| typhoon_tip |
| (Meteorologist) |
| Sat Sep 01 2007 04:04 AM |
Re: Tropical Depression #6 Forms East of Caribbean
|
Quote:
I found this exerpt of the TWD to be fairly insightful/interesting...
THE TRACK GUIDANCE IS TIGHTLY CLUSTERED
THROUGH 36 HR...THEN SOME SPREAD OCCURS. THE HWRF AND THE SHALLOW
AND MEDIUM MEAN LAYER BAM MODELS FORECAST A MORE NORTHWARD MOTION
TO A 120 HR POSITION NEAR THE YUCATAN CHANNEL...WHILE THE REMAINDER
OF THE GUIDANCE TAKES THE CYCLONE TO BELIZE AND THE SOUTHERN
YUCATAN PENINSULA. THE HWRF SHOWS THE STORM DISSIPATING DURING THE
FORECAST PERIOD...WHICH MAY ACCOUNT FOR ITS MORE NORTHWARD FORECAST
TRACK. THE TRACK FORECAST IS DOWN THE MIDDLE OF THE REMAINDER OF
THE GUIDANCE AND IS SIMILAR TO THE PREVIOUS FORECAST.
Traditionally, I have been used to seeing the models demonstrating a more westward biased when they see the system as weaker (shallower)...and a more northward biased when a stronger storm is forecast. In this case, the models are apparently showing the opposite (Per this NHC discussion blurb). I'd love to hear opinions on this, but my take is that a stronger system in this circumstance will strengthen the low/mid level ridge to the north and result in a more westward course....WIll be interesting to see if the other models move more toward the BAMS, BAMM, and the HWRF (Yucatan Channel)...or to see if the exact opposite will occur. Overall, the pattern does look like Dean revisited...but I'm sure there will be some interesting tracking differences in the coming days.
There is likely some "inside" info they are leaving out. This is a complex question, actually.
In general, your observations are correct for the longer view. There is a tendency for weaker systems to align more westerly...
However, there is also a bit of an on-going anomaly in the respective layers at those time intervals in question; namely, the low-level and mid-level steering fields. The ridge is in the process of migrating west through the Gulf by the time this system reaches the western Caribbean... If "Felix" beats the ridge to those longitudes, there is some prospect that the low level steering would be more NW if it is a weaker system. Why? Because the steering level will tend to turn W at low levels last for retrograding low-latitude ridges.
The modeled steering fields do have a W component nearing the longitudes of the western Caribbean at the time intervals in question; however, the low-level (H850) is pointing NW, and when combined with Beta Drift, and a weaker system, that could easily mean a northerly position.
However, if a stronger system is there, integrating more in the deeper layers where the majority of mass is blowing almost due W, this resumes a westerly course.
You have to look at the actual pattern for each individual circumstance. In this case, the complexities of all are dictating a bit of an anticorrelary correction need.