Dry Air Storm Hits the Pacific Northwest

You thought the BLOB was bad, with its gentle warming....

But now Northwest residents are facing a greater challenge:

an invasion of very dry air that will crack skin, irritate throats, increase itchiness of the eyes and skin, and enhance the risk of colds and flu.  Sparks will fly when you touch doorknobs and metal, and don't even think about getting near combustible or explosive substances.

What do we call this menace?   El Arido.

The problem is that cold, dry arctic air has spread over our region, with easterly flow moving down the Cascades resulting in even lower humidities.

A plot of the surface air relative humidities at 8 PM Tuesday shows low values of around 30-40% over NW Washington to around 50-60% around central Puget Sound (click on plot to expand).

But now let me show you the future, if you have the nerve to view it.  First, the UW WRF model forecast for OUTSIDE air relative humidity at 4 AM Wednesday.  Values below 35% (brown color) over much of Washington State and particularly over the western sides of the Cascades.

 At 1 PM Wednesday, El Arido is really cooking, with the entire state dry and the western part desiccated.

 By 4 AM Thursday, the situation has improved slightly over the West and the Columbia Basin is a bit humidified.

A plot of relative humidity over time at Seattle (blue line) show values getting down to around 20%.

But folks it is even worse than that.  Much worse.  These are OUTSIDE relative humidities, the air is MUCH drier inside of homes and buildings.

Why?   Buildings are leaky and outside air diffuses inside and is warmed by the heating system.    Imagine outside air of 30% relative humidity and a temperature of 32F.    The air is brought into the house and warmed, resulting in a further drop of relative humidity.   To understand this, one must think about the definition of relative humidity.

Relative humidity=  amount of water vapor in some air
                                 maximum amount of water in that air sample

Air moving indoors retains the moisture it started with, but the amount of water vapor the air can hold increases radically because the temperature has warmed (warm air can "hold" more water vapor).  Thus, relative humidities inside are far less than outside during the winter--sometimes less than 10%.

Tuesday afternoon, while the relative humidity outside my building was 40-50%, the relative humidity inside my building was about 20%.    With drier air outside today (Wednesday), the relative humidities inside heated buildings around Seattle will be around 10-20%.   A lot of sparks will fly and a some skin will become dry and itchy.   You may develop a dry cough.  Cracks may develop in your skin.

The suffering from El Arido will be much greater than any BLOB.

from Cliff Mass Weather Blog http://ift.tt/2i96ue0