Exploring weatherTAP's Aviation Weather
Written by weatherTAP.com
Last updated 11/6/2017, 12:30:58 PM
Severe Weather Outlooks
(under the Aviation Tab)
The next tab we move to is the aviation tab. Within this tab you will find the option for severe weather outlooks. You may be wondering what the different levels of advisory are all about. Well, this should help you out!
First of all, convective outlooks are issued by the Storm Prediction Center (SPC), which is located in Norman, Oklahoma on the campus of the University of Oklahoma. In addition, all severe thunderstorm and tornado watches (watch=conditions are favorable) are issued from the SPC. Every single one of them. Warnings (warning=severe wx is occurring) are then issued by your local National Weather Service (NWS) office.
The lowest level of threat is the general thunderstorm category. This is an area highlighted in light green that just shows that non-severe thunderstorms are possible.
If conditions look favorable for a couple of storms to reach severe limits, an area for the Marginal Risk of severe weather will be issued. The Marginal Risk is highlighted in a darker shade of green.
We get more concerned when a Slight Risk of severe weather is issued. This is an area that the SPC will highlight in yellow. Isolated severe storms are expected in a slight risk area, so everyone must pay attention to the weather when Slight Risk areas are highlighted.
We get even more concerned when an Enhanced Risk area is highlighted. This means that severe storms are likely, with slightly better coverage and/or intensity than we would expect in a Slight Risk area.
A Moderate Risk for severe storms really gets our attention. For many of us, this is the highest threat level for severe weather that our area will be issued in a severe wx season. Widespread severe weather is likely in a Moderate Risk and some of those storms are expected to be significant. Tornado outbreaks and/or widespread damaging wind events often take place in Moderate Risk environments. Most people in this threat level will be impacted in some way by severe weather.
In very rare instances a High Risk will be issued. This is the most concerning risk category of all. Widespread, destructive severe weather is likely within a High Risk area. Severe storms, some of which could be exceptionally strong, are possible. It is within these risk areas that folks see violent tornadoes (EF-4/EF-5) and/or significant, widespread, damaging winds.
That's generally the convective outlooks in a nutshell. Can there be a significant tornado in a slight risk area? Of course. Has there ever been a high risk area outlined and severe weather did not occur? Of course. But, most of the time the outlooks verify and severe weather occurs the way we generally expect it to.
Always be mindful of your threat level. The SPC updates these convective outlooks throughout the day. Knowing your risk can help you better prepare for the severe weather that may threaten your area.
As of now, these particular convective outlooks are only issued for Days 1-3. If warranted, each day will feature areas outlined for thunderstorm risks. A separate set of convective outlooks are issued for days 4-8, outlining possible threat areas in the coming week. Forecast accuracy for the 4-8 day outlook is low, but sometimes the signals are so strong and consistent that the SPC feels they can begin advising folks of the risk of severe weather.
Here's a handy chart describing convective outlooks, created by the SPC. Again, this particular set of threats are only highlighted on the days 1-3 outlook and not on the 4-8 day outlook.
Knowing where the jet stream winds are strongest, as well as how they are behaving, is key to any good forecast. The weather begins up high above our heads. What happens up there eventually influences what happens down here at the surface.
The atmosphere behaves like a fluid, so we often describe things in the atmosphere as we would a fluid. This is one reason why we often describe the jetstream as a river of air. It rarely flows in a straight line and often meanders, just as a river would do. The meanders tell us what to expect here at the surface (more on that in the next lesson).
The jet stream flows at an altitude of about 30,000 feet. It can easily contain winds that exceed 100 mph. So how does the jet stream influence our weather? I'm glad you asked!
Remember, the atmosphere is in layers as you go up. Each level influences the other in some way. When the winds at 30,000 feet increase, the air at another layer must rise to replace the air that is being carried away. Rising air cools and condenses, forming clouds. The farther down in layers that rising air can come from, the more those clouds can grow, possibly leading to intense precipitation and/or storms. That brings me to another point.
You may have heard the term "surface-based instability". This is exactly what that is referring to. If warm, moist air here the surface is lifted upward by a powerful jet stream, thunderstorms can result. Since warm, moist air is fuel to a storm, the more warm and moist our surface air mass is determines how big storms can get (generally). Air that is rising from the surface can also lead to tornadoes. A tornado is basically an updraft that can draw air from the surface. If it cannot draw air from the surface, it cannot reach the ground and cause damage.
When you use the jet stream analysis tool on weatherTAP, you can see the path the jet stream is taking. Like a river, the middle of the jetstream is fastest, with slower winds on the sides. Embedded within the jetstream's winds are areas of faster winds known as jet streaks. These are areas where the faster winds of the jetstream will cause localized areas to have air that is rising even faster, leading to precipitation or even storms. A typical jet streak may influence an area the size of the state of Tennessee. In areas with no coloration, that just means the winds are below 60 knots.
When you hear us forecasters say "there's a disturbance coming" we are often referring to a jetstreak that is on the way. We have to keep an eye on these jetstreaks because they can bring sudden changes in the weather.
The jet stream behaves like a river of air at around 30,000 feet above the surface of the earth. It crosses the earth and can generally be found high above a conflict between cold air and warm air at the surface. Cold air is often found to the north of the jetstream, while warm air is found south of the jetstream.
Like all wind, the wind of the jet stream is the result of pressure imbalances. The higher pressure of cold air and the lower pressure of warm air creates wind. Wind is Nature's way of trying to smooth out the pressure difference. The winds of the jetstream are a result of pressure/temperature imbalances between warm and cold air.
Temperature and pressure share a very close relationship with each other. Cold air is heavy and sinks, pushing down on surface objects and creating high pressure. Warm air is light and easily rises. This means that warm air is not pushing down on surface objects, creating low pressure.
In the winter time, the jetstream dips into the U.S. and brings changeable weather. If you find yourself south of the jetstream you have warmer weather. If you are north of the jetstream you have colder weather. In the summertime, the jetstream retreats to the north and has little influence on weather in the U.S. The U.S. is dominated by warm air in the summer and we don't see the conflict between warm and cold air. Therefore, the jetstream products are often most valuable to those of us in the U.S. during the winter time.
There are different aspects of the jetstream that are very important to us. Keep in mind, the jetstream is normally found at around 60 north latitude, which is right through the middle of Canada. During the winter, it sinks a bit farther south. During the summer, the jet stream moves farther north. As the winds of the jetstream circle the globe, they encounter obstacles that cause the jetstream to become wavy. One such obstacle that causes it to become wavy is the Rocky Mountains.
As the winds of the jetstream come across the Pacific Ocean, they are able to accelerate, since there are no obstacles across the ocean. As the jetstream comes into contact with the Rocky Mountains, it is forced to decelerate. This causes the jetstream to buckle and become very wavy. The speed at which those winds hit this obstacle determines how much of a wave the jetstream develops. Note: even though the jetstream winds are so high up, they are affected by the drag of the surface winds below them that are making contact with huge obstacles like the Rockies.
This is the cool part! The jetstream often hits the Rockies with similar intensity and in a similar location. Because mid-latitude cyclones (storm systems) like to form at the base of the bends in the jetstream, we can say with a fair amount of certainty where the next storm system will develop. In the U.S. that favored region for such cyclogenesis is southeast Colorado.
The image below shows favored area of cyclogenesis, with the one area in southeast Colorado being from the bending of the jetstream as it interacts with the Rocky Mountains.
And this diagram, found at www.atmo.arizona.edu, shows how this all comes together.
So, understanding what the jet stream is doing is vital to a good forecast. The faster the winds of the jetstream, the stronger the cyclogenesis may be, which affects all of us in the central and eastern U.S. Stronger storm systems can bring severe weather, blizzards, flooding, and a variety of weather across the U.S.
So, while the jetstream is high above us it certainly affects what happens here at the surface.
I hope this expands your understanding of the jetstream. If you have any questions don't hesitate to ask!