Practical Soaring Weather Forecasting

Colorado Performance Soaring Seminar 2012Boulder, 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

Dewpoint Temp vs Pressure

Forecast Hi Temp 93 F

Constant Mixing Ratio

Cloudbase

Dry Adiabatic Lapse Rate

Moist Adiabatic Lapse Rate

Thermal Forecast based on morning sounding

• Cu based around 14K ft• Good chance of T-storms

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 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

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