Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
228 FXUS66 KSEW 030343 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 843 PM PDT Sun Jun 2 2024 .SYNOPSIS...Atmospheric river pattern over Western Washington will continue through Tuesday, consisting of two separate waves of heavier rain moving through the area. The first will continue until early Monday morning and the second will start Monday evening and continue into Tuesday. The second wave will be weaker than the first. Upper level ridging builds over the area Wednesday and will remain in place through at least the weekend, bringing dry and much warmer weather to the region. && .SHORT TERM /TONIGHT THROUGH THURSDAY/...More rain on its way tonight with gusty SW to W winds. Current headlines remain intact. 33 Previous discussion...As per expectations, the radar has filled up with echoes with much of W WA seeing precipitation as of this writing. That said, forecast continues to be on track. Will reiterate key points for the short term forecast period before engaging in more detailed discussion. ...Key Messages for Upcoming Active Weather Pattern... * A late season atmospheric river pattern will develop today and continue with breaks through Tuesday. * Heavy rainfall amounts of 3 to 5 inches in the mountains, with possible 6 inch event totals over the southern slopes of the Olympics, and 2 to 4 inches along the coast. * Moderate to heavy rainfall of 1 to 2 inches across the lowlands during this timeframe. * Snow levels above 6500 feet will promote additional runoff with several area rivers flowing out of the Cascades forecast to enter action or even minor flood stage. A Flood Watch remains in effect across portions of Western Washington. The front currently over the area is only the first of two systems that are expected throughout this event. Even though the front will pass through W WA and move into E WA by late this afternoon/early this evening, the parent upper level low moves in quickly thereafter, ensuring persistent wet conditions tonight and much of Monday. Latest deterministic forecasts having a difficult time showing it, but ensembles clearly suggest perhaps a little bit of a break...or at the very least an easing up of activity...come Monday evening or Monday night, as there is the tiniest of gaps between the exiting low and the next incoming front. That is not to say that the CWA will dry out, but at the very least will see a dip in PoPs for a narrow 6-8 hour window. The next frontal system makes its way into W WA late Monday night/early Tuesday morning and will continue for much of the day before tapering off Tuesday evening and Tuesday night. Does not appear to be as much QPF with this system as with the first one but this one-two punch of AR-style systems will still merit keeping an eye on hydro concerns...which will be discussed below. Also meriting vigilance will be burn scars within the area. Although not expecting any significant activity there at this time, continued monitoring will be necessary should precip amounts or rates exceed those currently forecast. All of that said, the inherited Flood Watch will remain in place without any alteration. Along with precip, could see some breezy to locally windy conditions with occasional gusts throughout the short term with each of these frontal passages. Wind speeds do not look to merit any headlines at this time, however this combination of heavy rains and increased winds may allow for isolated instances of either branches or entire trees being felled. Caution is encouraged. Wednesday the switch gets flipped over to the summer setting as upper level ridging develops over the area, drying things out quickly. This feature will be discussed more in the Long Term Discussion. As would be expected, temps will take a bit of a hit today when compared to yesterday, with most locations only getting into the upper 50s to lower 60s. Highs cool even further Monday as most locations will sit in the upper 50s with the occasional spot hitting 60. A warming trend will kick off on Tuesday with highs returning to the lower 60s followed by a significant bump up on Wednesday with temps ranging from the mid 60s to lower 70s. Overnight lows will be pretty static tonight and Monday night, generally in the upper 40s to lower 50s. Clearing skies will help temperatures cool a little bit Tuesday night, ranging in the mid 40s to around 50 with lows Wednesday night looking similar. 18 .LONG TERM /FRIDAY THROUGH MONDAY/...While a broad ridge over much of the west will be responsible for at least the initial drying over W WA, it is the mammoth ridge that will build over just off the coast Thursday night and then progress slowly east that will be the main driver...or in this case blocker...of the forecast. GFS tries to be a little more progressive with this feature, having the ridge axis push inland starting Saturday, the ECMWF keeps this feature nearly stationary into at least the start of next week. Ensembles continue to lean in the direction of the ECMWF and as such, could be looking at enough of a dry stretch ahead to nullify or at least minimize the benefit from the rains in the short term. High temperatures will be the main discussion point for the extended, although NBM still coming in a little too cool for what this sort of pattern would dictate. Fortunately it does look like low level flow...both surface and mostly at 850mb...should remain generally onshore, helping to keep the expected increasing temperatures in check. That said, the warming trend is expected to continue with highs Thursday in the upper 60s to mid 70s, highs Friday in the lower 70s to around 80 and mid 70s to lower 80s on Saturday. 18 && .HYDROLOGY...Today through Tuesday forecasted rain totals are still 3 to 5 inches for the Olympics and the Cascades, with the possibility of 6 inch bullseyes along the southwest slopes of the Olympics. Snow levels will be in the 7000 to 8500 foot range for for the first wave of the atmospheric river and just a little lower for the second round. West southwesterly flow aloft favors the heaviest rainfall to be along the southwest slopes of the Olympics and the Central Washington Cascades. Several rivers flowing out of the Central Cascades and Southern Olympics are forecast to reach action stage. The river with the best chance to reach flood stage is the Skokomish River in Mason county. Even with some of the rivers only reaching action stage, these levels would be records for this time of the year. The flood watch for Skagit, Snohomish, King, Pierce, Mason and Lewis county remains in effect. In addition to the high flows on area rivers, the water temperatures in the rivers continues to be very cold. Most rivers flowing out of the Cascades are still in the mid 40s to lower 50s for water temperature. Felton && .CLIMATE...Here are some daily rainfall records for around the area today, Seattle 0.48 inches in 2001, Olympia 0.68 inches in 2010, Bellingham 0.48 inches in 1962, Quillayute 1.63 inches in 2010 and Hoquiam 1.35 inches in 1962. Felton && .AVIATION...An approaching frontal system will continue to spread of moderate to heavy rain, lower ceilings, and gusty winds to the area through Monday. Southerly surface winds will increase with gusts around 25 kt expected. Widespread MVFR conditions expected for much of the evening, with periods of IFR to LIFR possible in moderate to heavy rain.. Winds remain gusty Monday with continued low ceilings and restricted visibilities in rain. KSEA...Expect predominantly low MVFR to IFR conditions with steadier rain through the overnight. Little change through the period with continued low ceilings and restricted visibility and gusty southerly winds. Cullen/McMillian && .MARINE...A strong front continues to advance across the region. While a few gusts toward 35 kt may continue along the immediate coastline, conditions have largely been covered by the ongoing small craft advisory. In addition to winds, seas remain rather steep and will continue to build in response to the winds later today. Small craft advisories also remain in effect much of the interior zones through the evening or overnight with southeasterly winds now, transitioning to westerly behind the front into Monday. Additional rounds of advisory strength winds are expected late Monday and Tuesday with the potential (30-50% chance) for gale gusts through the Strait of Juan de Fuca. Expect wind gusts to also increase across Puget Sound this evening where a small craft advisory has been issued. While some isolated gusts will be observed, not as confident that these will be widespread or persistent enough. A better chance for the Sound will be on Tuesday. Overall, expect the pattern to remain active through the first half of the week, with yet another frontal system expected to move across the region on Tuesday. Latest guidance suggests the winds with this next system could be stronger, with more widespread gales possible across portions of both the Strait of Juan de Fuca and the coastal waters. Seas are expected to build into the 10 to 13 ft range and will remain rather steep with the dominant period around 8 or 9 seconds. Thus, even when the winds ease over the coastal waters, advisories will need to continue. High pressure then looks to build over coastal waters near midweek, with gradually subsiding seas and a less active weather pattern. Breezy northerlies may develop over the coastal waters, perhaps leading to additional small craft advisories late in the week and also maintaining steeper seas. Cullen/McMillian && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Flood Watch through Wednesday morning for Bellevue and Vicinity- East Puget Sound Lowlands-Everett and Vicinity-Hood Canal Area-Lower Chehalis Valley Area-Olympics-Seattle and Vicinity-Southwest Interior-Tacoma Area-West Slopes North Cascades and Passes-West Slopes North Central Cascades and Passes-West Slopes South Central Cascades and Passes- Western Skagit County. PZ...Small Craft Advisory until noon PDT Monday for Grays Harbor Bar. Small Craft Advisory until 2 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 5 AM PDT Monday for East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 11 PM PDT this evening for Admiralty Inlet-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 5 PM PDT Monday for Puget Sound and Hood Canal. Small Craft Advisory until 2 PM PDT 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. && $$