Trowal

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This animated GIF depicts a simplified, overhead view of a warm and cold front "occluding" (colliding), and lifting the warm air from the surface into the air, forming a trowal. Trowals will form in the dissipation stages of a low-pressure system with associated fronts. Forcing warm air into the atmosphere acts as a trigger for convection, and can generate associated precipitation, turbulence, and icing.

For a trowal to form, a mature low-pressure system must be present with associated fronts. A trowal is formed when these fronts occlude.

Occlusion occurs when fronts move at different speeds relative to each other and/or the low-pressure center itself, causing the surface fronts to collide into each other and form new features, both at the surface and above ground (in most cases, the cold front moves faster than the warm front). At the surface, the "occluded front" is the line between the cool air ahead of the warm front, and the cold air behind the cold front.

The trowal however, forms in the mid levels of the atmosphere. Due to the lower density of the air in the warm sector (warmer, more humid), this warm air is lifted off the ground by cool air ahead of the warm front, and the air behind the cold front and wraps around the low-pressure center. In Canada, it is conventional to mark the location of this warm air aloft with a trowal on weather maps.

Image Source: Environment and Climate Change Canada

This is a simplified side-view of cold air (behind the cold front) catching up with and overtaking cool air ahead of the warm front. The cold, dense air plows under the relatively warmer air ahead, forcing both warm and cool and upwards. The warm air, now settled on top of both air masses, forms a trough of warm air aloft (trowal). The point where the cold front, warm front, and trowal meet is referred to as the “triple point”. When the warm air is forced aloft, it is cools and condenses, generating precipitation/convection along the trowal.

Image Source: Environment and Climate Change Canada

While occluded fronts can often have strong and gusty winds associated with them at the surface, in Canada, the trowal is marked to indicate the active weather and convection that is generated aloft.

The vertical extent, and height off ground of the trowal can be quite variable, as it depends on the relative temperatures of the air ahead of the warm front and behind the cold front, and the relative stability of the air above the trowal;

Very cold cold-front air will cause very aggressive lift of the warm air.
When the cold-front air and the air ahead of the warm front are not significantly different, the warm air will be lifted more gently, potentially causing less significant convection and precipitation.

In both cases, this narrow region of warm, humid air and existing clouds along each front being forced into the upper levels provides prime, sustained conditions for precipitation to form. Significant precipitation rates are often noted along the trowal.

These precipitation rates are largely due to convection which forms in the trowal. Rapid movement of warm air into the cool/cold mid-levels allows the warm air to rise freely (since it is warmer and less dense than its surroundings), generating convective clouds. These convective clouds can generate significant precipitation, and thunderstorms.

This image is one of simplified relative precipitation rates around frontal features. Note the enhanced precipitation along the cold front and near the trowal/occlusion, due to the convection that is formed there. The low-pressure center also tends to have amplified precipitation, since it is responsible for large scale lift. Also, given the nature of cold air pushing underneath the warm air aloft, it is possible to get freezing precipitation in the low levels under a trowal in cold months.

Image source: SkybraryOpen a new window

Dissipation

Trowals will dissipate as they precipitate out all of the moisture that has been lifted. Since this is a region of continuous lift, however, this can often take quite a long time. This warm air also cools as it generates precipitation, gradually weakening the temperature difference between the warm air aloft and the cool/cold air below it, which weakens the strength of the trowal itself. Not only this, but trowals often get caught in the circulation around a low and curl around it. As this warmer air gets wrapped around the dissipating low-pressure system, it mixes with the circulating air and becomes less defined.

Duration

A trowal can exist for up to several days at a time, especially with particularly long surface fronts. A large relative temperature difference between the cold air and cool air ahead of the cold front can sustain the trowal precipitation for a long period of time. Very broad low-pressure systems with fronts will also tend to generate larger trowals.

As trowals are dependent on low-pressure systems with frontal structures, their climatology is strongly linked to that of low-pressure systems. While trowals can exist at any time of year, they are often strongest in the winter months, when the temperature difference between the equator and north pole is the strongest.

Image Source: SpringerLink

While trowals are generally quite easily distinguished on satellite imagery, their existence in the mid-levels can make them difficult to place exactly ahead of time. With few upper air observations over Canada at limited intervals, discerning the exact shape of the mid-levels can be difficult.

Occasionally, precipitation types can be difficult to discern under a trowal due to the intermixing of several air masses. Depending on the relative temperature and depth of the cool and cold air, rain/snow mixes can often be observed below the trowal. Freezing precipitation can also be a risk depending on the depth and temperature of the cold air at the surface, which can be difficult to verify in real time, especially over more remote locations.

As they are broad in scale, they are generally well captured and resolved by numerical guidance, but due to their narrow nature, if the predicted track is miscalculated, it can greatly alter precipitation forecasts for particular regions over which the trowal was or was not expected to pass.

The GFA panels valid for 1800Z on October 18, 2022, shows two low-pressure centers over southern Ontario and central Quebec, with a trowal extending north and west of the low in Quebec. This trowal is forecast to produce continuous precipitation across central Quebec and northeastern Ontario, with western most sections more likely to see a rain/snow mix. Conditions within this area of continuous precipitation are expected to be primarily MVFR to VFR, though occasional IFR conditions are possible in embedded convection, associated with ACCs with tops near 16,000ft. The thick cloud deck is also expected to be favorable for mixed icing conditions, as shown in the icing and turbulence panel. While the low-pressure center is expected to move at 10KT to the northwest, the fronts have a forecast speed of 20KT to the north. These features moving at different speeds and directions indicates dissipation of the system as a whole. This slow moving and degrading low and the associated trowal indicates longer, more disorganized of precipitation and MVFR/IFR conditions across the area.

Two TAFs shown here, Timmins, ON (CYTS) and Chapleau, ON (CYLD), capture the weather resulting from the passage of the trowal. As shown in the GFA, western-most regions are more likely to see a rain-snow mix, as is forecast and indicated in CYLD. CYTS, northwest of CLYD, is expected to remain in an area of continuous rain throughout the forecast period. Conditions at both sites are forecast to remain consistently stable and low, indicating the organized nature of the trowal. The duration of these low visibility conditions is a function of the speed of the low and its associated fronts, including the trowal. As shown in the GFA, the displacement is expected to be slow, likely translating to long events at certain sites seeing precipitation and low conditions as a result of the trowal.

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The nearly stationary areas of forecast precipitation associated with the trowal across northeastern Ontario and central Quebec are seen here in the RDPS model data forecast for 0600Z October 18 through 0000Z October 19. The low-pressure centers and expected trajectories shown in the GFA are captured here as well. The RDPS also captures the rain/snow mix mentioned both in the GFA and the Chapleau, ON (CYLD) TAF, with an area of snow just north of the Great Lakes expected to persist through the forecast period, while areas to the north and east remain as straight rain. This stationary trend for the location of the trowal means areas within the region of organized precipitation are likely to see a longer-lived event.

Image Source: College of DuPage

Here is a still from the forecast GIF for precipitation that displays the approximate location of the forecast trowal at 1800Z on October 18, as shown in the GFA panels.

Image Source: College of DuPage

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Multispectral GeoColorOpen a new window satellite imagery captured between 1730Z and 2020Z October 18 shows the nearly stationary nature of the lows over southern Ontario and Quebec, and the extensive cloud cover present along the trowal in northeastern Ontario and central Quebec. Additional, bubbly texture of this cloud deck is a sign embedded convection, as initially forecast in the GFA as ACCs.

Image Source: CIRA

A still of the satellite loop provided and valid at 1800Z is provided with the approximate location of the trowal. This area of cloud, associated with the continuous precipitation, was forecast within the GFA and TAF, and verified by local observations.

Image Source: CIRA

Radar imagery, valid 1830Z to 2130Z shows the areas of continuous precipitation along the trowal across central Quebec and northeastern Ontario. The TAFS shown for Timmins, ON (CYTS) and Chapleau, ON (CYLD) capture the actual precipitation that occurred that day, along with its continuous nature with the trowal stalled over the area.

Image Source: Environment and Climate Change Canada

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These observations confirm the continuous precipitation forecast by both the GFA panel and TAF for Timmins, ON (CYTS). The precipitation type forecast is also accurate, as continuous rain was observed throughout the period under the trowal’s influence.

Image Source: OGIMET

Chapleau, ON (CYLD) remained in the area of the GFA most likely to see a rain/snow mix, and in observations verified as straight snow though temperatures remained positive. The TAF indicated the likelihood that snow would be the primary precipitation type, though a rain/snow mix was also possible intermittently. The duration of this event under the influence of the trowal is also long-lived, persisting upwards of 13 hours, given the stationary nature of the entire system.

Image Source: OGIMET

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