Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 301016 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 316 AM PDT Mon Mar 30 2020 .SYNOPSIS...Unsettled weather will continue through much of the week and into the upcoming weekend. The most organized system will move across the area today, bringing a quick round of heavy lowland rain, mountain snow, and gusty winds to area. Rain will transition to widespread showers through the day, with a chance for a few thunderstorms. Unstable conditions with scattered showers will then continue through much of the week. && .SHORT TERM /TODAY THROUGH WEDNESDAY/...Water vapor imagery this morning suggests broad cyclonic flow in the upper levels of the atmosphere extending roughly from the Aleutian Islands/Western AK south/eastward into the northeastern Pacific and into the western US. From there, the flow becomes more split, with the northern stream extending into the northern Rockies and Canada, and the southern stream into the desert SW and southern Great Plains. In the lower levels, the most significant feature is an organized frontal system located just offshore the Washington and Oregon coasts. Focusing closer to home, an active period of weather is already underway. Within the broader scale cyclonic flow that was mentioned in the previous paragraph exists a robust area of closed low pressure, centered roughly just west of Haida Gwaii. Several vort maxes are also detected rotating around this area of circulation, one of which is currently /as of 10z/ taking on a negative tilt as it approaches the Washington Coast. This shortwave/vort max is aiding the larger scale lift, via PVA and attendant left exit region of jet streak, coincident with the sfc frontal boundary. The result is a rather organized swath of rain beginning to push into western Washington, per 10z radar imagery. Convective features have been noted, per data from KLGX, within this swath of precip. SPC mesoanalysis suggests sufficient deep layer and low level shear and helicity, which is suggestive of the possibility of a few rotating cells near the frontal boundary. Also given sufficient mid level lapse rates, a few lightning strikes are expected as well (In fact, a few strikes were noted earlier in the night across Grays Harbor County). The frontal boundary itself will be rather quick-moving this morning, with hi- res guidance suggesting its location near the Cascades by daybreak. Generally can expect increasing wind speeds/gusty winds and heavy rain with its passage in the lowlands, with isolated lightning also possible. For the higher terrain, decent snowfall is expected, especially above 2500ft. In the wake of the cold front, snow levels will tank to around 1500ft within an unstable environment by this afternoon. As mid level temperatures continue to fall, mid level lapse rates will steepen. This suggests the development of widespread post-frontal shower development. These showers will carry the threat for brief heavy downpours, small hail, and lightning in the lowlands, and brief bursts of heavy snowfall and lightning for the mountains. The overall winter weather headlines for the mountains will remain in effect through the day today with generally the highest snowfall amounts in the Northern Cascades. There will still be enough snowfall across the Central Cascades to create some travel headaches across the Passes. Unfortunately, not seeing a pronounced decrease in shower coverage into the overnight hours tonight. Unstable conditions will continue, along with disturbed mid and upper levels to support this trend. Given cool temps aloft, will opt to keep mention of thunderstorms in the forecast overnight tonight. Tuesday looks to similar to the latter half of Monday with unstable conditions continuing. The closed upper level low will begin to sink southward towards the local area. Ongoing scattered shower activity is likely to become enhanced as forcing from the approaching low comes into closer proximity. Thunderstorms will be possible yet again. The closed low will pass to the south into Wednesday, however, another shortwave will drop across the area from north. This feature will have the ability to spark a few additional showers through the day, though coverage currently does not look as high as the previous few days. Kovacik .LONG TERM /THURSDAY THROUGH SUNDAY/...The beginning of the long term forecast period will have the state of Washington embedded within broad scale upper level cyclonic flow extending from just offshore the coast of BC into the Northern Great Plains. A few pesky showers may be lingering early Thursday morning but overall coverage does not appear to be very impressive. A weak shortwave will pass through during the afternoon, which is likely to be strong enough to generate a few showers. Conditions will improve into Friday, with model guidance picking up on a closed area of low pressure offshore approaching the Pacific Northwest. This system is currently expected to pass south of the area into Oregon and northern California. A few showers still cannot be ruled out on Friday, given the proximity of this passing system as well as the overall larger scale pattern. Showers are likely to be more diurnally driven at this point in time. Unfortunately, this general pattern will take us through the weekend. Most ensemble guidance is suggestive of the broader scale trough lingering around the Pacific Northwest and perhaps even retrograding a tad through the weekend. This will likely continue the threat for showers off and on, but the good news is neither day appears to be a washout at this point. Kovacik && .AVIATION...Southwesterly flow aloft as the stronger upper level trough continues to move into the area overnight. Cigs will be on their way down as the rain showers spread inland. MVFR to IFR, down to LIFR in the stronger shower activity. The upper level pattern has destabilized some, lending itself to some isolated lightning strikes in convection moving onshore this evening, particularly in Grays Harbor County. Additional instability across all terminals will be limited overnight, but will increase into Monday and Tuesday. Southwesterly winds are expected to pick up across all terminals, with gusts from 10-20 knots becoming 25-30 knots associated with the frontal passage by early Monday morning. KSEA...Current conditions are low end VFR. Cigs are expected to deteriorate down to MVFR to IFR as rain shower activity spreads inland. A stronger frontal system is approaching the area overnight and will move through Monday morning, bringing stronger winds to the terminal. Winds remain predominantly southwesterly overnight, at 10- 16 knots, before becoming increasingly gusty ahead of the frontal boundary passing through. Gusts will increase to 20-25 knots at times. Kristell && .MARINE...A stronger frontal system is moving onshore overnight into Monday morning. Southerly gales for the coast, northern inland waters, and the Strait of Juan de Fuca tonight into Monday morning. Small Craft Advisory winds elsewhere as the front moves through. Winds will remain elevated until Monday evening into Tuesday, however the seas will remain elevated over the coastal waters in advance of another system possibly moving through late week. Kristell && .HYDROLOGY...River flooding is not expected over the next 7 days, despite an unsettled period of weather. A fast-moving frontal boundary will carry the threat for a short period of heavy rain and some mountain snow this morning. Given the progressive nature, total QPF is not expected to be impressive. Snow levels crash to below 2000 feet through the day today with rain turning to scattered showers. Some of these showers will produce short bursts of heavy rain and mountain snow, but with coverage remaining scattered, this will not cause river concerns. Showers then look like a good bet for much of the week, but again, given the overall scattered nature, not expecting river concerns. Kovacik && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Weather Advisory until 11 AM PDT Tuesday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and King Counties. Winter Storm Warning until 11 AM PDT Tuesday for Cascades of Whatcom and Skagit Counties. Winter Weather Advisory until 5 AM PDT Tuesday for Olympics. PZ...Small Craft Advisory from 8 AM this morning to 5 PM PDT this afternoon 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- Northern Inland Waters Including The San Juan Islands. Gale Warning until 8 AM PDT this morning 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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 2 PM PDT this afternoon for Central U.S. Waters Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 8 PM PDT this evening for Admiralty Inlet-Puget Sound and Hood Canal. && $$ www.weather.gov/seattle

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