Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
016 FXUS66 KPQR 232152 AFDPQR Area Forecast Discussion National Weather Service Portland OR 251 PM PDT Mon Sep 23 2024 .SYNOPSIS...The upper level trough has moved through the area, with no more remaining precipitation. High pressure returns tonight through Wednesday morning. Warm temperatures on Tuesday. Another front arrives on Wednesday night which brings higher probability for rainfall through the remainder of the week. && .SHORT TERM...Now through Thursday...The region has dried out as the previous front moves out of the area. Winds in the central and southern Willamette Valley are currently gusting up to 16-20 mph, but will continue decreasing and become light and variable overnight. Over Monday night, high pressure builds into the region, with dryer and warmer conditions Tuesday. The thermal trough shifts offshore late Monday night, with a brief period of easterly flow late Monday night and early Tuesday morning. Easterly wind gusts up to 20 mph will be possible through the Columbia Gorge during this time. By midday Tuesday winds will shift westerly as the thermal trough moves well inland, east of the Cascades. Tuesday sees much warmer temperatures, with around a 60% chance of high temperatures throughout the Willamette Valley being over 90 degrees. Chance of high temperatures being higher than 95% is less than 5%. Wednesday sees a pattern change as another trough enters the area. Model trends continue to show the jet stream shifting north, and it looks like wind speeds will remain under 25 mph for all regions. Rain amounts will be higher further north, with 80% chance of exceeding 0.25" in the Willapa Hills. This decrease down to 40% near Eugene. Thursday sees relatively zonal flow; some morning showers may be possible, but any accumulation will be minimal. Wednesday and Thursday both look much cooler, with highs around 70 degrees in the lowlands. /JLiu .LONG TERM...Thursday night through Monday...Brewing in the northeast Pacific is a low pressure system which will move eastward on Thursday and Friday. This incoming low will cause the flow to become more zonal which can be observed in the jet stream motion. The forecast area sits on the cool side of the jet with a 145 kt jet streak just to the north along the Washington-Canadian border. The low will enhance precipitation on Thursday into Friday. Because this low is moving inland to the north, we currently are sitting on the southern portion of the front. Will note that similar to the jet on Wednesday, this upper air pattern too has shifted northward which will result on lower probability for precipitation Friday night into Saturday. High pressure returns on Sunday into Monday. -Muessle && .AVIATION...High pressure with northwest to northerly flow aloft today will maintain predominately VFR conditions with variable high clouds. Widespread stratus over the waters from this morning has dissipated, but right along the coast streams of stratus (likely IFR cigs) extending downwind from coastal headlands. KONP forecast will be a bit of a challenge as winds have a slight offshore component, but with heating inland could see a change to onshore wind to bring clouds to the airport. Guidance keeps the coastline right on the edge of IFR conditions through about 05Z Tue then chances for IFR increase. Slightly different conditions at KAST where a weak surface trough just offshore maintaining a deeper marine layer there with MVFR cigs. May see SCT-BKN layer around 2000-2500 at KAST this evening, then a IFR later developing overnight. VFR prevails at inland TAF sites with mainly mid and high clouds. Then after about 12-14Z Tue there is about a 30-50% chance for MVFR or lower conditions, particularly south of KUAO in the Willamette Valley. PDX APPROACHES...VFR prevailing into Tuesday. But there is about 10% chance for MVFR cigs around 14-16Z Tuesday. Light northwest winds expected to increase to around 6-8 kt this afternoon. / mh && .MARINE...Small Craft Advisory wind gusts of 21 kt or more are expected across the coastal waters off the central coast of Oregon into early Tuesday morning. High pressure weakens on Tuesday while thermal troughing shifts inland, allowing for northerly winds to decrease substantially through the day. An approaching front will turn the winds southerly late Tuesday night into Wednesday. NBM Probabilistic guidance has backed off on chances for winds 20-25 kt range later Tue into Wed. The highest chances area now 25-35% for wind gusts above 21 kt Wed morning. However the latest deterministic NBM shows widespread gust 20-25 kt briefly Wed morning, though GFS and ECMWF are weaker. So still some room for models to come to better consensus. Further on, still uncertainty in forecast details late this week as a stronger low pressure system is expected to develop and rapidly intensify across the far NE Pacific, but there is much better agreement among the latest model runs. The surface low is very likely (>90% chance) to take the northerly track toward Haida Gwaii on Thursday. The majority of models suggest the associated front only brings Small craft Advisory level wind gusts to the coastal waters. However, there is a 10-20% chance of Gale Force southerly winds gusts 35 kt spreading across at least the northern waters. Seas around 5 to 7 ft today build to around 8 to 9 ft on Tuesday as a fetch of northwest swell moves into the coastal waters. Seas likely linger around 7 to 9 through Thursday. Another west to northwest swell is expected to move in behind the front on Friday with seas likely building in the 10 to 14 ft range. /mh && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ253. Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Tuesday for PZZ272. Small Craft Advisory until 5 AM PDT Tuesday for PZZ273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland