The predictability of
Tropical Storm Alma 2008
Dianna Nelson
University of Wisconsin - Madison
advisor: Michael C. Morgan
14th Cyclone Workshop
24 September 2008
Overview of Alma
• Advisories were issued on Alma beginning on 0300 UTC 29 May 2008
• Several days prior to actual genesis, many global NWP models suggested genesis in
the eastern Pacific or western Caribbean on or around 29 May 2008
Forecasts valid at 0600 UTC 29 May
East Pacific tropical cyclogenesis
Davis et al. 2008 examined analyses for the 2005 and 2006
hurricane season in a study of developing and non-developing
vortices in the eastern Pacific south of Mexico and central
America. Among the findings of this study were:
1) More vigorous vortices occur during periods of enhanced
convection along the ITCZ – within a zonally confined “Hadleytype” circulation with mean near-surface southerlies
(westerlies) south of (along) 10oN.
2) No development occurs in widespread easterly flow
3) Development occurs for westerlies at 10oN.
4) Barotropic instability is an unlikely mechanism for genesis.
5) Davis et al. suggest that horizontal deformation of the timeaveraged flow is a property that influences development
Focus: Predictability of Tropical Storm Alma
• Identification of key (large-scale?) precursors
– Easterly waves
– Upper-tropospheric troughs
• Characterization of background state within which
tropical cyclone formed
– Hadley cell
– Lower tropospheric zonal jet/PV strip
– MJO
Data and Tools
• GFS 1o x 1o global gridded forecast and
analysis data
• NOAA’s Climate Diagnostic Center and
interactive plotting tools
IR imagery from 0300 UTC 23 May 2008 – 0300 UTC 29 May
WV imagery from 0300 UTC 23 May 2008 – 0300 UTC 29 May
Hovmoeller of meridional wind for 1 May thru 15 August 2008
Hovmoeller of meridional wind for 1 May thru 15 August 2008
250 and 925 hPa analyzed winds at 0600 UTC 29 May
Anomalous near surface zonal wind
Anomalous 250 hPa geopotential height
Anomalous near surface meridional wind
Anomalous OLR
What have we found?
• Characterization of background state within which
tropical cyclone formed (e.g., Davis et al. 2008)
 Hadley cell
 Lower tropospheric zonal jet/PV strip
– MJO
• Identification of key (large-scale?) precursors
– Easterly waves
 Upper-tropospheric troughs
Composited forecasts valid at 0600 UTC 29 May
250 and 925 hPa GFS forecast winds at genesis
F24
250 and 925 hPa GFS forecast winds at genesis
F72
250 and 925 hPa GFS forecast winds at genesis
F192
Conclusions
• Findings agreed with Davis et al. study
– Enhanced “Hadley-like” circulation
– Enhanced low level zonal wind/PV strip
• Perhaps a unique feature of Alma’s
development was the contribution from
the upper-tropospheric trough
• Global models effectively predicted the
development of Alma
Future Work
• Investigate what role (if any) was played
by the MJO in this case
• Explore the use of tools like the NOGAPS
adjoint in identifying features to which
Alma’s intensity was most sensitive
• Apply Linear Discriminate Analysis to
several cases of east Pacific developing
and non-developing vortices
Acknowledgements
• This work was funded through a grant
from the Office of Naval Research