just to the north. Showers and thunderstorms are likely to become increasingly more prevalent beneath this vigorous cocktail ofÌýupper forcing, aided locallyÌýby daytime maxima climbing into the mid/upper 20's.

As the weekend advances, the trough expandsÌýup through southern Brazil into Saturday, before relaxing eastwards into Sunday.ÌýMeanwhile, just to the north, theÌýSACZÌý( - a critical component ofÌýBrazilian weather patterns) sits almost quasi-stationary, delivering frequent thunderstorms stretchingÌýover a vast band from the depths of the Amazon basin down towards SE Brazil. The intensity and positioning of the SACZ - shown to be allied to phasing of the (MJO) andÌý (ENSO) events - is the most important driving force behind any extreme spring-summer rainfall events occuringÌýin São Paulo.

Friday looks largely or entirely dry during the two free practice sessions, with sunny spells and turning somewhat breezier into the afternoon. Model continuity for this outcome has been essentially consistent throughout the medium range output from all operational centres. This dry spell precedes the increasing influence of the upper trough and surface cold frontÌýeasing northwards into Brazil later into the evening and night. Surface pressure steadily falls, the cloud cover thicken-upÌýand heralds a streadily increasingÌý% chance of showers later.

At the time of writing, the most pessemistic forecasts for SaturdayÌýcome from the very same models that performed so admirably when foretellingÌýof inclement conditions a fortnight ago in Korea.

ECMWF, UKMO and CMC are largely in agreement (albeit with nuances of timing differences)Ìýin developing a sharper, thunderyÌýfeature along the frontal bandÌýduring early hours and keeping it largely in-situ throughÌýthe day (almost a component of the SACZ), with resultant potential for more prolonged outbreaks of rain at times and generally a lot more cloud across Interlagos. Temperatures will fall back towards theÌýmid 20's. It's noteworthy how the wetter signal for Saturday is also reflected in the Brazilian Eta-20 regional modelling, especially into the afternoonÌý(see latest São Paulo meteogram, link at the top of this blog),Ìýthus offeringÌýpretty good support for the EC, UKMO and CMC solutions.Ìý ECMWF's EPSgram for São Paulo (NB: not available publically on the web)Ìýis a fairly decent replica of the Eta-20 solution, too.

GFS, on the other hand, is more progressive and takes the frontal band northwards quicker (it also wants the SACZ further north, too), with the shower threat easing away before qualifying and skies tending to clear somewhat. Of course, taking the precipitation distribution from GFS (or any other model) at face value in such a convective set-upÌýis a mug's game, because they're only ever providing a broad brush signal for the regional & local potential, not the actuality!

So, adopting the necessary 'wide-angle' viewÌýacross all the various model output, very clearly the likelihood of rain at some stage during Saturday is very high, butÌýthe comparative speed of this clearingÌýthrough Saturday isÌýthe critical element toÌýdictate wet weather potential into the windows of FP3 and qualifying. Some mainstream forecast providers (e.g., MeteoGroup)Ìýoffer up to 99% precipitationÌýchance for Saturday, which seems quite reasonable taken across the 24-hr window. However, for periods of track action it may prove closer to 20-40% in terms of site-specific probability. The outcome? Anyone's guess!! Better inter-model continuity is awaited...

But either way, the models do then tend to converge in offering Sunday's dry or largely dry prospects. The upper trough relaxes to a more zonal pattern aloft; the surface front/trough is by then further north and swallowed-up into the broader SACZ convective band over SE Brazil. Later into the afternoon, clouds will tend to bubble-up more readily across Interlagos, aided by insolation and a reasonable amount of CAPE, but any resultant showers are likely to be lighter affairs and more broadly scattered. So, there's more than a fighting chance for this year's race to remain a dry affair.

I'll of course keep you updated on how the model runs this week shape-up prospects for what promises to be a truly thrilling event - wet or dry!

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