Wx Watch: Cold Facts About Cold Fronts

A cold front is predicted to arrive soon or to be a factor in your flight. What does this mean to you? Perhaps nothing. Perhaps deviating a hundred miles. Perhaps canceling the flight altogether. It depends on the nature of the front, where it originated, its speed, the nature of the weather ahead of it, and the conditions high above the surface front's location. This is true in any season, but now that spring is fully upon us, we need to pay special attention to any mention of cold fronts. That's because spring cold fronts can spawn some of the most violent, tornado-laden severe thunderstorms.

Let's take a quick look at the variables that can spell out a bad-news cold front.

Nature and speed of the front. There are many types of cold fronts. Some pass by without any weather at all. With these, maybe a shift in the surface wind or a smattering of clouds will mark the front. Aloft, conditions may be so tame that the worst "weather" might be just a few bouts with turbulence, with ceilings and visibilities not being factors.

Other cold fronts, of course, can be violent killers. Be sure to ask your briefer for the nature of any cold front forecast for your flight. Is it moving quickly across the terrain? Is it increasing or decreasing in intensity? Or is it slow, weak, and dissipating? A fast-moving cold front (say, traveling across the ground at 25 knots or better) is apt to bring the worst turbulence, gust fronts, and thunderstorms. The speed of a fast mover is so great that the front's leading edge creates tremendous lifting and instability as it shoves away the air ahead of it. Your job: avoid fast-moving cold fronts.

You should also ask for any tops reports. Cold fronts with high tops (above 25,000 feet) are almost always sure signs of developing thunderstorms. Be sure to ask for updates on cloud tops. Early in the day tops may be at 10,000 feet or so, but if they are passing through 20,000 feet and climbing by midday, it's a safe bet that thunderstorms will soon materialize.

The clouds in a cold front, by the way, often cause clear icing, regardless of any convection that may be present. This by itself is a good reason to avoid flirting with a cold front's cumulus when outside air temperatures are in the 0 to minus 10 degrees Celsius range, where clear icing is most likely.

Origin and current location of the front. A cold front moving from Washington to North Dakota is a far cry from a cold front moving from Kansas to Ohio. The cold front in the Northwest will probably be a wet one without much convection until it encounters the mountains, then experience weakening and "drying" as it moves farther east into flatter land.

Kansas' — or any other midwestern state's — cold front is definitely one to watch. Here, the mixing of moist air from the Gulf of Mexico with drier air to the west and north can cause the mightiest of thunderstorms, with tops shooting to 40,000 feet or higher.

It's the difference between the two air masses on either side of the front that determines a front's intensity. If the temperature and humidity contrasts between the two are great, then expect fireworks. The greatest of these contrasts occur in the lower Mississippi Valley, so watch closely any fronts that are forecast to develop in, or pass through, this region.

California, the Southwest, the extreme Northeast, and Florida do not generally experience a significant number of cold fronts in the warmer months of the year. Thunderstorms caused by localized convection and upper-air disturbances, yes. But fronts, no. Why? The climates there are more stable, and therefore not conducive to the temperature and humidity contrasts that generate fronts.

It's often said that fronts weaken as the days pass and as they travel across regions with weather conditions tamer than those of their origin. That may be true sometimes, but don't count on this as a rule of thumb. The same Midwestern cold front that produced killer tornadoes, Level 6 thunderstorms, and riotous wind shear in Arkansas may not peter out by the time it reaches South Carolina a day later. Cold fronts and thunderstorms can regenerate as daily solar heating cycles boost lifting forces and increase instability. Besides, "weakening" of a spring cold front's thunderstorms is a relative concept. If you're trapped in a thunderstorm in Georgia, it probably doesn't matter much to you that it's just a tad less violent than it was the day before when it went through Memphis.

Conditions aloft. All of the ingredients for a vengeful cold front may be in place at the surface, but if there's no lifting force aloft, cold fronts won't intensify to impressive levels.

On a 500-millibar constant pressure chart (this represents an altitude of approximately 18,000 feet msl), look for troughs — southward bends in the height contours depicted on these charts. Troughs provide lift for unstable air beneath them. Find the southeast quadrant of the trough. That's where the maximum lifting forces usually are located. If there's not a front or storm complex beneath this part of the trough, there probably soon will be. If you can't obtain a 500-millibar chart, ask a briefer to describe any troughs that he might see on his copies.

Flight service specialists are required to tell you of any frontal activity during your preflight briefing, but it may be up to you to probe for more details. First of all, find out the front's current location. Then ask about the weather there. Be sure to ask for any pilot weather reports in the area and note what types of airplanes made the reports (light turbulence in a Boeing 757 can seem like spar-busting jolts in a piston single). After learning the front's forecast position and intensity for the time of your flight, make your go/no go decision.

Assuming that you decide to go, check back with flight watch (122.0 MHz) to keep tabs on the front's behavior. By the way, the earlier you can depart, the better. Storms build with the heat of the day, and a trip that can be completed by 10 a.m. in a cold front's scattered-to-broken condition is often much, much safer than one with a leisurely 11 a.m. departure that can put you in agressively building storm cells an hour or so after takeoff.

Finally, a little tornado geography to keep in mind as you prepare for your spring flights. Many tornadoes and the violent cold fronts that can produce them occur in a wide swath that stretches from northeast Texas to the lower Ohio River Valley. This is "Tornado Alley"; notice that its orientation is roughly along a northeast-southwest line — the orientation of most cold fronts. But in spite of its name, Tornado Alley doesn't take the prize for the area having the highest frequency of tornadoes. That honor goes to the area just east of Tampa, Florida — where there is seldom a cold front, but lots of juicy, hot air and about 90 tornadoes a year.