Hurricane Ivan -- Thursday 9/16 thread [Landfall!]

Hurricane Ivan Advisory Number 55

Statement as of 10:00 PM CDT on September 15, 2004

...Extremely dangerous Hurricane Ivan coming closer to the northern Gulf Coast...strong winds already moving onshore...

A Hurricane Warning is in effect from Grand Isle Louisiana to Apalachicola Florida...including the greater New Orleans area and Lake Pontchartrain. A Hurricane Warning means that hurricane conditions are expected within the warning area...generally within the next 24 hours. Preparations to protect life and property should be rushed to completion throughout the entire warning area.

A Hurricane Watch remains in effect from Morgan City Louisiana to west of Grand Isle.

A Tropical Storm Warning remains in effect from Morgan City to west of Grand Isle...and from east of Apalachicola to Yankeetown Florida.

At 10 PM CDT...0300z...the large eye of Hurricane Ivan was centered near latitude 29.3 north... longitude 88.1 west or about 65 miles south of the Alabama coastline.

Ivan is moving slightly east of north near 12 mph...and this motion is expected to continue for the next 24 hours. On the forecast track...the center of the hurricane will reach the coast early on Thursday.

Maximum sustained winds are near 135 mph...with higher gusts. Some fluctuations in intensity are possible prior to landfall...but Ivan is expected to make landfall as a major hurricane...category three or higher. Occupants of high-rise buildings within the Hurricane Warning area can expect higher winds than those experienced at the surface...about one Saffir-Simpson category higher at the top of a 30-story building. After landfall... hurricane force winds could spread inland up to about 150 miles near the path of the center.

People are strongly advised not to venture out from shelter during the calm conditions of the eye...as winds will increase rapidly with little or no warning when the eye passes.

Hurricane force winds extend outward up to 105 miles from the center...and tropical storm force winds extend outward up to 290 miles. The Dauphin Island C-man station reported sustained winds of 54 mph with a gust to 83 mph...and Pensacola Naval Air Station reported sustained winds of 51 mph with a gust to 68 mph.

The latest minimum central pressure measured by a NOAA hurricane hunter aircraft was 933 mb...27.55 inches.

Coastal storm surge flooding of 10 to 16 feet above normal tide levels...along with large and dangerous battering waves...can be expected near and to the east of where the center makes landfall. Lesser...but still significant surge values will be experienced where onshore flow occurs west of the center.

Dangerous surf conditions...including rip currents...are likely elsewhere along the Florida Gulf Coast.

Rainfall accumulations of 10 to 15 inches...with isolated higher amounts...can be expected in association with Ivan.

Tornadoes are possible over the next 24 hours in southern Alabama... the Florida Panhandle and Big Bend area...and southwestern Georgia.

Repeating the 10 PM CDT position...29.3 N... 88.1 W. Movement toward...north near 12 mph. Maximum sustained winds...135 mph. Minimum central pressure... 933 mb.

For storm information specific to your area...please monitor products issued by your local weather office.

Intermediate advisories will be issued by the National Hurricane Center at midnight CDT and 2 am CDT followed by the next complete advisory at 4 am CDT.

Forecaster Pasch

$$

Report from Weather Station on Dauphin Island:

Click on the graph icon in the table below to see a time series plot of the last five days of that observation.

	Wind Direction (WDIR):	ENE ( 60 deg true )
	Wind Speed (WSPD):	63 kts
	Wind Gust (GST):	78 kts
	Atmospheric Pressure (PRES):	28.81 in
	Pressure Tendency (PTDY):	-0.44 in ( Falling Rapidly )
	Air Temperature (ATMP):	76.6 °F
	Water Temperature (WTMP):	80.1 °F
	Dew Point (DEWP):	76.5 °F
	New! - Combined plot of Wind Speed, Gust, and Air Pressure

Continuous Winds
TIME (CDT)	WDIR	WSPD
11:00 pm	ENE ( 57 deg )	62 kts
10:50 pm	NE ( 50 deg )	63 kts
10:40 pm	NE ( 54 deg )	67 kts
10:30 pm	NE ( 53 deg )	66 kts
10:20 pm	NE ( 50 deg )	64 kts
10:10 pm	NE ( 50 deg )	64 kts

Off the top of my head this is how I would approach the problem.

Storm surge is a complex multivariate problem. Variables which effect its computation and modeling include: topology of the shoreline, topology of the seafloor gradients and inflexion points, air pressure, storm geometry (radius, distribution of wind velocities over the radii, velocity of rotation), storm velocity, tide levels, basin size, water currents, wave heights, and wind current distributions.

As a simple rule of thumb for Texan coast hurricanes, a storm with about 90 mph wind velocity, with 20 ft wave heights 100 miles out in the gulf might produce a storm surge of about 5 feet upon hitting land.

Other rules of thumb include one centimeter of storm surge for every millibar in reduced barometric pressure.

People should not be decieved by the wind velocities and relative comparisons they identify those wind speeds with during normal storms of less than 50mph winds/gusts.

Most structures built on the coast for hurricane codes may be designed for 100 mph winds or roughly 25 lbs/SF of pressure against an exterior wall. Inland structures are frequently built for about 80 mph winds or about 16 lbs/SF pressure from wind loads. Structurally, the design process typically multiplies the surface area of a building times that wind load, then multiplies that total times the height of the building 'center of mass' (centroid more accurately) to obtain a total moment for the design. The foundation of the structure is frequently designed to include any soil over the concrete foundation combined with the foundation to provide an overturning moment.

When posed with large winds, the wind forces vary proportionally with the square of the wind velocity. That means a 160mph wind doesn't merely double the wind forces' load on a structure from 80 mph, it quadruples the forces.

The forces of 70mph winds quadruple from 45 mph winds. Likewise they quadruple again to 135 mph.

Additional problems arise in that most structures, signs utilities, concrete sidewalks, you name it,,..really aren't deigned for much over 100mph (some specific designs might get designed to 120, but whether or not construction actually places them that rigorously is rarely tested, high rises for example usually get designed for higher wind loads).

For example, the increased water flows from storm water, rain, surf, greatly erodes soil conditions. Typically during hurricanes, grounds become saturated with water and the structural properties of the soil can change. Couple this with simple erosion where soil is physically removed, many designed structures might not even have the strength during storm conditions to remain intact in lessor wind loads.

Since these conditions aren't normal, they are rarely tested,..when they are, failures are expected as 'acts of God' and frequently lessons are too costly to find or redesign to overcome.

Additionally, people tend to tie things down to structures and previous design assumptions might not be obeyed by actual loading conditions. For example, somebody designs a signpost to withstand 125 mph winds, you get 100 mph winds and somebody ties down a trailer bewtween two signs,..the trailer might rip out the two signs along with the trailer flying,...or debris, not calculated for the impulse it renders when it strikes the signs, takes out everything that is tied together and the whole mess goes flying to strike something else.

Bottom line, is if you anticipate a large storm over 90mph winds,..don't hang around. There might not be anything to protect you from the elements.

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