textproduct: Portland

DISCUSSION...Now through Sunday

The weather pattern will undergo a fairly noticeable transition over the next couple of days. For today, a weak upper-level trough will move across the Pacific Northwest while moisture remains limited. Increasing mid and high level clouds overnight will likely be followed by areas of low clouds and patchy fog in the interior valleys this morning. Some cloud cover should persist into the afternoon, keeping temperatures in the upper 60s to low 70s across the interior and in the 60s along the coast. Despite the passage of the trough, measurable precipitation is unlikely for most locations. The best chances, around 30-50%, will be confined to the North Oregon Coast Range, Willapa Hills and Cascade crest, with probabilities around 5-15% elsewhere.

The trough will shift southward tonight into Wednesday as ridging strengthens offshore and begins to build into the Pacific Northwest. This will mark the start of a much warmer pattern, with interior highs climbing into the upper 70s to low 80s Wednesday. A developing thermal trough along the coast and Cascade foothills will also turn the low-level flow offshore, resulting in increasing easterly winds through the Columbia River Gorge and other east-west oriented terrain gaps. Pressure gradients between KTTD and KDLS are expected to fall into the -1 to -3 mb range, supporting gusts around 20-25 mph in the Gorge and locally breezier conditions over exposed terrain and ridgetops. The central and southern Willamette Valley should also become breezy as winds turn more northerly. There is a 30-50% chance of gusts exceeding 30 mph in the Gorge Wednesday, with probabilities around 5-15% across the central and southern Valley. Despite the warmer temperatures and increasing winds, afternoon humidity should remain above 30%, keeping fire-weather concerns minimal.

Thursday currently appears to be the warmest day of the week. Weak offshore flow will continue through the morning, with moderate to high confidence for interior temperatures reaching the low 80s. The thermal trough should then weaken Thursday afternoon, allowing onshore flow to return and limiting further warming. Meanwhile, the upper-level trough will shift east toward the Great Basin. Some wrap-around moisture could produce a 5-10% chance of thunderstorms along the crest of the Lane County Cascades Thursday, while the remainder of the forecast area remains dry and outside the thunderstorm threat. SPC has included a portion of the Lane County Cascades in a general thunderstorm risk, so this potential will need to be monitored as Thursday approaches.

The forecast becomes more uncertain Friday through the weekend as ensemble guidance diverges on how quickly the upper-level pattern evolves. Roughly half of the guidance maintains weak troughing over the Great Basin, which would keep temperatures closer to the low 70s, while the other half favors stronger ridging over the Pacific Northwest and warmer temperatures potentially reaching the mid 80s. Thus, there is still a fairly broad range in possible temperatures late in the week, but the overall signal continues to favor dry weather with above- seasonal-average temperatures. ~12

AVIATION...Through 12Z Wednesday

Westerly flow aloft will grdadually shift northerly as an upper level trough drops south towards northern California. High clouds continue to drift southeastward into the airspace, with ceilings above 20 kft gradually lowering through the TAF period. Expect predominantly VFR conditions through the TAF period, with the exception of a 30-50% chance of MVFR CIGs at coastal terminals before 20Z Tue and a 20-30% chance of brief fog at KEUG before 17Z Tue. Otherwise, expect light winds this morning to become north to northwesterly around 5-8 kt through the afternoon, possibly up to 10 kt at coastal terminals.

KPDX AND APPROACHES...VFR conditions are expected through the TAF period, with high clouds overhead. Light winds this morning will become northwesterly around 3-6 kt throughout the TAF period. ~12

MARINE

North to northwest winds continue today as high pressure builds offshore. A strengthening thermal trough along the coast is expected to increase pressure gradients across the coastal waters on Wednesday, increasing northerlies to 15-20 kt with at least an 80-90% chance for wind gusts exceeding 21 kt south of Cape Falcon. As a result, Small Craft Advisories have veen issued for the waters south of Cape Falcon from 2 to 11 PM Wednesday. Small Craft winds could hold on until 2 AM Thursday but confidence remains low at this time. Winds are likely to ease later in the week as low pressure drifts offshore. A persistent northwest swell around 4 to 5 ft at 7 to 8 seconds is expected to continue through Tuesday. Seas then build to around 6 ft on Wednesday as northerly winds strengthen. ~12/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM to 11 PM PDT Wednesday for PZZ252-253-272-273.