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FXUS66 KPQR 060418 AFDPQR

Area Forecast Discussion National Weather Service Portland OR 918 PM PDT Sun May 5 2024

SYNOPSIS
The broad low pressure system has shifted southeastward over the OR/ID border this afternoon and residual showers linger in it's wake. Active showers will persist through the next 36 hours with snow falling over the Cascades and rain elsewhere. Accumulations will be more spotty in nature than the previous days. A strengthening high pressure ridge builds in starting on Wednesday with increasing chances for some of the warmest temperatures observed this year so far.

SHORT TERM
Now through Wednesday Night...
Now through Tuesday...Current radar shows an intensification of rain showers along the Coast Range and the valleys, while precipitation has not really taken a break over the Cascades. While accumulations have been minimal, the persistent nature of the precipitation will keep conditions grey and dreary for a few days more. I mention "a few days more" because the next break is a bit farther off than previously expected. Starting well aloft at the jet stream, at 250 mb (~35,000 ft), the jet is beginning to dig southward with the forecast area sitting in the left exit region of the 150 kt jet streak. The negatively tilted jet will enhance upper level support for active weather at the surface. These conditions are coupled with a weak shortwave showing up at 500 mb (~18,000 ft)
and increased vorticity advection. These features together show an enhancement of energy which will encourage more active weather. Combine that with lower level onshore flow, orographic lifting will tap into these more active conditions aloft and thus, extend the precipitation.

At 850 mb (~5,000 ft) withs are forecast to be close to 40 kt early Monday morning during the time of the next round of rain.
Will note though that this system is not necessarily the most robust but it has stayed fairly consistent over the last few runs. Hi-resolution models are showing a period of increased precipitation Monday morning which will increase snowfall over the Cascades. Given the short time frame between systems, have decided to just extend the Winter Weather Advisory above 4000 ft through the "break" as to not over complicate things. Will say though, snow will lessen between 6 PM Sunday through around 2 AM Monday.

Conditions will become more showery on Monday. There is some tropofolding on Monday which can be associated with increased chances for thunderstorms. This feature combined with the active jet location, shortwave, and cooler temperatures aloft, have included slight chances for thunderstorms which encompasses most of the northwest Oregon and Southwest Washington. Thunder chances decrease Tuesday morning with chances for precipitation through the remainder of the day. -Muessle


LONG TERM
Wednesday through Sunday...If you're ready for a break in the rain, the long term forecast is for you. If you're looking for temperatures in the 70s, and 80s, then mid and late this week are definitely for you! A large area of high pressure builds over the Pacific starting on Wednesday. Clusters are in very good agreement of this ridging, and how amplified it is stretching from off the coast of California up through southern Canada. The extent of this high will increase easterly flow as thermal troughs form along the coast. There will be a slight gap winds through the Columbia River Gorge and downsloping off of the Coast Range. There is still a lot of room for conditions to change, but there are high probabilities of temperatures rising into the 80s on Friday. The coast will react a bit less. The NBM is showing around a 10% chance of temperatures in the 90s on Saturday within the Willamette Valley, but the other long range models have less than a 5% chance of temperatures greater than 85 degrees inland. This is because the cluster pattern would favor a transition to onshore flow. At this point, not banking on temperatures in the 90s as models have started to lower them a bit, but, it isn't a bad idea to start preparations for these warmer temperatures if they do manifest. -Muessle

AVIATION
A lull in precipitation has left mainly VFR conditions across the area and should continue for the next few hours before another frontal system off the coast comes onshore later tonight. Rain chances will increase from west to east, reaching the interior valley between 8-12Z. This will also bring MVFR CIGs back to the area with high probabilities of 80% or higher. Rain then breaks to post-frontal showers Monday morning, but confidence in the exact timing of any categorical improvement back towards VFR is low. It's worth noting there'll be slight chance(15-20%) for weak T-storm activity across the region Monday afternoon. Wind generally stay breezy out of the south at 10-25 kts - highest coastal sites.

PDX AND APPROACHES...Currently VFR with a break in precipitation ahead of an approaching frontal system. HRRR is indicating MVFR probabilities increasing (50-80%) after 8-10Z and probably persisting through about 14-16Z Monday before a switch to post- frontal shower activity. From there MVFR/VFR cigs will once again be modulated by showers. It's worth noting there'll be slight chance(15%) for weak T-storm activity locally Monday after 18-20z.
-Schuldt/Batz

MARINE
The active weather pattern continues into Monday before a gradual transition to to generally calmer/drier conditions and a much more summer-like pattern by the middle of the week. Our next and final(at least for awhile) frontal system arrives this evening into the overnight hours bringing yet another round of gusts up 25-30 kt as well as steepening seas across most of the waters through Monday morning. Following this weather disturbance high pressure begins to build over the northeast Pacific Tuesday and Wednesday leading to the development of breezy west to northwest winds (gusting 20-25 kt) across the waters come Tuesday morning. Also expect the arrival of a fairly pounced WNW 9-10 ft swell at near 12 seconds in the Monday night time-period. The broad area of high pressure shifts closer to the waters the second half of next week followed by a larger offshore component to the wind in the late Thursday through Saturday timeframe. -Schuldt

PQR WATCHES/WARNINGS/ADVISORIES
OR...Winter Weather Advisory until 11 PM PDT Monday for ORZ126>128.
WA...None.
PZ...Small Craft Advisory until 8 AM PDT Monday for PZZ210-251>253- 271>273.

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