5.1 A Five-Step Forecast Process

5.1.2 Identifying Predevelopment Conditions

Although the actual polar low is sub-synoptic, the systems that produce conditions favourable for polar low development are synoptic-scale.

Predevelopment conditions to watch for are:

   Areas where cold air is flowing over a water surface

   Large air/water temperature difference

   Cyclonic curvature or contours at the 700- and 500-hPa levels

   The formation of low-level cloud streets

   Surface winds of 15 knots or less

   Formation of low-level vortices. These can be individual vortices or may occur in families along a shear line or low-level baroclinic zone

   Within the cold air mass there may be evidence of low-level baroclinic zones. These can be induced by:

  Converging flows with different over-water trajectories
  Wrap-around warm air from a synoptic low
  Cold air flowing parallel to the water-ice or water-land edge

* Note: Not all of these conditions need to be present.

Numerical guidance is invaluable for long-range forecasting of these initial synoptic conditions. Actual surface analyses and satellite imagery are used to confirm the existence of the initial conditions.

5.1.3 Predevelopment Synoptics: Hudson Bay and Eastern Canada

For Hudson Bay and eastern Canadian waters, the initial northwesterly flow of cold arctic air is frequently the result of a major synoptic cyclone moving offshore from Labrador or crossing the Maritimes and Newfoundland on an east-to-northeast course that carries it south of Greenland.

Major fall and winter cyclones are often accompanied by a shift in the winter polar vortex to a location over Foxe Basin, central or southern Baffin Island, Hudson Bay, or northern Quebec. Low-level cold arctic air then streams from the continent and/or ice pack (depending on the time of year) over the warmer waters of Hudson Bay or Davis Strait and the Labrador Sea. Hudson Bay has been included here although the storm tracks could be northeastward across Quebec with the polar vortex northwest of Hudson Bay. The end result is often an air mass with surface temperatures as lows as -20 to -35°C flowing over a water surface.

5.1.4 Predevelopment Synoptics: Gulf of Alaska

Over the Gulf of Alaska the cold air will more often than not originate over the Arctic Ocean, Eastern Siberia, or Alaska and be pulled southward behind major synoptic cyclones moving onto the west coast or into the southeastern portions of the Gulf of Alaska. Businger (1987) shows that an upper vortex or major upper trough is generally positioned so that there are strengthening negative height anomalies over Alaska and the Gulf of Alaska some two to three days prior to polar low development.

5.1.5 Predevelopment Synoptics: North Atlantic

For polar lows to form in the eastern Atlantic and northern European waters, any of a number of synoptic situations may be present. In all cases, the lows develop within an arctic or polar air mass with the polar front situated south of the location.

In a cold air outbreak from the arctic ice sheet, convection is initially hindered by a strong surface inversion, often more than 5000 ft in depth. Therefore, the weather close to the ice edge is dominated by rather homogenous shallow convection, with low-level phenomena such as convergence lines, boundary-layer fronts, and minor troughs forming further downstream. Deep convection and polar lows start to form as the air reaches areas with sea surface temperatures of more than 2°C. Polar lows are thus mainly found in the areas east of the 0° meridian, west of 40° east, and south of 75° north, with some cases in the area southwest of Spitsbergen.

The west coast of Novaya Zemlya and the ice edge from the southern tip of Spitsbergen to Bear Island form stationary baroclinic zones during the winter season, and are important sources of baroclinic instability. In particular, the most western baroclinic zone probably influences polar low formation. Polar lows are rare east of 40 degrees east, mainly because of the low sea surface temperatures, but some cases have been recorded off the southwestern tip of Novaya Zemlya. The areas further to the north are no less weather exposed, as old decaying synoptic lows have a tendency to regenerate under the influence of the Novaya Zemlya baroclinic zone. This area is also prone to strong katabatic outflows in easterly winds, generating lee troughs over open water (Heinemann 2003). Lee lows off the southeast coast of Greenland can also provide the initial surface disturbance.

Alternatively, the baroclinic zone, which is the surface trigger, may be a trough or the remnants of a synoptic-scale system. Although many polar low outbreaks occur in a true arctic (northerly) airstream, this should not be treated as the only likely scenario. In some rare cases, the polar low has lasted long enough in northerly flows to reach the southern parts of the North Sea. More commonly, North Sea polar lows form in cold air outbreaks from the Scandinavian mainland, affecting the areas mainly in the western part of the North Sea.

5.1.6 Identifying Trigger Mechanisms

Upper-level triggering mechanisms to watch for are:

   Development and or movement of an upper-level negative height anomaly. For the Bering Sea and Gulf of Alaska the Naval Tactical Applications Guide suggests a 200 m anomaly

   Approach of a 500 hPa trough or vortex with temperatures below -30°C. This is a minimum value and most of the literature cites a sea surface-to-500 hPa difference of  > 40°C

   Approach of a comma cloud, jet streak, or area of positive vorticity advection

Numerical guidance can be used to identify larger-scale triggering features. Satellite imagery can confirm and supplement the numerical guidance by identifying cloud patterns.

5.1.7 Identifying Formation of Polar Lows

Surface reports and satellite imagery are the main tools the forecaster has at her/his disposal.

Items to look for include:

   Rapid or unexpected pressure falls in an otherwise benign environment

   Rapid or unexpected increase in surface winds

   Increased snow shower activity and possibly thundershowers at shore, island, or ship locations

   Satellite imagery showing enhanced and organized convective cloud development:

   Development of a symmetrical, spiral-shaped cloud formation
   A clear eye may be visible during the mature stage
   To reach Arctic hurricane status requires increased low-level convergence or upper-level divergence resulting in the cloud pattern becoming asymmetrical and an anticyclonically curved cirrus shield reflecting the outflow region (Naval Tactical Applications Guide 1992)

5.1.8 Forecasting the Motion of Polar Lows

Some of the tools and or techniques available for forecasting the motion of polar lows:

   Extrapolation:

• The experience of the Norwegians is that this method is only useful for a few hours

   Using a steering flow:

• The Norwegian Polar Lows Project reported that polar lows often follow the 850- to 700-hPa wind field. Midtbø, (1986) tested the method for the Norwegian and Barents sea area using the 850-hPa model-predicted wind and found an average error for an 18-hour forecast of 200 km compared with a mean path length of 800 km
• The Naval Tactical Applications Guide (1992) suggests that even strong polar lows will likely have a representative steering level closer to 850 hPa, and that for well-developed polar lows a speed of 30 knots is not unusual
• Polar lows have been observed to travel at between 1/3 and 1/2 of the wind speed at the steering level
• Where the steering-level winds are very light or variable, the low is often more influenced by the movement of the upper vortex
• As applicable, consider the possibility of binary rotation

   Numerical models

• Improved resolution makes it more likely that the model may show a trough or vorticity maximum associated with the surface development. Midtbø (1986) suggested following troughs and vorticity maximums on the charts. He found the prognoses to be better in stronger pressure fields. Due to better models this technique has more validity today
• Comparing computed tracks to past motions for consistency

   Climatology

• Although detailed studies have not been conducted, there appears to be a tendency for Davis Strait polar lows to follow the ice edge when the steering flow is light

5.1.9 Forecasting the Dissipation of Polar Lows

Although cold air vortices can persist for several days, the time that polar low criteria are met is often very short. By their nature cold air vortices and polar lows dissipate once they move over a land or ice surface. Vortices and polar lows also weaken and dissipate if they merge with frontal systems. Since they normally form deep in the cold air mass, this is not likely to occur until after the systems have already started to weaken.

Based on observations collected by Hanley and Richards (1991) and Atmospheric Environment Service, Prairie and Northern Region, 61 polar low and cold air vortices were examined for their time span of visibility in satellite imagery. Although this was not the point of their study, and these statistics cannot be taken as being 100% valid, they do point out that the tendency is for short-lived systems. It is also observed that many of the systems in the Gulf of Alaska gradually expanded into large spiral cloud forms that then drifted southeastward as they continued to dissipate.

References

Businger, Steven, 1987: The synoptic climatology of polar low outbreaks over the Gulf of Alaska and the Bering Sea. Tellus, 39A, 307-325.

Hanley, D. and W.G. Richards, 1991: Polar Lows in Atlantic Canadian Waters 1977 - 1989. Report: MAES 2-91. Scientific Services Division, Atlantic Region, Atmospheric Environment Service.

Midtbø, K.H., 1986: Polar low forecasting. Proc. Third International Conference on Polar Lows, Norway.

Naval Tactical Applications Guide. Vol. 8. Part 2, Arctic. East Siberian/Chukchi/Beaufort Seas. NRL/PU/7541 92-0005.

Parker, Neil, 1997: Cold Air Vortices and Polar Low Handbook for Canadian Meteorologists. Environment Canada.