textproduct: Portland

SYNOPSIS

Showers and isolated thunderstorms will dissipate this evening. Mainly dry and mild on Monday, aside from periods of light rain over southwest WA and far northwest OR. High confidence for a prolonged stretch of dry weather and above normal temperatures from Tuesday through at least Friday as a strong upper-level ridge builds overhead. Uncertainty remains regarding the eventual breakdown of this ridge and the timing of the next precipitation chance, however most guidance suggests rain will likely return by late Saturday night.

DISCUSSION...Now through Saturday night

Radar and surface weather observations from early Sunday afternoon depicted scattered rain showers occurring over northwest Oregon and western Washington along with breezy southerly to southwesterly winds. These showers are occurring behind a Pacific frontal boundary that moved inland earlier Sunday morning and progressed east of the Cascade crest in the late morning/early afternoon. There have also been three isolated thunderstorms observed so far early Sunday afternoon, one in the western Columbia River Gorge, one near Clatskanie, and one 5-10 miles east of Oregon City. However, these thunderstorms were generally weak and short-lived due to limited instability, producing only brief heavy rain and a few flashes of lightning per SKYWARN spotter reports. Nevertheless, there is a 10-15% chance of thunderstorms through 5 PM PST Sunday for locations to the north a line extending from Newport to Corvallis to Lebanan to Cascadia.

Hi-res model guidance and forecast soundings continue to suggest most of the ongoing convection will dissipate shortly after sunset over inland valleys, while isolated light showers linger in the Cascades and Coast Range through midnight. Light rain will then return to southwest Washington and far northwest Oregon Monday morning as a warm front lifts northward. There is a 80-95% chance locations to the south of a line extending from Lincoln City to Portland will stay dry with high temperatures in the mid 50s to 60 degrees.

Beyond Monday, models and their ensembles continue to show excellent agreement for upper level ridging over the Pacific Northwest, bringing an extended stretch of dry weather with above normal temperatures and light winds Tuesday through at least Friday. One exception in regards to winds will be in the western Columbia River Gorge and far eastern Portland/Vancouver metro where easterly winds will become quite gusty at times, strongest on Wednesday and Thursday when models suggest the surface pressure gradient from Troutdale to the Dalles will most likely peak somewhere between -6 to -8 mb according to the ENS ensemble. Given the stable conditions in lower levels of the atmosphere during that time, expect max wind gusts up to 80 mph at Crown Point, up to 60-65 mph at Corbett, and up to 40-45 mph at Troutdale. Note these wind speeds assume the pressure gradient will peak near -8 mb, which means observed winds will wind up being lower if the gradient winds up weaker than that. The NBM v5.0 seems to reflect this well and shows winds quite a bit stronger than the NBM v4.3, so this was utilized to produce the wind forecast Tuesday through Thursday night. Some tree damage and power outages will be possible if the higher gusts to materialize, however impacts shouldn't be overly significant for this time of year.

Note with much lighter winds away from the Columbia River Gorge, air stagnation will be a concern, especially in the central and southern Willamette Valley where winds will likely be calm and mixing heights will be low for 3-4 consecutive days. This may result in degraded air quality over time. It's also worth mentioning that uncertainty regarding the sky cover and temperature forecast is highest in the central and southern valley, as air stagnation patterns this time of year tend to favor stubborn low stratus and/or fog for these inland valley locations, which keeps temperatures quite a bit cooler compared to the coast, Coast Range and Cascades where warmer air aloft is tapped into along with sunshine. However, it appears sunshine is possible for much of the day even in the central and southern valley by late in the week.

The forecast becomes more uncertain for all locations on Saturday, as some model guidance suggests dry weather will continue while other guidance suggests precipitation will return to the area. There is currently a 20-40% chance rain will return as early as Saturday, and a 50-80% chance rain will return as early as late Saturday night. -23

AVIATION

Showers continue across the region, clearing across the valley by 00-03z Mon and along the coast and terrain by 03-06z Mon. There's a 15-20% chance of thunderstorms through 03z Mon, mainly for locations near and east of I-5. Main impacts with thunderstorms are frequent lighting, brief heavy rain, and gusty and erratic winds. Predominately VFR conditions expected through the TAF period for inland locations except for isolated MVFR CIG or VIS in showers. MVFR/IFR ceilings will linger along the coast into the evening, with widespread VFR conditions returning between 06-10z Mon. Broken cloud cover overnight could result in favorable conditions for fog/low stratus development around 06-10z Mon over portions of the Willamette Valley. There's around a 15-30% chance, mainly along and west of the I-5 corridor. Uncertainty remains as the amount cloud cover heavily influences such development. Any fog/low stratus that develops should improve to VFR by 17-20z Mon. Expect mostly southerly winds at 5-10 kt through 00z Monday, with gusts up to 20-25 kt along the coast and at a few inland terminals, then decreasing thereafter.

KPDX AND APPROACHES...Predominately VFR conditions with scattered showers continuing through 00z Mon. There's a 20-40% chance of MVFR CIGs returning between 09-15z Mon. Expect generally southerly to southeasterly winds around 5-10 kt through the TAF period, with gusts up to 20 kt possible before 23z Sun. ~12/03

MARINE

Southerly wind gusts have decreased below 20 kts this afternoon, but another weak front will graze the waters tonight into Monday, bringing increased south winds with gusts 20-25 kts. Winds will decrease below 20 kts again by late Monday afternoon, remaining calmer through the rest of the week. Expect wave heights of 10-13 feet at 11-13 seconds, through Monday evening, with the period increasing to 14-16 seconds late tonight into Monday morning. Due to a combination of winds and seas, a Small Craft Advisory is in effect through 1 AM PST on Tuesday. High pressure builds over the waters Tuesday with southerly winds less than 15-20 kts and seas less than 10 ft. By Wednesday, an increasing westerly swell is forecast to push into the waters, increasing seas to around 10-12 feet around 12-16 seconds through the end of the week. ~12/03

BEACH HAZARDS

A long period westerly swell will result in a high threat for sneaker waves persisting through Monday afternoon. Waves can run up significantly farther on a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning. Caution should be used when in or near the water, especially those who will be razor clamming this week. /42

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 1 AM PST Tuesday for PZZ210-251>253- 271>273.