11:00 a.m. update: The National Hurricane Center reports Karen’s maximum sustained winds have weakened to 50 mph. It expects minor fluctuations in intensity through Saturday with perhaps some modest strengthening in the hours prior to landfall.

From 10:27 a.m.: Tropical Storm Karen is about 300 miles south of New Orleans and will make landfall along the northern Gulf Coast most likely Saturday night.  Hurricane and tropical storm watches span four states, and tropical storm warnings are now up for eastern Louisiana.

New Orleans and Panama City, on the (west and east) edge of the storm’s likely track, are under tropical storm watches. Biloxi, Miss., Mobile, Ala., and Pensacola, Fl. – in the most likely zone for Karen’s landfall – are under hurricane watches.

The storm’s projected track and the extent of hurricane and tropical storm watches/warnings are shown here:

As of the 11 a.m. EDT advisory from the National Hurricane Center, Karen’s maximum sustained winds were 50 mph and was moving north-northwest at 10 mph.  It is forecast to maintain this intensity (+/- a few mph), and track north for another day or so before making a gradual turn to the northeast.

Storm is disorganized; modest intensification possible, but not guaranteed

At the moment, Karen is not the healthiest storm.  Due to a combination of strong vertical wind shear and extremely dry air nearby, Karen went from an ominous and intensifying storm Thursday to a skeletal system gasping for moisture.  That struggle continues today – the surface circulation is mostly exposed, and all of the thunderstorm activity is displaced to the east of the center.  If anything is certain in hurricane forecasting, it’s that a storm under these conditions will not rapidly intensify.

The plot below is a vertical slice though the center of the storm taken from the HWRF computer model (0600 UTC run, valid at 11 a.m. EDT today). The colored shading is the wind speed, the black line contours are the relative humidity, the x-axis is the distance from storm center (in kilometers), and the y-axis is the height (in millibars).  The key points illustrated by this are the asymmetry and shallowness of the circulation, and the insanely dry air to its west (mid- and high altitude relative humidity of less than 30 percent).

The vertical wind shear is close to 30 mph this morning, and is forecast to increase to about 40 mph by the time it makes landfall.  The environmental mid-level moisture should remain fairly constant, but constant means dry in this case.

There is a narrow window of time (Saturday evening) when the shear could relax a bit, giving the storm an opportunity to intensify just prior to landfall, but it would be a difference of 10-15 mph in peak wind speed at best.  Currently, no models are forecasting Karen to become a hurricane (however, since it is a small possibility, hurricane watches are in effect).

Landfall timing and location somewhat uncertain

The exact timing and location of landfall is admittedly quite uncertain now.  Track forecasts among the various models are diverging, telling us that they are very sensitive to small differences in the models as well as details pertaining to the timing and strength of a mid-latitude front sweeping down into the southeast U.S.

As of the model runs from early this morning, landfall location ranged from central Louisiana to the central Florida panhandle. The National Hurricane Center official track forecast (shown above) averages together the best performing models, which are in reasonably good agreement focusing landfall around southeast Louisiana.  The storm should cross the coastline in the early morning hours on Sunday (or late Saturday night).

Storm surge likely moderate and confined to narrow zone

The storm surge associated with Karen should not be a major threat.  The latest surge model guidance indicates a 10% probability of a 3-5′ surge along the Mississippi coast, but any slight shift in the forecast track will shift the surge location too.  The greatest surge will occur to the east of where the center comes ashore, and extend eastward from there (the areas of onshore flow).  As Hal Needham at Louisiana State University pointed out in his blog post this morning, this area is no stranger to destructive storm surge (see “Karen’s Surge May Be a 5 or 6 Year Coastal Flood Event for Portions of SE Louisiana and Mississippi“).

Inland rainfall flooding a more widespread threat

As Karen crawls up through the southeast U.S., it will be interacting with a cold front, streaming moisture out ahead of it and adding fuel to the mid-latitude system.  This is expected to result in widespread heavy rainfall that will most likely cause some flash flooding problems over many states.  The maps below break the forecast rainfall into chunks, with the total over the coming week shown in the bottom right.

Finally, a little bit of trivia that my colleague Phil Klotzbach at Colorado State University wanted to share.  So far this season, the strongest storm has reached 75 knots (or around 85 mph, in Humberto and Ingrid).  If that continues, it will be the first season since 1968 to have such a weak “most intense” storm.

Video: Gulf Coast preps for possible hurricane