The Good, the Bad, and the Ugly:

Ron McTaggart-Cowan, John Gyakum, and Lance Bosart

Numerical Prediction of Hurricane Juan

Outline

• Case Background

• Operational

Forecasts and NWP

• Improved NWP

Guidance

• Summary/Discussion

Case Background• Depression crosses Atlantic 14-20

September with weak convection

• Isabel’s outflow suppresses devel-opment from 21-24 September

• During 22-24 September, genesis occurred along an extended front to the northeast of the Lesser Antilles

Case Background• Tropical transition

completed by 25th

• Accelerates and develops strong asymmetry by 0000 UTC 29th

• Landfall near Halifax at 0300 UTC 29th as a

Category 2 Hurricane

NHC Best Track

Case Background

0000 UTC 28 September

• Still near maximum intensity (obtained at 1800 UTC 27th)

• Minimum SLP 970 hPa with est. 90 kt winds

Case Background

0600 UTC 28 September

• Moving W-N-westward and weakening slightly

• Minimum SLP 972 hPa with est. 85 kt winds

Case Background

1200 UTC 28 September

• Moving W-N-westward and developing strongly asymmetric structures

• Minimum SLP 970 hPa with est. 90 kt winds

Case Background

1800 UTC 28 September

• Moving northward and accelerating rapidly

• Minimum SLP 972 hPa with est. 80 kt winds

Case Background

0000 UTC 29 September

• Accelerating rapidly towards Halifax (landfall 3h later)

• Minimum SLP 974 hPa with est. 70-85 kt winds

Forecasting and NWP• Operational forecasting for Juan was

provided primarily by NHC and CHC

• Track forecast errors were below the 5 yr mean for all lead times (NHC)

• Intensity forecast errors were larger than average, but still reasonable

Forecasting and NWP• Forecasters at

both the CHC and the NHC issued excellent forecasts

• Lead times of over 24h were given for winds and rain associated with Juan

Forecasting and NWP• Forecasters noted

a bifurcation in model guidance from the 0000 UTC 28th run

• Common features at mid-levels are used to produce a good forecast

Forecasting and NWP• Over the past 5 years, the major

American, British, and European models have all been upgraded to deal with tropical features …

So what caused this potentially dangerous split in the guidance?

Forecasting and NWPModel Centre Quality* Availablity

GFS NCEP Low/Med Yes

Eta coord NCEP Low Yes

GFDL NCEP High No

UK Met UKMO High Yes

NOGAPS US Navy Med/High No

GEM CMC Low/Med Yes

MC2 McGill Low Yes* Quality is defined subjectively based on 24h

surface forecasts from the 0000 UTC 28th run

Forecasting and NWPInitialization GFS Sat Image

GFS SLP and surface winds (kts) at 0000 UTC 28th

Composite vapour/IR at 0000 UTC 28th

satellite

Forecasting and NWP24h Forecast GFS Sat Image

satellite

Quickscat winds at 2300 UTC 28th

GFS SLP and surface winds (kts) at 0000 UTC 29th

Forecasting and NWPInitialization Eta Sat Image

Eta SLP and surface winds (kts) at 0000 UTC 28th

Composite vapour/IR at 0000 UTC 28th

satellite

Forecasting and NWP24h Forecast Eta Sat Image

satellite

Quickscat winds at 2300 UTC 28th

Eta SLP and surface winds (kts) at 0000 UTC 29th

Forecasting and NWPInitialization GFDL Sat Image

Not Available

Forecasting and NWP24h Forecast GFDL Sat Image

Not Available

Forecasting and NWPInitialization UK Sat Image

UKMet SLP and surface winds (kts) at 0000 UTC 28th

Composite vapour/IR at 0000 UTC 28th

satellite

Forecasting and NWP24h Forecast UK Sat Image

satellite

Quickscat winds at 2300 UTC 28th

UKMet SLP and surface winds (kts) at 0000 UTC 29th

Forecasting and NWPInitialization Navy Sat Image

Not Available

Forecasting and NWP24h Forecast Navy Sat Image

Not Available

Forecasting and NWPInitialization GEM Sat Image

satellite

GEM SLP and 1000-500 hPa thickness 0000 UTC 28th

Composite vapour/IR at 0000 UTC 28th

Forecasting and NWP24h Forecast GEM Sat Image

satellite

GEM SLP and 1000-500 hPa thickness 0000 UTC 29th

Quickscat winds at 2300 UTC 28th

Forecasting and NWPInitialization MC2 Sat Image

MC2 SLP and 1000-500 hPa thickness 0000 UTC 28th

Composite vapour/IR at 0000 UTC 28th

satellite

Forecasting and NWP24h Forecast MC2 Sat Image

satellite

MC2 SLP and 1000-500 hPa thickness 0000 UTC 29th

Quickscat winds at 2300 UTC 28th

Forecasting and NWP

So what happened? …

• Forecast models initialized without a strong enough vortex failed to develop or maintain a closed circulation and favoured the western trough as the weather-maker

Forecasting and NWP• Weak initial

vortex without troposphere-deep PV column

• Weak poten-tial stability ahead of trough

Forecasting and NWP• PV tower tilts

downshear as development occurs over the Bay of Fundy

• Surface circu-lation tracks westward

Forecasting and NWP• Bogusing methodologies are very different

at the various operational centres

Centre Model Vortex Initialization

NCEP GFDL Grown balanced vortex

UKMO UK Met Synthetic observations

Navy NOGAPS Developed vortex insertion

NCEP GFS, Eta Vortex relocation

CMC GEM None

McGill MC2 None

complexity

most

least

Forecasting and NWP• Who produced “Good” guidance for

Hurricane Juan?

Centre Model Vortex Initialization

NCEP GFDL Grown balanced vortex

UKMO UK Met Synthetic observations

Navy NOGAPS Developed vortex insertion

NCEP GFS, Eta Vortex relocation

CMC GEM None

McGill MC2 None

complexity

most

least

Forecasting and NWPGFDL vortex initialization

• Vortex in GFS initial fields is removed using filters and optimal interpolation

• New vortex is grown in model

• Full treatment of SST evolution and interaction

• Strong, well-balanced vortex in initial fields

Forecasting and NWPUKMO vortex initialization

• Manual intervention when satellite or in-situ data is significantly different from first guess field

• Synthetic wind observa-tions are added

• Analysis cycle balances mass field to produce a realistic vortex

Forecasting and NWPUS Navy vortex initialization

• Vortex generation based on balanced hurricane vortex of Kurihara

• Fully-developed vortex is inserted in HPC-analyzed position

Forecasting and NWPNCEP vortex initialization

• Vortex removal is made using multiple filter passes

• Hurricane compon-ent moved to HPC-analyzed position

• Eta adds bogus winds (for now)

Juan?

Improved Guidance• Dense observations to the N/W/S of

the storm over the continent mean that upstream analysis should be good

• Quality of forecast depends on the strength/coherence of the initial vortex

Can we improve a bad forecast with a better initial vortex alone?

Improved GuidanceMcGill vortex initialization

• Insert a fully-developed Kurihara-type vortex at 0000 UTC 28th

• Rerun 48h forecast without any other modifications to the initial or boundary conditions (supplied by GEM)

• Both 12km and 3km domains are rerun

Improved Guidance

• Modified initial conditions at 0000 UTC 28th September

satellite

satellite

Improved Guidance

• Modified forecast SLP and winds fields valid 0000 UTC 29th September

satellite

satellite

Improved Guidance• Initial PV tower

is well-defined throughout the troposphere

• Convective instability over the continent is not changed

Improved Guidance• Vortex tube

remains upright and wraps potential instability into the circulation

• Much improv-ed track and structure

Improved Guidance• How much could

have been done in real time at high resolution?

Summary• The operational forecasts for Hurricane

Juan were of very high quality

• Operational models without a realistically- bogused vortex gave poor sfc guidance

• Synthesis of a strong (Category 2) vortex in the initial conditions of one of the “bust” models results in a much-improved forecast

Discussion• The NWP for Hurricane Juan reduces to

an initial value problem involving the tropical vortex itself:

A realistically-strong, balanced synthetic vortex was ALL that

was required for accurate numerical prediction of Juan in

real time