textproduct: Portland

SYNOPSIS

Expect one final day of abnormally warm conditions on Wednesday while onshore flow continues to increase eventually leading to more seasonable conditions for Thursday. A significant weather pattern change is still in store Thursday evening/night through Saturday as cooler and wetter conditions take hold; slight chance (15-20%) for thunderstorms Friday/Saturday afternoon as well. Conditions trend drier Sunday and Monday, but we'll maintain slight chances for light rain showers across the Coast Range and Cascades while temperatures remain near the below normal across the entire region.

DISCUSSION...Today through Monday

The upper-level ridge pattern which has been driving our warmer than normal conditions continues to break down and shift eastward opening the door for increasing onshore flow headed into Wednesday morning. It's worth noting models still show a weak upper-trough passing through the region during the afternoon. With the lingering dry airmass overhead, it's not expected to bring any shower activity and/or precipitation west of the Cascade crest-line. Wednesday afternoon highs will still be rather warm across the inland valleys but a couple degrees cooler than the day prior (today). Temperatures changes more noticeably come Thursday as onshore flow at the surface and westerly flow aloft push high temperatures into the 70s to near 80 across the inland valleys with upper 50s to 60s along the coast. The cooling trend only continues into the end of the week.

A large-scale pattern change to cooler and wetter conditions remains on track for Thursday night and Friday into the weekend as ensemble and deterministic guidance is in excellent agreement depicting an upper-level low pressure system progressing into the Pacific Northwest from the Gulf of Alaska. Most ensemble members have rainfall via a cold-frontal boundary reaching the coast by late Thursday afternoon/evening and then spreading into the I-5 corridor by later Thursday night into Friday morning. We'll remain in this cooler and wet pattern through at least Saturday, but at least total rain amounts at this time appear beneficial and generally non-impactful. Looking at the latest NBM, chances for 48-hour rain amounts exceeding 0.50 inch from 5 AM Friday to 5 AM Sunday have increased with forecast cycle and are around 80-90% for the coast and Coast Range, 40-70% along the I-5 corridor, and greater than 95% across the Cascades due to westerly flow bringing orographic enhancement. Will note that the western-most parts of the Willamette Valley (including McMinnville and Corvallis) only have a 30-40% chance to exceeding 0.50 inch during this timeframe due to potential rainshadowing from the Coast Range.

On Saturday the core of the upper-level low pressure centers itself right over-top the Pacific Northwest. Temperatures will further cool aloft allowing for increasing atmospheric instability during the daytime hours and facilitating CAPE values of 100-300 J/kg late Saturday morning into the afternoon. This instability plus sufficient lift from the low will support a slight chance (15-24%) for thunderstorms across northwest Oregon and southwest Washington. For now any convection appears rather weak and unorganized (our typical post-frontal pop-up storms) with deterministic guidance suggesting only 15-20 knots of effective shear - severe thunderstorms are unlikely (<5%). Any passing thunderstorms may produce lightning, brief heavy rain, erratic winds, and/or small hail. Also can't rule out a a cold air funnel cloud or two given the time of year.

The latter half of the weekend into early next week the majority of ensemble members are showing the broader upper- level low feature shifting eastward, returning relatively drier conditions and a warm-up of a few degrees. Will still maintain precipitation chances across the Coast Range and Cascades as westerly flow continues to support orographic enhancement of precipitation, however, any additional rain amounts appear light. Still, we'll have to keep an eye on subtle shortwave features rotating into the region north to south on the back side of the upper-level low which may expand the coverage of showers temporarily - low confidence on the placement/timing of these smaller shortwave features. Westerly winds throughout this cool and wet pattern will be breeziest along the central Columbia River Gorge and across Upper Hood River Valley, with chances for isolated wind gusts exceeding 45 mph around 10-35% from Friday to the end of the weekend. -99/10

AVIATION

Mostly clear skies and VFR conditions will continue for inland TAF sites through 06Z Thursday, with light and variable winds around 5 kt or less becoming northerly to northwesterly after 21Z Wednesday, with gusts up to 15-20 kt expected by late Wednesday afternoon.

Meanwhile, low marine stratus with ceilings around 400-600 ft will continue at the coast tonight and tomorrow. Similar to the last couple of days, KONP will likely fail to clear out tomorrow afternoon. Even if low clouds do clear out at KONP tomorrow, it would most likely be very short-lived. Meanwhile, there is an 80-90% chance KAST will clear out by 17-18Z Wednesday before low stratus returns to the terminal around 03Z Thursday.

KPDX AND APPROACHES...VFR conditions will continue through 06Z Thursday with scattered high clouds above 20 kft from time to time. Light and variable winds around 5 kt or less tonight will become northwesterly Wednesday afternoon with gusts up to 20 kt expected by 00Z Thursday with an incoming sea breeze. -23

MARINE

High pressure will maintain northwest winds across the coastal waters through Wednesday. Winds are generally expected to remain under 20 kt. Weakening high pressure will give way to a frontal system approaching the area on Thursday. Winds are expected to back west to southwesterly before the cold front pushes across the coastal waters late Thursday into Friday. Southwest winds are expected to be strongest across the inner coastal waters north of Cape Foulweather, but there is only around a 10-20% chance of wind gusts exceeding 21 kt at any given hour through Friday morning. West to northwest winds will persist behind the front later Friday and into the weekend as weak low pressure lingers near Vancouver Island. High pressure gradually builds across the waters late weekend. Seas are expected to remain less than 8 ft through Friday. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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