textproduct: Portland

SYNOPSIS

Conditions remain dry through this evening before a weak cold front brings low to moderate chances for light rain late tonight into Saturday morning. High pressure returns dry weather Saturday night into Sunday. Low chances for rain on Monday as a low pressure system likely remains well offshore. Seasonable temperatures and dry weather expected Tuesday and Wednesday before another chance for light rain returns Thursday.

SHORT TERM...Now through Sunday Night

Water vapor satellite imagery this afternoon shows the upper level ridge shifting east over the Canadian Rockies while southwest flow increases aloft over the Pacific NW. Warm and dry weather continues through this evening while offshore surface pressure gradients weaken and the flow turns back onshore.

Winds become more southerly ahead of a weak cold front that will approach the coast later this evening. As an upper level shortwave trough lifts across northwestern WA early Saturday, chances for light rain will increase along the coast and then spread inland through Sat morning. Mean QPF values range from a trace to 0.10 inch, generally north of Newport at the coast and from Eugene northward along the I-5 corridor. Conditions expected to dry out by the afternoon with northerly winds behind the front. Highs will be closer to normal, topping out in the mid to upper 60s across the interior valleys. Will need to keep an eye on Saturday night as clouds clear and temperatures drop into the 30s. Outside of the Cascades, the coolest temperatures are expected in the Upper Hood River Valley.

Dry weather is expected on Sunday as a relatively flat shortwave ridge passes over the region. Seasonable temperatures continue with highs peaking in the mid-60s for most interior lowlands and mid-50s to lower 60s along the coast. Sunday night into Monday morning will bring another chance for frost in the Upper Hood River Valley and the southern Willamette Valley as lows could drop into the mid to upper 30s. /DH

LONG TERM...Monday through Thursday Night

There is generally good agreement among models and their ensembles that a rather robust low pressure system will move across the northeastern Pacific toward the Oregon coast early Monday, but remain far enough offshore before dropping south to bring any meaningful impacts to the area. In fact, chances for even light rain have generally decreases to around 25% or less, mainly near the coast and Coast Range south of Astoria. The upper level trough over the Pacific is expected to split with the main low pressure area moving south toward the central California coast on Tuesday. Ensemble clusters remain in decent agreement for the upper level low to persist near the California coast on Wednesday, while shortwave ridging nudges into the Pacific NW. There is high confidence for dry conditions to persist across the area Tuesday through Wednesday, with temperatures warming back into the mid to upper 60s. More uncertainly returns later next week as another upper level trough begins to drop out of the Gulf of Alaska. Chances for rain increase on Thursday, though currently remain less than 40%. /DH

AVIATION...Through 06Z Sunday

Currently VFR inland and LIFR fog/stratus along the coast. A frontal system approaches from the northwest, expected to arrive at KAST by 07Z Sat, KPDX by 12Z Sat, and KEUG by 13Z Sat. This system will move through the airspace rather quickly, resulting in the possibility of only brief MVFR conditions and rain. There is limited confidence for rain at various terminals, with the highest confidence along the north OR/south WA coast (KAST, KTMK). While most coastal terminals will stay under LIFR/IFR conditions until at least 18Z Sat, KAST may be the one exception as the frontal system could provide enough mixing to clear the stratus out by 11-13Z Sat. As for inland terminals, there is a 30% chance of rain and 30-60% chance of MVFR CIGs between 11-16Z Sat. Otherwise, expect VFR conditions with high cloud coverage at inland terminals through the rest of the TAF period. While KAST will likely stay VFR through the remainder of the period after 11-13Z Sat, there little confidence regarding conditions at KONP, with probabilistic guidance suggesting low chances (15-25%) of LIFR/IFR conditions but deterministic guidance suggesting fog.

Expect mostly southerly winds under 8 kt overnight, then becoming north to northwesterly around 5 to 10 kt by the afternoon. KONP will be the one exception, with sustained winds around 8 to 12 kt and gusts up to 18 kt between 21Z Sat and 03Z Sun.

KPDX AND APPROACHES...Mostly VFR conditions with increasing high cloud cover through the TAF period. A passing front will bring chances for MVFR CIGs and light rain between 12-15Z Sat. Southerly winds under 8 kt ahead of the frontal system will become northwesterly by the afternoon. ~12

MARINE

Winds back today into tonight ahead of an approaching cold front which will traverse the waters late tonight, before veering out of the northwest Saturday morning behind the frontal passage. Seas of 5-7 ft at around 12 seconds otherwise continue through Saturday. The mid-period northwest swell will then build over the weekend, with seas rising to 8-10 ft. Chances for seas to exceed 10 ft reach 10-20% beyond 30 NM north of Cape Falcon. This swell will also pose a moderate risk for sneaker waves along the coast while overall seas remain relatively benign; caution should be used by those in or near the water along coastal beaches through this weekend.

As another weather system looks to track southward well offshore of the Oregon and Washington coasts early next week, and may bring a period of steeper seas Monday into Tuesday although there remains uncertainty in the timing and potential peak wave height. The latest forecast depicts the westerly swell reaching 11-12 ft at 12-14 seconds Monday afternoon and evening before easing below 10 ft by Tuesday morning, with a 10-20% chance significant wave heights reach 15 ft or higher beyond 20 NM. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.