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. --557FXUS66 KPQR 240340 AAAAFDPQRArea Forecast Discussion...UpdatedNational Weather Service Portland OR839 PM PDT Fri Aug 23 2024Updated Aviation....SYNOPSIS...An upper level low will maintain cool and showeryconditions through Saturday before the area starts to dry out as abroad, shortwave ridge starts to build over the Pacific NW for themajority of next week. There is also a 20%-25% probability ofthunderstorms across the forecast area through this evening. Offshoreflow develops late Wednesday/Thursday and will result in warm anddry conditions returning to the region.&&.DISCUSSION...Tonight through Thursday..Cool and moist conditionswill persist through Saturday as a broad area of low pressure slowlymoves eastward through the start of the weekend. This will result inshowers continuing through the evening hours along with a 20%-25%probability for thunderstorms across the CWA through the evening.Model soundings are showing CAPE values around 300-1000 J/kg andwhile the upper end of those values are from the GFS (which tends tobe somewhat heavy handed), the fact that we have already observedsome lightning within our CWA as well as on the east side of theCascades and across NWS Medford`s CWA, the overall thunder forecastand timing are continuing to look on track. We will continue tomonitor and adjust the very short term forecast as needed.Saturday, cool and moist conditions will persist, but theaforementioned low will be east of the Cascades and will result inless frequent showers and will be the start of the somewhat slowpattern shift which is expected through next week. Overall, thepattern change will look something like this. Sunday into Monday, theupper level pattern will become more zonal (west to east flow) as ashortwave ridge builds in from the eastern Pacific which will contendwith an upper level low moving eastward across Canada. This willmaintain weak onshore flow at the surface, but daytime temperatureswill start to warm up into the 70s and 80s. This upper upper levellow will also drag a weak front across the northern part of our CWA,which will likely bring very light rain to SW Washington and the verynorthern parts of NW Oregon late Monday/early Tuesday.By Tuesday the shortwave ridge will become the dominant synopticfeature and as such the warming and drying trend will persist.Wednesday and Thursday the shortwave ridge will build as it quicklymoves east of the Cascades and over the Great Basin. While it isstill very far into the future, multiple models like the GFS, NBM,ECWMF and as well as their respective ensembles, are showing athermally induced surface trough developing to the west or over thecrest of the Cascades. This means that easterly winds are in theforecast. Currently, models are showing the PDX to DLS gradientaround -2 to -3 mb. If these conditions manifest, that would likelyresult in easterly gusts around 20-25 mph for Thursday with a 20%-30%probability of 30 mph. Now, to put this into perspective, duringLabor Day of 2020 there were recorded gusts of 60-70 mph within theColumbia River Gorge. So, this is looking to be far below those LaborDay winds. Still, this forecast is well over 6 to 7 days in thefuture and the forecast will change.Overall, cool and moist conditions will continue through Saturday.However, a pattern change is expected early next week, which willbring warm and dry conditions through next week as well as east windsinto the region Thursday. /42&&.AVIATION...
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Showers will continue to decrease across SW Washingtonand NW Oregon tonight. VFR conditions are dominant with occasionalMVFR cigs with showers. High Resolution model guidance forecasts a 30percent chance of MVFR cigs redeveloping at KAST, KONP, and KEUG by10z Sat. A weak upper level wave will support isolated showerssaturday afternoon with occasional brief MVFR conditions.PDX APPROACHES...Isolated showers will continue to decrease tonightwith mostly dry conditions after 06z Sat. VFR conditions will prevailtonight. High Resolution model guidance forecasts a 25 to 35 percentchance of MVFR conditions 13-16z Sat. ~TJ
-- End Changed Discussion --&&.MARINE...Surface analysis places a low pressure center over thecoastal waters between Florence and Cape Blanco as of early Fridayafternoon, with buoy observations reporting 10 to 15 kt southerlybreezes and seas around 5 feet. Expect these general conditions topersist through Saturday afternoon as low pressure slowly pushesinland and a modest northwest swell maintains seas around 3-5 feetat 8-10 seconds. Winds will trend light and variable on Sunday asweak high pressure builds closer to shore. Stronger surface highpressure will then build offshore early next week and yield a moresummerlike northerly wind regime as inland warming helps totighten the coastal pressure gradient. Guidance continues todepict a 50-80 percent chance for Small Craft Advisory level windgusts across the waters by Tuesday, with the strongest windslikely to be found south of Cape Foulweather. Seas will also betrending upward early next week as building short period seascombine with a building northwest swell to produce 7 to 9 ft seasby Tuesday.In the meantime, strong ebb cycles will continue to produce choppyseas and make for potentially challenging bar crossings throughSaturday evening. However, background wave heights remain too lowto issue a Small Craft Advisory for the Columbia River Bar. /CB&&.PQR WATCHES/WARNINGS/ADVISORIES...OR...None.WA...None.PZ...None.&&$$www.weather.gov/portlandInteract with us via social media:www.facebook.com/NWSPortlandx.com/NWSPortland