The Washington Post Sunday - - OUTLOOK - By Jeffrey Halver­son JIM RASSOL/SUN-SENTINEL/ASSOCIATED PRESS RICKY CARIOTI/THE WASHINGTON POST Jeffrey Halver­son, a con­trib­u­tor to The Washington Post’s Cap­i­tal Weather Gang, teaches me­te­o­rol­ogy at the Univer­sity of Mary­land Bal­ti­more County.

The Gulf Coast has been badly bat­tered by hur­ri­canes in re­cent weeks. Har­vey sub­merged much of Hous­ton, caus­ing de­struc­tion that will take years and bil­lions of dol­lars to undo; Irma roared north through Florida, and with the power still out across much of the state, the dam­age is still be­ing tal­lied. The storms elicited a na­tion of ner­vous weather­watch­ers and ex­posed sev­eral myths.

A busy year of named storms will be dan­ger­ous and costly.

This seems in­tu­itive. The Weather Com­pany has raised the num­ber of named storms that it ex­pects to de­velop in its lat­est 2017 At­lantic hur­ri­cane sea­son out­look, warn­ing of “high­erthan-nor­mal land­fall risks in the north­east U.S.” Colorado State Univer­sity’s sea­sonal out­look said that the prob­a­bil­ity of ma­jor hur­ri­canes mak­ing land­fall in the United States was above nor­mal this year “due to the fore­cast for an above-av­er­age sea­son.”

Ac­tu­ally, there is only some cor­re­la­tion be­tween the to­tal num­ber of storms in a sea­son and the num­ber of storms mak­ing land­fall. In 2004, there were 15 named storms, and eight struck the United States. In 2010, there were 19 named storms but only two U.S. land­falls. (The to­tal in both years was well above the long-term av­er­age for the At­lantic.) It only takes one roar­ing storm for an in­ac­tive sea­son to be con­sid­ered aw­ful: 1992 saw just seven storms, well be­low the av­er­age, but one of them was Hur­ri­cane Andrew, at the time the na­tion’s costli­est hur­ri­cane.

How many storms make land­fall de­pends on trop­i­cal cur­rents, such as the trade winds, that steer hur­ri­canes. These sys­tems shift around and can strengthen or weaken. For in­stance, if the Ber­muda High cur­rent moves closer to the U.S. main­land, as it did in 2004 and 2005, more storms land on our shores. If the Ber­muda High moves closer to Africa, more tend to curve over the ocean in­stead of strik­ing the United States.

A Cat­e­gory 4 storm will cause more dam­age than a Cat­e­gory 2.

The well-known Saf­fir-Simp­son hur­ri­cane in­ten­sity scale de­fines five cat­e­gories of storms. These lev­els are based on the max­i­mum sus­tained wind, which seems like the most mean­ing­ful met­ric. “The lower the pres­sure, the more in­tense the hur­ri­cane, es­pe­cially in terms of wind speed and dam­age,” the ABC af­fil­i­ate in Chicago re­ported this month. And be­fore Har­vey, news out­lets sug­gested that as a Cat­e­gory 4 storm, it would be “stronger” than Ka­t­rina.

But size mat­ters, too. Both the size of a storm’s wind “foot­print” and the strength of its winds con­trol the height of the storm surge. Com­pare Hur­ri­canes Charley (2004) and Ike (2008). Charley packed 120 mph sus­tained winds and was very small. Ike was about 10 times larger but con­sid­er­ably weaker, with 95 mph winds. Charley’s surge along Florida’s west coast was in the six-to-seven-foot range, while Ike dev­as­tated the Texas-Louisiana coast with a very broad surge reach­ing 12 to 17 feet in height. The dif­fer­ence in surge heights largely re­flected Ike’s much larger size.

Storm rain­fall, which can be cat­a­strophic, also has lit­tle to do with wind in­ten­sity. Some of our worst coastal flood­ing dis­as­ters from trop­i­cal cy­clones have come from hum­ble trop­i­cal storms: Allison, for ex­am­ple, brought in­land Texas about 40 inches of rain in 2001.

In­land cities don’t need to worry about hur­ri­canes.

When hur­ri­canes strike the United States, at­ten­tion focuses on the coast­line, where watches and warn­ings are is­sued. Ac­cord­ing to LiveS­cience, the big cities at great­est risk are coastal ones: Hous­ton; New Or­leans; Mo­bile, Ala.; Tampa; Mi­ami; Bos­ton; New York and oth­ers. “If you live hun­dreds of miles in­land,” the au­to­mo­bile site Jalop­nik put it in an ar­ti­cle on how to es­cape a storm’s path, “. . . you don’t need to worry.”

It’s true that storms lose 50 per­cent of their wind in­ten­sity within 12 to 18 hours of land­fall. But hur­ri­cane rem­nants have sur­pris­ingly long reach. Many storms through­out U.S. his­tory have pro­jected for­mi­da­ble haz­ards hun­dreds of miles in­land, some­times days af­ter land­fall. And trop­i­cal rem­nants that join with pre­ex­ist­ing weather sys­tems, such as fronts and jet stream dis­tur­bances, have new sources of en­ergy to sus­tain them. In­land haz­ards have been par­tic­u­larly dev­as­tat­ing over the moun­tain­ous re­gions of the East Coast, where steep ter­rain lifts trop­i­cal mois­ture.

In­fa­mous ex­am­ples in­clude Camille (1969), which made land­fall over Louisiana but killed half its vic­tims days later in a hor­rific flood in Vir­ginia’s Blue Ridge Moun­tains. Agnes (1972) made land­fall in the Florida Pan­han­dle but un­leashed its worst flood­ing in cen­tral Penn­syl­va­nia. And Ivan (2004) first struck Alabama but spun up a record-break­ing swarm of dam­ag­ing tor­na­does across North­ern Vir­ginia and cen­tral Mary­land.

The big­gest threat from a hur­ri­cane is the wind.

Hur­ri­canes are huge, at­mo­spheric vor­tices, and they are prop­erly feared for their wide­spread, high-en­ergy on­slaughts of wind. The Weather Chan­nel holds that “wind is re­spon­si­ble for much of the struc­tural dam­age caused by hur­ri­canes.” Florid­i­ans un­der­stand­ably see wind as their en­emy: Hur­ri­cane Andrew was a land­mark storm, killing 65, to the tune of $27 bil­lion, most of this be­cause of winds ex­ceed­ing 175 mph.

But wind is only one of a deadly triad of im­pacts that hur­ri­canes de­liver. The storm surge — a sud­den rise in sea level along the coast, where ocean wa­ter is pushed in­land by the hur­ri­cane’s strong­est winds — causes far more fa­tal­i­ties. The max­i­mum surge is con­fined to the eye­wall of the storm, but its ef­fects can be broad: In 1876, a storm surge killed as many as 400,000 peo­ple in a sin­gle cy­clone along coastal Bangladesh. A re­cent study shows that, in the United States, the ma­jor­ity of hur­ri­cane-re­lated deaths come from wa­ter.

Some­times this means storm surge, but more of­ten peo­ple drown in fresh­wa­ter. Tor­ren­tial rains of­ten dump six to 12 inches or more, lead­ing to flash flood­ing. Hur­ri­cane Ka­t­rina (2005) pro­duced the high­est death toll of any hur­ri­cane — at least 1,000 — since the Okee­chobee hur­ri­cane in Florida in 1928, and most of the fa­tal­i­ties were drown­ings.

Fore­cast­ers hype the threat to push a cli­mate change agenda.

Rush Lim­baugh and Matt Drudge have both said since last year that the me­dia and the Na­tional Hur­ri­cane Cen­ter over­hype hur­ri­canes to play up global warm­ing.

But the mis­sion of the Na­tional Weather Ser­vice, which raised ur­gent warn­ings about the lat­est hur­ri­canes, is to pro­tect prop­erty and save lives. And it only makes sense for lo­cal news out­lets and pri­vate fore­cast­ers to fol­low their lead; it is a stan­dard, and re­spon­si­ble, prac­tice for these or­ga­ni­za­tions to post ev­ery of­fi­cial watch and warn­ing is­sued by the hur­ri­cane cen­ter.

Fur­ther­more, many sci­en­tists are cau­tious about the con­nec­tion be­tween hur­ri­cane ac­tiv­ity and cli­mate change. The Na­tional Oceanic and At­mo­spheric Administration, for in­stance, has said a de­tectable hu­man in­flu­ence has not yet been dis­cov­ered, though it projects in­creases in hur­ri­cane in­ten­sity dur­ing the com­ing decades.

A palm tree sways in Fort Laud­erdale as Hur­ri­cane Irma nears the Florida coast last week­end. Hur­ri­canes’ winds are fear­some but are not their dead­li­est threat.

Rough surf in Boyn­ton Beach, Fla., dur­ing Irma. Most hur­ri­cane deaths are caused by drown­ing.

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