The Most Extreme Warm Day in Seattle History: April 18, 2016

On Monday, Seattle experienced a stunning and unparalleled temperature extreme:

the warmest daily temperature relative to normal conditions ever observed for any date since record keeping began at Seattle-Tacoma Airport (1948).  Any date, any season, any year.

Specifically, temperatures on Monday surged to 89F,when the normal maximum temperature for that date was 58F:  a 31F anomaly (difference from the normal high).    Never had the daily maximum been so far from the average maximum.  Just amazing.

The daily records showed this was the largest maximum temperature anomaly on record, with April 1, 1987 coming in at second place (28F anomaly).   Here are the top ten daily maximum temperature anomalies provided by Mark Stoelinga, a research meteorologist at Vaisalla, Inc.   You will notice the most of the top events are in spring, with April being heavily represented.  Why spring?

Average highs are relatively low in April (mid 50s to lower 60sF) and the sun is getting quite strong (the sun's intensity and duration on April 21st is the same as on August 21st).  May and early June is also a good period for extreme maximum temperatures compared to normal highs for the same reason.

But even in the best season (spring), to get the truly extreme high temperature anomalies, the atmospheric circulation on both the large and local scales must be just right.

So why was Monday so amazingly anomalous?

We started with a strong upper-level high pressure ridge in the idea location:  just inland from the coast. (see upper level (500hPa, roughly 18,000 ft)  map at 5 PM Monday) On the western periphery of the ridge there is southwesterly flow which brought very warm air northward.

The 850 hPa map (around 5000ft) shows the warm air (red/orange colors) and southerly winds along the coast, with higher heights (pressures) to the east, lower to the west.  The temperatures that morning over Seattle were very warm--higher that all days but one for the period January 1 through May 1.  The existence of a very strong El Nino, with above normal water temperatures along the West Coast, contributed to the warmth.

At low levels there was a modest offshore pressure gradient, so that weak easterly flow was pushing the cool, marine influence away from Sea-Tac Airport.  To show this, let's look at the winds and temperatures (red lines) above Sea-Tac Airport that day (below is a time-height cross section, time increasing to the left, time is in UTC).  1821 is 2 PM on Monday, the y axis is pressure (850 is about 5000 ft).  Weak easterly flow dominated at low levels.  If the flow had been westerly, cool air would have moved over Sea-Tac.   Stronger easterly flow over the Cascades would have produced low pressure near the Cascade foothills that would have drawn in marine air near the surface.

To quote Goldilocks:  everything was "just right."

Some folks will suggest tjat this as a good example of the impacts of global warming, but they would be wrong.  Of the 31.5F anomaly that day, perhaps 1-2 F could be traced to human impacts, the rest are the result of natural variability.   Thus, without any human intervention it would still have been a record-breaking extreme day.

But you want to see this visually?   Here is an analysis done by Mark Stoelinga that shows the trend in yearly maximum extreme temperatures anomalies for the spring season (March, April, May)--blue lines.   We will talk about the other temperatures in another blog.  The biggest anomaly was this year (18C or 31F). And you will note that there is a very slow upward trend during the past 70 years:  about 1 C (1.8F).   Some of that trend is from human-increased greenhouse gases, some from urbanization, some from sensor changes, and some from natural variability.  In any case, the upward trend is small and eclipsed by the natural variability, with its jagged ups and downs.

As I noted in my Golden Rule of Climate Extremes:  the greater the climate anomaly, the larger the percentage of that anomaly due to natural variability.