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2008 hurricane season outlook

By: Dr. Jeff Masters, 2:56 PM GMT on April 13, 2008

It's too soon to find many clues about what the upcoming hurricane season might be like, since we are facing the famed "April Predictability Barrier". The atmosphere is not predictable enough to make a skillful forecast of seasonal hurricane activity in April (this changes by late May, when skillful predictions of the upcoming hurricane season CAN be made). Thus, we should put little faith in the predictions by Dr. Bill Gray's Colorado State University group and Tropical Storm Risk (TSR) Inc., made in April, calling for 15 named storms, 8 hurricanes, and 4 intense hurricanes. These numbers are exactly what the climatology has been in the 13 years since the current active hurricane period began in 1995, and are pretty unremarkable forecasts.

Figure 1. Sea Surface Temperature (SST) comparison between April 11, 2008 (top) and April 11, 2005 (bottom). SSTs were at record high levels in 2005, and are much cooler in 2008 over the Main Development Region (MDR) for hurricanes. The red wiggly line is the 26°C isotherm, which is the coolest temperature that hurricanes typically form at. Note that it stays warm enough to support hurricanes year-round in the Caribbean. High levels of wind shear prevent development in the winter and spring. Image credit: NOAA/AOML.

Sea Surface Temperature (SST) Outlook
Sea Surface temperatures are at the heart of any attempt to analyze seasonal hurricane activity, since SSTs change slowly and have a major impact on both the number of storms that form and their intensity. For example, the record-breaking Hurricane Season of 2005 had the warmest SSTs ever observed in the tropical Atlantic. Comparing the SSTs in April of 2008 with April of 2005 (Figure 1), we see that SSTs are more than 1 degree C cooler this year over a large portion of the Atlantic Main Development Region (MDR) for hurricanes. The MDR is where 85% of all major hurricane form. Overall, SSTs this April are near average compared to climatology when averaged over the Main Development Region. There is still time for a significant warming of SSTs to occur if we get a large decrease in the speed of the trade winds over the next few months. This would occur if the Bermuda High weakened to much below-average levels, driving slower clockwise winds around its center. TSR is forecasting only a modestly weaker Bermuda High than average, with trade wind speeds about 1 mph below average between now and hurricane season. This would allow SSTs in the MDR to warm to slightly above-average levels. Both Columbia University's IRI and NOAA's CFS model (Figure 2) are predicting average to slightly below average SSTs over the Caribbean and Main Development region during the peak hurricane season months of August, September, and October. The accuracy of these sort of long-range forecasts is a little better than chance, but not a lot. The bottom line is that SSTs should be near average this hurricane season, and should neither enhance nor inhibit hurricane formation. Note, though, that some of the regions outside the MDR, where the other 15% of major hurricanes form, have SSTs as warm or slightly warmer than 2005. For example, Katrina and Rita formed outside the MDR over the Bahamas, and SSTs there this year are comparable to 2005 levels. SSTs are also quite warm in the Gulf of Mexico this spring. Thus, we can expect plenty of fuel for any hurricane that might get loose in the Gulf of Mexico this hurricane season.

Figure 2. Sea Surface Temperature (SST) departure from average (left) and wind shear departure from average (right) from the April 12, 2008 run of NOAA's CFS model. The forecasts are for the 3-month period August-October spanning the most active part of hurricane season. The model predicts near average SSTs over the tropical Atlantic, and below average wind shear. Image credit:NOAA Climate Prediction Center.

El Niño/La Niña Outlook
Obviously, SSTs don't tell the whole story, since the second highest SSTs in the tropical Atlantic since the 1870s occurred in 2006. That was a very normal year with 10 named storms, 5 hurricanes, and 2 intense hurricanes, because an El Niño event occurred. El Niño events usually suppress Atlantic hurricane activity, by bringing increased wind shear and dryer, sinking air over the Atlantic. This is not always the case--recall 2004? El Niño conditions were even stronger that year (as measured by SST departures from average in the Equatorial Pacific), yet that year saw 15 named storms, 9 hurricanes, and 6 intense hurricanes in the Atlantic. Florida got walloped with four hurricanes.

Presence of La Niña conditions usually causes reduced levels of wind shear over the Atlantic, enhancing hurricane activity. The current strong La Niña event has begun weakening noticeably in the past few weeks, giving some hope that La Niña will be gone by hurricane season. However, there is probably not time for a full-fledged El Niño event to replace it by hurricane season, and it is expected that we will have weak La Niña or neutral conditions this hurricane season. Since the active period of hurricane activity that we are in began in 1995, both La Niña and neutral years have seen very high levels of hurricane activity (Figure 3). In fact, neutral years have had even higher hurricane activity than La Niña years (thanks in great measure to the Hurricane Season of 2005).

The NOAA CFS model (Figure 2) is predicting La Niña conditions and lower than average wind shear for the coming hurricane season. None of the computer models are forecasting a switch over to El Niño conditions this year. Keep in mind, though, that the accuracy of these long range models is poor, particularly for forecasts made in March and April. Still, La Niña is well enough established now that it would be a major surprise to see an El Niño arrive by hurricane season.

Figure 3. Observed numbers of named storms, hurricanes, and intense hurricanes (Category 3 and higher) for the 13-year period beginning in 1995. Background image is of Australia's Tropical Cyclone Monica, the most intense storm of 2006.

African dust outlook
African dust is thought to suppress Atlantic hurricane activity, although its role is not well understood. As I explained in a 2006 blog entry, research shows that the presence of drought conditions in the Sahel region of Africa the previous year will increase the amount of dust wafting over the Atlantic during hurricane season. This occurs because drought-damaged soil takes about a year to dry up and turn to dust that can blow away. Last year saw above-average rains during the rainy season (June-September) over the Sahel (Figure 4). This was also the case in 2005 and 2006, so in theory, three straight years of good rains in the Sahel should act to keep African dust levels over the Atlantic below average this hurricane season. The last significant drought years in the Sahel were 2001 and 2002. I made the same forecast last year, but we saw unexpectedly high levels of dust over the eastern Atlantic in July and August, which substantially cooled the ocean waters by blocking sunlight. Dust levels returned to near average levels in September.

Figure 4. Departure of precipitation from average in Africa for August 2007. The region in the red box is the Sahel region of Africa that accounts for most of the year-to-year variability in dust transport over the Atlantic Ocean. Image credit: NOAA Climate Prediction Center.

The outlook for the 2008 Atlantic hurricane season
If the forecasts of near normal sea surface temperatures, below average wind shear, no El Niño, and below average African dust come true, the 2008 Atlantic hurricane season should be well above average in activity. We are also in the midst of an active hurricane period that began in 1995. Since 1995, we've averaged 15 named storms, 8 hurricanes, and 4 intense hurricanes, and this is a reasonable forecast for 2008 (the 100-year climatology is 10 named storms, 6 hurricanes, and 2 intense hurricanes). By May, the atmosphere and ocean begin to give us significant clues about the upcoming hurricane season. Tune into the early June seasonal forecasts issued by NOAA, the Colorado State group, and TSR!

This will be my last blog until Monday April 21, as I'll be on vacation in Arizona.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.