Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
240 FXUS66 KPQR 252141 AFDPQR Area Forecast Discussion National Weather Service Portland OR 241 PM PDT Wed Sep 25 2024 .SYNOPSIS...Cold front is moving through the rest of today and tonight, which will bring widespread rainfall and breezy winds. Thursday will bring a brief lull in active weather, aside from lingering fog, Thursday afternoon. Another weaker front arrives overnight into Friday late Thursday into Friday morning. High pressure returns over the weekend. More active weather expected mid- week. && .SHORT TERM...Now through Saturday...The front has moved ashore, and rain has begun throughout the CWA. The initial leading edge of the front produced some thunderstorms in the Cascades that were slightly stronger than previously forecast, with some fairly frequent lightning production for about half an hour. However, these have moved over the Cascades, and with daytime heating cut off, instability is likely to fall apart over the rest of the afternoon. While further thunderstorms in the Cascades can`t be completely ruled out, weakening instability is likely to make any further thunderstorms very isolated. The front in general has also slowed in its progression, and some slightly more substantial accumulations are possible. The Willamette Valley will see between 0.25-0.5" of total accumulation over the entirety of Wednesday, and closer to 1" at the coast, Willapa Hills, and Coast Range. Thursday sees more zonal flow, with any residual showers ending in the morning. With clearing skies over Wednesday night, expect some fog throughout the region near the end of Wed night and early Thu morning. A very weak low moves through Thursday evening, but looks to be aimed towards central Washington; only southwest WA should see any accumulations. Friday and Saturday are very similar, with some weak lows coming in at times that could produce a few hundredths of an inch of accumulations at times. Cooler temperatures and higher RHs expected during this period, with generally north/northwesterly flow. /JLiu .LONG TERM...(Sunday through Tuesday)... No significant changes. High pressure will remain over the region, maintaining temperatures more typical of late September for Sunday. Bit of change for early next week, as the high pressure shifts east of the Cascades, and thermal low pressure develops along the coast. This will bring a bit of offshore flow, not much in way of wind, but enough to get warmer daytime temperatures, with inland highs back into the mid to upper 70s for Monday and Tuesday, and upper 60s to lower 70s along the coast. As is the case this time of the year, offshore flow will also bring dry air, allow temperatures to drop more quickly overnight. In areas where the east to northeast wind drops off (such as valleys in the Coast Range and Cascade foothills, as well as the Cowlitz Valley and Willamette Valley to south of Portland, temperatures will be somewhat cool, with overnight lows in the 40s. But, in areas with a bit of easterly wind, overnight lows will stay in the 50s. && .AVIATION...Predominantly VFR as of 21Z Wed as a front moves ashore. Heavier showers ahead of the front now moving into the interior valley and expect occasional MVFR conditions until the front moves through. Also thunderstorm activity is fairly active over the much of the Oregon Cascades. These storms should end or be east of the Cascades by 00Z Thu. Fog and low stratus on the coast has lifted but still a mix of IFR to VFR with the precipitation. Showers taper off this evening, with conditions expected to remain a mix of VFR and MVFR at the coast overnight. Inland areas - there is a mix of VFR and MVFR which is expected to continue through about 00Z-02Z Thu. Then for the remainder of tonight expect mainly VFR conditions at inland TAF sites. Early Thu morning, after 10Z or so, may see some MVFR cigs back build from the Cascade foothill to valley terminals. PDX APPROACHES...A mix of VFR and MVFR with rain chances with the incoming front peak through 00Z Thu. Southerly winds expected to increase to around 6-10 kt shifting to the west later this afternoon, around 00z. VFR expected overnight, but there is about a 30-50% chance for MVFR cigs to materialize after 10Z Thu. /mh /DH && .MARINE...Front moved ashore this afternoon and seas have increased to 10 to 13 ft with a northwest swell. As westerly winds ease behind the front expect seas to remain above 10 ft through the evening hours, so have a Small Craft Advisory through this evening. Winds turn southerly again on Thursday ahead of another frontal system. A developing low pressure will rapidly intensify across the far NE Pacific and move toward Haida Gwaii, while pushing a front toward the coastal waters later Thursday. The strongest winds associated with this system are expected across the northern waters. Model guidance shows probability of Gale Force southerly wind gusts exceeding 33 kt to around 50% Thursday afternoon and evening. The trailing cold front will likely weaken as it moves across the coastal waters Thursday night. A westerly swell moving in behind the front is expected to build seas to around 12 to 14 ft on later Thursday into Friday. High pressure also rebuilds on Friday, along with a strengthening thermal trough along the southern Oregon coast. This will result in a return to gusty northerly winds. Northerlies are likely to continue through the weekend, while seas settle to around 8 to 10 ft. /mh /DH && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 10 PM PDT this evening for PZZ251>253- 271>273. Small Craft Advisory from 11 AM Thursday to 2 AM PDT Friday for PZZ251-271. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland