Mesoscale Aspects of Winter Weather Forecasting:
Polar Lows - Ungava Bay, Northern Canada 01-03 December 2000

Overview

From the perspective of a forecaster, respond to this series of forecast questions. The questions go through the process of identifying predevelopment, formation, and evolution of a polar low. Each question entails the use of observations, model analysis, and model prognoses. Use the Data links to pop open a new browser window displaying the data. Use the links to the Supportnig Topics for access to background information.

Question 1

Current Time: 0300 UTC 01 Dec 2000

Examine the GEM regional NWP guidance (00, 12, 24, 36 and 48-hour forecasts) initialized at 0000 UTC 01 Dec. Identify any mid-level features in the area of Hudson Bay, Foxe Basin, Hudson Strait, Ungava Bay, and the Labrador Sea that might contribute to the development of a polar low during the next 36 hours. Choose all that apply:

Cold upper trough moving eastward from northern Hudson Bay
500-hPa vorticity centre moving eastward from northern Hudson Bay
Warm advection moving across Hudson Bay in the 12-48 hr period
Upper cold advection in westerly/northwesterly winds just west of the cold upper trough
Vertical motion at the base of the upper trough over central and southern Hudson Bay

(before viewing the discussion, be sure to look at the Data and Supporting Topics linked below each question)

Supporting Topics:
1.1 Development of Forcing Mechanisms
1.3.1 Predevelopment Conditions
2.1 Climatologogy: Reference Map and Seasonal Ice Coverage
2.2.3 Climatology for Northern and Eastern Canadian Waters
5.1.2 Identifying Predevelopment Conditions
5.1.3 Predevelopment Synoptics: Hudson Bay and Eastern Canada
5.1.6 Identifying Trigger Mechanisms

Question 2

Current Time: 0300 UTC 01 Dec 2000

Which of the following regions have surface and low-level conditions that might be favourable for the development of a polar low in the next 36 hours?

Northern Hudson Bay and Foxe Basin
Central Hudson Bay
Southeastern Hudson Bay
Western Hudson Strait
Eastern Hudson Strait
Ungava Bay
Labrador Sea

Supporting Topics:
1.1 Development of Forcing Mechanisms
1.3.1 Predevelopment Conditions
2.1 Climatologogy: Reference Map and Seasonal Ice Coverage
2.2.3 Climatology for Northern and Eastern Canadian Waters
5.1.2 Identifying Predevelopment Conditions
5.1.3 Predevelopment Synoptics: Hudson Bay and Eastern Canada
5.1.6 Identifying Trigger Mechanisms

Question 3

Current Time: 0300 UTC 01 Dec 2000

Based on the current and projected mid- and low-level conditions, determine the area(s) and timing where polar low development is most likely to occur.

Supporting Topics:
1.1 Development of Forcing Mechanisms
1.3.1 Predevelopment Conditions
2.1 Climatologogy: Reference Map and Seasonal Ice Coverage
2.2.3 Climatology for Northern and Eastern Canadian Waters
5.1.2 Identifying Predevelopment Conditions
5.1.3 Predevelopment Synoptics: Hudson Bay and Eastern Canada
5.1.6 Identifying Trigger Mechanisms

Question 4

Current Time: 1500 UTC 01 Dec 2000

It is now near the end of the shift, 1500 UTC on 01 Dec. It appears that the 1200 UTC run of the GEM regional model will be delayed due to circumstances beyond your control. You'll have to make do with the run from 0000 UTC on 01 Dec.

As expected, satellite imagery shows no evidence of polar low development over the Hudson Bay through 1400 UTC. In terms of polar low forecasting, your attention is concentrated on the Ungava Bay area. From previous evaluation you know that the most likely time of development is late on 01 Dec through 1200 UTC on 02 Dec. For your current assessment, focus on the 36 hour time period of 1200 UTC on 01 Dec through 0000 UTC on 03 Dec.

The GEM regional guidance initialized at 0000 UTC 01 Dec carries a weak elongated MSL low over northern Ungava Bay and eastern Hudson Strait. By 1200 UTC on 02 Dec, the MSL forecast shows a somewhat more organized system with a central pressure near 1005 hPa over Ungava Bay. In the subsequent 12-hour period, this system splits, with part of it remaining nearly stationary in Ungava Bay, and the other part moving eastward across the Torngat Mountains and into the Labrador Sea.

Assess the guidance for factors related to the expected depth of the low in the Ungava Bay area:

Model too weak because it contains no linkage between the model surface and the model atmosphere
Model too deep because of convective feedback effects due to the deep convection parameterization
Model too weak because of errors in its ice coverage field
Model too weak because of inadequate horizontal resolution

Supporting Topics:
4.1 Factors Contributing to the Quality of NWP
Ten Common NWP Misconceptions: "A 20-km grid accurately depicts 40-km features"

Question 5

Current Time: 0600 UTC 02 Dec 2000

It is now the second day of monitoring the region. The latest model run initialized at 0000 UTC 02 Dec is available. It maintains a weak area of low pressure over Ungava Bay. Examine the latest surface analysis and available observations and imagery through 0600 UTC to verify the model's six-hour forecast. Look for conditions that could lead to a polar low developing over the Ungava Bay area.

Supporting Topics:
1.3 Development of a Polar Low
5.1.2 Identifying Predevelopment Conditions
5.1.7 Identifying Formation of Polar Lows
5.1.8 Forecasting the Motion of Polar Lows

Question 6

Current Time: 0600 UTC 02 Dec 2000

There is pretty good evidence that a polar low has formed or is forming over Ungava Bay at this point in time. You must now forecast the track of this low. The following predicted tracks are based on three common approaches used to forecast the motion of polar lows. Before making an actual tracking forecast, match each approach to a given track.

Supporting Topics:
1.3.4 Motion of Polar Lows
3.1 Nowcasting
5.1.8 Forecasting the Motion of Polar Lows

Question 7

Current Time: 1430 UTC 02 Dec 2000

As suspected, the satellite imagery at this point in time shows that a polar low has fully formed and is moving across Ungava Bay. The low has the convection pattern associated with many polar lows: a spiral shape with arms composed of convective cloud elements with well-defined edges. Since it formed, right around 0600 UTC, it has been moving in the same direction as the upper-level trough.

Compare the GEM regional model initial pattern at 1200 UTC on 02 Dec to the latest surface observations and the satellite imagery at that time. Given this comparison, do you expect the guidance to correctly forecast the evolution of the low?

Yes No

Supporting Topics:
1.3.4 Motion of Polar Lows
1.3.5 Dissipation of Polar Lows
3.1 Nowcasting
4.1 Factors Contributing to the Quality of NWP
5.1.8 Forecasting the Motion of Polar Lows

Question 8

Current Time: 1430 UTC 02 Dec 2000

With the model forecast a bit suspect and the satellite imagery showing that the polar low has so far been following the direction of the upper trough, predict the track for the period from 1200 UTC on 02 Dec to 1200 UTC on 03 Dec. Which of these general scenarios most closely matches your forecast for the evolution of the polar low during this time period?

Stay quasi-stationary and slowly decay
Move approximately eastward and maintain strength as it passes over the Torngat Mountains and enters the Labrador Sea
Move approximately eastward over the Torngat Mountains and retain enough definition to cross the land and reform as a polar low over the Labrador Sea
Move approximately eastward over the Torngat Mountains and weaken as it passes over land without regaining strength over the Labrador Sea

Supporting Topics:
1.3.4 Motion of Polar Lows
1.3.5 Dissipation of Polar Lows
3.1 Nowcasting
5.1.8 Forecasting the Motion of Polar Lows