Forecasting Lake Effect Snow

Answer these 9 questions

Part I Lake Effect Basics

The three factors normally needed to form precipitation include (1) sufficient moisture, (2) lifting mechanism, and (3) instability. Lake effect snow forms when a cold air mass (cP, mP, or cA) moves over a relatively warm lake surface. This creates and/or enhances these three factors by:

(1) As cold (and possibly dry) air moves over the warm lake, some water evaporates modifying the air mass. (This can be important for lake effect snow over the Great Lakes, but is less important over the Great Salt Lake.)

(2) When the lake is warmer than the surrounding cold land a temperature difference is created. This temperature difference creates a pressure difference. This pressure difference causes air to move together toward the center of the lake. This is called Thermal Convergence. As the air converges it rises, creating a lifting mechanism.

(3) When a cold air mass moves over the warm lake a steep lapse rate is created. Steep lapse rates indicate an unstable atmosphere.

This video gives a brief overview of how lake effect snow forms over the Great Lakes:

3D Look at How Lake-Effect Snow Forms! (Links to an external site.)

Part II Great Salt Lake Effect Snow

Similar processes occur over the Great Salt Lake. However, because the Great Salt Lake (GSL) is much smaller than the Great Lakes, its lake effect tend to be less severe and less frequent. But at times it can still create heavy snowfall in northern Utah.

Recent research at the University of Utah has improved the techniques used to forecast GSL effect snow:

http://wasatchweatherweenies.blogspot.com/2012/02/new-perspectives-on-lake-effect.html (Links to an external site.)

Based on the article above, answer the following questions:

1. What two times of year is GSL effect snow most frequent?

2. What time of day is GSL effect snow most frequent?

3. The authors identified two key factors in forecasting GSL effect snow: (lake-700mb) temperature difference and (850-700mb) relative humidity. How do these two factors combine to determine the probability of GSL effect snow? (Hint: describe the graph that plots both of the factors.)

Part III Forecasting GSL Effect Snow

Based on the research above, the authors created a forecasting tool that includes (lake-700mb) temperature difference, (850-700mb) relative humidity, 700mb wind direction and speed, and 700-500mb lapse rate to create a probabilistic forecast of GSL effect snow. You can take a look at the tool here (it updates real time):

http://weather.utah.edu/text/LAKE_EFFECT.txt (Links to an external site.)

Another method forecasters use are check lists. Here is a check list forecasters use for forecasting GSL effect snow:

• Strong northwesterly flow (aligned with the long axis of the lake).
• Consistent wind direction from the surface to 700mb.
• Minimum 16°C (29°F temperature difference between the lake surface and 700 mb.
• Large lake-land temperature difference, favored at night.
• Minimum 65% 850-700mb relative humidity.
• Follows the passage of a surface low pressure system.

Now, you will be the forecaster!

• It is November 8th, 2010 at 6pm.
• A cold front moved through northern UT earlier in the day and the associated precipitation is starting to taper off.
• UDOT is wondering if they need to keep their plow drivers on for overtime from now through the morning since they heard a rumor there may be lake effect snow.
• They currently have 25 plows out working to cover the entire Salt Lake Valley.
• Your job: Determine the likelihood of Lake Effect snow and suggest to UDOT how many plows to keep out overnight in the Salt Lake Valley.
• Too many -> Lose money & Too few -> Safety hazard

 Use the following weather maps to help you forecast the likelihood of GSL effect snow:

GSLE_WeatherMaps

Actions

4. Using the maps above, find the value of ALL of the factors below at 6pm (the current time) AND 6am (the next morning). Describe how the value or character of each changes throughout the evening.

a. Wind Direction and Speed

b. Wind Direction change from surface to 700mb

c. Lake surface temperature

d. 700mb temperature

e. Lake surface-700mb temperature difference

f. 850-700mb relative humidity (hint: look at Discussion “IL-8 Calculating relative humidity from stuve diagram”)

g. Location of surface low

h. Location of 700mb trough

5. Based off the data you collected in Question 4, what do you think the likelihood is for Great Salt Lake Effect snow in the Salt Lake Valley during the night?

6. How many snowplows do you think UDOT should keep out overnight for the Salt Lake Valley (between 0 and 25)? Why did you choose this number?

Now, let’s see what happened! Take a look at the radar imagery for the evening, and read the precipitation summary:

GSLE_Precipitation
Actions

7. Did your lake effect forecast verify? What do you think went wrong or right with your forecast?

8. Do you think you chose the right number of snowplows for the Salt Lake Valley on this night? Why or why not?

9. Describe at least three reasons why weather forecasting is challenging.

6pm November 9th, 2010

Weather Forecast Data

Sfc pres & obs: 6pm

Sfc pres & obs: 6am

700mb hghts & wind: 6pm

700mb hghts & winds: 6am

500mb hghts & Wind: 6pm

500mb hghts & Wind: 6am

Stuve Diagram: 6pm

Stuve Diagram: 6am

Lake Temperature

Lake temp

~11C/52  F

Radar

Imagery

6:00 PM

7:00 PM

8:00 PM

9:00 PM

10:00 PM

11:00 PM

1

2:00 AM

1:00 AM

2:00 AM

3:00 AM

4:00 AM

5:00 AM

6:00 AM

7:00 AM

8:00 AM

9:00 AM

Precipitation Summary
• Around 6pm frontal precipitation began to taper off, and by 7pm

it was showery, with some lingering snowfall in the

mountains.

• At 11pm a lake effect snow band forms on the north end of the

Tooele valley. It produces some brief heavy snow, but soon

dissipates.

• From 3 to 5am lake effect snow forms and begins to fall over

the Great Salt Lake, but because of light wind speeds, the

snowfall is not carried into the surrounding valleys of

mountains.

• From 6 to 8am the lake effect snow over the lake begins to

move toward Ogden propelled by a westerly wind. Ogden

experiences some periods of heavy snowfall and accumulations

of 3-5 inches.

• By 9am most precipitation has ceased.

• Throughout the night, the Salt Lake Valley received only a trace

of snow.