textproduct: Seattle/Tacoma

SYNOPSIS

Dry weather continues into tonight. A deep trough develops offshore on Monday. Troughing remains across the Western US through much of the week, resulting in cooler temperatures, and precipitation at times for Western Washington.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

No major updates have been made this evening. There are a few light echoes showing up on radar along the north coast but are likely not precipitating yet. The rest of the previous discussion can be found below with an update to the aviation section.

A pattern change begins tonight as a very deep trough develops offshore of the Pacific Northwest Monday. Showers return tonight and will continue on Monday. Low temperatures will range in the low to mid 30s, with snow levels dropping to 1000 feet Monday morning. Given the convective nature of the precipitation, and colder air becoming more prominent aloft, a rain/snow mix may occur, particularly during the early morning hours. Probabilities remain low for accumulating snowfall for the lowlands tonight-Monday, with REFS/HREF mainly highlighting the US-101 corridor west of Lake Crescent with the best potential of 1" of snow (or more) through Monday PM (probabilities peaking around 30 to 70% on the REFS).

Showers will then continue through Tuesday as the deep upper low lingers offshore of Washington/Oregon. Temperatures aloft will continue to become colder into Tuesday (850mb temps around -5C), with snow levels Tuesday AM falling towards 200 to 500 feet. Although model guidance continues to struggle with the exact placement of showers Monday night and Tuesday, continued convective precipitation will allow for an increased potential of rain/snow mix (or just snow) in any shower. QPF does look to be a bit more enhanced Monday night along the Olympic Peninsula, and probabilities for light accumulating snowfall are increased along the west slopes of the Olympics Monday night- Tuesday. REFS probs range between 40 to 80% chance of 1" of snow or more from Lake Crescent towards Hoquiam during this period. However, this will again be dependent on the placement of heavier showers. In addition, another area of focus is also the Hood Canal with light S/SE surface winds and a bit of an upslope component potentially. Although probabilities remain lower elsewhere over Puget Sound due to less QPF, given the colder airmass, will need to monitor any additional showers Tuesday AM. Otherwise, light mountain snowfall is expected Monday morning through Tuesday.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

The pattern remains quite similar through approximately Friday as troughing is reinforced at times across the Pacific Northwest. This will again lead to cooler temperatures, as well as unsettled conditions with convective precip. Uncertainty remains in the ensembles in regards to the tracjectory of individual systems mid to late week, and thus, the confidence in both the timing and placement of precipitation remains lower. With that said, the colder airmass will continue to promote snow or rain/snow mix in heavier showers, especially during the overnight and morning hours. Although probabilities for accumulating snowfall remain low, localized accumulations will need to be monitored due the convective nature of precipitation, which may also be enhanced by localized banding or convergence as Fraser Outflow briefly deveops Wednesday into Thursday. Otherwise, ensembles are suggesting the return of more southwest flow aloft next weekend with troughing offshore of the West Coast. JD

AVIATION

Zonal flow aloft shifting to the SW tonight and into Monday as an upper level trough sinks southward over the coastal waters. VFR conditions area-wide as a mix of mid to high clouds begin to enter W WA in advance of a frontal system expected to pass through on Monday. Cigs still expected to lower to low-end VFR to higher-end MVFR cigs as rain showers arrive between 12-18Z Mon along the coast/northwest interior, then by 00Z Tue through the remainder of the CWA.

North winds 5-10 kt have already eased to light and variable for most terminals, only SEA and HQM abstaining. Both terminals are expected to join the others during the overnight hours. Surface winds area-wide switching to southerly by early Monday morning and increasing throughout the morning. Winds reach around 8-12 kt during the day Monday, with some gusts 15-20 kt possible at times.

KSEA...VFR conditions with high clouds moving in tonight with ceilings lowering overnight. Low end VFR to high end MVFR cigs expected by around 15-18Z Mon as a frontal system approaches. Rain showers move in after 18Z Mon and will be possible through the reminder of the period.

N winds up to 10 kt this evening will continue to ease, becoming light from 06-08Z tonight, switching to southerly after around 12Z and increasing through the morning. Winds peak after 18Z Monday up to 8-12 kt with gusts up to 15-20 kt possible. Winds start to ease after 03Z Monday evening.

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MARINE

The surface ridge present over the area will get pushed southwards as a frontal system moves across the area from the northwest late tonight into Monday. This front will bring increased winds across the area on Monday, with the outer coastal waters being the area most likely to see small craft advisory level winds (50-80% chance). Winds will decrease area-wide Monday evening. The parent low pressure will move southwards over the offshore coastal waters Tuesday into Wednesday. Broad low pressure look to remain in place through the end of the week. Ensembles are showing that a stronger frontal system may move through the area this weekend that could bring stronger winds and waves, but there are significant discrepancies in the evolution of this system that make the forecast uncertain at this point.

Seas 8 to 9 ft today will rise Monday into Tuesday up to 11 to 13 ft. Seas gradually lower once again Wednesday before another wave system arrives on Thursday, likely pushing waves back up to around 10 ft then. Larger waves may be possible if the system next weekend comes to fruition.

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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 from 4 PM Monday to 4 AM PST Tuesday 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.

Small Craft Advisory from 4 AM Monday to 4 AM PST Tuesday 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.