Starting this year, we have begun the issuing of a September-only forecast, an end-of-August seasonal update, and a prediction for the remainder of the hurricane season. Refer to our Web address (below) for more background information, term definitions, etc. These new forecasts are based on meteorological information through August 2002
by
Members of Colorado State University Hurricane Forecast Team
Including William M. Gray1,
Christopher W. Landsea2,
Philip Klotzbach3 and others
[Both this and prior forecasts are available at the following World Wide Web address:
http://typhoon.atmos.colostate.edu/forecasts/ - you also may contact:
Brad Bohlander and Thomas Milligan, Colorado State University media representatives who are available to respond to inquiries regarding this forecast (970-491-6432).
Department of Atmospheric Science
Colorado State University
Fort Collins, CO 80523
Phone Number: 970-491-8681
2 September 2002
Information obtained through August 2002 indicates that the 2002 Atlantic basin seasonal hurricane activity will be much below the average for the 1950-2000 period. The more active season anticipated in our earlier forecasts has been revised drastically downward due to the sharp increase in tropical cyclone inhibiting conditions during the last four months. Cyclone suppressing influences are expected to persist for the remainder of the 2002 hurricane season. These include below average Sea Surface Temperatures (SST), above average Sea Level Pressure (SLP), above average strength of easterly trade winds, a strengthening El Niño event (now judged to be moderate), and, most importantly, stronger than average upper tropospheric westerly winds throughout the Atlantic tropics.
Our September-only forecast calls for three named storms, two hurricanes, and one major hurricane and an Net Tropical Cyclone (NTC)4 activity of 26 which is only 54 percent of the mean September-only average.
We now anticipate that the Atlantic basin 2002 Tropical Cyclone (TC) season will have a total of eight named storms (average is 9.6), 25 named storm days (average is 49.1), three hurricanes (average is 5.9), 10 hurricane days (average is 24.5), one intense (category 3-4-5) hurricane (average is 2.3), two intense hurricane days (average is 5.0), Hurricane Destruction Potential (HDP) of 25 (average is 72.7) and overall Net Tropical Cyclone (NTC)activity of 45 percent of the long-term seasonal average for 1950-2000. These numbers are somewhat lower than our 7 August 2002 forecast. United States and Caribbean basin landfall probability for the remainder of the 2002 season is now estimated to be significantly below the long-term average.
| Full Season Tropical | Full Season | Observed | ||||
|---|---|---|---|---|---|---|
| Cyclone Parameters | Updated | Updated | 2002 Activity | New | Forecast | Revised Total |
| and their 1950-2000 | 31 May 2002 | 7 Aug 2002 | Through | Sept.-only | Activity | Seasonal |
| Climatology (in parentheses) | Forecast | Forecast | August | Forecast | After Sept. | Forecast |
| Named Storms (NS) (9.6) | 11 | 9 | 4 | 3 | 1 | 8 |
| Named Storm Days (NSD) (49.1) | 55 | 35 | 7 | 13 | 6 | 25 |
| Hurricanes (H)(5.9) | 6 | 4 | 0 | 2 | 1 | 3 |
| Hurricane Days (HD)(24.5) | 25 | 12 | 0 | 7 | 3 | 10 |
| Intense Hurricanes (IH) (2.3) | 2 | 1 | 0 | 1 | 0 | 1 |
| Intense Hurricane Days | ||||||
| (IHD)(5.0) | 5 | 2 | 0 | 2 | 0 | 2 |
| Hurricane Destruction Potential | ||||||
| (HDP) (72.7) | 75 | 35 | 0 | 15 | 10 | 25 |
| Net Tropical Cyclone Activity | ||||||
| (NTC)(100%) | 100 | 60 | 9 | 26 | 10 | 45 |
Colorado State University (CSU) has issued seasonal hurricane
forecasts for the last 19 years. These forecasts, which are issued in early
December of the prior year, and in the early part of the months of April, June, and August of the current year, have steadily improved through continuing research. CSU
forecasts now include individual monthly predictions of Atlantic basin
activity and seasonal and monthly U.S. hurricane landfall probabilities.
The National Oceanographic and Atmospheric Administration (NOAA) has also begun to issue Atlantic basin seasonal hurricane forecasts but do not issue monthly or landfall probability forecasts. The NOAA forecasts are independent of the CSU forecasts, although they utilize prior CSU research, augmented with their own insights. The NOAA and the CSU forecasts are not necessarily in agreement. Chris Landsea and Eric Blake, former CSU project members presently
employed by NOAA, are making important contributions to both forecasts. NOAA does not sponsor the CSU forecasts.
A new aspect of our climate research is the development of TC activity predictions for individual months. There are often monthly periods within active and inactive hurricane seasons which do not conform to the overall season. To this end, we have recently developed new schemes to forecast August-only and September-only Atlantic basin hurricane activity by the beginning of each respective month. These efforts have been recently documented in CSU project reports by Eric Blake (2002) for our August-only forecast and by Phil Klotzbach (2002) for our September-only forecast - see the last page for references. We also plan the development of a separate October forecast. On average, August, September and October have about 26, 48, and 17% or 91% of the Atlantic basin total NTC activity. Initial August-only forecasts have now been made by Blake for the last three seasons. This is the first year we are issuing a September-only forecast. We hope to make an October-only forecast next year.
Quite skillful August-only and September-only prediction schemes have been developed based on 51 years (1950-2000) of hindcast testing using a statistically independent jackknife approach. Predictors are derived from prior month, usually June and July (NCEP global reanalysis) data for the August-only forecast and with prior-month data through August for the September-only forecast.
Through August, the 2002 hurricane season has had only twenty percent of the NTC activity of the average hurricane season. As of 1 September only four weak tropical storms have formed and no hurricanes have developed. The average number of named storms through August is 4.2, the average number of hurricanes is 2.4, and the average number of major hurricanes is 0.7.
Table 1 and Fig. 1 list the nine September-only predictors with their location and sign for higher TC activity. Table 2 shows which of the nine predictors are used for each forecast parameter of NS, NSD, H, etc.
| Name of Predictor | Location | Abbreviated Predictor Name |
|---|---|---|
| 1) April 1000 mb U (-) | (12.5-30° S, 40° -10° E) | April S Atl 1000 mb U |
| 2) July 200 mb Geo Ht. (+) | (32-42°N, 100-160° E) | July Asian 200 mb Geo Ht |
| 3) July-Aug. 1000 mb U (+) (-) | (5-15N° , 30-50° W) - | |
| (22.5-35° N, 35-65° W) | July-Aug Atl 1000 mb U | |
| 4) Feb. 1000 mb U (-) | (20-30° N, 15° W-15° E) | Feb W Africa 1000 mb U |
| 5) April 200 mb U (-) | (67.5-85° N, 110-180° E) | April NE Siberia 200 mb U |
| 6) August SLP (-) | (0-30° S, 120-160° E) | Aug Indonesia SLP |
| 7) August SLP (-) | (20-45° S, 60-90° E) | Aug S Indian Ocean SLP |
| 8) May 200 mb V (+) | (0-20° S, 15-30° E) | May C Africa 200 mb V |
| 9) Jan-Feb 200 mb U (-) | (15-25° N, 120° E-160° W) | Jan-Feb W Pac 200 mb U |
|
|
| Abbreviated Predictor Name | Equations Used |
|---|---|
| 1) April S Atl 1000 mb U (-) | IH |
| 2) July Asian 200 mb Geo Ht. (+) | NSD, H, IH, TONS, TOH, NTC |
| 3) July-Aug. Atl 1000 mb U (+) (-) | H, HD, IH, IHD, TONS, TOH, NTC |
| 4) Feb W Africa 1000 mb U (-) | NS, NSD, HD, IHD, NTC |
| 5) April NE Siberia 200 mb U (-) | NS, NSD, HD, IH, IHD, TONS, TOH, NTC |
| 6) Aug Indonesia SLP (-) | NS, NSD, HD, TONS |
| 7) Aug S Indian Ocean SLP (-) | NS, H |
| 8) May C Africa 200 mb V (+) | NS, NSD, H, HD, IH, TONS, NTC |
| 9) Jan-Feb W Pac 200 mb U (-) | IHD, TOH |
Table 3 lists the value of each September-only predictor and whether its 2002 value indicates above or below September-only TC activity. Most of the July and August 2002 predictors are negative. Due to the unusual and unexpected massive rearrangement of global ocean and atmospheric conditions the last three to four months, we believe that the positive predictors before June are not typical of the coming September activity.
| Name of | Parameter Value in | Indication for Greater |
|---|---|---|
| Predictor | Standard Deviation (SD) | or Less September TC Activity |
| 1) April 1000 mb U (-) | +1.8 SD | Less |
| 2) July 200 mb Geo Ht. (+) | +0.6 SD | More |
| 3) July-Aug. 1000 mb U (+) (-) | -0.7 SD | Less |
| 4) Feb. 1000 mb U (-) | -0.9 SD | More |
| 5) April 200 mb U (-) | +1.5 SD | Less |
| 6) August SLP (-) | +0.4 SD | Less |
| 7) August SLP (-) | +1.8 SD | Less |
| 8) May 200 mb V (+) | +0.6 SD | More |
| 9) Jan-Feb 200 mb U (-) | -0.8 SD | More |
Our August-only 2002 forecast overpredicted August activity as shown in Table 4. We attribute this to the strong Tropical Upper Tropospheric Trough (TUTT) in the western Atlantic and its accompanying very strong upper-level westerly winds to the east side of this trough. And to the generally high values of Atlantic basin SLP and low values of SST. The current moderate El Niño event in the eastern tropical Pacific is also believed to have exerted an inhibiting influence on August 2002 TC activity.
| Statistical | Adjusted | ||
|---|---|---|---|
| Tropical Cyclone Parameters | August-only | August 2002 | August 2002 |
| and 1950-2000 August Ave. | Forecast | Forecast | Verification |
| Named Storms (NS) (2.8) | 2.09 | 4 | 3 |
| Named Storm Days (NSD) (11.8) | 18.9 | 10 | 7 |
| Hurricanes (H) (1.6) | 2.60 | 1 | 0 |
| Hurricane Days (HD) (5.7) | 7.11 | 4 | 0 |
| Intense Hurricanes (IH) (0.6) | 0.68 | 0 | 0 |
| Intense Hurricane Days (IHD) (1.2) | 1.06 | 0 | 0 |
| Net Tropical Cyclone (NTC) (26.4) | 31.8 | 18 | 7 |
We expect that these strong before September inhibiting conditions will persist through September but to be less restrictive than they have been in August. Table 5 shows both of our September-only statistical and adjusted forecast made in early August based on data through July (a) and our new September-only forecast based on data through August (b).
| (a) | (b) | |||
|---|---|---|---|---|
| September 1950-2000 | Statistical | Based on Data | Statistical | Adjusted |
| Climatology | Calculation | Through July | Calculation | September-only |
| (in parentheses) | Adjusted Forecast | Forecast | ||
| Named Storms (NS) (3.4) | 2.7 | 3 | 2.5 | 3 |
| Named Storm Days (NSD) (21.7) | 21.5 | 21.5 | 20.0 | 13 |
| Hurricanes (H) (2.4) | 2.5 | 2 | 2.3 | 2 |
| Hurricane Days (HD) (12.3) | 12.5 | 12 | 11.0 | 7 |
| Intense Hurricanes (IH)(1.3) | 0.8 | 1 | 0.7 | 1 |
| Intense Hurricane Days (IHD) (3.0) | 2.75 | 2.5 | 2.0 | 2 |
| Net Tropical Cyclone (NTC) (48) | 44.1 | 42 | 36 | 26 |
Table 6 is a summary of this year's hurricane activity through August and our projection for the rest of this season. During El Niño years (as 2002 is) October and November hurricane activity is typically suppressed. For example, during the strongest El Niño years of the last century, the average number of hurricanes forming in October and November has been only 35-40 percent of the number which developed in non-El Niño seasons. Allowing that 2002 now has a moderate El Niño event, we expect October and November hurricane activity for this year to be below average.
| Tropical Cyclone Parameters | Observed | Forecast | Revised | |
|---|---|---|---|---|
| and 1950-2000 Climatology | TC Activity | Sept. | Oct.-Nov. | Full Season |
| (in parentheses) | Through August | Forecast | Forecast | Forecast |
| Named Storms (NS) (9.6) | 4 | 3 | 1 | 8 |
| Named Storm Days (NSD) (49.1) | 7 | 13 | 5 | 25 |
| Hurricanes (H) (5.9) | 0 | 2 | 1 | 3 |
| Hurricane Days (HD) (24.5) | 0 | 7 | 3 | 10 |
| Intense Hurricanes (IH)(2.3) | 0 | 1 | 0 | 1 |
| Intense Hurricane Days (IHD) (5.0) | 0 | 2 | 0 | 2 |
| Hurricane Destruction Potential (HDP) (72.7) | 0 | 17 | 8 | 25 |
| Net Tropical Cyclone (NTC) (100) | 9 | 26 | 10 | 45 |
We now expect that the 2002 hurricane season will be even less active than we anticipated with our earlier forecasts. We now envisage a 2002 seasonal total of eight named storms, only three hurricanes, one major hurricane, and NTC of only 45 percent of the seasonal average. We also foresee a significant lower than average probability of U.S. and Caribbean basin hurricane landfall for the rest of this year.
Our forecasts of 2002 will be verified and critiqued in late November 2002. These verifications will be made available at our web address. Our first seasonal hurricane forecast for the 2003 season will be issued in early December 2002.
(http://typhoon.atmos.colostate.edu/forecasts/)
Eric Blake, John Sheaffer, John Knaff, Paul Mielke, Jr., Ken Berry, Todd Kimberlain and others have made important contributions to the conceptual and scientific background for this forecast.
1Professor, Department of Atmospheric Science
2Dr. Landsea is an employee of the NOAA Atlantic Oceanographic & Meteorological Laboratory. As part of his research to improve NOAA's climate forecasting ability, he collaborates with researchers at Colorado State University in developing CSU's climate forecast. The CSUclimate forecast is not endorsed by NOAA, nor should it be construed in any way as an official NOAA forecast.
3Graduate Student, Department of Atmospheric Science
4NTC is a combined measure of the yearly mean of six indices (NS, NSD, H, HD, IH, IHD) of hurricane activity as a percent deviation from the 1950-2000 year average.