textproduct: Seattle/Tacoma

SYNOPSIS

An upper-low will continue to spin offshore as the pattern remains active into the new week. Recurring spells of lowland rain and mountain snow is favored. Temperatures will tread slightly below to near average throughout the next several days.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

Latest radar shows rain associated with an incoming AR event over at least the southern two-thirds of the CWA with rainfall generally ranging light to some moderate spots, especially over portions of Grays Harbor and Mason counties at the time of this writing. As precip continues to spread northeasterly over the area, should see widespread lowland rain and mountain snow persist overnight and into Monday. Inherited forecast and headlines look good with no evening update needed.

For additional forecast details, please refer to the Previous Discussion section below. 18

From Previous Discussion...Broad troughing remains fixed across the NE PAC with ridging over the Intermountain West. In the Northwest, the aforementioned broad trough/upper-low complex continues to barrel an occluded 980mb frontal system into the region. This feature has lead to breezy winds, lowland rain and mountain snow. Low- topped thunderstorms have also been observed this afternoon as low to mid-level lapse rates range anywhere between 7 to 7.5 c/km. This environment along with meager CAPE has done just enough leading to lightning strikes over the coastal waters and WA coast.

For tonight, moisture associated with a weak to moderate AR will funnel into the region - increasing the coverage of widespread lowland rain and lifting snow levels up to 3,000 to 4,000 ft. Cross-Cascade gradients remain negative and with steadier precipitation rates in concert with offshore flow, snow should become the dominant precipitation type overnight as hi-res models reflect evaporative cooling very well. A Winter Weather Advisory has been expanded and now includes Snoqualmie and White Pass. Overnight lows are to bottom into the lower 40s.

Mountain snow and lowland rain will gradually progress off and away on Monday as steadier rain transitions to showers by the afternoon- evening. 24-hour rainfall totals ending Monday evening range between 0.50 to 1.00" of QPF for much of the lowlands. Lowlands north of Everett will see 0.25 to 0.50". NBM has a 30-40% chance of rainfall exceeding an inch Olympia south. The bulk of the moisture will remain over Oregon and eastward on Tuesday as a transient ridge replaces it. Can't rule out straggling showers during the day - mainly over the mountains with aid of orographics but it'll be the drier day here in the short-term forecast. High temperatures will top out in the mid to upper 40s with overnight lows in the mid to upper 30s.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Models have the upper-low center becoming closed off from the mean flow in the long-term. Guidance suggests a rex block forming over the NE PAC as a high center noses into the Bering Sea while sitting on the northern periphery of the upper-low. This set-up puts us under a regime of northwesterly flow aloft through Sunday with embedded systems, maintaining the chance of PoPs daily. No threat for heavy precipitation is on horizon according to the latest ECMWF/GFS ensemble runs but we'll keep a look out. The NBM suggests temperatures will be near average through the long-term forecast.

McMillian

AVIATION

A broad upper level trough will remain offshore into Monday producing west to southwest flow aloft over Western Washington. Rain will increase across the area overnight ahead of another frontal system with ceilings gradually deteriorating to MVFR thresholds most terminals. MVFR and a few pockets of IFR in rain at times will continue into midday Monday before lifting toward late afternoon or early evening as a frontal boundary shifts east of the area.

KSEA...Rain will increase overnight as a slow moving frontal system approaches the area. Ceilings are expected to lower to 1500 to 2500 feet toward early morning. The front will shift east of the area after 21Z with ceilings improving to low end VFR by late afternoon or early evening. Surface winds southeasterly 7 to 10 knots becoming southerly 10 to 15 knots before daybreak then easing in the afternoon. Winds will then veer to N/NE Monday evening/night.

27

MARINE

Winds across the outer waters will continue to gradually ease and shift more southwesterly today as a frontal system stemming from a deep low pressure system well offshore continues to move inland. Small Craft winds will hold on a little longer across the inland waters, with southerly winds tapering off overnight. Seas will also continue a lowering trend today as a large southerly swell moves out of the area, with seas easing to about 8 to 12 feet overnight tonight. Seas are expected to decrease to below 10 feet by Monday afternoon alongside easing winds offshore.

A weaker frontal system will then move across the area waters on Monday and will be followed by high pressure building back over the coastal waters in its wake. Another frontal system will cross area waters on Wednesday, with potential for westerly winds through the Strait of Juan de Fuca to reach gale force on Wednesday and Thursday. Gusts over the coastal waters may also reach Small Craft levels on Wednesday and Thursday.

Seas will gradually subside towards 6 to 8 feet through the day on Monday and further ease towards 5 to 7 feet by Tuesday. Waves will build to around 8 to 10 feet over the outer coastal waters by Thursday, with another round of seas reaching 9 to 12 feet over the coastal waters by the end of the week.

15

HYDROLOGY

No river flooding is expected over the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory until 2 PM PST this afternoon for San Juan County.

Winter Weather Advisory from 8 PM this evening to 1 PM PST Monday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County-Cascades of Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 10 PM PST this evening for Grays Harbor Bar-Puget Sound and Hood Canal.

Gale Warning until 4 PM PST this afternoon for Central U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm- Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 4 PM this afternoon to 4 AM PST Monday for East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 4 AM PST Monday for Admiralty Inlet- Northern Inland Waters Including The San Juan Islands.

Small Craft Advisory from 4 PM this afternoon to 4 PM PST Monday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.