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Posted by Ray
on 9/25/2008, 6:18 am
72.230.154.40
The weekend will not be a washout by any stretch. In the best-case scenario, both Saturday and Sunday could be VFR throughout. In the worst-case, occasional showers mixed in with the VFR. As for which day is preferable for flying, no way to tell just yet. Why is that?
Details: if our planet had negligible water (Mars for example), forecasting would be much easier. Add even a 5% coverage of Earth's surface with ponds and rivers and the situation is orders of magnitude more complicated, because then you have enough water vapor to form clouds, which of course feed back non-linearly and negatively to the incoming solar radiation. Now add oceans to most of the Earth's surface. They store incoming solar energy on time scales of weeks, years, and centuries. Their total energy reserves at any given time are not known, because the temperature that satellites see is only the skin of the oceans. Because the sun heats the ocean surface first, and because of hydrostatic stability, the average temperature of the deep oceans in not accurately known. When storms get going over water, waves form, and the resultant transfer of water vapor, heat, momentum, and friction is highly non-linear and hopelessly complicated to model from first principles, i.e. the basic laws of thermo and hydro dynamics. So what modelers do is called "parameterization". That's a fancy term for fudge factoring. People put in "adjustable" (read:guess) terms in those parts of the models for which the physics is too complicated, then tweak the models to correspond with what subsequently happens in a given case. Of course each case is different, so this process has to be continually repeated. Gradually, as the years go by, the computer models get better. Currently they are accurate for a few days at best. Forget about meaningfully forecasting, for example, the earth's average temperature decades from now.
Back to this weekend. Two of our best models now disagree markedly as to whether Saturday or Sunday will be more "VFR". The 60 hr. NAM prog (for 1200 UTC Saturday) has a substantive 500 mb vorticity max over western NY then, whereas the GFS does not. Interestingly, 24 hours further into the progs (1200 UTC Sunday), this discrepancy is preserved. Consequently, the respective precipitation forecasts, since that is primarily controlled by upper-level dynamics, differ similarly. The NAM would have both days (and especially Sunday) with intermittent showers, whereas the GFS has western NY rain-free all day both days. On the other hand, the main chunk of energy driving this sits over the Atlantic, and synoptic-scale dynamics suggests that since that entire ensemble (together with any associated, smaller-scale vorticity maxima) is being carried east and away from us by the Rossy wave, Saturday should be the better day.
Noticeably, the long-range NWS Buffalo forecast discussion isn't yet favoring either scenario either. Any wonder?
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