textproduct: Portland

SYNOPSIS

The region remains under the influence of a ridge of high pressure through Monday with additional low clouds/fog and limited clearing across the inland valleys. Clearer skies above the inversion in the Coast Range/Cascades, and along the coast. Mostly dry conditions likely persist through Tuesday as a weak frontal boundary grazes the coast. Then, an active and wetter weather pattern returns the second half of Wednesday through the rest of the week.

DISCUSSION...Now Through Saturday

This afternoon satellite imagery continues to show prolific low stratus across the interior valleys as a strong temperature inversion and weaker November sun-angle/insolation struggle to facilitate any appreciable clearing along the I-5 corridor. Those along the coast and above roughly ~1000-2000ft will continue to enjoy sunny conditions into the evening. As the axis of the upper- level ridge passes overhead tonight, the temperature inversion will strengthen further with continued low stratus and fog into Monday morning across the lower elevations. Once again, clearing may be a struggle across the interior lowlands Monday afternoon, presenting the biggest point of forecast uncertainty in the short term. Even the higher resolution models like the HRRR/UW-WRF/REFS/HREF/etc., notoriously struggle in these scenarios and almost always over-mix the boundary layer resulting in more clearing than actually occurs. Today (Sunday) was a great example of this. Should we clear out (unlikely) high temperatures would jump into the mid to upper 50s, however if low clouds remained pinned over the Willamette Valley and SW Washington, highs only rise into the 40s. Nonetheless, most of our impacts during this time period revolve around fog which will reduce visibility wherever present.

Then late Monday into Tuesday the latest model ensemble guidance is in good agreement showing a low pressure system off the northern California coast moving straight east, remaining south of the forecast area. While there is still a slight chance (10-15%) a weak frontal boundary on the northern periphery of the low is able to squeeze out some light rain along the north Oregon coast, the vast majority of western Oregon and southwest Washington should remain dry. At least the weak troughing overhead and cooling temperatures in the mid- levels by Tuesday afternoon should weaken the temperature inversion and help our daytime lingering low cloud/fog situation. It's also worth quickly noting the placement of this low pressure system likely facilitates a period of locally breezy/gusty east winds through the Cascade gaps and western Columbia River Gorge, peaking Tuesday morning.

Behind this weather system passing to our south, a shortwave trough quickly pushes a frontal system toward the coast later Wednesday, increasing precipitation chances across the area through Wed night. Most ensembles indicate this system will be weakening with just light amounts of rainfall most likely - generally 0.1-0.30" coast range/Cascades, <0.10" inland valleys. Cluster analysis of 500 mb heights are in relatively good agreement heading into the latter part of the week with zonal to southwest flow aloft allowing for a somewhat active pattern over the NE Pacific to approach the coast. There is still quite a bit of uncertainty in regards to exact rainfall coverage and amounts, but there is continued potential for multiple rounds of systems moving across the area into next weekend. At this point, no significant precipitation impacts are expected with any of these weather systems, and snow levels hold around 5000-6000ft. -99/DH

AVIATION...Through 00Z Monday

Satellite imagery and terminal observations as of early Sunday afternoon depicts a persistent stratus deck across the Willamette Valley bringing a mix of IFR/MVFR CIGs and some MVFR VIS. Given that we are past peak insolation and the sun will gradually be going down in the next few hours, there is low confidence for stratus clearing out. Will maintain these IFR/MVFR trends through this evening.

Tonight, high pressure will build aloft, resulting in a strong inversion overnight. Assuming the current stratus deck does not clear out, these low clouds will prevent radiational fog development. The only scenario where we could get fog is if the high pressure aloft pushes the stratus close to the surface that it essentially becomes fog. Either way, there is high confidence for IFR/LIFR conditions tonight into early Monday morning. Meanwhile, VFR conditions prevail along the coast as light offshore flow continues. Winds inland remain variable and around 5 kt or less.

KPDX AND APPROACHES...IFR/MVFR CIGs and VIS continue through this evening with low confidence for clearing and improvement to VFR. Tonight, high pressure builds and supports lower CIGs. Expect CIGs LIFR/IFR CIGs and potentially LIFR VIS if stratus drops to the surface. Variable winds under 3-4 kt, turning more easterly after 14-16z Mon. -10

MARINE

Benign conditions are expected across the coastal waters through this evening as high pressure returns. Winds remain light below 10 kt. Light offshore winds develop late tonight and persist into early next week. Seas remain around 3 to 4 ft this afternoon, but an incoming long period west-northwest swell is expected to build seas starting this evening. Seas build to around 8 to 11 ft at 15-19 seconds Monday morning. Small Craft Advisories remain in effect for the outer coastal waters beyond 10 NM offshore for wave heights peaking at around 10 to 11 ft through Tuesday morning. This long period swell will result in a moderate threat for sneaker waves at the coast Sunday evening through Tuesday. Caution should be used at beaches when in or near the water.

A weak frontal system approaches the waters on Wednesday, returning increasing southerly winds. By the end of the week, model guidance hints at a stronger system entering the waters, resulting in a 60-80% chance for maximum wind gusts exceeding 21 kt on Friday. In addition, a steeper, longer period swell will build seas into the mid teens with a 15-20% chance for seas exceeding 20 ft. -10/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 7 AM Monday to 7 AM PST Tuesday for PZZ271>273.