Weather analysis

[mingram]

Petr has been sharing his thoughts on weather and I thought it might be useful for the larger group. I got his permission to repost it.

This was from Saturday 11/9:
"Forecasts tend to be even lousier than usual in high pressure dominated weather as we had yesterday. Lows concentrate a lot of energy in the relatively narrow and long area of cold fronts and much of this action happens through the ascending air at the ground level. That can be more accurately predicted by the forecasting model than the motion of atmospheric high. If the high moves at all, the models don't do a good job in predicting the correct 3D angle of the incoming denser, descending air. That can make a big difference between the forecast and actual wind velocity. "

This is analysis about Woodstock for today 11/13:
"If this was an option for me, I would definitely go to WS and if I had your with your DHV1-2 glider, I would be prepared not to fly.

There is nothing spooky about the forecast, except for its low fidelity. Tomorrow will be nearly a midpoint of about a 5-7 day buildup of a huge high pressure system that is now centered somewhere between northern Texas and Missouri and will migrate to Atlantic over the weekend. That kind of situation wont create T storms or gust fronts that should definitely ground you. But this is also not a good weather system for reliable forecasts. Today the NOAA models were predicting MUCH weaker winds than those that were actually measured on the ground. The latest record: about 17mph actual vs 9 predicted in Winchester, close to WS, and that type of error makes a big difference to us
http://weatherspark.com/#!dashboard;a=USA/VA/Woodstock

If the trend continues, it might not be safely flyable because the winds could be too strong for PGs. So, consider the forecasts but trust your senses and instruments.

Tomorrow, what you will see on the launch is pretty much what you will get in the air throughout the afternoon. If you can launch safely, you will be OK, as long as you stay in front of the ridge and consider that the wind speed is going to increase as you go up. Look at the clouds before you launch: if they speed up like crazy, there is a stronger gradient of higher altitude wind speed than current models predict. Then you should think about staying local and not getting too high and should consider not launching at all. Likely, there will be enough of ridge and thermal lift. Think of your XC strategy, if the wind speed is manageable and if this is what you want to explore. Even on your relatively slow wing, this could turn out to be a great day to fly safely and in a controlled fashion from WS to Dickeys and beyond."

[Matthew]

I disagree as to today--

Here is today's text forecast for Woodstock--

THIS AFTERNOON
SUNNY. HIGHS AROUND 40. NORTHWEST WINDS 10 TO
15 MPH WITH GUSTS UP TO 25 MPH.

Doable for HG-- but no way for any PG.

Matthew

[mingram]

Agreed. Gust potential was added to the forecast today. I guess that's my point of reposting Petr's analysis. Specifically about how high pressure systems are more difficult to predict.

[sailin]

Good thread. I love weather stuff. I love learning about weather. There is so much to learn and so many local phenomenon and conditions that tweak all the forecasts and even nearby reporting weather stations. The old saying "you never know, unless you go" seems to always have some element of truth to it.

With Amy's help, I flew Woodstock today and it was most certainly PG'able. In fact it was very light. I had seen some gust earlier in the day from Winchester airport to 18mph but by the time I was driving past Front Royal it just wasn't that windy. I arrived at launch at 2pm and the biggest baddest stuff coming through was 10-12mph....and only occasionally. Things "ramped" down from there. I ended up launching at 4pm ( had to wait in the slot for a while just to get something that was going to give me some free airspeed.....the cycles were few and far between) Most of the stuff was 5-6mph and crossing from both the west and the north at different times. Above the ridge (I maxed out at 800' over) the air was buoyant and smooth......magic. I believe earlier in the day... maybe 10am-12pm, may have been a handful, but the rest of the day was very flyable by all wings. Man that ridge is beautiful in the last light of the day!!

Jon

[Petr]

Good to hear that WS was flyable today after all! I was having my doubts...

What I wrote last night was still valid early this AM when I briefly checked NOAA and saw about 11-12mph Winchester noon forecast with further decline in the PM, i.e. weaker than the day before. Of course, as it goes, just 2-3 hours later I could clearly see that trees in Bethesda were moving a lot more than what would correspond to such a forecast. At that time I already kept all my weather and flying links OFF, to get other things done and I was just hoping that the part of my message about NOT launching if things don't look right was not missed.

That made me also realize that I should have added strong, sustained and gusty winds from areas of high pressure (H) to the short list of spooky conditions to be aware of and to avoid. Especially because this fall we already had quite a few of flying days dominated Hs either in the North, like last Saturday and few times before, or in the SE, like today. The NOAA forecasts were usually all over the place with respect to the actual weather on those days. As those who were at Daniels this weekend, at least few times it was definitely worthwhile to get out there and check it out, against the NOAA odds. Needless to say, that does not mean that every time we do that in the future we will get to fly.

The bad thing about the H-driven strong winds is that they are really hard to correctly predict, as the physics behind this phenomenon apparently dictates, and as we see over and over again (and as anyone in Southern CA would tell you about Santa Ana weather forecasts, which is essentially that type of scenario). Today's NOAA pattern indicates to me that the models start to "predict" such winds just as they are being detected on the ground or only few hours before. And that's the relatively good thing about these winds: the H-driven wind speed usually ramps up over enough time for us to notice and avoid danger. In other words, our own senses and judgment are probably at least as good in locally measuring and forecasting this kind of weather than all the detectors and computers of NOAA combined.

Petr

[Danny Brotto]

I think the issue with wind prediction for the PG lot is the span of the (narrow) normal operation range. Off by a few knots and they can become blown out. HG have a wider range and sailplanes even more. (Not talking about soarable but flyable.) It is not unusual for the sailplane gang to accurately predict weather along a 300 mile long stretch of ridge days in advance of a frontal passage. Such an exercise is being carried out right now for next Tuesday/Wednesday. These prediction include wind velocities, direction, precipitation, cloud cover, inversion height, wave potential, etc. over the course of the entire day and across that long distance. (Some of you on this forum even have access to those e-mail threads.) If the SP prediction is off by a little bit, no worry as it's in the noise. PG & HG are more susceptible to the noise in the prediction.

Danny Brotto

PS: Disclaimer... not ment to offend any particular group of aviator or anything... just my thoughts.

[brianvh]

Though what happens along a front may be fairly clear (and front locations are tracked by satellite and surface observations and marked by eye more than modelled), the statement that winds can be predicted by weather models more accurately near the ground is simply untrue, especially for mountain areas. Weather models do better up high where there is less of the complicating interactions with the ground.