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 September with weak convection Isabel’s outflow suppresses devel- opment from September During September, genesis occurred along an extended front to the northeast of the Lesser Antilles

Case Background Tropical transition completed by 25 th Accelerates and develops strong asymmetry by 0000 UTC 29 th 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 27 th ) 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 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 28 th 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 NWP ModelCentreQuality*Availablity GFSNCEPLow/MedYes Eta coordNCEPLowYes GFDLNCEPHighNo UK MetUKMOHighYes NOGAPSUS NavyMed/HighNo GEMCMCLow/MedYes MC2McGillLowYes * Quality is defined subjectively based on 24h surface forecasts from the 0000 UTC 28 th run

Forecasting and NWP Initialization GFS Sat Image GFS SLP and surface winds (kts) at 0000 UTC 28 th Composite vapour/IR at 0000 UTC 28 th satellite

Forecasting and NWP 24h Forecast GFS Sat Image satellite Quickscat winds at 2300 UTC 28 th GFS SLP and surface winds (kts) at 0000 UTC 29 th

Forecasting and NWP Initialization Eta Sat Image Eta SLP and surface winds (kts) at 0000 UTC 28 th Composite vapour/IR at 0000 UTC 28 th satellite

Forecasting and NWP 24h Forecast Eta Sat Image satellite Quickscat winds at 2300 UTC 28 th Eta SLP and surface winds (kts) at 0000 UTC 29 th

Forecasting and NWP Initialization GFDL Sat Image Not Available

Forecasting and NWP 24h Forecast GFDL Sat Image Not Available

Forecasting and NWP Initialization UK Sat Image UKMet SLP and surface winds (kts) at 0000 UTC 28 th Composite vapour/IR at 0000 UTC 28 th satellite

Forecasting and NWP 24h Forecast UK Sat Image satellite Quickscat winds at 2300 UTC 28 th UKMet SLP and surface winds (kts) at 0000 UTC 29 th

Forecasting and NWP Initialization Navy Sat Image Not Available

Forecasting and NWP 24h Forecast Navy Sat Image Not Available

Forecasting and NWP Initialization GEM Sat Image satellite GEM SLP and hPa thickness 0000 UTC 28 th Composite vapour/IR at 0000 UTC 28 th

Forecasting and NWP 24h Forecast GEM Sat Image satellite GEM SLP and hPa thickness 0000 UTC 29 th Quickscat winds at 2300 UTC 28 th

Forecasting and NWP Initialization MC2 Sat Image MC2 SLP and hPa thickness 0000 UTC 28 th Composite vapour/IR at 0000 UTC 28 th satellite

Forecasting and NWP 24h Forecast MC2 Sat Image satellite MC2 SLP and hPa thickness 0000 UTC 29 th Quickscat winds at 2300 UTC 28 th

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 CentreModelVortex Initialization NCEPGFDLGrown balanced vortex UKMOUK MetSynthetic observations NavyNOGAPSDeveloped vortex insertion NCEPGFS, EtaVortex relocation CMCGEMNone McGillMC2None complexity most least

Forecasting and NWP Who produced “Good” guidance for Hurricane Juan? CentreModelVortex Initialization NCEPGFDLGrown balanced vortex UKMOUK MetSynthetic observations NavyNOGAPSDeveloped vortex insertion NCEPGFS, EtaVortex relocation CMCGEMNone McGillMC2None complexity most least

Forecasting and NWP GFDL 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 NWP UKMO 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 NWP US Navy vortex initialization Vortex generation based on balanced hurricane vortex of Kurihara Fully-developed vortex is inserted in HPC-analyzed position

Forecasting and NWP NCEP 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 Guidance McGill vortex initialization Insert a fully-developed Kurihara-type vortex at 0000 UTC 28 th 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 28 th September satellite

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