The touchdown of Hurricane Ida class 4 – wadding with it?

Reposted from the Cliff Mass Weather Blog

A severe hurricane hits the Gulf in exactly the wrong place for New Orleans and the surrounding area.

Hurricane Ida, a Category 4 hurricane with winds of up to 250 miles per hour, is now on land.

The latest weather radar image (which shows precipitation intensity) clearly shows the eye of the storm (letter E!) Just reaching the coast of Lousiana (below).

Ida has a compact eye, with the wall of the eye visible in the band of precipitation indicated by the blue arrow. A secondary eye wall appears to be forming (red arrow) which temporarily weakens the storm (it could have become a Category 5 storm if the second eye wall has not formed.

The infrared satellite image (which essentially gives the temperature of the clouds and surface) is impressive – the eye is small but well defined, with tall clouds (red colors) surrounding it.

The small size of the eye is a very good thing as the strongest winds are confined to the area in the storm’s eye wall. Here are the latest wind reports (10 a.m.) with the gusts displayed in red (click the image to enlarge). Gusts up to 89 miles per hour. Note the strong easterly winds (from the east) moving towards Lake Pontchartrain and New Orleans. Not good … pushes water ashore in the form of a storm surge.

The maximum wind gusts (see below) are up to 128 mph

The storm gets weaker as it moves overland, but expect extremely heavy rainfall, with totals 15-20 inches in southern Louisiana and large amounts of rainfall stretching into the northeastern United States (see NOAA / NWS forecast below).

Predictability of the storm

The forecast of Hurricane Ida is classic: extremely good forecasts for the storm follow many days, but the forecast of the storm intensity (central pressure and maximum winds) underestimated the storm.

The forecast tracks for the large US and Canadian models on Friday morning were quite good, but were a little too far to the west. (black line shows the trace being observed).

Yesterday morning the route forecast was excellent.

But what about the intensity forecast on Friday? A measure of the intensity is the central pressure of the storm (the lower the pressure, the more intense). The latest observations show that the lower center is down to 930 hPa (hPa is a unit of pressure). Here are the predictions from yesterday morning (black line indicates what was observed). None of the predictions brought the storm deep enough. As a result, their wind forecasts were too low.

Like this graphic from Dr. Brian Tang, University of Albany, shows the winds were about 25 knots too slow for the 47 hour forecast and about 50 knots too slow for the 72 hour forecast.

Why do our models often fail to get the intensification right? One reason is the lack of resolution. To simulate such storms, one has to simulate the hurricanes with model grids of about a kilometer or better, but the National Weather Service lacks the computer resources to do so.

Then there are flaws in the model physics, such as how sea spray humidifies the air over the breaking waves. Or how cooler water mixes vertically in the upper sea surface.

And then this is the need for model data to describe the hurricane structure off the coast.

An apparently significant problem for Hurricane Ida was a patch of deep warm water over the Gulf of Mexico, right on the storm’s trail (see below). Such warm water is very helpful in intensifying a storm.

Finally, there is one thing to note: our ability to predict the tracks and, to a lesser extent, the intensity of hurricanes, is an effective tool in reducing damage and casualties from tropical storms and hurricanes.
That is the reason for the deaths from tropical storms are much less today than 50-100 years ago, even with a lot more people on the planet.

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