Gulf of Mexico disturbance 93L a Lousiana flood threat; Katia a hurricane
Surface winds over the northern Gulf of Mexico are rising, pressures are falling, and heavy thunderstorms are building today thanks to a tropical disturbance (Invest 93L) that is the product of a tropical wave interacting with an upper-level low pressure system. At 8:35 am CDT, winds at the Mississippi Canyon 711 oil rig were south-southeast at 38 mph. This is just 1 mph below tropical storm force, but the wind instrument was 348 feet (106 m) above the ocean surface, and winds near the surface were probably considerably lower, near 30 mph. Long range radar out of Mobile, Alabama shows heavy rain showers building along the northern Gulf Coast, but these rain showers are not organized into spiral bands and show no signs of rotation. Strong upper-level winds out of the west-northwest are creating 30 knots of wind shear over 93L, keeping the storm's heavy thunderstorms disorganized. Strong onshore winds raising tides to 1 - 2 feet above normal are likely along the northern Gulf Coast through the weekend, and coastal flood statements have been issued for the region.
Figure 1. Predicted rainfall for the 5-day period ending at 8am EDT Sep 6, 2011. A large region of rains in excess of 15 inches is expected over Southeast Louisiana. Image credit: NOAA/HPC.
Figure 2. U.S. drought conditions on August 30. The rains from 93L have the potential to bring major drought relief to drought-stricken portions of the coast. Image Credit: U.S. Drought Monitor.
By late tonight, wind shear is expected to drop to the moderate range, below 20 knots, and 93L should begin to organize into a tropical depression. Wind shear is expected to remain moderate, 10 - 20 knots, into Monday. There is some cold, dry air aloft that will retard this process, and I think the earliest we would see a tropical depression is Friday afternoon. NHC is giving 93L a 70% chance of developing into a tropical depression by Saturday morning. All of the major models develop 93L near the Louisiana coast, and show a slow and erratic movement due to weak steering currents. Coastal Louisiana, Mississippi, Alabama, and the extreme western Panhandle of Florida will likely receive very heavy flooding rains beginning this afternoon and intensifying Friday and Saturday. The latest rainfall forecast from NOAA Hydrological Prediction Center (Figure 1) shows that a large area of 15+ inches of rain is expected over Southeast Louisiana. The region is under moderate drought, so flooding problems will be delayed compared to what we'd normally expect from heavy rains of over a foot. Nevertheless, minor to moderate freshwater flooding is likely from 93L, and flash flood watches are posted for New Orleans and surrounding areas, and rainfall amounts of 1 - 3 inches per hour are possible in some of the heavier rain squalls. Ocean temperatures are near record warmth, 88°F (31.3°C), which will provide plenty of moisture for heavy rains, and plenty of energy to help 93L strengthen into a tropical storm. Most of the models predict 93L will have some motion to the west by Saturday, which would bring rains to the Texas coast near the Louisiana border. Steering currents will be weak in the Gulf this weekend and early next week, making it difficult to predict where the storm might go. If 93L stays over water through Tuesday, like the ECMWF model is predicting, the storm would be a threat to intensify into a hurricane. Most of the other models predict 93L will move ashore over Louisiana by Sunday, limiting the storm's development to just tropical storm strength. I think it at least 50% likely 93L will be a tropical storm with 40 - 60 mph winds along the coast of Louisiana by Sunday.
Hurricane Katia
Hurricane Katia intensified into the 2nd hurricane of the 2011 season last night, and continues its long trek across the Atlantic Ocean today. Katia is expected to arrive at a position several hundred miles north of the Northern Lesser Antilles Islands by Monday. The islands are not in the cone of uncertainty, and it appears unlikely that they will receive tropical storm-force winds from Katia. Satellite images show that Katia is a well-organized storm with plenty of heavy thunderstorms, but the storm has been struggling with dry air and moderate wind shear of 10 -20 knots, and is looking less organized than it did last night. These problems will likely diminish by Friday night, as the upper low bringing the wind shear moves away. It is still unclear how much of a threat Katia may post to the U.S. Dr. Bob Hart's Historical Tropical Cyclone Probability web page suggests shows that tropical storms in Katia's current position have an 16% chance of hitting North Carolina, a 21% chance of hitting Canada, a 12% chance of hitting Florida, and a 54% chance of never hitting land. I suspect that Katia will turn north before reaching the U.S. and potentially threaten Bermuda and Canada, based on what past storms in similar situations have done, and assuming the jet stream maintains its current pattern of bringing frequent troughs of low pressure off the coast of the U.S. It will be another day or two before the models will begin to have a handle on the long-term fate of Katia, though.
Figure 3. Morning satellite image of Katia.
94L
A well-organized low pressure system with a surface circulation and limited heavy thunderstorm activity has developed between Bermuda and the Canadian Maritimes. This disturbance, (94L), is headed out to sea, and is being given a 10% chance of developing into a tropical depression by NHC. 94L is under a very high 30 - 40 knots of wind shear, and will not be able to intensify very much. However, Tropical Storm Jose formed from a similar type of system, and we might get surprised by 94L.
I'll have more on Irene in tomorrow's post.
Jeff Masters
Reader Comments
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actually yes
Not if it parks somewhere that forces them to close the gates.
Takes it over this area....washout ahead....
If they initialized advisors at the same time (at TS strength) then 93L being the first invest would get Lee and 94L Maria.
the one south of the one "racing" off to the west will win out. It is meandering, but pulling energy to it. It's about a few hundred miles south of the one that will NOT develop.
we will have a cat 3 hurricane in the GOM inside of 60 hours.
you heard it here first.
a cat 5 in 4 days
The Pumps, "pump" the Rainfall over the Gates Guy,, thats how they keep the pressure off the walls.
they will do fine with 93L..its the blog that will have issues with 93L and the NHC..
I just have a question, why do people post old graphics? This is from July. I see this quite often and am just curious.
Part of the job. They're used to it.
What keeps the pressure off the walls is reducing the flow capacity of the outfall pumps to match that of those ricky-ticky things the corps installed. They can't do a half inch an hour with the gates down, especially out the London Avenue Canal. I just hope that is the COC and it's off to visit TX and drags all that convection with it.
Fortunately New Orleans has extensive ponding storage areas. Unfortunately its where we park our cars.
DOOMcaster!
What are the chances of the large amounts of convection east of the system becoming organized?
No W winds,no Lee
Every once in a blue moon, the NHC screws the pooch on track. And I mean every once in a while. For the most part, they're pretty much scary accurate within 50-100 miles.
But when you have a steering pattern that would give even the best meteorologist a severe nosebleed, you do the best you can with the technology and inner gut that you can do.
Afterall, like us....They are only human.
Katia is attempting a comeback on the convection front but the low level center continues to push to the West. The combination of shear and a fast forward speed is preventing the new blow up convection from overtopping the low level center. I just looked at the ADT center fix and it has it right underneath the big blow up which clearly does not square away with the visible loop.
There is about 20 knots of Westerly shear ripping across the top half of Katia and stripping away the CDO to the Eastern semi-circle of the circulation. With 18 mph forward speed and 20 knots from the West over the top the convection will continue to lag behind.
The system is near 49W which is only about 660 miles to the East of the Leeward Islands and closing quickly. Several of the models will probably respond by taking Katia very near if not just over those Islands on the NE of the chain.
On monday we will use at least 2 of them ..... with only 8 names remaining the Greek Letter is more possible than ever !
NEXRAD Radar
Type
Velocity Azimuth Display Wind Profile ° Elevation
Range
124 NMI
They will likely find a closed circulation, IMO.
Haven't seen any W winds
72 hour 12Z ECMWF....
I agree.
Viewing: 401 - 451
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