1. Gray’s Forecasting Method Hurricane Notes, Part 2 ATS 553

2. Gray’s Forecasting Method Bill Gray’s Forecast Forecasts in December, June and August Forecasts: –Named Storms, Named Storm Days –Hurricanes, Hurricane Days –Intense Hurricanes, Intense Hurricane Days –Hurricane Destruction Potential –Maximum Potential Destruction –Net Tropical Cyclone Activity

3. Gray’s Forecasting Method Techniques Uses a combination of SUBJECTIVE and OBJECTIVE techniques.

4. Gray’s Forecasting Method Factors in Gray’s Forecast

5. Gray’s Forecasting Method 1. Phase of ENSO The Walker Circulation shifts eastward during El Nino. Tropical Atlantic experiences upper-level westerlies during El Nino—SHEAR El Nino means FEW hurricanes. La Nina means comparatively MORE hurricanes.

6. Gray’s Forecasting Method An El Nino Year

7. Gray’s Forecasting Method A La Nina Year

8. Gray’s Forecasting Method 2. The Quasi-biennial Oscillation An oscillation in: –The zonal winds –Along the equator –In the Stratosphere Period is about 23 months.

9. Gray’s Forecasting Method

10. 2. QBO When lower stratospheric or upper tropospheric winds are westerly, that’s SHEAR, which results in fewer hurricanes. However, the QBO is hard to measure near the tropopause. It’s easy to measure higher, like 30 mb…

11. Gray’s Forecasting Method 2. QBO 30mb winds: –From the east –Tropopause winds are from the west –SHEAR, fewer hurricanes –From the west –Tropopause winds are from the east –NO SHEAR, more hurricanes

12. Gray’s Forecasting Method 3. Sahelian Rainfall In drought years in the Sahel, Atlantic hurricane activity is suppressed. Why? –Maybe the same shear that is bad for hurricanes is also bad for AEWs. –Maybe dust from the Sahel is a “hurricane suppressant” (there is evidence of this)

13. Gray’s Forecasting Method 4. Caribbean Sea Level Pressure Anomalies When MSLP in the Caribbean is lower than normal, you get MORE hurricanes.

14. Gray’s Forecasting Method H When the pressure is unusually high in the Caribbean, it usually means that the ITCZ is far to the south, usually in a tradewind trough.

15. Gray’s Forecasting Method H When the pressure is unusually low in the Caribbean, it usually means that the ITCZ is far to the north, usually in a monsoon trough.

16. Gray’s Forecasting Method 5. Caribbean 200mb Zonal Wind Anomaly A very direct way to include shear in the Gray forecast –U’ from the west means FEWER hurricanes –U’ from the east means MORE hurricanes –Can be due to any cause: ENSO, QBO, a ridge, a trough, whatever

17. Gray’s Forecasting Method 6. SSTs in 3 regions of the Atlantic Warmer water means MORE hurricanes. Colder water means FEWER hurricanes. SST can change rapidly in the Atlantic, due to a generally shallow thermocline. Therefore, it is hard to predict how high the SSTs will be far in advance.

18. Gray’s Forecasting Method MANY other variables A total of 15 variables Also includes ANALOG YEARS in the forecast.

19. Gray’s Forecasting Method How Well Does Gray Do?

20. Gray’s Forecasting Method YearAugust ForecastObserved 19841012 19851011 198676 198777 19881112 1989911 19901114 199178 199286 1993108 199477 19951619 19961113

21. Gray’s Forecasting Method How Well Did Gray Do This Year?

22. Gray’s Forecasting Method NOAA Forecast 2004 12-15 tropical storms 6-8 hurricanes 2-4 intense hurricanes

23. Gray’s Forecasting Method Observations 2004: H. ALEX (3) T.S. BONNIE H. CHARLEY (4) H. DANIELLE (2) T.S. EARL H. FRANCES (4) T.S. GASTON T.S. HERMINE H. IVAN (5) H. JEANNE (3) H. KARL (4) H. LISA (1) T.S. MATTHEW T.S. NICOLE T.S. OTTO

24. Gray’s Forecasting Method Observations 2004: H. ALEX (3) T.S. BONNIE H. CHARLEY (4) H. DANIELLE (2) T.S. EARL H. FRANCES (4) T.S. GASTON T.S. HERMINE H. IVAN (5) H. JEANNE (3) H. KARL (4) H. LISA (1) T.S. MATTHEW T.S. NICOLE T.S. OTTO 15 tropical storms!

25. Gray’s Forecasting Method Observations 2004: H. ALEX (3) T.S. BONNIE H. CHARLEY (4) H. DANIELLE (2) T.S. EARL H. FRANCES (4) T.S. GASTON T.S. HERMINE H. IVAN (5) H. JEANNE (3) H. KARL (4) H. LISA (1) T.S. MATTHEW T.S. NICOLE T.S. OTTO 8 hurricanes!

26. Gray’s Forecasting Method Observations 2004: H. ALEX (3) T.S. BONNIE H. CHARLEY (4) H. DANIELLE (2) T.S. EARL H. FRANCES (4) T.S. GASTON T.S. HERMINE H. IVAN (5) H. JEANNE (3) H. KARL (4) H. LISA (1) T.S. MATTHEW T.S. NICOLE T.S. OTTO 6 intense hurricanes!

27. Gray’s Forecasting Method Problems with Gray’s Technique

28. Gray’s Forecasting Method 1. Multicollinearity Gray’s forecast has MANY problems with “multicollinearity”. Forecasting something using multiple variables that themselves are highly correlated. Leads to artificially high correlations, but doesn’t improve the forecast at all.

29. Gray’s Forecasting Method ENSO 200mb u’ Sahel drought MSLP’

30. Gray’s Forecasting Method QBO 200mb u’

31. Gray’s Forecasting Method Low latitude SSTs MSLP’ Higher latitude SSTs

32. Gray’s Forecasting Method Sahel drought Guinean Precip

33. Gray’s Forecasting Method Only ENSO, QBO, and SST anomalies in the Atlantic are truly independent. These are probably the only variables that add real value to the forecast. These factors change slowly and are relatively easy to forecast, which is good!

34. Gray’s Forecasting Method 2. Subjective Elements QBO ENSO Analog years

35. Gray’s Forecasting Method 3. Overforecasting Forecasts of # of hurricanes, # of hurricane days, etc. are pretty good. Many other #s are just junk.

36. Gray’s Forecasting Method 4. Garbage in, garbage out Forecast equations were “trained” using over a hundred years of hurricane data.

37. Gray’s Forecasting Method 1852

38. Gray’s Forecasting Method 1866

39. Gray’s Forecasting Method 5. Forecast VALUE? How do we act on this information?

40. Gray’s Forecasting Method Why Does Gray’s Technique Work So Well?

41. Gray’s Forecasting Method 1. ENSO, QBO, and SST anomalies dominate the equations, and there is no doubt that they ARE good predictors.

42. Gray’s Forecasting Method 2. Gray is a skilled forecaster. He makes excellent forecasts of ENSO, QBO, SST anomalies, etc.

43. Gray’s Forecasting Method 3. Subjectivity at the National Hurricane Center.