Brief Like a Weather Pro

Since I began my column over 10 years ago in IFR Magazine, I’ve strived to write as though you’re with me in the weather operations center.

During most of my Air Force career, I worked as an aviation forecaster. For many years, this was a constant succession of four days on, three days off, with rotating shifts. After driving to work, we would begin a shift-change briefing. If this was in the morning, the entire weather station would participate. If it was evening, the events would be more relaxed, with just me and the outgoing forecaster reviewing things.

The outgoing forecaster began by covering the current weather picture. We would go over their forecast, itemize any warnings and advisories and review their expiration times, then
discuss special aircraft operations, morning route packages, and other problems. Once I got settled at the desk, I reviewed the charts, working up my own mental picture of the atmosphere and putting together a forecast. This built up groundwork for a TAF and our flight weather products.

This is what I will cover—and hopefully it will help you be a safer pilot.

Upper-Air Charts

You might have heard about the “forecast funnel” that describes how the initial workup is done. Rather than going haphazardly through random charts, the forecast funnel begins with large scale processes and works down to the bits and pieces. This is much the same manner in which a car mechanic would start by looking over the engine and running it to look for obvious problems, rather than opening the hood and taking things apart.

The top of the forecast funnel is where we start, and it stresses the use of large-scale processes. By large scale, we refer not only to spatial dimensions but also that of time. In other words, we’re looking for patterns that are large, persistent, and slow moving. These form the framework for the rest of the picture.

The best place to start is with the upper tropospheric charts. Here we use actual charts from the National Weather Service’s AWIPS (Advanced Weather Interactive Processing System). This is the core computer system that has been used by all NWS offices for the 20 years. 

The best upper tropospheric chart to start with is the 300 millibar chart. This shows conditions at an altitude where the barometric pressure corresponds to 300 millibars, or 8.86 in Hg. This is usually near 30,000 feet msl and is designed to help put us at the jet stream level in our forecast area without going into the stratosphere itself. During the transition seasons and summertime we will select higher levels instead (250 and 200 millibars) since the troposphere grows in depth during the warm season.

Looking over the chart, we see a bunch of black lines. These are isoheights, or geopotential contours. Since the actual height of the 300 millibar surface is heavily influenced by the density of the column of air below, this gives us a rough temperature map of where the atmosphere, in general, is quite cold (low heights) and quite warm (high heights). Not surprisingly, we see a vast area of low height in the polar regions. This is the hemispheric polar vortex—not the big bad polar vortex from television weather, but a permanent, extensive feature centered on both poles. On this chart, the polar vortex takes up almost the entire chart, because it drives the prevailing westerlies in the U.S.

On the other hand, the tropics are made up of a massive, semi-permanent subtropical ridge that forms a belt around the Earth roughly between 30 degrees north and 30 degrees south. If you want to see it, just look at the entire left edge and bottom edge of the chart: These areas are deep within the subtropical ridge.

Closer to the U.S. we look for lobes of the subtropical ridge that are of immediate interest. The most significant one is at (A on diagram at the top of this article) , covering the Gulf of Mexico and the Caribbean. The expanse of the subtropical ridge is strongly correlated with warm VFR weather. Thunderstorms, showers, and other deep convection are rare. During the summer, lobes of the subtropical ridge often build into the U.S. and are commonly associated with heat waves.

In a similar manner, we look for lobes of the polar vortex and areas where it breaks up into smaller circulations. A very prominent example is the large polar low over the Hudson Bay (B). While it only measures 2,000 miles across, it is a smaller, more intense circulation and is closer to what the news media describes as the “polar vortex.” While you might be thinking that this feature is a strong storm system, it’s more correct to describe it as a large area of extremely cold troposphere. This has the potential for frozen fuel in some aircraft.

Another polar low is found in the Gulf of Alaska (C). We can see that it’s not very deep and almost opens up into a trough. This is part of an occlusion and is closer to the jet stream and more likely to be associated with bad weather.

Speaking of the jet stream, let’s look at the gradient. Lines packed close together indicate a strong height (or pressure) differential, implying a strong pressure gradient. If your ground school instructor covered all the important stuff, you’ll know that this means strong winds. And it’s here that we find the jet stream.

You might be thinking that if a cold troposphere and warm troposphere are close together, that puts the upper level highs and lows close together and pushes the contours closer, making the winds stronger. That’s correct. And it’s what provides the potential energy to power the jet stream winds.

The jet stream is indicated by colored shading, with green indicating winds of 100 knots, yellow (120), red (140), and white (160). The strongest winds in our example are over Ohio and Pennsylvania (D), with jet stream winds of 160 knots.

These winds represent the polar front jet. The strongest winds are called the “jet max” or “jet streak” and are usually found directly west of deep surface lows. In fact the area to the east, where the jet max opens up, is referred to as the “exit region” and is closely tied to the location of strong surface systems and bad weather. In this example, this area is off the coast of Maine and near Nova Scotia.

Working our way down the forecast funnel, we see smaller waves known as short waves and medium waves. Troughs are found where the contours point southward and contain low values of geopotential height. The region along and east of troughs is associated with bad weather. The most important example is at (E).This is a deep trough that is moving west to east toward Texas with the prevailing flow. 

Also we find short-wave and medium-wave ridges, which point northward and contain high geopotential height. The best example is at  (F) approaching the Pacific Northwest. Good weather is frequently along and ahead of these smaller-scale ridges, so when you see these approaching, chances are you’ll have a relaxing day flying the local pattern.

There are even smaller-scale troughs that are important and contain more intense circulations. They measure from tens to a few hundred miles in size. These are the true short waves, and they do not show up on the 300 millibar chart. Forecasters have to turn to the 500 millibar chart (about 18,000 feet MSL) where they show up better.

Diagnostic tools like vorticity, Q vectors, and three-hour pressure falls are essential for helping reveal these features as seen in the chart on the previous page. While these tools are too complex to discuss here and are not always reliable, you can often uncover short waves on your own by locating bands of organized precipitation on radar and well-defined, compact cloud fields approaching your area.

Surface Chart

The forecaster then turns their attention to the surface chart. Again, this chart is similar to what you would see at an NWS forecast desk. Isobars (lines of equal pressure) are in black. The red lines indicate 100-500 millibar thickness. This is a direct measure of atmospheric density in a layer between roughly the surface and 18,000 feet msl, so in a sense it measures the average temperature in that particular layer. Because of this, we use these red lines for delineating the shape and extent of individual air masses.

The pressure field is dominated by a large surface high (G on diagram above). This represents bitterly cold air moving out of Canada. Other large highs are found on the map. There’s a stagnant plateau high over the Great Basin region, an oceanic high off the coast of California, and an older high over the Bermuda Triangle that represents polar air transitioning into warmer air over the Gulf Stream waters. Obviously there’s a lot taking place.

An unusual feature for the Southwest region is the deep low crossing the Rockies in northwest Mexico (H) . This is a Pacific low that is crossing over to the Eastern states, but at a low latitude. 

The low pressure area is part of the polar front that extends across Mexico into the Gulf. Farther to the east it joins up with another wave (I). This is an older frontal low that migrated across Texas and the Southeast states sometime earlier, where it produced wintry weather from Dallas up to Nashville. When lows reach that region, they have to be watched carefully during the wintertime for the development of a coastal low or nor’easter. This becomes less of a concern during the warm season since the air mass temperature contrasts are much weaker.

The weather map shows extensive ridging pointed southward from the high over Oklahoma. If you’re familiar with the terrain in Texas and Mexico, you can sort of visualize this ridge being banked up against the mountains of Mexico and New Mexico. This is “cold air damming,” where shallow cold air has pushed westward against the mountains without crossing over. It has also pushed south into northeast Mexico.

Forecasters will watch for such a surface low in Mexico to eventually lose definition and “ride over” the top of the cold air as it moves east. Then, it will enter the Gulf of Mexico and redevelop if the upper dynamics cause it to link up with the polar front.

While the weather system crosses the ridge, weather in Texas and the Gulf Coast will go downhill and low ceilings and precipitation will develop. Meanwhile forecasters in Florida will be alert to the possibility of a surface low approaching from the west. If the low deepens rapidly in the Gulf, a squall line can develop and move across Florida. One of the strongest such events took place in March 1993, producing a nighttime tornado outbreak across Florida.

This column first appeared in the May Issue 958 of the FLYING print edition.