On an unusual February day in Portland, Oregon, the temperature is 18°C (65°F) and it is dry and sunny. The winter climate in Portland is usually chilly and rainy. How could you explain a warm, dry day in Portland in winter?

The Willamette Valley and Columbia River Gorge is sufficiently complex that is among the most difficult places to predict the weather in the world.

Some of the variables are:

* Direction of surface airflow—usually from the west, but if it changes to be from the southwest it becomes warmer than usual.  If it changes to come from the northwest, it becomes colder than normal.  If high pressure builds in the western Rockies or even east of the Cascades, easterly flow through the Gorge completely blows NOAA's computer models, though our weather is usually clear and cold in winter and extra hot in summer.

* The position of the jet stream normally meanders from north to south and back over us every few days to weeks.  In winter, if it is north of us it pulls in upper level systems from the south possibly bring warm moist air from the subtropics.  If the jet stream moves south of us it usually attracts cold moisture-saturated Arctic air causing overcast and cooling.

* If cold air pools in the valleys, it can resist being "blown away" for several days.  The colder the harder to move.

* The Coast Range and Tualatin mountains and the Cascades have subtle effects on wind direction and air moisture saturation.  Those can change the temperature of the air significantly.

The bottom line is that slight differences in temperature, wind direction, and system movement can have a radical effect on our weather.