Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
369 FXUS66 KSEW 020255 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 800 PM PDT Wed May 1 2024 .SYNOPSIS...Another weak system will slide to the south tonight bringing in light shower activity. Drier and warmer conditions are expected Thursday. More precipitation arrives in time for Friday, the weekend, and beyond. && .SHORT TERM /TONIGHT THROUGH SATURDAY/...Current satellite showing a mix of high and mid level clouds this evening. Zooming out to the entire Pac NW, IR loop shows the system passing to the south of the area, however the aforementioned clouds are the northern edge of it. This segues nicely to current radar, showing echoes mainly over the coastal waters with maybe just the slightest of spillover onto the far Southwestern corner of the CWA. While there may be a little more slight eastward seepage to this precip, it is generally expected to be pretty limited as this system continues its way southward. That said, forecast on track for a dry overnight period and upper level ridging for Thursday. As such, no evening updates needed at this time. For additional forecast details, please refer to the Previous Discussion section below. 18 From Previous Discussion...A weak frontal system will clip SW WA tonight for a chance of rain (mainly coast and south of Olympia). We`re seeing a mix of mid and high clouds on satellite associated with this system with sunnier skies toward the north. Temperatures will be chilly again overnight and in the mid to upper 30s over the south sound. Patchy frost is possible although should be brief. A weak ridge will build offshore on Thursday and we`re looking for mainly dry weather for western WA. We could see a few pop-up showers in the Cascades otherwise dry in the lowlands. The air mass will be a little warmer with temperatures in the 60s. The ridge axis moves inland Thursday night with dry weather continuing. The pattern changes as we move into Friday. By then the ridge shifts farther east and inland while a deep upper low settles down over the NE Pac. Attached to this low is a frontal boundary with rain reaching the coast in the morning then spreading inland during the afternoon. The upper low shifts south off the coast on Saturday but we`re still under moist/diffluent flow over western WA. This looks like more rain spreading over the region through much of the day (and not much of a break). With cooler and cloudier conditions we`re only expecting highs in the lower to mid 50s. 33 .LONG TERM /SUNDAY THROUGH WEDNESDAY/...From Previous Discussion...The broad upper level low shifts inland over the weekend and is still meandering over the West as we move into Sunday. There`s still plenty of wrap-around moisture moving into western WA with additional showers expected. Snow levels are around 4500 ft for rain at the Cascade passes. Moving on, we`re under solid onshore flow early next week keeping us cool and wet through the period. Onshore flow eases by Wednesday and drier N/NW flow may give us a brief break in the action. 33 && .AVIATION...North to northwesterly flow aloft tonight and Thursday as an upper level trough continues to move eastward to make way for an incoming ridge. VFR conditions in place over W WA this evening with some high to mid level clouds overhead due to passing system to the south. Majority of terminal sites likely to remain this way through the overnight before additional clearing is expected during the day Thursday. Mix of high to mid clouds should keep lower cigs or fog out of OLM, however timing on clearing might just be early enough to allow for IFR conditions to emerge at PWT. Will evaluate this further for 06Z TAF issuance. Widespread VFR conditions expected late Thursday morning to the end of the TAF period. Low level winds starting to see some transition to northerly for northern half of the CWA and should start to see remainder of sites from SEA south transition to this direction by 06Z tonight and remain that way for much of Thursday. Wind speeds generally staying around 4 to 8 knots. KSEA...VFR conditions persist with mid to high level clouds lingering this evening and overnight. Clearing skies expected by mid morning Thursday and remaining that way for the remainder of the TAF period. Lingering S/SW winds as of 02Z at 4 to 8 knots will shift to the north by 06Z and generally maintain the same speeds into Thursday afternoon. 18 && .MARINE...Onshore flow will persist with surface high pressure over the coastal waters. A weak low pressure system will continue to move south into the Oregon border tonight, allowing for winds over the coastal waters to increase and turn southerly. Latest guidance keeps winds over the coastal waters around 10 to 20 kts, remaining under any headline thresholds. Weak high pressure will move through the waters Thursday into Friday, before a stronger system moves in Friday into Saturday with small craft southerlies. Another system looks to move through during the weekend. Combined seas this afternoon are around 5 to 7 feet and will subside to around 3 to 5 feet tonight. Seas will remain 3 to 5 feet before increasing on Friday. Maz/18 && .HYDROLOGY...No river flooding is expected over the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$