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

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)

Caribou Tool (Cobb Technique)

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

Caribou Tool (Cobb Technique) Ex. Normalizing the Layer Snow Ratios LSR = 12 %ω = -5/-20 = 25% LSR = 22.5 %ω = -10/-20 = 50% LSR = 9.6 %ω = -5/-20 = 25%

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 4.0 + 11.3 + 2.4 = 17.7 or (18 : 1)

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

ER-Thickness Tool Snow ratios calculated from the 850-700 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.

ER-Thickness Tool

ER-Thickness Tool

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.