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.