Seattle Weather

900 PM PDT Friday sep 3 2010

Synopsis: An upper trough will move over the region Saturday. Onshore flow will increase this evening and persist through Saturday. Another upper trough will reach the area Monday and Tuesday. Cool and unsettled weather will continue trough much of next week.

Short Term: the flow is becoming strongly onshore with +4 Mb oth-sea and also through the strait. Swly winds have picked up through the chehalis gap reaching to the southern Puget Sound. The last visible image shows low stratus already pushing down the strait and through the chehalis gap, so expect low clouds to fill in quickly across just about all of the interior by late tonight. Models pick up on some spotty light qpf around indicating some showers around as the main upper trough digs swd over the area. there may also be some morning drizzle or light rain, especially along the west slopes of the Cascades/foothills Given the strength of onshore flow. Max temps will be dramatically cooler than today, in some cases down some 20-25 degrees. The lowlands will generally struggle out of the low 60s Saturday afternoon. some late day Sunday breaks are possible but don`t Look for much clearing.

Models dig the main trough axis southeast from British Columbia through Eastern Washington Saturday night and Sunday but keep Western Washington under cyclonic flow. Upper level short wave energy on the back side of the trough should produce a few showers into Sunday, especially over the mountains. The gfs brings another disorganized shortwave across the area Sunday night into Monday which might also generate a few showers. However, this system will likely be decaying as it moves across due to a much stronger upper flow forming along the British Columbia coast, heading toward wa. Onshore flow through early next week will keep daytime temps below average with lots of clouds around. No changes to the forecast this evening.

mercer Long Term: Previous discussion, an upper trough digs over the pacnw thru Tuesday, precip is light and scattered in the gfs thru Tuesday. By midweek the trough has dug into calif and the chance of showers is pretty minimal for Western Washington so i pulled Wednesday and Thursday out of the fcst and made those days merely partly cloudy. By the end of the week however the chance of showers returns and the gfs has a couple of possible weather systems that could bring pcpn to the area. 19

Aviation: The thermally induced trough of low pressure at the surface that was over the Puget Sound lowlands this afternoon has now shifted east of the Cascades. Strong onshore pressure gradients are ushering cool, moist, and stable marine air at the lower levels this evening. Expect ifr stratus along the coast to spread inland quickly after about 08z and engulf much of the interior 10z-10z tonight. Cloud bases along the coast are running 003-007, so inland expect bases to come in around 007.

a weak cold front now seen from southwest British Columbia across central Vancouver Island then southwest to 46n/130w Will move southeast across the area toward morning. Some lift with this front combined with convergent low level onshore flow will likely give some 3-5sm -dz br and possibly lower cigs to the central Puget Sound tafs 12z-18z. Abundant moisture will remain through much of the day Saturday, but expect sufficient destabilization aloft with the incoming upper trough and some diurnal heating to lift cigs to 025-030 20z through 08z or so. Lower cigs will settle in early Sunday morning. These trends are depicted well by the latest nam12 solution.

Ksea, convergent low level flow will give variable wind this evening, but expect winds to trend southwest 6-10 kt by about 08z. Stratus now moving in through the chehalis gap will spread to the terminal between 10 and 12z. The combination of the marine air and the passage of a weak front will likely give a broad convergence zone from the terminal northward to kpae, and may produce 3sm -dz br with cigs ovc003-005. Uncertainty is high enough that a broad brush 5sm -dz br ovc006 will be carried in the terminal forecast, then amended as conditions develop. Conditions are expected to improve to bkn020-025 ovc050 18z-20z. albrecht

Marine: The thermally induced trough of low pressure that was over the inland waters this afternoon has shifted east of the Cascades giving strong onshore pressure gradients in its wake. Small craft advisories are up through the early morning hours for most of the marine zones with exception to the central and eastern Strait of Juan de Fuca where gale conditions are occurring.

A weak front will move through the area during the early morning hours, and will be followed by another strong surge of onshore flow Saturday afternoon through Saturday night. With exception to Puget Sound and Hood Canal, the warnings and advisories for this evening will be carried into Saturday afternoon and Saturday night.

Onshore flow will weaken somewhat Sunday. Then another weak front will move through the waters from the northwest later Monday or Monday night. High pressure will rebuild offshore with lower pressure inland on Tuesday. albrecht

Watches/Warnings/Advisories: Wa, none. Pz, Gale Warning central and east strait. Small Craft Advisory coastal waters, west entrance, admiralty inlet, northern inland waters this evening then again Saturday afternoon and Saturday night. Small Craft Advisory Puget Sound and Hood Canal through 11 PM this evening.