Archive for August 27th, 2012
Isaac Still a Tropical Storm
The 10 p.m. advisory package is in…
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LOCATION…27.1N 87.0W
ABOUT 190 MI…305 KM SE OF THE MOUTH OF THE MISSISSIPPI RIVER
ABOUT 255 MI…410 KM SSE OF BILOXI MISSISSIPPI
MAXIMUM SUSTAINED WINDS…70 MPH…110 KM/H
PRESENT MOVEMENT…NW OR 310 DEGREES AT 10 MPH…17 KM/H
MINIMUM CENTRAL PRESSURE…979 MB…28.91 INCHES
The pressure is low for a tropical storm, but that probably reflects the large size and relaxed pressure gradient.
The recon planes in the storm still can find no signs of significant strengthening.
Issac will likely become a hurricane overnight.
Tropical storm force winds are about to reach the mouth of the Mississippi River.
The projected track is shifted slightly back east by a few miles, over the city of New Orleans.
The projected intensity at landfall is back to 90 mph. Landafall should occur near Buras LA around 7 p.m. tomorrow night.
Strong tropical storm force winds (>58 mph) will reach New Orleans about midnight tomorrow night. Hurricane force winds will affect the city around 6 a.m. Wednesday, for about 4 hours, with slowly improving conditions Wednesday night and Thursday.
The forecasted forward speed has picked up a little bit after landfall, which will help somewhat with rainfall. Isaac wil be in southern Arkansas by Thursday evening, at which time it should no longer be tropical in nature.
A Center Exposed
Isaac is still right on the verge of being classified as a hurricane tonight. But the Hurricane Center has made a good cal so far not upgrading it, yet.
Here are the FAST FACTS on the storm at 7 PM:
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LOCATION…26.7N 86.5W
ABOUT 230 MI…370 KM SE OF THE MOUTH OF THE MISSISSIPPI RIVER
ABOUT 295 MI…470 KM SSE OF MOBILE ALABAMA
MAXIMUM SUSTAINED WINDS…70 MPH…110 KM/H
PRESENT MOVEMENT…NW OR 305 DEGREES AT 10 MPH…17 KM/H
MINIMUM CENTRAL PRESSURE…981 MB…28.97 INCHES
After a strong burst of convection during the day, dry air wrapping into the circulation really knocked the storms near the center down and that center is exposed tonight. But new deep convection was wrapping around the southern side of the center and it will be interesting to see if the convection can hold together overnight.
Top winds are 70 mph. The reconnaissance planes have found lots of 70 knot flight level winds in both semicircles of the storm. But the observations from the planes have not indicated SFMR data that supported upgrading it to hurricane strength. Still, don’t be surprised to see that happen shortly.
Extrapolating the fixes still indicates a steady northwest motion. Several of the models and the official forecast all indicate that the storm will slow over its forward motion over the next 24 hours. Isaac is expected to make landfall near or just west of the Mississippi river late tomorrow afternoon. It will move slowly northwest, passing New Orleans late tomorrow night. It will then take nearly 24 hours for it to make the short jog to Baton Rouge.
This could lead to some biblical rainfall amounts over southeastern Louisiana. At least two of our models show the storm stalling just northeast of New Orleans, keeping South Mississippi and Southwest Alabama in the area of extreme rainfall. Let’s hope we don’t have another Hurricane Danny, which dumped over 36 inches of rain in 24 hours on Dauphin Island in 1997 as it stalled over Mobile Bay.
The big storm, its slow movement and extended duration of onshore winds in the eastern semicircle will push large amounts of water onto the coast in the funnel near the Louisiana/Mississippi border, the Mississippi coast and into Mobile Bay. Tides may reach 6-12 feet in parts of this area. The probability of a surge six feet or greater is currently 10% in Mobile Bay, 30% at Biloxi, 40-50% at Gulfport, and up to 60% on the eastern facing shores of the parishes east and southeast of New Orleans.
Tornadoes will be possible tomorrow over southern Alabama, southern Mississippi, southeastern Louisiana, the Florida Panhandle and southwestern Georgia. This area will expand northward to a iine along US-78/US-280 in Alabama on Wednesday.
A wide area over Mississippi, southeastern Louisiana and southwestern Alabama will see between 10-20 inches of rain from Isaac. Over Central Alabama, we are forecast to see 3-5 inches of rain as heavier outer bands set up across the area at times.
Join us on the WeatherBrains podcast for a special Isaac show at 8:30.
Almost a Hurricane
The new package is out on Isaac. Stops just short of upgrading to hurricane.
Pressure down to 981 millibars. Winds on the advisory at 70 mph. Movement is NW at 12.
The plane found a widespread area of 60+ knot flight level winds all along their flight path from west to east through that major convective mass south of the center.
The storm still looks like it is still being sheared and there is dry air wrapping into it on the eastern side. This is keeping it from really strengthening rapidly.
The forecast does make it a category two with top winds of 100 mph right before landfall.
The official track was shifted ever so slightly to the left with the skinny black line we’re not supposed to pay attention to running just west of New Orleans now.
FROM TWITTER:
Tweet from NWS Mobile: Large swells from TS #Isaac with 11 second periods set to impact the area beaches tonight with breakers up to 8 feet.
Amtrak stopping City of New Orleans at Memphis.
Alabama Power: We’re bringing in 300 line-crew workers from across the Southeast, in case #Isaac brings trouble to Alabama:
Tropical Soaking Ahead For Alabama
An all new edition of the ABC 33/40 Weather Xtreme video is available in the player on the right sidebar of the blog. You can subscribe to the Weather Xtreme video on iTunes by clicking here.
Be sure and watch the afternoon Weather Xtreme video for all of the graphics that go along with this discussion….
New NHC track just out…
RIGHT NOW: Most of Alabama is rain-free, although the first feeder bands of Isaac have moved into the southeast part of the state. From here on this, our weather will be determined by Isaac.
ISAAC NOW: A new NHC update will be issued soon, probably by the time you read this, and it could be a minimal hurricane. It is certainly close now with sustained winds of 65 mph. Isaac is moving NW at 14 mph.
TRACK: We are now very confident that Isaac will move into Southeast Louisiana, around the mouth of the Mississippi, tomorrow, followed by a recurving track through Louisiana and Arkansas with gradual weakening as the system moves inland. Don’t expect much change in this between now and time of landfall. Alabama will be on the wet, unsettled side of the circulation (the east side).
INTENSITY: Three things I believe have prevented Isaac from growing stronger rapidly… dry air (clearly seen on the southern and eastern sides now on satellite images), the large nature of the circulation (it needs to be smaller and tighter for rapid deepening), and the shallow nature of the warm Gulf of Mexico temperatures. The SSTs are pretty warm, but the warm water is not very deep. Accordingly, I think NHC is correct in showing Isaac as a category one hurricane at the time of landfall.
COASTAL ISSUES: No, there won’t be major structural damage with a category one hurricane, but I do think there will be some power outages along the Gulf Coast, especially from New Orleans over to Orange Beach, with some potential for downed trees from the gradient wind. A few tornadoes are possible from New Orleans all the way around to Apalachicola tomorrow as Isaac makes landfall.
But, the biggest issue to me is the potential for flooding. NHC suggests a 6 to 12 foot storm surge will effect the MS/AL Gulf Coast late tomorrow, which means some coastal flooding. No, nothing like Ivan or Katrina, but still this is significant.
And, the rains will be tremendous. Guidance shows a very slow motion through Louisiana, and some of the QPF charts are showing rains of 10 to 20 inches over the next 72 hours for places like Gulfport, Biloxi, Mobile, Daphne, Foley, Gulf Shores, and Orange beach. Flooding, most likely, will be the biggest problem associated with Isaac for the Gulf Coast. Conditions will slowly improve by Thursday along the coast.
UP THIS WAY: Let’s go to bullet points….
*POWER OUTAGES? I don’t think the gradient wind will enough over North/Central Alabama for major tree damage, or widespread power outages. No doubt it will be breezy tomorrow night into Wednesday, with south winds of 15-25 mph, but that isn’t enough to really cause serious issues. We are not recommending that people in mobile homes leave.
*FLOODING POTENTIAL: With the slower motion, I now think the heaviest rain here will come Wednesday, and possibly into the day Thursday. For the I-20/59 corridor (Tuscaloosa, Birmingham, Anniston, Gadsden), rain amounts of 3 to 5 inches are likely, with lighter amounts up north over the Tennessee Valley. Not sure we see all that much flooding around here, but it is possible.
The most significant flooding threat for inland areas of Alabama will come over the southwest part of the state, generally south of a line from Livingston to Demopolis to Greenville and south to Andalusia. This part of Alabama could see 5-12 inches of rain through Thursday.
*TORNADO THREAT: I do believe a few small, isolated tornadoes are possible across North and Central Alabama Wednesday and Thursday. Remember, the tornadoes in spiral bands around a decaying tropical system tend to be small, short lived, low topped, and hard to detect. They are not like the truly violent tornadoes we see in spring or fall, but that can produce significant damage, and the NWS does issue tornado warnings if they believe one has formed. So, be close to a good source of severe weather information this week in case warnings are needed.
*TRAVEL: I believe incoming and departing flights from Birmingham-Shuttlesworth Airport will not be hampered too much by Isaac, but there might be a ground stop or two along the way in event of torrential rain, or a tornado warning. If you are flying to the Alabama/Michigan game in Dallas this week you should be able to get there just fine, although your flight from Birmingham might be delayed a bit.
If you are driving somewhere by car, just be aware of the flooding potential; that is the biggest concern. And, be sure you can hear tornado warnings if they happen to be needed.
*SCHOOL CLOSINGS? I honestly don’t think they will be needed, but if the flooding threat becomes more serious that could change.
*LABOR DAY WEEKEND: Isaac will be long gone by the holiday weekend, so it won’t interfere with a beach trip. But, very tropical air rich in moisture should linger across the Deep South, meaning you will have to dodge at least scattered showers and storms through Monday with a fairly limited amount of sunshine each day.
WEATHER BRAINS: Don’t forget you can listen to our weekly 90 minute netcast anytime on the web, or on iTunes. This is the show all about weather featuring many familiar voices, including our meteorologists here at ABC 33/40. We will produce this week’s show tonight at 8:30 as usual… you can watch it live here.
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Stay tuned to the blog for running updates on Isaac through the week. Also, thanks to my friends at the Walker County ARC in Jasper for allowing me to drop by for a while today. They were great audience.
Look for the next Weather Xtreme video here by 7:00 a.m. tomorrow…
Hurricane Force?
The Air Force Hurricane Hunter plane is finding a large area of +70 knot flight level winds on the southwest side of the storm in that large convective mass.
I saw one 72 knot flight level wind, which would discount to a little over 65 knots, which is hurricane force.
We may see Isaac upgraded to a hurricane shortly.
