Practical Soaring Weather
Forecasting
Colorado Performance Soaring Seminar 2012
Boulder, CO
Dave Leonard
Overview
• What do you need to know?
• What tools are available?
• How do you use the tools?
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.)?
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
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)
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
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
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
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
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
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)
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)
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
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
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
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
• Early lift map
shows late
start in
Boulder Valley
– Tow into the
hills to get an
early start
• Early start
from Kelly
likely
• 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
• Sfc Solar Radiation
– Shows extensive
cloud shadowing,
mostly over the
high ground, but
pushing out over
the plains
– Correlates with
dead lift areas
• 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
• Cu where
Potential > 0
– Grey areas are
forecast blue
holes
– Shows high
bases,
dropping to
the east
• Surface winds
light E-NE over
plains, W-NW
over northern
mountains
– Winds are calm
in blue hole
south of
Arkansas river
• 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
• BL Up/Down
Motion shows
broken
convergence line
from border to
border
– Blue areas
indicate grid
scale dumps,
likely virga
showers
• CAPE map shows <
1000 J/Kg along Front
Range
– Probably no heavy tstorms
– 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
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
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
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
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