textproduct: Portland

SYNOPSIS

Going through the weekend into next week Western Oregon and Southwest Washington remain under the influence of a progressive westerly flow pattern with little to no notable weather impacts. Expect a weak weather disturbance to arrive late tonight into Saturday morning ushering in occasional light precipitation mainly along the coast and at higher terrain features. Then after a temporary return to drier/warmer weather Sunday into Monday, model guidance continues to trend towards more appreciable widespread rainfall around the middle of next week. However, beyond the broader favorable set-up for rainfall, details regarding the exact timing/amounts are uncertain.

DISCUSSION...Today through Thursday

Through the first half of the weekend upper-level troughing remains the dominant feature over the far ENE Pacific while helping to keep persistent onshore flow in place. Expect the axis of this upper-level trough feature to progress overhead Saturday morning coupled with a weak cold-frontal boundary which likely leads to a period of light rainfall/shower chances for a portion of the region. Overall the best relative chance for light rain (20-50%) appears to be along the coast, coast range/Willapa Hills, and Southwest Washington, particularly in the Cascades and Cascade foothills, where orographic forcing is present. Can't completely rule out the Portland Metro receiving a shower in the morning too ( 15-25% chance) but confidence is high any rainfall amounts stay rather light (<0.05in). The additional onshore flow also maintains fairly prolific cloud cover across the inland valleys into the western slopes of the Cascades Saturday helping to keep daytime temperatures 5-10 degrees lower than normal.

By Sunday, the upper-level trough axis shifts eastward of the Cascades followed by a shortwave ridge building over the region for the start of the week. This transition likely results in high temperatures jumping a degree or two day-to-day on Sunday followed by further warming on Monday with daytime highs reaching into the upper 70s to low 80s across the interior, 60s along the coast and into the 70s for the Cascades. It's worth highlighting most models depict a weak near surface thermal trough developing during Sunday/early Monday before quickly shifting eastward. This would result in a period of slightly breezier northerly winds across the central and southern Willamette Valley Sunday afternoon followed by light easterly winds in the Cascade/Cascades foothills Sunday night into Monday morning. Beyond the aforementioned winds, slightly degraded overnight RH recoveries due to the light easterly/offshore flow, and a 12-18hr window for haze/smoke to drift towards the northern Willamette Valley from local wildfires can be anticipated. At least all of these impacts appear minimal and rather temporary as winds switch back westerly by Monday afternoon into Tuesday raising RHs and pushing smoke/haze back towards the Cascade crest-line.

Looking into the middle of next week, ensemble guidance continues to heavily favor the development of an early fall- like cooler and wetter weather pattern although confidence in the exact details are only low to moderate at this time. This model uncertainty is almost solely derived from the placement/amplitude of the upper level low progression diving southward from the Gulf of Alaska with some ensembles increasing rainfall as early as Tuesday morning (weaker trough feature), while others hold it off until later Tuesday and Wednesday (stronger trough). With that said, at this point almost all deterministic models and their respective ensembles show at least a wetting rain (>0.25in) in the late Tuesday through late Thursday night timeframe. Exploring the latest NBM probabilistic guidance points to a 60-80% chance for greater than 0.5in of rainfall across the entire region, and even a 40-60% chance (slightly higher at the coast) for greater than 1.0in of rainfall over that same 48 hr timeframe - very promising overall. While this isn't shaping up to be a "fire season ender" unless amounts trend much higher, the forecast rainfall will definitely work to heavily moderate fire weather related concerns. We'll take what we can get this time of year. -99

AVIATION...Through 21Z Saturday

West to southwest flow aloft continues as an upper level trough remains over the region. Satellite imagery reveals scattered to broken low level clouds across the area this morning while terminal observations show predominately VFR CIGs. Expect skies to become mostly sunny this afternoon as clouds mix out by 21z. A weak front will move across the area Saturday morning bringing lower clouds and light rain showers to northern portions of the area. Most inland terminals are expected to remain predominately VFR, although there are low chances (10-30%) for MVFR CIGs after 12z Sat. Higher chances for at least MVFR conditions at the coast, with the highest chances near KAST. North to northwest winds increase this afternoon to around 10 kt before easing overnight.

KPDX AND APPROACHES...Broken to scattered stratus with low-end VFR CIGs early this afternoon is expected to clear by 21-22z. A weak front will bring another round of lower clouds Saturday morning, with around a 20-30% chance for MVFR conditions after 12z Sat, along with low chances for rain showers. Northwest winds increase to around 8-10 kt this afternoon before easing overnight. /DH

MARINE

High pressure offshore will maintain predominately north to northwest winds across the coastal waters through the weekend and into early next week. A weak front pushes across the northern waters late tonight into Saturday morning with little to no impact. Winds are generally expected to remain around 15 kt or less. Predominately wind driven seas remain at around 3 to 5 ft.

The pattern likely becomes more active by the middle of next week as a stronger frontal system approaches the region later Tuesday into Wednesday, followed by low pressure moving southeast from the Gulf of Alaska. There is at least a 50% chance that this system brings low end gale force wind gusts in excess of 34 kt, as well as resulting in seas building towards 10 ft. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.