Soaring Forecast

[Bob Kuczewski]

Hi Zack,

I'd be interested in knowing what to look for as well. I've been flying good old reliable Torrey for so long, that I haven't kept up with the latest in forecasting technology and web sites. I'd sure appreciate a tutorial on what to look for if you can spare the time.

Thanks!
Bob K.

[ZackC]

The purpose of sites like this is to do the interpretation for you (so you don't have to read Skew-Ts and what not)...you just have to read the values for the parameters. There are three I consider really important. The first is Thermal Updraft Velocity (W*), which is the average thermal strength. You have to subtract your sink rate from this (usually ~200 for flex wing HGs) to get your expected climb rate. Because thermals vary in strength even on the same day, I don't look at it as a hard number for expected climb rates but rather a general idea of how good the day is. I expect good conditions if I see 500+.

The Buoyancy/Shear Ratio (B/S) gives you an idea of how workable the thermals are given the wind. B/S increases with thermal strength and decreases with wind shear (which can break up thermals). A high W* but a low B/S means it's probably going to be a rowdy day and hard to work any lift. I like days with a B/S of 9 or above, but I've had good days with it as low as 7.

Height of Critical Updraft Strength (Hcrit) is the altitude at which thermal strength drops to about the sink rate of a hang glider. Thus, it's roughly the
maximum altitude you can expect to attain. This value may be below cloudbase, in which case you can expect you won't make it to cloudbase.

To complicate things, there are two models used to compute these values. The NAM model can forecast out to two days while the RUC can only give the current day. However, RUC has a higher resolution.

To complicate things even more, each model gives forecasts for several times throughout the day (RUC giving more). The times are given in GMT so for Central time you'll need to subtract 6 hours (or 5 once DST ends). Sometimes the forecast for later in the day is much better than earlier.

Often good soaring days have a significant chance of overdevelopment or thunderstorms, so you still need to check rain chances...also see Dr. Jack's OvercastDevelopment Potential parameter.

The wind parameters are useful as well. They show the vector average wind velocity through the boundary layer (where thermal activity occurs). Sometimes winds aloft are much stronger than surface winds, which will be reflected by these parameters.

I really don't pay a lot of attention to these forecasts because I'll fly if it's flyable, but if you have any specific questions I'll try to answer them.

Zack

[Bob Kuczewski]

Thanks Zack!!

I'm getting ready for a trip, so I don't have much time to explore.

But I did visit the site for the first time, and I'm already appreciating the information available. Thanks for opening my eyes to this great resource!!

Bob K.

[SamKellner]

Yes, thanks Zack.

Going over the info you specified as important, and making a guess and then checking back to see if we all come up with the ~same
forecast. That is what it will take for me to gain confidence.

This is gonna help.