Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 140433 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 833 PM PST Fri Dec 13 2019 .UPDATE...No changes were necessary to the forecast this evening, as weather conditions remain much quieter this evening than 24 hours ago. Water vapor loop reveals a shortwave trough in the vicinity of 130W and moving eastward. Enough lift has been supplied by this disturbance just to its east where some organized shower activity is taking place just offshore the Washington Coast. Hi-res model guidance does not pick up well on this area of precipitation, but given the weak nature of the shortwave trough, do not expect it to really hold together into the overnight hours. Elsewhere, local radar imagery continues to pick up some scattered rain shower activity across the lowlands and light snow shower activity across the Cascades. Going forward tonight, do not expect much in the way of impactful weather. As the shortwave trough pushes east into western Washington, there should be sufficient lift for a continuation of scattered showers across the area. Shortwave then appears to get hung up over the area into the evening hours. Again, spotty showers likely to continue. Could see very light snow accumulation in the Cascades tonight-tomorrow night but impacts appear minimal. Previous discussion included below. Kovacik && .PREV DISCUSSION... /issued 228 PM PST Fri Dec 13 2019/ .SYNOPSIS...Snow showers will gradually taper off in the Cascades through this evening. Scattered showers and perhaps an isolated thunderstorm in the lowlands will continue into this evening before tapering off. A shortwave ridge will bring calmer weather to the region this weekend with a more active weather pattern potentially making a return mid to late next week. && .SHORT TERM /TONIGHT THROUGH MONDAY/...Scattered rain and snow showers will gradually taper off across Western Washington through Saturday morning as a shortwave ridge builds off shore and pushes across the region. A northwesterly flow regime will set up in its wake, yielding generally quiet weather across the region through the remainder of the weekend. The upper level pattern will gradually amplify through the end of the period as a ridge builds off of the northwest coast. A cold airmass will remain in place, with model ensembles showing low to mid 30 low temperatures in the low lands on Sunday and Monday mornings. Highs are generally progged to run around normal for mid December. .LONG TERM /TUESDAY THROUGH FRIDAY/...Ensemble guidance is consistent in the aforementioned upper level shortwave ridge pushing across the Pacific Northwest as the first of a potential series of shortwave troughs ejects out of the Gulf of Alaska. This will bring a return of wet and active weather to western Washington by midweek. GEFS, EPS, and GEPS members show a healthy fetch of moisture ahead of second wave later next week into next weekend which would bring another round of rain and mountain snow to the region. Stay tuned to the latest forecast through the weekend into next week as details with these potential systems become clearer. Pullin && .AVIATION...Tricky TAF night in store for western Washington. Another evening of mixed ceilings will likely continue through the overnight hours. Expect the predominate category overnight to be MVFR but think conditions will largely improve to VFR Saturday afternoon. Another issue overnight will be fog. OLM currently down between 1/4-3/4 SM. Have this continuing into Saturday morning. Will also watch PWT and possibly SEA and PAE for fog development. A few spotty light rain showers may linger across most TAF sites thru the period as well. Winds will be under 10kts thru the period, with the dominant direction S/SW. KSEA...Ceilings expected to drop to MVFR overnight, tho the possibility of a period of IFR and fog in the vicinity may occur. Conditions are expected to improve back to VFR Saturday. Spotty light rain showers likely in the vicinity thru most of the TAF period. Winds under 10kts from the S, likely light and variable at times. Kovacik && .MARINE...No changes were necessary to the marine forecast this evening. Winds remain light across all waters this evening and conditions will be similar through the weekend. Main forecast concern over the next 12-24 hours will be the hazardous seas across the coastal waters and western Strait as swells remain high. Previous forecast discussion highlights this well: Small craft advisories for hazardous seas continue into Saturday for the coastal waters and west entrance to the strait. Seas will gradually diminish over the weekend. Otherwise, no wind related concerns are anticipated over area waters. Winds will become increasingly offshore early next week as surface high pressure gets more established east of the Cascades. Kovacik/27 && .HYDROLOGY...There are no flooding concerns at this time as shower activity will remain scattered and light. Generally quiet conditions will prevail through the weekend and into early next week before the next storm systems affect the region by mid next week. Pullin && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 4 PM PST Saturday 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-Grays Harbor Bar-West Entrance U.S. Waters Strait Of Juan De Fuca. && $$ www.weather.gov/seattle

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