In Seatac this morning, the weather is cool, overcast and still. The entire sky is blanketed with stratus clouds.
The National Weather Service (NWS) in Seattle forecasts a cloudy day today, with a high temperature of 67 F and a 70% chance of rain. New rain accumulation will be less than 0.10 inches. Winds will be from the west-southwest at 6-13 mph. This evening will be cloudy, with a low temperature of 59 F and an 80% chance of rain. New rain accumulation will be between 0.10-0.25 inches. Winds will be 8-10 mph from the west, becoming west-southwest by the late evening.
The visible satellite imagery shows that most of the state is under some cloud cover. The bands of clouds are moving into a low pressure system just off the west coast of British Columbia.
The enhanced infrared satellite imagery shows that some of these clouds are thicker than we’ve seen so far this week, though on the grand scale, they are still thin with warm, low tops.
The water vapor satellite imagery shows a clearer image of the low pressure system. Notice the moisture that is being drawn into the center of circulation.
The 12Z upper air sounding from UIL shows an incredibly humid sounding, with the entire column near saturation until around 200 mb. There was 1.29 inches of precipitable water and no 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 4.6 C/km.
The deep-layer shear was 26 kts, and the low-level shear was 10 kts. The shear at all levels is due to speed changes, for the most part.
The surface observations (from the SPC Mesoscale Analysis Map) show the still winds, cloudy skies, and high humidity (as represented by low surface dewpoint depressions), throughout the state. There are no major frontal boundaries present in this image.
The surface pressure map shows no sharp pressure gradients over the state this morning. According to the RAP, none are expected to develop in the next six hours.
Synoptically speaking, the 300 mb NAM chart (from Unisys) shows the trough around the low pressure system. There is a weak jetstreak that will nose into the southwestern corner of the state later this afternoon, and a stronger one that will hit later this week.
The 500 mb NAM chart shows some weak Positive Vorticity Advection (PVA) working its way into the southwestern corner of the state. Otherwise, there is no strong vorticity advection over the state today.
The 700 mb NAM chart shows no rapidly-rising air over the state today. This chart has been excluded from today’s post.
The 850 mb NAM chart shows no strong thermal advection over the state today, but the thermal gradient remains in place, perhaps as a stalled frontal boundary.
The precipitation chart shows light precipitation over the area around the Puget Sound, but not much precipitation outside of this area.
Overall, I expect a rainy, messy day. Overall, this is a great day to leave Seattle for some place that has more sunshine. We will travel to Spokane this evening, and then to Casper, WY tomorrow.
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