| spinup |
| (Weather Watcher) |
| Fri Aug 17 2007 01:18 AM |
ensemble intensities, ofci intensity
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I typically go to the Colorado State Univ. Operational Model Guidance webpage and the SFWMD Forcast Models webpage for the first postings of the track and intensity model forcasts.
In response to the previous post, Met Clark included a link to the NHC/ATCF model output webpage.
2 questions;
On the NHC/ATCF site, why is it that the "ensemble" intensities for Dean start so low (35 kts) and then increase a bit for 12 hrs, and then predict steady decrease in intensity to 10 kts in 48 hrs and then holding steady? the "current" intensity forcast has Dean steadily increasing to 115 kts in 5 days.
And, why does the OFCI (does this mean Official? Can't be) intensity graph go down to 45kts in 5 days when the NHC forcast has Dean going to 115 kts in the same time frame?
I've seen some wide differences in the intensity forcasts but this is especially broad.
| Clark |
| (Meteorologist) |
| Fri Aug 17 2007 06:00 AM |
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Re: ensemble intensities, ofci intensity
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Regarding the ensemble intensities, those are taken straight from the GFS ensemble raw wind fields. The ensemble members are all run at a relatively coarse resolution, about 100km, which does not allow them to represent tropical cyclones very well. While they can and often do have a decent initial intensity -- that they get from the main GFS analysis as a first guess -- they often can't sustain it at later times. It's mostly included in the suite of plots for completeness; the CSU page does not output those intensity fields.
OFCI is the 6hr interpolated NHC official forecast, meaning it is from the previous forecast cycle but interpolated 6 hours ahead in time. I see that the 18z intensity guidance plot had the storm going down to 55kt. The only reason it does so is because the 12z package put out by the NHC (the 11a ET advisory, basically), went from 120kt to 80kt in a 24-hr period at the end of the 5-day forecast period. Interpolated ahead another six hours, that leads to further weakening of the storm (though 55kt is overkill). As with any of the various model pages out there, the raw values should be taken with caution.
(Note that the CSU plot at the same time had the same OFCI intensity.)Hope this helps and isn't too technical...
| SeaMule |
| (Weather Hobbyist) |
| Fri Aug 17 2007 02:18 PM |
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a spinning top on a ball....?
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I have heard in the past that the further north a hurricane goes, the more it has a tendency to curve northward....that is....at the equator...they go west...and further north...they tend to curve. that being said, irrespective of the high pressure Dean is being steered by...as it leaves that influence, wouldn't the tendency to curve, if there is no other influences...become realized...
I'd love a met's comments on this. perhaps the models do NOT take this in effect....perhaps I don't have a clue what I'm talking about...
| spinup |
| (Weather Watcher) |
| Fri Aug 17 2007 02:24 PM |
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Re: ensemble intensities, ofci intensity
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Thank you Clark for your response.
The "Ensemble" intensities were especially puzzling. Now I understand.
My confusion was compounded as a result of my failure to notice the difference between OFCI and OFCL!
Another question;
Why is OFCI called "interpolated"? Isn't it actually "extrapolated" ?
Technical works for me.
| Clark |
| (Meteorologist) |
| Fri Aug 17 2007 04:45 PM |
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Re: ensemble intensities, ofci intensity
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Glad I could help. Not sure why it's called interpolated rather than extrapolated; it might be from how they compute all but the last forecast time. Either way, it's really just a naming artifact that has stuck for quite some time.
| Goosus |
| (Verified CFHC User) |
| Fri Aug 17 2007 09:51 PM |
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Re: Major Hurricane Dean Moving Westward
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In regards to the long range trend for Dean, I thought I had read here in the past that the more intense a hurricane becomes, the more likely it will try to veer northward in it's forward motion?
Is this a correct assumption, and if so, why?
| Brian B |
| (Registered User) |
| Fri Aug 17 2007 10:36 PM |
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Re: Major Hurricane Dean Moving Westward
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I really wish I could remember the exact explanation for this. Dr. Reasor would be very disappointed in me right now. Clark should be able to answer this one.
| Clark |
| (Meteorologist) |
| Fri Aug 17 2007 11:00 PM |
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Re: Major Hurricane Dean Moving Westward
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More intense hurricanes have deeper vertical circulations, meaning that they extend to higher altitudes. At higher altitudes, the mid-latitude steering patterns are often a bit better defined than they are at lower altitudes. This manifests itself in the preferred steering layers for tropical cyclones of various intensities (see http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/winds-dlm.html for more). Thus, for deeper storms, there is an increased likelihood of a more northerly track.
Larger, more intense storms also often have an increased magnitude to the Beta drift, a northwestward steering flow component (just a few mph in magnitude at its peak, but still significant) that can lead to an increased poleward motion of these storms. The dynamics of the beta drift and how it comes about are beyond the scope of this discussion, but basically there are gyres within each hurricane that impart a background flow toward the NW across the center of the storm's circulation.
| Clark |
| (Meteorologist) |
| Fri Aug 17 2007 11:02 PM |
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Re: Major Hurricane Dean Moving Westward
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Quote:
I really wish I could remember the exact explanation for this. Dr. Reasor would be very disappointed in me right now. Clark should be able to answer this one.
Ha! Brian, I don't think he covers most of that in his dynamics course, unless it changed in the few years since I've taken it! Nothing to worry about on your end...