textproduct: Portland

SYNOPSIS

Cooler and wetter conditions are expected moving into the weekend as a significant pattern change takes hold while leaving our previously warm/hot conditions in the rear- view mirror for the foreseeable future. After several rounds of widespread shower activity and a 10-20% chance for thunderstorms Saturday afternoon, conditions trend a little drier and slightly warmer Sunday through early next week. However, regionally we'll still maintain slight chances for rain showers mainly across terrain features like the Coast Range and Cascades.

DISCUSSION...Today through Wednesday

Mid to high cloud cover continues to stream into the Pacific Northwest this afternoon thanks to an approaching low pressure system ushering in a period of cooler and showery conditions. While most the region remains dry for the next few hours, a cold-frontal boundary likely brings rain to the coast by the late afternoon/evening hours before it quickly spreads eastward into the I-5 corridor and then the Cascades tonight. We'll experience cooler than normal temperatures and shower chances through at least Saturday into early Sunday although rainfall amounts appear beneficial and generally non-impactful at this time. Looking at the latest NBM, chances for 48-hour rainfall amounts exceeding 1 inch from 5 PM today to 5 PM Saturday are around 30-55% for the Coast and Coast Range, 5-10% along the I-5 corridor, and 60-80% or higher across the Cascades due to westerly flow bringing orographic enhancement. It's also worth quickly noting snow-levels (that's not a term we've used in a awhile) bottom out around 5000-6000ft on Saturday so those in the higher elevations of the Cascades have a shot at seeing some wet snow in the air - minimal impacts.

We continue to monitor the potential for weak thunderstorms the next several days as the upper-level low pressure originating from the Gulf of Alaska tracks overhead. The latest guidance has backed off on T-storm chances Friday afternoon (<10% chance) due in-part to a lack of convective depth that does not appear conducive to electrification. While models depict 50-200 j/kg of CAPE, deterministic model soundings confine this instability around and below 0C. You generally need instability to extend well into the -10C to -25C mix phased region of the atmosphere to produce lightning, and it appears we won't meet that requirement on Friday. The shallow nature of the convection may end up being an issue on Saturday as well but there's at least a model or two suggesting slightly higher convective depth comparatively, especially right under the core of the low passing overhead during the afternoon hours, so the 15-20% chance for thunderstorms remains. High resolution guidance during this period suggests effective shear remains below 20-25 kt too, therefore, chances for any thunderstorms becoming severe are very low (<5%). Otherwise, any strong shower or passing weak thunderstorm may produce brief heavy rain, gusty and erratic winds, and a lightning strike or two. Also can't rule out an isolated cold air funnel cloud given the time of year, especially on Saturday.

The latter half of the weekend into early next week, the majority of ensemble members are showing the broader upper- level low feature shifting eastward and lingering along the interior mountain west, returning relatively drier conditions and a warm-up of a few degrees to our area. This will put high temperatures just below the seasonal normals with low 70s for the interior lowlands and upper 50s to low 60s for the coast). Will still maintain precipitation chances across the Coast Range and Cascades as westerly flow continues to support orographic enhancement of precipitation, though any additional rain amounts appear very light. Still, we'll have to keep an eye on subtle shortwave features (like late on Sunday) rotating into the region north to south on the back side of the upper-level low which may expand the coverage of showers temporarily. There's only low to moderate confidence on the placement/timing of these smaller shortwave features. Ensemble members then suggest another upper-level trough dropping down into the area on Wednesday into Thursday, however, there is also very high uncertainty with how much precipitation we get, if any. Westerly winds throughout the week will be breeziest along the central Columbia River Gorge and across Upper Hood River Valley, with chances for isolated wind gusts exceeding 45 mph around 10-35% from Friday through early next week, with the highest chances from Sunday to Tuesday. -99/10

AVIATION

Cool and moist onshore flow will bring widespread, light precipitation and cloud cover across the airspace through the TAF period. For coastal locations, expect IFR/LIFR conditions through around 18Z-20Z Friday with conditions improving towards MVFR/IFR conditions. For inland locations expect a mixture of VFR/MVFR conditions, with brief periods of IFR/LIFR possible (10-20% chance) at any given hour through the TAF period. Winds are expected to slowly become west/southwest less than 10 kt across the airspace with gusts up to 25 kt along the coast through around 12Z Friday.

KPDX AND APPROACHES...Mixture of VFR/MVFR conditions through the TAF period as light precipitation spreads across the region. 10-20% chance for brief periods of IFR/LIFR conditions at any given hour through the TAF period. West/southwest winds less than 10 kt. /42

MARINE

Expect increasing south to southwesterly winds across the coastal waters through tonight as an occluding front approaches the area. Strongest winds with gusts to around 25 kt are expected across the inner coastal waters north of Cape Foulweather and up to 10-20 NM offshore. Latest model guidance suggests winds increase slightly earlier, so have adjusted the Small Craft Advisory to begin at 9 PM this evening. Still expect winds to ease below SCA thresholds by 5 AM Friday as winds turn more westerly as the front pushes onshore.

West to northwest winds will persist behind the front later Friday and into Saturday as weak low pressure near Vancouver Island slides south near the Washington coast. High pressure gradually builds across the waters Sunday into early next week. Seas around 4 to 5 ft this afternoon build to around 7 to 9 ft this weekend as a west-northwesterly swell moves through the waters. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 AM PDT Friday for PZZ210-251-252.