textproduct: Portland

SYNOPSIS

High pressure across the region maintains dry conditions into the middle of next week. Gusty winds continue through the Cascade and Coast Range gaps, and along the Columbia River Gorge into portions of the Portland metro into the weekend. Expect overnight/morning low stratus and fog to remain limited to valley areas more protected from east winds like the southern Willamette Valley. Offshore winds decrease a touch the second half of the weekend with mostly clear skies, chilly mornings, and mild afternoons, albeit with fog increasing in coverage.

DISCUSSION...Friday morning through Thursday

Satellite imagery along with observations at 2 AM PST show fog has developed in the southern Willamette Valley as well as near the I-5 corridor in Cowlitz and Columbia counties. This fog could spread north into the central Willamette Valley and south along the I-5 corridor through sunrise, though due to strong east winds through the Columbia River Gorge, don't expect it to spread much further. A strong ridge of high pressure remains situated over the West Coast with the axis over the eastern Pacific, resulting in dry weather and warmer than normal days across most of NW Oregon and SW Washington. One difference is locations that see fog and stratus linger into the late afternoon hours won't see near normal (cooler) daytime temperatures than surrounding areas. Additionally, morning low temperatures will be near or below freezing in 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.

At the surface, a thermally induce trough has formed under the strong upper level high pressure, leading to a tightening of surface pressure gradients. Current pressure gradient from PDX to DLS is around -8 to -9 mb, which is resulting in the aforementioned strong easterly wind flow through the western Columbia River Gorge. This pressure gradient is forecast to remain around -8 to -10 mb through most of the weekend. This will result in peak wind gusts up to 40-55 mph into the Troutdale area, 35-45 mph in the eastern Portland metro, and 55-65+ 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 eastern Vancouver metro and Portland/Vancouver metro areas outside of the Wind Advisory, especially in the eastern Washougal area, but not expecting them to be widespread or long long enough to include in the advisory. Breezier east winds with peak gusts up to 35-45 mph are also possible through some Coast Range gaps and peaks of the Coast Range and Cascades. Most ensemble guidance indicates the pressure gradient begins to slowly weaken on Sunday as the thermal trough decreases in strength. Breezy winds are still expected into Sunday, just not as strong as Friday and Saturday, and will be slowly decreasing through the day. Elevated east winds are likely to continue in the Gorge into early next week, but not expecting much more than peak gusts up to 25-30 mph making it farther west than the Troutdale area beyond Sunday.

A pattern change looks to be on the horizon sometime mid to late 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. Some GFS, Euro, and Canadian ensemble members indicate this could be as early as Wednesday, which is leading to a 20-40% chance of precipitation in the NBM over the coast and Terran on Wednesday. Probabilities increase to 20-30% across the interior lowlands and 30-50% over the coast and terrain on Thursday as more ensemble members suggest precipitation on Thursday. 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. -03

AVIATION

Dry northerly flow aloft as an upper level ridge of high pressure sits just offshore. A strong inversion remains in place with cooler, moist air near the surface below the dry and warm air aloft. Offshore winds have also increased, especially through gaps in the terrain such as the Columbia River Gorge. Areas influenced by these winds are likely to remain well mixed and thus fog and cloud free. VFR conditions are expected to persist across the Portland metro area, as well as at the coast. Other areas more protected from these winds are likely to see a return of fog and/or stratus overnight, though the northern extent remains somewhat uncertain. As of 10z, satellite imagery and surface observations across the southern Willamette Valley show increasing coverage of dense fog and LIFR conditions, as well as in the lowlands of Cowlitz and Columbia counties. Expect these conditions to persist at KEUG through 20z Friday. There are lower chances, around 20-30%, for IFR conditions to spread northward across portions of the central Willamette Valley by 14-16z Friday, with similar probabilities for the Tualatin Valley. But, conditions are likely to improve quickly after 20z Friday for most locations. The main exception would be the southern valley where northerly, upslope flow maintains the moisture there. Winds will be strongest out of the Columbia River Gorge, with gusts up to 40 kt likely at KTTD and up to 45-60+ kt east of KTTD, continuing into Saturday morning.

KPDX AND APPROACHES...VFR with mostly clear skies expected through the period. Surface winds at the terminal will likely remain relatively light through 18-20z Friday, though gusty east winds may mix down at times. Will maintain the mention of LLWS as east winds around 40 kt are expected to persist between 1000-2000 ft MSL. Approaches to the east are more likely to see these gustier winds near the surface. East winds are likely to increase at the terminal after 18z Friday with gusts up to 25 kts continuing through Friday night. -DH/03

MARINE

Strong upper level high pressure continues into the weekend with a surface thermally induce trough just over the coast. Winds will shift easterly this morning due to the thermal trough. Widespread winds have decreased below 10 kts, but gusts up to 20 kts, occasionally up to 25 kts, are possible downwind of gaps in the coastal terrain, especially in zone PZZ271. Will likely end the Small Craft Advisory for all zones by 4 AM PST today, though occasional gusts over 21 kts are possible through today. Winds remain easterly through the weekend, generally less than 15-20 kts. Seas should remain below 10 ft around 10-13 seconds through early next week. -03

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.

Dense Fog Advisory until noon PST today for ORZ116>118.

WA...None. PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ271.

Small Craft Advisory until 4 AM PST early this morning for PZZ272-273.