textproduct: Portland

SYNOPSIS

High pressure remains anchored over the region through Wednesday. Cool and dry conditions lead a favorable environment for frost and fog development during the overnight and morning hours through Thursday morning. A pattern shift is in store beginning Thursday with relatively warmer air and precipitation returning, continuing into early next week.

DISCUSSION...Tuesday Afternoon through Monday

Visible satellite imagery Tuesday afternoon depicts mostly clear skies across NW Oregon and SW Washington except for a few lingering areas of stratus in interior valleys. Upper level high pressure remains over the region today with the axis over western OR/WA, and a thermally induced trough remains at the surface. This has caused a tightening of pressure gradients over the Cascades, leading to gusty east winds through Cascade valleys, especially the western Columbia River Gorge. Gusts up to 25-35 mph are expected in the western Portland metro area with up to 35-45 mph possible around Troutdale through this afternoon. Local gusts up to 50-55 mph have been observed at the typical windiest locations such as Corbett, Crown Point, and Three Corner Rock. The upper level ridge axis begins moving east of the Cascades later this afternoon and through Wednesday, allowing the thermal trough and subsequent surface pressure gradients to weaken. Breezy east winds will subside slightly this evening, though gusts up to 35-40 mph are still possible east of Troutdale. Winds continue subsiding through the day Wednesday.

Cold temperatures are expected again tonight into Wednesday morning and again Wednesday night into Thursday morning due to mostly clear skies with temperatures in the upper 20s to low 30s across inland valleys, mid to upper 30s along the coast, and low 20s to low 30s in the Cascades. Coldest temperatures will be tonight into Wednesday morning. Additionally, fog and low stratus once again have the potential to form during the same time periods, mainly along the I-5 corridor from the southern Willamette Valley north through Cowlitz County. Both fog and frost are less likely in the Portland metro area, especially on the east side, due to the increased east winds from the Gorge.

Another area of concern is stagnant air under this ridging. A strong inversion remains in place through Wednesday afternoon with stable air in place near the surface. This will allow for an increase in air quality concerns, especially for locations outside of the windier eastern Portland metro area. If you are susceptible to air quality concerns, be sure to monitor air quality through Air Now.

A pattern change is on the way for Thursday, with wet conditions returning and continuing into early next week. Ensemble guidance indicates a longwave trough deepening over the eastern Pacific on Thursday as a separate closed low approaches central/southern California from the west. Some uncertainty remains in the specifics of the pattern on Thursday into Saturday, but the general consensus amongst the GFS, Euro, and Canadian ensembles is on Thursday, the low approaching California will continue moving east with a secondary weak shortwave moving north along the main longwave trough into the PacNW. This shortwave will carry enough moisture from the closed low to bring precipitation chances spreading north into NW Oregon and SW Washington sometime on Thursday, though uncertainty remains on whether the rain will start earlier in the day on Thursday or late Thursday night into Friday morning. Then the longwave trough slowly moves east through the eastern Pacific on Friday into Saturday, with rain continuing in the region. Additionally, warmer air will accompany the rain beginning Thursday with 850 mb temperatures remaining above 0 deg C through Saturday, and snow levels will rise to above 4500-5500 feet, meaning snow is only likely over the high Cascades. Ensembles indicate a frontal system will move inland from the west sometime on Saturday accompanied by a stronger wave along the longwave trough. This could bring a period of breezy winds along the frontal passage, but strong winds are not expected. NBM indicates around a 50% chance along the coast of gusts above 40 mph with a 10-25% chance for inland areas.

The specifics of the weather pattern become much more uncertain in ensemble guidance Sunday into Monday, but in general, upper level troughing remains, bringing continued rainy conditions. None of the rain Thursday into Monday is expected to be impactful or cause flooding concerns and instead will be more of a typical winter rain for the PacNW. Looking at NBM's potential 72 hour rain totals from 4 AM Friday through 4 AM Monday, the mean scenario (or most likely scenario) suggests around 0.75-1 inch for interior lowlands and 0.75-2 inches for the coast and Cascades. The high end scenario (10% chance of exceeding these values) indicates around 1.25-1.75 inches for the interior lowlands, 1.3-3 inches for the Cascades, and 2-4 inches for the Coast. For the low end scenario (90% chance of exceeding these values), NBM indicates 0.25-0.4 inches for the interior lowlands and 0.4-0.9 inches for the Cascades and coast. -03

AVIATION

Persistent upper-level ridging over the region will continue to favor largely VFR flying conditions through the period. As temperatures fall to near-freezing overnight, there remains uncertainty whether fog or frost will be favored across inland valleys. Dry easterly flow will limit the fog potential west of the Columbia Gorge including across the Portland area, but Willamette Valley terminals will have at least a 10-20% chance of IFR vis within fog overnight, most likely from 13-17z Wed. The highest chances for fog will be across the southern Willamette Valley including KEUG. Otherwise, light east winds continue aside from northerly flow along the Willamette Valley and much stronger east flow near the Columbia Gorge at KTTD, where sustained winds to 20 kt with gusts to 30 kt may continue through Wednesday before local pressure gradients begin to weaken substantially. Winds at 1-2 kft across the Portland area remain out of the east, but have eased below 35-40 kt and below thresholds for low-level wind shear inclusion in TAFs.

KPDX AND APPROACHES...VFR conditions expected to persist through the period. East winds at 5-10 kt continue, with stronger surface flow out of the Columbia Gorge expected to remain confined to the east metro. Winds at 1-2 kft remain stronger, but have eased below 35-40 kt and below thresholds for low-level wind shear inclusion in TAFs. Dry east flow will minimize chances for overnight fog near KPDX terminal, but patchy fog may encroach from downstream along the Columbia River near sunrise. -36

MARINE

High pressure remains over the region through the middle of the week. Stronger high pressure inland will maintain offshore winds across the coastal waters through tonight, before becoming more southerly on Wednesday. Winds expected to remain around 10 kt or less, while seas persist around 6 to 7 ft through Wed night. Southeasterly winds increase on Thursday to around 15 kt as a low pressure system begins to develop well offshore from the northern California coast. This low pressure system is expected to strengthen offshore late in the week, although there remains considerable uncertainty regarding the strength and position of the low into the weekend. That said, there is around a 60-70% chance of Gale force wind gusts occurring across the waters sometime late Friday into Saturday. These stronger winds, combined with a developing fresh southerly swell will likely push seas into the lower to mid teens by Saturday, with a 10-15% chance of exceeding 20 ft. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from noon to 4 PM PST Wednesday for PZZ210.