textproduct: Portland

SYNOPSIS

High pressure over the area today with seasonably warm and dry conditions. Showers increase on Tuesday ahead of a cold frontal system which will push across the area Tuesday night, bringing widespread rain and Cascade snow through Wednesday. There is a slight chance of thunderstorms late Wed morning through afternoon. Showers continue Wednesday night into Thursday as snow levels fall to around 3000 ft. Anomalously strong high pressure develops over the NE Pacific Ocean later this week, though moisture riding over the ridge will maintain chances for precipitation through the end of the week and into the weekend.

DISCUSSION...Now through Sunday

Mostly cloudy skies are giving way to a few sun breaks this afternoon, as temperatures are expected to warm into the upper 50s to near 60 degrees. A weak, transient ridge of high pressure will continue to shift across the region today with mostly dry conditions expected through tonight.

Most, southwest flow increases over the area by Tuesday morning, while orographic effects allow for light showers to develops at the coast and across the terrain. As an upper level trough approaches the region, precipitation chances increase through the day. However, most lowland areas will either remain dry, or receive very light rain through Tuesday afternoon as showers remain fairly scattered. More widespread rain arrives Tuesday night into Wednesday as the upper trough pushes a cold front across the area. Showers redevelop in the post-frontal environment by late Wed morning. Cooler air aloft associated with the upper trough may allow for some deeper showers to develop through Wed afternoon. Forecast soundings are showing skinny CAPE profiles of up to 200 J/kg. Will maintain a slight chance (10-20%) of thunderstorms during this period. Total precipitation amounts will likely vary due to the showery nature, but chances for exceeding 0.50 inch of liquid precip remains high across the area, at around 70-90%. Though, chances for exceeding 1 inch over the 24-hour period along the I-5 corridor are much lower, at around 5-15%. At this point, not expecting wind impacts with this system, though could see some southwesterly wind gusts up to 30-35 mph along the coast and to around 25 mph inland, with locally stronger gusts over higher terrain as the front passes through on Wednesday.

Potential for advisory-level snow also returns to the Cascade passes as snow levels drop to around 3500-4500 feet on Wednesday. The bulk of the snow is expected between 4 AM Wednesday to 4 AM Thursday, with highest snowfall rates likely occurring Wednesday morning into Wednesday afternoon. Chances for 6+ inches of snowfall during this 24 hour period are around 50-70% along the Santiam and Willamette Passes and 20-30% for Highway 26 at Government Camp. Since we'll be in mild onshore flow, snow would most likely be wet as temperatures are marginally near freezing along the Santiam and Willamette Passes (850 mb temps fall to around 0 to -1 C Wed morning). Snow amounts likely lower north of Santiam Pass as temperatures remain a few degrees above freezing near Government Camp. Those planning to travel through the Cascades mid-week should check road conditions before leaving and carry an emergency supply kit.

Showers likely continue through Thursday as moist northwest flow persists, though precipitation amounts are expected to be much less. Snow levels drop to around 2500-3500 ft on Thursday, but any snow accumulations at elevation are expected to be minimal. More forecast uncertainty arrives later in the week and into the weekend as the majority of ensemble members suggest anomalous, broad upper-level ridging develops offshore over the northeast Pacific. The offshore placement of this ridge will not be favorable for a return to dry and sunny weather. Most ensemble guidance suggests that moisture will ride the northern or eastern periphery of the ridge and track towards the Pacific Northwest, maintaining chances for precipitation through the end of the week. How much precipitation reaches our area will largely depend on the positioning and strength of the upper ridge. Though mean QPF amounts remain relatively light, there are a few scenarios where more substantial precipitation occurs, but chances for those is currently around 5-10%, highest to the north. DH/10

AVIATION

Conditions remain varied across the region as stratus continues to dissipate. MVFR/IFR conditions along the coast may improve late this afternoon with an 80% chance of VFR conditions around 00-03z Tuesday before CIGs drop again this evening and overnight. Conditions over the southern Willamette Valley are mainly IFR with low CIGs impacting KSLE and KEUG though satellite imagery indicates clouds starting to break up. Clouds have largely cleared up in the central and northern Willamette Valley, leading to VFR conditions. Winds are generally light around 5 kt through the period.

An approaching cold front will bring lower CIGs over the region tonight. Probabilities of MVFR or lower CIGs inland increase to 20-40% after 06z Tuesday with even higher chances after 12z at 60-80%.

KPDX AND APPROACHES...VFR with light and variable winds around 5 kt expected. MVFR CIGs will spread in from the west tonight ahead of an approaching cold front. Chances after 06z are 20-40% and increase further to 60-80% after 10-12z. -19

MARINE

Benign conditions expected through tonight. Winds turn more southerly today ahead of the next system, but remain light. Seas remain around 3-4 ft at 11-12 seconds. The next frontal system arrives on Tuesday, returning breezier southerly winds. A Small Craft Advisory has been issued for all waters including the Columbia River Bar from 7 AM Tuesday to 4 AM Wednesday for southerly wind gusts up to 25 kt. Seas will also gradually build on Wednesday and Thursday as a westerly swell moves in. Chances for seas exceeding 10 ft are 40-60% during this time, with the highest chances beyond 10- 20 NM offshore. Benign conditions return toward the end of the week and into the weekend with seas dropping below 10 feet and northwest- west winds less than 20 kt. -19/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 4 AM to 10 PM PST Wednesday for ORZ127-128.

WA...None. PZ...Small Craft Advisory from 7 AM Tuesday to 4 AM PST Wednesday for PZZ210-251>253-271>273.