textproduct: Portland

SYNOPSIS

A Pacific frontal system is moving across the region early this morning, bringing periods of light rain while mild temperatures persist. Conditions trend mainly dry tonight before light rain returns to southwest Washington and far northwest Oregon on Monday. High confidence exists 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.

DISCUSSION...Now through Friday night

Early this morning, a weak Pacific front is progressing inland, producing light stratiform rain across much of the area. Rain will continue through the morning hours before transitioning to more intermittent, light post-frontal showers by this afternoon and evening. Rainfall amounts remain light, and impacts are expected to be minimal given mild temperatures and weak forcing. Snow levels remain elevated, generally near or above 5000 feet, resulting in rain or a rain/snow mix at pass level.

Showers gradually diminish this evening, with conditions trending drier overnight as the post-frontal air mass weakens. This break in precipitation is expected to persist for much of the region, though southwest Washington and far northwest Oregon will see increasing chances for light rain on Monday as a warm front brushes the area. Probabilities for rain on Monday are highest north of the Columbia River, decreasing southward into northwest Oregon. Much of the southern Willamette Valley and Lane County Cascades and foothills are likely to remain dry given the northward track of the system. Where precipitation does occur, amounts will remain light and impacts negligible. Probabilistic guidance for rain accumulation greater than 0.01 inch over 24 hours on Monday: 80-100% chance in most of southwest Washington and Astoria, 60-80% chance along the coast from Tillamook to Newport, 40-60% chance from the Portland/Vancouver metro to Aurora, and a 10-30% chance south of Aurora. Snow levels will continue to stay above pass elevations.

By Monday night, ensemble guidance is in strong agreement that an upper-level ridge builds rapidly over western Oregon and Washington, shutting off any remaining precipitation and ushering in a prolonged dry period with unseasonably warm temperatures.

Beyond Monday, the regime is dominated by a strong and anomalous upper-level ridge centered over the western CONUS. Ensemble means indicate 500 mb heights climbing into the 97th to 99th percentile for this time of year from Tuesday to Thursday, supporting high confidence in dry conditions from Tuesday through at least Friday. Similarly, ensemble means indicate 850 mb temperatures climbing into the 97th to 99th percentile for this time of year from Tuesday to Thursday. Temperatures are expected to run well above normal, with widespread highs in the 50s and 60s across the region. Persistent easterly flow associated with the ridge will promote breezy conditions through the western Columbia River Gorge and far eastern portions of the Portland and Vancouver metro area, though wind speeds are expected to remain below impactful thresholds. Specifically for Wednesday and Thursday, current ensemble guidance suggests a -5 to -6 mb pressure gradient from KPDX to KDLS and current probabilistic guidance suggests a 10-30% chance of wind gusts over 35 mph around Troutdale and in the Gorge. Offshore flow will also contribute to notably warm conditions along the coast, with highs reaching the 60s at many coastal locations.

Forecast uncertainty is highest within the Willamette Valley, where low-level inversions may support persistent low stratus, particularly during the early portion of the week. This could result in cooler daytime temperatures in valley locations compared to surrounding higher terrain that remains above the cloud layer. Confidence is higher for more widespread stratus Tuesday and Wednesday, with improved clearing potential by Thursday and Friday. Given the strength of the ridge and warm air mass aloft, some locations may approach or exceed daily temperature records, especially where sunshine prevails. Prolonged stagnation under the ridge raises concerns for air quality degradation in the Willamette Valley, particularly in central and southern portions. While offshore flow may provide some mixing for northern valley locations, air stagnation will remain a growing concern as the week progresses. ~12

AVIATION

As a frontal system moves through the airspace this morning, expect rain and MVFR conditions to persist at most terminals until around 23Z Sunday. Guidance suggests around a 50-70% chance of MVFR conditions until 23Z Sunday, as well as a as a 10-30% chance of IFR CIGs. Foggy LIFR conditions earlier this morning have come to an end as the front brings increasing winds and mixing at the surface. While conditions will improve back to VFR in the afternoon as the front exits the airspace, broken cloud cover may bring back favorable conditions for fog development around 06Z Monday. Uncertainty remains whether fog will redevelop after 06Z Monday as cloud cover heavily influences such development. Expect mostly southerly flow 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...Expect VFR through most of the period, with the exception of a passing front bringing rain and possible MVFR conditions until 23Z Sunday. Guidance suggests a 40-70% chance of MVFR conditions, with the highest probability between 16-19Z Sunday (additionally, a 20% chance of IFR conditions). Expect conditions to improve back to VFR around 21-23Z Sunday. MVFR CIGs may return around 09Z Monday (20-30% chance). Otherwise, expect generally southerly to southeasterly winds around 5-10 kt through the TAF period, with gusts up to 20 kt possible before 23Z Sunday. ~12

MARINE

South winds have increased as a surface front slowly moves through the waters this morning. This is not expected to be a particularly strong front with sound winds 15-20 kts with gusts up to 20-25 kts through the morning, then decreasing below 20 kts late this afternoon. Another weak front will graze the waters late today into Monday, with another quick increase in south winds with gusts 20-25 kts, decreasing below 20 kts by late Monday afternoon. Expect increased wave heights of 10-13 feet at 11-13 seconds with the period increasing to 14-15 seconds late today into Monday. Due to a combination of winds and seas, a Small Craft Advisory is in effect through 4 PM PST on Monday. 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 4 PM PST Monday for PZZ210-251>253- 271>273.