textproduct: Portland

SYNOPSIS

Today will be the last day of abnormally warm temperatures. Conditions remain dry while onshore flow increases. A significant weather pattern change is on track beginning Thursday as a frontal system swings through and returns widespread chances for rain beginning Thursday night and through the weekend. There is also a 10-25% chance for thunderstorms Friday afternoon and Saturday afternoon. Conditions trend drier and slightly warmer Sunday through early next week, but we'll maintain slight chances for light rain showers across the Coast Range and Cascades.

DISCUSSION...Today through Tuesday

Satellite imagery as of early Wednesday morning depicts widespread marine stratus along the coast and mostly clear skies inland. As daytime heating progresses today, we'll see increased mixing and some stratus break-out along the coast by late morning. Guidance is optimistic for stratus breaking out along the entire coast, however, based on the persistent pattern and observations from the last few days, trends favor the north Oregon coast clearing out after 17-18z Wed while stratus keeps hold along the central Oregon coast. Whether stratus breaks out or not today, there is high confidence that marine stratus will fill back in later this evening.

Today is the final day of very warm and dry conditions as the upper-level ridge that brought the heat the last couple days is breaking down and shifting eastward. Model guidance continues to show a weak upper-level trough passing through the region today, however, it's not expected to bring any shower activity west of the Cascade crest-line given the lingering dry airmass over our area. Even though we're expected to stay dry, this trough will bring increased onshore flow and afternoon high temperatures a few degrees cooler than the last couple days - upper 80s to low 90s across interior valleys. As a result, there remains widespread Moderate HeatRisk across the Willamette Valley, SW Washington lowlands, and Columbia River Gorge. This level of heat will affect those who are sensitive to heat, especially those without adequate cooling or hydration. Make sure to stay hydrated, wear light-colored and loose-fitted clothing, wear sun protection, taking frequent breaks from the heat, and limit strenuous outdoor activities during the hottest part of the day.

A large-scale pattern change to cooler and wetter conditions remains on track for Thursday night through the weekend as ensemble and deterministic guidance is in excellent agreement of 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 late 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 1 inch from 5 PM Thursday to 5 PM Saturday are around 40-70% for the coast and Coast Range, 10-30% along the I-5 corridor, and greater than 80% 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 5-10% chance of exceeding 1 inch during this timeframe due to potential rainshadowing from the Coast Range.

By Friday afternoon, the upper-level trough moves further south and eventually centers itself right over-top the Pacific Northwest as a closed low on Saturday. Temperatures will further cool aloft allowing for increasing atmospheric instability during the daytime hours and facilitating CAPE values of 50-100 J/kg Friday afternoon and 100-200 J/kg late Saturday morning into the afternoon. This instability plus sufficient lift from the low will support a 10-20% chance for thunderstorms across NW Oregon and SW Washington Friday afternoon, and a 15-25% chance for thunderstorms Saturday afternoon. For now, any convection appears rather weak and unorganized (our typical post-frontal pop-up storms). LREF guidance suggests effective shear remains below 35 kt; therefore, chances these thunderstorms become severe are unlikely (<5%). Otherwise, any passing thunderstorms may produce lightning, brief heavy rain, erratic winds, and/or small hail. Also can't rule out 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 very 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 through early next week, with the highest chances on Sunday and Monday. -10/99

AVIATION

Satellite imagery as of early Wednesday morning depicts widespread marine stratus along the coast resulting in LIFR/IFR CIGs. Expect stratus to hold on along the entire coast through at least 17-18z Wed. After 18z Wed, increased mixing from daytime heating will support stratus breaking out for parts of the coast, returning VFR conditions. Based on the persistent pattern the last couple days, trends favor KAST clearing out after 18z Wed, while IFR stratus holds at KONP. If KONP does clear out, it would be very brief and sometime between 19-23z Wed. Either way, there is high confidence that marine stratus will fill back in and return widespread IFR conditions along the coast after 02-03z Thu. For Willamette Valley terminals, VFR conditions prevail throughout the TAF period with FEW/SCT high clouds around 25 kft. Light winds under 5 kt through 18z Wed, becoming west-northwesterly and increasing around 8-11 kt with gusts up to 18-20 kt across most terminals between 22z Wed-03z Wed. Winds gradually ease in the evening.

KPDX AND APPROACHES...VFR with FEW/SCT high clouds through the TAF period. Light winds under 5 kt, becoming northwesterly and increasing to 9-11 kt with gusts up to 20 kt between 23z Wed-03z Thu. -10

MARINE

High pressure will maintain northwest winds across the coastal waters through Thursday morning. 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 Thursday afternoon before the cold front pushes across the coastal waters Thursday night into Friday. Southwest winds are expected to be strongest across the inner coastal waters (up to 10 NM offshore) and 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 around 3-5 ft through Friday, building to 6-9 ft this weekend as a west- northwesterly swell moves in. -10/DH

PQR WATCHES/WARNINGS/ADVISORIES

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