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Can the temperature/dew point forecast be used to get a "rough" idea of the
ceiling? They have those forecasted for several days in advance on most popular weather sites. -Ali "Jonathan" wrote in message news:cPand.115563$R05.55665@attbi_s53... I've been wondering about this for a while now, and am sure someone here will know... How can I find/figure out what the forecast cloud bases will be 3 or 4 days out? I've looked on ADDS, AOPA's weather, NOAA.gov, etc but not found anything. In other words, if I'm going flying in a few days, And I go to weather.com and see 'mostly cloudy' in the forecast, how do I know if that's "2000' better-file-an-ifr-flight-plan Cloudy" or "10,000' VFR-is-no-problem" cloudy? I've tried looking at forecast temperatures and dewpoints, and using lapse rate calculate cloud bases, but that hasn't worked out. Are there any sites which give this info? or other info I could use to figure this out? TIA for your help -Jonathan |
#12
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"Ali Ghorashi" wrote in message ... Can the temperature/dew point forecast be used to get a "rough" idea of the ceiling? They have those forecasted for several days in advance on most popular weather sites. -Ali Well, yes and no. If you have a reasonable suspicion that the cloud forecast is for convective cumulus in a well-mixed boundary layer...then you could use the standard "400 times (the spread in degrees celsius)" or "222 times (spread in degrees F)", to get the base AGL in feet. Remember: this is ONLY accurate for convective cumulus in a well mixed boundary layer. One thing that you CAN use the temperature-dewpoint forecast for (if they are accurate).... is to try to determine what airmass the forecasters are putting you in, relative to the airmasses you see around the area. That can help tell you whether you are "behind the cold front", or if it is still to come... or are you "in front of the warm front" and can expect lowering ceilings in the warm-front precipitation, etc. Remember that, in many ways, the dewpoint forecast may be a better indicator of the airmass than the temperature. If they are forecasting "cloudy", or especially "rain", they will depress the forecast "High-temperature" quite a bit due to no sun. But the dewpoint is conserved fairly well within an airmass, regardless of other properties. If you get a good estimate of your forecast airmass, and look at the relationship of the site to the forecast pressure pattern, you certainly might have a good guess whether to expect IFR, MVFR or VFR, especially if you also have access to a "text" forecast specifying the cloud and precipitation. For example if your site is forecasting a temperature/dewpoint of 14/10 C and "cloudy, chance of rain" as the outlook for that day... .... and to the south of you, you see forecasts of 25/12 C and "partly cloudy", then you and him could actually be pretty close to the same airmass... .... but if you also check the pressure-pattern forecast and find that you pretty close to a low pressure system and slightly to the east or northeast of it, and he is quite a bit south of there.... .... then it is a pretty good bet that you are being forecast as being right *at* the warm front, and are probably going to be IFR or at best MVFR. Will the ceiling be 200, 700, or 1200....??? I wouldn't hazard a guess, even with the dewpoint forecast. (He, being well south, in the warm sector, will probably be VFR in hazy Cumulus and/or an isolated airmass thunderstorm, and I could certainly guess the bases at 5000-5500 there.) ....but say that the outlook for that day, rather than "cloudy, chance of rain", was simply "cloudy"? Well, the estimate for the cloud base with that 4-degree spread could be anything. With the warm front, you can probably expect the 8-thsnd-plus sort of middle cloud that has cut off the sun and will hold the temperature down. But its not forecast to rain so will there be any low level moisture to form low cloud? And if there is, will it form the cloud at 3 thsnd or 3 hundred? I don't believe the dewpoint spread will really help you there. For me, I would simply be prepared for IFR, only because it is at the front, with a low nearby, and the potential for the very low cloud exists. Beware that you are not using the pressure forecasts from one agency, "text" forecasts from another, and "dewpoint" forecasts from still another. In these days of free-enterprise meteorology, the media produce some pretty bizarre stuff with their own in-house staff. Choose who you trust, I guess, but in North America, I would stick to NWS in the US, and Environment Canada.... These are, I believe, the only providers of "official" information to the FSSs. As I said before.... YMMV ... greatly. |
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