Rain Sounding
A simple sounding that shows Temp(Red) and Dew Point(black). Notice how they area close together from the surgace to aloft. This shows saturation and is likely precipitating. The blue line ignore at this time. Now notice the slanted lines. On a Skew-T sounding these are the isotherms(lines of equal temp). You want to find the 0C isotherm. In this case, the temp profile is to the RIGHT of the 0C line from 700mb to the surface (the isobars are horizontal). This indicates RAIN. It starts out as snow aloft, but hits the warm layer and melts. Thus falling to the earth as rain.
Now we will look at some forecasted soundings. I chose the Eta which in theory does better than the rest of the models because it has better resolution and deals with low-level temps the best. Notice that Temp profile is in Red and the Dew Point is in Green. You will find many different colors depending on the site. Just remember the Dew Point is always to the LEFT of the temp profile because the dew point can never be warmer than the actual temperature. Left on a Skew-T means cooler if you look at the isotherms.
Looking at this sounding, the first thing you see in an inversion. This occurs when there is warming aloft. The warming is from about 700 mb or so down to 850 mb or so. Above this the snow falls, then hits this warm layer and melts. Then the temp profile cools below 0C again all the way to the surface. Since this depth is large relatively speeking, the rain has time to freeze into a solid, in this case sleet. Below the picture I will include a text file that shows at different pressure levels, the temperature. This will show the inversion and how it re-freezes towards the surface. Scroll down for the info. You will see how the temp at the surface is just above freezing. That doesnt matter since this layer is not thick enough to melt the sleet.
Sleet Text File
Snow Sounding
This sounding is as simple as the rain sounding. Notice the temp profile dictates all levels are below 0C. Thus when the snow falls from aloft in the cloud, it does not melt, it simply continues to fall to the ground. 2 things to notice here. First, this is not a deep layer of moisture. Just looking at the temp/dew point profile, it is not together from the surface to a very high height. It only goes up to about 800mb or so. This is because the sounding is around the time when snow is winding down across the area. So there is no deep moisture but a smaller layer. Had this been a height of a snowstorm, the layer of saturation would be much deeper, much like the rain sounding above. Also note the winds aloft. They are NW which is a drying wind. This coincides with the drying that is taking place on the sounding. Cold air advection is taking place. As there is a N wind around 850 mb which on the previous sounding showed an inversion. At that time there was a SW wind which is a warm wind. The NE flow in the lower layers will help to cool that layer off to allow fro snow. Included is a text file so you can see the layers below 0C. When you see this, forecast snow.
Snow Text File
Sleet to Snow Sounding (Loop)
This is an example of a loop that shows the 2 instances above. A situation where cold air advection aloft allows for a change from sleet to snow. It starts out as sleet. Again see the inversion and then the cooling towards the lower layers. Then notice the winds change from southerly to northerly and watch that inversion dissapear. So by the end, its a snow situtation. Included is the text file so you can see this taking place with the real numbers.
Sleet to Snow Text File
Freezing Rain / Rain Sounding
This sounding shows a combination of things. Its a rarity. It shows a large inversion so the liquid is there. Then it cools to 0C between 900 and 1000mb. In this small layer it will freeze into ice, but not into sleet because the layer is not thick enough like seen before. It will likely go to freezing rain but then towards the surface it goes above 0C again. So a combination of freezing rain and rain is the outcome. Had the layer at the surface been below 0C, then it would be all freezing rain.
Freezing Rain / Rain Text File
Vertical Velocity (Omega)
This is a map of Omega. This is the vertical motion in the atmosphere. In this case its at 700mb which is a common layer to observe this. Also 500mb is another layer to observe Omega. The College of Dupage has both of these layers for viewing. In this case the NWS site only has 700mb.
Looking at the map, there is strong omega over the NE states. This is usually associated with strong precipitation. Now omega means nothing unless you have 2 other things. One is moisture, and 2 is some sort of lifting mechanism like a front or a coastal low like we have here. Point is, if you don't have an inversion on the profile, and its cold aloft, and you see strong omega, there is a good change for Dynamic Cooling. This is simply when the precip is very strong and it actually pulls down the cold air from aloft so that surface temps that where once possibly in the low 40s with rain, can go quickly to snow and into the low 30s. So when forecasting, be wary of the omega field and what the upper levels are doing.
There are 2 maps here to look at for the Eta. First there is the surface map with thicknesses and precip. Notice the placement of the surface low off the NE coast. At this time, per sounding analysis, the precip show be changed over to snow. Notice the thickness 5400 line. Its not over the CT region where we observed the soundings from. This shows that the thickness option doesnt always work. Soundings are far better. Use thinkness with caution. You will see the same thing on the AVN below. Below this is a meteogram. This is started at 9Z and goes to the late afternoon. It shows the precip getting cut off by 12Z. I started this time slot because according to the soundings, 9Z is when the snow started. So we know that everything from this point on is snow. Then simply calculate the total precip and you know how much the Eta is putting out for snow at that location. Note the precip is in mm, so you have to convert the total to inches by dividing the mm by 25.4. Then it comes out to inches.
Example AVN Forecast Plots
Here we have the equivilent of above with the surface map for the AVN. Notice the difference in the storm in the NE. It is deeper with more moisture. This can have ramifications on your forecast. Why? Well we know that in the morning it will change over to snow behind the system. Or do we? That snow solution came from using the Eta profiles. If you look at the AVN profiles, it might be different cause after all the models are depicting the storm differently. Just look at the meteogram for the AVN. Its much wetter. One is depicting a couple of inches the other over 5. This is where you have to look at other models for comparison and also look at the upper level features like the 500mb flow and the jet stream flow to see which model is believable. Its a science. Its never perfect but we try to get it as best as possible. Good Luck!
