September 1, 2010
Of all the products that make up a good preflight weather briefing, the Terminal Aerodrome Forecast (TAF) must surely rank very near the top in importance. TAFs give us 24- or 30-hour views into the anticipated weather conditions at select airports around the world. Where else can you find such a rich trove of surface forecasts? On meteograms, that’s where.
Some meteograms take the TAF concept in another direction—and others are depictions of past surface observations over 24-hour time frames. Where TAFs and METARs are strings of alphanumeric information, meteograms chart visual pictures of essentially the same data. Although they don’t serve as substitutes for TAFs or METARs, a check of meteograms lets you easily see 24-hour weather trends in graphic form. They are also helpful as learning tools, because they let you see the relationships between several weather variables. These variables can mark critical events such as frontal passages, the onset of precipitation, a plunge in visibility, or a drop in cloud bases, to name a few.
Meteograms, like so many other forecast products, live on the Internet and come from computer-generated forecast models. In fact, they’re produced by more than one model. The National Weather Service’s National Center for Environmental Prediction (NCEP) uses the NAM (North American Mesoscale) model on its meteogram website. The National Center for Atmospheric Research (NCAR) uses its RUC (Rapid Update Cycle) model, and Unisys and the University of Wyoming tweak government data to make their own meteograms. Codes and symbols for interpreting meteograms can vary between meteograms—but links on meteogram websites can give you clues to cracking these abbreviations.
Another caveat is that some meteo-gram websites offer more airport observations and forecasts than others. Unisys, for example, has just a few airports to click on. There are more on the NCEP’s NAM forecast meteogram map sites, and the University of Wyoming takes the cake with worldwide coverage—and even posts a meteogram archive. A little surfing will give you a better idea of what’s available.
From the University of Wyoming website, I called up the meteogram (select “GIF Meteogram” in the “Type of Output” drop-down menu at the top of the page) for Green Bay, Wisconsin (GRB), between June 8 and June 9, 2010. I picked that day because an occluded front was passing through at the time, and the meteogram’s observations certainly reflect that. Now let’s take a quick look at what was going on in that time frame. (Detailed callouts on the opening page illustration touch on all the data.)
I recommend starting with the bottom chart. This shows the dates and times along the horizontal axis of the chart, with the date coming first, then a slash, followed by the time (in UTC, or Zulu time). So right away we can see that the forecast time frame runs from 1053Z on June 8 to 1153Z on June 9. Times are posted in three-hour intervals, and refer not just to the bottom chart, but extend upwards for the observation times on the top and middle charts.
That bottom chart shows a steady drop in barometric pressure—or altimeter setting—from a high of 1020 millibars or so at 12Z on June 8, to a low of about 1007 mb at 06Z to 07Z on June 9. Then comes a slight rise. Right off the bat, you can assume that low pressure began to move in just before dawn on June 9. Was it a front? Let’s look at the other two charts for more information.
The middle chart plots all the above elements, and the evidence does indeed strongly suggest a frontal passage on June 9, at around 07Z. Look at the station models lined up across the top of the chart. See how southerly winds switched around to westerlies at the time in question? Also, notice the lowering of the cloud bases to some 1,000 feet at 06Z on June 9 (the “O” stands for overcast; “B” means broken; “S” means scattered; and “C” means clear skies).
As for visibility, look at the trace drop to less than three statute miles at approximately 23Z on June 8 (visibility values are listed at the right edge of the chart). Why did this happen? For that, look at the “WSYM” (precipitation type) line above the center chart. The green symbols show that continuous moderate rain occurred between 23Z and 02Z during the night of June 8 through 9. Then fog formed a few hours later. By the way, that rainfall arrived on southerly winds gusting to 18 and 19 knots, as shown in the “Gust” line.
The top chart shows the temperature-dew point spread narrowing during those predicted rain and fog events, and relative humidity rising to the 90-percent level by the end of the forecast period. Hourly forecast temperatures—listed along the bottom of the chart—show lows of 53 and 54 degrees Fahrenheit at 22 and 23Z at the end of June 8, and highs of 60 and 61 degrees as the west winds kicked in.
So what happened aviation-weather-wise? At a glance, we can see that instrument weather arrived in Green Bay early on the morning of June 9. By mid-morning (UTC) on the ninth, skies cleared, and visibilities increased. It was a cool, damp morning after more than a half-inch of nighttime rain (the “P061” line shows the maximum precipitation level—0.61 inches of rain—for the previous six hour interval), but other than that, a nice spring day in the offing.
Next time you’re planning a flight into a data-sparse region, bear in mind that a lack of TAFs may not mean a total lack of forecast information. For example, forecast meteograms from NCEP’s NAM data show plenty of airports not covered by TAFs, so there may be a meteogram that’s close enough to your destination to provide an idea of the forecast weather. Although they may not be up to legal-briefing TAF standards, forecast meteograms offer useful, educational supplementary advice. And meteograms of surface observations are great for answering those “What happened?” questions concerning past weather. Just remember: be sure to check valid dates and times!
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