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35 What is Prof. Gray's seasonal hurricane forecast for this year and what are the predictive factors?




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This article is from the Storms FAQ, by Chris Landsea landsea@aoml.noaa.gov with numerous contributions by others.

35 What is Prof. Gray's seasonal hurricane forecast for this year and what are the predictive factors?

Prof. Bill Gray at Colorado State University in Fort Collins, Colorado
(USA) has issued seasonal hurricane forecasts for the Atlantic basin since
1984. Details of his forecasting technique can be found in Gray (1984a,b)
and Gray et al. (1992, 1993, 1994). Landsea et al. (1994) also provides
verifications of the first 10 years of forecasting. A quick summary of the
components follows:

* El Nino/Southern Oscillation (ENSO) - During El Nino events (ENSO warm
phase), tropospheric vertical shear is increased inhibiting tropical
cyclone genesis and intensification. La Nina events (ENSO cold phase)
enhances activity.

* African West Sahel rainfall - In years of West Sahel drought conditions,
the Atlantic hurricane activity is much reduced - especially the intense
hurricane activity (Landsea and Gray 1992). Wet West Sahel years mean a
higher chance of low-latitude "Cape Verde" type hurricanes. This is also
due to higher tropospheric vertical shear in the drought years, though there
may also be changes in the structure of African easterly waves as well to
make them less likely to go through tropical cyclogenesis.

* Stratospheric quasi-biennial oscillation (QBO) - During the 12 to 15
months when the equatorial stratosphere has the winds blowing from the
east (east phase QBO), Atlantic basin tropical cyclone activity is reduced.
The east phase is followed by 13 to 16 months of westerly winds in the
equatorial stratosphere where the Atlantic activity is increased. It is
believed (but not demonstrated) that the reduced activity in east years
is due to increased lower stratospheric to upper tropospheric vertical
shear which may disrupt the tropical cyclone structure.

* Caribbean sea level pressure anomalies (SLPA) - During seasons of lower
than average surface pressure around the Caribbean Sea, the Atlantic
hurricane activity is enhanced. When it is higher than average, the
tropical cyclone activity is diminished. Higher pressure indicates
either a weaker Inter-tropical Convergence Zone (ITCZ) or a more
equatorward position of the ITCZ or both.

* Caribbean 200 mb zonal wind anomalies (ZWA) - The 200 mb winds around
the Caribbean are often reflective of the ENSO or West Sahelian rainfall
conditions (i.e. westerly ZWA corresponds to El Ninos and West Sahel
drought conditions). However, the winds also provide some independent
measure of the tropospheric vertical shear, especially in years of neutral
ENSO and West Sahel rainfall.

Dr. Gray and his forecast team issues seasonal forecasts in late
November, early June, and early August of each year with a verification of
the forecasts given in late November. To obtain these forecasts, surf
to: http://tropical.atmos.colostate.edu/forecasts/index.html

Also available (via unix machines) a finger command to get a table with
the latest forecast info and what the observations have been of the season
so far. Available via: finger forecast@typhoon.atmos.colostate.edu

 

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