Onshore Flow

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There are mainly two processes that can cause onshore winds to form.

A sea (or lake) breeze circulation can develop. This is caused by a relatively cloudless day allowing for the ground to heat up much more significantly than the water beside it, allowing air to readily rise over the land. As air over land rises, air from the water is pulled in to replace it on shore, redirecting surface winds onshore from over the lake.

Alternatively, synoptic winds (such as ahead of a low-pressure system, or behind a high-pressure system) can be directed from water toward the land. When synoptically driven (caused by the overall pressure gradient), these winds can be stronger, and more long lasting.

The concept of onshore flow is itself very simple, but it can have some marked impacts. The main impact of onshore flow is increasing humidity where wind from the water meets the shore. Depending on the time of day and atmospheric stability, this can cause different effects.

In situations where cool air from the water is moving over warmer land (often daytime, spring/summer/fall conditions), cool air is rapidly heated when it comes in contact with the ground, and is able to lift higher into the atmosphere, where it condenses, and forms clouds. This process, effectively a lake breeze circulation, can generate convection in onshore flow if clouds are able to freely rise high enough in the atmosphere. A similar process can occur when a cool air mass with a cloud deck moves over very warm land; if the ground is significantly warmer than the invading cloud-filled air mass, heat from below can force clouds upwards, improving cloud ceilings. This is generally only observed in summer with maximum surface heating.

This animation shows an idealized lake-breeze circulation developing, generating onshore flow and clouds inland, as well as potential convection if conditions are unstable enough.

Image Source: Environment and Climate Change Canada

On the other hand, when cold air moves over cool or cold land, the opposite occurs. When onshore flow moves over a cooler surface, a surface inversion forms, creating a very stable layer which inhibits upward motion. As this cool, humid air moves over cold land, it is cooled from below, bringing the air closer to its dew point temperature. With sufficient cooling, mist or low clouds can develop near the surface, and continue to lower as long as humidity and cooling continue. This effect is often seen in coastal regions at or after sunset in onshore flow.

When a cool air mass filled with low clouds moves over a cold surface in stable conditions (often seen in winter, or at night in all seasons), the gradual increase in ground height causes slight cooling in the air mass as it is lifted. However as the layer is stable, the clouds cannot build upward, so they build down toward the surface instead, lowering cloud ceilings. As onshore winds continue to push onshore, ceilings can continue to fall, especially through the overnight period. In winter, in onshore flow conditions, this phenomenon can be responsible for maintaining low cloud ceilings in coastal regions through all hours of the day. This setup is particularly significant in the winter, as water, and the air directly in contact with the water are warmer than land temperatures, which can create or maintain very strong surface inversions all day long.

This animation is a simplified depiction of fog developing in warm onshore flow over a cool surface. Warm, humid air moving onshore raises the humidity, while the cool surface cools the air above it, both raising humidity and lowering temperature simultaneously, bringing the air rapidly closer to its dew point temperature until fog forms and spreads out onshore.

Image Source: Environment and Climate Change Canada

Moreover, when onshore flow encounters a barrier (e.g. moving directly onshore and up a significant hill or mountain slope; orographic lift), one of two things can happen depending on atmospheric stability:

Unstable conditions: onshore flow is lifted aggressively up the sudden slope and is allowed to freely rise, condense and cause convective precipitation.
Stable conditions: Air is pushed up the mountain side up to the inversion height. It progressively cools and condenses, but does not necessarily surpass the top of the terrain, generating a stagnant cloud deck that builds downward. These clouds can generate drizzle, or rain (if cloud tops are cold enough).

This animation is a simplified depiction of clouds in onshore flow interacting with elevated topography in land. In unstable conditions, humid air or clouds can be pushed up into an unstable atmosphere where they can freely grow and become convective clouds. In stable conditions, clouds will only lift as high as the inversion height (here, the mountain top), and as the air mass cools at altitude, and additional humid air is lifted up the mountain face, cloud ceilings will build down toward the surface, potentially generating drizzle or rain.

Image Source: Environment and Climate Change Canada

Dissipation Parameters

In the case of a lake/sea breeze circulation, this system will break down as surface temperatures cool and become more equal with those over water. This is often linked to the sun starting to set, or setting completely, which removes the strong pressure gradient that allowed the circulation to form in the first place. This could also occur if clouds move over the system, cutting off the solar radiation (although this slows the rate of cooling overnight, which might allow a weak circulation to last a few hours longer).

For synoptically driven onshore flow to cease, the overall pressure gradient must shift, such that winds are no longer directed straight onto the land from the water. Usually this requires synoptic scale features (low/high-pressure systems, troughs) or patterns to change locations, changing the shape of the pressure pattern at the surface, and redirecting winds.

The most significant wind shift that will improve low conditions generated by onshore flow is a shift to a directly “offshore” flow. Conversely to how onshore flow cools, condenses, and increases humidity at the surface, offshore winds cause slight warming, drying conditions. Offshore winds very effectively clear out low conditions, or improve cloud ceilings. Due to the drying effect, fog or mist can improve rapidly once offshore winds take hold.

Duration

Lake breeze circulations usually only survive on the order of hours, generally setting up in the early afternoon and diminishing close to sunset. They may last slightly longer if surface conditions can stay quite warm compared to the temperature of the water, but often break down overnight.

Synoptic circulation can cause onshore winds for several hours to several days at a time, depending on the overall speed and movement of the system.

Predictably, onshore flow can only occur in regions where wind is moving from over water, over land. This limits the occurrence to only regions that have significant water features close by.

Lake breeze circulations are particularly strong in spring and summer, but can occur in fall on particularly strong, sunny days under clear skies, where there is maximum temperature difference between land (warm) and water (cool).

Onshore flow creating stable conditions (warmer air from water moving over cooler land) can occur any time of year. In warm seasons, it is much more likely to occur overnight, when there is maximum surface cooling. In the winter, it can occur at any hour of the day, as waters are generally warmer than land.

This image is a map of all waters in Canada. Onshore flow is possible from all waters, but most effective at transporting humid air in more southern regions, where it is warmer. Arctic air tends to be much more dry and cold, though cloud regions moving onshore are still subject to cooling due to lifting, and can still support low conditions.

Image Source: Environment and Climate Change Canada

While determining when synoptic winds will blow onshore from the water is fairly straightforward, determining the details of how and how quickly weather will change onshore can remain tricky.

Another forecasting challenge is determining whether the energy of the sun above the stratus is strong enough to break up the cloud deck. Intense sunshine can often allow just enough light to reach the surface such that it heats and lifts cloud ceilings to break up the cloud deck. However, a strong inversion can maintain very solid stratus decks. These two opposing forces are often difficult to quantify, and provided that solar energy is seasonally-dependent, it is not always evident which force will dominate.

Also challenging is determining exactly how quickly surface conditions will deteriorate in onshore flow. Data is often more limited over the ocean, so it is not always evident exactly how humid an incoming air mass is (more humid generally means it will condense and form low conditions more quickly). In cloudy situations, this is exacerbated further, since there are often few or no direct observations of conditions below the cloud deck, so it can be difficult to assess exact conditions that will be affected. Knowing exactly how quickly the surface will cool the air above it, too, can be challenging.

Convection associated with onshore flow can be difficult to diagnose, as characteristics of air moving onshore are not always known (temperature or humidity). More complicated, is when air moves up into mountainous regions, where real data is sparse, and model data has a hard time capturing surface features due to the extreme variability of the terrain. Determining exactly where and if convection might form in onshore flow can be incredibly difficult under these conditions.

Lastly, forecasting a lake breeze circulation comes with its own challenges. Trying to determine whether or not the thermal circulation will be able to overpower the existing pressure gradient is not always obvious.

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This GFA clouds and weather panel valid at 1200Z on August 29, 2023 captures onshore flow in multiple regions of Atlantic Canada. Events at Yarmouth, Nova Scotia (CYQI), indicated by a red star, will be discussed in this example.

The region is under a high-pressure system centered southeast of Nova Scotia and Newfoundland, which is generating weak, anticyclonic (counterclockwise) flow in the region. Note the “H” is not drawn out as the center is outside of the GFA domain.

Onshore flow is forecast in the GFA for southwestern Nova Scotia, southern New Brunswick within the Bay of Fundy, Prince Edward Island, the southern tip of the Gaspé Peninsula, the Quebec North Shore (Sept-Îles (CYZV), Natashquan (CYNA), Havre-Saint-Pierre (CYGV)), and the western coast of Newfoundland.

Onshore flow can cause or enhance various weather phenomena. A frequent feature of the Maritime region forecast, as advection fog is drawn in the orange dashed line affecting the southwestern tip of Nova Scotia and regions along the Bay of Fundy.

By 0000Z on August 30^th, the high-pressure system was expected to move further into the Atlantic, and the anticyclonic winds now expected to create onshore flow conditions along the southern shores of Nova Scotia, Cape Breton, Newfoundland, and the Strait of Belle Isle. The GFA notes this in the locally reduced ceilings and visibility comment seen over Newfoundland, as well as the orange dashed line.

The orange dashed line indicating the forecast presence of advection fog and mist has stretched to cover most of the coast of the southern shore of Nova Scotia. This is an example of synoptic flow that can support low ceiling and visibility conditions, especially if the pattern stagnates over the region.

Onshore flow is forecast for the entire 12-hour period of the TAF for Yarmouth, Nova Scotia, located at the southwestern tip of the island. As shown in the GFA, it is already expected to be within the orange dashed line at 1200Z on August 29th. As is typical with summer advection fog, the TAF describes an improving trend from 1200 to 1700Z as daytime heating increases. However, as the station is expected to see onshore flow throughout the day and the fog bank is expected to stay just offshore all day, the risk of low visibility and ceilings remains even in the afternoon hours, as indicated by the PROB30 from 1700-2200Z. As the sun sets and onshore winds are expected to persist, low conditions are expected to push back over the airport by 2200Z, putting Yarmouth back in the orange dashed line noted on the 0000Z GFA on August 30th.

It is important to note that onshore flow does not guarantee IFR conditions. Though onshore flow can reduce ceilings and visibilities to the low-IFR range and remain there for hours or days at a time (depending on the season, and specific conditions of the day), conditions often vary through the day. This can depend on factors like consistent winds, solar heating, geography and local effects, or other weather systems moving through the area. While the onshore flow mechanism is well defined, any phenomena (precipitation, low visibility or ceilings, etc) resulting from onshore flow depend highly on the specific conditions of the day.

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Atmospheric profiles for Yarmouth, NS (CYQI) are shown for a 4-day period to show the possible persistence of onshore flow and resultant weather conditions under consistent synoptic conditions. The time period shown in this loop is August 29^th at 0600Z until August 31^st at 1800Z, a 60-hour period.

Image Source: Pivotal Weather

Predicted onshore flow can be seen at the surface by looking at the wind barbs on the right-hand side of the graphic in the yellow circle. Any southeasterly to southwesterly flow at CYQI means onshore off the Atlantic, which can result in low conditions when there is high humidity over the ocean. Resultant weather in this onshore flow event is expected to be a saturated atmosphere at low-levels between 0600Z August 29^th and 0600Z August 30^th, giving the low ceilings and visibilities forecast in both the GFA and the TAF (orange circle).

Image Source: Pivotal Weather

Overnight on the 30^th, a low-pressure system was forecast to move into the Maritimes, bringing precipitation and deep clouds, as noted by the saturated column of air (orange circle). Despite this change in air mass, the additional moisture to the low levels supported by continuous onshore flow (yellow circle) from more humid regions were expected to keep conditions quite low at Yarmouth. Winds were expected to remain south-southwesterly and onshore until 2100Z August 30^th – a total of 39 hours.

Image Source: Pivotal Weather

After 2100Z on August 30th, winds were forecast to shift gradually to the west, and then again at 0900Z August 31^st to the north-northwest in the wake of the cold front moving through. As these winds shift, the temperature (red) and dew point temperatures (green) begin to adjust as wind is now blowing the fog bank away from the station. While there is a small onshore component even in northwesterly flow at CYQI off the Bay of Fundy, northwest synoptic winds typically bring in much drier air which results in the dissipation of low clouds and restrictions to visibility. However, they can easily bring phenomena such as convective showers from the bay over the airport in the right conditions, for example. This is why forecasters consider significant amounts of data before and while writing their forecasts, as factors such as onshore flow can, but do not always, cause significant weather in a particular location.

Image Source: Pivotal Weather

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This is the RDPS model forecast loop for mean sea-level pressure (mb) and 10-meter AGL wind (kt) between 0600Z August 29^th and 1800Z August 31^st. A reminder that CYQI is located on the southwestern tip of Nova Scotia (see GFA for a visual reference).

At the start of the loop, the model derived synoptic pattern forecast over the Maritimes shows anticyclonic (clockwise) winds with the high centered south of Newfoundland, similar to the GFA. The high then begins to weaken and shift eastward, yet the model keeps CYQI in onshore flow as shown in the wind barbs forecast in the vicinity.

In the wake of the high, the RDPS shows a low-pressure system (mentioned in the atmospheric profiles section) approaching from the Great Lakes region. Ahead of this low, CYQI remains in onshore flow in what is now considered cyclonic flow (counterclockwise), determined by the curvature of the isobars.

It is only after the RDPS forecast cold frontal passage that winds are expected to radically shift west and then northwestward. This wind direction is far more likely to bring in much drier air (see next image), resulting in the dissipation of low conditions seen throughout the period, even if the winds remain “onshore”. This is because the dry air being moved over the station often overpowers and flushes out remaining humidity at the surface over the water, which cuts off available moisture which could turn into low conditions.

Image Source: Pivotal Weather

This RDPS model loop of 2-meter AGL relative humidity and 10-meter AGL winds is shown for 0600Z August 29^th-1800Z August 31^st (same as the previous loop). The wind barbs shown at individual timesteps are also identical to those seen in the previous loop.

While purely under the influence of the high early in the period, notice the localized regions of higher relative humidity (Bay of Fundy and Atlantic waters south of Nova Scotia) predicted by the model. This matches the 1200Z GFA panel identifying those regions as likely to see reduced ceilings and visibilities. By 0000Z August 30^th the model derived zone of moisture extends up to the southern edge of Cape Breton and Newfoundland, which supports lower conditions forecast in onshore flow in the 0000Z GFA panel shown.

Once the low begins to influence the region, the model predicts that localized humidity will be overpowered by the moisture and large-scale dynamics associated with the low. However, it is in the wake of its passage where a distinct change in air mass is forecast. This calculated lower humidity, along with the wind shift at CYQI specifically, would help break down any low ceilings and visibilities in the area and usher in clear skies.

Image Source: Pivotal Weather

This is a Nighttime MicrophysicsOpen a new window multispectral satellite image loop over Atlantic Canada showing clouds between 0600Z and 1000Z August 29^th. We can see the anticyclonic flow across Nova Scotia and southern Newfoundland, associated with the high-pressure center located offshore, by looking at the slight clockwise curve in the overall cloud structure across the area.

The low-level stratus over southern waters giving fog and low ceilings/visibilities at Yarmouth, NS (CYQI) is mostly obscured by the presence of higher clouds. Cirrus clouds often make it difficult or impossible for the satellite to pick up what’s underneath them, though hints of low warm clouds and fog (light blue green, see legend underneath the image) poke through here and there.

This low-level cloud can however be seen across other areas: Vermont, Maine, New Hampshire, New York, and portions of southern Quebec. Looking back at the forecast GFA valid for 0000Z August 29^th, an additional orange dashed line for fog/mist was located off the north shore of Newfoundland. This can be seen in the satellite loop, as this low-level stratus appears light blue green overnight, becoming fuchsia after sunrise.

Image Source: CIRA

Observations from Yarmouth, NS (CYQI) on August 30^th between 1500Z and 2020Z show low conditions in fog and mist in onshore flow. This specific timeframe aligns with the passage of the low-pressure system over the region, adding moisture and additional factors that can amplify the low conditions seen.

Image Source: OGIMET

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