Sounding 5 Discussion

Discussion on Sounding 5 - Sleet

The wind structure: Similar to the the early soundings, this wind profile is more interesting than the snow sounding in number 4. Low level northeaterlies indicate that either a developing low center is to the south or a surface ridge is building south and east of the area. However, it is consistent to say a low is to the south with the upper level trough located further upstream (west). The southwesterly flow aloft leads me in this direction.

Again apparent, as seen in many of the other soundings, a layer of warm advection is present above the surface cold dome located near 750 mb. This layer has winds veering from southeasterly to southwesterly and increasing in speed to 50 knots through only a 100 mb deep layer.

The temperature and moisture structure: coincident with the geostrophic warm advection region in the wind profile is the elevated warm layer existing from approximately 720 mb to near 800mb. Below this layer, temperatures are below freezing to the surface. Above the warm layer, the atmosphere is moist and the cloud top is cooler than the -12C to -15C range indicating that ice is present.

Using the chart from the sounding 3 discussion, the warm layer temperature would fall into the 1 to 3C range, the precipitation would have ice introduced from upper levels, and the surface temperature is less than 32F. Thus, it would fit that the precipitation type is sleet or ice pellets...with a possible mix if the inversion top max temperature temperature warms to 3C.


Assuming the surface temperature is <32F:

Warm Layer Max Temperature

Precipitation Type
with ice introduced

Precipitation Type
without ice introduced

<1C

Snow Freezing Rain

1C to 3C

Sleet (1C) to Mix (3C) Freezing Rain

> 3C

Freezing Rain Freezing Rain

Therefore, the supercooled droplets of water freeze via heterogenous nucleation and produce ice above about 580 mb and begin to fall. The snowflakes then warm in the layer above 0C to melt into partially frozen and liquid particles and then they refreeze to sleet below the warm layer. The forecaster would need to watch for continued warming in the 780 mb layer to possibly switch the precipitation type to freezing rain.

This may occur in an approaching east-west warm frontal case where the warm air aloft continues to increase. In this case, you would begin with snow well to the north of the surface warm front, change to sleet moving further south, and then change to a mix slightly further south. Finally the change to all freezing rain would occur as the warm layer coninues to both warm and deepen aloft, provided the surface temperature was below 32F. Finally you would go into the warm sector and change to all rain.


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.