Wx Watch: Thundersnow!

Web site of the month

Finding archived weather data for long ago can be a challenge. But some information for dates as early as 1980 is available online. Just fill in the blanks and use the dropdown menus. To obtain graphics for the thundersnow flight I went to NOAA’s Earth Systems Laboratory Web site. Select the variables you want from the “variables” dropdown menu (I used sea-level pressure) then pick the altitude from the “analysis” menu. (I used 1,000 mb). Follow the directions for entering the date(s) and time(s) that interest you, skip down to “scale plot size” and enter 150 percent, then enter “USA” in the “region of globe” field. Then hit “create plot” at the bottom of the page and you’re done. Up pops your chart. Just don’t expect lows and fronts to be drawn for you; they must be inferred using pressure patterns and experience.

Do you note weather conditions in your logbook entries? I do. It’s a great way to look back over the conditions that prompted memorable >weather events. It also helps you quickly identify exactly when and where adverse weather occurred.

Originally, this article was going to address snow. That raised memories of snowy flights, and a logbook hunt for one in particular. One that featured a conglomeration of ever-worsening variables including a low pressure system moving much faster than predicted, a rapidly lowering ceiling and visibility, a diversion to an alternate, mishandling by air traffic control, and the closure of a major international airport. And how did I find this long-ago flight? By looking for this standout entry—dated March 8, 1984—in the “wx” column of my logbook: - 3 ¾ SF TRW++ (I used the old method for recording surface reports, from the pre-METAR days). Translation: sky partially obscured, ceiling 300 overcast, ¾-mile visibility in snow and fog, thunderstorm strong and increasing.

That’s right, a winter thunderstorm with snow. Meteorologists call this type of thunderstorm thundersnow. The skies boomed, lightning flashed, and all the while temperatures near the surface were just below freezing. What an odd combination! We usually think of thunderstorms as creatures of warm weather, but like so much in nature, this can be a misleading generalization. That’s because air mass thunderstorms depend on surface heating to trigger the rising air currents that cause moisture-laden air to condense as it’s lifted in an unstable atmosphere. But what could cause a thunderstorm when temperatures are cold enough for snow?

This is where a phenomenon known as elevated convection becomes relevant. As the name suggests, the convection in this process occurs at altitude. Typically, the lifting force is supplied by warm, supersaturated air riding up and over—a process called overrunning—a retreating cold air mass. Sound familiar? It should, because these are the dynamics behind the weather associated with warm fronts. If the air is comparatively warm and juicy enough along and above the warm frontal surface, and the air beneath the front is at or below freezing, then moisture falling through the frontal surface aloft becomes snow or sleet. Meanwhile, the lifting created by the ride up the same frontal surface causes convection aloft. Presto, elevated convection.

Thundersnow is a rare event. Memorable, too, because it tends to happen near areas of deep low pressure fed by moist flows of air, and distinguished by the low pressure system’s proximity to cold, continental air masses. Within the low pressure complex, the area known as Larko’s triangle is a favorite haunt of thundersnows—and the worst warm-weather thunderstorms, too! So are occluded fronts that extend from the northerly quadrants of a parent surface low.

Occluded fronts mark the boundary regions where advancing cold fronts catch up with preceding warm fronts. Result? The cold air mass helps squeeze that overrunning warm air aloft even higher, which really gives a convective boost. Occluded fronts, or occlusions, are often visible on satellite imagery as a “comma” of clouds wrapping around the surface low.

LFI to FDK, March 8, 1984

The purpose of this flight was to visit the NASA Langley Research Center in Hampton, Virginia, where then- AOPA Pilot Executive Editor Steve Thompson and I were to get a briefing about an experimental, stallproof glider. The day began with a weather briefing advertising a low pressure system forecast to reach my return route well after the flying day was done—at about midnight. Otherwise, the conditions were good for the relatively short flight from the Frederick (Maryland) Municipal Airport (FDK) to Langley Air Force Base (LFI) in Hampton. The 170-nm route would take us south through Washington, D.C., airspace then on to the Brooke and Harcum VORs.

Flying a Cessna 172RG, we launched into skies that featured only a high overcast, and the trip down was uneventful. After we finished the visit at about 3 p.m., I rechecked the weather for the return trip. The low and its fronts were picking up speed, ceilings were coming down, and an amended TAF mentioned 1,500-foot ceilings with good visibility beneath for the D.C. area. So I filed IFR and took off.

By the time we approached Brooke VOR, I sensed that the amended TAF was too optimistic. We were in clouds at 5,000 feet, it was snowing, and flight watch was saying that pressures were falling rapidly, along with ceilings and visibilities. With each handoff, ATC told the bad news. Ceilings were 1,000 feet, then 800 feet, then 500 feet, and snow had begun to reach the surface at Washington-Dulles International Airport, south of Frederick.

Because the weather was going downhill quickly, I figured that continuing to Frederick was a bad idea. It was dark now. The ILS at Frederick had published minimums set at 450 feet HAT (height above the touchdown zone)—not the usual 200 feet—and if I missed the approach I’d be flying for another half-hour or so to get to an alternate airport. In icing conditions. Except the weather situation just past Brooke was definitely not textbook icing; the snow was so wet that it was accreting on the airplane’s leading edges.

I requested and was cleared for the ILS approach to Dulles’ Runway 1R, crossed the outer marker (Tille, as I recall), and started down final. It was snowing very heavily now, but there was little in the way of turbulence. I was feeling pretty good at this point, when the tower told me to break off the approach. An inbound airliner was cleared for the same approach, and he would overtake me. I should have refused the clearance and landed, but I foolishly did as I was told. Vectors to Tille followed, and then I was cleared for my second low-IFR ILS of the day. I was definitely worried because I thought a missed approach would be next. I was approaching minimums and still didn’t have the runway lights in sight. Did I mention that the autopilot didn’t work?

Not to worry—the rabbit was a welcome sight at minimums. Now the snow was a factor. Nearing the threshold I could see that snow had completely covered the runway. The landing was smooth, thanks to two inches of wet snow, but taxiing involved a lot of brake-pumping and sliding to slow the ship down, get it off the runway, and to the general aviation tiedown area (which is now the tarmac between the main and midfield terminals). While taxiing, ground control gave the news: The airport was closed. I was the last airplane in.

While waiting for the van, Thompson and I were checking out the snowfall when it happened. A huge bolt of lightning hit a light pole in short-term parking, followed by a thunderclap. It was my first and I hope only experience with thundersnow.

E-mail the author at [email protected].