textproduct: Seattle/Tacoma

SYNOPSIS

Drier conditions will develop across Western Washington today and continue 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 /TODAY THROUGH MONDAY/

A few showers continue to shift onshore across Western Washington early this morning as an upper level trough shifts into the coastal waters in the wake of a frontal system that swept across the area Friday evening. Conditions will start to dry out across the CWA as the day progress with most shower activity confined to the higher terrain by later this afternoon. High temperatures will be cooler than recent days...mostly mid and upper 40s across the lowlands. Overnight lows will dip into the 30s.

Apart from a few snow showers in the Cascades, the mostly dry conditions continue into Sunday as a cutoff upper low slowly shifts onshore across Northern California and southwest Oregon. And then the changes begin. A 160 to 170 knot jet core diving southward across the eastern Gulf of Alaska will carve out a very deep upper trough that will merge with upper troughing already present over the Pacific Northwest. This will result in a vertically stacked trough parked just offshore Monday night. South to southwest flow will pull moisture northward across the area Monday into Monday night. Temperatures aloft will cool dramatically through the course of Monday...falling to around -5C or -6C at 850 millibars. This puts snow levels at 1000 to 1500 feet. The showery nature of the precipitation makes QPF a tough call at this stage, but it'll definitely be all snow in the mountains. High temperatures across the lowlands on Monday will struggle to get much beyond the lower 40s.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

The upper trough axis offshore Monday night will gradually shift onshore late Tuesday. Overall QPF will remain on the light side through the period, but the cold air aloft will ensure that snow levels remain well below the pass levels. The low level flow remains almost entirely southerly...which doesn't favor lowland snow accumulation apart from some heavier showers depositing a quick skiff on some of the higher hills. The anomalously cool and unsettled pattern is slated to continue into the end of the next week, but subtle differences in some of the ensembles lend increasing uncertainty to some aspects of the forecast. 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 in terms of both potential mountain snowfall...and, yes, lowland snowfall potential too.

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.

27

AVIATION

Southwest flow aloft will continue across Western Washington today as broad upper troughing remains offshore. Low level southerly flow will become northerly late this afternoon into tonight. Areas of MVFR ceilings in scattered showers this morning will improve to VFR most areas as shower activity diminishes. VFR is then expected under varying amounts of mid level clouds into this evening. Areas of MVFR ceilings are expected to reform on Sunday morning.

KSEA...Period of MVFR ceilings in and around scattered showers will lift to predominantly VFR by midday and continue into this evening. A return of MVFR ceilings in stratus is expected by around 12Z Sunday. Surface winds southerly 7 knots or less this morning will veer north/northeasterly after 22Z and continue into tonight.

27

MARINE

A building surface ridge over the southern interior of British Columbia tonight into early Sunday will turn the flow northerly across area waters. A deepening trough over the coastal and offshore waters Sunday night into Monday will shift winds back to south to southeasterly. Broad troughing remains over the waters into the early portion of next week, but is not expected to generate much in the way of wind-related headlines.

Coastal seas will briefly drop below 10 feet over the weekend before additional swell trains generated by activity offshore push seas back into double digits Monday night and Tuesday.

27

HYDROLOGY

No river flooding is expected over the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 4 AM PST early this morning for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-Grays Harbor Bar.

Small Craft Advisory until 4 PM PST this afternoon 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.