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Posted at 11:15 AM ET, 09/02/2011

Tropical storm Lee poses significant flooding risk for New Orleans, central Gulf Coast


Rainfall projected over southeast Louisiana by the National Weather Service. (National Weather Service)

1:45 p.m. update: Tropical depression 13 has strengthened and been given the name tropical storm Lee. Positioned 200 miles southwest of Cameron, La., it has maximum sustained winds of 40 mph. Tropical storm warnings stretch from Pascaguola, Ms. to Sabine Tx, including New Orleans. The National Hurricane Center warns of “extensive flooding, especially in urban areas.”

Earlier:As tropical depression 13 drifts towards the central Gulf Coast, southern Louisiana - including New Orleans -, Mississippi, and Alabama face a dangerous flooding threat. Long lasting, torrential rains are possible as this very slow moving system, forecast to become tropical storm Lee, ingests deep tropical moisture from the warm Gulf waters. Some locations may be inundated by 10-20” of rain or even a little a more over the next several days.

Pinpointing exactly where the heaviest rain falls is difficult due to the weak steering currents guiding this system’s progression. NOAA’s Hydrometeorological Prediction Center cautions:

THIS IS A RATHER LOW CONFIDENCE FORECAST GIVEN THE CURRENT STAGE OF DEVELOPMENT OF THE T.D. [TROPICAL DEPRESSION] AND WEAK STEERING FLOW.


Projected track of tropical depression 13, forecast to become tropical storm Lee.
But the National Weather Service Office in New Orleans is taking no chances. In addition to the tropical storm warning (which extends inland over southeast Louisiana) issued for much of the central Gulf Coast, it has issued a Flash Flood Watch through the weekend stating:

MODEL ESTIMATES AND THE NOAA HYDROMETEOROLOGICAL PREDICTION CENTER INDICATES AN AVERAGE OF 10 TO 15 INCHES MAY OCCUR THIS WEEKENDACROSS THE WATCH AREA. LOCALIZED HIGHER AMOUNTS NEAR 20 INCHES ARE POSSIBLE...ESPECIALLY TO THE SOUTH OF LAKE PONTCHARTRAIN AND ALONG THE MISSISSIPPI COAST

And it offers this advice:

RESIDENTS AND BUSINESSES IN THE WATCH AREA SHOULD ENSURE THAT DRAINAGE DITCHES...CATCH BASINS...AND CULVERTS ARE CLEARED OF DEBRIS BEFORE RAINS ONSET TO ALLOW FOR ADEQUATE DRAINAGE.

AccuWeather is sounding the alarm bell about the threat, stating the Gulf System may cause “epic flooding” and be the next billion dollar weather disaster in the U.S. It writes:

It is possible Lee could be a Louisiana version of Allison (2001). Tropical Storm Allison stalled around the upper Texas coast in June of 2001 and unloaded up to 40 inches of rain and disastrous flooding, with damage at over $5 billion. Governor Bobby Jindal has placed Louisiana in a state of emergency in anticipation of Lee’s impact.

Given the uncertainty in this system’s development and motion, AccuWeather’s dire pronouncements may be the premature, but it’s difficult to deny at least the potential for serious rainfall.

In addition to rain, portions of the central Gulf Coast may als face tropical storm winds and a tidal surge. Around New Orleans, the NWS states:

MAXIMUM WINDS ARE FORECAST TO BE IN THE 35 TO 45 MPH RANGE WITH GUSTS TO 70 MPH.

As for coastal flooding and storm surge, the NWS warns:

THERE IS AN INCREASING CHANCE FOR COMBINED STORM SURGE AND ASTRONOMICAL TIDE WATERS UP TO 4 FEET ABOVE MEAN SEA LEVEL WITHIN AREAS CLOSER TO THE COAST

This surge would not top hurricane protection levees, but the NWS cautions outside the levees:

MINOR TIDAL FLOODING OF PROPERTY AND SOME FLOODING OF LOW LYING ROADS MAY OCCUR

Will this depression become tropical storm Lee and pack strong winds? Why is this so hard to track and predict?

by Greg Postel

In order for this depression to form, the widespread thunderstorm activity that has persisted over the Gulf for the last couple of days apparently tightened up the pre-existing broad rotation into a small, closed vortex sufficiently defined to be classified as a tropical depression.

Since the vortex is still relatively diffuse, the models aren’t so sure what to track. The low-level center could easily jump around a considerable distance until a dominant thunderstorm cluster appears.


Satellite pictures over the Gulf of Mexico are not particularly revealing. Though transient groups of cumulonimbus clouds well-populate the local atmosphere, there is still no coherent central core of long-lived convection (thunderstorms). Some of this may have to do with the fact that anarea of upper-level low pressure is situated over the northwestern Gulf Coast (yellow L in the picture).

This system may be partly responsible for enhancing the thunderstorm activity over the northern part of TD13 (in a non tropical way), while at the same time imparting an ostensibly destructive westerly shear across the rest of the depression to the south.

Evidence of this shear is noted by the stark eastern bias in the overall cloud pattern. The satellite imagery clearly shows the sharp cutoff in the cold cloud tops west of a New Orleans longitude.

In addition to, but not unrelated to, the sheared environment over the Gulf, dry air, as (as shown by the dark and bronze areas in the picture) continues to be drawn into TD13 by the westerly winds on the south side of the upper low. Given that the inhalation of dry air can be potentially fatal to a fledgling tropical cyclone, the overall development prospects for TD13 don’t look good unless things change.

With NHC expecting the shear to gradually relax and the environment to slowly humidify, the consensus is that TD13 will intensify into a tropical storm very soon. As far as I can tell, this is predicated on the anticipation that the upper vortex will back off and leave the Gulf under a more tranquil environment. If that doesn’t happen, we would expect TD13 to continue to struggle in the less than hospitable environment that currently surrounds it.

As such, the biggest threat TD13 poses is excessive rainfall as discussed above. Winds could strengthen to tropical storm force, but the NHC and NWS are rightly placing emphasis on the rain threat.

Hurricane Tracking Center

By and  |  11:15 AM ET, 09/02/2011

Categories:  Latest, Tropical Weather

 
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