1. How the NWS Des Moines Uses IFPS Smart Tools to Issue Snow Forecasts
Rod Donavon October 31, 2006

2. Caribou Tool (Cobb Technique)
Snow ratio is considered a function of temperature, vertical motion and relative humidity.
Vertical motion of a layer is weighted based on RH. RH > 80% =1, RH = 0 = 0
Adjusted positive vertical motions are summed
Each layer snow ratio is normalized by the layer’s percent of upward motion contribution to the total.
Normalized layer snow ratios are added to give the cloud base snow ratio.
Clouds > -8 C are assumed to be composed of super cooled liquid. No Ice, No Dice. (Baumgardt 1999)
Snow ratio reduction due to wind speed (optional)

4. Caribou Tool (Cobb Technique)

5. Caribou Tool (Cobb Technique) Ex. Layer Temperature and Vertical motion.
Temp. = -22C ω = -5 μbs-1
Temp. = -14C ω = -10 μbs-1
Temp. = -8C ω = -5 μbs-1

6. Caribou Tool (Cobb Technique) Ex. Normalizing the Layer Snow Ratios
LSR = %ω = -5/-20 = 25%
LSR = %ω = -10/-20 = 50%
LSR = %ω = -5/-20 = 25%

7. Caribou Tool (Cobb Technique) Ex. Normalizing Snow Ratios Continued
Top Layer
12.0 * 0.25 = 4.0
Middle Layer
22.5 * 0.50 =11.3
Lower Layer
9.6 * 0.25 = 2.4
Cloud base
snow-ratio
= 17.7 or (18 : 1)

8. Caribou Tool (Cobb Technique) Sub-Cloud Adjustments
SFC Temperature (F)
Reduction Factor
31.0
1.00
32.0
0.56
33.0
0.25
34.0
0.05
35.0
0.00
SFC Wind (kt)
Reduction Factor
15 -24
0.87
>24
0.75

9. ER-Thickness Tool
Snow ratios calculated from the mb thickness forecast.
Reductions also made for temperatures 32 degrees or above.
Wind Reduction also optional.
Limitation, study was conducted over the mid Atlantic States.

10. ER-Thickness Tool

11. ER-Thickness Tool

12. Other Snow Tools
Garcia Tool: Mixing Ratio on an isentropic sfc multiplied by a constant (typically 2).
Snow Amount Tool: Can use a user specified snow ratio or a snow ratio based on surface temperature.