Record Warmth and The El Nino Blahs

I should start by noting the return of much warmer than normal conditions after a few days of cooler conditions (see graph, with average highs and lows and observed temps). It is now certain that Seattle will beat its all time temperature record--we are now experiencing the warmest January in Seattle recorded history! Not a single day has had a mean temperature below normal. And this warmth has been observed over much of our region. The mean tempeature this month will come in close to 47F...that is warmer than an average March day (45.15F), but not quite April (49.4F). My garden is really responding right now...my grass is really growing and bulbs are pushing out of the ground.

The weather situation that we have been seeing this month, and which appears to be dominant in our future, is classic late-winter El Nino. In such a pattern there is unusual troughing offshore, with the jet stream..the current of strong winds...going south of us into California. They get more storms and precipitation. We get split flow and a dribble of weak systems. Really dead. Below I have put a graphics of the flow at 500 mb--roughly 18,000 ft for you to examine. The lines tell how high that pressure level is above sea level. Winds parallel the lines with higher heights to the right of the winds. The closer the lines are together, the stronger the winds. With all that said, you see the obvious--trough/low offshore and the main winds going to California.




We are running out of time for winter. In three weeks, we pass the unofficial end to NW winter...after which we rarely get major storms or flooding events. And our computer forecasts can see well 1/3 of the way and nothing is going happen here.

Finally, want to see a wonderful video of mountain wave clouds...lenticular clouds..downstream of the Olympics? Check this out!

http://www.drdale.com/lapse/lapse100128.mov

Inversions and Inverse Barometers

Well folks, there will not be any active weather for a long, long time. But there is always something meteorological that is interesting. Weather is like a fine wine...the subtleties are often the most rewarding. And a lot less expensive.

Take today. A high pressure area has been overhead, with relatively clear skies aloft and light winds. This time of year you know what that spells? Temperature inversions and fog.

Why? Under clear skies there is good infrared radiation cooling to space from the surface, allowing it to cool faster than air above. This produces an inversion in which temperature INCREASES with height. Want to see the effect in action? Look at the figure below from the Seattle Sand Point profiler near Magnuson Park in NE Seattle. In this figure you see temperature (in C) plotted with height for various hours (this is in GMT, 12 is 4 AM, 18 is 10 AM, etc). You can see a good inversion in the morning in the lowest 400m, which gets "burned off" when the sun finally got in to heat the surface. When the low-level air gets cooled to saturation we get fog.
Its just not fair...we get high pressure over us and it gets colder in the morning and some fog.

What about inverse barometers? I had number of emails and blog comments about the tide tables being WAY off last week--at some locations the water level was nearly TWO FEET higher than predicted! Here are two examples of this effect at Neah Bay and Astoria. The red line is observed and the blue is predicted. The green line is the difference.

Tide tables are normally pretty reliable, so what is going on? Fuzzy math? Nope...the answer has been the unusually low pressure this month. Take a look at the pressure at Seattle, Astoria, and Sea Tac in the plot below. From roughly the 15th to the 23rd the pressure was amazingly low...roughly 980 mb (mb is millibars, a unit of pressure). This is approximately37 mb below normal (1017 mb). So why is this important? It turns out that low atmospheric pressure results in a higher sea level. This is known as the "inverse barometer effect", with water level rising about 1 cm for every 1 mb drop in pressure (see the figure below). Thus, with a difference of 37 mb from normal, sea level would be 37 cm or 14.5 inches below normal. Thus, the extraordinarily unusual low pressure, something not considered in the tide models, resulted in the predicted water level height being much too low! There are also secondary factors, such as the sustained unusual winds offshore and higher than normal wave activity...but the pressure anomaly is the main driver.