1st December marks the start of the northern hemisphere meteorological winter, when routing vessels across the North Pacific and North Atlantic Ocean basins becomes a little more challenging as more benign and predictable summer weather conditions are replaced by a stormy and often more volatile pattern. So right on cue one of the most common features associated with this change in pattern is the occurrence of large cut-off low pressure systems that frequently attain storm force. There is a good example due in the NE Pacific in the next few days which ships will want to avoid.
Cut-off lows are different in nature to other low-pressure systems (such as tropical, extratropical as well as primary and secondary lows) as they bring active low pressure systems which remain near stationary, or some even retrograde and move westwards, over a period of several days, counter to the usual progressively easterly flow of weather systems in the Northern hemisphere. Such behaviour is characteristic of low pressure systems that become separated from the upper-level steering winds, such as a jet stream, resulting in free-form movements that can be haphazard and unpredictable which weather models can find difficult to predict.
Figure 1 Sequence shows how a meander develops in a flow and then can get cut-off
Figure 1. shows how a meander develops in a thermodynamic flow and then can get cut-off. The jet streams seldom flow in straight lines and typically display a sinuous pattern with differing wavelengths and amplitudes. The associated ridges and troughs translate down through the coupled atmosphere to the surface as high-pressure ridges and troughs generating surface lows respectively. For mariners, an active cut-off low that is near stationary can also cause the development of a fully arisen sea due to the persistent winds, which is counter to more normal faster moving winter storms that have a shorter duration, over a particular area.
As we move into early December, models have been in agreement on the development of a cut-off low in the eastern North Pacific, where favourable atmospheric conditions are present for cyclogenesis.
Figure 2. GFS 250mb chart initialized Dec 01/12z valid at Dec 05/00z. Source: Tropical Tidbits
From Figure 2, we can see polar jet stream digging sharply south in a marked trough over the central N Pacific. This will support the development of a rapidly deepening cyclone at the surface, along with strong ridging off the western coast of the USA which will block any eastward movement of the system.
Figure 3. GFS surface pressure chart initialized Dec 01/12z valid at 05/00z
Moving down to the surface forecast at the same time we can see from Figure 3 the corresponding cut-off storm force low pressure system is expected to form about 1000 nautical miles to the north of Hawaii by 5 December. With persistent northerly gale to storm force winds along the western side of the system we can expect to see fully arisen seas peaking over 10-11 meters, with 5-6 meter seas extending as far south as Hawaii. Consequently, this intense slow moving and slow filling system creates a large hole to avoid for ship routing.
Cut-off lows will always pose a challenge for forecasters and routers. Due to the fact that they are separated from the main atmospheric flow means that their movement can be highly unpredictable and their slow moving nature can result in fully arisen seas, impacting a large area which could necessitate large route deviations to avoid the worst weather. We will be closely monitoring this upcoming feature in the eastern Pacific and will be on the lookout for more to come as we enter the active winter season.
Stay connected and safe.