textproduct: Portland

SYNOPSIS

Dry weather continues through the workweek as high pressure strengthens over the Pacific Northwest. Frost remains a concern across interior valleys tonight, followed by a warming trend late in the week as ridging amplifies and easterly flow develops. A return to cooler, wetter conditions is possible over the weekend as a trough approaches from the Gulf of Alaska.

DISCUSSION...Now through Sunday

High pressure will maintain generally quiet weather across northwest Oregon and southwest Washington through midweek. An upper-level low over eastern Oregon and Washington continues to weaken and shift south, while ridging builds inland from the northeast Pacific. Afternoon temperatures will gradually recover from the mid 60s today and Tuesday into the lower 70s Wednesday.

The primary near-term concern is another round of frost tonight as clear skies and light winds favor efficient radiational cooling. Interior valleys away from urban centers are expected to see temperatures fall into the mid 30s, with colder sheltered locations potentially approaching freezing. Frost Advisories have been issued for portions of northwest and west central Oregon and southwest Washington for tonight into Tuesday morning. Temperatures within the Portland metro should remain somewhat milder due to the urban heat island effect. Additionally, the Upper Hood River Valley is expected to see sub-freezing temperatures as low as 28 to 32 degrees, therefore a Freeze Warning has been issued from tonight into Tuesday morning. Expect the coldest temperatures near Parkdale. Frost and freeze conditions could kill crops, other sensitive vegetation and possibly damage unprotected outdoor plumbing. Consider temporarily covering sensitive outdoor plants with a lightweight cotton fabric (such as bed sheets, thin blankets), as this will help retain heat and provide frost protection for your plants.

Dry conditions are expected to persist through Wednesday for most of the forecast area. A weak system tracking along the northern periphery of the ridge toward British Columbia may bring a light chance of light showers to the south Washington coast Wednesday morning, but precipitation is not expected elsewhere. Frost potential should decrease after tonight as increasing cloud cover and a gradual change in the low-level wind pattern support milder overnight temperatures.

The upper-level ridge is forecast to strengthen further Thursday and Friday, allowing temperatures to climb into the mid to upper 70s across the interior lowlands. Thermal troughing is also expected to redevelop, bringing a return of easterly offshore flow through the Columbia River Gorge and Cascade gaps late in the week. While the Columbia River Gorge will likely stay gusty through each day, winds in the Cascades will likely weaken by the afternoon each day, followed by strengthening again overnight. The strength of this flow will need to be monitored as the pattern evolves.

Heading into the weekend, guidance increasingly favors an upper-level trough moving southeast from the Gulf of Alaska and displacing the ridge eastward. This would bring cooler temperatures and a renewed chance for precipitation across the region, potentially extending into early next week. Probabilistic guidance suggests a decent amount of rain over the weekend, with chances of 0.25 inches over 48 hours around 25-50% along the coast, 20-30% within the interior lowlands, and 15-30% over the Cascades. As for chances of 0.01 inches, 40-70% along the coast, 40-60% within the interior lowlands, and 30-60% over the Cascades. With considerable uncertainty in the timing, strength, and placement of the trough, confidence in the details of the weekend forecast remains limited. ~12

AVIATION...Through 18Z Tuesday

High pressure will maintain VFR conditions with clear skies through the TAF period for most locations. There's a 15-25% chance of localized areas of stratus with MVFR/IFR conditions forming again around 11-14z Tue for inland valleys and along the coast with the highest chances in the Portland area. Not enough confidence to add to the TAFs at the moment, but something to keep an eye on. Any stratus that does form should dissipate and improve to VFR by 16-18z Tue. However, a weak front brushing the PacNW will bring increased high level cloud cover from the NW after 10-12z Tue. Depending on the timing of these clouds, this could impact stratus development Tuesday morning. Northwesterly winds this afternoon around 5-7 kt or less, becoming light and variable after 02-04z Tue.

KPDX AND APPROACHES...VFR through the TAF period, though there's a 15-25% chance of MVFR/IFR stratus forming Tuesday morning between 11-14z. If this stratus does form, it should dissipate by 16-18z Tue. Northwesterly winds under 5 kt. -03

MARINE

Benign marine conditions expected this week. A Small Craft Advisory is in effect for the main channel of the Columbia River Bar from 4-9 PM this evening for a strong ebb current bringing seas to 6-7 ft. Otherwise, high pressure will maintain light northerly winds across the waters through tonight. Tuesday to Wednesday, winds turn more southerly from a weak front, then return northerly by the end of the week. Seas remain around 4-6 ft this week with periods of 13-16 seconds through Wednesday and 10-13 seconds late this week into the weekend. A long-period west/northwesterly swell will result in a moderate threat for sneaker waves this week. Caution should be used when in or near the water. -10/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from midnight tonight to 8 AM PDT Tuesday for ORZ102>109-112>119-123>125.

Freeze Warning from midnight tonight to 8 AM PDT Tuesday for ORZ121.

WA...Frost Advisory from midnight tonight to 8 AM PDT Tuesday for WAZ202-204>208.

PZ...Small Craft Advisory from 4 PM this afternoon to 9 PM PDT this evening for PZZ210.