Pattern Recognition in Meteograms

This article initially appeared in The Avalanche Journal, Volume 125, Winter 2020-21

By Uwe Gramann

METEOGRAMS, THE DISPLAY OF MULTIPLE weather parameters along a time series, have recently become a very useful decision-making tool for weather-related decisions in avalanche operations. Meteograms are traditionally evaluated based on absolute numbers, but these numbers alone are unable to unlock additional information that is contained in them, and that is the weather patterns within.

As general patterns repeat themselves much more frequently than numbers, the pattern recognition method is efficient and intuitive, especially for people who recognize visual changes. Counter-intuitively, pattern recognition can also result in better decision making since patterns or changes in parameters are often predicted with higher accuracy than the underlying absolute numbers.

In order to recognize patterns, one has to have seen them first. This article provides a first look at the most common patterns together with an initial interpretation so that you can find them later in real-world applications.

BASIC SINGLE PARAMETER PATTERNS

In all meteograms time progresses from left to right. You can picture yourself ‘riding’ on any curve, experiencing the change in conditions as you go. As a result, a line, or curve that is slanted from the bottom left upwards to the top right indicates a parameter that is increasing over time. Conversely, when a curve progresses downwards from left to right, the parameter decreases. This may be obvious, but always pay attention to the intensity as well: it could be a gradual five degree temperature change over three days, or a sudden five degree change over three hours. In this context it also becomes important that you note the scale of the parameter on the y-axis, which changes between meteograms.

In general, it can be said the steeper the line is, the more intense the rate of change is and the more attention you may want to pay to that time period. A good example would be a drastic pressure increase behind a frontal passage, which is directly tied to wind intensity in the wake of the front. Just as much as a slanted line indicates change, a flat line indicates persistent conditions. This may again be obvious, but it is often overlooked how important it can be to identify a period of persistent conditions. If the at line starts at the current time, powerful statements can be made such as, “What you see is what you get for the next …” Additionally, if you can identify two different but persistent time periods with different values (Figure 1), chances are that you found a pattern change of sorts located between them (e.g. from a warmer subtropical pattern to a cooler polar pattern).

The third basic pattern, minima and maxima, also occurs at times when some change is taking place in the atmosphere that you need to pay attention to. It could be that a frontal system is moving through the region, allowing for clearing in its wake (a pressure dip), or that a ridge of
high pressure is starting to break down, allowing clouds and wind to increase (a pressure peak). Just like before, be aware of the time frame within which changes happen and the associated intensity and scale.

A special kind of wave pattern that has a wavelength (peak-to-peak or dip-to-dip) of exactly 24 hours is the diurnal pattern that is caused by solar radiation, like temperature. The amplitude is dependent on the intensity of the solar radiation. A strong diurnal signal can be seen during clear days. A lack of a diurnal pattern in temperature (a at line) hints at a lack of solar radiation, and thus, clouds.

Diurnal patterns also show up in sea level pressure when the solar radiation is still strong (during shoulder seasons and summer), particularly over continental regions away from the moderating influence of an ocean. It shows up more in the Rocky and Columbia Mountains than the Coast Mountains. Keep in mind that since diurnal patterns are created by solar radiation, they become weak during the darkest days of the year and vanish entirely over the far north.

COMPLEX PATTERNS AND TRANSITIONS

The patterns discussed above become more complex when nature changes or combines them into new patterns. The most common of these are:

• Changes in intensity: In the left panel of Figure 2, isotherms are displayed in grey tones (pink-blue regularly) colours decreasing with time throughout much of Monday. However, a distinctly more intense temperature drop can be seen around 7 p.m. on Monday between 800 hPa and 500 hPa. Simultaneously, relative humidity decreases and winds veer from southwesterly to westerly. These are all hallmarks of a passing cold front.
• Transitions from one pattern into another: The top right panel in Figure 2 shows at-lining cloud cover and solar radiation, indicating persistent conditions for the first half of the period. As cloud cover decreases, solar radiation begins to show a diurnal pattern with a decent amplitude of about 700 watts per meter squared (W/m2). An important next step is to understand what caused this change in cloud cover and what it means to your operation(s).
• Two patterns appearing at the same time simultaneously: The bottom right panel of Figure 2 shows a diurnal temperature pattern overlain by a gradual temperature increase over the first three-quarters of the time period. Notice how both daytime highs as well as overnight lows increase over that time. This hints not just at clearing skies but also at a generally warming air mass.

The Pressure Dip

Sea level pressure is the most powerful single indicator of atmospheric changes. Almost all low-pressure systems and frontal passages are indicated by temporary low points in pressure, known as a pressure dip, followed by a pressure increase. The dip traditionally occurs together with changes in wind direction, precipitation, and cloud. The timing of the dip allows for surprisingly precise timing of surface frontal passages.

• The deeper and more intense the pressure dip, the more intense the weather that may occur. The top diagram in Figure 3 is from a hurricane passage in Nova Scotia; this should be easy to pick out. Check the scale and how intense the pressure drop is.
• If the pressure flatlines or decreases only slowly before suddenly increasing, chances are that a surface frontal passage lies right at the very beginning of the pressure increase. This is displayed in the middle diagram in Figure 3. Since this dip bottoms out near 1,015 hPa, weather associated with this feature is not expected to be intense. Note also how different the scale is compared to the hurricane case.
• If pressure also shows a diurnal pattern, as in the bottom diagram of Figure 3, it becomes tricky to see the pressure dip, but examining conditions at the time of the lowest pressure dip is a very good first approach.

PUTTING PATTERN RECOGNITION TO WORK IN A METEOGRAM

The real power of any meteogram reveals itself when you take the above patterns and apply them across all available parameters and interpret their meaning together. Over time, you will learn to find fronts and ridges quite literally with one quick look. The meteogram in Figure 4 is a good practice example to start with.

The top panel shows an obvious white-green-white pattern that helps to divide the forecast time into three different weather periods: a dry (white) period lasting until 8 a.m. on Thursday, a moist (green) period lasting until about noon on Friday, and drier (white) conditions again.
The isotherms (pink-blue) in the first period show a mostly gradual increase (warming) that becomes much steeper (strong warming) when the moisture starts picking up below 700 hPa (about 3,000m), suggesting a warm front associated with the second period. Temperature then
flatlines (stays persistent) in the third period but shows a definitive warmer environment than in the first period. A closer look at the outer envelope of the green shading in period two reveals it tilts slightly from top left to bottom right, therefore clouds can be expected to appear
in the mid atmosphere several hours before they reach lower terrain.

Notice also how the sea level pressure shows a broad maximum in period one around 11 p.m. on Wednesday indicating a ridge or high-pressure system moving into the area, peaking at 11 p.m. and then moving out again making room for the approaching front, increasing clouds, and backing surface winds from westerly to southerly.

In period two, you can see two precipitation periods associated with the cloud, the second of which is strongly correlated with a pressure dip near 5 a.m. on Friday. The pressure dip at the frontal passage is bottoming out around 1,007 hPa, which is not impressively low and reflected in the associated listless precipitation amount of four millimetres. It coincides with a shift in surface wind direction from southwesterly to northwesterly and thus strongly hints at a frontal passage that will trigger the clearing into period three. I would expect some improvement in sky conditions to start by 8 a.m.

Everything we have found so far is backed up by the 2m temperature that shows a distinct diurnal trend (clear skies) under the high-pressure ridge of period one. The diurnal trend becomes weaker (increasing cloud) and even disappears (overcast) into a gradual incline due to the warm front during period two, and is then followed by a warmer pattern with a diurnal trend reoccurring (cloud breaking up) in period three.

Meteograms are great tools to show exact forecast conditions at a location, but only by seeing beyond the numbers and recognizing the patterns will they reveal the story behind the numbers. With a little bit of practice and patience you will notice that these patterns occur again and again, allowing you to interpret the picture faster and with greater accuracy.

ABOUT THE AUTHOR

Uwe Gramann is a Canadian professional meteorologist and practice area lead for RWDI’s climate data and weather forecasting teams. He has worked his entire professional life in the mountains of western Canada and is passionate about mountain weather, forensic studies, weather models, and, particularly, about communicating meteorological information for decision making—like meteograms.