Precipitation Rate

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In order to start considering a precipitation rate, one must first have a process to generate precipitation. Whether it be a low-pressure system, convective showers, orographic lift induced precipitation, or any other method, one must have some sort of precipitation generated before rate can be considered.

Precipitation rate is highly impacted by the process that is generating the precipitation itself.

Synoptic scale precipitation (e.g. associated with a low-pressure system) forms under large scale, stable processes. The energy, moisture and lift is spread out over a larger area, creating precipitation rates that are generally steady and light to moderate. High accumulations may still be caused by the system, but more so due to its longevity. Heavier amounts of synoptic precipitation tend to be associated with additional factors such as significant orographic lift, support of frontal features or embedded convection within the system.
Convection (widespread or embedded), however, tends to produce much higher precipitation rates. Moisture and lift is concentrated in much smaller, or narrower regions, allowing much more precipitation to fall much faster over a small space.
Mesoscale or smaller processes that enhance lift in particular regions, such as onshore and upslope flow, small troughs, snow squalls, etc cause enhanced lift and cause very high precipitation rates over small regions.

While long lasting systems with low precipitation rates can be impactful, it is particularly disruptive when high precipitation rate events persist for a long time over a single location. Events like lake effect snow squalls, atmospheric rivers, or stationary thunderstorms can cause enormous accumulation and possible flooding.

Image source: Royal Meteorological SocietyOpen a new window

While precipitation rate measures the depth of precipitation expected to accumulate in an area over a specific time period, this information is not always straightforward to measure. When considering rain, rain showers, and freezing drizzle, this is much easier to measure with on-site instruments. However, when considering snow, flurries, snow grains, ice pellets as well as drizzle/freezing drizzle, fall rate becomes much harder to measure, due to the light and blowing/bouncing nature of frozen precipitation, exacerbated by wind and/or turbulence affecting fall rate. More factors must be assessed in order to precisely determine the actual depth of frozen precipitation expected to reach the ground.

Most liquid precipitation types (rain, rain showers, freezing rain) use precipitation rate to determine the intensity (-/ /+) that is noted in METAR/SPECI.

Liquid precipitation is also frequently accompanied by mist at the surface, so while precipitation intensity indicates how much liquid is falling per hour, it does not always have a direct link to the associated visibility. Additionally, a high precipitation rate will more quickly alter the condition of critical surfaces, rendering them wet, contaminated, etc.

This image is a simplified depiction of rainfall rate as noted in MANOBS Chapter 6.

Image source: Environment and Climate Change Canada

For frozen precipitation however, as well as drizzle and freezing drizzle, it is the visibility that is used to determine the precipitation intensity, as opposed to the precipitation rate itself. Since one must consider the snow-to-liquid ratio (SLR, discussed further in the Snow term) before the snowfall rate can be properly assessed, the readily observable visibility is used to determine intensity.

This image is of frozen precipitation and drizzle/freezing drizzle intensities and symbols, as defined in MANOBS (Manual of Surface Weather Observations Standards) Chapter 6.

Despite snowfall intensity and snow to liquid ratios, frozen precipitation accumulation depends on factors such as melting which takes into consideration the temperature of the air and the underlying surface (ex. runway) and any treatments that may have been applied to it. A treated runway, for example, might be experiencing a snowfall rate of 4cm/hr, while only 1-2cm is able to accumulate due to the chemical treatment of the tarmac.

Moderate freezing rain occurs infrequently and typically briefly in Canada, and heavy freezing rain is exceedingly rare. Light freezing rain, with high visibility is most frequently observed. Heavier rates tend not to last, since associated downdrafts (air being pulled down with precipitation as it falls) are often from the warm air aloft, which raises temperatures at the surface.

Image source: Environment and Climate Change Canada

Dissipation

As a weather feature moves away from its original source region, into an environment that is no longer conducive to its growth, it will begin to weaken and dissipate in intensity. As this support is removed, less moisture and energy is being taken in to produce more precipitation, and the precipitation rate will weaken, and eventually fizzle out.

In situations where orographic lift, or the existence of embedded convection/frontal features were supporting high precipitation rates, once the wind shifts away from the high terrain, or the supporting frontal features start to weaken, precipitation along those boundaries will weaken as well.

This is an image of typical cyclone tracks observed in North American winters. Summertime is dominated by more convective precipitation rates and less organized weather systems. Convection is responsible for higher precipitation rates, but not generally spread out over large regions. Weak low-pressure systems give lower precipitation rates due to their weak synoptic support, but can give more widespread rainfall.

During the wintertime, organized weather systems having strong dynamics are the major contributors to heavy precipitation and high precipitation rates. Convection tends not to be as deep, but due to colder temperatures it can be triggered easily, though not always with high snowfall rates. Winter demands consideration of snow-to-liquid ratios however, which can hugely impact the precipitation rate.

Where a low-pressure system originates and its’ typical storm track are two factors in determining the amount of moisture and heat readily available to the storm.

Storms that are generated in moisture rich locations, such as in south regions or near warm waters (such as Gulf Lows and Nor’easters) tend to have higher precipitation rates through the duration of the event, due to their ability to draw more moisture from their surroundings.

Mackenzie Lows for example, as well as Alberta Clippers, form in colder, drier regions with very low moisture, usually resulting in low precipitation rates and small accumulations.

Additional moisture can be pulled into low-pressure systems/convective systems from lakes, rivers, recent rain or snow fall, leads/cracks in the ice over a body of water, and many other surface sources. This can either help to sustain the precipitation, or increase precipitation rate.

Image source: SpringerLinkOpen a new window. Note the image was modified a bit.

Forecasting precipitation rate tends to be quite a bit easier for synoptic systems, as their size is much easier to resolve by numerical models, and there is a lot of upstream data to refer to. However, small variations, mesoscale and small systems, and the details within each system (embedded convection, small regions of enhanced precipitation caused by local processes) can often be smoothed out due to their small size. This is similar to convection, whose small or narrow size can make it difficult for numerical models to pick out small, but very important details. In many cases, it’s up to the forecaster to add in additional scientific reasoning to account for discrepancies, and they rely heavily on upstream data, radar, satellite, PIREPs and real soundings to see if the model is performing well and adjust the forecast as needed.

Determining the snow to liquid ratio and the associated visibilities can be quite challenging, as it is not always uniform across different sectors of a low-pressure system.

Summer precipitation rate forecasting can be challenging as well, since the season is usually dominated by convective precipitation, and less organized low-pressure systems. Convective precipitation, amounts and rates are difficult to predict in particular, partly because the amounts can be so variable over a relatively small area.

One of the most difficult questions to answer is often how much of the precipitation falling will wind up accumulating on the ground. Rain situations are slightly easier, since the water lands and drains off critical surfaces. In snow situations however, accumulation forecasts take many conditions into account (snow-to-liquid ratios, in-cloud temperatures, convection, local effects, wind strength, anticipated snowfall rates, etc) and often result in a range of snow expected to fall and collect on frozen ground. However, it is incredibly difficult to predict how much snow will survive and accumulate on treated ground/runways, without knowing the nature and schedule of runway treatments, nor the temperature of the runway.

Precipitation rates can at times vary considerably from one location to the next, even in close proximity, and especially when embedded convection is present. This is an example of GFA panels valid at 0000Z on August 24, 2023, in which a stationary upper-level front is forecast to stagnate over Ontario. This front is predicted to support a region of intermittent rain over southern Ontario and provide atmospheric instability over the Great Lakes region, generating occasional showers and isolated thunderstorms. The cloud deck is deep enough to support rain and an icing risk in areas of the cloud that are below freezing, as is shown in the icing and turbulence panel.

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TAFs do not provide a direct indication of hourly precipitation rates, and though basic inferences can be made users should always refer to their standard procedures and/or call the FIC for a weather briefing. In this CYYZ TAF, light showers are expected throughout much of the period up until 1300Z on August 24^th. Differentiating between light (-), moderate (no symbol), and heavy (+) is covered in the science explained section, MANOBS, as well as NC-SWOP and is a good “first guess” of precipitation rate when looking at a TAF. One caveat to keep in mind is the presence of VCTS, is that precipitation rates within thunderstorms can be much higher and a qualifier of intensity (-/+) is not used with VCTS to indicate predicted rainfall rates. The VCTS in the CYYZ TAF between 2000 and 0300Z highlights the possibility of higher accumulations in the vicinity due to thunderstorms.

The Kitchener (CYKF) TAF highlights a few differences when compared to the CYYZ TAF shown. Though the CYKF TAF is shorter, both cover forecast conditions from 1400Z August 23 until 0200Z August 24. From a precipitation rate perspective: intermittent moderate showers are forecast at CYKF between 1400Z and 1800Z, defined as a rainfall rate between 2.6-7.5mm/hr. Later in the period, between 2000Z and 0200Z, thunderstorms are forecast as a PROB30. Should these storms occur at the station, potentially much higher rainfall rates would be expected.

The atmospheric profiles forecast for CYYZ between 1200Z August 23^rd and 0600Z August 24^th capture a few interesting signatures. There is a well-defined inversion through the low/mid-levels that remains in place throughout all profile hours, starting off in the 850-700mb range and gradually lowering to the 950-800mb range after 0000Z on August 24^th. Above the inversion the atmosphere is forecast to be unstable, which when associated with moisture and a trigger, can lead to thunderstorms being generated high off the ground (known as “elevated convection”). These storms, along with intermittent rain, can greatly impact the hourly precipitation rate over any site depending on their movement and evolution.

Image Source: Pivotal Weather

This is a graphical loop of the precipitation type and radar reflectivity (in dBz) forecast by the RDPS from 1100Z August 23 until 0400Z August 24. The expected intermittent nature of the rain showers is shown by the gaps in forecast precipitation, along with the variable precipitation intensity between rain and rain showers in embedded convection. Radar reflectivity can be translated into precipitation rate and is an option within graphical forecast model output as shown here. It will be discussed more in the radar section.

Image Source: College of DuPage

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Cloud-Top PhaseOpen a new window satellite imagery valid 1100Z August 23^rd until 0100Z August 24^th shows the progression of cold, high-topped clouds (often highlighting thunderstorms and strong convective showers), as coldest cloud tops shown in blues, greens, oranges, and reds. Between 1200Z and 1500Z individual cells can be distinguished between Lake Ontario and Lake Huron, implying variability in precipitation rate between clouds. After 1500Z, most of the organized precipitation becomes centered over southwestern Ontario and Lake Erie. The nature, path and distribution of precipitation can make a significant difference on precipitation rates and accumulations observed on the ground over small distances. While continuous rain with little convection embedded will result in similar precipitation rates over the path of the system, intermittent precipitation, and systems with embedded convection will result in highly variable precipitation rates across an area. The result can be precipitation intensity and accumulations varying significantly over a short distance.

It should be noted, however, that satellite alone is not enough to assess precipitation. Checking satellite imagery against surface-based observations is the best way to assign precipitation ability to clouds.

Image Source: CIRA

GeoColorOpen a new window multispectral satellite imagery valid between 1200Z and 1600Z August 23rd is shown as the early morning images are a great display of embedded convection along the stationary boundary shown in the GFA, in comparison to the surrounding clouds suspected to generate intermittent precipitation near the Great Lakes. Shadows on the southwestern edges of storms capture the towering height of convective clouds, with bubbling texture along their tops. Fall rates from convective clouds can generate higher intensity precipitation than continuous and intermittent precipitation on the ground (although stratiform precipitation can certainly generate high fall rates as well), as will be shown in the METAR section.

Image Source: CIRA

Radar imagery between 0800Z and 1500Z August 23^rd shows the intermittent nature of the rain and the variability in radar reflectivity across the region. This variability can translate to high differences in total precipitation between sites, whether it be per hour or for the whole event. Consider radar echoes moving over Barrie starting near 0800Z, discontinuous rain with diminishing precipitation intensity as echoes approach the station. Many breaks, and lesser values of precipitation rate per hour than stations close by (Kitchener) demonstrate how variable conditions can be within any one system. Pick any two sites here yourself and see if you could guess the difference in possible accumulation. You can always test yourself and check observations either using historical METAROpen a new windows or historical weather dataOpen a new window.

Early on in the loop we see the majority of precipitation moving in a line between St. Catharines to Owen Sound, which was shown in the Cloud-Top phase satellite imagery as blue-green cloud top convective cells. As the day progresses, the most active convection and more intense precipitation drifts southwestward into the London/Kitchener area. This was also shown in satellite imagery with explosive growth and cold, organized cloud tops.

For a deeper dive, here is more information on how radar reflectivityOpen a new window can be translated to precipitation rate.

Image source: Environment and Climate Change Canada

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METARs from CYYZ on August 23^rd show intermittent light showers between 0800Z and 1800Z, verifying the TAF, minus one SPECI of moderate showers issued at 1050Z. The observer also notes convective cloud embedded throughout most of the observations pictured here. Some stations will also include hourly precipitation accumulations (CYYZ is not one of them), or precipitation every 6hrs in METARs known as “synoptic observationsOpen a new window”. These 6hr periods are at 0000Z, 0600Z, 1200Z, and 1800Z, and are cumulative precipitation from the previous six hours.

Image Source: OGIMET

If one particular site does not include precipitation accumulation reports in the METAR, historical weather dataOpen a new window from nearby stations may be available (from aviation or non-aviation stations). Here, for example, is the total for Toronto City Centre, which saw a total accumulation of 2.4mm on August 23. However, using a station in proximity to the one of interest is that the precipitation total may vary greatly, depending on the type of weather seen that day, such as scattered showers or thunderstorms, and additional local factors such as mountains or water sources.

Image source: Environment and Climate Change Canada

Kitchener, ON (CYKF) is located southwest of CYYZ and as an auto station has a pluviometer that measures hourly precipitation. As seen in the observations, multiple hours saw moderate or heavy rain, as shown in the TAF, as well as thunderstorms. Comparing precipitation rates between intensities: light rain (-RA) gave 2.0mm of accumulation between 1600Z-1700Z, while moderate/heavy rain (RA/+RA) and thunderstorms in the 1500Z-1600Z hour yielded 6.5mm. The total rain accumulation that day was 17.3mm, a stark difference from the 2.4mm seen in Toronto City. This is only one example of how much variability is present even within a few hours, and also between two sites 85km apart. Forecasters take into consideration many parameters when determining precipitation accumulation, and this is where detailed analyses and an expertise of both weather and local climatology makes a difference.

Image Source: OGIMET

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