65th IHC Booklet/Program (pdf - 4.9MB) - Office of the Federal ...

Intraseasonal to Seasonal Prediction of Tropical Cyclogenesis:
A Statistical-Dynamical Forecast System
Tom Murphree and David Meyer
(murphree@nps.edu)
Department of Meteorology, Naval Postgraduate School
We have developed a statistical-dynamical prediction system for forecasting the
probability of tropical cyclone (TC) formation at intraseasonal to seasonal scales and 2.5°
horizontal resolution across the western North Pacific (WNP), with lead times out to 90 days.
We use five large scale environmental factors (LSEFs) to represent the favorability of the climate
system for tropical Cyclogenesis (low level relative vorticity, sea surface temperature, vertical
wind shear, upper level divergence, and planetary vorticity). We use logistic regression to
develop a statistical model representing the probability of TC formation based on the LSEFs.
Data for the model development was obtained for the LSEFs from the NCEP R2 reanalysis, and
for the corresponding TC activity from the JTWC best track data set. We use the NCEP Climate
Forecast System (CFS) to obtain forecasts of the LSEFs at lead times of several days to several
months, which we then use to force the regression model. The forecasts are ensemble means
based on extensive ensembling of individual forecasts with multiple initial conditions and
multiple lead times.
We have conducted independent hindcasts for 1982-2008 which show the forecast system
has skill and potential value to risk adverse customers (e.g., positive Brier skill scores, skillful
ROC values). For 2009 and 2010, we have generated and verified forecasts out to leads of 90
days. The skill of these forecasts is very positive, as indicated by several metrics. For example,
for 2009, the 30 day lead forecasts had a probability of detection (POD) of 0.81 and a Heidke
skill score (HSS) of 0.38. For 2010, the 90 day lead forecasts had a POD of 0.82 and a HSS of
0.26 (the 2010 skill results are preliminary until the release of the JTWC best track data in 2011).
The 2009 and 2010 forecasts also correctly predicted several intraseasonal and interannual
variations. For example, the 30-90 day lead forecasts for 2010 forecasted a northward and
westward shift in the location of the main formation region from its long term mean position, and
much lower formation probabilities than normal, consistent with the observed formations for
2010 and with the development of La Nina conditions in 2010. These formation anomalies were
forecasted first by the 90 day lead forecasts issued in Mar-May 2010, three (five) months prior to
the first La Nina watch (advisory) issued by NOAA/CPC.
We have also developed a corresponding system for forecasting North Atlantic TC
formations, with positive early results. We are presently upgrading the forecast system to take
advantage of NCEP’s recently released and higher resolution Climate Forecast System
Reanalysis (CFSR) and Climate Forecast System version 2 (CFSv2).
Session 10 – Page 9