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/

Dry weather and mostly sunny conditions continue this afternoon with lingering high clouds across Lewis County and the southern Cascades. 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

An upper level trough moving in from the north will maintain SW to southerly flow aloft today. VFR conditions prevail across the vast majority of the region, with the exception of some scattered to broken clouds over the northern Salish Sea including CLM. These clouds will continue to persist but slowly break up throughout the day. High clouds will begin to filter in from the northwest tonight, with ceilings gradually lowering overnight as a front moves into the area. Ceilings should 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 interior.

North winds 5-10 kt will ease, becoming light and variable this evening, switching to southerly overnight and increasing through the early morning hours on Monday. Winds reach around 8-12 kt during the day Monday, with some gusts 15-20 kt possible at times.

KSEA...VFR conditions with mostly clear skies this afternoon. High clouds will begin to filter 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 afternoon will continue to ease, becoming light from 03-06Z Mon, switching to southerly after around 08Z and increasing through the morning. Winds peak after 18Z up to 8-12 kt with gusts up to 15-20 kt possible. Winds start to ease after 03Z Tue.

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MARINE

The surface ridge present over the area today 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...None.