Local Seattle / Pacific Northwest Weather
Recent Weather Facts
With the recent low pressure area over the region, conditions have been cooler and wetter than normal for early September. In fact, the high temperature of 59 at Sea-Tac International Airport yesterday, September 8, was tied for the fifth coolest in early September (September 1-10) on record at the airport, dating back to 1948, and only one degree from the current coolest high temperature on record for that time period (58).
Forecast
Generally westerly flow from the Pacific will bring a train of dry days mixed with weak systems moving through the region through the middle of next week, around September 15. Beyond that, a large but fairly disorganized and slow-moving low pressure system approaches the west coast with periods of rain possible during this time. However, timing and location of enhanced rain chances are not yet clear. A wetter pattern may develop for late September as indications show the Gulf of Alaska becoming seasonally more active.
A low over the Intermountain West will continue to move eastward through the Rockies and into the Plains today, ending shower chances across the Northwest, even in the mountains. The Puget Sound lowlands will be dry. Onshore flow will keep clouds around most of the day with a few sunbreaks possible by late afternoon, and mostly sunny skies breaking out in spots this evening before the sun sets. Seattle area high temperatures will be in the mid to upper 60s.
On Friday, expect clouds to begin the day with onshore flow, but these will break by late morning to early afternoon, with more sun through the afternoon. Saturday will be more of the same, as the region remains between two weak systems. Highs both days will remain in the mid to upper 60s.
By Sunday, a trough currently over the Yakutat Borough of Alaska will have moved southeast to southern British Columbia, bringing a chance of light precipitation to Washington State in westerly flow. The greatest chances of precipitation will be over Vancouver Island and north of the Canadian border, with rain chances quite minimal south of Seattle. Clouds will prevail Sunday along the Oregon coast and north and west of a line from Astoria, Oregon to the north Cascades, with some sun possible late afternoon and evening in the lowlands from Seattle southward. Though warmer air moves in, clouds will keep temperatures in the upper 60s for highs.
After morning clouds Monday, skies will clear out for a majority of the Monday through Wednesday time period, with the exception of cloudier conditions along the immediate coastlines of Oregon and Washington Monday and Tuesday. A couple weak waves moving northeast ahead of the approaching Pacific low may give some rain to primarily northern Vancouver Island late Tuesday into Wednesday, but otherwise the region will remain dry until clouds move into the region with this low on Thursday with enhanced shower chances late next week. High temperatures will rise from the low 70s on Monday to the mid to upper 70s by Wednesday.
Significant National Weather
Autumn on the Way
Medium and long-range models show some indications of the approaching fall season. The low that just moved through the Pacific Northwest will move into the southern Canadian Prairies tomorrow morning then to western Hudson Bay by Sunday. While in Canada, the system will establish a connection to some more autumnal air in northern Canada, sending some of this air southward into the Upper Midwest and northern Great Lakes toward the middle of next week. Widespread high temperatures in the 50s will be found across northern Minnesota, northern Wisconsin and the Michigan Upper Peninsula by next Wednesday and Thursday, with low temperatures in the 30s producing frost and possibly freezing conditions in some areas. Beyond this, both the major GFS and European meteorological computer models dislodge an area of colder air near the North Pole, in extreme northern Canada, moving this area southward to encompass a large area of Canada by the weekend of September 18-19. While this air will modify and looks to stay mostly in Canada at this point, the northern tier of the United States along the border may experience a good taste of autumn within the 10-15 day time period.
2010 Hurricane Season
The 2010 Hurricane Season has been active and this looks to continue through September. The latest Global Forecast System (GFS) model indicates an active hurricane month in the Atlantic. By September 20, the model shows four active tropical storms in the Atlantic basin at the same time (image courtesy of the National Centers for Environmental Prediction):
In this image, only the second tropical storm from the west has formed and been named as of today; it is currently Tropical Storm Igor. The next three names to be used are Julia, Karl, and Lisa. Because of stronger than normal extratropical flow across the United States, more characteristic of later in the season, and a weaker Bermuda High in the Atlantic, the GFS does not bring any of the three Cape Verde systems it forms (the three eastern tropical storms in the image) anywhere close to land, recurving them eastward far from any land mass. The only system it depicts making landfall is the western-most system, which the GFS forms toward the middle of next week in the Carribean Sea, south of any westerly extratropical steering winds. Very early indications and several model runs show this may be Tropical Storm or Hurricane Karl, making landfall along the Gulf Coast somewhere from Mexico to Louisiana around September 20. However... there is a ton of time for this to change, with almost a week before the formation of the system is even shown. Therefore, it is also entirely possible there will not be a system in the Carribean, but it is worth keeping an eye on.
