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. --761FXUS66 KPQR 090416AFDPQRArea Forecast DiscussionNational Weather Service Portland OR916 PM PDT Sat Jun 8 2024UPDATED AVIATION DISCUSSION.SYNOPSIS...Overall the upper-level weather pattern staysfairly progressive through next week although weather distancesthrough this period are largely shunted to our north. Thusexpect high pressure of varying amplitude over the region todictate continued near to above normal temperatures and mostlydry conditions through the middle of next week. Little to noappreciable weather impacts to speak of going forward as aresult outside of slight chances(15-20%) for T-storms over theCascades tonight. Any limited chances for precipitation(15-35%)peak across the Cascades into early Sunday morning and againover far northwest Oregon and southwest Washington on Tuesdaymorning - high confidence in dry conditions for the WillametteValley through this period. Slight better chances (15-40%) forprecipitation return late next week..SHORT TERM...Today through Sunday night...This afternoonwater vapor satellite imagery depicts an upper-level troughcontinuing to march towards the Pacific Northwest coastline withSW-SSW flow aloft ahead of this feature streaming mid to highbased cloud cover over the region. The radar has shown some highbased returns this afternoon as well however a dry layer nearthe surface has ensured this has just remained virga and hasntreached the ground - surface observations under these radarreturns have all shown continued dry conditions. As the upper-level wave progresses overhead this evening and overnight, itlikely provides enough forcing to tap into accompanying mid-level moisture/instability leading to shower and isolatedthunderstorm development over the Cascades. At least locally theextent of any such activity appears fairly limited.Jumping further into mid to high based shower/T-storm chances,deterministic guidance like the NAM/GFS/Canadian showsincreasing MUCAPE values into the 300-800j/kg range between700-300mb tonight over the Cascades collocated with decentmoisture within this same layer. Its worth noting modelsoundings confirm this instability would have to be initiatedaloft between roughly 700-600mb and wont be surface based, butthis is typically the case for most nocturnal thunderstormformation. At least the aforementioned shortwave should berobust enough to provide the upper-level forcing needed anditll just come down the placement of the moisture/instabilitywhich guidance keeps pinned near the Cascade crests andeastward. Exploring high resolution guidance like the HREF andUW-WRF does add confidence to the development and placement ofhigh based convection this evening/overnight near the Cascadecrests with the best chances(15-25%) in eastern Lane/LinnCounty. Steering winds aloft will push any activity further intoeastern Oregon - sorry Willamette Valley lightning enthusiasts.Once the axis of the upper- level trough moves directlyoverhead, winds aloft switch northwesterly and the field of mid-level moisture shifts out of the area ending any chance for highbased shower/T-storm activity over the western side of theCascades by mid Sunday morning.From there temps will continue their gradual cooling trendmoving through the rest of Sunday with highs forecast in the midto upper 70s for inland valleys and 60s along the coast. Thelatest NBM guidance still suggests a 25-50% chance that theWillamette Valley will meet or exceed 80 degrees; highest in thePortland/Vancouver metro and near the base of the Cascadefoothills. By Sunday night, high pressure will re-build behindthe exiting trough now moving into Rockies which will lead togenerally decreasing cloud cover outside of coastal stratus.-Schuldt.LONG TERM...Monday through Saturday...Ensemble systems are inagreement of 500mb heights remaining above normal throughWednesday/Thursday of next week supporting near to above normaltemperatures and generally dry conditions over the region. Afteran upper-level ridge briefly rebuilds for Monday the ridgebegins to flatten Tuesday-Wednesday as a trough near the Gulf ofAlaska pushes lower 500mb heights into British Columbia andnorthern Washington. This would result in a slight coolingtrend and a slight chance (15-25%) for light precipitation overfar NW Oregon/SW Washington Tuesday morning as the trough axisgrazes us to the north.By Thursday, most ensemble members (75%)suggest that ridgingwill persist over the Pacific Northwest. However, 25% of memberssuggest that the aforementioned trough near Alaska will beginto dip southward and bring more southwest flow over us. It stillappears by Friday, the majority of ensemble members have thetrough dropping close enough to the Pacific Northwest to returnrain chances to our area. Meanwhile, 20% of the ensemble memberskeep the ridging on Friday with drier conditions persisting. BySaturday almost every ensemble member has a trough in thevicinity of the Pacific Northwest with higher confidence in awetter weather pattern by this point. -Schuldt/Alviz&&.AVIATION...
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A weak upper level disturbance is supporting aweakening cold front just off the OR/WA coast, which will comeonshore with little fanfare tonight. Scattered clouds across thearea remain above 15-20 kft and should remain at or above thislevel through the period with the exception begin areas along thecoast. Probabilities are very high (90% or higher) for the returnof a marine stratus deck along the coast towards sunrise onSunday, with cigs most likely ranging between 500-1000 ft. Therehas already been a few observations from KNOP of brief IFR CIGsthis evening. While this marine push looks to be weak with limitedinland extent, there is a 15-25% chance the marine stratus deckwill end up extending as far inland as KHIO/KPDX/KTTD (however fora short period of time between 15-18Z Sunday). Even if thestratus deck is able to push that far inland, conditions would beMVFR at worst and would quickly improve back to VFR after 18ZSunday. Winds inland will be light and variable and northwesterlyalong the coast around 5 knots overnight. Northwest winds between5-10 knots are expected on Sunday.PDX AND APPROACHES...VFR with typical northwesterly flow over thenext 24 hours with periods of broken high clouds (cigs over 20kft).Light and variable winds overnight will become northwesterlyat 5-10 knots on Sunday. There is around a 25% chance for a briefperiod of 2000-3000 foot cigs between 15-18Z Sunday. -Batz/TK
-- End Changed Discussion --&&.MARINE...Skies were clearing out a bit over the coastal watersSaturday afternoon in the wake of a decaying cool front. Buoyobservations continued showing seas around 4-5 feet at 11 to 15seconds with northerly winds between 10-20 kt. Still expectingseas to rise a few feet from the current conditions as guidancecontinues to suggest seas will peak around 8-9 ft at 12 to 14seconds late Saturday night due to a combination of increasednortherly wind waves and westerly swell.Conditions will settle again late Sunday into Monday as highpressure rebuilds over the waters. Another weak cool front movesover the waters on Tuesday, bringing a brief shift to south winds.Northerly winds return mid week and ramp up late in the week as athermal trough deepens along the south Oregon coast. As usualfor this time of year, northerly winds will be strongest over thecentral and southern waters during the afternoon and eveninghours. The strongest northerly winds are likely to occur onWednesday and Thursday when there is a 50-70% chance for smallcraft advisory level wind gusts up to 25 kt. -TK&&.PQR WATCHES/WARNINGS/ADVISORIES...OR...None.WA...None.PZ...Small Craft Advisory from 5 AM to 11 AM PDT Sunday for PZZ210.&&$$www.weather.gov/portlandInteract with us via social media:www.facebook.com/NWSPortlandwww.twitter.com/NWSPortland