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Pastimes : Hurricane and Severe Weather Tracking -- Ignore unavailable to you. Want to Upgrade?

To: Benny-Rubin who wrote (11396)	2/28/2010 9:29:05 AM
From: carranza2	Respond to of 25997

I wonder if the winter sea temps in the Caribbean have anything to do with the severity of the following hurricane season. I doubt it but have not yet studied the issue. Bill Gray had this to say about Dec. 1 forecasts but it may be outdated [ENSO means El Nino/Southern Oscillation]: 74.125.95.132 The two West African rainfall indices are needed for Atlantic tropical cyclone forecasting because of the intimate tie between concurrent seasonal amounts of intense hurricane activity and seasonal rainfall in the Sahel of West Africa (Landsea and Gray 1992). We have found that the previous year rainfall along the Gulf of Guinea and in the Sahel itself provides a very dependable indication of future Sahel rainfall (and thus Atlantic hurricane activity). The Sahel rainfall correlation to its previous year rainfall is reflected in the strong tendency for anomalies of precipitation to continue from year to year. This persistence is likely due to a combination of global sea surface temperature forcing (Lamb 1978; Folland et al. 1986) and changes in the land surfaces including desertification which may reinforce drought conditions (Nicholson 1988; Xue and Shukla 1993). The positive feedback between the Gulf of Guinea rainfall in August through November to Sahel rainfall/Atlantic hurricanes the following year appears to result from changes in available moisture for the North African monsoon through long-term storage in the soil and biosphere (Gray et al. 1992a). While the previous year Sahel rainfall accounts for only about 5% of the intense hurricane variability, the Gulf of Guinea rainfall anomalies provide a much stronger predictor of up to 40% of the variability forecasted in the intense hurricane activity. Finally, the latest addition to the 1 December forecast methodology is an inclusion of the forecasted state of ENSO 10 months later. The previously observed eastern equatorial Pacific SSTA, sea level pressures measured at Darwin, Australia, 100 mb temperature anomalies at Singapore, and the earlier described QBO winds and Western Sahel rainfall can be used to give a quite skillful extended range forecast of ENSO conditions. The physical link between ENSO and Atlantic hurricanes is in the alteration of the upper tropospheric flow patterns. In general, when a warm episode (i.e. El Niño) is occurring, the atmosphere over the tropical North Atlantic and Caribbean are observed to have much higher tropospheric vertical shear due to increased westerlies at 200 mb. Conversely, when a cold episode (i.e. La Niña) is observed during the hurricane season, there is reduced vertical wind shear due to easterly 200 mb wind anomalies. As stated earlier, large vertical wind shear is detrimental to tropical cyclone formation and intensification. Moderate to strong El Niño events have been observed to reduced the number of hurricanes by 44% from non-El Niño years (Gray 1984a). Overall, the 1 December forecasting should be able to provide insight into about 40% to 50% of the variability (or 45% to 60% in the newest version including ENSO effects) of the tropical cyclone activity. Fig. 3 demonstrates the observed differences in intense hurricanes for the ten hindcasts for the most active TC seasons and the ten hindcasts for the quietest TC seasons. Note the very large differences in observed intense hurricane tracks indicating a very substantial amount of skill present in these hindcasts. We feel that this is a startling result considering that this forecast is issued six months before the start of the ``official" hurricane season and eight months before the active portion of the hurricane season. Here is Gray's current December forecast, though he denigrates its accuracy a bit: tropical.atmos.colostate.edu