12/24/2023 0 Comments Short range surface prognostic chartThis will cause a strong “cold overlying warm air” situation in the lowest mile of the atmosphere, which translates to strong but shallow instability. One is when there is a combination of a fresh cold-air outbreak and very strong heating. The cold air mass driving the cold front naturally tends toward clear weather, good visibilities, gusty winds and no precipitation, but there are exceptions. The area below this frontal surface contains cooler temperatures, and if temperatures are already close to freezing you may be looking at significantly increased icing hazards if you descend through the frontal surface. Unless you are dealing with a “dry front” you can expect to find broken and overcast cloud layers close to and especially just above this altitude. What does the frontal surface mean to you? This contact zone is near the region of strongest lift. In practical terms, this is a 6000-foot rise per 100 nm, but over mountainous terrain, the slope is often steeper, since the lowest layers tend to get tangled up in the terrain while the higher layers sail along unobstructed. In the case of a cold front, a good rule of thumb is that this averages 1:100. The surface of this blob, where it contacts the warm air above and in front, is the “frontal surface.” Meteorologists occasionally talk about the slope of this frontal surface in terms of rise/run. With any front-cold, warm, occluded, or stationary-there is always a cold air mass involved, and the front marks the edge of this cold “blob.” The blob always slopes upward with height toward the cold air. What should you be thinking when you see this? First off, it’s important to avoid two-dimensional thinking, “There’s a front across Idaho I need to watch out for.” Instead, think of the three-dimensional structure. Its frontal marking is outlined by triangles pointing along the direction of advance. In the Pacific Northwest we find a cold front (A). Let’s go through the weather systems piece by piece and discuss some of the things appearing on this chart that you’ll likely see again. I’ve selected a random late summer chart to review. In Wx Smarts we give you the “why,” and as we’ve said in countless articles over the years, understanding and situational awareness are your ace in the hole when things don’t go according to plan. This gives you the “how” and “what” of reading the chart and understanding what you’re seeing. FAA AC 00-45G, Aviation Weather Services Section 8-1 provides a solid foundation on these charts. Of course, IFR is written for the “accomplished pilot” who often goes beyond the basics. There is also a separate chart set under the “Surface” tab that ranges out to 8 days in the future, but there are no precipitation markings after 48 hours. They’re found many places, but the source is the NOAA Aviation Weather Center website under Forecasts > Prog Charts > Low Level. Progs are published four times a day and provide a comprehensive overview of weather across the United States out to 24 hours. Prognostic charts (“progs”) rank among the most used weather charts in aviation.
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