Storm Impacting the Northwest U.S.; Fire Weather Conditions in Southern California; Severe Weather in the South

A Pacific storm is bringing areas of low elevation rain, moderate to heavy mountain snow, and high winds to the Northwest. Strong Santa Ana winds and very dry conditions are producing elevated to critical fire weather conditions in southern California. Isolated strong to severe thunderstorms are possible through early Wednesday morning across parts of northeast Texas into western Tennessee. Read More >

Winter Precipitation Types

Winter weather is certainly not a stranger to the Texas and Oklahoma Panhandles from late fall through early spring. More often than not, the main precipitation type that is observed is snow. However, sleet and occasionally freezing rain also affect the Panhandles. What causes these different types of precipitation? The vertical temperature profile is the most important aspect in controlling whether snow, sleet, or freezing rain occurs. Just as important, in order for ice crystals (snow) to form, there needs to be sufficient saturation (relative humidity values of 70% higher) present within a layer known as the dendritic growth zone, or snow growth zone. This is a layer in the atmosphere with temperatures between 10.4 and 0.4° F (-12° and -18°C) where snow forms. If there is not enough saturation present, snow and sleet will not form.

In order for the surface precipitation type to be snow, the atmospheric temperature (dashed red line in Figure 1) must be at or below 32°F (0°C) to ensure that no melting occurs. However, there are other special circumstances when snow can occur at the surface despite the entire atmosphere not being below freezing. The first situation occurs when there is a very shallow melting layer aloft with a maximum temperature in the melting layer less than 33.8°F (1°C). The second situation occurs when the wet bulb zero height (the height where the evaporatively cooled temperature, goes below 32°F) is less than 1,500 feet. Wet snow is most likely to occur in both circumstances since the snow flake has been partially melted.

Figure 1. Vertical temperature profile supportive of snow.

Figure 2. Vertical temperature profile supportive of sleet.

In Figure 2, notice that the entire atmosphere is not below freezing. In fact, there is a thin layer above the surface that is above freezing. In this example, sleet would be the dominant precipitation type. Sleet occurs when a snow flake partially melts and then refreezes. For partial melting to occur, the maximum temperature in the melting layer is typically between 33.8 and 37.4°F (1 and 3°C) and for a relatively thin layer (less than 2,000 feet thick).

When the snow flake completely melts and the surface temperature is below freezing, freezing rain will be the dominant precipitation type. In Figure 3, notice that there is a deep melting layer above the surface and a shallow freezing layer just above the surface. When the maximum temperature in the melting layer exceeds 37.4°F (3°C), the snow flake completely melts and becomes a rain drop. Since the freezing layer just above the surface is shallow, there is not enough time for the rain drops to freeze into sleet. As a result, the rain drops freeze on contact with the surface and can create very hazardous conditions. Freezing rain or freezing drizzle can also occur when the surface temperature is below freezing and there is not sufficient moisture present in the snow growth zone.

Figure 3. Vertical temperature profile supportive of freezing rain.

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