Disclaimer and Cautionary Note
These forecasts are prepared using published NWS data and commonly accepted forecasting tools. They are offered as guide to the likely quality of soaring flight in the vicinity of Allentown, PA. Pilots making use of these forecasts should also follow the good advice of sections 91.103 and 91.155 of the FAR’s.
Last Revised
Monday, March 22, 1999
Forecast Area
The Govenors’ Cup forecast is for the Allentown-Bethlehem-Easton Airport. There are good reasons for this: ABE is pretty much in the middle of the more or less circular task, it is a METAR site, it has an ATIS, and it is an official NWS observing site. All this means that a good deal of data is available.
It would be reasonable, on most days, to assume that the forecast will be valid within the confines a 50 mile circle centered upon ABE. Be aware however that the terrain to the North of the forecast area is very different from that to the South of ABE, and also that 50 miles can make a lot of difference.
Issuing Times and Valid Times
We plan to post both 30 hour forecasts and 6,9, and 12 hour forecasts. Expect the latter to be of higher quality.
The 30 hour forecasts are valid for 18Z the following day, and 6,9, and 12 hour forecasts are valid at 15Z, 18Z, and 21Z on the day of issuance.
We will try to post 30 hour forecasts by 2200Z. We will try post 9 hour forecasts by 1300Z on Saturday and Sunday, and by 1300Z on any week day which threatens to be a good enough day to warrant taking time off from work.
Methodology
Quite a lot goes into a seemingly simple enough forecast, and I thought it might be of interest to document my approach to preparing gliding forecasts.
Data
There are only two RAOB sites of potential utility for our area – Washington and Pittsburgh. Both are rather far away, and in any event, since the balloons are released at 12Z, don’t give me data soon enough to be helpful. Fortunately, the GOES8/9 satellites are able to measure the atmospheres temperature and dew point profile. This is quite amazing, and the technology is very impressive. From my perspective, the best thing is that I can get a sounding less than 2 hours old from sites reasonably close to the forecast area. I use the soundings for
UNV (State College) and PHL (Philadelphia).
I get surface conditions from the NWS which operates an observing station at ABE.
Forecast Conditions
In preparing forecasts I rely heavily upon numerical models. For a basic discussion of the models, take a look at the USA Today site. For a more comprehensive approach, Kevin’s Numerical Model Page provides links to just about anything you might want to know about numerical weather prediction.
I use the ETA model and NGM models for 30 hour forecasts, and the RUC-2 model for 6, 9, and 12 hour forecasts.
All three of these models spit out a lot of information and it is possible to spend a good deal of time looking at this data. Sometimes I do, sometimes I don’t – it depends on the day. However, one thing I always look at is the "Soundings" produced by these models. These are not soundings in the sense that a RAOB is, since they are calculated, not measured, but they use the same format (skewTlogP chart). The models are able to produce a "sounding" because they all divide the atmosphere up into multiple levels.
From the GOES soundings I am able to get a complete picture of the temperature and dew point at all levels of the atmosphere. I use this data and the forecast soundings generated by the RUC-2 model to prepare my own skewTlogP chart.
I make use of the observations report from ABE to check on the performance of the models. Typically, I will get a 12Z forecast of surface temperature and dew point and compare these values to reported values. If, as is often the case, there is a significant discrepancy, I will make adjustments.
Caveats
There are days when my confidence level in the forecasts is high, others when it is low. I will generally indicate my confidence level.
Of all of the inputs affecting the forecasts, surface temperature and dew point are susceptible to the largest errors.
Errors in the forecast surface dewpoint are particularly troublesome, since they give rise to errors in cloudbase height predictions, and all too often are large enough to make the difference between a blue day and a day with cumulous cloud.
Errors in the forecast surface temperature are some times significant, sometimes not, depending upon the state of the air aloft. On days when small differences in surface temperatures are likely to cause large differences in soring conditions, I will issue a caution. On such days, the prudent pilot will, from time to time, check the ABE ATIS.
Winds
I will generally provide wind data for the surface, and for the 950, 900, 850, and 800 mb pressure levels, corresponding, approximately, to 1770, 3240, 4780, and 6390 feet msl respectively.
Be advised that wind forecast directions are TRUE, and that the ATIS winds are magnetic.
Since I will be forecasting lift strength and height, it seems reasonable to forecast ridge conditions too, but I caution that not everyone understands the same thing by the phrase "working ridge".
When wave seems likely, I will note this.
Flight Precautions
I will note the possible presence of any or all of the following: Poor visibility, thunderstorms, high winds, and rapidly changing conditions.
Richard Kellerman, QV
Please Email the weatherman with suggestions and observations on actual vs. forecast weather. mailto:quovadis@ix.netcom.com
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