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jcvitte
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(Registered User)
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Wed Aug 22 2007 07:00 PM
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ULL for a novice.
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How do I regognize a ULL? ie. I am looking at water vapor images of what was/us invest92L, I see a clear area to the east, which the images seem to be wrapping around, yet I believe it was mentioned that the ULL that would affect this system had moved into the GOM. Is there another map I should be watching? How do I know what I am looking at is the ULL? Do the low pressure systems push or pull a wave? I can recognize the high pressure ridge to the north in the Atlantic. More importantly, is there a site where I can begin to understand this information. Thanks
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Allison
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(Weather Guru)
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Wed Aug 22 2007 08:36 PM
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Re: ULL for a novice.
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Quote:
How do I regognize a ULL? ie. I am looking at water vapor images of what was/us invest92L, I see a clear area to the east, which the images seem to be wrapping around, yet I believe it was mentioned that the ULL that would affect this system had moved into the GOM. Is there another map I should be watching? How do I know what I am looking at is the ULL? Do the low pressure systems push or pull a wave? I can recognize the high pressure ridge to the north in the Atlantic. More importantly, is there a site where I can begin to understand this information. Thanks
As a fellow amateur, I can tell you that I've learned to recognize many types of features, including ULL's, by taking the NHC Tropical Weather Discussion (example here) and comparing it with the relevant satellite loops, etc. It is also very helpful to follow along with the lengthy discussions provided by the mets on the forums.
As for what you're seeing on the WV loops, my understanding is that it's no longer a ULL, but an open tropical wave. You are correct that there was previously a ULL over the Bahamas while Dean was still in the Caribbean, but that has since moved through the GOM and into Texas.
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Brian B
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(Registered User)
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Wed Aug 22 2007 11:16 PM
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Re: ULL for a novice.
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One thing to know is the characteristics of the source you're using. WV imagery is obviously different from visible and IR. When you're looking at WV, you're looking at the amount of moisture in the atmosphere between about the 700-400 mb level. Upper level lows are seen quite easily in water vapor. Surface features are better seen in IR and enhanced IR, or visible. This is why it is important to look at a number of data sources when forecasting.
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Myles
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(Weather Hobbyist)
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Thu Aug 23 2007 12:22 AM
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Re: ULL for a novice.
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Like Brian B said, you can tell by which Sat source you're using and by the way the feature behaves. Also as Brian B said, WV sat shows you the moisture in the middle to upper atmosphere, so it shows ULL's very clearly. In general, ULL's have very broad circulations. They wrap around for hundreds of miles while surface lows are generally much, much smaller and nearly impossible to see on WV - of course that's unless you have a mature hurricane w/an eye) On the Vis and IR channels you can see from the sfc to the upper levels. Sfc features are easiest to distinguish on Vis, but as your eyes get better at seeing the details you'll also be able to see them on IR. You'll also learn to see ULL on those channels as well.
Also, the ULL that was ahead of Dean and could have allowed him move north did move through the GoM and into Mexico. (I think that's the ULL you remembered being mentioned) A new one formed behind Dean and fed off his outflow. During that time it began interacting with an already present tropical wave and created the mess we see now.
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jcvitte
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(Registered User)
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Thu Aug 23 2007 06:37 PM
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Re: ULL for a novice.
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Thanks so much for the info. I appreciate your time and knowledge. The answers you provided have helped me tremendously, next system I will be able to follow much more closely and have clarity in interpreting what I am seeing and reading. To everyone who contributes to this site, a big thanks.
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