The failure of preseason predictions for the hurricane season of 2006
The preseason predictions of an extremely active hurricane season were spectacularly wrong. Only nine named storms and five hurricanes formed in the Atlantic, one below the average of ten named storms and six hurricanes. We ended up with the quietest hurricane season since 1997, much to the relief of regions ravaged by the unprecedented activity of 2004 and 2005. What happened to make the prognostications such a abysmal failure?
Dry air and dust
A significant reason for the failure appears to be the unusual amount of dry air laden with African dust that came off the Sahara Desert during July and August. Hurricanes need moist air at mid-levels of the atmosphere in order to form, and recent research suggests that the dust within the dry air may act as an inhibiting factor as well, through some not well-understood process. In addition, these dry air outbreaks are frequently accompanied by a strong jet of easterly winds that brings hostile wind shear over the Atlantic. As seen in a plot of the relative humidity at 500 mb (roughly 18,000 feet altitude) in August of 2006 (Figure 1), there was much drier air than usual over a large portion of the tropical Atlantic where hurricanes like to form.
Why was there so much dry air and dust? During the early part of the rainy season (May-July) in the southern Sahel region of Africa, precipitation was well below average (Figure 2). Rainfall was also below average in this region in 2005, and these factors could have contributed to more Saharan dust being blown out over the tropical Atlantic in 2006.
Figure 1. Departure from normal of relative humidity for the month of August, at 500 mb (about 18,000 feet altitude).
Figure 2.Departure of precipitation from normal for May-July 2006 for Africa. Note that the southern Sahel region (approximately 10-15ºN, 0-15ºW) had much below average precipitation, and this likely contributed to the dry air and widespread Saharan dust outbreaks observed over the tropical Atlantic this year. Image credit: Bill Gray and Phil Klotzbach's Summary of 2006 Atlantic Tropical Cyclone Activity and Verification of Author's Seasonal and Monthly Forecasts at Colorado State University.
El Niño
The other reason the hurricane season of 2006 was so mild is probably due to the arrival of El Niño conditions in September. It is well known that when the warming of the Equatorial Pacific waters off the coast of South America brings about an El Niño event, the number and intensity of Atlantic hurricanes is sharply reduced. Conversely, action in the Eastern Pacific is enhanced, and we saw both of these effects in 2006. The reason usually given for this lack of activity in the Atlantic is an increase in wind shear. The warm waters of the eastern Pacific lead to more rising air than usual there, and when that rising air hits the top of the troposphere (the lower atmosphere), it spreads out and creates strong upper-level winds that blow from east to west towards the Western Pacific, and west to east over the Atlantic. These strong upper-level winds create hostile wind shear that tears apart developing hurricanes. However, in 2006, it appears that El Niño-induced wind shear was not a serious impediment to Atlantic hurricane formation. Wind shear was near average over the Atlantic during most of hurricane season (Figure 3). There are additional reasons El Niño suppresses hurricane activity, and foremost among these is the introduction of stable, sinking air over the Atlantic. It is likely that El Niño brought such conditions to the Atlantic during large portions of the 2006 hurricane season, significantly inhibiting hurricane formation.
Figure 3. Wind shear over the tropical Atlantic. Except for a big above-normal spike in October, wind shear in 2006 (blue line) was near normal (black line) over the tropical Atlantic during hurricane season. Image credit: NOAA/CIRA.
So why did the long range forecasts fail?
The long range seasonal hurricane predictions are statistical in nature--they look for early season patterns in winds, pressures, and ocean temperatures that occurred in years past that one can combine to make a skillful prediction of the hurricane season. One of the variables these prediction schemes typically do not include is the early season rainfall in Africa. If it is an unusually dry and dusty year over the Atlantic like 2006 was, then the forecast is going to be wrong. The other problem was the unusual nature of the El Niño event that developed this year. We went from La Nina conditions in March to a full-fledged El Niño in September. This was by far the largest percentage warming of SST anomalies between June-July and August-September in the tropical Pacific for a year that had El Niño conditions in August-September. In addition, the the timing was unusual--it is uncommon for El Niño events to start in the Fall. Since the historical record had very few cases mimicking the behavior of this year's El Niño event, it is no wonder that the statistical models which rely on past years' data to come up with forecasts of hurricane activity failed. El Niño behaved too strangely this year to anticipate, and the computer models had no idea it was coming until about March or April.
Jeff Masters
Reader Comments
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Any chance that global warming could introduce a variable into hurricane development that nobody has considered - specifically an increase in African dust? It would seem that if global warming increased drought in Africa, as many models suggest, then this inhibitor could be come more frequent, making hurricanes less frequent. Might help a bit if warmer SSTs lead to more intense hurricances.
AFTERNOON...WITH A GOOD COVERAGE OF CUMULUS AND INCREASING AMOUNTS
OF HIGH-LEVEL CLOUD COVER COURTESY OF DEVELOPING CYCLONE BETWEEN
THE WESTERN TIP OF CUBA AND THE YUCATAN PENINSULA. SHORT TERM
MODELS ARE IN GENERAL AGREEMENT ON A SLOW DEEPENING OF THE LOW-
LEVEL CYCLONE...AS WEAK HEIGHT FALLS GRADUALLY SPREAD EASTWARD
ACROSS THE GULF IN ADVANCE OF MID-LEVEL SHORTWAVE TROUGH AND
ASSOCIATED UPPER-LEVEL SPEED MAX CURRENTLY MIGRATING ACROSS
NORTHERN MEXICO. AS THIS OCCURS...LOW-LEVEL FLOW WILL VEER TO THE
SOUTH...WITH RICH TROPICAL MOISTURE ADVECTED NORTHWARD ACROSS THE
FLORIDA PENINSULA. THE COMBINATION OF DEEP WARM ADVECTION AND
MODERATELY STRONG SYNOPTIC SCALE FORCING WILL RESULT IN
DEVELOPMENT OF A WIDESPREAD AREA OF RAINFALL/THUNDERSTORMS BY
THURSDAY AFTERNOON...WITH THIS ACTIVITY LIKELY BEING MOST
CONCENTRATED ALONG SURFACE BAROCLINIC ZONE LOCATED SOMEWHERE
ACROSS THE CENTRAL PORTION OF THE FLORIDA PENINSULA. THUS...WILL
PLACE LIKELY POPS ACROSS THE NORTHWESTERN COUNTIES OF THE FORECAST
AREA...AND GRADUALLY TAPER THEM OFF TO THE SOUTHEAST...AS WEAK
UPPER RIDGING WILL BE MORE ESTABLISHED IN THIS AREA.
CURRENT INDICATIONS ARE THAT RAIN AND EMBEDDED THUNDERSTORMS WILL
CONTINUE FOR MUCH OF THE DAY ON FRIDAY AND FOR THE FIRST HALF OF
SATURDAY...BEFORE THE ENTIRE SYSTEM SLOWLY DRIFTS EASTWARD INTO
THE ATLANTIC. DESPITE FAIRLY CONSISTENT SOLUTIONS OFFERED BY THE
NAM/GFS...AS WELL AS THE UKMET/CMC...IT MUST BE EMPHASIZED THAT
THE PREVIOUSLY MENTIONED UPPER LOW OVER NORTHERN MEXICO MAY TEND
TO CUT OFF AND MOVE SLOWER THAN CURRENT GUIDANCE IS INDICATING...
WHICH WOULD RESULT IN RAIN CHANCES LINGERING THROUGH MORE OF THE
WEEKEND. THIS TREND IS EXHIBITED MOST BY THE LATEST RUN OF THE
ECMWF. OVERALL THE MAIN THREATS FROM THIS ACTIVITY WILL BE
LOCALLY HEAVY RAINFALL...AS PWAT VALUES ARE PROGGED TO APPROACH
OR SLIGHTLY EXCEED 2 INCHES...AND CLOUD TO GROUND LIGHTNING
STRIKES WITH ANY EMBEDDED CONVECTIVE CELLS.
main concern is the drastic change of the 12z and 18z GFS and
European model runs for this weekend's storm. The latest model runs
indicate that the trough will not be as deep as once advertised. The
closed low is now well to the east over Arizona by Monday. Although this has
been just a couple of runs...confidence is greater that this will
become more of a wind event then rain event
This fall has been just like a La Nina more then an El Nino down here and this is just more evidence.
Here is the CMC rainfall forcast showing some descent rainfall across the area.
000
FXUS62 KMFL 140115
AFDMFL
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MIAMI FL
815 PM EST WED DEC 13 2006
LOW LEVEL EASTERLY WINDS...THOUGH WEAKENING...STILL ADVECTING
ISOLATED TO WIDELY SCATTERED LIGHT RAIN SHOWERS UPON THE E COAST.
BUT THIS IS NOT THE ONLY SHOWER ACTIVITY AFFECTING S FLA AS LIGHT
SHOWERS MOVING NE ACROSS THE SE GULF OF MEXICO FROM A DEVELOPING LOW
PRES SYSTEM JUST NW OF THE W TIP OF CUBA. IR SATL LOOPS INDICATE
COOLING TOPS MEANING STRENGTHENING CONVECTION. AS THE EVENING
PROGRESSES...CURRENT ISOLATED LIGHT SHOWERS WILL INCREASE AND BECOME
SCATTERED...AFFECTING MAINLY THE W COAST FROM NAPLES N AND INLAND
TOWARD LAKE OKEE AND PALM BEACH. CURRENT ZFP PACKAGE HAS THIS TREND
AND NO CHANGES ARE ANTICIPATED.
Chanchu, Bilis, Xangsane, Saomai, and Durian
Also the following names were replaced from the names retired in 2005
Longwang "Dragon King" was replaced by Haikui "sea anemone"
couldn't find info on the other two that were retired last year, must still be thinking of names.
Link
NWS discussion from 3am.
.DISCUSSION...MODELS CONTINUE TO BE IN DISAGREEMENT ABOUT THE WX
FOR S FLA NEXT 48 HOURS. THE GFS SHOWS SHORTWAVE TROUGH CURRENTLY
DIGGING INTO S TX/NORTHERN MEXICO AND THEN DEVELOPING A SFC LOW
OVER THE EASTERN GOFMEX WHILE THE NAM SHOWS NOTHING OF THE KIND.
WATER VAPOR IMAGERY DOES SHOW SUBTROPICAL JET MOVING INTO THE
WESTERN GOFMEX WITH SLIGHT CYCLONIC CURVATURE DEVELOPING SO WILL
LEAN TOWARDS THE GFS SOLUTION. AS THIS THEN TRAVERSES ACROSS THE
FL PENINSULA COULD BE A DECENT RAIN PRODUCER AND WE ARE IN DIRE
NEED OF THAT. THE HEAVIEST RAIN WILL TEND TO OCCUR THURSDAY NIGHT
INTO FRIDAY AS ALL OF THIS INTERACTS OVER S FLA. THE GFS HAD BEEN
SLOWING THE SPEED OF THE LOW DOWN BUT THE 00Z RUN NOW SPEEDS IT UP
AGAIN AND BY FRIDAY EVENING WILL BE EXITING THE SE CST. THIS IN
TURN WILL END OUR PRECIP EARLIER SO HAVE LOWERED THE POPS FOR
FRIDAY NIGHT AND SATURDAY. AS THIS OCCURS STRONG SUBTROPICAL RIDGE
WILL BUILD ACROSS THE GOFMEX AND EXTEND ACROSS FLA AS A LARGE
SHORTWAVE DIGS INTO THE DESERT SW. AS THIS LOW THEN MOVES INTO TX
BY THE MIDDLE OF NEXT WEEK, SOME OF THE MOISTURE COULD GET CAUGHT
BACK UP AND INCREASE OUR POPS AT THAT TIME. FOR NOW, HAVE NOT
CHANGED MUCH OF THE EXTENDED PERIOD.
According to the MLB Local all the models went with an open wave but the gfs.
Patrap~ everyday should begin like that.
...great!
RADAR
Water temps still 80F at this buoy,which is even further north of the wave...
24 FRAME VISIBLE LOOP OF THE GOM
Link
Adrian's weather
Still got a long shot, outside chance of a freak, late season one over the Azores, nogaps, gfs, cmc (phase anylisis). The models have actually been leaning more toward it as the event approaches.
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