Case 5: Cascade Mountains
Links for the case: Data Viewer, Forecast Guide, Data Form
You’ve been asked to make an avalanche weather forecast for Mt. Baker Ski Resort, which is in the northern Cascade Mountains of Washington. All you know is that:
- Mt. Baker Ski Resort is in near latitude 49°N, longitude 122°W, with a ridge-top elevation of approximately 5,000 feet (1500 m)
- The forecast elevation is 4,500 feet (1400 m)
- You’ll be forecasting from 1200Z April 2 to 1200Z April 3
Here’s the process to follow:
Step 1: Open the Data Viewer and examine the following data.
- Satellite imagery: 0000Z to 1500Z on April 2
- Composite radar reflectivity: 0000Z to 1500Z on April 2
- GFS model output: From 1200Z April 2 through 1200Z April 3. Six-hourly forecast charts beginning
with the initial analysis at 1200Z April 2 and going through 1200Z April 3
- 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 use at one session 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 A.
The total GFS QPF is approximately 0.97 in (25 mm), with QPF during the four 6-hour time periods of 0.06 in (1.5 mm), 0.13 inches (3 mm), 0.40 inches (10 mm), and 0.38 inches (10 mm). Since the winds were strong and, during the latter half of the forecast period, aligned perpendicularly to the Cascade Mountains, it’d be reasonable to increase the forecast amount to over 1.0 inches (25 mm). Actual instrument data recorded 1.25 inches (32 mm).
2. What’s the minimum and maximum temperature range for the 24-hr forecast period? (Choose the best answer.)
The correct answer is B.
The elevation of the Mt. Baker Ski Resort is closest to the 850-mb pressure surface. On the GFS 850-mb forecast charts, the temperature rises for the first half of the forecast period and falls after that. The temperatures get close to freezing (-2 or -1°C) between 1800Z April 2 and 0000Z April 3. Then they fall to -5 or -6°C by 1200Z April 3. Actual instrument data had a maximum of 29°F (-2 °C) the afternoon of April 2 and a minimum of 22°F (-6 °C) at 1200Z on April 3.
3. What’s your estimate of new snow density based on temperature-density table? (Choose the best answer.)
The correct answer is A.
The forecast temperatures at 850 mb imply a fairly dense snow (at least 0.09). But this might be an overestimate since temperatures in the snow-producing portion of the clouds were probably colder. Actual data from the snow study plot at 4500 feet (1372 m) showed an average snow density of 0.07.
4. What is the forecast for the average wind speed for the 24-hr forecast period? (Choose the best answer.)
The correct answer is B.
This is tricky because much higher winds just offshore are circulating around the low pressure system coming onshore. The wind speeds on the 850-mb charts go from about 15 knots (17 mph or 8 m/s) at 1200Z April 2 to 25 knots (29 mph or 13 m/s) at 1200Z April 3. The average appears to be in the low-20 knot range (mid-20 mph range or around 12 m/s). Actual instrument data from that day had average wind speeds around 25 mph (11 m/s) and gusts to 75 mph (34 m/s).
5. What is the average wind direction for the 24-hour forecast period? (Choose the best answer.)
The correct answer is C.
Winds are southerly to southeasterly during the first half of the forecast period and shift to the southwest and west-southwest during the latter half. Actual instrument data from that day had the average wind direction from the southeast in the morning and south-southwest for the rest of day and overnight.
6. What is the average sky condition for the 24-hr forecast period? (Choose the best answer.)
The correct answer is A.
High relative humidity at 850, 700, and 500 mb persists throughout the forecast period, virtually guaranteeing overcast conditions. The satellite data show a well-developed, comma-shaped cloud coming onshore with heavy cloudiness covering northwestern Washington. The observations had cloudy skies throughout the forecast period with a few breaks after 0600Z April 3.
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 A.
To calculate the 24-hour total new snowfall, divide the estimated total water by the estimated new snow density (1.10 to 1.30 water / 0.07 density). This yields a snowfall forecast of 12 to 16 inches (30 to 40 cm). The actual 24-hour total snowfall at 4,500 feet (1400 m) at Mt. Baker was 17 inches (43 cm). Since all of the forecast values exceed critical thresholds, avalanche potential is expected to increase. (To review, the forecast is for more than 12 inches or 30 cm of snow in 24 hours, more than 1 inch or 25 mm of water in 24 hours, and average wind speeds greater than 20 mph or 9 m/s.)
8. Was the ridge-top wind direction consistent over the 24-hr forecast period? (Choose the best answer.)
The correct answer is B.
It shifted from southeast to southwest.
9. Based on your ridge-top wind direction forecast, which slope aspects have the best chance of being wind loaded? (Choose the best answer.)
The correct answer is C.
The southerly component to the winds would load the north-facing slopes. Later on, the westerly component would load the east-facing slopes.
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 increase or stay the same. Access the synopsis to review the case.
Links for the case: Data Viewer, Forecast Guide, Data Form
The avalanche hazard rating issued by the Northwest Avalanche Center for these elevations on April 2 was considerable and forecast to slightly increase the next morning due to expected new snow and wind.
Earlier in the week (from March 28 to 30), one to two feet (30 to 60 cm) of snow fell in the Cascade Mountains, accompanied by strong winds. A two-day break between storms allowed the snow to settle and stabilize. Crusts formed on south-facing slopes. In the overnight hours, during the break in the weather, some surface hoar was reported to have formed on some wind-protected, north-facing slopes at around 5,000 feet (1500 m).
On April 3, two young snowboarders were buried in a small avalanche just outside a ski area to the south of Mt. Baker in the North Cascades. It happened at 5,200 feet (1585 m) on a northeast aspect whose slope angle ranged from 37 to 41 degrees. Thankfully both snowboarders survived unhurt. But later in the day, skiers triggered a second avalanche in the area. The crown depth averaged almost three feet (1 m) and released on the surface hoar that had formed between the storms.