The trough off of the West Coast is present in all cases, but the significant ridge near Alaska in the composite mean is present in only about half of the individual events. There is more variation between these plots than is apparent for the hardest freezes events shown in Fig. 4 but showing the 500-hPa geopotential height anomaly patterns of each member of the heaviest prolonged rain target group at the time of heavy rain onset. Composite maps shown are (a) 72, (b) 48, and (c) 24 h prior to the onset of the heavy rain, and (d) at the time of event onset. Contours use 60-m intervals for the target group ensemble mean. 2 but showing 500-hPa geopotential height pattern typical of the heaviest prolonged rain target group. This field is a good indicator for the hardest freezes because significant features of the composite are found in all or almost all members of the ensemble. The upper-left map is the longest event, the event length decreases across and then down the page. The larger panel is the target group ensemble mean.
Contours use 60-m intervals dashed contours are below zero solid thin contours with darker shading are above zero thick solid contour is for the zero value.
The 500-hPa geopotential height anomaly patterns of each member of the hardest freezes target group at the time of hard freeze onset. Composite maps shown are (a) 72, (b) 48, and (c) 24 h prior to the onset of the hardest freezes, and (d) at the time of hardest freezes onset. Contours use 2-K intervals for the target group ensemble mean. 2 but showing the temperature pattern typical of the hardest freezes target group shown is the 700-hPa level. Light shading with a dark edge denotes positive significant anomalies (unusually high heights) dark shading is for significantly low heights. Shaded areas denote areas passing a 1% significance test.
Geopotential height pattern typical of the hardest freezes target group shown is the 500-hPa level. The 99% of the values stand between the two dashed lines. The target group mean has been added and is shown by a heart-shaped symbol. This histogram refers to the grid point closest to Sacramento, and compares the means of 1000 random groups. Null distribution generated while assessing the significance of the 850-hPa mean temperature for the start of hottest heat wave events.