Snow is falling from a Nor' Easter. The howling wind is rattling the windows and drifting the snow. Then a sudden flash of lightning and then the low rumble of thunder. This is called thundersnow and I have only seen it once in my lifetime. It is an exciting experience to see the electrifying blue flash with the whiteout! The ingredients for thundersnow to occur are hard to come together as first there has to be a Nor' Easter to develop. First, a vigorous upper level disturbance has to track along the jet stream. Then the water temperatures have to be warm enough to produce great temperature differences along the New England coastline. It is the combination between the cold air coming down from a huge high pressure to the north combined with the warm moist advection coming from the south. It is the combination between these two that creates a significant storm along the east coast. Now, it is at this time that the upper air disturbance continues to energize the system and make it stronger and stronger. as the snow falls heavy. Then the storm is referred to as a "bomb". These storms are called this because it is an area of low pressure in which the barometric pressure in the center of the cyclone falls at the rate of twenty four millibars in 24 hours. Some of these Nor' easters have had the center of the storm fall sixty millibars in a twenty four hour time span! When these storms become very intense on the satellite picture sometimes there can even be an eye forming, just like that of a hurricane. It is when the storm is at this intensity thundersnow can be a factor. The Nor' easter forming is the first step. Now, I will go over the factors going into thundersnow. Thundersnow is a very dynamic and interesting weather event. When I witnessed it the thunder seemed to be not as loud as if it happened during the summer. Elevated convection is a factor for thundersnow. Convective Available Potential Energy is one factor that could be present in forming these thunder and lightning events. The convective available potential energy must be elevated therefore considering it elevated convective potential energy. This happens when the CAPE is not found at the surface, but instead found aloft. Now, this happens because the warm, moist air from the ocean overrides the colder air at the surface. Thus, it may look stable at the surface just looking at a sounding, but actually a person should be looking at levels above the surface. This should look almost like a slant on a skew-T showing the cold temperatures at the bottom and the warmer temperatures at the top. This can also be referred to as slantwise convection. If CAPE is not present then a person can look at the lapse rate. The lapse rate is when warm air overrides the cold air. This causes an Elevated Warm Layer. This instability of the warm air wedging on top of the cold air can cause thundersnow. With all this convection happening it is because of the dynamic lifting of the air as the water like I had said is very warm and the cold air is settled at the surface. This is exactly like in the summer as the cold air bumps into the warm air. There is just as much lifting in the storm of the winter as there is in the summer as the Nor' easter is very dynamic. So even if the elevated cape is not present there may be so much lifting inside the atmosphere going on that the thundersnows will develop anyways. Two charts that I like to look at for the formation of thundersnow are the 500 millibar vorticity chart and the 700 millibar vertical velocity chart. Another is the lapse rate. The 500 millibar vorticity chart shows positive and negative vorticity. In order for conditions to become more unstable there has to be lifting in the downstream of the vorticity maximum. This area will be to the right of the trough axis causes more lift over the ocean. It is here where the lifting is very, very strong and is feeding lots of energy into the system. This is where the 700 millibar chart comes into play as vertical velocity is a major factor into the lifting of the air. The 700 millibar chart shows exactly what is ahead of the vorticity maximum from the 500 millibar chart. This is the strong vertical velocities ahead of the vorticity maximum. These vorticity maximums cause high velocities. These high velocities cause air to rise just like in the spring and summer time with overshooting thunderheads. The other chart that is of great importance is the 300 millibar chart. This chart is just as important as the 500 millibar and 700 millibar charts because this shows more clearly where really active lifting is from the location of jetstreaks. The jetstreak is a very local region of very fast wind. These can reach wind speeds in excess of 160 knots. How a jet streak works is when air enters it, the jet streak inside the jet stream will speed up. Once that same air exits the jet streak will slow down. The slowing up and speeding up of the jet streak itself combined with the curvature of the jet stream and wind shear will cause the air to add up in some areas causing the air to converge at the surface. The divergence will happen over the ocean which a lot of times during a Nor' easter will cause that thundersnow above the surface causing the lifting and keep making the storm stronger/ just like the thunderstorms that happen over the Summer and also in the Spring. If divergence happens the storm becomes much more stronger and the threat of thundersnow becomes more and more likely. The problem with trying to forecast thundersnow is actually trying to forecast thundersnow. Thundersnow is something that is more difficult to forecast than the actual major Nor' easter itself only because thundersnow is a real mesoscale event which even the computer models can have a pretty difficult time in forecasting. My experience into looking at thundersnow is when a major winter storm such as a Nor' easter is threatening us here in southeastern New England I first look at how strong the actual surface low pressure is going to be. The major issue here is similar to what happens with hurricanes. As a hurricane comes across the ocean the models have a difficult time in forecasting them because there are not a lot of observing stations out there. This is somewhat similar in a Nor'easter as the models seem to have a difficult time over the water especially when the water is warmer than normal. When this happens the models could be showing a 980 millibar low, when actually the storm bombs out and is a 960 millibar low causing thundersnow. Also, the model may not show real heavy precipitation and show light precipitation. This is because of the mesoscale problem of banding. Just like with a hurricane, banding is when the storm becomes so intense that it is at a very small scale. It is in these bands that thundersnow can occur and snowfall rates could br as high as 5 inches an hour! I remember one time we had a major storm and in northeastern Massachusetts banding occurred and the snowfall rates were 3 inches to 5 inches an hour. The snowfall amounts were expected to be a total of 3 inches to 6 inches. Instead places there received 8 inches to 12 inches in a short amount of time. It is the thundersnow in these bands, just like summertime downpours that create these incredible amounts. A way of finding these areas of thundersnow is by looking at the 700 vertical velocity chart. The reason is because this chart shows the upward vertical velocity which has the greatest uplift. The more uplift the more chance there is of thundersnow. What I have learned from Nor' easters are a few and important things. Number one, forecasting thundersnow is not an exact science yet as there are still a lot of questions still not answered. However, I can now know when the possibility of it happening can. Number 2, always check to see how much the water temperature is above or below normal. The temperature anomaly chart is a good place to check this. Number 3, not to rely on the surface chart for precipitation amounts known as QPF because thundersnow is a mesoscale event and just like in a summer thunderstorm heavy amounts can be very isolated. Number 4, look at the 500 millibar chart for areas of greatest positive vorticity where to the right of that is the greatest uplift. That brings me to number 5, to look at the 700 millibar chart which shows the greatest vertical velocities and greatest uplift. This will not show how much precipitation a locality will receive, but it will show where the heavy precipitation will likely occur. It will show up as colors of red, purple and yellow indicating the highest uplift. It is in these areas where the 3 inches to 5 inches of snow will likely occur. Also, increasing the possibility of thundersnow. Number 6 the jetstream level which shows the jetstreaks and divergence making the storm stronger and creating shear so the thunderstorm tops will sometimes overshoot over the stratocumulus clouds. This is also a region of greatest uplift, especially is the jetstreak is coming your way to help bend the trough more and more making it stronger. Lastly, even though it is something looked at during the Spring and Summer months are CAPE which is elevated cape. Also, soundings are important to look at for the CAPE and also the Elevated Warm Layer because it indicates how deep the lapse rate really is. So what I learned from this report is that forecasting thundersnow should not be considered as a one dimensional chart to forecast it. A person has to look at more than one chart and see the other factors such as instability indexes. Looking at all these things will hopefully make thundersnow not much of a surprise and the next time it threatens I will be ready for it! References 1. USA Today 2. Haby Hints 3. The Weather Channel 4. Thunderstorms, Tornadoes, and Hail by Peter R. Chaston 5. Weathermaps Third Edition by Peter R. Chaston 6. ABC News 7. National Weather Service, KBLU 8. University of Wyoming |