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Our Faith in NEXRAD

September 16, 2025/by Jeff Van West

Choosing the Right Alternate: IFR Rules vs. Real-World Decisions

September 12, 2025/by Bret Koebbe

Filing an alternate airport often feels like just another step in the IFR paperwork shuffle. You type something in the box, hit “file,” and move on. But when the weather doesn’t cooperate, that alternate airport can quickly become the most important part of your plan.

Our Faith in NEXRAD

September 16, 2025/by Jeff Van West

Choosing the Right Alternate: IFR Rules vs. Real-World Decisions

September 12, 2025/by Bret Koebbe

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Our Faith in NEXRAD

September 16, 2025/by Jeff Van West

In my younger days, my concept of weather data in flight was watching the clouds get darker as I trundled along the VOR radial, and wondering how rough the next 30 minutes might get. Now, I feel naked without in-cockpit weather, and will ask for deviations around denser precip just to keep from spilling my coffee.

But such confidence can get a harsh reality check now and then. Let me share a story of a flight that could be titled, “A Man with Two Watches.”

The route was direct Albany VOR (ALB), when two hours of hand-flying through IMC looked like it would end. (Yeah, that’s a photo in a Cirrus with the autopilot off. It’s not digitally altered. We all need the practice to stay sharp.) The iPad in my lap displayed ADS-B weather with stronger returns right of my course. The smoothest ride might have been to head a bit left (north), before crossing over ALB as planned. However, the XM weather on the MFD displayed a 20-mile wide area of low-energy returns left of the course, implying that a course southeast of ALB offered the smoothest ride. The ideal path using the MFD as my guide would be right on the edge of what I’d purposefully avoid using the iPad.

This dilemma wasn’t life-and-death. Perhaps it wasn’t even even spilling-the-coffee or not. For simplicity, I just stayed on course to ALB, feeling equivalent bumps to driving down a dirt road. That’s not the point.

It’s concerning that with only one of the weather presentations, I might have asked for a deviation with complete confidence. Had the returns been stronger, that misplaced confidence would have had real consequences. Using only one of the NEXRAD images, I would have asked for different things. Surely, both displays can’t be correct. Or, can they?

Let’s eliminate some potential sources for the discrepancy. The weather was not a rapidly changing convective environment, and the timestamps on both NEXRAD images were nearly identical. Both were “composite” reflectivity but came from different weather providers. We tend to simplify our thinking of NEXRAD as “base” or “composite” as if base always shows returns from the lowest part of the sky and composite displays all returns from the ground to the stratosphere. It’s not that simple.

Base reflectivity usually means “lowest tilt” reflectivity. This angle has the best chance of showing rain that would reach the ground, but it isn’t always low — once the beam is 125 miles from the station, the lowest tilt of the radar is looking about 17,000 feet in the air. That’s one of the reasons it’s essential to combine images from multiple stations into a mosaic, which is a single NEXRAD image.

Composite reflectivity adds multiple scan angles into the mosaic, but not necessarily all of them. What data is included — and what is left out — is part of the “secret sauce” different weather providers use, and part of the reason why we may see such different images.

The radar images, also, might not be as different as they first appear. The orientation on the MFD was heading-up. The iPad was track-up, but I had panned around, so now it was on some odd orientation. The MFD had a current direct-to ALB after a previous deviation. The iPad still displayed an old course because it hadn’t been updated.

When you rotate and scale the iPad image to match the MFD (above), the two images better align. The iPad still displays heavier returns where the MFD displays fewer, but the gaps line up better, and the logic of just staying on the course to ALB looks more consistent. Note to self: Think hard about the map scale and orientation when making any weather deviation based on in-cockpit images.

The two images would agree more if the color scales were the same. On the iPad using ADS-B weather, the colors decode as: light green is 20-30 dBz, dark green is 30-40 dBz, yellow is 40-45 dBz and amber is 45-50 dBz. On the MFD, green is 20-30 dBz, yellow is 30-40 dBz, and red is 40-50 dBz. (The color scales are different for internet-based weather. Check the manual for whatever app you use to know what colors equate to in dBz for all situations.)

The danger zone is usually considered to be 40 dBz and higher, so staying clear of the yellow on the iPad is a usually good policy. But the MFD paints yellow for 30-40 dBz returns that might be fine to fly though, and that the iPad would paint dark green.

Given the differences in radar color scales, it, now, makes sense that the iPad displays a wide swath of dark green with bits of yellow to the right of the airplane, while the MFD displays a wide area of yellow with bits of red in the same locations. The largest area of red on the MFD (on that dashed, curved distance ring) is the same return as the yellow and orange shown on the iPad (furthest right of the screen, just to the right of ACOVE).

DBz isn’t the only factor worth considering. The distance between the “contour lines” reveal how much the dBz increases in intensity over a certain distance. The greater the change in intensity over a shorter distance, the greater the hazard. On this flight, the distance between contour lines is relatively wide–that’s pretty benign.

Even after all this alignment, there’s still a discrepancy. The iPad shows yellow and amber by V213 and V489 — a no-fly of over 40 dBz on ADS-B dBz scales — yet the MFD shows some yellow (30-40 dBz) surrounded by green (20-30 dBz), which I’d been flying through for nearly two hours. Remember, yellow and amber on my iPad would be red on the MFD. So, there’s an area I refuse to cross using only the iPad, but would bisect without much concern using only the MFD. That’s a little spooky.

Which display was right? Beats me. I could have asked ATC, but I didn’t. That might have shed light, or the controller might have called it “light to moderate precip,” leaving me as uncertain as before.

The takeaway is that all such information should be considered an educated opinion, not a fact. It’s data that’s been collected from various sources, processed automatically, checked by a human briefly, and then pumped out to our cockpits. It’s neither completely right nor completely wrong. And, of course, NEXRAD mosaics shouldn’t be used for sole-source choices on navigating any kind of weather, especially for close-in tactical decisions.

Having that extra info sure beats just watching the clouds get darker, though.

Watch This Video

Misleading Lightning

Stormscopes don’t seem as common as they once were, and that’s too bad because they’re a great adjunct to NEXRAD. A sferic device like a stormscope does a great job showing the relative bearing of the electrical activity, but it’s not always so hot on distance. Stronger lightning strikes appear closer to the aircraft than they really are. Weak ones appear further away than they really are — and, from the perspective of light aircraft, no lightning should be considered weak.

stormscope

By combining the lightning and the NEXRAD, you get a better picture. The bearing to the lightning should also be the bearing to the cell that generated it. If the lighting is between you and the cell, in apparently clear air, it’s likely those strikes are extra-strong, and that cell should be given extra distance. It’s also possible there’s activity in that spot but it’s growing fast and the NEXRAD isn’t painting any precip yet. Either way: Stay far away.

ForeFlight Question of the Month

Choosing the Right Alternate: IFR Rules vs. Real-World Decisions

September 12, 2025/by Bret Koebbe

Filing an alternate airport often feels like just another step in the IFR boxes to check. You type something familiar in the field, hit “file,” and move on. But when the weather doesn’t cooperate, that alternate airport can quickly become the most important part of your plan.

The regulations are straightforward enough. FAR 91.169 says you don’t need to file an alternate if, for one hour before and after your estimated time of arrival, the forecast calls for a ceiling of at least 2,000 feet and visibility of at least three statute miles. Many pilots know this as the “1-2-3 rule.” If either number is lower, you must designate an alternate, and that alternate must have at least one instrument approach available.

But while the regs define the minimum requirements, smart IFR flying is about going beyond the minimums. Choosing a useful alternate requires a blend of weather savvy, chart knowledge, and practical judgment. Here’s how to make that decision with confidence.

Step One: Big-Picture Weather Awareness

Don’t just skim the TAF for your destination. Consider what’s causing the forecast conditions:

Cold fronts may create pockets of low visibility right along your arrival route. Sometimes the best alternate is just ahead of or behind the frontal boundary.
Fog often requires a much larger diversion, especially if there’s no wind or a second cloud layer preventing solar heating.
Time of day matters. Conditions often improve after sunrise and deteriorate after sunset.
Terrain and elevation can affect ceilings — an airport 20 miles away but 500 feet lower might give you the clearance you need.

Modern planning tools like ForeFlight and Garmin Pilot make it easy to visualize these trends. A quick look at the Graphical Forecast for Aviation (GFA) can help confirm whether you’re picking an airport in better air or setting yourself up for the same problem twice.

Step Two: Legal Minimums

After narrowing down candidates, check the regulatory boxes. FAR 91.169 specifies standard alternate weather minima:

Precision approach available (ILS/LPV) → 600-foot ceiling and 2 miles visibility.
Non-precision approach only (VOR or LNAV) → 800-foot ceiling and 2 miles visibility.

But the chart may show higher, nonstandard requirements. Look for the black “A” symbol on the approach plate, which indicates exceptions. Often it’s as simple as “NA when local weather not available,” but sometimes the required ceiling and visibility are higher than the standard 600-2 or 800-2.

Remember: these requirements are for planning purposes only. Once airborne, your decision comes down to actual approach minimums, not planning numbers.

Step Three: Practical Considerations

A legal alternate isn’t always a good alternate. Ask yourself:

Does the airport have reliable weather reporting?
Is there fuel or FBO support if you divert at night or on a weekend?
Is the runway long enough and in good condition for your aircraft?
Would a towered field with ATIS make life easier in busy weather?

Thinking about services ahead of time can save you from an inconvenient or unsafe diversion.

Step Four: Avionics Limitations

Your panel also plays a role. If you’re flying with a first-generation IFR GPS (KLN94, non-WAAS GNS 430/530), you can only plan one RNAV approach between the destination and alternate — the other must have a non-GPS approach.

With WAAS navigators (modern GTN/G1000 NXi), you can file RNAV at both airports, but for planning purposes you must assume LNAV minimums. In reality, you can still fly the LPV if it’s available when you arrive.

Step Five: Planning vs. Reality

Flight planning happens hours before you actually arrive. By the time you’re established on an approach, the forecast may be irrelevant. That’s why datalink weather is invaluable: ADS-B allows you to confirm ceiling and visibility reports in real time and pick the best option.

Keep ATC in the loop, but remember they don’t see your filed alternate. It’s up to you to evaluate the current picture and decide where to go. Sometimes your filed alternate is still the best choice; other times it’s wiser to pick something new based on conditions.

Making Your Alternate Count

The alternate box on your flight plan is not a paperwork chore — it’s a risk management tool. Yes, the regulations set minimums, but the safest IFR pilots know how to combine those with weather knowledge, avionics awareness, and real-world practicality. When the destination isn’t an option, you’ll be glad you treated your alternate like more than a backup plan.

Half Sized Blog Element (Single Author Style)

Half Sized Blog Element (Multi Author Style)

Our Faith in NEXRAD

Choosing the Right Alternate: IFR Rules vs. Real-World Decisions

Our Faith in NEXRAD

Choosing the Right Alternate: IFR Rules vs. Real-World Decisions

Full Sized Blog Element (Big Preview Pic)

Our Faith in NEXRAD

Watch This Video

Misleading Lightning

ForeFlight Question of the Month

Choosing the Right Alternate: IFR Rules vs. Real-World Decisions

Step One: Big-Picture Weather Awareness

Step Two: Legal Minimums

Step Three: Practical Considerations

Step Four: Avionics Limitations

Step Five: Planning vs. Reality

Making Your Alternate Count

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