The VFR Rain Trap

It’s only 36 miles from the Muskegon, Michigan airport to Grand Rapids. The terrain is flat and both airports have lots of long runways. It’s hard to think of an easier flight to make.

The METAR sequence report for Muskegon showed calm wind, visibility of 10 miles, and scattered clouds at 10,000 feet with a higher overcast. It was raining lightly.

The METAR at Grand Rapids showed scattered clouds at 11,000 feet and 10 miles visibility. It was not raining.

The terminal forecasts for both airports called for showers in the vicinity but ceiling and visibility were predicted to remain good VFR with viz no lower than 6 miles and ceiling no lower than 4,000 feet for several hours.

What pilot looking at the reports of actual weather at both airports, and the forecast for continued good weather at both ends, would think that it would be a problem to make the short flight under VFR?

The Nexrad radar did show light to moderate returns over Muskegon airport, and scattered along the route to Grand Rapids. But the visibility was 10 miles or more even in the rain at Muskegon airport. There were students shooting landings in the pattern. And the Nexrad returns didn’t look any different along the route than over the airport.

I pulled my airplane out of the hangar in the light rain and called for an IFR clearance. I expected to be in visual conditions for the whole flight at the cleared altitude of 3,000 feet.

As I climbed through about 1,500 feet just a few miles east of Muskegon airport I flew into a cloud. During breaks in the clouds I could see a lower cloud/fog layer below me. The Nexrad picture hadn’t changed since takeoff. It showed just level one green with some areas of level two yellow to the south of my route. But I was on solid instruments just 1,000 feet above the ground.

I remained in clouds and rain at 3,000 feet most of the way to Grand Rapids. About six miles from the airport the clouds parted, the rain stopped and Grand Rapids had the good VFR conditions reported and forecast.

What happened? How can two airports so close together both reporting and forecasting good VFR have unexpected IFR conditions in between?

The most likely reason is that the rain cooled the air enough to bring the dew point and air temp together forming clouds. Rain cooled clouds are not uncommon, but can be difficult to forecast.

Terminal forecasts (TAFs) attempt to predict the weather only in the immediate airport area, a radius of about five miles. The TAFs were getting the forecast mostly right over both airports, but did not apply to the short distance in between.

The area forecast covers huge swaths of territory and deals in generalities, including predictions of VFR or IFR conditions. The area forecast for Michigan covers big chunks of the state, such as “SW QTR” for southwestern quarter. My route was in that quarter of the state, but the area forecast did not predict conditions going below marginal VFR.

This short flight is the perfect example of why only the windshield is a reliable weather observation for VFR flying. I only had two data points for weather, one on each end of the trip. Forecasters only had the same data points, too. What conditions actually were along the route at takeoff were a mystery.

Since it was already raining at Muskegon but the weather was still good VFR why did the rain create different conditions between the two airports? If the rain was going to create clouds, why didn’t that happen at Muskegon?

The answer is probably the effect of Lake Michigan. The water is still very cold and its presence can actually create its own weather, just as any coast can be impacted by air flowing over water. For reasons that are almost impossible to forecast the “sea breeze” clouds can form just off shore, or just a few miles on shore. On this day they formed on shore east of the weather observation at Muskegon airport.

The reminder for me on this flight is that if the dew point is high, and fairly close to the air temp, and there is rain showing on the radar, you have to expect clouds and IFR conditions to form unexpectedly no matter what the weather observations may be.

The great frustration is that most of the time light rain showing on Nexrad does not significantly change the ceiling and visibility. You can fly for miles and miles in good VFR with the Nexrad showing lots of green and even yellow returns. But then on a few days the same amount of rain can create clouds that are a very real risk for VFR flying.

If you’re flying VFR and see light rain returns show up on Nexrad you don’t need to panic. But you sure need to look ahead for any signs of low clouds forming, and be ready for an instant diversion to a nearby airport.


This entry was posted in Mac Clellan's Left Seat Blog. Bookmark the permalink.

10 Responses to The VFR Rain Trap

  1. Kayak Jack says:

    Thanks, Mac. I fly VFR only, out of Mason Jewett, (KTEW) just S’ly of Lansing. I’ve run into similar surprises with both clouds and turbulence. After not turning tail the first time, and foolishly pressing on, I now am more cautious. My buddy calls it “being a wuss”. I’m guilty.

  2. Bill Tomlinson says:

    My ab-initio instructor used to make me practice low-flying until I could do it in my sleep. Good insurance for conditions like this. (He and I once washed the windscreen by flying through an irrigation sprinkler – but that’s another story.)

  3. Roger Halstead says:

    As “they say”, been there, done that” and didn’t even get a t-shirt.

    If the dew point is “anywhere” near the temperature use caution and don’t forget the lapse rate. The closer the temp and dew point or the laps rate predicts they will be the same anywhere near your altitude, (the more you need to know about meteorology!

    It may have been lake Michigan and it may not. Without going into a long winded spiel explaining all the possibilities (which is difficult for me!), along with the dew point caveat, a couple of examples and a warning that, rain, snow, and wind do not require the movement of a front anywhere near your flight path.

    My wife and I were headed from Midland MI (KIKW) to Florida around late fall or early winter. Forecast was to be clear sailing all the way to Knoxville. Then light showers somewhere on toward the FL/GA border.

    Solid overcast showed up around Northern Kentucky and light showers by Northern Tennessee, but still high ceilings The forecast was still the same as when we started out. IE: They were a tad off. At-any-rate, we set down at Knoxville in steady light rain, but high ceilings and visibility greater than 10.

    We topped off the tanks (110 gallons with tip tanks), I’m paranoid about gas, so if the weather turned to crap, I could make it all the way home, non stop with everything full.

    After a snack, the forecast looked like we could make it close to Florida and then a short VFR over the top to clear sky in Florida. It was still a light, but steady rain as we walked out to the Deb. Time for a good preflight and we fired up. Take off was to the West and a turn back East to Snowbird VOR.

    We had just lifted off, with good visibility, when at roughly 50 feet the Heavens opened up. We went from greater than ten to where I couldn’t see the prop. No way was I going to set back down when I could barely see the runway lights to the side “down there”. We were IMC, the tower knew we knew and we knew they knew. You could hear the relief in the controllers voice when he acknowledged, we broke on top, between layers about 3000 AGL. We told them we’d head for Snowbird and play it by ear. If it got worse, or no better, we’d head back North. Definitely not what was forecast. Had that rain hit before we landed, I’d have headed back to Kentucky for an extra day.

    A few years later, in the summer, I was just going to take the Deb out to play. Forecast was for just a few scattered showers. No frontal movement anywhere near us. There was one shower over Grand Rapids (Heat island?). The typical, summer, Green, Yellow, Red bulls-eye that indicate the likely hood of a thunder shower. The rest of the state was clear, but it was hot and humid and the laps rate indicated a “top, or cap”. So I decided to wait for the next scan display. That showed the one storm was larger, but that’s all. So I threw everything in the 4-Runner and checked the RADAR again. There were a few new, small, green spots, but still between GR and Muskegon. Laps rate, temp, humidity, and high dew point made me uneasy so I waited a few more minutes.

    I could hardly believe what I saw. Draw a line from Muskegon to to about 20 miles South of Alpena. Make that a solid band about 50 miles wide, north to South filled with level 3 to maybe 5 thunderstorms. It had changed that much between display updates. When the rising air punched through the cap, that whole area went down. There was none of this scud running into trouble, or IMC sneaking up in either case.

    I was glad “the signs” had made me uneasy. The winter storm was more benign it’s not something I’d have flown into intentionally, while I’d have hated to have gotten caught in that summer change.

  4. Ted Jackson says:

    The old saying about the MKG area years ago was: ‘You have to be rated for IFR to fly from Coopersville to MKG’ Still a good idea unless lots of sun..

  5. Keith Nalley says:

    Something similar happened to me when I was a student pilot. I was flying from KBIV (West Michigan Regional) to 3GM (Grand Haven ). I may have have had about 15 solo hours at the time. I was flying at 3000 agl with 6 mile vis and then with almost no notice I was IFR! I slowly descended to about 2200 agl when I could again see the ground! I never forgot this and I did not put off my IFR ticket because of this!

  6. Dan Brown says:

    For most of my 60 years of flying, I filed IFR when going anywhere — especially at night.
    There were too many flights when the weather was worse than predicted.
    These days I’m flying under the sport pilot rules and am perhaps overly conservative in undertaking any trip out of the local area. I fail to see the rationale of limiting the proposed 3rd class medical exemption to VFR day only. It seems to me that being able to file and fly IFR when needed is a safety improvement. And keeping IFR currency works to increase flying skills in genera.

  7. Frank Giger says:

    “What pilot looking at reports….would think it would be a problem?”

    The one flying an open cockpit airplane! :)

    Seriously, though, the point that weather can change or be radically different between even two close points is valid. All the gizmos in the world can’t replace the Mark One Eyeball when it comes to clouds and rain, or the conventional wisdom of the Local Pilot who knows the typical patterns of the area.

    I’ve seen St. Claire County (PLR) here in Alabama go from CAVU to zero-zero in fifteen minutes when an inversion layer sets up fog over the lake that shifts a half mile to cover the field in the morning, particularly in the spring….at 0900 hours! Nothing for it but to either sit and wait it out or divert to Tuscaloosa ten miles away and sit it out there…or if one is lucky see that the wall of fog shifts to one end of the runway for a landing.

  8. Frank Giger says:

    Oops, forgot to address Dan, who makes a valid point… long as the aircraft is equipped for IFR (which mine ain’t).

  9. Pingback: The VFR Rain Trap | Left Seat | David A. Buser

  10. Greetings! Very useful advice inn this particcular post!
    It’s the little changes that will make the biggest changes.
    Thanks a lot for sharing!

Leave a Reply

Your email address will not be published. Required fields are marked *


You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>