FXUS66 KSEW 132114 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 214 PM PDT Sun Oct 13 2024 .SYNOPSIS...An upper level ridge with high pressure will leave one more nice day across western Washington today. An active weather pattern takes over from Monday through next weekend with multiple systems bringing in periods of precipitation across the region. Temperatures will also drop several degrees, becoming more seasonal for this time in October. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY/...One more nice day to wrap up the weekend today. An upper level ridge over Alberta and eastern Washington begins to depart the area today to the east. One more dry and warm day is on tap this Sunday across western Washington (mostly clear skies with pockets of high cirrus in the air). Temperatures have broken 70 in several places already this afternoon (primarily in Puget Sound and west slopes of the Cascades), but with cooler and wet weather on the way, this may be the last 70 degree day for the foreseeable future. The first of a series of weather makers is already visible on satellite just off the Pacific Coastline. A visible cloud band/moist conveyor belt stretches from the central Pacific (around Hawaii) up into a low just off the Alaska/British Columbia coastline. IVT analysis shows limited moisture funneling ahead of a low/trough for Monday into Tuesday. This will aid a cold front expected to pass through the region Monday into Tuesday for rain showers. This first "atmospheric river" is expected to be the weakest one this week, with limited QPF totals ranging from a tenth to a quarter in the lowlands, and up to around half an inch to an inch in the north Cascades, west Olympics and the coast. The rain will pass through Monday morning from west to east, and taper back going into Monday night to light showers. In addition, sneaker waves will be possible with this system, so beach goers should be on the lookout for high waves along the coastline. A second, more vigorous trough digs off the Pacific Wednesday, and swings around a stronger cold front into the Pacific Northwest Wednesday. A 140 kt jet max pushes this front/trough inland with much cooler air behind. High temperatures will drop from the low 60s into the upper 50s Wednesday, with snow levels expected to drop to around 4,000 feet by late Wednesday. The jet will help funnel in moisture for additional rounds of precipitation throughout the day Wednesday. Given the stronger temperature gradient with this front, and cool air aloft, thunder will be possible on Wednesday. This will likely create localized heavier rainfall rates with any convection that is able to form. Flooding is not expected to be a concern with this next system, but rivers/streams will likely see rises with this system. Additionally, many of the mountain areas above 4,000 feet will likely see accumulating snowfall, with potential of some of the snow sticking at Stevens Pass. .LONG TERM /THURSDAY THROUGH SUNDAY/...The Wednesday system will continue into Thursday with more rounds of precipitation continuing through Thursday night. Amounts are quite a bit spread from the mean, so it is too early to advertise potential amounts with this Wednesday/Thursday system (but will most likely be in the magnitude of an inch to a few inches, with the heavier amounts in the Cascades and the coast/Olympics). The pattern becomes a little bit more uncertain next weekend. Most of the ensembles are showing another trough moving through Saturday and Sunday (alternative would be ridging in the Southwest underneath the trough). IVT analysis shows a more classic moisture plume moving in from the central Pacific into the Pacific Northwest. This will likely shift some between now and next weekend, but the potential exists for a more "classic" atmospheric river event next weekend with even more precipitation. The CPC does have a slight risk of heavy precipitation next weekend. This will be monitored throughout the week for potential hydro impacts. HPR && .AVIATION...Increasing southwesterly flow into Monday with upper troughing over the NE Pacific and an approaching frontal system tonight into Monday morning. High clouds are filtering into the region this afternoon ahead of the next system, but VFR cigs will prevail. Rain is expected to arrive along the coast by 12Z, spreading to inland terminals around 17Z to 18Z. Largely MVFR ceilings are forecast, though there could be pockets of IFR and LIFR in locations more susceptible to lower cigs, such as OLM, PWT and coastal terminals. Winds will be southeasterly for much of the period, picking up slightly and turning more southerly late tonight into early Monday morning as the front approaches. KSEA...VFR cigs with high clouds overhead will prevail throughout the rest of the afternoon. Light surface flow generally out of the south to southwest will continue, but will experience a slight increase in speeds as the front approaches on Monday morning. The 50th percentile ceiling forecast brings ceilings over SEA to around 1500 ft come 12Z Monday morning, with further reductions possible as the rain sets in closer to 18Z. Rain will be light, but consistent through the late morning into afternoon hours tomorrow, marking the beginning of a wetter and more unsettled pattern through the rest of the week. Kristell && .MARINE...Light southerly winds expected for the interior waters today, with continued south to southeast winds for the Coastal Waters. Southerly winds will then increase tonight into Monday for all area waters as a front moves across Western Washington. Small Craft Advisory has been issued for the Coastal Waters with the south winds into Monday morning. Conditions were borderline headline worthy in the west entrance of the Strait of Juan de Fuca for winds and possibly seas, but the part of the zone that will be most impacted by heightened seas close to 10 feet and stronger winds will be the western most extent, namely just off shore of Neah Bay. South winds will also increase for portions of the interior waters, especially for Admiralty Inlet Monday morning, hence, the rationale for posting a Small Craft Advisory to cover the timing of the frontal passage. Otherwise, winds will become lighter Monday night through Tuesday. Another, potentially stronger front, is expected to move through the area Tuesday night into Wednesday. Additional headlines possible for increasing south to southwest winds during this period. A post-frontal westerly push is forecast on Wednesday for the Strait of Juan de Fuca as well. Seas 5 to 7 feet will build to 10 to 13 feet later tonight into Monday. For this reason, Small Craft Advisories continue for the Coastal Waters due to seas Monday into Monday night. Seas are expected to then briefly subside below 10 feet later Tuesday before increasing to near 10 feet Wednesday and potentially into Thursday as the more active and unsettled weather pattern continues. Kristell && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 11 PM this evening to 11 PM PDT Monday for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm. Small Craft Advisory from 5 AM to 11 PM PDT Monday for Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Small Craft Advisory from 11 PM this evening to 8 AM PDT Tuesday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. && $$