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Saturday, July 23, 2005 |
| At 5 PM EDT, the center of Tropcial Storm Franklin was at 29.9 North 74.3 West, 295 miles northeast of Grand Abaco, Bahamas and moving to the east-northeast at nine mph. Maximum sustained winds are 70 mph and minimum central pressure is 1001 millibars (29.56"). There has not been much change in Franklin's appearance on satellite since the recon plane left this afternoon. Wind profiles from dropsondes suggest that the usual practice of deduction 20% from the winds observed at the flight level of the recon plane is excessive in this case; it appears that there should be no deduction. Because central pressure never fell below 1000 millibars, the initial intensity is unchanged. 12Z model guidance causes problems for the track forecast. They have the trough push through and leaving behind a circulation at the mid-levels of the atmosphere. That remaining circulation is then pushed to the south-southwest while the surface circulation goes northeast. If Franklin remains intact through Tuesday, then it is possible that he will move south. Offical forecast is to the right (south and east) of the previous forecast and slower than previously. Franklin could become a hurricane in the next 24 hours before shear starts to weaken it. SLIGHTLY LATER: Bah, didn't mean to send this out without comments. A fun day for the forecaster. Two systems to worry about with one being a total pain in the butt that the models are hosed with. The GFS model was wrong in so many ways with its initialization (the depiction of current weather before the forecast actually begins) it is amazing that the forecaster found it of any use. NOGAPS also had a lousy initialization of Franklin's position, placing it too far to the north. The only real certain thing with the forecast is that if Franklin survives the passage of the trough then it will stick around a bit longer than originally anticipated. The storm's structure is especially pecuiliar with the strongest winds in the southeast quadrant (instead of the southeast). Winds in other quadrants are significantly weaker. The forecaster's assertion that the flight level winds (i.e. the wind recorded by the plane, which flies at a height of about 5000 feet) are representative of the surface wind is also unusual. He has put more weight on the dropsonde observations and the visual estimates of the flight meteorologist than is normal. As I think I have mentioned before, the dropsonde observations are usually viewed with a touch of suspicion because they are instantaneous winds, not an average over 10 or 30 seconds. Because of that, there is no way of telling if it recorded a true sustained wind or if it recorded a peak gust. In this case, however, the forecaster seems to be accepting the dropsonde observations at face value. In doing so he may be overestimating the strength of the storm by 10-15 mph. 2005 Hurricane Season, Weather comment []5:36:25 PM  trackback []
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| At 4 PM CDT, the center of Tropical Depression Seven was at 19.4 North 93.2 West, 180 miles west of Campeche Mexico and moving to the west-northwest at six mph. Maximum sustained winds are 30 mph and minimum central pressure is estimated to be 1011 millibars (29.85"). Satellite imagery and surface observations indicate that Tropical Depression Seven has developed. Wind data from the observations supports the initial intensity of 30 mph. The motion estimate is a very rough guess due to the uncertainty of the center. The upper air situation does suggest that a west-northwest movement should occur until landfall occurs on the coast of Mexico. Track forecast is similar to the consenus of NHC's guidance models. Conditions are favorable for development. However, with so little time over water, the depression is expected to strengthen only modestly. The intensity forecast maxxes out the system at 50 mph; the forecaster notes that if the initial intensity estimate is too low or if the system moves slower, then it could become stronger. Not much to say yet as the plane sent out to investigate the system has not yet reached its center. 2005 Hurricane Season, Weather comment []5:02:36 PM trackback []
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| I mentioned yesterday that an system then over the Yucatan Peninsula was under scrutiny. It now appears that the system has formed into a tropical depression: ...SEVENTH TROPICAL DEPRESSION OF THE SEASON FORMING OVER THE BAY 2005 Hurricane Season, Weather comment []3:16:07 PM trackback []
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| At 11 AM EDT the center of Tropical Storm Franklin was at 29.4 North 75.0 West, 245 miles north-northeast of Great Albaco Island. Movement is to the northeast at 9 mph. Maximum sustained winds are up to 70 mph and minimum central pressure is 1001 millibars (29.56") Reports from the recon plane indicate that Franklin has become much better organized. The flight crew has been (visually) estimating 60-65 knot winds, while actual observations are a little bit lower. Despite the relatively high pressure of 1001 millibars, forecaster opted to up the intensity to 60 knots. Initial motion is estimated to be 055 degrees at eight knots. Organization is better than what global forecast models had been indicating. While the trough that was expected to pick up Franklin does reach far enoough south to do so, the majority of the energy associated with it resides further north. This combined with the fact that Franklin is further south and moving slower than previously forecast by NHC caused the forecast track to be adjusted to the south and at a slower pace. The track is consistent with the consensus of the GFDL, UKMET, and NOGAPS models (GUNS consensus). Because of how Franklin has persevered in the face of tough conditions, it appears now that Franklin will become a hurricane in the next 12-24 hours. Current intensity forecast maxes out Franklin out at 65 knots (75 mph). Looked at the forecast models from last night and comparing their 12 hour forecast for this morning, I saw that none of them did well. Global models were too far north by 50+ miles while were too far west by a similar amount. The small size of the storm, as well as the subtletly of some of the features around it aren't making for an easy forecast for the models. One feature that they seem to have missed is an upper level low that is currently centered over southern Florida. It developed over northern Florida overnight and has since slid south. This has done wonders for Franklin's health as it has not only protected it from shear that was affecting him from the southwest, but it has also provided diffluent flow southwest of the cyclone, which has greatly enhanced the convection associated with Franklin. Right now, the low does not appear to have affected the course of Franklin. A possibility that will have to be monitored, however, is the Fujiwara effect, in which two lows start rotating along the same axis together and eventually combine. The question in something like that ocurring is, which would be the dominant low? If Franklin were to be dominant then the upper level low would get pulled out to sea along with Franklin. However, if the upper level low were to be dominant, then Franklin would get looped around. The scenario of Fujiwara interaction is not handled well by models at all. It tends to be something the forecaster has to be on the lookout for himself (and even then, from what I understand, we don't know enough about how and why it happens to really forecast it with confidence; it's more like something that the highest confidence level is merely: 'this could happen'. A fun storm to look at, even if it is minus some excitement. 2005 Hurricane Season, Weather comment []11:39:51 AM trackback []
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© Copyright
2005
Charles Fenwick
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Last update: 8/2/2005; 2:00:45 PM .
Last update: 8/2/2005; 2:00:45 PM .
