Joel Gratz | Vail, CO | 2009-01-07
Weather Forecasting for Joel Gratz, Meteorologist
Powder Lovers

In the name of
powder...
• Weather 101: Getting High
• Predicting snow in Colorado
• Case study: Today’s storm
• Forecasting Tricks
• Websites
• Q & (.5)*A
Joel Gratz | Meadow Mountain, CO | 2009-03-11

WEATHER 101: Weather happens
“Weather happens” when moist air gets high.
1. Moist air rises.
2. Less pressure = air expands = cools.
(air pressure decreases as you go up)
3. Water vapor (gas) condenses into water drops
(liquid) = a cloud forms.

WEATHER 101: Weather happens
“Weather happens” when moist air gets high.
“Weather happens” when moist air gets high.
“Weather happens” when moist air gets high.
“Weather happens” when moist air gets high.
“Weather happens” when moist air gets high.
“Weather happens” when moist air gets high.

WEATHER 101: Finding Moisture
It depends on the path of the storm. Storms from the
west and southwest bring the most (heaviest) snow.
Little moisture
(land source)
OK moisture
Lots of moisture (ocean + plants)
(ocean source)
http://www.virtualsources.com/Countries/North%20America%20Countries/us-map.gif

WEATHER 101: Finding Moisture
Satellite Image: Water Vapor, 12a on Oct 19, 2009
Lots of moisture
(ocean source)
Less moisture
http://www.atmos.washington.edu/~ovens/loops/wxloop.cgi?wv_central_enhanced+/24h/

WEATHER 101: Five ways to get high
C is for convection.
L is for low pressure.
O is for orographic.
U is for undercut.
D is for dynamic.

WEATHER 101: Five ways to get high
C is for convection.
cold air
warm air

WEATHER 101: Five ways to get high
L is for low pressure.
Low
Pressure

WEATHER 101: Five ways to get high
O is for orographic.
http://www.clker.com/cliparts/e/4/9/a/12401693261710228990Kliponius_Mountain_in_a_Cartoon_Style.svg.med.png

WEATHER 101: Five ways to get high
U is for undercut.
Cold front moving left-to-right
Cold Air
Warm Air
http://www.clker.com/cliparts/e/4/9/a/12401693261710228990Kliponius_Mountain_in_a_Cartoon_Style.svg.med.png

WEATHER 101: Five ways to get high
D is for dynamic.
This is complicated - it’s all physics.
Jet Stream 30,000ft
(river of fast air) 300mb
Induces upward motion by “pulling” surface air
upward into the void left by the fast-moving air
above. Similar to opening a car window while
moving and having a piece of paper “sucked” out.
http://www.clker.com/cliparts/e/4/9/a/12401693261710228990Kliponius_Mountain_in_a_Cartoon_Style.svg.med.png

WEATHER 101: Five ways to get high
D is for dynamic.
This is complicated - it’s all physics.
18,000ft Vorticity
500mb counter-clockwise
spin
As vorticity centers (“vort max”)
move, they induce upward
motion in front and downward
motion behind.
http://www.clker.com/cliparts/e/4/9/a/12401693261710228990Kliponius_Mountain_in_a_Cartoon_Style.svg.med.png

WEATHER 101: Five ways to get high
C is for ___________.
L is for ___________.
O is for ___________.
U is for ___________.
D is for ___________.

SNOW in CO: A state of Extremes
• More than half of the total annual precipitation falls
in only 20% of the days when precipitation occurs.
• The difference between a wet year and a dry year can
come down to the presence or absence of just a few
major storms.
• In the arid Western states, “normal weather” is just the
average of 30 years of extreme events.
Joe Ramey, National Weather Service Grand Juntion, Slide #8 2009-2010 Winter Outlook for Colorado

SNOW in CO: Precipitation is Elevation
ph ic!
ro gra
fo ro
is
O Park Range
Indian Peaks / RMNP
Flattops
Grand Elk
Mesa Mtns
Sangre de
N&S Christo
San Juans
East
San Juans
Joe Ramey, National Weather Service Grand Juntion, Slide #8 2009-2010 Winter Outlook for Colorado

SNOW in CO: Jamming the radar

SNOW in CO: Jamming the radar
The “magic” of radar is rather useless in the mountains
since the beam is often blocked. Snow also doesn’t
show up very well on radar.
Beam is blocked by the
first mountain and can’t
“see” to the second
mountain.
0.5 deg
http://www.iconspedia.com/uploads/2058636058.png
http://www.drawingcoach.com/image-files/cartoon_clouds_1.gif
http://images3.wikia.nocookie.net/funorb/images/0/06/Radar_Dish.PNG

SNOW in CO: Overshooting
The radar can also overshoot the snow if it looks too
far away. Radar beam at Steamboat = 16,000ft after
starting at 10,000ft from Grand Mesa (GJT radar).
Beam overshoots most
of the snow and you 16,000ft
can’t “see” it on radar.
10,600ft
0.5 deg
10,000ft
http://images3.wikia.nocookie.net/funorb/images/0/06/Radar_Dish.PNG

SNOW in CO: Jamming & Overshooting
The Denver, Pueblo, and Grand
Junction radars are often jammed
at the low levels, and overshoot
the falling snow at higher levels.
Steamboat
RMNP
Denver Radar = 5,400ft
Pueblo Radar = 5,300ft Vail
Grand Junction Radar = 10,000ft Summit Co.
Aspen
Telluride
Wolf Creek
http://www.ncdc.noaa.gov/nexradinv/

SNOW in CO: Upslope vs. Downslope
Upslope is good: Downslope is bad:
Air cools, condenses, Air warms, dries, and
and clouds are born. clouds are killed.
http://thumbs.dreamstime.com/thumb_73/1153789284jI008B.jpg

SNOW in CO: The upslope wind direction
East
RMNP
North
Steamboat
Northwest
Summit Co. West
Aspen Vail
Southwest
Telluride
Wolf Creek

SNOW in CO: What winds to watch?
As a standard, winds are forecast (and measured) at 850mb,
700mb, 500mb, 300mb, and 250mb.
JET STREAM 30,000ft
300mb
VORTICITY 18,000ft
500mb
UPSLOPE 10,000ft
700mb
5,000ft
850mb

Case Study 10/21/2009: Storm Track & Vorticity
500mb Vorticity, Mon AM > Wed Noon

Case Study 10/21/2009: Upslope
700mb Wind, Tues AM > Wed Noon

Case Study 10/21/2009: Rain + Snow (liquid)
Precipitation Tues AM > Wed Night

Case Study 10/21/2009: Snow
Snow Tues AM > Wed Night

Case Study 10/21/2009: Verification
1230 PM
SNOW
2 S CUCHARA 37.36N
105.10W
10/21/2009 M14.0 INCH
HUERFANO CO
TRAINED SPOTTER
14 INCHES OF SNOW...STILL SNOWING.
TOTAL WATER EQUIVALENT APPROXIMATELY
2.25 INCHES.
0404 PM
SNOW
2 S CUCHARA
37.36N 105.10W
10/21/2009 M17.0 INCH
HUERFANO CO
TRAINED SPOTTER
http://www.nws.noaa.gov/view/prodsByState.php?state=CO&prodtype=public

Forecast Tricks
• Watch the weather every day, and match the forecast
to actual conditions.
• Read the “Area Forecast Discussion” from the National
Weather Service. Available for Boulder (BOU), Pueblo
(PUB), and Grand Junction (GJT) areas.
http://www.nws.noaa.gov/view/prodsByState.php?state=CO&prodtype=discussion
• Ready yourself for the “Steamboat Surprise”.
• Understand how the computer model forecast is
trending as the storms moves closer. Computers are
never exactly correct.

Websites
• Current Snow:
SNOTEL: http://www.wcc.nrcs.usda.gov/snotel/Colorado/colorado.html
COCORAHS: http://www.cocorahs.org/
NWS PUBLIC INFO: http://www.nws.noaa.gov/view/prodsByState.php?state=CO&prodtype=public
RADAR/SATELLITE: http://www.rap.ucar.edu/weather/
• Current Temp/Wind:
MESOWEST: http://mesowest.utah.edu/index.html
WEATHER UNDERGROUND: http://www.wunderground.com/wundermap/
• Forecasts:
FIREWEATHER: http://www.fireweather.info/
TWISTERDATA: http://www.twisterdata.com/
NCAR: http://www.rap.ucar.edu/weather/