In Seattle this morning, the weather was cool, overcast and still. The sun keeps trying to peek out, but is mostly unsuccessful.
The National Weather Service (NWS) in Seattle forecasts a partly sunny day today, with a high temperature of 68 F and a 20% chance of rain. Winds will be from the southwest at 6 mph, becoming light and variable in the afternoon. This evening will be mostly cloudy, with a low temperature of 56 F, and a 20% chance of precipitation util 11 PM. Winds will be from the north at 5-15 mph.
The visible satellite imagery shows that most of the state has some clouds, but the heaviest cloud cover is in the areas around the Puget Sound, including my location at Seatac.
The enhanced infrared satellite imagery shows that the stationary front became a cold front. The cold front moved south, and there is a line of clouds just behind the leading edge of the front.
The water vapor satellite imagery shows the cold front as well. Notice the area of dry air just behind the frontal boundary.
The 12Z upper air sounding from UIL shows a humid, mixed sounding with 0.94 inches of precipitable water and low dewpoint depressions throughout the column.
The deep-layer shear was 44 kts, and the low-level shear was 13 kts. There was some directional shear near the surface, but most of the shear is due to directional changes.
The surface observations (from the SPC Mesoscale Analysis Map) tell a similar tale to yesterday’s surface observations. We have heavier cloud cover and higher dewpoints in the coastal region, and drier, clearer skies in the west.
The surface pressure map shows no strong pressure gradients over the state this afternoon. The pressure gradient will tighten up in California, Oregon and southern Washington over the next six hours, according to the RAP.
Synoptically speaking, the 300 mb NAM chart (from Unisys) shows that the upper-level low has not moved much over the past day. The initial jetstreak has cycled through, and will be moving out of our area today.
The 500 mb NAM chart shows that there will be some Negative Vorticity Advection (NVA) pushing southeast from the Puget Sound. This may clear out some of the existing clouds, and may help explain the clear area southwest of Seattle.
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 Cold Air Advection (CAA) continuing throughout the day. Notice the winds bowing across the thermal gradient from cold to warm.
The precipitation chart shows that the area around the Puget Sound can expect rain today, but farther east will remain dry.
Overall, I expect a chilly, cloudy day today. The CAA will keep the temperature cool, the slightly tightening of the isobars may increase the wind speeds.
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