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Friday, September 03, 2004 |
| At 5 PM EDT, Hurricane Frances was 90 miles of Freeport (on Grand Bahamas Island) and 200 miles east-southeast of Florida. Forward motion is set at west-northwest (300 degrees) at 8 mph (note that it gets difficult to tell what the heading is when the storm is moving slower than 8 mph). Winds remain at 115 mph and as noted in my previous post, minimum central pressure has fallen to 959 millibars (28.32"). Hurricane warnings extend from Florida City to Flagler Beach. A tropical storm warning and hurricane watch are up for the area from Flagler Beach to Fernadina Beach (north of Jacksonville). A tropical storm warning is also in place from Florida City south to the Seven Mile Bridge in the Florida Keys. Without examining the coordinates exactly, it's diffiicult to say exactly, but it looks like the forecast track brings Frances ashore near Vero Beach Saturday afternoon, passing over Orlando and entering the Gulf as a tropical storm north of Tampa. The discussion says none of the guidance models have the storm strengthening, but the possibility certainly exists. For this reason the intensity forecast is persistence (strength will be unchanged). This is a good line: THE BOTTOM LINE IS THAT FRANCES...A POWERFUL HURRICANE...IS I ran back to the meteorology building to take a peak at the latest superensemble output. It is just a touch south of (on landfall) and west of (when the storm turns north) of the NHC forecast. It's going to be an interesting 24 hours... Weather comment []5:26:18 PM  trackback []
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| A somewhat technical discussion follows. I heard from someone that was in on the National Hurricane Center's conference call this morning that they weren't sure why conditions became unfavorable so quickly yesterday. Of course, they knew what had happened (Weatherbug meteorologist Mark Hoekezma gives a description in his interview with Weatherblog and what was ocurring, but the why of it was quite unclear. This afternoon, my synoptic meteorology professor gave a couple of the working theories. Earlier in the week we had noted a small bit of anticyclonic circulation over the Bahamas (tuft cell? I think that's what he called it) With 20/20 hindsight, this circulation was found over south Florida/Cuba yesterday (that is, the images from yesterday were being reexamined today). This circulation was probably too small to get picked up by the models or for the forecasters to consider important if they noticed it. It is plausible that when Frances got on the eastern edge of this circulation the initial interaction between the two was for the cell to cause shear across the southwestern quadrant of Frances, which brought about the weakening. (The thought that something was causing shear is without doubt). Another possibility (that probably ties into the first) is that the interaction with land (Bahamas islands) had a greater effect than anticipated. As mentioned before, Andrew had some weakening when he came across the islands in '92. This shear was so severe that it would have killed off a smaller storm. But Frances was big and strong so it only dropped a couple of categories. Looking at the most recent satellite images, it appeared that the cell was being drawn to the southeast (and ultimately merging into Frances' circulation). It looks like that shear is decreasing (outflow in the southwestern quadrant appears to be expanding outward again after being non-existent earlier today). The most recent recon message (from the Hurricane Hunter plane) shows the pressure starting to fall again. My professor predicted strengthening over the next six hours. Everything seems to be breaking her way again. One worrying thing is the apparent slowness in movement. Obviously, if the conditions are getting favorable again , people wanting a weak storm would want it to move as soon as possible (so that it doesn't strengthen much before landfall). At the moment it appears that Frances will take her sweet old time getting to the coast. Also, the latest images seem to indicate more of a westward motion (indicating landfall further south). From what I've heard, the 1800Z of the FSU superensemble has a 'noticable' shift to the left (south) in its latest run. The Florida peninsula, of course, becomes smaller in the southern portion. It would be very troubling if Frances were able to make a short transit to the Gulf of Mexico where Sea Surface Temperatures are a few degrees warmer than in the Atlantic. Now let's see what the latest update from the National Hurricane Center says.... Weather comment []5:00:29 PM trackback []
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| One undercovered part of the story regarding Frances is the potential for heavy rainfall and the effects that would have. Note that the first rain bands of rain from Frances started coming ashore about an hour ago. With Frances moving as slowly as it is (9 mph at 2:00 PM report), that rain is going to accumulate quickly. My synoptic meteorology professor gave the rule of thumb for rainfall from a hurricane as being 100 / forward speed. So, if the storm is moving at 20 miles per hour, one could approximate 5 inches of rain; 10 inches if it's moving at 10 mph, and so on down. Obviously when you get a storm moving slower than 10 miles per hour the amount of rain becomes incredible. A disconcerting aspect of this is the potential for Frances to move over areas that are already quite damp (from Charley). This soil moisture graphic shows that a fair bit of southern Florida's soil is already wetter than normal. This makes it more susceptible to flooding. (In an earlier post I mentioned the flooding caused by Hurricane Floyd. That flooding was enhanced by another hurricane/tropical storm passing over the same region a few weeks earlier). The Southeast River Forecast Center in Atlanta predicts extensive impact from flooding as Frances moves inland. Another worrying aspect of this is the fact that the uprooting of trees is enabled when the soil becomes moist (and therefore looser). This is how enormous, seemingly unmovable trees get pulled out of the ground from winds. Obviously, with a long period of rain this effect is going to occur in spades, which is quite worrisome because even if the winds stay where they are, the increased moisture in the soil would enhance the effect of the winds to an extent equal to that of higher winds (without the weakened soil). The flooding factor causes worries where there wouldn't normally be. When people think flooding from a storm, they tend to think of the coast, not inland. But as Gaston recently demonstrated in Richmond, and Floyd demonstrated in North Carolina, the flooding associated with heavy rains can occur in any low-lying area. People in south and central Florida should keep this in mind when making plans. Weather comment []4:39:54 PM trackback []
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| You find yourself kind of wishing you had class on the weekend. My synoptic meteorology class was so interesting today that it left me wishing we had class this weekend so I could hear more from my professor on his thoughts regarding Frances. Anyhow, I've a lot to talk about and the 5:00 PM update is coming soon, so there will be a few posts coming up in fairly short order. Weather comment []4:21:07 PM trackback []
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| At 11 AM EDT... Hurricane Frances was somehwere over the Bahmas. (National Hurricane Center hasn't posted the public advisory yet and I must be going...) minimum central pressure has risen to 959 millibars and winds are lowered to 115 mph. Intensity forecast downgraded to a 115 mph winds at land fall (meaning no change in strength from now until then. Looks like the big worry now is the large amount of rain that a slow moving hurricane can drop on an area (ask people from northeastern North Carolina and southern Virginia about Hurricane Floyd). The National Weather Service office in Melbourne has put up flood watches for all east central Florida counties. Some blog stuff: Weatherbug is in Melbourne, FL Fresh Bilge reads between the lines of the 0500 discussion from the NHC Florida Cracker mentions talk of the 1928 Lake Okeechobee hurricane I'm off to my atmospheric dynamics class... Weather comment []11:11:10 AM trackback []
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| The reaction of normal people to the news that Frances has weakned to 125 mph winds: "That's good to hear!" Reaction of meteorologists, meteorology majors, et al. "Dammit! How did we miss on the intensity forecast?!" Weather comment []12:43:36 AM trackback []
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| At 11 PM EDT, Hurricane Frances was 25 miles east-southeast of Northern Cat Island in the Bahamas or 330 miles east-southeast of Florida. Forward motion is to the west-northwest at 10 mph. Winds have weakened to 125 mph and the minimum central pressure is 948 millibars (27.99") The track forecast is essentially unchanged as all models are agreeing with each other to a reasonable extent. The timing, however, is slower, as it does not have Frances making landfall until Saturday evening. The intensity forecast is for winds to remain at 125 mph for the next 12 hours and then for some strengthening to occur after that as Frances crosses over the Gulf Stream. According to observations, the upper level winds are somewhat unfavorable to Frances at this time, hence the sudden weakening (at 8:00 PM, winds were still 140 mph) Weather comment []12:04:45 AM trackback []
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© Copyright
2004
Charles Fenwick
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Last update: 10/2/2004; 11:03:31 PM .
Last update: 10/2/2004; 11:03:31 PM .
