In Seattle this morning, the weather has been another stereotypical Pacific Northwestern day; cool, rainy, and overcast. The skies are a blanket of stratus clouds, and there is small mist falling. I went for a 3 mile run this morning, only to find that my body didn’t know how to process the extra air (running close to sea-level instead of the 5700 ft mesa near my house), or the extra humidity.
The National Weather Service (NWS) in Seattle forecasts a partly sunny day today, with a high temperature of 69 F and a 20% chance of rain showers. Winds will be from the east at 6 mph, shifting to the north-northwest by this afternoon. This evening will be mostly cloudy with a low temperature of 53 F and an 8-10 mph north wind, shifting to the east after midnight. No hazardous weather is expected.
The visible satellite imagery shows clouds over the land near the coast, but most of the state is under clear skies this morning.
The enhanced infrared satellite imagery shows that all clouds are thin, with low, warm tops. This image has been excluded from today’s post.
The water vapor satellite imagery shows a slightly dryer atmosphere in the southern part of the state, perhaps swirling into a low pressure system over very northern Idaho and southeastern British Columbia.
The 12Z upper air sounding from UIL shows a nearly-saturated boundary layer under 850 mb. Above 850 mb, the air dries considerably, as shown by the separation in the dewpoint trace (green) and the temperature trace (red). There was 0.85 inches of precipitable water and 1 J/kg of Convective Available Potential Energy (CAPE) present in the column this morning. There was no strong thermal inversion, and the 0-3 km average lapse rate was 4.1 C/km.
The deep-layer shear was 47 kts, and the low-level shear was 7 kts. Low-level shear was a mix of speed and directional changes, and deep-layer shear was due almost entirely to speed changes.
The surface observations (from the SPC Mesoscale Analysis Map) show cool temperatures (at least by my New Mexico standards!) throughout the state. Skies are clear west of the Puget Sound; the dewpoints are also lower in this region. There is a dryline/stationary front that extends all the way through Oregon along this area as well, with the damp side west and the dry side east.
The surface pressure map shows some bunching of the isobars from the bend in the Columbia River near Portland up through the western shore of the Puget Sound. The RAP shows that this pressure gradient will increase slightly over the next six hours.
Synoptically speaking, the 300 mb NAM chart (from Unisys) shows an upper-level closed low over eastern British Columbia. Around this low, the polar jet dips south, and a small speed max is passing through Washington state today.
The 500 mb NAM chart shows no significant vorticity advection today over the state. There is some vorticity to our north, but it isn’t advecting south. This chart has been excluded from today’s post.
The 700 mb NAM chart shows no rapidly-rising air over the state today. In fact, there is some sinking air. However, this chart has been excluded from today’s post.
The 850 mb NAM chart shows strong Cold Air Advection (CAA) moving southeast through the Puget Sound. Notice how the winds blow directly across the thermal gradient from cold to warm.
The precipitation chart shows that the whole western part of the state can expect some light precipitation.
Overall, I expect a messy day today, at least by New Mexico standards. Winds near the sound may increase, but not enough to cause a stir. The CAA may cool things off a little, but given the unmixed morning sounding, and the layer of clouds, I don’t know that the CAA will have a big impact on the area temperatures.
Thank you for reading my forecast.
The upper air soundings and mesoscale analysis plots are from the Storm Prediction Center website.
The forecasted upper air soundings are from TwisterData.com.
The surface observation and upper level charts are from Unisys Weather.
The satellite data is from NASA – MSFC