TC Forecasting
National Hurricane Center (NHC), Miami, FL
Tropical
M. D. Eastin
Outline
The Challenge
Current Position and Intensity
Track Forecasting
• Climatology and Persistence
• Numerical Models
• NHC Forecast Errors
• Improvements from Targeted Observations
Intensity Forecasting
• SHIPS
• Models
• NHC Forecast Errors
Seasonal Forecasting
Tropical
M. D. Eastin
TC Forecasting: The Challenge
Limitations
• General lack of observations (most TCs are far from land)
• Only the Atlantic Basin has routine aircraft penetrations
• Rest of world relies on satellites and occasional ships and buoys
• Limited observations are often inadequate for forecast needs
• At times, discerning current intensity and location can be difficult
• Some numerical models don’t even “know the TC exists”
and a “bogus” vortex must be inserted
• Most models struggle forecasting evolution of the environment
• Numerical models limited by resolution and processing speed
• Most forecast models don’t resolve the eye or eyewall
• 3-6 day forecasts of the region must be complete in a few hours
• Incomplete understanding of meso- and convective-scale dynamics
Tropical
M. D. Eastin
TC Forecasting: The Challenge
NHC Responsibilities:
Two TC Basins
• Atlantic and East Pacific
Each Tropical Cyclone
• Current position and intensity
• 12, 24, 36, 48, 72, 96, & 120 hr
forecasts of position, intensity,
and size
• Public Forecast
• Marine Forecast
• Forecast Discussion
• Issue watches and warnings
• Coordination with local, state,
federal, and international groups
• New forecasts are every 6 hrs
NHC Forecast Process
Tropical
M. D. Eastin
TC Forecasting: The Challenge
TPC/NHC Responsibilities
Hurricane Watches:
Hurricane conditions (winds > 65 knots) are possible
in the specified area within 48 hours
Hurricane Warnings:
Hurricane conditions are expected in the specified
area within 36 hours
Analogous watches/warnings are initially issued for Tropical Storms (TS)
Only NHC is authorized to issue tropical storm or hurricane watches and
warnings
Coordination with local and state governments often begins well before the
watch is issued….Why?
Tropical
M. D. Eastin
TC Forecasting: Current Position & Intensity
Observational Sources (away from land)
• Geostationary Operational Environmental Satellite (GOES)
• Provides continuous 24/7 coverage
• Visible and IR imagery
• Dvorak Classification
• Polar orbiting satellites (ASCAT/OSCAT, TRMM, SSMI, AMSU, MODIS)
• Provide greater detail with more types of data
• Snapshots of storm 1-4 times per day
• Aircraft reconnaissance (Atlantic only)
• Flight-level observations (multiple radial profiles across storm)
• Dropwindsondes (vertical profiles at select locations – eye and eyewall)
• Stepped Frequency Microwave Radiometer (SFMR)
(estimates of surface winds similar to ASCAT/OSCAT)
• Ships
• Surface observations on the periphery
• Buoys
• Few in number and most located near land
Tropical
M. D. Eastin
TC Forecasting: Current Position & Intensity
Where’s the Center?
Location
GOES IR
• Center of eye in satellite imagery
• Center of circulation in animated
satellite imagery (when no eye)
• Wind minimum from aircraft
• Center of eye on aircraft radar
Intensity
85 GHz PCT
• Reduction of max flight-level wind
to the surface (~90% of FL max)
• Dropwindsondes in the eyewall
• SFMR (routinely available in 2007)
• Dvorak Classification Scheme
• Estimate intensity from cloud patterns in GOES VIS and IR imagery
• Fairly accurate (errors of ~10 knots)
• Very difficult to apply (only a few people are good)
• Used globally (primary intensity estimate outside the Atlantic Basin)
• Only intensity estimate for TCs in remote oceans
Tropical
M. D. Eastin
TC Forecasting: Current Position & Intensity
What’s the Intensity?
Dvorak Classification Scheme
GOES IR
• Takes advantage of two basic aspects:
• TCs of similar intensity tend to have
similar cloud patterns
• TCs develop in a slow predictable manner
• Intensity is determined from an assessment
of the total cloud pattern, which is related to
a “T-number” and then to an intensity
• Cloud patterns:
• Curved Band (T1.0 – T4.5)
• Shear Pattern (T1.5 – T3.5)
• Central Dense Overcast (T2.5 – T5.0)
• Banding Eye Pattern (T4.0 – T4.5)
• Eye Pattern (T4.5 – T8.0)
• The final T-number is arrived at by taking into account
the length and curvature of the banding as well as the
temperature difference between the eye and the
surrounding cold cloud tops of the eyewall
Tropical
Atlantic Basin – Dvorak Table
Max Winds
MSLP
T-number
(knots)
(mb)
===============================
1.0 - 1.5
25
--2.0
30
1009
2.5
35
1005
3.0
45
1000
3.5
55
994
4.0
65
987
4.5
77
979
5.0
90
970
5.5
102
960
6.0
115
948
6.5
127
935
7.0
140
921
7.5
155
906
8.0
170
890
M. D. Eastin
TC Forecasting: Current Position & Intensity
Dvorak Classification Scheme
Tropical
M. D. Eastin
TC Forecasting: Current Position & Intensity
Dvorak Classification Scheme
Tropical
M. D. Eastin
TC Forecasting: Track
Track Forecast Models
• CLIPER (CLImatology and PERsistence)
• Statistical model
• Based on the motion of all previous tracks of similar TCs in the same area
• Predictors are storm location, motion, intensity, and Julian date
• Benchmark for assessing other model’s track forecast skill
• BAM (Beta Advection Model)
• Simple Dynamical model
• Uses vertically averaged steering flow from GFS model + Beta effect term
• TC is initialized as a “point vortex” at a location with a motion vector
• Three averaging layers:
Deep (850-200 mb)
Medium (850-400 mb)
Shallow (850-700 mb)
• LBAR (Limited area BARotropic)
• Simple dynamical model – incorporates the Beta effect
• Initialized using an idealized symmetric vortex and a storm motion vector
• Uses vertically averaged steering flow and heights from the GFS model
• Uses “nested grids” for greater detail of storm circulation
Tropical
M. D. Eastin
TC Forecasting: Track
Track Forecast Models
• ECMWF (European Center for Medium-range Weather Forecasting)
• Hydrostatic, global spectral model
• Incorporates a no “bogus vortex”
• Includes the “4-D assimilation” of environmental observations
• GFS (Global Forecasts System)
• Hydrostatic, global spectral model (NCEP’s model)
• Incorporates “vortex-relocation” based on initial model runs
• Includes the “3-D assimilation” of environmental observations
• UKMET (U.K. Meteorological Office)
• Non-hydrostatic global spectral model
• Incorporates a more complex “bogus vortex” based on synthetic data
• Includes the “4-D assimilation” of environmental observations
Tropical
M. D. Eastin
TC Forecasting: Track
Track Forecast Models
• HWRF (Hurricane Weather Research and Forecasting model)
• Non-hydrostatic, limited-area, finite-difference model
• Employs three “nested grids” (the inner two grid follow the TC)
• Highest resolution is 3 km grid spacing
• Incorporates a complex “bogus vortex” based on observations
• Includes the “3-D assimilation” of environmental observations
• Coupled to the ocean
• GFDL (Geophysical Fluid Dynamics Laboratory)
• Non-hydrostatic, limited-area, finite-difference model
• Employs three “nested grids” (the inner two grids follow the TC)
• Highest resolution is 5 km grid spacing
• Incorporates a complex “bogus vortex” based on observations
• Includes the “3-D assimilation” of environmental observations
• Coupled to the ocean
• Consensus “Models” (different models at the same forecast time)
• TCOA (mean of GFDL, HWRF, UKMET, and GFS)
• TVCA (mean of GFDL, HWRF, UKMET, GFS, and ECMWF)
Tropical
M. D. Eastin
TC Forecasting: Track
Mean Absolute Error – 2012 – Atlantic
Tropical
Forecast Skill – 2012 – Atlantic
M. D. Eastin
TC Forecasting: Track
Tropical
M. D. Eastin
TC Forecasting: Track
Tropical
M. D. Eastin
TC Forecasting: Track
Improving Track Forecasts with Targeted Observations:
• Models initialized from a dense network of observations make better forecasts
• Basic Idea:
Identify regions of large uncertainty in the model forecasts of
steering flow, and go drop a network of sondes in the area
NOAA G-IV Aircraft
Variance of the Deep Layer Mean Wind
Aircraft
flight path
and drop
locations
A deployed
GPS dropsonde
Tropical
Areas of Large
Uncertainty
Forecast track of
TS Bonnie 082198
M. D. Eastin
TC Forecasting: Track
Improving Track Forecasts with Targeted Observations:
• Goal: Extra observations over data sparse regions will reduce model forecast
errors, which will lead to a reduction in official NHC forecast errors
GFDL model
track forecasts
Forecast track
All GPS Sondes
Observed track
Forecast Track
No GPS sondes
Tropical
M. D. Eastin
TC Forecasting: Track
Recent improvements
largely due to impact of
targeted observations
and better model physics
Tropical
M. D. Eastin
TC Forecasting: Intensity
Intensity Forecast Models
• SHIFOR (Statistical Hurricane Intensity FORecast)
• Analogous to CLIPER
• Based on intensity changes of all previous similar TCs in the same area
• Predictors: current intensity, recent intensity trend, location, and Julian date
• Benchmark for assessing other model’s intensity forecast skill
• SHIPS (Statistical Hurricane Intensity Prediction Scheme)
• Statistical / dynamical model
• Uses a simple multiple regression on a variety of predictors**
• Most skillful intensity forecast model
• GFS model
• HWRF model
• GFDL model
Note: Intensity forecasts are often heavily based on forecaster experience
with (and recent trends in) convective patterns, vertical shear patterns,
moisture patterns, and the impact of deep warm ocean features.
Tropical
M. D. Eastin
TC Forecasting: Intensity
Predictors for the Operational SHIPS Model
Tropical
M. D. Eastin
TC Forecasting: Intensity
Small improvements
largely due to a better
SHIPS model, better bogus
vortex and data
assimilation, and better
numerical model physics
related to convection
Tropical
M. D. Eastin
TC Forecasting: Track and Intensity
Track and Intensity Forecast Models
• Additional information on these models and others, as well as details concerning each
model’s set-up, parameterizations, and run times can be found at:
http://www.nhc.noaa.gov/modelsummary.shtml
• An in-depth presentation from NHC forecasters is also provided on the course website
Tropical
M. D. Eastin
TC Forecasting: Seasonal
• Pioneered by Dr. William Gray at Colorado State University
• Uses winter and springtime “signals” to forecast upcoming
summer/fall Atlantic basin TC activity
• Uses simple linear-regression techniques
• Predictors: ENSO
Feb-Mar
West African rainfall
Previous Aug-Nov
West African east-west ΔT & ΔP
Feb-May
Stratospheric QBO winds
Apr-May
Caribbean SLP anomaly
Apr-May
Caribbean 200mb U anomaly
Apr-May
Tropical
Bill Gray
M. D. Eastin
TC Forecasting: Seasonal
ENSO (El Nino - Southern Oscillation)
• Based on SST anomalies in the eastern equatorial Pacific and the surface
pressure difference between Darwin and Tahiti in February and March
• Warmer (Colder) waters produce more/stronger (less/weaker) convection
over the tropical Pacific
• Increased (Decreased) outflow from the convection increases (decreases)
vertical shear over Atlantic basin
Warm waters (El Nino)
Cold Waters (La Nina)
Tropical
=
=
Less TC Activity
More TC Activity
M. D. Eastin
TC Forecasting: Seasonal
West African Rainfall
• Based on rainfall totals in two regions (Western Sahel and Gulf of Guinea)
during the previous fall (August through November)
• More (Less) rainfall acts to enhanced (suppressed) the following summer
monsoon through enhanced (suppressed) evaporation / evapotranspiration
• A stronger monsoon will produce stronger convection and easterly waves,
increasing the likelihood of TC genesis
More Rainfall =
Less Rainfall =
Tropical
More TC Activity
Less TC Activity
M. D. Eastin
TC Forecasting: Seasonal
East-West Surface Temperature and Pressure Gradients in West Africa
• Based on mean surface T and P differences between the western Sahel
and Gulf of Guinea regions in the spring (February – May)
• A negative (positive) P-gradient and a positive (negative) T-gradient acts to
enhance (suppress) the onshore monsoonal flow that supplies the
easterly-wave convection with warm moist air
• More/Stronger convection leads to increased likelihood of TC genesis
Negative P-gradient and a positive T-gradient
Positive P-gradient and a negative T-gradient
Tropical
=
=
More TC Activity
Less TC Activity
M. D. Eastin
TC Forecasting: Seasonal
Stratospheric QBO (Quasi-Biennial Oscillation) Winds
• Zonal winds at ~50 mb reverse direction every 13 months (the QBO)
[More on the QBO later in the course]
• Based on QBO wind direction over the equatorial Atlantic in April and May
• Westerly (Easterly) QBO winds decrease (increase) the vertical shear near
the tropopause and thus help promote (inhibit) TC genesis
West-phase QBO
East-phase QBO
Tropical
=
=
More TC Activity
Less TC Activity
M. D. Eastin
TC Forecasting: Seasonal
Caribbean Sea-Level Pressure Anomaly
• Based on island station sea-level pressures during April and May
• Lower (Higher) pressures indicate a further poleward (equatorward)
excursion than normal of the ITCZ and/or a stronger (weaker) ITCZ
• A strong-poleward (weak-equatorward) ITCZ brings enhanced (suppressed)
low-level convergence, convection and vorticity to the TC genesis region.
Low Pressure
High Pressure
Tropical
=
=
More TC Activity
Less TC Activity
M. D. Eastin
TC Forecasting: Seasonal
Caribbean 200-mb Zonal Wind Anomaly
• Based on Caribbean island rawindsonde sites during April and May
• Since the easterly low-level trade winds are nearly constant, stronger
(weaker) upper-level easterlies lead to increased (decreased) vertical
shear over the Atlantic’s primary TC genesis region
• The predictor is slightly related to the phase of ENSO
Positive anomalies (weak easterlies)
=
Negative anomalies (strong easterlies) =
Tropical
More TC Activity
Less TC Activity
M. D. Eastin
TC Forecasting: Seasonal
CSU Atlantic Basin Forecast Verification
Number of Hurricanes
Old Method (discussed in class)
Tropical
New Method
(not discussed)
M. D. Eastin
TC Forecasting: Seasonal
Other Seasonal Forecasts:
Atlantic Basin:
CSU (http://typhoon.atmos.colostate.edu/)
NOAA (http://www.cpc.ncep.noaa.gov/products/outlooks/hurricane.shtml)
Tropical Storm Risk (TSR) (http://www.tropicalstormrisk.com)
Cuban Institute of Meteorology (CIM) (http://www.insmet.cu/)
NC State (http://cfdl.meas.ncsu.edu/research/TCoutlook_2011.html)
UKMET (http://www.metoffice.gov.uk/weather/tropicalcyclone/northatlantic.html)
West Pacific:
Univ. HongKong (http://weather.cityu.edu.hk/tc_forecast/2011_forecast_JUN.htm)
TSR (http://www.tropicalstormrisk.com)
Australia:
TSR (http://www.tropicalstormrisk.com)
Each forecast uses slightly different regional and global predictors
Check out what each organization forecast for this year…
Tropical
M. D. Eastin
TC Forecasting: Seasonal
Purpose:
• Benefits organizations operating
over large spatial scales with
diverse assets/responsibilities
(i.e., governments / companies)
• Do NOT provide “point” forecasts
for the individual family
Tropical
M. D. Eastin
Tropical Cyclone Forecasting
Summary
• Limitations
• TPC / NHC Responsibilities
• Observational Sources
• Estimating Storm Location (methods)
• Estimating Storm Intensity (methods)
• Dvorak Classification Scheme (basic method)
• Track Forecast Models (similarities / differences, performance)
• Targeted Observations (goal, impact)
• Intensity Forecast Models (similarities / differences, performance)
• Seasonal Forecasts (predictors, physical linkages)
Tropical
M. D. Eastin
References
Aberson, S. D., 2003: targeted observations to improve operational tropical cyclone track guidance. Mon. Wea. Rev.,
131, 1613-1628.
Aberson, S. D., and B. J. Etherton, 2006: Targeting and data assimilation studies using Hurricane Humberto (2001).
J. Atmos. Sci., 62, 175-186.
Blake, E. S., and W. M. Gray, 2004: Prediction of August Atlantic basin hurricane activity. Wea. Forecasting, 19,
1044-1060
DeMaria, M. and J. Kaplan, 1994: A statistical hurricane intensity prediction Scheme (SHIPS) for the Atlantic basin.,
Wea. Forecasting, 9, 209-220.
DeMaria, M. and J. Kaplan, 1994: An updated statistical hurricane intensity prediction Scheme (SHIPS) for the Atlantic
and eastern north Pacific basins., Wea. Forecasting, 14, 326-337.
Demaria, M., M, Mainelli, L. K. Shay, J. A. Knaff, and J. Kaplan, 2005: Further improvements to the statistical hurricane
intensity prediction scheme (SHIPS), Wea. Forecasting, 20, 531-543.
Gray, W. M., C. W. Landsea, P. W. Mielke, and K. J. Berry, 1992: Predicting Atlantic seasonal hurricane activity 6-11
months in advance. Wea. Forecasting, 7, 440-455.
Gray, W. M., C. W. Landsea, P. W. Mielke, and K. J. Berry, 1994: Predicting Atlantic basin seasonal tropical cyclone
activity by 1 June. Wea. Forecasting, 9, 103-115.
Klotzbach, P. J., and W. M. Gray, 2003: Forecasting September Atlantic basin tropical cyclone activity. Wea. Forecasting,
18, 1109-1128.
Klotzbach, P. J., and W. M. Gray, 2004: Updated 6-11 month prediction of Atlantic basin seasonal hurricane activity.,
Wea. Forecasting, 19, 917-934.
Velden, C. , and Coauthors, 2006: The Dvorak tropical cyclone intensity estimation technique.
Bull. Amer. Met. Soc., 87, 1195-1210.
Tropical
M. D. Eastin