Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
472 FXUS66 KSEW 221042 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 342 AM PDT Sun Sep 22 2024 .SYNOPSIS...Flat upper level ridge over Western Washington with weather system moving by to the north tonight. Ridge strengthens Monday before moving off to the east Tuesday. Western Washington still on the backside of the ridge Tuesday with thermally induced surface trough along the coast in the morning for a short spell of offshore flow. Thermally induced trough moving inland later Tuesday. Cold front arriving Wednesday with another system later Thursday into Friday. && .SHORT TERM /TODAY THROUGH TUESDAY/...Satellite imagery shows high clouds over the northern portion of the area this morning. Winds are calm but so far no fog seen on the satellite imagery or in the surface observations. Temperatures at 3 am/10z ranged from the mid 40s in the Southwest Interior to the upper 50s in the Seattle metro area. Flat upper level ridge over Western Washington today with a weak shortwave trying to move by to the north late in the day. Dry this morning with some high clouds along with patchy fog. Clouds thickening up over the northern portion of the area this afternoon with a chance of showers along the north coast and near the Canadian border. Highs today similar to Saturday, in the mid 60s to lower 70s. Shortwave stalls to the north tonight keeping the chance of showers in the forecast for the coast and the interior from about Everett northward. Mostly cloudy skies for the remainder of the area. Lows in the 50s. Upper level ridge strengthening Monday with 500 mb heights in the mid 580 dms by 00z Tuesday. This will push the shortwave well north of the area. Surface gradients remaining northwesterly so even with the warming temperatures aloft highs will just be a couple of degrees warmer than today, mid 60s to mid 70s. Thermally induced surface trough along the Central Oregon coast Monday expanding northward to the Washington coast by early Tuesday morning. Low level offshore flow developing keeping lows in the 50s. Thermally induced surface trough moving inland during the day Tuesday. Cross Cascade gradients never turn negative combined with the upper level ridge moving east will keep highs from getting too warm. Warmer locations like the Southwest Interior highs pushing 80 degrees while most of the remainder of the area stays in the 70s. Not a very big spread in high temperatures this time of year. The normal high for Seattle Tuesday is 69 degrees but only 12 out of the previous 79 years ( 15 percent ) has there been a high on the 24th 75 degrees or warmer. .LONG TERM /WEDNESDAY THROUGH SATURDAY/...Extended models showing good consistency this morning. With the upper level ridge well to the east by Wednesday this opens up the door for a cold front to move through the region. Current timing has the front moving through Western Washington Wednesday afternoon. Highs cooling into the lower to mid 60s. Cool air mass behind front for Wednesday night. Air mass unstable enough to a chance of evening thunderstorms in the Cascades. In the lowlands, shower activity will taper off overnight. Lows around 50. Next frontal system arriving Thursday into Friday keeping rain chances in the forecast Thursday with likely pops Thursday night into Friday. Upper level ridge trying to build behind the front Friday night into Saturday for the possibility of a dry weekend. Felton && .AVIATION...Westerly flow expected aloft this morning as a weak upper ridge flattens over the area. Flow aloft will become more northwesterly again tonight into Monday as the ridge rebuilds over the area. Surface winds generally light northerly early this morning, but expect winds to transition back to southerly and increase to 4-8 kts by late morning. Current conditions across area terminals are VFR. Mid to high level cloud cover will increase through the day today with gradually lowering cigs as a frontal system moves into British Columbia. Low level moisture will be abundant enough for some stratus/fog development early this morning for terminals along the coast and across the southern Sound (HQM, OLM, and PWT). This could result in a few hours of MVFR to IFR conditions early this morning. While OLM and PWT will likely return to VFR conditions by late morning, HQM may remain socked in as the approaching system draws closer. A few isolated showers will be possible this afternoon across the far northern interior terminals, generally from KPAE northward. Shower activity may briefly drop ceilings and visibilities to MVFR at times. Ceilings will then become more widespread MVFR at the area terminals as low clouds push into the region overnight into Monday morning. KSEA...VFR conditions with increasing mid to high clouds today. Ceilings will slowly lower through the day, nearing MVFR by late tonight. Light northerly winds will transition to the SW and increase to 4-8 kts this morning. Latest NBM probabilistic guidance indicates a 70 percent chance of ceilings lowering to MVFR by Monday morning and a 30 percent chance of ceilings lowering towards IFR. 14 && .MARINE...High pressure over the coastal waters will weaken today as a frontal system moves into British Columbia. High pressure will then build back into the region Monday and Tuesday while a thermally induced trough expands northward along the Oregon coast. This will allow for flow to turn weakly offshore and northerly. A frontal system will then move into the coastal waters on Wednesday, bringing the return of onshore flow. Onshore flow will strengthen in the wake of the front Wednesday evening and may bring small craft strength westerlies to portions of the central and eastern Strait of Juan de Fuca. Another, stronger frontal system looks to approach the area waters Thursday into Friday. Seas of 4 to 6 feet will continue today, before building to near 7 feet across the outer coastal waters on Monday. Seas will then build later Tuesday into Wednesday, with GEFS probabilistic wave guidance indicating a 70 to 80 percent chance of seas approaching 9-10 feet on Wednesday across the coastal waters. Seas then look to build even more next Friday, with GEFS guidance hinting at a 60-80 percent chance of seas building to greater than 12 ft across the coastal waters. 14 && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$