Little change to Isaac, but storm poses a serious storm surge threat

By: Dr. Jeff Masters , 3:25 PM GMT on August 27, 2012

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Tropical Storm Isaac has changed little in strength or organization this morning, as the storm heads northwest at 14 mph towards the Central Gulf Coast. There are two hurricane hunter aircraft in the storm, and Isaac's central pressure held steady at 989 mb at the 8:30 am and 9:15 am center fixes. Top surface winds remain near 65 mph. Infrared and visible satellite loops show that Isaac is a very large storm, but isn't very symmetric. Heavy thunderstorm activity is lacking on the southeast side, where 10 knots of wind shear is driving dry air into the circulation. The center is surrounded by a ring of echoes now, which was not the case on Sunday. However, the echoes are weak. The 8:30 am center report from the Hurricane Hunters reported about half of a ragged eyewall, but the 9:15 am report did not mention any evidence of an eyewall. Isaac will have to form an eyewall in order to intensify significantly.


Figure 1. Morning satellite image of Isaac. Note the lack of heavy thunderstorm activity on the storm's southeast side, where dry air and wind shear are combining to interfere with development.

Isaac's rains
Isaac's heaviest rains have fallen along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. West Palm Beach received 7.57" of rain from Isaac as of 10 am EDT this morning. A trained spotter in Western Boynton Beach reported 10" of rain from midnight to midnight Sunday. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least nineteen, and two died in the Dominican Republic.


Figure 2. Radar-estimated rainfall from Melbourne, Florida radar shows that Isaac has dumped a wide swath of 3+ inches of rain (orange colors) across the state.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show major differences in what happens after that. It is still uncertain if a trough of low pressure passing to the north of Isaac will be able to turn the storm due north, as the ECMWF model is predicting, or bypass the storm, allowing a more west-northwest motion into western Louisiana, like the GFS model is predicting. In either case, Isaac is likely to slow down as it approaches the coast, which will increase the damage potential fro m its wind, storm surge, and rains. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. The ECMWF model predicts that these heavy rains will fall more over Mississippi. It appears likely that Arkansas will see some heavy rains from Isaac late in the week, which would help put a dent in the exceptional drought conditions there.


Figure 3. Predicted precipitation for the 8-day period from 2 am Monday August 27 to 2 am Tuesday September 4, from the 2 am EDT August 27 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Louisiana. Additional very heavy rains are predicted for the Midwest, as moisture from Isaac interacts with a cold front. Image credit: NOAA/NCEP.

Intensity forecast for Isaac
Isaac is currently crossing over a relatively cool eddy of water, which will keep intensification slow today. By tonight, the total heat content of the waters increases, which should aid intensification. Low wind shear of 10 knots or less is likely until landfall. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow to the north is not as strong as yesterday, which should also slow intensification today. The models forecast the upper-level outflow should improve by Tuesday. A storm this large will have trouble undergoing rapid intensification, and Isaac's most likely intensity at landfall will be as a Category 1 hurricane, which is what most of the intensity models are forecasting.


Figure 4. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Storm surge forecast for Isaac
Storm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1' this morning. Conversely, water levels have fallen by 2' this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac's counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac's storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac's predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5' high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extending pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

The affect of storm size and angle of approach on storm surge
A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Isaac's storm surge will provide the first test of the newly-completed New Orleans levee system upgrade. In the wake of the disastrous storm surge flooding from Hurricane Katrina in 2005, Congress appropriated $14.5 billion to upgrade the New Orleans levee system to withstand a Category 3 hurricane storm surge. Katrina was a Category 3 storm at landfall, but the storm passed far enough to the east of the city that its storm surge was characteristic of a Category 1 - 2 storm at the places where the city's flood walls and levees failed. The new flood defenses were only partially completed in time for the arrival of Hurricane Gustav in 2008, which hit Central Louisiana as a Category 2 storm. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. Since that time, the imposing 2-mile long IHNC Flood Barrier has been completed to block off the funnel-shaped pair of canals on the east side of the city. I expect New Orleans' new flood defenses will be able to hold back Isaac's surge, but areas outside the levees are at risk of heavy storm surge damage.


Figure 5. A portion of New Orleans' new $14.5 billion dollar flood defenses, as taken from an Army Corps of Engineers map.

New Orleans flood defense info
Army Corps of Engineers map of the new flood defenses
Army Corps of Engineers video showing the flood defenses
New York Times article, Vast Defenses Now Shielding New Orleans

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1050 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Wednesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands, and high wind shear should begin to tear the disturbance apart on Tuesday.

Another tropical wave that moved off the coast of Africa on Sunday is located just south of the Cape Verde Islands. This disturbance is moving west at 10 - 15 mph, and could arrive in the Lesser Antilles around September 2. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Wednesday morning.

Angela Fritz will have a new post here by 6 pm EDT. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will doing a few 3-minute tropical updates at 30 minutes past the hour between 2:30 - 7:30 pm EDT today.

Jeff Masters

Tropical Storm Isaac (chelina)
View from the north side of the Sunshine Skyway Bridge.
Tropical Storm Isaac
our street at noon today (seflagamma)
Aug 26, 2012: Isaac floods our street really bad. Now nearly 11
our street at noon today

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Quoting Methurricanes:
at 949 with thsi storm thats what 80mph?


949 Mb pressure is a strong Cat.3
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Quoting Kristina40:
We just had a quick shower here in Panama City and the breeze is picking up a bit. Some of the bands are getting close now.
Getting darker and breezier here too, but so far no rain yet. Very gloomy.
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I see we just had a WU guy on TWC. He did a good job.
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Quoting seflagamma:


Hard to be here when you are trying to keep water out of your house and vehicles...

plus so many are now without power...


Yeah, there is a crisis developing out it Wellington, severe flooding. I do apologize the coverage of Isaac is minus a few bloggers. No disrepect and not intentional on my part at least.

Heading out to Wellington in 30min to try and rescue my mother in law from work and check the flooding situation at her house. I'll get pics and post them however, I may not be able to get there.
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Quoting StormTracker2K:



NW side power flickered when and intense squall moved thru at about 5:30am.



At news time the outages were down to around 500. That was only OUC and Progress Energy Customers in CFL. Don't know about FPL Customers in the area
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Quoting Aquaimage13:
How bad do you guys think this will affect that massive sinkhole in LA that is overtop the butane reserves? Crazy to think about..
http://www.examiner.com/article/louisiana-activat es-4000-national-guardsmen-securing-sinkhole-equip mentThey are not taking Isaac lightly but there's really only so much they can do. If it blows during the storm and all that stuff gets dispersed. I'm not even sure, what is under it in the cavern? Butane? By products from the refining process?
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Quoting seflagamma:


Hard to be here when you are trying to keep water out of your house and vehicles...

plus so many are now without power...
When I had water coming in my house, I threw the main so I wouldn't get electrocuted.
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Member Since: August 6, 2011 Posts: 4 Comments: 39309
John Oldshoe!!! Great Job!!!
Member Since: August 16, 2007 Posts: 0 Comments: 397
Quoting WeatherMSK:


I really think you should be taking care of business rather than posting online about your problems, otherwise i would say you are making a bigger deal out of nothing.
FYI, as if it is any of yours, I have help here and am disabled. Pfft! Pics soon for such a person so concerned with my welfare.
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Flash flood warning issued till 430pm eastern broward and palm beach counties.
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Quoting sarahjola:
wow! asked a weather question and got 2 answers and none were weather related. this blog used tohave people who wanted to talk about weather, but now it seems as though all people want to do is disagree and not really get into weather discussions. i'll just keep checking imagary and forget about asking questions. thanks anyway weather blog :)


I never saw your question. Feel free to ask again.
Member Since: June 24, 2012 Posts: 0 Comments: 472
Sage advice:

Link

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Quoting MississippiWx:
Might have been posted already, but HWRF shifts to Mississippi Coast.

at 949 with thsi storm thats what 80mph?
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Quoting LargoFl:
good luck ok, this is far from over for us here on the gulf coast


TS warnings now dropped up all west coast of FL to App bay.
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Got us off the rig yesterday evening. From Morgan city to Denham springs La there was either no gas or lines at the pumps. Got gas this AM, ran the generators, 3 cases of beer, tequila, and margarita mix........I'm ready now.
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Quoting Dsntslp:
Well, the drain is not clogged. It is just too much water coming in too fast at once down the street and down the small canal at the same time and it is too much for the drain so it is taking the path of least resistance over the top of the drain. Nothing can be done so I am off to go try and figure out how I can...ohhhhhhh!!! I can open the pump valve on the pool! It will purge water from the pool into the already overflowing canal behind my house but it will allow the water in my home to go back in the pool! BBL!! :)


I really think you should be taking care of business rather than posting online about your problems, otherwise i would say you are making a bigger deal out of nothing.
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wow! asked a weather question and got 2 answers and none were weather related. this blog used tohave people who wanted to talk about weather, but now it seems as though all people want to do is disagree and not really get into weather discussions. i'll just keep checking imagary and forget about asking questions. thanks anyway weather blog :)
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The ECMWF model seems to have predicted the landfall area better. It helps to have greater resolution using spectral methods.
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Quoting NCHurricane2009:

Last night we were sayin the same thing...

But realistically...I still believe he'll be a cat 1 before landfall...but I just think its amazing that a storm in apparently favorable conditions is takin its sweet time gettin its act together....


There's a difference in looking at infrared and visible to determine an eye wall. Visible imagery is much easier to determine.
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Quoting NativeSun:
How strong is the front over Central Texas and Arkansas? Looks to be moving South and East, which if strong enough should push the storm towards the N/NE as it approaches the coast near the AL/Miss line.


I don't think it's that strong. I don't believe it's progged to make it through the HGX area.
Member Since: June 24, 2012 Posts: 0 Comments: 472


More detailed look at MSWX was pointing out about closing off an eye...
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Quoting WxLogic:
Isaac starting to moisten up from the S... on its way to be a Cat 1 soon.


Tonight may be his last chance at optimal conditions, it would be concerning if he got his core built this afternoon entering the diurnal maximum tonight into tomorrow morning.

If you ask me he's getting better and better defined. I just hope people on the Gulf Coast don't let their guards down. This thing still has plenty of time to do mischievous things.
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I suspect a lot of people are burnt out from late nighters tracking Isaac or those no longer in the path have lost some interest in him, but I suspect they will be back when he is about to make landfall. I'm still keeping my eyes on him to see where he will make landfall. Weather here in Tampa isn't too bad with the occasional rain bands and it really wasn't ever expected to be bad. The trajectory of Isaac would have made it extremely difficult for us to get a direct hit and even then the most it would have been was a minimal hurricane.
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Well, the drain is not clogged. It is just too much water coming in too fast at once down the street and down the small canal at the same time and it is too much for the drain so it is taking the path of least resistance over the top of the drain. Nothing can be done so I am off to go try and figure out how I can...ohhhhhhh!!! I can open the pump valve on the pool! It will purge water from the pool into the already overflowing canal behind my house but it will allow the water in my home to go back in the pool! BBL!! :)
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Quoting LouisianaWoman:


That's not necessarily true. A good bit of the posters here yesterday are from Louisiana, Mississippi, and Alabama. They are quite possibly doing finishing touches on their Hurricane preparations. I know that's what we've been doing for a good bit yesterday and today.



I am taking a lunch break while reading the blog, then have just a little bit left to do.
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Quoting bucyouup68:


I am right on the bay in St Pete, the water is getting high and the gusts are starting to push the water over the seawall a bit now. I will get the GoPro out a bit later if it gets worse, trying to work now...Not having much success at the moment.
good luck ok, this is far from over for us here on the gulf coast
Member Since: August 6, 2011 Posts: 4 Comments: 39309
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How strong is the front over Central Texas and Arkansas? Looks to be moving South and East, which if strong enough should push the storm towards the N/NE as it approaches the coast near the AL/Miss line.
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Quoting seflagamma:


Hard to be here when you are trying to keep water out of your house and vehicles...

plus so many are now without power...

I saw your pics Gamma, hope it doesn't get any higher for you, do you have a bilge pump set up in your house yet, no really, all jokes aside, stay safe
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Quoting StormTracker2K:



NW side power flickered when and intense squall moved thru at about 5:30am.



I heard some heavier rain about 30 minutes before that, but we didn't really get a squall, more an 'afternoon shower' level of wind and rain. I guess so far we have been very lucky compared with those a few miles further south.
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Quoting hurricanehanna:

Hey Cajun - Lafayette here too. We are under a TS Warning...we'll be on the "good side" of the storm...lots of rain and some TS wind gusts....mostly a lot of rain. If you love in a flood prone area, take precautions.
But keep an eye on the updates...track could change.


You can send it over here to Houston... We will take it.
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Might have been posted already, but HWRF shifts to Mississippi Coast.

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Quoting LargoFl:
well alot of people lost power and do not forget, this is a work day, for me i dont care so much about what some post about, its their right to do so but for most of us, we care about the people in danger and we do post about it..IF..it came to tampa, we would be posting about that..when it goes to new orleans we will be posting alot about that too..dont worry so much about what other people post about ok..only causes trouble and its meaningless in times of danger like it is today
You have valid points that I didn't think about . You have been doing a good job on keeping ppl informed....
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Harbor Master here in Melbourne Florida and DANG! I have been at it since 4am dealing with gust and rain. We had a 51mph gust a few hours ago, hasnt stopped raining since 7am. Have had to adjust docklines 4 times on almost all of my 89 boats here from wind stretching them out. Have seen many spiraling clouds and doesnt look like it is letting up. I told people to prepare but as soon as they heard storm was westbound they all went to the bar. HAHA, now Im doing it all!

On another note....
How bad do you guys think this will affect that massive sinkhole in LA that is overtop the butane reserves? Crazy to think about..

Ok, back to it, another squall! PEACE
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Quoting CajunCrawfishhunter:
Can anyone tell me what we would expect here in Lafayette LA?

Hey Cajun - Lafayette here too. We are under a TS Warning...we'll be on the "good side" of the storm...lots of rain and some TS wind gusts....mostly a lot of rain. If you love in a flood prone area, take precautions.
But keep an eye on the updates...track could change.
Member Since: September 5, 2007 Posts: 0 Comments: 3488
We just had a quick shower here in Panama City and the breeze is picking up a bit. Some of the bands are getting close now.
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42099
SCRIPPS
Location: 27.34N 84.245W
08/27/2012 1657 UTC
Significant Wave Height: 14.4 ft
Dominant Wave period: 8 sec
Mean Wave Direction: E (84°)
Water Temperature: 84.2 °F
Station 42099
SCRIPPS
Location: 27.34N 84.245W
08/27/2012 1657 UTC
Significant Wave Height: 14.4 ft
Dominant Wave period: 8 sec
Mean Wave Direction: E (84°)
Water Temperature: 84.2 °F
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Quoting bucyouup68:


I am right on the bay in St Pete, the water is getting high and the gusts are starting to push the water over the seawall a bit now. I will get the GoPro out a bit later if it gets worse, trying to work now...Not having much success at the moment.
keep us posted
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Strong rain band and wind now in South Palm Beach County--rain gauge up to 13" since Midnight Sunday
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Quoting MississippiWx:


He has never looked like this.

Last night we were sayin the same thing...

But realistically...I still believe he'll be a cat 1 before landfall...but I just think its amazing that a storm in apparently favorable conditions is takin its sweet time gettin its act together....
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Quoting Sirena7cs:


Thanks Largo, for this important update.

Dr. Masters said early this morning tides were 2' below normal in St.Pete, but I was thinking that this would change when Issac bands stop blowing us offshore from the East, and we start to get winds from the backside of Issac coming up at us from the West.


I am right on the bay in St Pete, the water is getting high and the gusts are starting to push the water over the seawall a bit now. I will get the GoPro out a bit later if it gets worse, trying to work now...Not having much success at the moment.
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Quoting seflagamma:


Same here the water has no place else to go. We've been under that line of stormy and heavy rain now since early this morning and it rarely lets up.

water not yet in my house but probably in others in our neighborhood.

The East Coast of Florida has been hammered by Isaac with all the rain.
Wish it would move out soon and allow others to get some rain.



Hey Gamma....couldnt agree more...this tail wont move and now on the latest radar views it looks like its filling in all the way down toward Cuba. Dont wish it on anybody, but its getting pretty flooded around here.
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Quoting dmh1026:
All that rain training on the east coast of Florida is slowly moving towards the west coast. I think we're going to get a drenching soon....


I hope we do here, since our rainy season will be coming to an end soon enough and I would like to end it with a good surplus for a change. No flooding though!
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414. 7544
Quoting LargoFl:
guys there may yet be something trying to form off the east coast of florida..nws has a watch area for it..be careful over there.............


agree looks like the tail , or band has detach from issac and is moving nne maybe right up the se coast of fla ealirer looked like it moving west but shifted back to the east i could bee wrong but that how i see it stay tuned isacc is not done with south or north fl yet imo
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Quoting MississippiWx:
Isaac is currently in the process of wrapping deep convection from the south around to the northeast quadrant. He appears to have more than half of an eye wall completed.



That feeder band developing on its north side will be key in whether or not he can close that off.
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Isaac starting to moisten up from the S... on its way to be a Cat 1 soon.
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Quoting BZCTX:
304. Dunkman 12:00 PM CDT on August 27, 2012 +0
Interesting mapping note. From the last recon fix a motion of 307 degrees takes it to Morgan City, LA. 300 degree motion takes it to Beaumont, TX and a 295 motion takes it to Houston.
Action: Quote | Ignore User
Member Since: February 6, 2010 Posts: 0 Comments: 197

Stupid question, I am sure but....
Can you give me a quick summary on understanding degree in movement, sorry, I never have understood what they mean when giving the motion of degrees. tia
http://www.cardinalpoints.co.uk/ A bit off topic but that's a really nifty education app that can visually show you how to orientate yourself with a map/compass. Gives you a good idea what the degrees and directions are.
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Quoting OviedoWatcher:


Where in Orlando? Absolutely no problems on the east side that I could see on my way in to work



NW side power flickered when and intense squall moved thru at about 5:30am.

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Quoting wunderkidcayman:
by the way do we had 98L tagged on the African wave yet

No....

I use this page to find out what is tagged an Invest:

http://www.nrlmry.navy.mil/tc_pages/tc_home.html
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.