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Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
782 FXUS66 KPQR 141833 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 1133 AM PDT Wed Aug 14 2024 Updated Aviation Discussion .SYNOPSIS...Westerly onshore flow will continue over the next seven days, bringing near to below normal temperatures. Scattered showers and thunderstorms possible over the central Oregon Cascades Thursday and Friday. It is looking increasingly likely that widespread rain is possible Friday into Saturday, with chances for thunderstorms on Saturday. Slight precipitation chances continue into early next week as an upper level low settles near the area. However, exact timing and amounts remain uncertain. && .DISCUSSION...Wednesday through Monday...Observations indicate mid level cloud cover never fully dissipated for locations generally north of McMinnville on Tuesday, and IR satellite imagery shows these clouds beginning to build back to the south early this morning in the Willamette Valley as northerly winds returned Tuesday evening. Model guidance is not picking up this cloud cover well, but as winds around and south of Salem are more from the northwesterly direction and expected to remain this direction in addition to low-level and mid-level water vapor satellite imagery showing a drier slot moving into the area south of McMinnville, not expecting this stratus to move much farther south than Salem. The upper trough from earlier today continues to slowly move through eastern Oregon, and as it moves east of Oregon by around daybreak, clouds are expected to dissipate from south to north as shortwave transient ridging builds over NW Oregon and SW Washington. Temperatures today will bounce back to near normal under mostly clear skies by the afternoon, generally in the low 80s for the Willamette Valley and mid to upper 60s along the coast. Model guidance is in good agreement of a negatively tilted low moving through southern and central Oregon tonight through Thursday. This will bring a 30-50% chance of scattered showers along the Lane and Linn county Cascades with a 15-20% chance for the Cascades north of Linn County Thursday. Latest guidance is honing in on the best chances for thunderstorms being east of the central Oregon Cascade crest, but can`t rule out enough orographic forcing for a 15% chance of thunderstorms right along the Lane County Cascade crest. Widespread cloud cover will also return to the majority of NW Oregon and SW Washington, allowing temperatures to drop back below normal into the mid to upper 70s across the Willamette Valley and low 60s across the coast. Latest GFS, Euro, and Canadian ensemble guidance is coming into better agreement in the general pattern for Friday into the weekend, though some uncertainty remains in specific details. All 500 mb WPC clusters indicate a longwave trough deepening over the eastern Pacific Friday into Saturday, lingering in this area into early next week, with the closed low generally lingering somewhere west of the Oregon coast. The uncertainty comes in the exact location of the low, whether it will be closer to the coast to the east or remain farther west over the Pacific as well as whether it will be closer to the Oregon/Washington border or the Oregon/California border. These seemingly small differences in position will lead to differences in rain amounts, timing, and thunderstorm chances as shortwaves move along the low. Clusters continue to indicate Friday will be dry. Precipitation chances increase Friday night with NBM indicating a 30-50% chance of widespread rain pushing into the region from the south to the north early Saturday morning through Saturday afternoon as a shortwave moves through the region. As this will bring south to southwesterly upper level flow along with instability and moisture, there`s a 10-20% chance of thunderstorms east of the Coast Range and a 20-25% chance over the Oregon Cascades Saturday afternoon and evening. However, some ensemble members show this shortwave bringing rain later on Saturday into Sunday, so shower chances continue into Sunday. Shower chances linger into Monday and Tuesday, mainly for the coast, as ensembles depict the closed low more offshore after the shortwave passes. Temperatures through early next week will remain very similar to Thursday`s temperatures. -HEC && .AVIATION...Northwesterly flow will maintain predominantly VFR conditions across most of northwest Oregon and southwestern Washington through the TAF period. Inland cloud cover around 4000-6000 ft continues to dissipate this morning and should leave mostly clear skies for the afternoon. An upper level disturbance will move in from the southwest, bringing the chance for marine stratus along the coast after 00-06Z this evening. Model guidance also suggests some scattered MVFR cloud cover developing inland closer to 10-12Z Thursday morning. Winds remain N/NW around between 5-10 kts. PDX AND APPROACHES...Onshore flow will produce predominantly VFR conditions through 18Z Thursday. There is a 30-50% chance MVFR CIGs develop after 12Z Thursday morning. -Batz && .MARINE...Expect benign conditions over the waters this week. This afternoon, high pressure will re-build and return northerly to northwesterly winds. Wind gusts will remain around 10 kt or less this week across all waters, with wind-driven seas of 2 of 4 feet. -Alviz/Sala && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland