Wx Watch

Chart Reading Between the Lines

How surface analysis charts reveal forecasting basics

By Thomas A. Horne (From AOPA Pilot, April 2001.)

Every pilot ought to be able to interpret weather charts and make some simple forecasting assumptions — without any help from a flight service briefer or a DUATS printout. Why? Well, you never know when you'll be stuck somewhere without a full array of weather briefing products, without Internet access, and be forced into a briefing that relies on rapid-fire FSS verbal barrages over a pay telephone and no more graphics than a 12-hour-old surface analysis chart tacked on a bulletin board. Think that won't happen? Then try going on a long cross-country flight into some of the more remote areas of the United States (or, even better, the rest of the world — especially the Third World). Besides, all pilots should know more about the weather than his or her ground-pounding brethren for the simple reason that it affects us much, much more.

Here's a quick course in the lost art of reading weather charts. Let's say someone slaps down a surface analysis chart and asks you: What's going on now? What will the weather be in, say, 12 or 24 hours? Could you answer with any degree of confidence?

In such a case, the first step in weather forecasting is to identify the three or four largest pressure systems on the chart. It's rare that you'll find much more than this number of weather-makers over the contiguous United States. So you see, for example, a low-pressure center over the Great Lakes, a small high-pressure center over the Northeast, a large area of high pressure over the Southwest, and another low entering the Pacific Northwest from the Pacific Ocean. Those are your four systems.

Now it's time to scrutinize any fronts associated with these pressure systems. Sure enough, the Great Lakes low has a cold front stretching from its center and extending to the southwest at a 45-degree angle. To the east of the low, a warm front reaches out, then turns into a stationary front with a southward bulge in it, over the Mid-Atlantic states.

The low in the Pacific Northwest has a cold front, too. It's depicted as a bulging line running from Puget Sound to San Francisco. The high in the Southwest doesn't have any fronts — yet.

What can you deduce from this smattering of information? Quite a lot. Here are a few assumptions we can safely make about this situation:

The frontal complex emanating from the Great Lakes low is typical. The area between the warm front and the leading edge of the cold front is known as the warm sector, and is characterized by a flow of warm, moist air out of the south as the circulation moves counterclockwise around the parent low. Ahead of the cold front, we can expect the strongest winds, and the warm sector is likely to contain thunderstorms if the atmospheric moisture content and surface temperatures are high. Up near the low itself, expect the worst icing zones to wrap around the northeast quadrant if temperatures aloft are in the icing range. It being the Great Lakes, the icing here is apt to be of the large-droplet kind identified as a contributing factor in the October 1994 crash of an ATR–72 in Roselawn, Indiana. That landmark crash led to a reclassification of icing types, to acknowledge the severity of large-droplet icing (see "WxWatch: News From the Icing Front," February 1998 Pilot).

Heading north in the warm sector? Then expect tailwinds. Heading west, beyond the cold front? Sorry, it'll be northwesterly headwinds for you, and probably a rough ride.

The warm/stationary front looks like another potential problem. It's reasonable to assume that instrument meteorological conditions would prevail in this frontal zone, and that bulge in the stationary part of the front — caused by the flow around the high to its northeast — could create widespread shallow fog in the Mid-Atlantic, dropping ceilings and visibilities to low IFR (LIFR — ceilings and visibilities below 500 feet and one mile, respectively) values. In the colder months, low-level freezing rain is possible. This is a result of the colder air penetrating southward, from the clockwise circulation around that high in the northeast. The cold air lowers temperatures to the dew point, and all that moisture in the front can turn into a dense, persistent fog. This is a common setup in the fall months, when cold air from the north can flow all the way down to Georgia, damming up cold air, fog, and rain against the Appalachians. Any freezing rain will be caused by rain falling through a cold layer of air. The cold front associated with the Pacific Northwest low will also have a lot of moisture, so it would be reasonable to assume that it would contain widespread areas of IMC in rain and fog. The high in the Southwest will make for the best flying weather. Surface winds would be light, but in the warmer months convection could produce bone-jarring turbulence and air-mass thunderstorms.

Care to make any predictions? Given the usual movements of these kinds of complexes, you could safely predict that:

The Great Lakes low will move to the east, dragging the cold front behind it. That high in the Northeast may slow its easterly progress, though, and cause the instrument weather we talked about to linger in the Mid-Atlantic. The high in the Southwest will move east, too, and will eventually collide with moist air from the Gulf of Mexico. The result: a frontal boundary between the dry, high-pressure air to the west and the Gulf mugginess. Rain showers and thunderstorms will follow, and any low-pressure centers forming along the front will likely progress to the Northeast, bringing the storminess with them. The front in the Pacific Northwest will eventually move into the Rockies, where one of two things usually happens. One option is that the front stagnates and dies. The other is that its force regenerates on the lee side of the Rockies, and a new low-pressure center can form.

Finally, we can learn a lot about the nature of a depicted front simply by its alignment. Cold fronts that run north and south on a straight line are apt to be the fastest moving, and carry the greatest potential for the worst thunderstorms and wind shear. Those that head away from their parent low at an angle move more slowly and aren't as bombastic. Those with bulges — like the other one proffered here, in the Pacific Northwest — can be the slowest moving, which means that the weather they bring can last for days as the system plods eastward.

As for warm fronts, they move slower than cold fronts — by definition. Warm fronts with curves or deep southward extensions are most problematic. This is an indication that the warm front is turning to a stationary one — and that the lousy weather it brings will hang around until a sufficiently strong high or cold front can blow it away. Fortunately, it's also a sign that the entire low-pressure complex — low-pressure center, cold fronts, warm fronts, and anything else in the system — is dying from a lack of lifting forces aloft. This loss of upper-air support, it can be inferred, is the result of the jet stream's strongest winds (the source of the lifting aloft) moving to the south of the surface low's position.

So there. From one glimpse at one surface analysis chart we've come up with some pretty good guesses about the current and forecast weather. We have a darned good idea about where the instrument weather could be, where the winds aloft will be blowing from, how fast the fronts will move, how strong the fronts are, and where the worst thunderstorms and icing may lurk. It won't be a perfect assessment or prediction, but nobody's perfect. Keep track of official predictions and you'll see that they can come up short, too.

 

E-mail the author at tom.horne@aopa.org.

 

 

Posted Friday, March 16, 2001 12:48:10 PM

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