Yesterday's high of 93°F at the Salt Lake International Airport was a record for the day but not really all that exceptional when you consider that the record high on the previous day (September 11) is 99°F and a couple days later (September 15) is 97°F. Somehow September 13 has slipped has never gone out-on-the-limb for maximum temperature compared to the surrounding days. Such are the realities of climate statistics.
That said, yesterday's maximum temperature was a out-on-the-limb in terms of what was forecast. Several days ago, we posted that the forecast for tomorrow (Sunday) was uncertain. Some models, like the GFS, where bringing in a cold front on Saturday, resulting in much cooler weather for Sunday. Others, like the ECMWF AIFS brought the front in during the day on Sunday. I'm not sure if any called for the cold front to come in Sunday evening.
These days one can process the forecast from well over 100 ensemble forecasts to produce a range of possible forecast outcomes. This is what the National Weather Service does for their National Blend of Models (NBM) forecast. On Tuesday we posted about the enormous spread for the maximum temperature forecast for yesterday (see An Uncertain Sunday) with the middle 50% of high temperature forecasts lying between 68° and 84°F (see the tallest red box in the left-hand graphic below).
That middle 50% is sometimes referred to as the interquartile range. However, half the forecasts lie outside of that box. The whiskers surrounding the box extend through the middle 80% of forecasts (or from what is called the 10th to 90th percentile). In this case, the top whisker extended to 92°F. That means that 90% of the forecasts called for a maximum temperature at or below 92°F.
I went back to that NBM forecast distribution and it turns out it ended at 92°F. In other words, not a single forecast member called for a maximum at or above 93°F. The observed maximum temperature on Sunday was outside of the entire predicted ensemble range.
Ideally that would not happen except in the most exceptional circumstances, but our ensembles are not perfect. Forecasts do sometimes verify outside of the ensemble range and that's a sign that we still have work to do.
Part of what made the forecast for Sunday challenging was the occurrence of something called anticyclonic wave breaking (AWB). If you have been to the beach, you have experienced wave breaking. As water waves approach the shore, the decelerate and steepen. Eventually the waves steepen to a point where they overturn and break down into turbulence, or what scientists call wave breaking.
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| Breaking wave. Source: NOAA. |
The waves you experience at the beach are gravity waves that occur at the interface between the high-density water and the lower-density atmosphere. Gravity waves also occur in the atmosphere where gravity wave breaking is one important cause of aircraft turbulence.
In AWB the waves that are breaking are not gravity waves but very large-scale atmospheric waves known as Rossby waves. Rossby waves are named after Carl-Gustaf Rossby, who did pioneering work on the jet stream and the dynamics of upper-level waves in the middle 20th century. He was even featured on the cover of Time Magazine in December 1956.
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| Source: https://content.time.com/time/covers/0,16641,19561217,00.html |
Rossby waves are the large-scale upper-level troughs and ridges that we observe at upper-levels (i.e., jet-stream level) in the atmosphere. These waves are so big (1000 km or more across) that the rotation of the Earth strongly affects their behavior.
Like waves at the beach, Rossby waves can also "steepen." In contrast to water waves that steepen in the vertical, Rossby waves "steepen" in the north-south direction, which is what happened last week over the north Pacific. At 1200 UTC 10 September, there was a ridge over western Alaska and a trough over the northeast Pacific. This ridge-trough system amplified over the next 24 hours, with the 500-mb height contours "steepening" and turning clockwise (anticyclonically) on the downstream side of the ridge. Eventually, the 500-mb height contour "broke" in the sense that higher 500-mb heights were north of lower 500-mb heights. This is consistent with the red contour crossing the purple meridian three times at the end of the plot sequence below. Think of the ridge pushing over that meridian as an overturning wave with the surfer in the the tube or trough at lower elevations.
AWB is associated with the amplification of a strong upper-level ridge, which is often related to the release of heat by condensation in precipitation systems just upstream of the ridge. Although AWB events are often predicted reasonably well in terms of their occurrence, the details (i.e., the "turbulence") can be hard to get right. Sunday was a hard forecast because the strength and position of the trough and cold front was challenging to anticipate. The 93°F we hit on Sunday reflects the fact that the AWB produced a stronger trough than anticipated, delaying its progression across the Pacific Northwest.
Surfers can be surprised by breaking wave behavior. So can meteorologists.
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