FXUS66 KSEW 291611 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 911 AM PDT Sun Sep 29 2024 .UPDATE...Current temperatures are in the 40s and 50s across W WA. Conditions are dry as of now but showers (15-30% chance) are possible throughout today, mainly for the northern half of the CWA. No changes made to the inherited forecast as the previous discussion remains below along with an updated aviation section. && .SYNOPSIS...A subtle trough will enter western Washington today increasing precipitation chances along the Canadian border and in the Cascades, but little impact elsewhere. Upper level ridging starts the new week off dry, however a series of troughs starting Tuesday will influence the pattern. A more organized frontal system looks to arrive on Friday. && .SHORT TERM /TODAY THROUGH TUESDAY/...Current IR satellite showing clouds moving into W WA this morning, although impact will be dependent on location: the northern half of the Olympic Peninsula has either clear skies or some high clouds, the southern half a mix of high and low clouds with some clear spots, east of the Sound from Snohomish county south seeing low to midlevel clouds socking in the area and lastly, Skagit and Whatcom counties are generally clear in their western halves with clouds in place over the Cascades. All of this thanks to a weak upper level trough passing through the area this morning. Forecast expectation is that there will be some low- end PoPs as this feature passes through...mainly focused over the Cascades and northern portions of the area. Current radar loop supports this, showing a line of weak echoes developing over central Skagit and perhaps even drooping down into north-central Snohomish. This would be slightly further south than most progs hinted at, but given that this development really does not gel together until the start of more complex topography, can likely just lump this is with the PoPs associated with the Cascades. This feature will not linger terribly long, with any impacts wrapped up by late tonight/early Monday morning. Upper level ridging will bring a dry Monday, but this ridge will flatten throughout the day ahead of a dissipating front associated with low pressure over the Gulf of Alaska. Deterministic models in some disagreement here as the ECMWF keeps the feature together long enough for PoPs, albeit low ones, to emerge throughout the day Tuesday while the GFS completely scuttles the feature. Ensembles in general agreement with a trend that seems the most likely, that being low PoPs and even lower QPF for the northern half of the CWA while points from Everett south will likely remain dry...yet cloudy. The forecast splits the difference here with widespread low-end PoPs for much of the area /barely topping 30 pct/, aside from the northern tied and two-thirds of the Cascades, where 40-50 pct PoPs are present. To offset this, QPF values throughout the area are minute with only a few hundredths expected for those locations that do see precip. Daytime highs today running pretty similar to yesterday with most spots in the upper 50s to lower 60s. The upper ridge Monday however will give temps a bit of a boost with most lowland locations falling in the 60s and the prospect for some locations in the SW interior to get close to 70. Overnight lows will retain their autumn-like nature, with lower to mid 40s expected tonight, then a little warmer Monday night and Tuesday night being in the mid to upper 40s in most spots. 18 .LONG TERM /WEDNESDAY THROUGH SATURDAY/...Generally dry conditions Wednesday and Thursday as upper level ridging moves into the area, however another low over the Gulf of Alaska will move eastward and fizzle out. While portions of its associated front may bring some very slight PoPs to the very northernmost reaches of the area, likely not sinking any further south than Bellingham, overall the precip threat is minimal. Models remain consistent in bringing in a frontal system to W WA Friday, however latest runs look less and less convincing...suggesting this will also suffer the same fate as systems already mentioned in this discussion...mainly fizzling out either near or over the CWA. Not quite ready to bite onto this solution just yet as this is a fairly recent development. Will be more willing to chew it over if this solution is present again in 24 hours. Ensembles, however, remain consistent suggesting some precip for much of the area...granted expected amounts are a tenth of an inch or less. A return to upper level ridging will wrap up the forecast period with dry conditions. Temps remain fairly static throughout the long term with daytime highs returning to the lower to mid 60s and overnight lows mostly in the upper 40s to lower 50s. 18 && .AVIATION...West to southwesterly flow aloft will become northwesterly late today as an upper trough axis shifts east of the Cascades. Low level onshore flow continues near the surface this morning. Areas of MVFR ceilings have improved thus far to VFR and will continue to hold throughout the TAF period. The low level flow will turn northerly early this afternoon into tonight. KSEA...VFR at the terminal this morning with additional scattering expected. Surface winds southerly 7 to 11 knots will veer north/northeast after 21Z. 27/Kristell && .MARINE...Onshore flow continues this morning as a surface ridge expands into the coastal waters. This will likely keep small craft advisory westerlies going a little longer in the central/east strait and have opted to extend the advisory into late this morning. The ridge will shift onshore tonight turning the low level flow more northerly. Thermally induced low pressure expanding northward along the Oregon coast on Monday will turn the flow weakly offshore at times. Onshore flow resumes on Tuesday as a weak front dissipates over the coastal waters with little to no impact. Some degree of onshore flow continues into midweek as surface ridging remains poised offshore with lower pressure east of the Cascades. Hazardous double digit seas are expected to reach the coastal waters later Tuesday into Wednesday as the result of a deep surface low passing well north of the area in the Gulf of Alaska. 27 && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 11 AM PDT this morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$