FXUS66 KSEW 040429 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 829 PM PST Tue Dec 3 2024 .UPDATE...Fog and stratus continues to spread across Puget Sound this evening, with lowering visibilities. In addition, temperatures are generally in the low to mid 30s, with lows dropping into the upper 20s in some areas tonight. This may result in areas of freezing fog, along with dense fog, which will continue to be monitored. If you're traveling tonight, give yourself extra time through the morning commute. No major forecast updates this evening. JD && .SYNOPSIS...High pressure aloft will maintain dry conditions with cool overnight temperatures, morning fog and low stratus through Wednesday evening. The upper level pattern will begin to shift Thursday into Friday as the next storm system approaches, bringing widespread precipitation across the region through the weekend. && .SHORT TERM /TONIGHT THROUGH SATURDAY/...The persistent upper level ridge over the region will remain in place through Wednesday, maintaining the status quo of dry and stable conditions with widespread morning fog and low stratus with pockets of greatly reduced visibility possible again overnight. Cold overnight lows will also give way to the development of freezing fog in areas that fall below freezing overnight, primarily south of the Puget Sound. Lows will again dip to near freezing across the lowlands including the Cascade valleys and highs Wednesday will rise into the upper 40s to lower 50s. The large scale pattern is expected to shift Thursday as an upper level trough moves into the Pacific Northwest, displacing the persistent upper level ridge. This weak system will allow light rain to move into western Washington late Wednesday night into Thursday morning, perhaps more importantly it will also boost mixing in the boundary layer and limiting fog into Thursday morning. On Friday a warm front will lift across the region, bringing even more widespread rainfall and a warmer air mass - boosting snow levels to near 7000-9000 ft. Temperatures Friday will peak well into the 50s across the lowlands. .LONG TERM /SUNDAY THROUGH WEDNESDAY/...Warm frontal precipitation will continue into Saturday morning. A trailing cold front will follow closely behind on Saturday as an upper level trough settles over the region, bringing in more precipitation and lowering snow levels to 3000-3500 ft by Saturday night. The deterministic precipitation forecast from 06z Saturday through 06Z Sunday - capturing both the heavier warm frontal precipitation and expected accumulations from the cold front is highlighting over 3 inches in portions of the North Cascades. This has trended towards the wetter portion of the ensemble distribution somewhere near the 80th percentile mark. Given that the area has been relatively dry for the last week or so, this will be a noticeable shift to more typical Pacific Northwest Fall weather. Showers are expected to continue into Sunday under the trailing upper level low, with orographic enhancement over the Cascades and snow levels at or below pass level. Ensembles and medium range models continue to highlight a shift back to a drier, more benign patter early next week with high pressure aloft and northerly flow aloft. && .AVIATION...Another round of widespread fog is expected to fill into the majority of the terminals this evening bringing IFR/LIFR conditions through the night and into Wednesday morning. Some guidance suggests that KCLM and KBLI may remain MVFR/VFR as guidance hints at lower probabilities for LIFR/IFR conditions. Conditions should begin to improve to VFR after 21Z, the exception being KOLM, as they may improve to MVFR. KSEA...Fog has started to fill in around the terminal this evening, with LIFR conditions taking over through majority of tonight into Wednesday morning. Winds will be from the north/northeast, however guidance at this time indicates that winds will be light. Similarly to today, model guidance has LIFR fog beginning to lift around 21Z (20%) into VFR conditions on Wednesday afternoon. MGF/Mazurkiewicz && .MARINE...Another round of fog tonight over the interior waters into Wednesday morning. Visibilities may decrease below 1 NM. Light winds will keep waves between 5-8 ft. A series of storm systems are expected to move through the area beginning on Thursday, followed by a stronger system on Friday. Southerly flow will allow for steep seas to develop before waves build with the Friday system. Stronger winds and westerly/northwesterly winds will help waves build between 14-17 ft on the coastal waters during the weekend. Waves should gradually begin to subside on Monday. Currently there are no advisories out for the coastal waters, however ensemble guidance does hint at winds possibly reaching Small Craft criteria on Friday. && .HYDROLOGY...An upper ridge will remain in place across Western Washington through Wednesday. A frontal system will move through on Thursday for lighter QPF amounts. Another weather system Friday into Saturday is forecast to bring heavier QPF amounts. In addition, snow levels are expected to rise later Friday into early Saturday to 7000 to 9000 feet. This, combined with rainfall, will result in rises on rivers across Western Washington. At this time, the Skokomish River is the main river of concern for potential river flooding during this period. There is a fair amount of uncertainty as to the highest amounts of rainfall that are possible with Saturday's storm, with some models producing over 4 inches over the Olympics and Central Cascade north. This could drive some rivers to close to or over minor flood. Due to this and the higher snow levels Friday into Saturday, other area rivers will need to be monitored for flood potential as well. JBB/JD && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$