Posted: 2:00 pm Thursday, June 1st, 2017
By Kirk Mellish
Tropical cyclone season begins June 1st and runs through the end of November.
An early out of season tropical storm already formed back in April named Arlene. It was only the second April tropical storm since the satellite era, that was Ana in 2003.
Rare, earlier than normal storms also developed last season in January and May.
Last year Hurricane Mathew killed 46 people in the Southeast U.S. The 2016 season was the most active since 2012, with 15 named storms, including 7 hurricanes and 4 major hurricanes last year.
The theme of preliminary outlooks for the 2017 season has been that an oncoming El Nino in the Pacific and slightly cooler Atlantic waters would mean a below-normal number of storms this season. We take a look at past history to get a possible picture of the future as this will be a season where we transition from La Nina to El Nino.
However, recent modeling has backed off on the strength and timing of any El Nino, and waters have warmed in much of the Atlantic Ocean and are expected to be warmer than normal in the Gulf of Mexico and off the East Coast.
So forecast numbers have been inching up and that trend may continue in the months ahead depending on signals from June and July data.
The season reaches a peak by mid September:
It must be noted that months in advance forecasters can only estimate the total number of storms that may form, and if that number is average, above average or below normal. Weeks and months in advance it is NOT possible to predict if or where any future storm may strike.
There is only a weak correlation between the total number of storms that form and how many strike land, if any. Many or NONE of the storms that form could strike land, but it only takes one hit.
How many is normal? The long-term average number since 1950 has been for 12 named storms, 7 of those becoming hurricanes, and 3 of those to be major category hurricanes.
2017 HURRICANE OUTLOOK NUMBER:
The consensus forecast calls for a near-average number of hurricanes this season.
The reason the season is expected to be somewhat muted and possibly a little below average in total storms is the expected higher than normal pressure and elevated wind shear in the prime tropical breeding grounds. But that could change.
There has been a relative hurricane “drought”, no major hurricane has made a direct eye hit on the U.S. since 2005, which is a record long streak.
“The odds of going 11 years without a major hurricane landfall in the US is around 1 in 2,000,” said Phil Klotzbach, a hurricane research scientist at Colorado State University.
The climatology numbers say at least one storm will hit the U.S. each year on average with the frequency of a major hurricane strike once every 2 years.
The following charts show the forecasts from a variety of prediction outlets, with the mean plotted on the far left axis.
2017 TROPICAL SEASON FORECASTS:
So the mean or average of those predictions is for 13 named tropical systems, 6 hurricanes, 3 of the 6 being major.
These analog years have produced some landmark hurricanes, particularly in the Gulf of Mexico, including Audrey (1957), Camille (1969), and David and Frederic (1979; David remained east of the Gulf but affected eastern Florida). CSU emphasizes: “…these forecasts do not specifically predict where within the Atlantic basin these storms will strike. The probability of landfall for any one location along the coast is very low.” In its landfall probability estimates, CSU departs only slightly from the long-term average, calling for a 1% – 3% increase over the climatological odds of a hurricane making landfall for each state from Texas to North Carolina.
NOAA/NWS outlook yesterday calling for an above-normal season:
If we take the midpoint of these ranges, NOAA called for 14 named storms, 7 hurricanes and 3 major hurricanes.
BEWARE of focusing only on the numbers. One caveat must be mentioned about the 2017 season. Past years like this moving from a weak La Nina to a Weak El Nino have shown a concentration of more favorable conditions for development, tracks and intensification closer to the U.S. coastlines. So even if the total number of tropical storms and hurricanes ends up near the two decade average the mainland is more vulnerable than in some years.
This year marks the 25th anniversary of Andrew, which tore into South Florida on Aug. 24, 1992.
RETURN INTERVAL FOR HURRICANE CONDITIONS BASED ON CLIMATE RECORDS:
IF there is an early season tropical cyclone in June, the map below shows where they typically form this time of year:
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