For roughly the last decade, a special version of multiple linear regression has been used by Dr. William Gray and his team to make forecasts separately for each of several parameters of tropical storm activity in the Atlantic Basin (Gray et al. 1992). The least absolute deviation (LAD), rather than the least squared deviation (where "deviation" represents the error), is used as the criterion to develop the prediction model for these forecasts. LAD regression was discussed briefly in the December 1992 issue and is described more fully in Gray et al. (1993) and references therein. Forecasts for each year's storm season are made at three times: in late November of the preceding calendar year, in early June, and finally in early August at the beginning of the storm season. Forecast parameters include (1) named storms, (2) named storm days, (3) hurricanes, (4) hurricane days, (5) intense hurricanes, (6) intense hurricane days, (7) hurricane destruction potential, (8) net tropical cyclone activity, and (9) maximum potential destruction. The predictors used, and the skill expected, at each of the three forecast times have been revised a number of times as new opportunities for skill increases have emerged. For example, in 1994 prediction of the ENSO condition expected for the forthcoming fall storm season was largely objectified and incorporated into the regression equation (Gray et al. 1994b). In fact, the present forecast includes a cluster of Atlantic Ocean regional predictors that has not been used previously (Landsea et al. 1997). Expected predictive skill is improved with the inclusion of the new predictors when compared with skills using only the previous set of predictors. The following list identifies the six predictor clusters, their expected influence on Atlantic tropical storm activity, and their status regarding the 1996 storm season. More detail on the influences of these predictor groups is found in Gray et al. (1993, 1994a).

(1) The Quasi-Biennial Oscillation (QBO) at 50 and 30 mb in the northern tropics expected at the onset of the hurricane season: Westerly QBO winds enhance storm activity, while easterly winds suppress it (Gray et al. 1992). During fall 1996 the QBO is easterly, which is expected to suppress storm activity this season.

(2) El Nino/Southern Oscillation (ENSO): Warm east-central equatorial Pacific sea surface temperature, (SST), or the warm phase of ENSO, reduces storm activity, while anomalously cool SST enhances it. Going into fall we currently have a near-zero to weak negative SST anomaly in the Nino 3.4 region. This should very slightly enhance hurricane activity.

(3) African rainfall: Intense hurricane activity is enhanced when the Western Sahel receives above normal rainfall in June-July immediately preceding the current hurricane season, and when the Gulf of Guinea region had been wet during the August-November period of the previous year. Conversely, activity is suppressed when precipitation in those two regions/periods is below average (Landsea et al. 1993). This June-July 1996 features weak drought conditions (-0.6 standard deviations) in the western Sahel. Gulf of Guinea rainfall for August through November of 1995 was near to very slightly above average. The net effect of these two indicators is from very slightly suppressing to near neutral.
(4) West Africa west-to-east surface pressure and temperature gradients: Above average west-to-east surface pressure and (often associated) east-to-west surface temperature gradients from February to May are associated with enhanced hurricane activity later that year. For February through May of this year, both the east-to-west temperature and west-to-east pressure gradients were slightly negative (-0.3 standard deviations), indicating a slight suppressing influence on storm activity.

(5) Caribbean basin sea level pressure anomaly (SLPA) and upper tropospheric (12 km) zonal wind anomaly (ZWA): Negative anomalies of either one of these in Jun-Jul weakly imply enhanced storm activity, while positives weakly associate with reduced activity. For Jun-Jul both SLPA was somewhat higher than normal (suppressing storm activity) but ZWA was slightly below average, tending to enhance storm activity. The net indication is for average 1996 storm conditions.

(6) Atlantic Ocean Regional Predictors (ONR, SSTA-MATL, SSTA-TATL): When the previous year's October-November northeast Atlantic (30-40N, 20-30W; near Azores) subtropical ridge (ONR) is weaker than normal, seasonal hurricane activity is enhanced (see Landsea et. al 1997 for physical reasoning). The SLP anomaly in Oct-Nov 1995 was quite low (-1.8 standard deviations), indicating high storm activity for 1996. The Atlantic SST anomalies in two regions (MATL: 30-50N, 10-30W; TATL: 6-22N, 18-82W) in the April to June period is positively correlated with storm activity several months later through the enhancement of deep oceanic convection. The anomalies in these regions were slightly positive, encouraging slightly positive storm activity for 1996. Together, the three predictors indicate a somewhat active 1996 storm season.

The LAD multiple regression predictions, made first in November 1995 and then in early June and early August 1995, are shown in Table 1 for each tropical storm parameter. (The interim April forecasts are not shown here.) The August forecasts are shown both as objective results of the LAD regression equations ("obj") and as a final forecast ("fnl") which includes a qualitative human/intuitive adjustment. The earlier forecasts also include such an adjustment. The August forecasts are the sum of the 1996 storm activity observed before August 1 and the model-predicted amount of activity expected from August 1 through the balance of the 1996 storm season. The mean values over the 1950-95 period are shown. The right-hand column provides a 50% confidence interval for the forecasts--i.e., the 25 and 75 percentile points of the estimated probability distribution of the final August forecast. This distribution is estimated by empirical examination of 1950-95 hindcast errors, and not from a theoretical calculation based on a statistical skill score. Regarding the latter, the first column shows expected skill for the forecasts in terms of percent variance explained. The first coefficient is based on the development (dependent) sample, and as such it is an overestimate of the skill expected for a forecast for a future season or for any year outside of the development sample. The second coefficient, based on exhaustive resampling simulations, is a best estimate of the skill expected on truly independent data (Mielke et al. 1995). The decrease from the dependent to independent sample skill occurs because of the fitting of a fairly large number of predictors to the predictand using only a small to moderate sample (46) of hurricane seasons. The dependent versus independent sample skill differences have encouraged the authors to choose a set of predictors judiciously, resulting in retention of only the most valuable predictors for each tropical storm parameter. The number of elemental predictors now used ranges from 4 to 7 out of the pool of 12 candidates (composing the six clusters described above), depending on the predictand. By economizing on the number of predictors, the penalty for overfitting on the development sample is minimized.

Near to very slightly above average tropical storm activity is predicted for 1996, as it has been since the late November 1995 forecast. The best analog years to 1996 are 1954, 1956, 1970, 1974, 1979, and 1989.

Table 1. Expected forecast skill (agreement coefficient, or percentage variance explained, for early August forecasts) and predictions for the 1996 Atlantic tropical storm season, as of late November 1995, early June and early August 1996. The last two columns show the 1950-90 climatology and the 50% confidence interval for the final August forecast. The number in parentheses after the forecast parameter shows the number of predictors used for that parameter.

Gray, W.M., C.W. Landsea, P.W. Mielke, and K.J. Berry, 1992: Predicting Atlantic seasonal hurricane activity 6-11 months in advance. Wea. Forecasting, 7, 440-455.

Gray, W.M., C.W. Landsea, P. Mielke and K. Berry, 1993: Predicting Atlantic basin seasonal tropical cyclone activity by 1 August. Wea. Forecasting, 8, 73-86.

Gray, W.M., C.W. Landsea, P. Mielke and K. Berry, 1994: Predicting Atlantic basin seasonal tropical cyclone activity by 1 June. Wea. Forecasting, 9, 103-115.

Gray, W.M., J.D. Sheaffer, P.W. Mielke, K.J. Berry and J.A. Knaff, 1994: Predicting ENSO 9-14 months in advance. Proceedings of the 18th Annual Climate Diagnostics Workshop, Boulder, Colorado, November 1-5, 1993, 390-393.

Landsea, C.W., W.M. Gray, P.W. Mielke and K.J. Berry, 1993: Predictability of seasonal Sahelian rainfall by 1 December of the previous year and 1 June of the current year. Preprints, 20th Conference on Hurricane and Tropical Meteorology, AMS, San Antonio, Texas, 473-476.

Landsea, C.W., W.M. Gray, K.J. Berry and P.W. Mielke, Jr., 1997: Revised Atlantic basin seasonal tropical cyclone prediction methods for 1 June and 1 August forecast dates. Wea. Forecasting, 12, in preparation.

Mielke, P.W., K.J. Berry, C.W. Landsea and W.M. Gray, 1995: Artificial skill and validation in meteorological forecasting. Wea. Forecasting, 11, 153-169.