National Weather Service (2024)

Area Forecast Discussion
Issued by NWS Portland, OR

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-- Discussion containing changed information from previous version are highlighted. --860FXUS66 KPQR 311038 CCAAFDPQRArea Forecast Discussion...CorrectedNational Weather Service Portland OR337 AM PDT Fri May 31 2024.SYNOPSIS...Warm and dry weather will prevail today as high pressurepersists over SW Washington and NW Oregon. The high pressure weakensenough Saturday to allow a weak frontal system to clip the regionwith clouds and a few showers, mainly along the coast and over thehigher terrain. A stronger system will spread rain throughout theforecast area Sunday; this rain may be locally heavy as a moderate tostrong atmospheric river of moisture will be involved. Unsettledweather will linger through the first half of next week, then a turntoward warmer and drier weather appears likely by the end of theweek.&&.SHORT TERM...
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Today through Sunday...High pressure will lead to apleasantly warm and dry day to close out the month of May, withtemperatures a few degrees above normal inland. With 850 mb tempsclimbing to around +10 deg C over much of the Willamette Valley andlittle cloud cover to speak of, inland highs will push well into the70s with a couple locations potentially touching 80 degrees thisafternoon. In fact, latest NBM probabilistic guidance shows 60-80%probs of reaching 80 degrees this afternoon across much of the PDXmetro area. These chances decrease to the south of Salem, with thecentral/south Willamette Valley in the 20-50% chance range.Regardless of whether or not the inland valleys reach 80 degrees,this afternoon looks warm with plenty of sunshine inland. As is oftenthe case, coastal temps will be a few degrees cooler due to afternoonsea breezes.The high pressure responsible for the warm and sunny weather todaywill weaken tonight into Saturday, allowing a weak shortwave systemto move onshore near the WA/BC border. This system will deepen themarine layer, potentially bringing some drizzle or light rain tonorth coastal areas Saturday morning. Just enough moisture andlow-level instability could linger into Saturday afternoon for a fewshowers, with the best chances being along our north coastal zonesand across the higher terrain.Whatever rain falls on Saturday will likely pale in comparison to therain expected with the next, stronger system Sunday. Models and theirensembles continue to suggest a deep tap to subtropical moisture,somewhat associated with a tropical system which emerged from thewestern Pacific over the past 24-48 hours. GEFS members suggest ahigh likelihood (>90%) of integrated vapor transport (IVT) exceeding500 kg/m/s, with about a 50/50 chance of IVT exceeding 750 kg/m/salong the Oregon coast Sunday evening. Meanwhile the forecast areawill be in or near the left-exit region of an unseasonably strong jetstreak; this will enhance lift as the deepest moisture moves acrossthe region. As such, Sunday afternoon and evening look quite wet forSW Washington/NW Oregon, with NBM probabilistic QPF suggesting thevast majority of the CWA has a better than 50% chance of receiving 1inch or more of QPF, with these chances at around 70% for the Salemand Portland metro areas. The same guidance suggests 1 inch or moreof rain is a near certainty in the Coast Range and Cascades north ofHighway 20, with NBM means suggesting 1.5-3.0 inches of QPF for thesame area Sunday night into Monday. High-end (95th percentile) QPFsuggest a couple locations could reach 4 inches of QPF if everythingwere to line up properly. With this in mind, it is not out of thequestion that there could be rapid rises or even minor flooding on acouple of our flashier creeks and drainages (the Grays River comes tomind), but based on latest guidance it appears the chances of thisoccurring at any given location are 10% or less. Weagle.LONG TERM...Monday through Thursday...The unseasonably strong jetstream over the NE Pacific remains aimed toward the Pac NW throughTuesday, keeping the weather unsettled across SW Washington and NWOregon. Latest NBM thunder probs are in the 15-20% range for much ofthe CWA Monday afternoon, which seems reasonable given the pocket ofcool air aloft swinging through the region and the strong early Junesun. Depending on how much sunshine can break through the cloudsMonday, a couple of the thunderstorms could become strong enough toproduce small hail and/or gusty winds. Either way, Monday will likelybe somewhat blustery given the 30-40 kt SW winds at 850 mb shown bythe 06z NAM. Guidance suggests another system potentially moving intothe Pac NW Tuesday, though there is a lot of variability on thestrength of this system.Starting midweek, models are suggesting a high likelihood of a strongupper level ridge of high pressure developing somewhere over theWestern United States. Looking at the 00z WPC cluster analyses, thereis still a lot of variability in where models and their ensembles areplacing the upper ridge axis...which will be key in determining justhow warm it will be toward the end of next week. Based on the 00z WPCclusters, it appears there is only about a 20-30% chance of a patternthat would avoid significant positive 500 mb height anomalies overWA/OR (i.e. significantly above-normal temperatures), with theremainder of guidance suggesting temps several degrees above normal(at least inland) by the end of the week. By next Friday, NBMprobabilistic guidance suggests a 30-50% chance of highs reaching the90s in the Willamette Valley, with a 5-15% chance of highs exceeding100 degrees. With this in mind, those sensitive to heat should payclose attention to the forecast the next few days, and perhapsconsider where they may go to stay cool late next week should thehotter forecast guidance prevail. Weagle
-- End Changed Discussion --&&.AVIATION...VFR conditions under mostly clear skies prevailthrough most of the TAF period as high pressure continues over theregion. The high pressure begins to breakdown later today as afront approaches off the northeast Pacific. HREF guidanceindicating 40-60% chance for MVFR or lower conditions at the coasttowards 12Z Sat. Winds generally north to northwest less than 8kt in the morning, then picking up in the afternoon 8 to 12 ktwith gusts to 25 kt along the coast and 15 to 20 kt for inlandlocations.PDX AND APPROACHES...VFR conditions through the entire TAF periodwith mostly clear skies. Winds diurnally driven with northwestwind to 5 kt, then 8 to 12 kt from 00Z Sat. /mh&&.MARINE...Surface high pressure continues offshore today with athermal trough along the south Oregon coast. This will drivenortherly winds across the waters. Expect Small Craft Advisorywinds gusting to 25 kt south of Cape Falcon (marine zones PZZ272,273,252,253). The high pressure breaks down later thisafternoon and evening as a weakening Pacific front approaches.The weak front is expected to move through the waters onSaturday, shifting winds westerly to southwesterly. A moreorganized and stronger storm system will move into the northeastPacific Sunday shifting winds southerly. Small Craft Wind gustsare likely with 90+% chance of gusts exceeding 21 kts. There`salso a 20-30% chance of a brief period of Gale Force winds of 35kts Sunday afternoon or evening. /mh&&.PQR WATCHES/WARNINGS/ADVISORIES...OR...None.WA...None.PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ252-253-272-273.&&$$www.weather.gov/portlandInteract with us via social media:www.facebook.com/NWSPortlandwww.twitter.com/NWSPortland