Monday, October 17, 2011

Tomorrow Will be the Warmest Day for Several Months

Tuesday, October 17, 2011
4:04 P.M.

That was a bold title, and I may end up eating my own words at some point in the next couple weeks. Nevertheless, I firmly believe that tomorrow will be the warmest day until spring of next year.

One commonly associates low pressure with cloudy, rainy, and sometimes downright stormy conditions. Tomorrow, however, low pressure will be our friend.

Winds in the atmosphere are the result of pressure differences in the atmosphere. Wind generally flows from high pressure to low pressure.Tomorrow, we will have a weak trough off of our coast and higher pressure in Eastern Washington, so we will see an offshore wind (easterly) wind. This type of wind generally gives us relatively warm and dry conditions, and if a big offshore wind event sets up in the summer, Western Washington can see some serious heat.

If you look at the "Chinook Wind" diagram, you can see some text that reads "adiabatic warming." Believe it or not, everybody has experienced some sort of adiabatic effect in their life. Adiabatic cooling refers to the cooling and expansion of air as it rises, and adiabatic warming refers to the warming and condensing of air as it sinks. Adiabatic cooling is visible as clouds, so you have certainly experienced adiabatic cooling. Adiabatic warming is associated with clear skies, warm temperatures, and dry air, and it is how we get our heat waves in the summer.

On the windward side of the Cascades, air rises, but on the leeward side, it sinks. When we have low pressure to our west, we are on the leeward side of the Cascades because the air coming in to "fill" the low pressure originates from our east. Tomorrow, we will be on the leeward side, and the air coming over the Cascades will sink and warm when it gets to Puget Sound. Hence, we will experience our last "warm wave" for a long time!

Let's take a look at the models for tomorrow.

Valid 11:00 am PDT Tue, 18 Oct 2011 - 30hr Fcst - UW 12km 12z WRF-GFS Sea-Level Pressure, 10m winds, 925mb temp

Take a look at the pressure difference over the Cascades. Higher pressure in Eastern Washington and lower pressure over us.

The next graphic shows the 10 meter wind speed at the same time. This is with the 4/3 km resolution model. You can really see some details in this one.

Valid 11:00 am PDT Tue, 18 Oct 2011 - 30hr Fcst - UW 4/3km 12z WRF-GFS 10m wind speed

You can see fairly strong offshore winds around the foothills, with the strongest ones near Enumclaw. Enumclaw often gets very windy during offshore flow events. On Christmas Eve of 1984, they saw winds over 100 from an offshore wind event. I'll talk about Enumclaw windstorms in a future blog post.
My favorite graphics for tomorrow are below. These show the temperature forecast at various points throughout the day.

8 A.M.

Valid 08:00 am PDT Tue, 18 Oct 2011 - 27hr Fcst - UW 4/3km 12z WRF-GFS 2m temp

11 A.M.

Valid 11:00 am PDT Tue, 18 Oct 2011 - 30hr Fcst - UW 4/3km 12z WRF-GFS 2m temp

2 P.M.
Valid 02:00 pm PDT Tue, 18 Oct 2011 - 33hr Fcst - UW 4/3km 12z WRF-GFS 2m temp

You can see the interior getting really hot. One thing I really want to point out is how warm the foothills on the western sides of the Cascades and Olympics are. You can definitely see this in the 8AM and 11AM  frames. The foothills get a stronger adiabatic warming effect than places by the sound or off of the coast, explaining the warmer temperatures. 

Bottom line: enjoy tomorrow! 

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