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Posted by Ray
on 8/28/2008, 3:24 pm
72.230.154.40
For the many: A lovely three-day weekend weather wise for all outdoor activities. Only one forecasting challenge for FLSC: when will the remnants of the front coming through Friday night exit Dansville sufficiently for blue sky to appear?
For the few: Synopticians are fond of saying "What happens at the surface is a reflection of what happens aloft". Here, "surface" means essentially the planetary boundary layer or PBL (where we normally thermal), and "aloft" means part of the troposphere above that, including the "level of non-divergence" (at ~ 500 mb at this latitude), and particularly the jet stream higher up. We are interested in making money giving rides, and people are more likely to come out when it's sunny, even if partly. To have blue skies (albeit with embedded cumuli) requires synoptic-scale subsidence above the PBL, and as previously mentioned the magnitude of that subsidence need only be a few centimeters per second. Or conversely, a few cm/sec of ascent above the PBL means over-development and overcast. Of the 28 available plots for the Eta (= NAM) model, probably the most useful in this context is "925/850 Mixing Ratio + 700/500 Vert motion". The current 48 hr prog, valid (?) for 1200 EDT Saturday, shows minus 2 to minus 5 cm/sec over western NY at both 700 and 500 mb. Also, the mixing ratio maps on the same panel show relatively dry air over us at both levels. So that's good. But a couple of more items one should check. First, the 500 mb NAM and GFS vorticity maps. They both display the small feature swinging through today causing the dreary conditions right now (4 PM Thursday). For Saturday, they both have us free of any big vort maxima; however the GFS (but not the NAM) has a small vort max passing rapidly through during the day. Second, it's good to study the 700 mb winds, scrutinizing them for any short-wave mini troughs: the NAM has a small one which has passed to our east by 1200 Sat, whereas the GFS has one right over us at the same time. So, the model output is a bit contradictory: mixing ratio and vertical velocity being favorable, and the GFS 700 mb flow and 500 mb vorticity data, very slightly unfavorable for early clearing. All things considered, I'll stick my neck out and give a 50% chance of seeing at least some blue patches over Dansville by 10 AM , and a 90% chance of mostly sunny with embedded cumuli by the afternoon. But then, I try to be an optimist.
PS I'm glad I don't live in New Orleans!
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