Practical Soaring Weather Forecasting

1. Practical Soaring Weather Forecasting Colorado Performance Soaring Seminar 2012 Boulder, CO Dave Leonard

2. Overview • What do you need to know? • What tools are available? • How do you use the tools?

3. What’s important? • What the FAA wants you to know • Big picture, bad weather • What the typical glider pilot wants to know • Which day will be better this weekend? • How good will today be? • When will it start, when will it end, how strong, how tall? • Any chance of really adverse weather (storms, winds, etc.)?

4. You don’t have to be a weather man • How much weather knowledge is required? • A little fundamental knowledge helps a lot • Understanding how a system works makes predicting what it will do much easier • Practical experience helps even more

5. Basic atmospheric info • Basic useful facts about the atmosphere • We worry mostly about the troposphere (< 35K ft) • Pressure and temperature drop with altitude • Air absorbs very little energy from the sun • Air temperature is primarily driven by interaction with the earth’s surface • Air is a poor conductor of heat • The water vapor capacity of air is limited • Water vapor changes to liquid when capacity is exceeded • This condensation releases heat energy • Thermal energy and moisture are primarily transported by air motion • Air moves in response to pressure gradients and density differences (buoyancy)

6. How Thermals Start • Thermals start when a parcel of air near the earth’s surface is warmer than the surrounding air • The air is warmed by the surface, not directly by the sun • The air parcel has no walls, so the pressure inside and out is the same • The warmer air is less dense than the surrounding air – making it buoyant • The warmer parcel of air moves up in response to the buoyancy force applied to it

7. What happens in the thermal? • The warm parcel is surrounded by air at a lower pressure as it rises • The lack of walls means the pressure in the parcel drops to equal the surrounding air • This results in an increase in volume of the parcel • The parcel absorbs very little direct solar energy • The combination causes the temperature of the parcel to drop • The rate of temperature decrease with altitude is known as the Dry Adiabatic Lapse Rate = 9.8 deg C / km

8. What else happens? • Water vapor content stays fairly constant • But the pressure drop reduces the dewpoint temp • About 2 deg C / km • If the parcel rises high enough temperature drops to dewpoint • Moisture condenses and cloud forms • Condensation releases heat into the parcel • Increases buoyancy of the parcel • Lapse rate drops as heat is now added as more vapor continues to condense out of the parcel • These processes continue until surrounding air density is greater than the density in the parcel

9. What does that have to do with anything? • For thermal forecasting you need to know • Temperature and dewpoint of air at the surface • Temperature and dewpoint of the air at altitude • Measured or modeled • If there are cumulus, cloudbase will be at ~(Surface Temp – Dewpoint Temp) (deg C) * 400 ft • Cumulus forecasting is most easily done graphically (or via computer) • Thermal strength is highly correlated with thermal height and whether there are cumulus or not • Empirical models do pretty well

10. Temp vs Pressure • Thermal Forecast based on morning sounding • Cu based around 14K ft • Good chance of T-storms Moist Adiabatic Lapse Rate Dewpoint Temp vs Pressure Cloudbase Constant Mixing Ratio Dry Adiabatic Lapse Rate Forecast Hi Temp 93 F

11. Rolling together local soundings and bigger picture • To refine this forecast, need to take into account bigger picture • Both to account for airmass changes during the day and terrain influenced air circulation patterns • There are several good soaring forecast services that do this and more based on numerical weather model outputs • XCSkies • Dr Jack BLIP Maps • Dr Jack’s RASP BLIP Maps (run locally under license)

12. Refining the forecast – Big Picture Tools • National surface and upper air charts • Looking for big changes coming • Local NWS forecast – “picnic weather” • Professional guess at t-storms and winds • Satellite images • Looking for unforecast cloud, weather system position and movement • Local AWOS / ASOS / ATIS reports • A simple check against forecast for temp, dewpoint, wind • NWS “Forecast Discussion” • Professional assessment of whats going on, what about the forecast is worrisome, which model(s) are doing the best currently • DUAT or FSS (for the record)

13. Understanding the Soaring Forecast Products • NWS Soaring Forecast • Based on morning sounding at DEN • Sounding plots – raw and various models • BLIP Maps – NAM, RUC, RASP, GFS • Note local Front Range RASP is NAM based • All 3 services produce similar products • Different combinations of numerical models • Different calculations of soaring parameters from the model outputs • Dr Jack’s BLIP Maps and XCSkies forecast out a couple days ahead for picking your weekend day by Thursday or Friday • Same products, but less forecast accuracy the farther ahead

14. My morning pre-flight weather brief • I have a set of links collected in a bookmark folder that I open all at once, then click through each tab • Unisys Weather - http://weather.unisys.com/ • NWS 7 day forecast- http://forecast.weather.gov • Kelly Airpark AWOS - http://www.nws.noaa.gov/obhistory/KMNH.html • Dr Jack BLIP map - http://www.drjack.info/BLIP/univiewer.html • NWS Forecast Discussion - http://www.nws.noaa.gov/view/prodsByState.php?state=co&prodtype=discussion • Kevin Ford’s DEN Sounding - http://www.soarforecast.com/ti.cgi?SUBJECT=TI&Upperstation=DNR&Surfacestation=DEN&Forecasthigh=&MaxAltitude=25000 • NWS Soaring Forecast -http://www.crh.noaa.gov/bou/include/showProduct.php?product=algbou&parentdir=routine • FAA DUATS - https://www.duat.com/ • Satellite Images -http://weather.rap.ucar.edu/satellite/ • And of course the Colorado Front Range RASP

15. Pulling it together • Following is a walk through of the various types of BLIP map products for a mid summer soaring day to predict soaring conditions • Included are some notes on how to interpret, correlate, and maybe understand the various products • The trick is to review all the data thoroughly enough to get a good idea of what to expect • But not to spend all day looking at the data and miss a great flight

16. Sample review of RASP forecast • July 3, 2011 • Start with Thermal Updraft Velocity Map for mid-afternoon • Note strong lift • Lots of small dead areas in the mountains

17. Early lift map shows late start in Boulder Valley • Tow into the hills to get an early start • Early start from Kelly likely

18. Check same map later afternoon • Looking for extent of dead areas • RASP normally shows earlier end of convection than actual • Shows movement of dead areas off mountains • Expect scattered OD

19. Sfc Solar Radiation • Shows extensive cloud shadowing, mostly over the high ground, but pushing out over the plains • Correlates with dead lift areas

20. Check mid-afternoon sounding for Boulder • Looking at upper winds • Steer storm movement and cirrus blowoff • Note Temp and Dew Point lines merge at about 18K ft • Cu won’t go away when they die • Wide inverted V indicates possible high based T-storms, microbursts

21. Cu where Potential > 0 • Grey areas are forecast blue holes • Shows high bases, dropping to the east

22. Surface winds light E-NE over plains, W-NW over northern mountains • Winds are calm in blue hole south of Arkansas river

23. Surface dewpoint map shows lots of moisture in the Arkansas and Platte river valleys • Correlates with the varied cloudbase • Doesn’t explain blue hole south of Arkansas river

24. BL Up/Down Motion shows broken convergence line from border to border • Blue areas indicate grid scale dumps, likely virga showers

25. CAPE map shows < 1000 J/Kg along Front Range • Probably no heavy t-storms • Microbursts, lightning, wind possible based on other maps • Above about 1500 J/Kg can get ugly with hale and heavy rain • Farther east can go up to several thousand with really nasty results

26. Forecast Summary for July 3, 2011 • Looks like an interesting day • High pressure building to the west • Previous day was pretty punk • Forecast Strong, tall lift • Good stretches of convergence • Early starts possible • Scattered to broken high based storms with significant dead areas by late afternoon • Some risk of microbursts

27. So how did this one work out? • Several pilots flew from Boulder down to the NM border and back • Pedja flew his first 1000 km OLC flight of the weekend from Boulder • Speeds were not outrageous due to detours around cells and dead areas and some weak climbs • Several pilots had an interesting encounter with a high based t-storm cell moving across Boulder at landing time • The RASP model hit it pretty well

28. You really don’t have to be a weather man • Need to understand generally what’s going on • Need to know which models to trust at any given time • Need to understand what the models are telling you • Need to recognize when the models are not helping and trust what you see out the window

29. Great reference material • Understanding the Sky – Dennis Pagen • Meteorology and Flight: Pilot's Guide to Weather - Tom Bradbury • Meteorology for Glider Pilots - C.E. Wallington • http://www.gliderpilot.org/Weather