Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 202150 AFDPQR Area Forecast Discussion National Weather Service Portland OR 248 PM PDT Thu Sep 20 2018 .SYNOPSIS...A weakening storm system will spread light rain showers across the region late Friday and Saturday before much warmer and drier weather return early next week. && .SHORT TERM...Today through Sunday...Water vapor satellite imagery this afternoon shows a positively tilted shortwave trough over the northeast Pacific. Underneath this storm system, a weak surface low pressure is currently beginning to develop near 145W and 45N and will help to turn a broad west to east oriented baroclinic zone over the northeast Pacific into a warm front. Models are in good agreement this will result in rain spreading into Vancouver Island this afternoon. Based on radar and satellite trends suspect rain will have a hard time reaching as far south as Pacific County, but cannot completely rule out it out so have kept some low PoPs just in case. Either way, the threat of rain will decrease across our far northwestern zones overnight as the warm front lifts more solidly north of the region. This should allow most of the area to warm up a few degrees Friday when compared to the past few days with inland high temperatures rising into the mid to upper 70s. As the aforementioned low pressure moves closer to the region, a weakening front will drop slowly southeastward into the area Friday afternoon through early Saturday. Models are in good agreement light rain will move onto our north coast Friday afternoon and evening and push inland Friday evening into Friday night. Rainfall amounts will be highest across the north and lowest towards Lane County. Lingering low level instability, particularly coupled with daytime heating, should keep the threat of showers going into Saturday afternoon and early evening. A stabilizing northwesterly flow pattern under a building shortwave ridge should result in shower chances decreasing Saturday night into Sunday. Temperatures should remain several degrees below average Saturday and Sunday, though. /Neuman .LONG TERM...Sunday night through Thursday...The operational models and their ensembles continue to support the idea of an extended period of dry weather and above average temperatures next week as shortwave ridging builds over the Pacific Northwest. Northerly winds should develop early in the week as thermal low pressure builds northward into northwest Oregon and southwest Washington. Temperatures appear likely to warm into the 80s either Monday or Tuesday. Depending on the exact placement of thermal low pressure, we could see these warm temperatures progress westward to coastal locations as well, but it`s still too early to be confident which days this will or will not occur. In addition, gusty east winds should develop at some point across the western Columbia River Gorge and Cascade ridgetops, but timing differences between models remain. /Neuman && .AVIATION...The area should remain in VFR conditions through the evening hours. Inversions should develop overnight, but generally remain shallow enough to prevent IFR and MVFR restrictions from developing. The main exception to this may be along the coast where some patchy fog could develop towards 12z Friday. KAST will more than likely develop a lowering stratus deck over night bringing MVFR cigs. KPDX AND APPROACHES...VFR conditions expected through until 00Z Saturday. Winds will be on the light side, with a slight easterly direction through the overnight hours. Occasional cigs could be seen overnight, but should remain within VFR thresholds. /42 && .MARINE...A weak warm front was moving through the Washington and far North Oregon waters early this afternoon. An 18Z ASCAT pass indicated generally 20 to 25 kt south wind close to the low center and 10 to 15 kt over the Washington waters out near 60 nm. The 12Z NAM and GFS are similar, showing 20 kt boundary layer wind speeds clipping the far nw part of PZZ270 around 00Z Fri. Would not be surprised if buoy 089 reported a couple hours of 25 kt gusts late this afternoon or early evening. However, since this looks to be a localized and short-lived event, an advisory will not be issued. A stronger surface low develops well offshore tonight and is forecast to move inside 130W 18z Fri. The NAM continues to be slower than the GFS or ECMWF, but has come more in line with the central pressure. Confidence is growing that small craft advisory wind speeds will develop over the north zones late Fri morning and continue through 12Z Sat and have issued a small craft advisory to cover this. There is a chance the far north part of PZZ275 could see 25 kt gusts Fri night, but this will be marginal. North wind returns Sunday and continues through the first half of next week. Gusts up to 30 kt will be possible for PZZ275 late Mon through Tue night. Seas will remain 3 to 5 feet through tonight, but will build to 6 to 9 ft by late Fri night. Expect the largest seas north of Cascade Head and beyond 20 nm from shore. Seas then ease to around 5 ft Saturday night and hold through the start of next week. NW wind wave will be the dominant feature Sun through the middle of next week, with a much smaller longer-period background swell. This will result in choppy conditions and could lead to square-sea conditions. Weishaar && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory for winds from noon Friday to 5 AM PDT Saturday for Coastal Waters from Cape Shoalwater WA to Cascade Head OR out 60 NM. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. This area is commonly referred to as the CWA or forecast area.

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