A Guide to Tropical Cyclone Guidance Will Lewis 11 APR 2017

Dynamical and Statistic Guidance Products Dynamical guidance originates from the numerical solution of sets of Partial Differential Equations, i.e. Numerical Weather Prediction (NWP) models. NCEP’s GFS is a dynamical guidance product. Statistical guidance uses “Big Data” techniques (e.g. regression, classification) to exploit the relationships between observed atmospheric / oceanic quantities (wind shear, SST, etc.) and, most often, TC intensity. Statistical-Dynamical guidance takes the output of NWP models and seeks to extract further value by correcting biases or forming regressions / classifiers for phenomena the NWP models have a difficult time predicting (e.g. intensification, secondary-eyewall formation). We’ll focus on Dynamical and Statistical-Dynamical Guidance Products.

Deterministic and Probabilistic Guidance Deterministic products rely on single realizations (i.e. “runs”) of NWP models. Good examples are the GFS and NAM products on the AOS webpage. Probabilistic products use multiple realizations (of the same model or of different models) to attempt to define the “second moment” of the PDF (i.e. the variance). GEFS products are a good example of probabilistic guidance. Both Deterministic and Probabilistic products are important in TC forecasting.

TC Genesis Within the last several years, the global models (especially ECMWF and GFS) have become increasingly skillful at TC genesis forecasts. Ensemble runs of the global models are very helpful in identifying genesis potential. Pgenesis = n / N, where n: # members with TC formation = true, N: total number of ensemble members

Deterministic Track and Intensity Guidance Track Guidance comes from both global and regional models: HWRF, COAMPS-TC, etc.10-m track forecast for TC Cook GFS, GEM track forecast for TC Cook

Deterministic Track and Intensity Guidance Intensity Guidance comes almost exclusively from regional models: HWRF, COAMPS-TC, etc.10-m wind forecast for TC Cook

Probabilistic Track and Intensity Guidance Global models provide excellent guidance on the uncertainty of track forecasts: 21-member GEFS track forecast

Probabilistic Track and Intensity Guidance Regional models provide probabilistic guidance for both track and intensity: 16-member UW-NMS track forecast 16-member UW-NMS 10-m wind forecast

TC Structure Guidance Regional models provide detailed (if not terribly reliable) guidance on TC structure: 12-hr forecast of SLP and 10-m wind 12-hr forecast of SLP and Zcomposite

Statistical-Dynamical Guidance These products are almost exclusively produced for the Western Atlantic (AL) and Eastern Pacific (EP) basins (so we don’t have many good examples of it right now). SHIPS and LGEM are used to provide intensity forecast guidance and, until quite recently, often outperformed dynamical guidance. pERC (developed at UW-Madison by Kossin et al.) uses a Bayes’ Classifier method to the predict the probability of an eyewall replacement cycle occurring (a significant and operationally important form of structure change). Many other S-D products exist.

SHIPS text file (SHIPS, LGEM) Intensity Forecasts GFS Predictors

SHIPS text file (pERC) Climatological pERC and Modeled pERC

Online Sources of Guidance Products The Tropical Tidbits site is overwhelmingly the best ”one-stop shop” for TC guidance products. It’s Current Storms page provides a summary of active TCs and areas of suspicion (aka Invests): Individual regional model pages provides greater detail for their forecasts: Genesis forecasts are available from EMC: Tropical Tidbits Current Storms HWRF COAMPS-TC UW-NMS NCEP / EMC Cyclogenesis Tracking Page

Links Tropical Tidbits: http://www.tropicaltidbits.com/storminfo/ HWRF: http://www.emc.ncep.noaa.gov/gc_wmb/vxt/HWRF/index.php COAMPS-TC: https://www.nrlmry.navy.mil/coamps-web/web/tc UW-NMS: http://cup.aos.wisc.edu/will/HFIP/ EMC/NOAA Cyclone Tracks: http://www.emc.ncep.noaa.gov/gmb/tpm/emchurr/tcgen/ SHIPS text in real time: ftp://ftp.nhc.noaa.gov/atcf/stext/