This morning was odd. Walking outside at 7 AM here is Seattle, it was noticeably warmer than the previous afternoon.
And moist. Almost tropical.
The moisture over us now had streamed northward out of the tropics across Hawaii and then north and westward, as shown by the water vapor image shown below (for 7 PM tonight). The lighter colors indicates more water vapor. We are located under the red star, and if you look closely you can see the swirl of water vapor into an intense low pressure center to our Northwest.
A satellite image of total water vapor content in the vertical (below) shows the reservoir of moist air over the tropics (read and purple colors) and the tendrils of moisture moving north…..one of them towards red. Such moisture pathways are known as atmospheric rivers. And when they are forced to rise by our mountains, heavy precipitation occurs.
Temperatures today rose into the mid-60s today over Puget Sound and around 70F in the Portland area, while the dew points–a good measure of water vapor content–got to near 60F. All under generally cloudy conditions.
We did not need the sun to warm today; instead strong southerly flow brought in warm air from the southwest.
Tonight is a very windy night as a deep low pressure center passes to the north and a cold front extending from the low approaches Washington State. Below is an official NWS analysis (top) and a model simulation (bottom) for 4 PM and 10 PM PST, respectively, that shows the situation. The current of warm air from the south is particularly strong in front o of the cold front–and that is what we are experiencing tonight.
Winds are gusting to 40-50 mph around Puget Sound right now, and there are already some power outages.
Enjoy the warmth–it won’t last. This weekend the atmospheric configuration will change and cold air from the north will dominate. Today may be the last time you enjoy highs in the 60s for the remainder of the year.