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. --263FXUS66 KPQR 212155AFDPQRArea Forecast DiscussionNational Weather Service Portland OR255 PM PDT Wed Aug 21 2024.SYNOPSIS...
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A weak surface trough continues to bring light showersthis afternoon for the coast and Coast Range, then expected tospread inland overnight into Thursday. Late Thursday into Friday,an upper level low will slide south and stall just off theOregon coast, then shift eastward over Washington, Oregon, andnorthern California. This low will bring widespread soakingrain, keeping conditions cool and showery over the upcomingweekend.
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Now through Friday night...Currently, a weaksurface trough located just off the coast of British Columbiacontinues to dive southward along the B.C. coast towards WA andOR coast. This low continues to bring chances of light showersover the southwest WA and northwest OR coast and Coast Range.Expect showers to reach inland overnight into Thursday morningas the low approaches and begins to stall in our coastal waters.This low is then set to slowly move inland late Thursday intoFriday, moving over Oregon, Washington and northern California.This slow-moving low will send several rounds of moisture intothe area, bringing chances for a more widespread soaking rain.In general, expect occasional light to moderate rain showersThursday and Friday. Cannot rule out heavier showers or even afew thunderstorms if enough cloud breaks and daytime heatingoccur, however confidence is low as most guidance suggestsmostly cloudy and low CAPE/instability values. Nonetheless, NBMthunder probabilities remain around 10-20%, highest over theCascades. NBM 12 hour probabilities for 0.25" of rain or morefrom 11PM Wednesday-11PM Friday remain between 50-70%. Theexpected precipitation for Thursday and Friday should helpfurther dampen wildfire activity across the area. High temps arealso expected to be quite cool, around the upper 60s and barelyreaching 70 at some inland locations.-Sala/TK.LONG TERM...Saturday through Tuesday...The forecast continuestrending towards cool and showery conditions on Saturday as the upperlevel low responsible for the rain expected earlier in the weekremains in place over the Pacific Northwest, albeit finally shiftingeast of the Cascade crest. Nevertheless, excess cloud cover andshowers will likely keep temps stuck in the 60s to lower 70s for boththe coast and inland valleys, around ten degrees below normal forthis time of year.This low will shift even farther to the east on Sunday, likelypushing into Idaho and Montana. A ridge will build over westernWA/OR as this low exits east, bringing a return of dry weatherand temps warming up a bit. How much temps will warm up remainsuncertain as model spread remains fairly large. The NBM 10thpercentile for high temps ranges from the lower 70s to lower80s Sunday, mid 70s to upper 80s Monday and mid 70s to lower 90sTuesday. The outcome will ultimately depend on how amplifiedthe ridge will be, which most ensemble guidance suggests will bea weak ridge. Deterministic NBM suggests highs will warm backinto the upper 70s to lower 80s Monday and Tuesday and thiswould be right around normal for this time of year. As such,seems like a reasonable middle ground scenario for now.-Sala/TK
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Satellite imagery and current observations as of 21ZWednesday show widespread VFR across the airspace. Expectsouth/southwest winds along the coast and west/southwest windswith gusts up to 15 kt.A weak front coming from the NW will slowly bring showers acrossthe airspace. Showers expected to start around 21Z-23Z Wed forKAST and then slowly spread southeastward. Showers should reachKPDX around 07Z-09Z Thursday, KONP around 00Z-02Z Thursday, KSLEand KEUG around 07Z-10Z Thursday. Generally VFR across theairspace over the next few hours. However, expect a mixture oflow-end VFR/high-end MVFR conditions to develop as showers moveinto the region through the overnight hours. Also, around a 15%probability for thunderstorms around KAST starting around 00ZThursday through 08Z Thursday.APPROACHES... Predominately VFR conditions. Showers should reachKPDX around 07Z-09Z Thursday and will likely result in a mixtureof low-end VFR/high-end MVFR conditions through the overnighthours. /42
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A series of weak low pressure disturbances willmaintain southerly to westerly winds across all waters throughSaturday. Winds around 5-10 kt with gusts up to 20 kt and seas3-5 feet. Saturday night into Sunday, high pressure returnsresulting in a northerly wind shift for the start of next week.Strong to very strong ebb currents will be possible throughThursday evening. Therefore passage across the Columbia River barcould be challenging at times. /42
-- End Changed Discussion --&&.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