SPC Winter Weather Analysis

Data Courtesy Storm Prediction Center

Precipitation Type

Precipitation type relies on the "top down" method, where the initial precipitation type is modified as it falls to the ground. Our algorithm first searches for the highest 50 mb layer (in the lowest 5 km AGL) with RH > 80% and upward motion. The initial precipitation type in this layer is defined as: rain if temperature > 0 C; freezing rain if temperature > -5 C; a mix of freezing rain and snow if temperature between -5 and -9 C; and snow if the temperature is less than = -9 C.

After establishing the initial type, precipitation then falls through the temperature profile and changes if it encounters layers with temperatures above freezing (melts) or below freezing (remains frozen or re-freezes). Deeper and warmer layers result in melting to all rain, while shallower above-freezing layers result in only partial melting. This process continues down to the ground where the final precipitation type is assigned.

Legend:
0 - RA: Rain
1 - ZL-/ZR: Freezing Drizzle/Light Freezing Rain
2 - ZR: Freezing Rain
3 - ZR/IP: Mix of Freezing Rain and Sleet
4 - IP-: Light Sleet
5 - IP: Sleet
6 - IP/SN: Mix of Sleet and Snow
7 - SN-: Light Snow
8 - SN: Snow
9 - SN+: Heavy Snow
10- UK: Unknown

Near Freezing Surface Temp

Surface Wet Bulb Temperature

The surface through 6-km above ground level shear vector denotes the change in wind throughout this height. Thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells are commonly associated with vertical shear values of 35-40 knots and greater through this depth.

Critical Thickness

Thickness is the vertical distance between two isobaric surfaces, which is often proportional to the temperature of that layer. As such, empirical studies have shown that certain values of differing thickness have shown some skill in differentiating between rain or snow.

1000-500 mb (5400 m)
1000-700 mb (2840 m)
1000-850 mb (1300 m)
850-700 mb (1540 m)

Near Freezing Surface Temperatures

Dendritic Layer RH and Omega

The relative humidity (RH) and upward vertical motion (Omega) are displayed for the dendritic growth layer (defined here as the layer with temperatures from -12 to -17 C). Strong ascent and saturated conditions in this layer supports rapid growth of dendritic cyrstals, which favors heavy snow production.

Potential melting below the dendritic layer must be considered when evaluating snowfall potential. Also, please keep in mind that these real-time fields reflect the 1-h RAP model forecast of temperature, moisture, and omega aloft, and are subject to any biases in the model forecast itself.

Maximum Wet Bulb Temperature

The maximum wet bulb temperature in the lowest 400 mb of the atmosphere is displayed as a simple way to estimate the presence of potential melting layers in winter precipitation forecasting. Values less than 0 C suggest that the entire profile in the lowest 400 mb will remain colder than the melting point of ice, after accounting for potential evaporative cooling with precipitation falling through layers that are sub-saturated.

0-9 km AGL skew-T diagram for Winter Weather

Display depicts the vertical profiles of temperature (red), in the form of a standard skew-T/logP diagram for the lowest 9 km above ground level.

The 0 C isotherm (diagonal dashed black line) is plotted for reference, with any "warm nose" shaded in light red (layer warmer than 0 C) and any lower "cold layer" shaded in light blue (colder than 0 C, below a warm nose). The purpose of these plots is to assist with winter precipitation type forecasts.