CENTRAL REGION TECHNICAL ATTACHMENT 97-04
Composite 500-mb Arctic Outbreak Patterns
Richard L. Van Ess
Weather Service Forecast Office
Bismarck, North Dakota
Post-cold front strong winds with accompanying ground blizzard conditions, very cold temperatures and extreme wind chill (arctic outbreak) can be a significant forecast problem in the Northern Plains. The meteorological parameters that lead to arctic outbreaks have been previously defined, and a checklist has been developed to help forecast arctic outbreak strong wind events (Van Ess 1985; Van Ess and Kapela 1989; Kapela et al. 1995). The checklist can be used as an aid to predict severe cold outbreaks across the Northern Plains in the short term. A longer lead time would be beneficial. Five hundred-millibar (500-mb) pattern recognition would be one way to obtain a longer lead time, and would also easily translate into the shorter term arctic outbreak checklist.
Historical weather maps from the NWS/NCEP grid point data set between 1950 and 1989 were used in this study. Evolutions of 500-mb heights (initial through 48 hrs) associated with arctic outbreaks were compared. One hundred winter wind events were analyzed, which were reduced to fifty significant arctic outbreaks. The final composite products defined three 500-mb patterns leading to the most severe cold intrusions:
The 500-mb northerly and northwest composite flows will be discussed while the southwest flow will not be included. The southwest flow scenario usually is a large synoptic scale cyclone that has significant snow or mixed precipitation. It is likely preceded by watches or warnings and considerable forecaster concern. The arctic air intrusion only adds to the already harsh and complicated weather. The north and northwest 500-mb flow patterns have limited precipitation and are seldom preceded by watches or warnings. Wind, cold temperatures, dangerous wind chill and blowing snow are the main concerns of these outbreaks.
Discussion and Composites
The two composite 500-mb flow patterns can be used by a forecaster to help define when a future arctic outbreak is possible. The following is a brief description of each composite flow.
In the northerly composite flow (Figures 1 and 2), a large closed 500-mb low initially forms near the west side of Hudson Bay in central Canada. The subsequent movement (12-48 hours) is almost due south toward the Great
CR TA 97-04
Lakes. This moves a short wave trough with strong cyclonic flow over the Northern Plains. The west side of this closed upper low (over the Northern Plains) is the favored area for maximum low and mid level tropospheric cold air advection and the main invasion area of arctic air. A southward moving closed 500-mb low directly out of central Canada in winter will always have (based on observational experience) an abundance of very cold air associated with it. Once this northerly composite flow is recognized, the forecaster can then concentrate on the shorter term arctic outbreak checklist.
Figure 1. Initial 500-mb composite heights (dm) in a northerly flow.
Figure 2. Composite 500-mb heights (dm) in a northerly flow 48 hours later, which would result in an arctic outbreak over the Northern Plains.
This 500-mb composite pattern starts with a rapidly intensifying short-wave trough moving through a larger scale northwest flow pattern (Figures 3 and 4). The short-wave trough moves across Alberta and Saskatchewan and strengthens into a closed low in southern Manitoba. Once the system closes off, it transports cold air southward along its western periphery into the Northern Plains, allowing needed ingredients for an arctic outbreak to come together.
As the northwest flow scenario unfolds, there are two important questions a forecaster should answer: 1) is there an ample supply of arctic air the system can ingest; and 2) will all the ingredients come together at the right time? The northerly flow 500-mb closed low outbreak always has a supply of very cold air that it transports south. This key ingredient can be limited in northwest flow situations. However, the composite of the stronger arctic outbreaks showed that once the system became a closed low, the greater baroclinic development dramatically increased the transport of arctic air. If it were to remain an open short-wave trough, the speed of the system would be too fast to create a prolonged strong wind event and overall strength of the system too weak to develop a significant arctic air surge.
Figure 3. Initial 500-mb composite heights (dm) in a northwest flow.
Figure 4. Composite 500-mb heights (dm) in a northwest flow 48 hours later, which would result in an arctic outbreak over the Northern Plains.
A composite of 850-mb heights and temperatures during a northerly flow arctic outbreak is shown in Figures 5 and 6. This composite was included to show how rapidly the 850-mb frigid air temperatures can be transported into the Northern Plains.
Figure 5. Initial 850-mb composite heights (dm) and temperatures ( C) in a northerly flow.
Figure 6. Composite 850-mb heights (dm) and temperatures ( C) in a northerly flow 48 hours later.
CR TA 97-04
The use of the 500-mb composites in north and northwest winter flow can provide advanced notice that an arctic outbreak is possible. Not all systems will develop into significant outbreaks, but knowing these patterns can alert a forecaster to the potential. Once the possibility is there, a forecaster can look for more short time signatures on the strength of the system.
Kapela, A.F., P.W. Leftwich, R.L. Van Ess, 1995: Forecasting the impacts of strong wintertime post-cold front winds in the Northern Plains. Wea. Forecasting, 10, 229-244.
Van Ess, R.L., 1985: Forecasting Ground Blizzards. Central Region Technical Attachment 85-22. DOC/NOAA/NWS Central Region Hdqters, Scientific Services Division, Kansas City, Mo.
____________, and A.F. Kapela, 1989: Arctic Outbreaks. Central Region Technical Attachment 89-20. DOC/NOAA/NWS CR Hdqters, Scientific Services Div., Kansas City, Mo.