textproduct: Portland

SYNOPSIS

High pressure sits across the region into at least the middle of next week with dry weather prevailing. Gusty easterly winds continue through the Cascade and Coast Range gaps, and along the Columbia River Gorge into portions of the Portland metro this weekend as well. Expect overnight/morning low stratus and fog to remain limited to valley areas more protected from east winds like the southern Willamette Valley come Saturday morning before coverage gradually increases day to day going forward due to a offshore winds backing off a touch. Overnight lows near/below freezing in combination with any fog may lead to locally slick conditions - something to keep in mind.

DISCUSSION...Now through Thursday night

Currently this afternoon, visible satellite imagery depicts mostly clear skies across the region as the last of the fog in the southern Willamette Valley and southwest Washington has dissipated. A strong ridge of high pressure remains situated over the West Coast with the axis over the eastern Pacific, resulting in the continuation of an abnormally long stretch of dry and warm weather across NW Oregon and SW Washington. That said, locations which see fog and stratus linger into the late afternoon hours will stay cooler than surrounding areas albeit still dry. Additionally, morning low temperatures will be near or below freezing across our low lying areas away from the east winds from the Gorge. However, elevations around ~1000-5000 ft will see much warmer than normal low temperatures due to a strong subsidence inversion. Ensemble guidance shows high confidence that this general pattern will continue into next week.

As far as our impacts are concerned, the placement of the ridge has lead to a strong pressure gradient across the Cascades with around a -9 to -10 mb difference between KPDX to KDLS this afternoon. We'll likely a pressure gradient of this magnitude persist through most of Saturday resulting in peak wind gusts up to 40-55 mph into the Troutdale area, 25-45 mph in the eastern Portland metro, and 55-70 mph for our climatologically windiest spots like Corbett, Crown Point, Three Corner Rock, etc. A wind advisory remains in effect for the eastern Portland metro area into Saturday afternoon when winds will be strongest. Some isolated wind gusts up to 45 mph are possible in the far southeastern Vancouver metro and Portland/Vancouver metro areas outside of the Wind Advisory, especially in the eastern Washougal area and Hilltop locations in Portland, but not expecting them to be widespread or long lived enough to include in the advisory. Breezier east winds with peak gusts up to 35-45 mph are also expected through some Coast Range gaps and peaks of the Coast Range and Cascades. Most ensemble guidance indicates the pressure gradient begins to slowly weaken Saturday night into Sunday as the ridge and a thermal trough near the surface weakens. Breezy winds are still expected into Sunday and early next week just not as strong as today/Saturday. Elevated east winds are likely to continue in the Gorge into Wednesday of next week (potentially longer although lighter still), but not expecting much more than peak gusts up to 25-30 mph making it farther west than the Troutdale area beyond Sunday.

Finally a pattern change looks mix things up and revert us back to "normal" January weather sometime late Wednesday to Friday of next week. Ensemble guidance begins breaking down the high pressure by midweek, though significant uncertainty remains in the exact pattern for Wednesday into Thursday. Precipitation is likely to return to the region at some point. Around 30-40% of ensemble members from the ECMWF, GEFS, and GEPS indicate this could occur as early as late Wednesday. With this in mind the NBM brings a 15-20% chance of precipitation over the coast and higher terrain on Wednesday evening. Probabilities increase to 20-35% across the interior lowlands and 30-50% over the coast and higher terrain on Thursday as more ensemble members lock into a pattern conducive to precipitation. Even by Friday Jan 23rd, probabilities of precipitation returning are on the low to moderate side, though there is high confidence (80%) in the ensemble members in the breakdown of the strong ridge by then. At this point, no major weather systems are expected with this pattern change in the 7-10 day forecast, and any precipitation is expected to be lighter and more beneficial than impactful for most areas. However, given the time of year and antecedent conditions of a cooler airmass near the surface coupled with lingering easterly flow, we'll need to be on the lookout for a period of wintry precipitation, mainly in the Columbia Gorge and Hood River Valley as this transition takes places - more specific details at this point are rather unclear. -99/03

AVIATION

Persistent upper-level ridging will yield continued widespread VFR conditions for most terminals through the period. Sheltered portions of the southern Willamette, Lower Columbia, and Cowlitz Valleys as well as smaller adjacent valleys will see nightly fog or low stratus, with 40-50% chances or more of IFR vis within fog at KEUG after 06z Sat.

Ongoing offshore flow will maintain largely east winds in the Portland area as well as along the coast, with northerly flow instead expected along the Willamette Valley. The gustiest east winds will be constrained to the western Columbia River Gorge including near KTTD where gusts of 35-45 kt are likely to continue through the period. Higher terrain areas to the east of KTTD will be the most likely to see easterly gusts in excess of 60 kt. Gap flows through the Coast Range will also yield areas of gustier east winds at favored locations along the coast, including near KONP. Where surface winds are sheltered from stronger easterlies to the west of the Columbia River Gorge, low-level wind shear of 35-40 kt in the lowest 2 kft will continue.

KPDX AND APPROACHES...VFR flying conditions with largely clear skies are expected through the period. East winds of 10-15 kt with gusts to 25 kt continue into tonight, easing to 10 kt or less by 12z Sat as mixing ceases and the surface decouples. -36

MARINE

Strong upper level high pressure continues into the weekend with a thermally-induced surface trough along the coast favoring persistent easterly offshore flow. While winds are largely at 10 kt or less, areas downwind of gaps in the coastal terrain may see stronger flow with gusts as high as 20-25 kt. The most widespread area of these stronger gusts continues west of the mouth of the Columbia River, between Cape Foulweather and Cape Falcon beyond 10 NM, where a Small Craft Advisory remains in effect through 9 PM this evening. East winds otherwise persist through the weekend, generally less than 15-20 kt. Seas remain below 10 ft around 10-13 seconds through early next week. -03/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory from 7 PM Saturday to 10 AM PST Tuesday for ORZ109-114>118. Wind Advisory until 3 PM PST Saturday for ORZ112. WA...None. PZ...Small Craft Advisory until 9 PM PST this afternoon for PZZ271.