textproduct: Seattle/Tacoma

SYNOPSIS

Drier conditions will persist into Sunday as a frontal system moves onshore well south of the region. A deep upper level trough will drop southward into the area on Monday and remain largely in place through much of the next week for unsettled conditions, colder temperatures, and low snow levels.

SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/

Broad upper-troughing is positioned over the coastal waters of the US West Coast. Downstream, ridging is in place over the Intermountain West. A frontal system pushing well to our south along with subtle ridging overhead is leaving drier conditions across W WA this evening. Most showers are on tap to end but a few stragglers may remain over the southern Cascades tonight. Overnight lows will dip into the upper 20s to mid 30s.

Drier weather remains into Sunday as we'll be in between systems. Models have a cut-off low just off the coast of CA and the next disturbance further north as it slides down the BC coast. Then, the latter disturbance will arrive just offshore by Monday evening, as a trough digs and pulls moisture off the Pacific and channeling it towards W WA. Here we'll see our next best chance of widespread PoPs with snow levels around 1,000-1,500 ft during the day before falling to 500 ft early Tuesday morning. The showery nature of the precipitation makes QPF a tough call at this stage, but it'll definitely be all snow in the mountains. Can't rule out the potential for wet flakes to fall in showers across lowland areas overnight Monday into Tuesday morning but any accumulation will be insignificant given current indications.

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LONG TERM /TUESDAY THROUGH SATURDAY/

QPF has trended lower through midweek, likely due to the core of the troughing remaining over the coastal waters and not tracking directly over WA. Also, guidance has most of it's associated PVA focused well towards our south as CA receives the brunt of the action. The anomalously cool and unsettled pattern is slated to continue into the end of the next week, but much uncertainty remains in precipitation amounts. Some ensemble members shift the mean trough position eastward into the Northern Rockies toward Thursday/Friday while others hold it closer to the Pacific Northwest. This can have significant implications with regard to the trajectory of weather systems diving southward on the back side of the trough...meaning the difference between an over water trajectory (more moisture) or "inside slider" (less moisture) across the interior of British Columbia. This difference is bared out with significant differences among ensemble members (particularly in the GFS ensembles) in terms of both potential mountain snowfall...and, yes, lowland snowfall potential too. The latest ECMWF ensembles have shown a significant decrease in QPF towards the second half of the week compared to its previous runs.

So, while confidence is high that the cooler than normal pattern will prevail through the whole of next week, actual precipitation totals, especially late in the week, are far from a lock.

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AVIATION

Southwest flow aloft tonight will gradually switch more northwestly by Sunday evening. Low level southerly flow will become northerly tonight before switching more onshore on Sunday. Mostly VFR under varying amounts of mid level clouds this evening. Areas of MVFR ceilings are expected to reform on Sunday morning but again, transition to more VFR Sunday afternoon.

KSEA...VFR will continue into this evening. A return of MVFR ceilings in stratus is expected by around 12-14Z Sunday morning. Surface winds turning more out of the N-NW this afternoon and continue into Sunday afternoon. VFR likely to rebound by midday Sunday.

MARINE

A surface ridge over southern British Columbia will slowly be pushed southward tonight as a low pressure system begins to move southward along the B.C. coast. This system will begin to deepen as it moves offshore on Monday and further strengthen Tuesday as it moves through the coastal waters. Low pressure looks to remain over the area through much of the week, with additional systems moving across the area Friday into next weekend.

Despite the active pattern, winds generally do not look to be of concern through this period. Winds may briefly surpass 20 kt in the outer coastal waters early on Monday. The frontal systems late-week may also produce stronger winds that may require headlines. The evolution of the pattern will continue to be monitored.

Seas 9 to 10 ft today will very slowly ease to around 7 to 8 ft tomorrow. Waves rise back to around 10 to 13 ft Monday before slowly easing by the middle of the week.

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HYDROLOGY

No river flooding is expected over the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 10 PM PST this evening for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.