Forecasting Extreme Weather

Forecasting Extreme WeatherForecasting Extreme WeatherTim Hewson Met Office, Exeter

© Crown copyright Met Office

Royal Met Soc Meeting – “Understanding the Weather of 2011” – Birmingham, 3 Feb 2011

Layouty

O f Off f• Overview of Met Office forecasting responsibilities regarding severe weather

• The National Severe Weather Warning Service

• Tools for forecasting

• Case Studies – focusing on shorter range forecasts

A t t• A storm at sea

• A multi-hazard storm over land

• Thunderstorms over Wimbledon

• Questions and answers


Responsibilities regarding severeResponsibilities regarding severe weather


Strategygy

• The Met Office is broadening its horizons, encompassing hazards of different types, and impacts, and working together with different partners:partners:

• e.g. flooding issues are now addressed in partnership with the Environment Agency

F th hi f f t th i it i t f ti f• For the chief forecaster the priority remains correct forecasting of adverse weather

• Over land the focal point is the (new) “National Severe Weather Warning Service”Over land the focal point is the (new) National Severe Weather Warning Service (NSWWS)

• Impacts-based from April 2011; the old system was threshold-based

I l d id bl li i ith t d th ili it t• Includes considerable liaison with government and the resilience community, to help save lives and preserve infrastructure

• For sea areas Gale Warnings are provided, via with the shipping forecaster (a marine forecasting requirement originally lead to foundation of the Met Office – via Admiral


forecasting requirement originally lead to foundation of the Met Office – via Admiral Fitzroy)

• NSWWS

S h t• Snapshot from Met Office website


• Snapshots from Metfrom Met Office website

“RISK” = (likelihood) x (impact)


RISK = (likelihood) x (impact)

The Weather Impact Matrixp

• Important to understand!• Important to understand!

• Well in advance of an event confidence is commonly low, but one has ideas of potential impactspotential impacts

• Often warning areas in advance will be larger, but less extreme (eg yellow)

• As an event approaches and confidence increases, warning areas will become smaller but the level may become more extreme (eg amber or red)extreme (eg amber or red)

• In some instances warnings will be dropped completely, as a potential event becomes less likelyless likely

• Ideally we should not jump from one impact level to another as an event approaches, although the complexities of different


although the complexities of different situations means that this can happen

fTools for Forecasting


Computer Forecastsp

• Numerical (computer) models

• Comprise a mathematical representation of the physics of the atmosphere-• Comprise a mathematical representation of the physics of the atmosphere-land-ocean system

• Simultaneous equations are solved on a grid to derive a forecast

• Large suite of different models run at the Met Office

• Deterministic (single) model runs( g )

• At resolutions (=gridpoint spacing) of 1.5, 4, 12, 24km (2 to 6 days)

• Ensemble (multiple) model runs, using slightly different starting conditions( p ) , g g y g

• At lower resolutions, eg 18, 60km (3 to 15 days)

• Data from other centres (eg ECMWF) is also used extensively


Data from other centres (eg ECMWF) is also used extensively

Forecasting extreme weather at longer ranges (i.e. days ahead)

• Is this possible ?

• Generally no (exceptions might be protracted heatwaves in summer, protracted cold in winter)

• All we can usually do is indicate an increased probability of an extreme event –for a given point this might be, for example, a 10% risk at a lead of 5 days, instead of a climatological risk of say 0.1%g y

• So why is this?

• Extreme events commonly cover small areas, and depend on the positioning of y , p p gweather systems, such as fronts and depressions

• We cannot predict the locations of weather systems with sufficient accuracy. This is because of ‘chaos’ – tiny changes now to the state of the atmosphereThis is because of chaos – tiny changes now, to the state of the atmosphere, can grow and have a huge impact in a few days time. Ensembles represent this uncertainty..

C t di i thi t lk th f f h t f ti (< 1 d )


• Case studies in this talk therefore focus on short range forecasting (<~1 day)

Dalmatian Chart (Spots show cyclone centres, colour denotes intensity, brighter = more extreme)

Forecast sequence, 52 ensemble members, from day 0 to day 9

52 forecast runs (EPS + Control + Deterministic)

Additional Information used by the forecaster

• Imagery – from satellites and radar

• Observations from the surface (SYNOPs, ships, buoys) and from upper levelsObservations from the surface (SYNOPs, ships, buoys) and from upper levels (aircraft data, etc.)

• Radiosonde (balloon) ascents remain very useful, giving a comprehensive picture of the whole depth of the atmosphere at a given point in temperaturepicture of the whole depth of the atmosphere, at a given point, in temperature, humidity and wind

• Commonly these data are compared with the computer forecasts, to see how f f fthose forecasts are performing, and thereby reject or accept aspects of those

forecasts

• Knowledge of the weaknesses/biases of different computer models is also g pcritical

• Uncertainties increase markedly when severe weather is possible – hence the chief forecaster continues to play a very important role as we shall see


chief forecaster continues to play a very important role….as we shall see….

C SCase Studies


1. A storm at sea


0000Z 17th June 2011A h bl t l t i t d th th Atl ti ith• A changeable westerly type existed across the north Atlantic, with signs that a cyclone could develop on a front…


• Satellite image showed a developing cyclone that was not present in the right location in short range (3h) Met Office deterministic computer forecastsp

• Surface buoy measurements didn’t help much on this occasion

Lightning strikes, implying a very active system


• Ensemble forecasts showed a huge range of outcomes at short range from a weak cyclone with gusts to 40kts (force 7) to anrange, from a weak cyclone with gusts to 40kts (force 7) to an extreme cyclone with gusts of over 70kts (force 11)


• Imagery matched up better with the psuedo-imagery from the extreme ensemble member and also with the ECMWFextreme ensemble member, and also with the ECMWF deterministic run, which showed a large area of gusts > 60kts

Gusts >60kts 00-06Z


ECMWF 15h forecast for 03Z ECMWF Max gusts 00-06Z

The OutcomeThe Outcome

• Force 10 gale warnings were issued for sea areas Fitzroy and Biscay

• This is very unusual for June

S f• So far as we can tell this was correct..

• Shows the benefits of imagery, multiple models, g y pensembles, and forecaster input, in a severe weather case

• There were also significant (rainfall) implications for the UK due to the development of this storm…


2. A Multi-hazard Storm over Land


0000Z 2nd Jan 2012• A depression was just beginning to appear in the western north• A depression was just beginning to appear in the western north

Atlantic

A lA new low was developing here


This low was disappearing

0000Z 3nd Jan 2012• 24 hours later• 24 hours later….

The new low deepened rapidly


p p y

Early hours of 3rd Jan…y

• A rapidly developing depression is heading for• A rapidly developing depression is heading for Scotland

• How strong could the winds get?• How strong could the winds get?

• Is the amber warning that is in force for parts of Scotland sufficient?Scotland sufficient?

• And does it cover the right areas?

• Should a red warning be issued?

• Additional warnings for snow and heavy rain, andAdditional warnings for snow and heavy rain, and strong winds across the south of the UK had to be monitored too!


Windstorm Conceptual Model• Damage Swathes attributable to passage of an intense,

developing North Atlantic cyclone.

COLD CONVEYOR (CCB)-Very Strong Winds (gusts ~70kts)Very Strong Winds (gusts 70kts)-In cold airmass

STING JET (SJ)-Exceptionally Strong Winds (gusts ~80kts)- Just in cold airmass

WARM CONVEYOR (WCB)-Strong Winds (gusts ~ 60kts)


g (g )- Warm sector / on cold front

Interpretation of Windstorm pConceptual Model

• Overall picture should be considered as ‘malleable’

Oft l 1 2 f th 3 d th ill• Often only 1 or 2 of the 3 damage swathes will occur

• Relative sizes, shapes and magnitudes of swathes vary by case

• So for the current case how relevant is this conceptual model ?conceptual model ?


Hi-resolution models can now capture the sting jet phenomenacapture the sting jet phenomena..

Thi tit t j03Z – 08Z 3/1/2011

• This constitutes major progress, since October 1987!

• But is the model’s overall l i ?evolution correct?

© Crown copyright Met Office 4km run from 21Z 2nd


Gusts of 80kts+

Decision processp

• Concern regarding the possibility of the sting jetConcern regarding the possibility of the sting jet phenomena affecting Scotland had been expressed in forecasts from several days previously

• Conceptual models, rapid deepening of the low, hallmark imagery signatures, and hi-resolution model output all suggested there could be a sting jetsuggested there could be a sting jet..

• But this is a very rare phenomena, and the area affected would likely be very narrow, with only a low risk that thewould likely be very narrow, with only a low risk that the populated Central Lowlands would be hit

• The model forecast suggested areas further north wereThe model forecast suggested areas further north were more vulnerable

• Red warnings require high confidence


g q g(note that impact level stays the same)

The Outcome• A red warning was issued for the Central Lowlands of

Scotland at 0815Z, triggered in part by observations from Islay showing extreme gustsIslay showing extreme gusts

EdinburghPrevious max gusts since site

d i 1999 Islay Edinburghopened in 1999


SFrom Sting Jet to Cold Conveyor Belt…

Note fingers ofNote fingers of cold top cloud disappearing as they move forward from the

l d h dcloud head

Note also breaks in low level cloud

Both signify evaporation, which can fuel the sting jet

East of Scotland these features are absent – this is the cold conveyor belt phase


belt phase

Resumé• This was an extreme windstorm in a heavily populated region – perhaps a 1 in 20 year

event for affected sites

• Disruption was experienced right across the UK, even with lesser winds in the south. p p g ,Snow and heavy rain were additional hazards

• These hazards had been clearly flagged within the forecasting process in the preceding daysy

• Management of warnings and forecasts in this multi-hazard environment presented quite a challenge!

Whil t i i bl t it ‘ f t’ h b tt l d• Whilst warning issue was arguably not quite ‘perfect’, we are now much better placed than hitherto for warning of these rare events - a tribute to model developments and forecaster expertise

The next big step at the Met Office will be the introduction of a 2 2km ensemble this


• The next big step at the Met Office will be the introduction of a 2.2km ensemble – this should enable the sting jet region to be located more precisely in real time

3. Summer ThunderstormsProbabilistic forecasts for Wimbledon-Probabilistic forecasts for Wimbledon


00Z Tue 28th June 2011• A potentially thundery airstream lay over Wimbledon ahead of a• A potentially thundery airstream lay over Wimbledon, ahead of a

cold front advancing from the west


Setting the gScene

• This was the Tuesday in the second week of Wimbledon; delays the previous week had left a backlog of matchesg

• The general forecasts for Monday, whilst expressing uncertainty, had warned of heavy showers and thunderstorms. These had not materialised.

• Forecasts from different models/centres varied massively in their handling of the weather for Tuesday – the Met Office global model 6h forecast for example showed widespread thunderstorms at 06Z on Tuesday over E l d h thEngland, when there were none.

• Wimbledon staff arrived an hour or so after the forecaster, looked at the radar, saw nothing and thought it would again be fine and dry.g g g y

• So there were many issues to deal with – scepticism of the forecast of rain, the need to get over the match backlog, huge variations in model guidance..


• This was the ideal scenario in which to use detailed probabilistic forecasts in the short range to good effect..

Forecast Guidance from Exeter(based on 12km model with manual adjustments – cloud, pressure, rainfall – brighter colours = heavier rain)


Unstable=> Thunderstorms

• Forecast ‘tephigram’

ht• Model representation of the atmosphere over SE England at 12Z ei

gh• Unusual situation –

daytime thunderstorms

He

cold

unrelated to sun heating the ground S winds

warm


Temperature cold N winds

Official Forecast

F t i d T 28th J 2011 t 0930• Forecast issued on Tue 28th June 2011, at 0930• Wimbledon Championships Forecast

• Valid for TuesdayValid for Tuesday

• Some sunny intervals are expected this morning, and after a dry start there is distinct possibility that showers will build and affect the Wimbledon area in the late morning to early afternoon period. Whilst the showers, if we catch them, could be heavy, slow moving and perhaps thundery, it is also possible that they will remain confined to areas to the east of Wimbledon. So the probability of one or more disruptions to play, in the period up to 1500, is rated at 60%.

• From about 1500 to 1800 it will probably be dry, if rather cloudy.

• Then in the evening period, from 1800 to the close of play, there is a renewed threat of some rain, which would probably be light, but could be sufficient, again, to interrupt play. So in this time window the likelihood of one or more interruptions to play is rated at 40%.

• As we approach 2100 in the evening, light conditions are likely to deteriorate rather more quickly than they did yesterday, due to cloud cover.

• Maximum temperature a warm and humid 24C, but turning cooler this afternoon


• Confidence in details for today is not high, and regular updates will accordingly be provided during the day.

OutcomeOutcome

• Play stopped from about 12:30 to 15:30, due to thunderstorms The centre court roof was usedthunderstorms. The centre court roof was used.

• Play recommenced from about 15:30 and continued to about 19:00 when drizzle set inabout 19:00, when drizzle set in.

• Light rain and drizzle and ultimately bad light prevented any further play from about 19:00.any further play from about 19:00.

• A successful forecast, with probabilities used to good effect, if not exactly the outcome Wimbledon wanted!e ect, ot e act y t e outco e b edo a ted

• Required understanding of a complex meteorological situation. Watching the clouds moving in opposite


g g ppdirections at different levels was quite striking..

SummarySummary

• Met Office forecasters regularly have to deal with wide ranging types of severe weather

• Computer model forecasts are the backbone of the forecasting effort, but often show wide-ranging solutions in the lead up to severe events, even at shortsolutions in the lead up to severe events, even at short ranges

• The chief forecaster’s role is to reconcile differences,The chief forecaster s role is to reconcile differences, and issue forecasts and warnings accordingly, using probabilistic guidance where necessary

• The new NSWWS system conveniently combines ‘impact level’ and ‘likelihood’ to arrive at the different warning tiers (yellow / amber / red)


warning tiers (yellow / amber / red)

Questions & answers


Forecasting Extreme Weather

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