textproduct: Portland

SYNOPSIS

Dry conditions with near normal temperatures continue Monday. A frontal system moves in on Tuesday bringing widespread rain and breezy south winds. Showers continue through Wednesday before tapering off late in the week. Dry weather with a warming trend late week into the weekend. Chances increasing for another weather system late weekend into early next week.

DISCUSSION...Monday through Sunday

Satellite imagery early Monday morning shows mid to high level clouds streaming over NW OR and SW WA from the west. These clouds are preventing temperatures from cooling quite as much as last night, and observations as of 2 AM indicate temperatures across the coast and lowlands are around 4-8 degrees warmer than 24 hours ago, generally in the low to mid 40s. The only exception is the far southern Willamette Valley where temperatures have already fallen to the mid 30s. Patchy frost is possible this morning across the lowlands for locations where temperatures do fall to around 34 degrees or lower. Upper level flow remains zonal today with high temperatures similar to or a few degrees cooler than yesterday, in the mid to upper 50s across the interior lowlands. However, high cloud cover will continue through the day ahead of an incoming weather system.

An upper level low pressure system currently located just south of Alaska deepens in the northeastern Pacific today into tomorrow. A surface closed low pressure system with an associated upper level shortwave will move northeast along the flow into western Canada today into Tuesday. The cold front associated with this low will stretch south from western Canada to offshore of northern California today, moving inland on Tuesday and producing widespread rain and breezy winds for NW OR and SW WA. Expect periods of moderate to locally heavy rain at times, but overall, total rain amounts are not expected to produce flooding impacts. 24 hour rain amounts from 5 AM Tuesday to 5 AM Wednesday are around 0.25-0.6 inch for the interior lowlands, 0.5-1 inch along the coast, 0.75-1.5 inch for the terrain north of Lane County, and 0.3-0.75 inch for the Lane County terrain.

Expect winds to peak along the coast between 8 AM to 5 PM and inland between 12 PM to 5 PM. Strongest winds will be along the coast, Coast Range, and Willapa Hills where there is a 60-80% chance of maximum wind gusts exceeding 40 mph. Inland, this probability drops to only a 10-20% chance, highest in the central Willamette Valley south of the Portland metro area to the Salem area. Chances for maximum wind gusts exceeding 30 mph for inland areas increase to around 50-75%, so most likely scenario is peak wind gusts of 30-35 mph for inland locations and 40-45 mph along the coast.

Another upper level shortwave moves east along the flow Wednesday into early Thursday. Showers will continue Wednesday, slowly tapering off Wednesday night into early Thursday, but amounts will be much lower than Tuesday. Only expect an additional 0.05-0.15 inch for interior lowlands, 0.05-0.25 for the coast, Coast Range, and Willapa Hills, and 0.1-0.4 inch for the Cascades. Snow levels are now not expected to begin falling to below pass level until early Wednesday morning when the heaviest of precipitation is tapering off, so the snow forecast continues to diminish over the Cascades. The probability of at least 6 inches of snow over the Oregon Cascades is less than 5-15%, even for the highest elevations. This increases slightly to 15-25% for the SW Washington Cascades. Snow levels do continue to lower to 1500-2000 ft Wednesday night into early Thursday morning, with a 10% chance of snow levels falling to the valley floor. However, precipitation probabilities along with amounts are so low by this point that even if some snow does fall, it would not accumulate. Additionally, roadways will be too warm, and snow would melt on impact.

By Thursday afternoon, the shortwave moves east of the area with upper level flow becoming more zonal behind it. Generally zonal flow or slight ridging continues Friday into Saturday, bringing dry conditions with clearing skies by midday Thursday into Saturday. Expect a small warming trend with daytime temperatures in the upper 50s on Thursday, and into the low to mid 60s for Friday, and upper 60s on Saturday for the interior lowlands. Saturday is expected to be the peak day of the warming trend, and NBM indicates a 20-40% chance of temperatures reaching 70 degrees across the Willamette Valley, except for a 50-65% chance for the southern Willamette Valley in Lane County.

Ensemble guidance indicates a deep Pacific trough moving east through the Pacific and approaching the West Coast on Sunday, bringing increasing chances of precipitation. However, this far out, there is quite a bit of uncertainty on the forecast solutions for Sunday as it looks to be a transition day between patterns. The 25th-75th percentile high temperature spread is anywhere from the upper 50s to the mid 70s for the Willamette Valley, and rain could begin anytime between Sunday afternoon and Monday. Will have to see how this forecast resolves. -03

AVIATION

VFR conditions expected through the TAF period under westerly flow aloft with increasing BKN high level cloud cover. Dry weather continues Monday with generally light and variable winds less than 10 kts. Temperatures between 09-16z Mon expected to drop to the low to mid 30s across the coast and Willamette Valley, leading to potential frost development on metal surfaces. A cold front approaches the coast after 10-12z Tue, with OVC mid to high level clouds. After 12z Tue, expect widespread rain and breezy south winds with lowering CIGs along the coast.

KPDX AND APPROACHES...VFR conditions through the TAF period with high level BKN cloud deck. Light northwest winds less than 10 kts. These light winds support the potential for frost development over metal surfaces between 09-16Z Mon. -03

MARINE

High pressure over the waters will maintain northerly winds through Monday. The next strong ebb will occur around 818 AM Sunday, so a Small Craft Advisory is in effect for the the Columbia River Bar between 6-11 AM Monday as the ebb builds seas around 7 to 8 ft along the Main Channel of the Bar.

A weak thermal trough along the southern Oregon coast will last through early this morning. This is tightening pressure gradients across the waters and leading to breezier northerly winds with gusts up to 25 kt. A Small Craft Advisory remains in effect for all waters except for the Columbia River Bar through at least 5 AM Monday. Northerly winds may last longer for the outer waters beyond 10 NM offshore, so the Small Craft Advisory for these zones will go through 8 AM Monday. Seas build to 7-11 ft at 11 sec as wind waves increase from the breezy northerly winds.

The next robust weather system returns Tuesday into Wednesday. This system will bring a return of breezy southerly winds across all waters. Current guidance suggests a 50-70% chance for widespread and frequent Gale force wind gusts of 34 kt or greater on Tuesday, with the highest chances beyond 20 NM offshore. Latest guidance suggests the strongest winds occur between 8 AM to 5 PM Tuesday. A Gale Watch remains in effect for all waters including the Columbia River Bar from 5 AM to 11 PM Tuesday. Seas on Tuesday and Wednesday are also expected to build to around 12-15 ft in response to increasing wind waves along with an increasing westerly swell. There's a 10-15% chance seas could exceed 15 ft with a 2-6% chance for seas exceeding 20 ft. Chances increase from south to north, peaking across the waters north of Cape Falcon and mainly beyond 40 NM offshore. -10/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 6 AM to 11 AM PDT this morning for PZZ210.

Gale Watch from late tonight through Tuesday evening for PZZ210- 251>253-271>273.

Small Craft Advisory until 5 AM PDT early this morning for PZZ251>253-271.

Small Craft Advisory until 8 AM PDT this morning for PZZ272-273.