textproduct: Portland

SYNOPSIS

Upper level ridging will bring warmer spring-like temperatures and light winds through at least Thursday with high temperatures mainly in the 60s, except lower 70s for inland valleys in Lane County and mid 50s to lower 60s over southwest WA and the north Oregon coast. Mainly dry conditions will persist each day south of a line extending from Lincoln City to McMinnville to Troutdale to Stevenson, while locations north of that line will see occasional rain Monday night through Thursday, especially in Pacific County and along the north Oregon coast. Trending a touch cooler on Friday with a 30-60% chance of rain showers, except 60-80% along the coast, coastal mountains, and south WA/north OR Cascades. The forecast becomes more uncertain over the weekend in regards to temperatures and precipitation amounts.

DISCUSSION...Now through Sunday night

There are no signs of any high impact weather this week across northwest Oregon and southwest Washington. The forecast remains highlighted by upper level ridging with relatively warm spring-like temperatures, light winds, and mainly dry conditions across most of northwest Oregon now through at least Thursday. However, rain is likely at times (>60% chance) Monday night through Thursday over northern portions of the forecast area, particularly to the north of a line extending from Lincoln City to McMinnville to Troutdale to Stevenson. Rain will be most persistent over Pacific County and the far north Oregon coast; these areas have forecast rain totals ranging between 1.25-2.50 inches from 11 PM PDT Monday through 5 AM PDT Friday. Since these rain amounts are extended over a prolonged period of time with unimpressive hourly rain rates, flooding is not a concern. Confidence is high flooding will not be a concern even if the worst case scenario verified, as the NBM 90th percentile for 72-hour rain amounts ending at 5 AM PDT Friday is not high enough to suggest flooding will occur (max amounts up to 3.0 inches, rather than 1.25-2.50 inches like the official deterministic forecast suggests). Nevertheless, these areas will be much wetter than the rest of southwest Washington and northwest Oregon this week, and will therefore experience relatively cooler temperatures. Expect high temperatures mainly in the mid to upper 60s across the lowlands each day through Thursday or Friday, except lower 70s for inland valleys in Lane County and mid 50s to lower 60s over southwest WA and the north Oregon coast.

The forecast becomes more uncertain over the weekend as model ensemble guidance begins to differ on how deamplified the upper level ridge over the western CONUS will become. WPC's cluster analysis for 500 mb heights and anomalies highlights this uncertainty well, showing around a 20-40% chance the ridge will deamplify enough to result in a transition from southwest flow aloft to nearly zonal flow, and a 60-80% chance the ridge will remain amplified enough to result in continued southwest flow aloft. If the ridge does deamplify enough for zonal flow aloft to develop, expect relatively cooler temperatures will occasional light rain showers. If the ridge remains amplified with continued southwest flow aloft, expect mainly dry conditions with warmer temperatures. The NBM 10-90th percentile for high temperatures reflects this uncertainty well, ranging from the lower 50s to mid 70s on Saturday and from the lower 50s to lower 60s on Sunday. Large model spread is evident for low temperatures too, ranging from the lower 30s to mid 50s Saturday night/Sunday morning. Regardless of the outcome, confidence is high there will be no high impact weather to worry about. NBM PoPs seem to reflect the most likely outcome of continued southwest flow aloft, only peaking between 20-40% west of the Cascades on Saturday and Sunday and 40-50% in the Cascades. -23

AVIATION

A broad upper level ridge with mid-level moisture will maintain west to northwest flow aloft. Light scattered rain showers return to the north Oregon and southwest Washington coast after 08- 10z Tue, then spread inland for locations generally north of Lane County over the following few hours, continuing through around 03- 06z Wed. Expect widespread VFR conditions to continue through the period at most terminals. Conditions along the coast may drop to MVFR/IFR with higher chances north of KTMK with showers. There's a 70-90% chance of MVFR cigs after 06-09z Tue with a 30-50% chance of IFR cigs. South of KTMK, there's a 40-60% chance of MVFR and IFR cigs. Generally southerly winds less than 5 kt expected through the TAF period except for at KAST where gust up to 20-25 kts continue through 21-23z Tue.

KPDX AND APPROACHES...Mainly VFR conditions expected with light and variable winds less than 5 kts. Light rain showers move in after 10- 12z Tue along with a low (10-20%) chance of MVFR cigs. -03/19

MARINE

Southerly winds across all waters expected through the middle of the week. Wind gusts up to 25 kt will be possible through Tuesday as a cold front sags southward toward the northern Oregon and southern Washington coasts. Small Craft gusts will be possible from Cape Foulweather to Cape Shoalwater, including the Columbia River Bar. Small Craft Advisories are out through 4 am Wednesday. Seas will build tonight to between 8 and 10 feet at 10 to 12 seconds as winds increase and a southwesterly to westerly swell moves into the waters.

Conditions improve by the middle of the week. Another weak front may pass through the region Thursday, bringing a chance (30%) for low end Small Craft gusts. -19

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 AM PDT Wednesday for PZZ210-251- 252-271-272.