Case 4: Teton Mountains
Links for the case: Data Viewer, Forecast Guide, Data Form
You’ve been asked to make an avalanche weather forecast for Jackson Hole Mountain Resort in Wyoming. You have little to no information about the area’s snowpack and weather history. All you know is that:
- Jackson Hole is near latitude 43°N, longitude 111°W, with a ridge-top elevation of approximately 10,000 feet (3050 m)
- The forecast elevation is 9,500 feet (2900 m)
- You’ll be forecasting from 1200Z April 1 to 1200Z April 2
Here’s the process to follow:
Step 1: Open the Data Viewer and examine the following data.
- Satellite imagery: 0000Z to 1500Z on April 1
- Composite radar reflectivity: 0000Z to 1500Z on April 1
- GFS model output: From 1200Z April 1 through 1200Z April 2; six-hourly forecast charts beginning
with the initial analysis at 1200Z April 1 and going through 1200Z April 2
- 300-mb charts (near jet-stream level)
- Geopotential heights (m): Black lines with contour intervals of 120 m
- Isotachs (kts): Color-filled contours beginning at 50 kts (57 mph, 26 m/s) with contour intervals of 20 kts (23 mph, 10 m/s)
- 500-mb, 700-mb, 850-mb charts
- Geopotential heights (m): Black lines with contour intervals of 60 m
- Winds (kts): Standard wind barbs in black (pennant = 50 kts or 26 m/s; long staff = 10 kts or 5 m/s; short staff = 5 kts or 3 m/s)
- Temperatures (°C): Red lines, dashed for < 0°C, solid otherwise, with contour intervals of 5°C (9°F)
- Relative humidity (%): Color-filled contours beginning at 50%, with contour intervals of 10%
- 300-mb charts (near jet-stream level)
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- QPF charts: 6-hour accumulated liquid precipitation; color-filled contours beginning at 0.05 inch (1.3 mm), with contour intervals of 0.05 inch (1.3 mm)
Step 2: Forecast the following avalanche weather variables, using the Data Form to keep track of your findings. (Note that you can enter data in the form but not save it. It’s for single-session use only.)
- Total 24-hour water amount (add up each 6-hour period)
- Minimum and maximum temperature range (convert to °F)
- From your temperature forecast, estimate the new snow density using the temperature-density table; divide the estimated total water by the estimated density to calculate a 24-hr snowfall accumulation
- Average wind speed and direction for the 24-hr period
- Average sky condition for 24 hours
Step 3: When you’ve finished examining the data, access the Questions tab. We’ll walk you through the avalanche weather forecast process to determine the impact of expected weather conditions on avalanche potential.
Step 4: When done, access the Synopsis tab and review the case.
Links for the case: Data Viewer, Forecast Guide, Data Form
1. What’s the estimated total accumulated water amount for the 24-hr forecast period? (Choose the best answer.)
The correct answer is C.
The GFS QPF shows about 0.15 inches during each of the first two 6-hour forecast periods, a small amount (0.02 in) during the third period, and no precipitation in the fourth period. Adjusting the total of 0.32 inches slightly upwards to account for orographic enhancement yields a forecast of at least 0.40 inches. Actual instrument data recorded 0.42 inches.
2. What’s the best approximation for the minimum/maximum temperature range for the 24-hr forecast period? (Choose the best answer.)
The correct answer is A.
The forecast elevation is closest to 700 mb. Using the 700-mb forecast charts, the area is under cold advection through most of the forecast period. The GFS 700-mb temperature drops from about -9°C at 1200Z April 1 to -13°C at 1200Z April 2. Adjusting for weak daytime heating and nighttime cooling yields a max/min of approximately -8°C/-14°C (18°F/7°F). Actual instrument data had a maximum of -8°C (17°F) on April 1 and a minimum of -15°C (5°F) at 1200Z on April 2.
3. What’s the estimated new snow density based on the temperature-density table? (Choose the best answer.)
The correct answer is C.
Temperatures between -8°C and -14°C support an estimated snow density of about 0 .04 or 0.05. Actual data from the snow study plot at 9,500 feet showed an average snow density of 0.04.
4. What’s the average wind speed for the 24-hr forecast period? (Choose the best answer.)
The correct answer is B.
Using the GFS 700-mb forecast charts, wind speeds are forecasted to be in the range of 10 to 20 kts (12 to 23 mph or 5 to 10 m/s) and strongest between 0000Z and 0600Z on April 2. Actual instrument data from that day had average wind speeds of 18 mph (8 m/s) and gusts to 44 mph (20 m/s).
5. What’s the average wind direction for the 24-hour forecast period? (Choose the best answer.)
The correct answer is C.
The winds will start off from the north but change to northwest and west-northwest. Actual instrument data from that day had average wind direction from WNW.
6. What’s the average sky condition for the 24-hr forecast period? (Choose the best answer.)
The correct answer is A.
The GFS RH forecast at both 700 and 500 mb average above 90% through 0600Z on April 2. After that, drier air begins to move into the forecast area from the west. Overcast skies can be expected with such high RH values. Actual observations showed mostly cloudy skies until just after 0600Z with clearing thereafter.
7. Is enough new snow, water, or wind loading forecasted in the next 24 hours to increase avalanche potential? (Choose the best answer.)
The correct answer is B.
To calculate the 24-hour total new snowfall, divide the estimated total water by the estimated new snow density (0.30 to 0.40 water / 0.04 density). This yields a snowfall forecast of 7.5 to 10 inches (18 to 25 cm). The actual 24-hr total snowfall at 9,500 feet (2900 m) was 10 inches (25 cm). Given that the forecast for the 24-hour period calls for less than 12 inches (31 cm) of new snow, less than one inch (25 mm) of water equivalent, and not exceedingly strong winds, avalanche potential is not expected to increase.
8. Is there enough snow to cause sluffing on slopes steeper than 45 degrees? (Choose the best answer.)
The correct answer is A.
Some minor sluffing could occur due to the low-density nature of the forecasted snow.
9. Given the low-density snow that’s expected to fall, could the 20 mph (9 m/s) average winds create new soft slabs on southeasterly facing slopes? (Choose the best answer.)
The correct answer is A.
The winds are expected to be strong enough to transport the low-density snow. Soft slabs may be created on leeward slopes but shouldn’t be much more than a foot or so deep.
10. Based on all of this information, what do you expect avalanche potential to be for the next day? (Choose the best answer.)
The correct answer is A.
The avalanche potential is expected to stay the same. See the synopsis for more information.
Links for the case: Data Viewer, Forecast Guide, Data Form
Based on 20 inches (50 cm) of new snow and high winds on March 29 and 30, the Bridger-Teton National Forest Avalanche Center issued an avalanche hazard rating of “considerable” on the morning of April 1. After the additional snow that day, the hazard rating remained the same on April 2. Unfortunately, a snowmobiler was killed on April 2 in an avalanche in the mountains just south of the Teton Range.
In looking at the radar loop, you may have noticed that the Salt Lake City (SLC) radar data were not available. Since you couldn’t see precipitation moving into the region from the southwest, you may have underestimated your initial analysis of the amount of precipitation that fell in the mountains just before the forecast period began. By using the satellite loop in combination with the radar data, you could have surmised that more precipitation was occurring over the mountains upstream of the forecast area than was being shown on the radar loop.