textproduct: Portland

SYNOPSIS

An anomalous area of high pressure remains planted over the Pacific Northwest into next week likely (90%) maintains dry weather through at least Jan. 20-21st. For valley locations below roughly 1000 ft fog (dense at times), low stratus, and cool temperatures continue into Thursday. Elevations above 1000 ft will remain much warmer and drier, with clear skies going forward. However, as offshore flow strengthens on Friday breezy/gusty east winds are expected to impact the Cascades gaps/foothills, western Columbia River Gorge and portions of the Portland metro. At least this change should also bring an end to the prolific fog and low stratus in the lowlands within reach of those easterly winds - more protected areas like the southern Willamette Valley may still see fog linger overnight.

DISCUSSION...Now through Tuesday night

An unusually strong area of high pressure will drive our weather impacts for the next 6-7 days as it generally sits overhead. This high pressure is coupled with a rather defined temperature inversion which already has resulted in vastly differing weather conditions for elevations below 1000 feet compared to elevations above 1000 feet - this trend continues through at least Thursday for much of the region. Above 1000 ft expect clear skies and warmer temperatures compared to below 1000 feet where cooler, cloudier and foggier persist. Headed into Thursday morning areas of dense fog are likely yet again for the lowland valleys with the REFS projecting a 70-95% chance to see <0.25 mile visibility for much of the I-5 corridor including the Portland/Vancouver. If you woke up with dense fog this (Wednesday) morning, you'll more than likely get a repeat tonight. Be prepared to drive slowly if you must commute as fog will pose a travel hazard. Pedestrians and bikers should wear reflective clothing. Note with the continued stagnant airmass in place, air quality may become degraded at times over the lowlands through Thursday night as well.

As mentioned earlier, elevations above 1000 feet will be warmer and significantly drier as they poke into the upper portions of the low-level temperature inversion and see plenty of sunshine. In fact, high temperatures in the low to mid 60s are in the forecast for the Cascade foothills, Coast Range lowlands and central Oregon coast, warmest in the Lane County Cascade foothills. It's worth highlighting humidity values between 2000-4000 feet where the driest portion of the inversion interfaces with the terrain will experience extremely poor afternoon and nighttime relative humidity values for this time of year with many RAWS stations in the exposed portions in the Cascades and coast range reporting RHs in the single digits to teens. Given these dry antecedent conditions the thin strip between the low level moisture in the valleys and where snow is present at elevation will be a place to monitor when east winds increase.

By Thursday night/Friday morning, models and their ensembles continue to suggest low-level offshore flow will rapidly strengthen, bringing breezy/gusty east winds to the Cascades, Cascade foothills, tops of the Coast Range, western Columbia River Gorge, and portions of the Portland metro. East winds are forecast to reach the coast as well as a surface thermal trough along the southern/central OR coast begins to shift slightly offshore. This will bring an end to fog and air stagnation for these locations. Meanwhile, the central and southern Willamette Valley may see at least some patchy fog occur again Thursday night/Friday morning again as this wind-sheltered area will have minimal influence from the increasing offshore flow. Offshore flow will most likely remain in place Friday night through the upcoming weekend, albeit in a weaker state as the cross-Cascade pressure gradient eases. This should maintain mostly clear conditions over the area, aside from the central and southern Willamette Valley where low stratus and fog concerns will likely persist. Note overnight temperatures are expected to remain chilly each night through at least Monday night, particularly in the Willamette Valley and Upper Hood River Valley where below freezing low temperatures are expected. That said, apparent temperatures are not forecast to be cold enough to warrant any cold weather advisories at this time. Ensemble guidance hints at the ridge finally breaking down around Wednesday of next week however there is significant uncertainty regarding the exact timing of this change. -99/23

AVIATION

High pressure remains firmly in control over the area, supporting IFR/LIFR stratus and areas of dense fog throughout the Willamette Valley. Locations south of the metro have pretty much been socked in through the day with no improvement expected until possibly Thursday afternoon. Conditions along the coast are expected to fluctuate a bit more. MVFR stratus covers KAST with guidance suggesting possible VIS reductions overnight but this may not persist for long. In the southern Willamette Valley around KEUG, temperatures will fall near or below freezing again. Therefore there is an increased chance for FZFG through the morning.

Easterly winds will begin to increase after 12z due to an intensifying pressure gradient. Through the Columbia River Gorge, gusty winds will initiate as early as 15Z Thu with speeds of 20 kt or greater. At KTTD, by 18Z Thu, models are suggesting gusts up to 30 kt.

KPDX AND APPROACHES...Stratus briefly scattered out earlier this evening but is already beginning to reform. Fog is also expected to develop again with models suggesting 1/2 mi visibility developing between 6-8z. Easterly winds from the Gorge will make it into the terminal Thursday morning. Will not see speeds nearly as strong, but could see gusts up to 20 kt, and VIS improving considerably. Models are also suggesting CIGs improving before noon as the stronger winds mix out the clouds. /19

MARINE

High pressure over the area will maintain more settled conditions through the next week. In the short-term, seas have begun to rise with heights of 8-11 ft at 13 seconds. These seas took a bit longer than previously forecast, but are well on their way for continued Small Craft Advisory heights through at least Thursday morning. On Thursday, the thermal trough will develop with a northerly wind increasing. Will see the wave based Small Craft Advisory transition into one based on wind speeds. Due to the location of the ridge, the northern and inner waters are less likely to experience gusts greater than 25 kt. Therefore, the advisory remains in place for the central and southern waters.

On Friday, winds will ease and unremarkable conditions will prevail. No concerns in the long-term forecast. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 4 AM PST Friday for ORZ109>111- 114>118. Dense Fog Advisory until noon PST Thursday for ORZ114>118. WA...Air Stagnation Advisory until 4 AM PST Friday for WAZ205-206. PZ...Small Craft Advisory until 1 AM PST Thursday for PZZ251-271. Small Craft Advisory until 10 PM PST Thursday for PZZ252-253- 272-273.